T Cell–Mediated Rejection

CTLs kill cells in the grafted tissue, causing parenchymal and endothelial cell death (the latter resulting in thrombosis and graft ischemia). Cytokine-secreting CD4+ T cells trigger inflammatory reactions resembling DTH in the tissues and blood vessels, with local accumulation of mononuclear cells (lymphocytes and macrophages). Activated microphages can injure graft cells and vasculature. The microvascular injury also results in tissue ischemia, which contributes to graft destruction.

Antibody-Mediated Rejection

Although T cells are of paramount importance in allograft rejection, antibodies also mediate some forms of rejection. Alloantibodies directed against graft MHC molecules and other alloantigens bind to the graft endothelium and cause vascular injury through complement activation and recruitment of leukocytes. Superimposed on the resulting endothelial damage and dysfunction is thrombosis, adding further ischemic insult to the injury.

Hyperacute rejection is a special form of rejection occurring if pre-formed anti-donor antibodies are present in the circulation of the host before transplantation. This may happen in multiparous women who have anti-HLA antibodies against paternal antigens encountered during pregnancy, or in individuals exposed to foreign HLA (on platelets or leukocytes) from previous blood transfusions. Obviously, such antibodies also may be present in a patient who has previously rejected an organ transplant. Subsequent transplantation in such patients will result in immediate rejection (within minutes to hours) because the circulating antibodies rapidly bind to the endothelium of the grafted organ, with resultant complement activation and vascular thrombosis. With the current practice of screening potential recipients for pre-formed anti-HLA antibodies and cross-matching (testing recipients for the presence of antibodies directed against the donor’s lymphocytes), hyperacute rejection occurs in less than 0.4% of transplant recipients.

Morphology

On the basis of the time course and morphology of rejection reactions, they have been classified as hyperacute, acute, and chronic (Fig. 4–24). This classification is helpful for understanding the mechanism of rejection, because each pattern is caused by a different type of dominant immunologic reaction. The morphology of these patterns is described in the context of renal transplants; however, similar changes are encountered in other vascularized organ transplants.

Figure 4–24 Morphologic patterns of graft rejection. A, Hyperacute rejection of a kidney allograft associated with endothelial damage and thrombi 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. D, Chronic rejection in a kidney allograft with graft arteriosclerosis. The arterial lumen is replaced by an accumulation of smooth muscle cells and connective tissue in the intima.

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

Hyperacute Rejection

Hyperacute rejection occurs within minutes to a few hours after transplantation in a presensitized host and typically is recognized by the surgeon just after the vascular anastomosis is completed. In contrast with a nonrejecting kidney graft, which regains a normal pink color and tissue turgor and promptly excretes urine, a hyperacutely rejecting kidney rapidly becomes cyanotic, mottled, and flaccid and may excrete only a few drops of bloody fluid. The histologic picture is characterized by widespread acute arteritis and arteriolitis, vessel thrombosis, and ischemic necrosis, all resulting from the binding of preformed antibodies to graft endothelium. Virtually all arterioles and arteries exhibit characteristic acute fibrinoid necrosis of their walls, with narrowing or complete occlusion of the lumens by precipitated fibrin and cellular debris (Fig. 4–24, A).

Acute Rejection

Acute rejection may occur within days to weeks of transplantation in a nonimmunosuppressed host or may appear months or even years later, even in the presence of adequate immunosuppression. Acute rejection is caused by both cellular and humoral immune mechanisms, and in any one patