Mechanisms of Recognition and Rejection of Allografts

Rejection is a complex process in which both cell-mediated immunity and circulating antibodies play a role⁹⁹; moreover, the contributions of these two mechanisms are often reflected in the histologic features of the rejected organs.

T Cell–Mediated Reactions

The critical role of T cells in transplant rejection has been documented both in humans and in experimental animals. T cell–mediated graft rejection is called cellular rejection, and it involves destruction of graft cells by CD8+ CTLs and delayed hypersensitivity reactions triggered by activated CD4+ helper cells. The major antigenic differences between a donor and recipient that result in rejection of transplants are differences in highly polymorphic HLA alleles. The recipient’s T cells recognize donor antigens from the graft (the allogeneic antigens, or alloantigens) by two pathways, called direct and indirect (Fig. 6-39).¹⁰⁰

• In the direct pathway, T cells of the transplant recipient recognize allogeneic (donor) MHC molecules on the surface of APCs in the graft. It is believed that dendritic cells carried in the donor organs are the most important APCs for initiating the antigraft response, because they not only express high levels of class I and II MHC molecules but also are endowed with costimulatory molecules (e.g., B7-1 and B7-2). The T cells of the host encounter the donor dendritic cells either within the grafted organ or after the dendritic cells migrate to the draining lymph nodes. CD8+ T cells recognize class I MHC mdecules and differentiate into active CTLs, which can kill the graft cells by mechanisms already discussed. CD4+ helper T cells recognize allogeneic class II molecules and proliferate and differentiate into T_H1 (and possibly T_H17) effector cells. Cytokines secreted by the activated CD4+ T cells trigger a delayed hypersensitivity reaction in the graft, resulting in increased vascular permeability and local accumulation of mononuclear cells (lymphocytes and macrophages), and graft injury caused by the activated macrophages. The direct recognition of allogeneic MHC molecules seems paradoxical to the rules of self-MHC restriction: If T cells normally are restricted to recognizing foreign peptides displayed by self-MHC molecules, why should these T cells recognize foreign MHC? The probable explanation is that allogeneic MHC molecules, with their bound peptides, resemble, or mimic, the self-MHC–foreign peptide complexes that are recognized by self-MHC–restricted T cells. Thus, recognition of allogeneic MHC molecules is a cross-reaction of T cells selected to recognize self-MHC plus foreign peptides.

• In the indirect pathway of allorecognition, recipient T lymphocytes recognize MHC antigens of the graft donor after they are presented by the recipient’s own APCs. This process involves the uptake and processing of MHC and other foreign molecules from the grafted organ by host APCs. The peptides derived from the donor tissue are presented by the host’s own MHC molecules, like any other foreign peptide. Thus, the indirect pathway is similar to the physiologic processing and presentation of other foreign (e.g., microbial) antigens. The indirect pathway generates CD4+ T cells that enter the graft and recognize graft antigens being displayed by host APCs that have also entered the graft, and the result is a delayed hypersensitivity type of reaction. However, CD8+ CTLs that may be generated by the indirect pathway cannot directly recognize or kill graft cells, because these CTLs recognize graft antigens presented by the host’s APCs. Therefore, when T cells react to a graft by the indirect pathway, the principal mechanism of cellular rejection may be T-cell cytokine production and delayed hypersensitivity. It is postulated that the direct pathway is the major pathway in acute cellular rejection, whereas the indirect pathway is more important in chronic rejection. However, this separation is by no means absolute.

FIGURE 6-39 Recognition and rejection of organ allografts. In the direct pathway, donor class I and class II MHC antigens on antigen-presenting cells in the graft (along with costimulators, not shown) are recognized by host CD8+ cytotoxic T cells and CD4+ helper T cells, respectively. CD4+ cells proliferate and produce cytokines (e.g., IFN-γ), which induce tissue damage by a local delayed hypersensitivity reaction. CD8+ T cells responding to graft antigens differentiate into CTLs that kill graft cells. In the indirect pathway graft antigens are picked up, processed, and displayed by host APCs and activate CD4+ T cells, which damage the graft by a local delayed hypersensitivity reaction and stimulate B lymphocytes to produce antibodies.

Rejection of Kidney Grafts

Because kidneys were the first solid organs to be transplanted and more kidneys have been transplanted than any other organ, much of our understanding of the clinical and pathologic aspects of solid-organ transplantation is based on studies of renal allografts.

Morphology. On the basis of the morphology and the underlying mechanism, rejection reactions are classified as hyperacute, acute, and chronic. The morphologic changes in these patterns are described below as they relate to renal transplants. Similar changes may occur in any other vascularized organ transplant and are discussed in relevant chapters.

Hyperacute Rejection. This form of rejection occurs within minutes or hours after transplantation. A hyperacutely rejecting kidney rapidly becomes cyanotic, mottled, and flaccid, and may excrete a mere few drops of bloody urine. Immunoglobulin and complement are deposited in the vessel wall, causing endothelial injury and fibrin-platelet thrombi (Fig. 6-40A). Neutrophils rapidly accumulate within arterioles, glomeruli, and peritubular capillaries. As these changes become diffuse and intense, the glomeruli undergo thrombotic occlusion of the capillaries, and fibrinoid necrosis occurs in arterial walls. The kidney cortex then undergoes outright necrosis (infarction), and such nonfunctioning kidneys have to be removed.

FIGURE 6-40 Morphology of hyperacute and acute graft rejection. A, Hyperacute rejection of a kidney allograft showing endothelial damage, platelet and thrombin thrombi, and early neutrophil infiltration in a glomerulus. B, Acute cellular rejection of a kidney allograft with inflammatory cells in the interstitium and between epithelial cells of the tubules. C, Acute humoral rejection of a kidney allograft (rejection vasculitis) with inflammatory cells and proliferating smooth muscle cells in the intima.

(Courtesy of Dr. Helmut Rennke, Department of Pathology, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA.)

Acute Rejection. This may occur within days of transplantation in the untreated recipient or may appear suddenly months or even years later, after immunosuppression has been used and terminated. In any one patient, cellular or humoral immune mechanisms may predominate. Histologically, humoral rejection is associated with vasculitis, whereas cellular rejection is marked by an interstitial mononuclear cell infiltrate.

Acute cellular rejection is most commonly seen within the initial months after transplantation and is heralded by clinical and biochemical signs of renal failure (Chapter 20). Histologically, there may be extensive interstitial mononuclear cell infiltration and edema as well as mild interstitial hemorrhage (Fig. 6-40B). As might be expected, immunohistochemical staining reveals both CD4+ and CD8+ T lymphocytes, which express markers of activated T cells, such as the α chain of the IL-2 receptor. Glomerular and peritubular capillaries contain large numbers of mononuclear cells that may also invade the tubules, causing focal tubular necrosis. In addition to causing tubular damage, CD8+ T cells may injure vascular endothelial cells, causing a so-called endothelitis. The affected vessels have swollen endothelial cells, and at places the lymphocytes can be seen between the endothelium and the vessel wall. The recognition of cellular rejection is important because, in the absence of an accompanying humoral rejection, patients respond well to immunosuppressive therapy. Cyclosporine, a widely used immunosuppressive drug, is also nephrotoxic, and hence the histologic changes resulting from cyclosporine may be superimposed.

Acute humoral rejection (rejection vasculitis) is mediated by antidonor antibodies, and hence it is manifested mainly by damage to the blood vessels. This may take the form of necrotizing vasculitis with endothelial cell necrosis, neutrophilic infiltration, deposition of immunoglobulins, complement, and fibrin, and thrombosis. Such lesions are associated with extensive necrosis of the renal parenchyma. In many cases, the vasculitis is less acute and is characterized by marked thickening of the intima with proliferating fibroblasts, myocytes, and foamy macrophages (Fig. 6-40C). The resultant narrowing of the arterioles may cause infarction or renal cortical atrophy. The proliferative vascular lesions mimic arteriosclerotic thickening and are believed to be caused by cytokines that cause proliferation of vascular smooth muscle cells. Deposition of the complement breakdown product C4d in allografts is a strong indicator of humoral rejection, because C4d is produced during activation of the complement system by the antibody-dependent classical pathway.^101,102 The importance of making this diagnosis is that it provides a rationale for treating affected patients with B cell–depleting agents.

Chronic Rejection. In recent years acute rejection has been significantly controlled by immunosuppressive therapy, and chronic rejection has emerged as an important cause of graft failure.¹⁰³ Patients with chronic rejection present clinically with a progressive renal failure manifested by a rise in serum creatinine over a period of 4 to 6 months. Chronic rejection is dominated by vascular changes, interstitial fibrosis, and tubular atrophy with loss of renal parenchyma (Fig. 6-41). The vascular changes consist of dense, obliterative intimal fibrosis, principally in the cortical arteries. These vascular lesions result in renal ischemia, manifested by glomerular loss, interstitial fibrosis and tubular atrophy, and shrinkage of the renal parenchyma. The glomeruli may show scarring, with duplication of basement membranes; this appearance is sometimes called chronic transplant glomerulopathy. Chronically rejecting kidneys usually have interstitial mononuclear cell infiltrates of plasma cells and numerous eosinophils.

FIGURE 6-41 Chronic rejection of a kidney allograft. A, Changes in the kidney in chronic rejection. B, Graft arteriosclerosis. The vascular lumen is replaced by an accumulation of smooth muscle cells and connective tissue in the vessel intima.

(Courtesy of Dr. Helmut Rennke, Department of Pathology, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA.)

Transplantation of Other Solid Organs

In addition to the kidney, a variety of organs, such as the liver (Chapter 18), heart (Chapter 12), lungs, and pancreas, are also transplanted. The rejection reaction against liver transplants is not as vigorous as might be expected from the degree of HLA disparity. The molecular basis of this “privilege” is not understood.

Transplantation of Hematopoietic Cells

Use of hematopoietic stem cell transplants for hematologic malignancies, certain nonhematologic cancers, aplastic anemias, thalassemias, and certain immunodeficiency states is increasing. Transplantation of genetically engineered hematopoietic stem cells may also be useful for somatic cell gene therapy, and is being evaluated in some immunodeficiencies. Hematopoietic stem cells are usually obtained from the bone marrow but may also be harvested from peripheral blood after they are mobilized from the bone marrow by administration of hematopoietic growth factors. In most of the conditions in which bone marrow transplantation is indicated, the recipient is irradiated to destroy the immune system (and sometimes, cancer cells) and to create a graft bed. Several features distinguish bone marrow transplants from solid-organ transplants. Two problems that are unique to bone marrow transplantation are graft-versus-host (GVH) disease and immunodeficiency.

GVH disease occurs in any situation in which immunologically competent cells or their precursors are transplanted into immunologically crippled recipients, and the transferred cells recognize alloantigens in the host.¹⁰⁶ It is seen most commonly in the setting of bone marrow transplantation but, rarely, may occur following transplantation of solid organs rich in lymphoid cells (e.g., the liver) or transfusion of unirradiated blood. When immune-compromised recipients receive normal bone marrow cells from allogeneic donors, the immunocompetent T cells present in the donor marrow recognize the recipient’s HLA antigens as foreign and react against them. To try to minimize GVH disease, bone marrow transplants are done between donor and recipient that are HLA-matched using sensitive DNA sequencing methods for molecular typing of HLA alleles.

Acute GVH disease occurs within days to weeks after allogeneic bone marrow transplantation. Although any organ may be affected, the major clinical manifestations result from involvement of the immune system and epithelia of the skin, liver, and intestines. Involvement of skin in GVH disease is manifested by a generalized rash that may lead to desquamation in severe cases. Destruction of small bile ducts gives rise to jaundice, and mucosal ulceration of the gut results in bloody diarrhea. Although tissue injury may be severe, the affected tissues are usually not heavily infiltrated by lymphocytes. It is believed that in addition to direct cytotoxicity by CD8+ T cells, considerable damage is inflicted by cytokines released by the sensitized donor T cells.

Chronic GVH disease may follow the acute syndrome or may occur insidiously. These patients have extensive cutaneous injury, with destruction of skin appendages and fibrosis of the dermis. The changes may resemble systemic sclerosis (discussed earlier). Chronic liver disease manifested by cholestatic jaundice is also frequent. Damage to the gastrointestinal tract may cause esophageal strictures. The immune system is devastated, with involution of the thymus and depletion of lymphocytes in the lymph nodes. Not surprisingly, the patients experience recurrent and life-threatening infections. Some patients develop manifestations of autoimmunity, postulated to result from the grafted CD4+ helper T cells reacting with host B cells and stimulating these cells, some of which may be capable of producing autoantibodies.

Because GVH disease is mediated by T lymphocytes contained in the donor bone marrow, depletion of donor T cells before transfusion virtually eliminates the disease. This protocol, however, has proved to be a mixed blessing: GVH disease is ameliorated, but the incidence of graft failures and EBV-related B-cell lymphoma and the recurrence of disease in leukemic patients increase. It seems that the multifaceted T cells not only mediate GVH disease but also are required for engraftment of the transplanted marrow stem cells, suppression of EBV-infected B-cell clones, and control of leukemic cells. The latter, called graft-versus-leukemia effect, can be quite dramatic. Deliberate induction of graft-versus-leukemia effect by infusion of allogeneic T cells is being used in the treatment of chronic myelogenous leukemia when patients relapse after bone marrow transplantation.

Immunodeficiency is a frequent complication of bone marrow transplantation. The immunodeficiency may be a result of prior treatment, myeloablative preparation for the graft, a delay in repopulation of the recipient’s immune system, and attack on the host’s immune cells by grafted lymphocytes. Affected individuals are profoundly immunosuppressed and are easy prey to infections. Although many different types of organisms may infect patients, infection with cytomegalovirus is particularly important. This usually results from activation of previously silent infection. Cytomegalovirus-induced pneumonitis can be a fatal complication.

X-Linked Agammaglobulinemia (Bruton’s Agammaglobulinemia)

X-linked agammaglobulinemia is one of the more common forms of primary immunodeficiency.¹¹⁰ It is characterized by the failure of B-cell precursors (pro-B cells and pre-B cells) to develop into mature B cells. During normal B-cell maturation in the bone marrow, the Ig heavy-chain genes are rearranged first, in pre-B cells, and these are expressed on the cell surface in association with a “surrogate” light chain, where they deliver signals that induce rearrangement of the Ig light-chain genes and further maturation. This need for Ig-initiated signals is a quality control mechanism that ensures that maturation will proceed only if functional Ig proteins are expressed. X-linked agammaglobulinemia is caused by mutations in a cytoplasmic tyrosine kinase, called Bruton tyrosine kinase (Btk); the gene that encodes it is located on the long arm of the X chromosome at Xq21.22.⁹⁵ Btk is a protein tyrosine kinase that is associated with the Ig receptor complex of pre-B and mature B cells and is needed to transduce signals from the receptor. When it is mutated, the pre-B cell receptor cannot deliver signals, and maturation stops at this stage. Because light chains are not produced, the complete antigen receptor molecule (which contains Ig heavy and light chains) cannot be assembled and transported to the cell membrane.

As an X-linked disease, this disorder is seen almost entirely in males, but sporadic cases have been described in females, possibly caused by mutations in some other gene that functions in the same pathway. The disease usually does not become apparent until about 6 months of age, as maternal immunoglobulins are depleted. In most cases, recurrent bacterial infections of the respiratory tract, such as acute and chronic pharyngitis, sinusitis, otitis media, bronchitis, and pneumonia, call attention to the underlying immune defect. Almost always the causative organisms are Haemophilus influenzae, Streptococcus pneumoniae, or Staphylococcus aureus. These organisms are normally opsonized by antibodies and cleared by phagocytosis. Because antibodies are important for neutralizing infectious viruses that are present in the bloodstream or mucosal secretions or being passed from cell to cell, individuals with this disease are also susceptible to certain viral infections, especially those caused by enteroviruses, such as echovirus, poliovirus, and coxsackievirus. These viruses infect the gastrointestinal tract, and from here they can disseminate to the nervous system via the blood. Thus, immunization with live poliovirus carries the risk of paralytic poliomyelitis, and echovirus can cause fatal encephalitis. For similar reasons, Giardia lamblia, an intestinal protozoan that is normally resisted by secreted IgA, causes persistent infections in persons with this disorder. In general, however, most intracellular viral, fungal, and protozoal infections are handled quite well by the intact T cell–mediated immunity.

The classic form of this disease has the following characteristics:

Autoimmune diseases, such as arthritis and dermatomyositis, occur with increased frequency, in as many as 35% of individuals with this disease, which is paradoxical in the presence of an immune deficiency. It is likely that these autoimmune disorders are caused by a breakdown of self-tolerance resulting in autoimmunity, but chronic infections associated with the immune deficiency may play a role in inducing the inflammatory reactions. The treatment of X-linked agammaglobulinemia is replacement therapy with immunoglobulins. In the past, most patients succumbed to infection in infancy or early childhood. Prophylactic intravenous Ig therapy allows most individuals to reach adulthood.

Common Variable Immunodeficiency

This relatively common but poorly defined entity represents a heterogeneous group of disorders.^111,112 The feature common to all patients is hypogammaglobulinemia, generally affecting all the antibody classes but sometimes only IgG. The diagnosis of common variable immunodeficiency is based on exclusion of other well-defined causes of decreased antibody production.

As might be expected in a heterogeneous group of disorders, both sporadic and inherited forms of the disease occur. In familial forms there is no single pattern of inheritance. Relatives of such patients have a high incidence of selective IgA deficiency (see later). These studies suggest that at least in some cases, selective IgA deficiency and common variable immunodeficiency may represent different expressions of a common genetic defect in antibody synthesis. In contrast to X-linked agammaglobulinemia, most individuals with common variable immunodeficiency have normal or near-normal numbers of B cells in the blood and lymphoid tissues. These B cells, however, are not able to differentiate into plasma cells.

Both intrinsic B-cell defects and abnormalities in T helper cell–mediated activation of B cells may account for the antibody deficiency in this disease. Families have been reported in which the underlying abnormality is in a receptor for a cytokine called BAFF that promotes the survival and differentiation of B cells, or in a molecule called ICOS (inducible costimulator) that is homologous to CD28 and is involved in T-cell activation and in interactions between T and B cells.¹¹¹ However, the known mutations account for a minority of cases.

The clinical manifestations of common variable immunodeficiency are caused by antibody deficiency, and hence they resemble those of X-linked agammaglobulinemia. The patients typically present with recurrent sinopulmonary pyogenic infections. In addition, about 20% of patients present with recurrent herpesvirus infections. Serious enterovirus infections causing meningoencephalitis may also occur. Individuals with this disorder are also prone to the development of persistent diarrhea caused by G. lamblia. In contrast to X-linked agammaglobulinemia, common variable immunodeficiency affects both sexes equally, and the onset of symptoms is later—in childhood or adolescence. Histologically the B-cell areas of the lymphoid tissues (i.e., lymphoid follicles in nodes, spleen, and gut) are hyperplastic. The enlargement of B-cell areas probably reflects defective regulation, that is, B cells can proliferate in response to antigen but do not produce antibodies, and therefore the normal feedback inhibition by IgG is absent.

As in X-linked agammaglobulinemia, these patients have a high frequency of autoimmune diseases (approximately 20%), including rheumatoid arthritis. The risk of lymphoid malignancy is also increased, and an increase in gastric cancer has been reported.

Isolated IgA Deficiency

Isolated IgA deficiency is a common immunodeficiency. In the United States it occurs in about 1 in 600 individuals of European descent.¹¹³ It is far less common in blacks and Asians. Affected individuals have extremely low levels of both serum and secretory IgA. It may be familial, or acquired in association with toxoplasmosis, measles, or some other viral infection. The association of IgA deficiency with common variable immunodeficiency was mentioned earlier. Most individuals with this disease are asymptomatic. Because IgA is the major Ig in external secretions, mucosal defenses are weakened, and infections occur in the respiratory, gastrointestinal, and urogenital tracts. Symptomatic patients commonly present with recurrent sinopulmonary infections and diarrhea. Some individuals with IgA deficiency are also deficient in the IgG2 and IgG4 subclasses of IgG. This group of patients is particularly prone to developing infections. In addition, IgA-deficient patients have a high frequency of respiratory tract allergy and a variety of autoimmune diseases, particularly SLE and rheumatoid arthritis. The basis of the increased frequency of autoimmune and allergic diseases is not known. When transfused with blood containing normal IgA, some of these patients develop severe, even fatal, anaphylactic reactions, because the IgA behaves like a foreign antigen (since the patients do not produce it and are not tolerant to it).

The basic defect in IgA deficiency is impaired differentiation of naive B lymphocytes to IgA-producing cells. The molecular basis of this defect in most patients is still unknown. Defects in a receptor for the B cell–activating cytokine, BAFF, have been described in some patients.

Hyper-IgM Syndrome

In hyper-IgM syndrome the affected patients make IgM antibodies but are deficient in their ability to produce IgG, IgA, and IgE antibodies. It is now known that the defect in this disease affects the ability of helper T cells to deliver activating signals to B cells and macrophages. As discussed earlier in the chapter, many of the functions of CD4+ helper T cells require the engagement of CD40 on B cells, macrophages and dendritic cells by CD40L (also called CD154) expressed on antigen-activated T cells. This interaction triggers Ig class switching and affinity maturation in B cells, and stimulates the microbicidal functions of macrophages. Approximately 70% of individuals with hyper-IgM syndrome have the X-linked form of the disease, caused by mutations in the gene encoding CD40L located on Xq26.¹¹⁴ In the remaining persons the disease is inherited in an autosomal recessive pattern. Most of these patients have mutations in the gene encoding CD40 or the enzyme called activation-induced deaminase, a DNA-editing cytosine deaminase that is required for class switching and affinity maturation.

The serum of persons with this syndrome contains normal or elevated levels of IgM but no IgA or IgE and extremely low levels of IgG. The number of B and T cells is normal. Many of the IgM antibodies react with elements of blood, giving rise to autoimmune hemolytic anemia, thrombocytopenia, and neutropenia. In older patients there may be uncontrolled proliferation of IgM-producing plasma cells with infiltrations of the gastrointestinal tract. Although the proliferating B cells are polyclonal, extensive infiltration may lead to death.

Clinically, individuals with the hyper-IgM syndrome present with recurrent pyogenic infections, because the level of opsonizing IgG antibodies is low. In addition, those with CD40L mutations are also susceptible to pneumonia caused by the intracellular organism Pneumocystis jiroveci, because of the defect in cell-mediated immunity.

DiGeorge Syndrome (Thymic Hypoplasia)

DiGeorge syndrome is a T-cell deficiency that results from failure of development of the third and fourth pharyngeal pouches. The latter give rise to the thymus, the parathyroids, some of the clear cells of the thyroid, and the ultimobranchial body. Thus, individuals with this syndrome have a variable loss of T cell–mediated immunity (resulting from hypoplasia or lack of the thymus), tetany (resulting from lack of the parathyroids), and congenital defects of the heart and great vessels. In addition, the appearance of the mouth, ears, and facies may be abnormal. Absence of cell-mediated immunity is caused by low numbers of T lymphocytes in the blood and lymphoid tissues and poor defense against certain fungal and viral infections. The T-cell zones of lymphoid organs—paracortical areas of the lymph nodes and the periarteriolar sheaths of the spleen—are depleted. Ig levels may be normal or reduced, depending on the severity of the T-cell deficiency.

DiGeorge syndrome is not a familial disorder. It results from the deletion of a gene that maps to chromosome 22q11.¹¹⁵ This deletion is seen in 90% of patients, and DiGeorge syndrome is now considered a component of the 22q11 deletion syndrome, discussed in Chapter 5. One mutation that has been associated with the DiGeorge syndrome affects a member of the T-box family of transcription factors, which may be involved in development of the branchial arch and the great vessels.

Severe Combined Immunodeficiency

Severe combined immunodeficiency (SCID) represents a constellation of genetically distinct syndromes, all having in common defects in both humoral and cell-mediated immune responses.¹¹⁶ Affected infants present with prominent thrush (oral candidiasis), extensive diaper rash, and failure to thrive. Some patients develop a morbilliform rash shortly after birth because maternal T cells are transferred across the placenta and attack the fetus, causing GVH disease. Persons with SCID are extremely susceptible to recurrent, severe infections by a wide range of pathogens, including Candida albicans, P. jiroveci, Pseudomonas, cytomegalovirus, varicella, and a whole host of bacteria. Without bone marrow transplantation, death occurs within the first year of life. Despite the common clinical manifestations, the underlying defects are quite different in different forms of SCID, and in many cases the genetic lesion is not known. Often, the SCID defect resides in the T-cell compartment, with a secondary impairment of humoral immunity.

The most common form, accounting for 50% to 60% of cases, is X-linked, and hence SCID is more common in boys than in girls. The genetic defect in the X-linked form is a mutation in the common γ-chain (γc) subunit of cytokine receptors. This transmembrane protein is part of the signal-transducing components of the receptors for IL-2, IL-4, IL-7, IL-9, IL-11, IL-15, and IL-21. IL-7 is required for the survival and proliferation of lymphoid progenitors, particularly T-cell precursors. As a result of defective IL-7 receptor signaling, there is a profound defect in the earliest stages of lymphocyte development, especially T-cell development.¹¹⁷ T-cell numbers are greatly reduced, and although B cells are normal in number, antibody synthesis is severely impaired because of lack of T-cell help. IL-15 is important for the maturation and proliferation of NK cells, and because the common γ chain is a component of the receptor for IL-15, these individuals often have a deficiency of NK cells as well.

The remaining cases of SCID are inherited as autosomal recessive. The most common cause of autosomal recessive SCID is a deficiency of the enzyme adenosine deaminase (ADA). Although the mechanisms by which ADA deficiency causes SCID are not entirely clear, it has been proposed that deficiency of ADA leads to accumulation of deoxyadenosine and its derivatives (e.g., deoxy-ATP), which are toxic to rapidly dividing immature lymphocytes, especially those of the T-cell lineage.¹¹⁸ Hence there may be a greater reduction in the number of T lymphocytes than of B lymphocytes.

Several other less common causes of autosomal recessive SCID have been discovered:

The histologic findings in SCID depend on the underlying defect. In the two most common forms (ADA deficiency and γc mutation), the thymus is small and devoid of lymphoid cells. In SCID caused by ADA deficiency, remnants of Hassall’s corpuscles can be found, whereas in X-linked SCID the thymus contains lobules of undifferentiated epithelial cells resembling fetal thymus. In either case other lymphoid tissues are hypoplastic as well, with marked depletion of T-cell areas and in some cases both T-cell and B-cell zones.

Currently, bone marrow transplantation is the mainstay of treatment, but X-linked SCID is the first human disease in which gene therapy has been successful.¹²² For gene therapy a normal γc gene is expressed in bone marrow stem cells of patients using a retroviral vector, and the cells are transplanted back into the patients. The clinical experience is small, but some patients have shown reconstitution of their immune systems for over a year after therapy. Unfortunately, however, 20% of these patients have developed acute T-cell leukemias, which appear to have been triggered by the activation of oncogenes by the integrated retrovirus,¹²³ highlighting the dangers of this particular approach to gene therapy. Patients with ADA deficiency have also been treated with bone marrow transplantation and, more recently, with gene therapy to introduce a normal ADA gene into T-cell precursors.

Immunodeficiency with Thrombocytopenia and Eczema (Wiskott-Aldrich Syndrome)

Wiskott-Aldrich syndrome is an X-linked recessive disease characterized by thrombocytopenia, eczema, and a marked vulnerability to recurrent infection, ending in early death.¹²⁴ The thymus is morphologically normal, at least early in the course of the disease, but there is progressive secondary depletion of T lymphocytes in the peripheral blood and in the T-cell zones (paracortical areas) of the lymph nodes, with variable loss of cellular immunity. Patients do not make antibodies to polysaccharide antigens, and the response to protein antigens is poor. IgM levels in the serum are low, but levels of IgG are usually normal. Paradoxically the levels of IgA and IgE are often elevated. Patients are also prone to developing non-Hodgkin B-cell lymphomas. The Wiskott-Aldrich syndrome is caused by mutations in the gene encoding Wiskott-Aldrich syndrome protein (WASP), which is located at Xp11.23. This protein belongs to a family of proteins that are believed to link membrane receptors, such as antigen receptors, to cytoskeletal elements. The WASP protein may be involved in cytoskeleton-dependent responses, including cell migration and signal transduction, but the essential functions of this protein in lymphocytes and platelets are unclear. The only treatment is bone marrow transplantation.

Genetic Deficiencies of the Complement System

The complement system plays critical roles in host defense and inflammation. Hereditary deficiencies have been described for virtually all components of the complement system and several of the regulators.¹²⁵ A deficiency of C2 is the most common of all. With a deficiency of C2 or the other early components of the classical pathway (i.e., C1 [C1q, r, or s] or C4), there is little or no increase in susceptibility to infections, but the dominant manifestation is an increased incidence of an SLE-like autoimmune disease, as discussed earlier. Presumably, the alternative complement pathway is adequate for the control of most infections. Deficiency of components of the alternative pathway (properdin and factor D) is rare. It is associated with recurrent pyogenic infections. The C3 component of complement is required for both the classical and alternative pathways, and hence a deficiency of this protein results in susceptibility to serious and recurrent pyogenic infections. There is also increased incidence of immune complex–mediated glomerulonephritis; in the absence of complement, immune complex–mediated inflammation is presumably caused by Fc receptor–dependent leukocyte activation. The terminal components of complement C5, 6, 7, 8, and 9 are required for the assembly of the membrane attack complex involved in the lysis of organisms. With a deficiency of these late-acting components, there is increased susceptibility to recurrent neisserial (gonococcal and meningococcal) infections; Neisseria bacteria have thin cell walls and are especially susceptible to the lytic actions of complement. Some patients inherit a form of mannose-binding lectin, the plasma protein that initiates the lectin pathway of complement, that does not polymerize normally and is functionally defective. These individuals also show increased susceptibility to infections.

A deficiency of C1 inhibitor gives rise to hereditary angioedema.¹²⁶ This autosomal dominant disorder is more common than complement deficiency states. The C1 inhibitor is a protease inhibitor whose target enzymes are C1r and C1s of the complement cascade, factor XII of the coagulation pathway, and the kallikrein system. As discussed in Chapter 2, these pathways are closely linked, and their unregulated activation can give rise to vasoactive peptides such as bradykinin. Although the exact nature of the bioactive compound produced in hereditary angioedema is uncertain, these patients have episodes of edema affecting skin and mucosal surfaces such as the larynx and the gastrointestinal tract. This may result in life-threatening asphyxia or nausea, vomiting, and diarrhea after minor trauma or emotional stress. Acute attacks of hereditary angioedema can be treated with C1 inhibitor concentrates prepared from human plasma.

Deficiency of other complement-regulatory proteins is the cause of paroxysmal nocturnal hemoglobinuria. In this disease there are mutations in enzymes required for glycophosphatidyl inositol linkages, which are essential for the assembly of decay-accelerating factor and CD59, both of which regulate complement.¹²⁷ Uncontrolled complement activation on the surface of red cells is believed to be the basis of hemolysis (Chapter 14). Mutations in the complementregulatory protein factor H underlie about 10% of cases of a renal disease called hemolytic uremic syndrome, which is characterized by microvascular thrombosis in the kidneys (Chapter 20).

Epidemiology

Epidemiologic studies in the United States have identified five groups of adults at risk for developing AIDS. The case distribution in these groups is as follows:

The epidemiology of AIDS is quite different in children under age 13. Close to 2% of all AIDS cases occur in this pediatric population, and worldwide over 500,000 new cases and almost 400,000 deaths were reported in children in the year 2006. In this group the vast majority acquired the infection by transmission of the virus from mother to child (discussed later).

It should be apparent from the preceding discussion that transmission of HIV occurs under conditions that facilitate exchange of blood or body fluids containing the virus or virus-infected cells. The three major routes of transmission are sexual contact, parenteral inoculation, and passage of the virus from infected mothers to their newborns.

In addition to male-to-male and male-to-female transmission, there is evidence supporting female-to-male transmission. HIV is present in vaginal secretions and cervical cells of infected women. In the United States this form of heterosexual spread is approximately 20-fold less common than male-to-female transmission. By contrast, in Africa and parts of Asia, the risk of female-to-male transmission is much higher. This observation is believed to be attributable to the presence of concurrent sexually transmitted disease. All forms of sexual transmission of HIV are enhanced by coexisting sexually transmitted diseases, especially those associated with genital ulceration. In this regard, syphilis, chancroid, and herpes are particularly important. Other sexually transmitted diseases, including gonorrhea and chlamydia, are also cofactors for HIV transmission, perhaps because in these genital inflammatory states there is greater concentration of the virus and virus-containing cells in genital fluids, as a result of increased numbers of inflammatory cells in the semen.

Transmission of HIV by transfusion of blood or blood products, such as lyophilized factor VIII and factor IX concentrates, has been virtually eliminated. This fortunate outcome resulted from increasing use of recombinant clotting factors and from three public health measures: screening of donated blood and plasma for antibody to HIV, stringent purity criteria for factor VIII and factor IX preparations, and screening of donors on the basis of history. However, an extremely small risk of acquiring AIDS through transfusion of seronegative blood persists, because a recently infected individual may be antibody-negative. Currently, this risk is estimated to be 1 in more than 2 million units of blood transfused. Because it is now possible to detect HIV-associated p24 antigens in the blood before the development of humoral antibodies, this small risk is likely to decrease even further.

Much concern has arisen in the lay public and among health care workers about spread of HIV infection outside the high-risk groups. Extensive studies indicate that HIV infection cannot be transmitted by casual personal contact in the household, workplace, or school. Spread by insect bites is virtually impossible. Regarding transmission of HIV infection to health care workers, an extremely small but definite risk seems to be present. Seroconversion has been documented after accidental needle-stick injury or exposure of nonintact skin to infected blood in laboratory accidents. After needle-stick accidents, the risk of seroconversion is believed to be about 0.3%, and antiretroviral therapy given within 24 to 48 hours of a needle stick can reduce the risk of infection eightfold. By comparison, approximately 30% of those accidentally exposed to hepatitis B–infected blood become seropositive.

Etiology: The Properties of HIV

AIDS is caused by HIV, a nontransforming human retrovirus belonging to the lentivirus family. Included in this group are feline immunodeficiency virus, simian immunodeficiency virus, visna virus of sheep, bovine immunodeficiency virus, and the equine infectious anemia virus.

Two genetically different but related forms of HIV, called HIV-1 and HIV-2, have been isolated from patients with AIDS. HIV-1 is the most common type associated with AIDS in the United States, Europe, and Central Africa, whereas HIV-2 causes a similar disease principally in West Africa and India. Specific tests for HIV-2 are available, and blood collected for transfusion is routinely screened for both HIV-1 and HIV-2 seropositivity. The ensuing discussion relates primarily to HIV-1 and diseases caused by it, but the information is generally applicable to HIV-2 as well.

Structure of HIV

Similar to most retroviruses, the HIV-1 virion is spherical and contains an electron-dense, cone-shaped core surrounded by a lipid envelope derived from the host cell membrane (Fig. 6-43). The virus core contains (1) the major capsid protein p24; (2) nucleocapsid protein p7/p9; (3) two copies of genomic RNA; and (4) the three viral enzymes (protease, reverse transcriptase, and integrase). p24 is the most readily detected viral antigen and is the target for the antibodies that are used for the diagnosis of HIV infection in the widely used enzyme-linked immunosorbent assay. The viral core is surrounded by a matrix protein called p17, which lies underneath the virion envelope. Studding the viral envelope are two viral glycoproteins, gp120 and gp41, which are critical for HIV infection of cells.

FIGURE 6-43 The structure of the human immune deficiency virus (HIV)–1 virion. The viral particle is covered by a lipid bilayer derived from the host cell and studded with viral glycoproteins gp41 and gp120.

The HIV-1 RNA genome contains the gag, pol, and env genes, which are typical of retroviruses (Fig. 6-44). The products of the gag and pol genes are translated initially into large precursor proteins that are cleaved by the viral protease to yield the mature proteins. The highly effective anti-HIV-1 protease inhibitor drugs prevent viral assembly by inhibiting the formation of mature viral proteins. In addition to these three standard retroviral genes, HIV contains several other accessory genes, including tat, rev, vif, nef, vpr, and vpu, that regulate the synthesis and assembly of infectious viral particles and the pathogenicity of the virus.^128-130 For example, the product of the tat (transactivator) gene causes a 1000-fold increase in the transcription of viral genes and is therefore critical for virus replication. The functions of other accessory proteins are indicated in Figure 6-44.

FIGURE 6-44 The HIV genome. Several viral genes and the functions of the encoded proteins are illustrated. The genes outlined in red are unique to HIV; others are shared by all retroviruses.

Molecular analysis of different HIV-1 isolates has revealed considerable variability in certain parts of their genome. Most variations are clustered in particular regions of the envelope glycoproteins. Because the humoral immune response against HIV-1 is targeted against its envelope, such variability poses problems for the development of a single antigen vaccine. On the basis of genetic analysis, HIV-1 can be divided into three subgroups, designated M (major), O (outlier), and N (neither M nor O). Group M viruses are the most common form worldwide, and they are further divided into several subtypes, or clades, designated A through K. Various subtypes differ in their geographic distribution; for example, subtype B is the most common form in western Europe and the United States, whereas subtype E is the most common clade in Thailand. Currently, clade C is the fastest-spreading clade worldwide, being present in India, Ethiopia, and Southern Africa.

Pathogenesis of HIV Infection and AIDS

While HIV can infect many tissues, there are two major targets of HIV infection: the immune system and the central nervous system. The effects of HIV infection on each of these two systems are discussed separately.

Profound immune deficiency, primarily affecting cell-mediated immunity, is the hallmark of AIDS. This results chiefly from infection of and a severe loss of CD4+ T cells as well as impairment in the function of surviving helper T cells.^131,132 As discussed later, macrophages and dendritic cells are also targets of HIV infection. HIV enters the body through mucosal tissues and blood and first infects T cells as well as dendritic cells and macrophages. The infection becomes established in lymphoid tissues, where the virus may remain latent for long periods. Active viral replication is associated with more infection of cells and progression to AIDS. We first describe the mechanisms involved in viral entry into T cells and macrophages and the replicative cycle of the virus within cells. This is followed by a more detailed review of the interaction between HIV and its cellular targets.

Life Cycle of HIV

The life cycle of HIV consists of infection of cells, integration of the provirus into the host cell genome, activation of viral replication, and production and release of infectious virus (Fig. 6-45).¹³³ The molecules and mechanisms of each of these steps are understood in considerable detail.

FIGURE 6-45 The life cycle of HIV, showing the steps from viral entry to production of infectious virions.

(Adapted with permission from Wain-Hobson S: HIV. One on one meets two. Nature 384:117, 1996. Copyright 1996, Macmillan Magazines Limited.)

Infection of Cells by HIV

HIV infects cells by using the CD4 molecule as receptor and various chemokine receptors as coreceptors (Fig. 6-45). The requirement for CD4 binding explains the selective tropism of the virus for CD4+ T cells and other CD4+ cells, particularly monocytes/macrophages and dendritic cells. Binding to CD4 is not sufficient for infection, however. HIV gp120 must also bind to other cell surface molecules (coreceptors) for entry into the cell. Chemokine receptors, particularly CCR5 and CXCR4, serve this role.¹³⁴ HIV isolates can be distinguished by their use of these receptors: R5 strains use CCR5, X4 strains use CXCR4, and some strains (R5×4) are dual-tropic. In approximately 90% of cases, the R5 (M-tropic) type of HIV is the dominant virus found in the blood of acutely infected individuals and early in the course of infection. Over the course of infection, however, T-tropic viruses gradually accumulate; these are especially virulent because T-tropic viruses are capable of infecting many T cells and even thymic T-cell precursors and cause greater T-cell depletion and impairment.

Molecular details of the deadly handshake between HIV glycoproteins and their cell surface receptors have been uncovered by elegant studies and are important to understand because they may provide the basis of anti-HIV therapy. The HIV envelope contains two glycoproteins, surface gp120 noncovalently attached to a transmembrane protein, gp41. The initial step in infection is the binding of the gp120 envelope glycoprotein to CD4 molecules. This binding leads to a conformational change that results in the formation of a new recognition site on gp120 for the coreceptors CCR5 or CXCR4. Binding to the coreceptors induces conformational changes in gp41 that result in the exposure of a hydrophobic region called the fusion peptide at the tip of gp41. This peptide inserts into the cell membrane of the target cells (e.g., T cells or macrophages), leading to fusion of the virus with the host cell.¹³⁵ After fusion the virus core containing the HIV genome enters the cytoplasm of the cell. The requirement for HIV binding to coreceptors may have important implications for the pathogenesis of AIDS. Chemokines sterically hinder HIV infection of cells in culture by occupying their receptors, and therefore, the level of chemokines in the tissues may influence the efficiency of viral infection in vivo. Also, polymorphisms in the gene encoding CCR5 are associated with different susceptibility to HIV infection. About 1% of white Americans inherit two defective copies of the CCR5 gene and are resistant to infection and the development of AIDS associated with R5 HIV isolates.¹²⁵ About 20% of individuals are heterozygous for this protective CCR5 allele; these persons are not protected from AIDS, but the onset of their disease after infection is somewhat delayed. Only rare homozygotes for the mutation have been found in African or East Asian populations.

Viral Replication

Once internalized, the RNA genome of the virus undergoes reverse transcription, leading to the synthesis of double-stranded complementary DNA (cDNA; proviral DNA) (see Fig. 6-45). In quiescent T cells, HIV cDNA may remain in the cytoplasm in a linear episomal form. In dividing T cells, the cDNA circularizes, enters the nucleus, and is then integrated into the host genome. After this integration, the provirus may be silent for months or years, a form of latent infection. Alternatively, proviral DNA may be transcribed, with the formation of complete viral particles that bud from the cell membrane. Such productive infection, when associated with extensive viral budding, leads to death of infected cells.

In vivo, HIV infects memory and activated T cells but is inefficient at productively infecting naive (unactivated) T cells. Naive T cells contain an active form of an enzyme that introduces mutations in the HIV genome. This enzyme has been given the rather cumbersome name APOBEC3G (for “apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like editing complex 3”).¹³⁶ It is a cytidine deaminase that introduces cytosine-to-uracil mutations in the viral DNA that is produced by reverse transcription. These mutations inhibit further DNA replication by mechanisms that are not fully defined. Activation of T cells converts cellular APOBEC3G into an inactive, high-molecular-mass complex, which explains why the virus can replicate in previously activated (e.g., memory) T cells and T-cell lines. HIV has also evolved to counteract this cellular defense mechanism; the viral protein Vif binds to APOBEC3G and promotes its degradation by cellular proteases.

Completion of the viral life cycle in latently infected cells occurs only after cell activation, and in the case of most CD4+ T cells virus activation results in cell lysis. Activation of T cells by antigens or cytokines upregulates several transcription factors, including NF-κB, which stimulate transcription of genes encoding cytokines such as IL-2 and its receptor. In resting T cells, NF-κB is sequestered in the cytoplasm in a complex with members of the IκB (inhibitor of κB) protein. Cellular activation by antigen or cytokines induces cytoplasmic kinases that phosphorylate IκB and target it for enzymatic degradation, thus releasing NF-κB and allowing it to translocate to the nucleus. In the nucleus, NF-κB binds to sequences within the promoter regions of several genes, including those of cytokines that are expressed in activated T cells. The long-terminal-repeat sequences that flank the HIV genome also contain NF-κB–binding sites that can be triggered by the same transcription factors.¹³⁷ Imagine now a latently infected CD4+ cell that encounters an environmental antigen. Induction of NF-κB in such a cell (a physiologic response) activates the transcription of HIV proviral DNA (a pathologic outcome) and leads ultimately to the production of virions and to cell lysis. Furthermore, TNF and other cytokines produced by activated macrophages also stimulate NF-κB activity and thus lead to production of HIV RNA. Thus, it seems that HIV thrives when the host T cells and macrophages are physiologically activated, an act that can be best described as “subversion from within.” Such activation in vivo may result from antigenic stimulation by HIV itself or by other infecting microorganisms. HIV-infected people are at increased risk for recurrent exposure to other infections, which lead to increased lymphocyte activation and production of pro-inflammatory cytokines. These, in turn, stimulate more HIV production, loss of additional CD4+ T cells, and more infection. Thus, it is easy to visualize how in individuals with AIDS a vicious cycle may be set up that culminates in inexorable destruction of the immune system.

Mechanism of T-Cell Immunodeficiency in HIV Infection

Loss of CD4+ T cells is mainly because of infection of the cells and the direct cytopathic effects of the replicating virus (Fig. 6-46).¹³⁸ Approximately 100 billion new viral particles are produced every day, and 1 to 2 billion CD4+ T cells die each day.¹³⁹ Because the frequency of infected cells in the circulation is very low, for many years it was suspected that the immunodeficiency is out of proportion to the level of infection and cannot be attributed to death of infected cells. In fact, many infected cells may be in mucosal and other peripheral lymphoid organs, and death of these cells is a major cause of the relentless, and eventually profound, cell loss. Also, to a point the immune system can replace the dying T cells, and hence the rate of T cell loss may appear deceptively low, but as the disease progresses, renewal of CD4+ T cells cannot keep up with the loss of these cells. Possible mechanisms by which the virus directly kills infected cells include increased plasma membrane permeability associated with budding of virus particles from the infected cells, and virus replication interfering with protein synthesis.

FIGURE 6-46 Mechanisms of CD4+ T-cell loss in HIV infection, showing some of the known and postulated mechanisms of T-cell depletion after HIV infection. APC, antigen-presenting cell; CTL, cytotoxic T lymphocyte.

In addition to direct killing of cells by the virus, other mechanisms may contribute to the loss of T cells (see Fig. 6-46).¹⁴⁰ These include:

• HIV colonizes the lymphoid organs (spleen, lymph nodes, tonsils) and may cause progressive destruction of the architecture and cellular composition of lymphoid tissues.

• Chronic activation of uninfected cells, responding to HIV itself or to infections that are common in individuals with AIDS, leads to apoptosis of these cells by the process of activation-induced cell death.^140,141 Thus, the numbers of CD4+ T cells that die are far greater than the numbers of infected cells. The molecular mechanism of this type of cell death is not known.

• Loss of immature precursors of CD4+ T cells can also occur, either by direct infection of thymic progenitor cells or by infection of accessory cells that secrete cytokines essential for CD4+ T-cell maturation.

• Fusion of infected and uninfected cells with formation of syncytia (giant cells) can occur. In tissue culture the gp120 expressed on productively infected cells binds to CD4 molecules on uninfected T cells, followed by cell fusion. Fused cells develop ballooning and usually die within a few hours. This property of syncytia formation is generally confined to the T-tropic X4 type of HIV-1. For this reason, this type is often referred to as syncytia-inducing (SI) virus, in contrast to the R5 virus.

• Apoptosis of uninfected CD4+ T cells by binding of soluble gp120 to the CD4 molecule, followed by activation through the T-cell receptor by antigens. It has been suggested that such cross-linking of CD4 molecules and T-cell activation lead to aberrant signaling and activation of death pathways. CD8+ CTLs may kill uninfected CD4+ T cells that are coated with gp120 released from infected cells.

Although marked reduction in CD4+ T cells, a hallmark of AIDS, can account for most of the immunodeficiency late in the course of HIV infection, there is compelling evidence for qualitative defects in T cells that can be detected even in asymptomatic HIV-infected persons. Reported defects include a reduction in antigen-induced T-cell proliferation, a decrease in T_H1-type responses relative to the T_H2 type, defects in intracellular signaling, and many more. The loss of T_H1 responses results in profound deficiency in cell-mediated immunity, leading to increased susceptibility to infections by viruses and other intracellular microbes. There is also a selective loss of the memory subset of CD4+ helper T cells early in the course of disease, which explains poor recall responses to previously encountered antigens.

Low-level chronic or latent infection of T cells (and macrophages, discussed below) is an important feature of HIV infection. It is widely believed that integrated provirus, without virus expression (latent infection), can remain in the cells for months to years. Even with potent antiviral therapy, which practically sterilizes the peripheral blood, latent virus lurks within the CD4+ cells (both T cells and macrophages) in the lymph nodes. According to some estimates, 0.05% of resting CD4+ T cells in the lymph nodes are latently infected. Because these CD4+ T cells are memory T cells, they are long-lived, with a life span of months to years, and thus provide a persistent reservoir of virus.

CD4+ T cells play a pivotal role in regulating both cellular and humoral immune responses. Therefore, loss of this “master regulator” has ripple effects on virtually every other component of the immune system, as summarized in Table 6-12.

TABLE 6-12 Major Abnormalities of Immune Function in AIDS

LYMPHOPENIA

Predominantly caused by selective loss of the CD4+ helper T-cell subset

DECREASED T-CELL FUNCTION IN VIVO

Preferential loss of activated and memory T cells Decreased delayed-type hypersensitivity Susceptibility to opportunistic infections Susceptibility to neoplasms

ALTERED T-CELL FUNCTION IN VITRO

Decreased proliferative response to mitogens, alloantigens, and soluble antigens Decreased cytotoxicity Decreased helper function for B-cell antibody production Decreased IL-2 and IFN-γ production

POLYCLONAL B-CELL ACTIVATION

Hypergammaglobulinemia and circulating immune complexes Inability to mount de novo antibody response to new antigens Poor responses to normal B-cell activation signals in vitro

ALTERED MONOCYTE OR MACROPHAGE FUNCTIONS

Decreased chemotaxis and phagocytosis Decreased class II HLA expression Diminished capacity to present antigen to T cells

HLA, human leukocyte antigen; IFN-γ, interferon-γ; IL-1, etc., interleukin-1; TNF, tumor necrosis factor.

Natural History of HIV Infection

HIV disease begins with acute infection, which is only partly controlled by the adaptive immune response, and advances to chronic progressive infection of peripheral lymphoid tissues (Fig. 6-47). Virus typically enters through mucosal epithelia. The subsequent pathogenetic events and clinical manifestations of the infection can be divided into several phases: (1) an acute retroviral syndrome; (2) a middle, chronic phase, in which most individuals are asymptomatic; and (3) clinical AIDS (Figs. 6-47 and 6-48).^131,132

FIGURE 6-47 Pathogenesis of HIV-1 infection. The initial infection starts in mucosal tissues, involving mainly memory CD4+ T cells and dendritic cells, and spreads to lymph nodes. Viral replication leads to viremia and widespread seeding of lymphoid tissue. The viremia is controlled by the host immune response (not shown), and the patient then enters a phase of clinical latency. During this phase, viral replication in both T cells and macrophages continues unabated, but there is some immune containment of virus (not illustrated). There continues a gradual erosion of CD4+ cells and ultimately, CD4+ T-cell numbers decline, and the patient develops clinical symptoms of full-blown AIDS. CTL, cytotoxic T lymphocyte.

FIGURE 6-48 Clinical course of HIV infection. A, During the early period after primary infection there is dissemination of virus, development of an immune response to HIV, and often an acute viral syndrome. During the period of clinical latency, viral replication continues and the CD4+ T-cell count gradually decreases, until it reaches a critical level below which there is a substantial risk of AIDS-associated diseases. B, Immune response to HIV infection. A cytotoxic T lymphocyte (CTL) response to HIV is detectable by 2 to 3 weeks after the initial infection, and it peaks by 9 to 12 weeks. Marked expansion of virus-specific CD8+ T-cell clones occurs during this time, and up to 10% of a patient’s CTLs may be HIV specific at 12 weeks. The humoral immune response to HIV peaks at about 12 weeks.

(Redrawn from Fauci AS, Lane HC: Human immunodeficiency virus disease: AIDS and related conditions. In Fauci AS, et al (eds): Harrison’s Principles of Internal Medicine, 14th ed. New York, McGraw-Hill, 1997, p 1791.)

Primary Infection, Virus Dissemination, and the Acute Retroviral Syndrome

Acute (early) infection is characterized by infection of memory CD4+ T cells (which express CCR5) in mu-cosal lymphoid tissues, and death of many infected cells. Because the mucosal tissues are the largest reservoir of T cells in the body, and a major site of residence of memory T cells, this local loss results in considerable depletion of lymphocytes.^147,148 Few infected cells are detectable in the blood and other tissues.

Mucosal infection is followed by dissemination of the virus and the development of host immune responses. Dendritic cells in epithelia at sites of virus entry capture the virus and then migrate into the lymph nodes. Once in lymphoid tissues, dendritic cells may pass HIV on to CD4+ T cells through direct cell-cell contact. Within days after the first exposure to HIV, viral replication can be detected in the lymph nodes. This replication leads to viremia, during which high numbers of HIV particles are present in the patient’s blood. The virus disseminates throughout the body and infects helper T cells, macrophages, and dendritic cells in peripheral lymphoid tissues.

As the HIV infection spreads, the individual mounts anti-viral humoral and cell-mediated immune responses.¹⁴⁹ These responses are evidenced by seroconversion (usually within 3 to 7 weeks of presumed exposure) and by the development of virus-specific CD8+ cytotoxic T cells. HIV-specific CD8+ T cells are detected in the blood at about the time viral titers begin to fall and are most likely responsible for the initial containment of HIV infection. These immune responses partially control the infection and viral production, and such control is reflected by a drop in viremia to low but detectable levels by about 12 weeks after the primary exposure.

The acute retroviral syndrome is the clinical presentation of the initial spread of the virus and the host response.¹⁵⁰ It is estimated that 40% to 90% of individuals who acquire a primary infection develop the viral syndrome. This typically occurs 3 to 6 weeks after infection, and resolves spontaneously in 2 to 4 weeks. Clinically, this phase is associated with a self-limited acute illness with nonspecific symptoms, including sore throat, myalgias, fever, weight loss, and fatigue, resembling a flulike syndrome. Other clinical features, such as rash, cervical adenopathy, diarrhea, and vomiting, may also occur.

The viral load at the end of the acute phase reflects the equilibrium reached between the virus and the host response, and in a given patient it may remain fairly stable for several years. This level of steady-state viremia, or the viral “set point,” is a predictor of the rate of decline of CD4+ T cells, and, therefore, progression of HIV disease. In one study, only 8% of patients with a viral load of less than 4350 copies of viral mRNA per microliter of blood progressed to clinical AIDS in 5 years, whereas 62% of those with a viral load of greater than 36,270 copies had developed AIDS in the same period.¹⁵¹ From a practical standpoint, therefore, the extent of viremia, measured as HIV-1 RNA levels, is a useful surrogate marker of HIV disease progression and is of clinical value in the management of people with HIV infection.

Because the loss of immune containment is associated with declining CD4+ T-cell counts, the Centers for Disease Control (CDC) classification of HIV infection stratifies patients into three categories on the basis of CD4+ cell counts: CD4+ cells greater than or equal to 500 cells/μL, 200 to 499 cells/μL, and fewer than 200 cells/μL (Table 6-13). For clinical management, blood CD4+ T-cell counts are perhaps the most reliable short-term indicator of disease progression. For this reason, CD4+ cell counts and not viral load are the primary clinical measurements used to determine when to start combination antiretroviral therapy.

TABLE 6-13 CDC Classification Categories of HIV Infection

Chronic Infection: Phase of Clinical Latency.

In the next, chronic phase of the disease, lymph nodes and the spleen are sites of continuous HIV replication and cell destruction (see Fig. 6-47). During this period of the disease, few or no clinical manifestations of the HIV infection are present. Therefore, this phase of HIV disease is called the clinical latency period. Although the majority of peripheral blood T cells do not harbor the virus, destruction of CD4+ T cells within lymphoid tissues continues during this phase, and the number of circulating blood CD4+ T cells steadily declines. More than 90% of the body’s approximately 10¹² T cells are normally found in lymphoid tissues, and it is estimated that HIV destroys up to 1 × 10⁹ to 2 × 10⁹ CD4+ T cells every day. Early in the course of the disease, the body may continue to make new CD4+ T cells, and therefore CD4+ T cells can be replaced almost as quickly as they are destroyed. At this stage, up to 10% of CD4+ T cells in lymphoid organs may be infected, but the frequency of circulating CD4+ T cells that are infected at any one time may be less than 0.1% of the total CD4+ T cells. Eventually, over a period of years, the continuous cycle of virus infection, T-cell death, and new infection leads to a steady decline in the number of CD4+ T cells in the lymphoid tissues and the circulation.

Concomitant with this loss of CD4+ T cells, host defenses begin to wane, and the proportion of the surviving CD4+ cells infected with HIV increases, as does the viral burden per CD4+ cell. Not unexpectedly, HIV RNA levels may begin to increase as the host begins to lose the battle with the virus. How HIV escapes immune control is not entirely clear, but several mechanisms have been proposed.^152,153 These include destruction of the CD4+ T cells that are critical for effective immunity, antigenic variation, and down-modulation of class I MHC molecules on infected cells so that viral antigens are not recognized by CD8+ CTLs. During this period the virus may evolve and switch from relying solely on CCR5 to enter its target cells to relying on either CXCR4 or both CCR5 and CXCR4. This coreceptor switch is associated with more rapid decline in CD4+ T-cell counts, presumably because of greater infection of T cells.

In this chronic phase of infection, patients are either asymptomatic or develop minor opportunistic infections, such as oral candidiasis (thrush), vaginal candidiasis, herpes zoster, and perhaps mycobacterial tuberculosis (the latter being particularly common in resource-poor regions such as sub-Saharan Africa). Autoimmune thrombocytopenia may also be noted (Chapter 14).

AIDS

The final phase is progression to AIDS, characterized by a breakdown of host defense, a dramatic increase in plasma virus, and severe, life-threatening clinical disease. Typically the patient presents with long-lasting fever (>1 month), fatigue, weight loss, and diarrhea. After a variable period, serious opportunistic infections, secondary neoplasms, or clinical neurologic disease (grouped under the rubric AIDS indicator diseases, discussed below) emerge, and the patient is said to have developed AIDS.

In the absence of treatment, most but not all patients with HIV infection progress to AIDS after a chronic phase lasting from 7 to 10 years. Exceptions to this typical course are exemplified by rapid progressors and long-term nonprogressors. In rapid progressors the middle, chronic phase is telescoped to 2 to 3 years after primary infection. About 5% to 15% of infected individuals are long-term nonprogressors, defined as untreated HIV-1–infected individuals who remain asymptomatic for 10 years or more, with stable CD4+ T-cell counts and low levels of plasma viremia (usually less than 500 viral RNA copies per milliliter).¹⁵⁴ Remarkably, about 1% of infected individuals have undetectable plasma virus (50–75 RNA copies/mL); these have been called elite controllers. Individuals with such an uncommon clinical course have attracted great attention in the hope that studying them may shed light on host and viral factors that influence disease progression. Studies thus far indicate that this group is heterogeneous with respect to the variables that influence the course of the disease. In most cases, the viral isolates do not show qualitative abnormalities, suggesting that the course of the disease cannot be attributed to a “wimpy” virus. In all cases there is evidence of a vigorous anti-HIV immune response, but the immune correlates of protection are still unknown. Some of these individuals have high levels of HIV-specific CD4+ and CD8+ T-cell responses, and these levels are maintained over the course of infection. Further studies, it is hoped, will provide the answers to this and other questions critical to understanding disease progression.

Clinical Features of AIDS

The clinical manifestations of HIV infection can be readily surmised from the foregoing discussion. They range from a mild acute illness to severe disease. Because the salient clinical features of the acute early and chronic middle phases of HIV infection were described earlier, here we summarize the clinical manifestations of the terminal phase, AIDS. At the outset it should be pointed out that the clinical manifestations and opportunistic infections associated with HIV infection may differ in different parts of the world. Also, the course of the disease has been greatly modified by new antiretroviral therapies, and many complications that were once devastating are now infrequent.

In the United States, the typical adult patient with AIDS presents with fever, weight loss, diarrhea, generalized lymphadenopathy, multiple opportunistic infections, neurologic disease, and, in many cases, secondary neoplasms. The infections and neoplasms listed in Table 6-14 are included in the surveillance definition of AIDS.

TABLE 6-14 AIDS-Defining Opportunistic Infections and Neoplasms Found in Patients with HIV Infection

CNS, central nervous system.

Tumors

Patients with AIDS have a high incidence of certain tumors, especially Kaposi sarcoma (KS), non-Hodgkin B-cell lymphoma, cervical cancer in women, and anal cancer in men.¹⁵⁶ It is estimated that 25% to 40% of untreated HIV-infected individuals will eventually develop a malignancy. A common feature of these tumors is that they are all believed to be caused by oncogenic DNA viruses, that is, kaposi sarcoma herpesvirus (Kaposi sarcoma), EBV (B-cell lymphoma), and human papillomavirus (cervical and anal carcinoma). Even in healthy people, any of these viruses may establish latent infections that are kept in check by a competent immune system. The increased risk of malignancy in AIDS patients exists mainly because of failure to contain the infections and reactivation of the viruses, as well as decreased immunity against the tumors.

Kaposi Sarcoma

Kaposi sarcoma, a vascular tumor that is otherwise rare in the United States, is the most common neoplasm in patients with AIDS. The morphology of KS and its occurrence in patients not infected with HIV are discussed in Chapter 11. At the onset of the AIDS epidemic, up to 30% of infected homosexual or bisexual men had KS, but in recent years, with use of HAART there has been a marked decline in its incidence, from 15 cases per 1000 person years to less than 5 cases.¹⁵⁷

The lesions of KS are characterized by the proliferation of spindle-shaped cells that express markers of both endothelial cells (vascular or lymphatic) and smooth muscle cells (Chapter 11). There is also a profusion of slitlike vascular spaces, suggesting that the lesions may arise from primitive mesenchymal precursors of vascular channels. In addition, KS lesions display chronic inflammatory cell infiltrates. Many of the features of KS suggest that it is not a malignant tumor (despite its ominous name).¹⁵⁸ For instance, spindle cells in many KS lesions are polyclonal or oligoclonal, although more advanced lesions occasionally show monoclonality. The spindle cells in many KS lesions are diploid and are dependent on growth factors for their proliferation. When these cells are implanted subcutaneously in immunodeficient mice they do not form tumors but transiently induce slitlike new blood vessels and inflammatory infiltrates in the surrounding tissue. These elements recall features of human KS but surprisingly are of murine origin, and when the human KS cells involute, these elements also regress. Based on these observations, the current model of KS pathogenesis is that the spindle cells produce pro-inflammatory and angiogenic factors, which recruit the inflammatory and neovascular components of the lesion, and the latter components supply signals that aid in spindle cell survival or growth (Fig. 6-49).

FIGURE 6-49 Postulated pathogenesis of Kaposi sarcoma (KS). Proposed roles of HIV, KS herpesvirus (KSHV; HHV8), and cytokines in the development of KS. Cytokines are produced by the mesenchymal cells infected by KSHV, or by HIV-infected CD4+ cells. B cells may also be infected by KSHV; they are the probable cells in body cavity lymphomas, also associated with KSHV infection, but their role in KS is unclear.

But what initiates this cycle of events? There is compelling evidence that HIV itself is not the culprit, and that KS is caused by the KS herpesvirus (KSHV), also called human herpesvirus 8 (HHV8).¹⁵⁹ Epidemiologic and molecular studies have established the link between KSHV and KS development. KSHV DNA is found in virtually all KS lesions, including those that occur in HIV-negative populations, and in the lesions, KSHV is strikingly localized to the spindle cells, which display predominantly latent infection. However, KSHV infection, while necessary for KS development, is not sufficient, and additional cofactors are needed. In the AIDS-related form, that cofactor is clearly HIV. (The relevant cofactors for HIV-negative KS remain unknown.) Debate continues over exactly how HIV contributes to KS development. The simplest model is that HIV-mediated immune suppression allows widespread dissemination of KSHV in the host, allowing it to access more spindle cells and set them on the path to uncontrolled growth. Another idea is that HIV-infected T cells produce cytokines or other proteins that promote spindle cell proliferation and survival. Clearly, these possibilities are not mutually exclusive.

Exactly how KSHV infection leads to KS is also still unclear.¹⁵⁸ Like other herpesviruses, KSHV establishes latent infection, during which several proteins are produced with potential roles in stimulating spindle cell proliferation and preventing apoptosis. These include a viral homologue of cyclin D and several inhibitors of p53. Such proteins could give latently infected cells a survival and growth advantage in vivo that would allow them to begin proliferating. But in addition to latent infection, a small subpopulation of cells in KS is undergoing lytic viral replication, with cell death and the release of viral progeny. The KSHV lytic cycle is remarkable for its production of numerous paracrine signaling molecules, including a viral homologue of the cytokine IL-6 and several chemokines. The latter probably play prominent roles in eliciting the inflammatory infiltrates that are an important feature of KS. The contribution of viral IL-6 is not yet clear. Another viral protein produced during lytic infection is a constitutively active G protein–coupled receptor. This protein has attracted attention because its expression activates the release of vascular endothelial growth factor (VEGF), which can promote angiogenesis in the surrounding tissue. Interestingly, expression of the viral G protein–coupled receptor in transgenic mice leads to the development of neovascular spaces vaguely reminiscent of those in KS. Thus, there is ample reason to believe that both latent and lytic KSHV infection contributes to KS pathogenesis.

KSHV infection is not restricted to endothelial cells. The virus is related phylogenetically to the lymphotropic sub-family of herpesviruses (γ-herpesvirus); in keeping with this, its genome is found in B cells of infected subjects. In fact, KSHV infection is also linked to rare B-cell lymphomas in AIDS patients (called body cavity-based primary effusion lymphoma) and to multicentric Castleman disease, a B-cell lymphoproliferative disorder.

Clinically, AIDS-associated KS is quite different from the sporadic form (Chapter 11). In HIV-infected individuals the tumor is usually widespread, affecting the skin, mucous membranes, gastrointestinal tract, lymph nodes, and lungs. These tumors also tend to be more aggressive than classic KS.

Lymphomas

AIDS-related lymphomas can be divided into three groups on the basis of their location: systemic, primary central nervous system, and body cavity–based lymphomas.¹⁶⁰ Systemic lymphomas involve lymph nodes as well as extranodal, visceral sites; they constitute 80% of all AIDS-related lymphomas. The central nervous system is the most common extranodal site affected, followed by the gastrointestinal tract and, less commonly, virtually any other location, including the orbit, salivary glands, and lungs. The vast majority of these lymphomas are aggressive B-cell tumors that present in an advanced stage (Chapter 13). In addition to being commonly involved by systemic non-Hodgkin lymphomas, the central nervous system is also the primary site of lymphomatous involvement in 20% of HIV-infected patients who develop lymphomas. Primary central nervous system lymphoma is 1000 times more common in patients with AIDS than in the general population. Body cavity lymphomas are rare, but they attract attention because of their unusual presentation as pleural, peritoneal, or pericardial effusions.

The pathogenesis of AIDS-associated B-cell lymphomas probably involves sustained polyclonal B-cell activation, followed by the emergence of monoclonal or oligoclonal B-cell populations. It is believed that during the frenzy of proliferation, some clones undergo mutations or chromosomal translocations involving oncogenes or tumor suppressor genes, and subsequent neoplastic transformation (Chapter 7). There is morphologic evidence of B-cell activation in lymph nodes, and it is believed that such triggering of B cells is multifactorial. Patients with AIDS have high levels of several cytokines, some of which, including IL-6, are growth factors for B cells. In addition, there seems to be a role for EBV, known to be a polyclonal mitogen for B cells. Half of the systemic B-cell lymphomas and virtually all lymphomas primary in the central nervous system are latently infected with EBV. Other evidence of EBV infection includes oral hairy leukoplakia (white projections on the tongue), resulting from EBV-driven squamous cell proliferation of the oral mucosa (Chapter 16). In cases in which molecular footprints of EBV infection cannot be detected, other viruses and microbes may initiate polyclonal B-cell proliferation. There is no evidence that HIV by itself is capable of causing neoplastic transformation. The rare body cavity–based primary effusion lymphomas are uniformly latently infected with KSHV, discussed earlier.

With prolonged survival, the number of AIDS patients who develop non-Hodgkin lymphoma has increased steadily. It is currently believed that approximately 6% of all patients with AIDS develop lymphoma during their lifetime. Thus, the risk of developing non-Hodgkin lymphoma is approximately 120-fold greater than in the general population. In contrast to KS, immunodeficiency is firmly implicated as the central predisposing factor. It seems that patients with CD4+ T-cell counts below 50 per microliter incur an extremely high risk.

Other Tumors

In addition to KS and lymphomas, patients with AIDS also have an increased occurrence of carcinoma of the uterine cervix and of anal cancer. This is most likely due to reactivation of latent human papillomavirus (HPV) infection as a result of immunosuppression.¹⁶¹ This virus is believed to be intimately associated with squamous cell carcinoma of the cervix and its precursor lesions, cervical dysplasia and carcinoma in situ (Chapters 7 and 22. HPV-associated cervical dysplasia is 10 times more common in HIV-infected women as compared with uninfected women attending family planning clinics. Hence it is recommended that gynecologic examination be part of a routine work-up of HIV-infected women.

Properties of Amyloid Proteins

Physical Nature of Amyloid

By electron microscopy amyloid is seen to be made up largely of continuous, nonbranching fibrils with a diameter of approximately 7.5 to 10 nm. This electron-microscopic structure is identical in all types of amyloidosis. X-ray crystallography and infrared spectroscopy demonstrate a characteristic cross-β-pleated sheet conformation (Fig. 6-51). This conformation is seen regardless of the clinical setting or chemical composition and is responsible for the distinctive Congo red staining and birefringence of amyloid.

FIGURE 6-51 Structure of amyloid. A, An amyloid fiber schematically showing four fibrils (there can be as many as six in each fiber) wound around one another with regularly spaced binding of the Congo red dye. B, Congo red staining shows apple-green birefringence under polarized light, a diagnostic feature of amyloid. C, Electron micrograph of 7.5- to 10-nm amyloid fibrils.

(Reproduced from Merlini G and Bellotti V. Molecular mechanisms of amyloidosis. N Engl J Med 349:583–596, 2003, with permission of the Massachusetts Medical Society.)

Pathogenesis of Amyloidosis

Amyloidosis results from abnormal folding of proteins, which are deposited as fibrils in extracellular tissues and disrupt normal function.^165,166 Misfolded proteins are often unstable and self-associate, ultimately leading to the formation of oligomers and fibrils that are deposited in tissues. The reason diverse conditions are associated with amyloidosis may be that each results in excessive production of proteins that are prone to misfolding (Fig. 6-52). The proteins that form amyloid fall into two general categories: (1) normal proteins that have an inherent tendency to fold improperly, associate and form fibrils, and do so when they are produced in increased amounts; and (2) mutant proteins that are prone to misfolding and subsequent aggregation.

FIGURE 6-52 Pathogenesis of amyloidosis, showing the proposed mechanisms underlying deposition of the major forms of amyloid fibrils. See text for abbreviations.

Normally, misfolded proteins are degraded intracellularly in proteasomes, or extracellularly by macrophages. It seems that in amyloidosis these quality control mechanisms fail so that too much of a misfolded protein accumulates outside cells. This proposed mechanism may explain most forms of amyloidosis. For instance, SAA is synthesized by the liver cells under the influence of cytokines such as IL-6 and IL-1 that are produced during inflammation; thus, longstanding inflammation leads to elevated SAA levels. However, increased production of SAA by itself is not sufficient for the deposition of amyloid. There are two possible explanations for this. According to one view, SAA is normally degraded to soluble end products by the action of monocyte-derived enzymes. Conceivably, individuals who develop amyloidosis have an enzyme defect that results in incomplete breakdown of SAA, thus generating insoluble AA molecules. Alternatively, a genetically determined structural abnormality in the SAA molecule itself renders it resistant to degradation by macrophages.

In familial amyloidosis the deposition of TTRs as amyloid fibrils does not result from overproduction of TTRs. It has been proposed that genetically detemined alterations of structure render the TTRs prone to misfolding and aggregation, and resistant to proteolysis.

Classification of Amyloidosis

Because a given biochemical form of amyloid (e.g., AA) may be associated with amyloid deposition in diverse clinical settings, we follow a combined biochemical-clinical classification for our discussion (Table 6-15). Amyloid may be systemic (generalized), involving several organ systems, or it may be localized, when deposits are limited to a single organ, such as the heart.

TABLE 6-15 Classification of Amyloidosis

On clinical grounds, the systemic, or generalized, pattern is subclassified into primary amyloidosis, when associated with some immunocyte disorder, or secondary amyloidosis, when it occurs as a complication of an underlying chronic inflammatory or tissue-destructive process.¹⁶⁷ Hereditary or familial amyloidosis constitutes a separate, albeit heterogeneous group, with several distinctive patterns of organ involvement.

Amyloid of Aging

Several well-documented forms of amyloid deposition occur with aging. Senile systemic amyloidosis refers to the systemic deposition of amyloid in elderly patients (usually in their 70s and 80s). Because of the dominant involvement and related dysfunction of the heart, this form was previously called senile cardiac amyloidosis. Those who are symptomatic present with a restrictive cardiomyopathy and arrhythmias (Chapter 12). The amyloid in this form is composed of the normal TTR molecule. In addition to the sporadic senile systemic amyloidosis, another form, affecting predominantly the heart, that results from the deposition of a mutant form of TTR has also been recognized. Approximately 4% of the black population in the United States is a carrier of the mutant allele, and cardiomyopathy has been identified in both homozygous and heterozygous patients. The precise prevalence of patients with this mutation who develop clinically manifest cardiac disease is not known.

Morphology. There are no consistent or distinctive patterns of organ or tissue distribution of amyloid deposits in any of the categories cited. Kidneys, liver, spleen, lymph nodes, adrenals, and thyroid as well as many other tissues are classically involved. Macroscopically the affected organs are often enlarged and firm and have a waxy appearance. If the deposits are sufficiently large, painting the cut surface with iodine imparts a yellow color that is transformed to blue violet after application of sulfuric acid.

As noted earlier, the histologic diagnosis of amyloid is based on its staining characteristics. The most commonly used staining technique uses the dye Congo red, which under ordinary light imparts a pink or red color to amyloid deposits. Under polarized light, the Congo red–stained amyloid shows a green birefringence (see Fig. 6-50B). This reaction is shared by all forms of amyloid and is due to the cross-β-pleated configuration of amyloid fibrils. Confirmation can be obtained by electron microscopy. AA, AL, and TTR amyloid can be distinguished in histologic sections by specific immunohistochemical staining. Because the pattern of organ involvement in different clinical forms of amyloidosis is variable, each of the major organ involvements is described separately.

Kidney. Amyloidosis of the kidney is the most common and potentially the most serious form of organ involvement. Grossly, the kidneys may be of normal size and color, or, in advanced cases, they may be shrunken because of ischemia caused by vascular narrowing induced by the deposition of amyloid within arterial and arteriolar walls.

Histologically, the amyloid is deposited primarily in the glomeruli, but the interstitial peritubular tissue, arteries, and arterioles are also affected. The glomerular deposits first appear as subtle thickenings of the mesangial matrix, accompanied usually by uneven widening of the basement membranes of the glomerular capillaries. In time the mesangial depositions and the deposits along the basement membranes cause capillary narrowing and distortion of the glomerular vascular tuft. With progression of the glomerular amyloidosis, the capillary lumens are obliterated, and the obsolescent glomerulus is flooded by confluent masses or interlacing broad ribbons of amyloid (Fig. 6-53).

FIGURE 6-53 Amyloidosis of the kidney. The glomerular architecture is almost totally obliterated by the massive accumulation of amyloid.

Spleen. Amyloidosis of the spleen may be inapparent grossly or may cause moderate to marked splenomegaly (up to 800 gm). For completely mysterious reasons, one of two patterns of deposition is seen. In one, the deposits are largely limited to the splenic follicles, producing tapioca-like granules on gross inspection, designated sago spleen. In the other pattern, the amyloid involves the walls of the splenic sinuses and connective tissue framework in the red pulp. Fusion of the early deposits gives rise to large, maplike areas of amyloidosis, creating what has been designated lardaceous spleen.

Liver. The deposits may be inapparent grossly or may cause moderate to marked hepatomegaly. Amyloid appears first in the space of Disse and then progressively encroaches on adjacent hepatic parenchymal cells and sinusoids (see Fig. 6-50). In time, deformity, pressure atrophy, and disappearance of hepatocytes occur, causing total replacement of large areas of liver parenchyma. Vascular involvement and deposits in Kupffer cells are frequent. Normal liver function is usually preserved despite sometimes quite severe involvement of the liver.

Heart. Amyloidosis of the heart (Chapter 12) may occur in any form of systemic amyloidosis. It is also the major organ involved in senile systemic amyloidosis. The heart may be enlarged and firm, but more often it shows no significant changes on gross inspection. Histologically the deposits begin as focal subendocardial accumulations and within the myocardium between the muscle fibers. Expansion of these myocardial deposits eventually causes pressure atrophy of myocardial fibers. When the amyloid deposits are subendocardial, the conduction system may be damaged, accounting for the electrocardiographic abnormalities noted in some patients.

Other Organs. Amyloidosis of other organs is generally encountered in systemic disease. The adrenals, thyroid, and pituitary are common sites of involvement. The gastrointestinal tract may be involved at any level, from the oral cavity (gingiva, tongue) to the anus. The early lesions mainly affect blood vessels but eventually extend to involve the adjacent areas of the submucosa, muscularis, and subserosa.

Nodular depositions in the tongue may cause macroglossia, giving rise to the designation tumor-forming amyloid of the tongue. The respiratory tract may be involved focally or diffusely from the larynx down to the smallest bronchioles. It involves so-called plaques as well as blood vessels (Chapter 28). Amyloidosis of peripheral and autonomic nerves is a feature of several familial amyloidotic neuropathies. Depositions of amyloid in patients on long-term hemodialysis are most prominent in the carpal ligament of the wrist, resulting in compression of the median nerve (carpal tunnel syndrome). These patients may also have extensive amyloid deposition in the joints.

Clinical Features.

Amyloidosis may be found as an unsuspected anatomic change, having produced no clinical manifestations, or it may cause death. The symptoms depend on the magnitude of the deposits and on the particular sites or organs affected. Clinical manifestations at first are often entirely nonspecific, such as weakness, weight loss, lightheadedness, or syncope. Somewhat more specific findings appear later and most often relate to renal, cardiac, and gastrointestinal involvement.

Renal involvement gives rise to proteinuria that may be severe enough to cause the nephrotic syndrome (Chapter 20). Progressive obliteration of glomeruli in advanced cases ultimately leads to renal failure and uremia. Renal failure is a common cause of death. Cardiac amyloidosis may present as an insidious congestive heart failure. The most serious aspects of cardiac amyloidosis are conduction disturbances and arrhythmias, which may prove fatal. Occasionally, cardiac amyloidosis produces a restrictive pattern of cardiomyopathy and masquerades as chronic constrictive pericarditis (Chapter 12). Gastrointestinal amyloidosis may be entirely asymptomatic, or it may present in a variety of ways. Amyloidosis of the tongue may cause sufficient enlargement and inelasticity to hamper speech and swallowing. Depositions in the stomach and intestine may lead to malabsorption, diarrhea, and disturbances in digestion.

The diagnosis of amyloidosis depends on the histologic demonstration of amyloid deposits in tissues. The most common sites biopsied are the kidney, when renal manifestations are present, or rectal or gingival tissues in patients suspected of having systemic amyloidosis. Examination of abdominal fat aspirates stained with Congo red can also be used for the diagnosis of systemic amyloidosis. The test is quite specific, but its sensitivity is low. In suspected cases of immunocyte-associated amyloidosis, serum and urine protein electrophoresis and immunoelectrophoresis should be performed. Bone marrow aspirates in such cases often show monoclonal plasmacytosis, even in the absence of overt multiple myeloma. Scintigraphy with radiolabeled serum amyloid P (SAP) component is a rapid and specific test, since SAP binds to the amyloid deposits and reveals their presence. It also gives a measure of the extent of amyloidosis and can be used to follow patients undergoing treatment.

The prognosis for individuals with generalized amyloidosis is poor. Those with immunocyte-derived amyloidosis (not including multiple myeloma) have a median survival of 2 years after diagnosis. Persons with myeloma-associated amyloidosis have a poorer prognosis. The outlook for individuals with reactive systemic amyloidosis is somewhat better and depends to some extent on the control of the underlying condition. Resorption of amyloid after treatment of the associated condition has been reported, but this is a rare occurrence. New therapeutic strategies aimed at correcting protein misfolding and inhibiting fibrillogenesis are being developed.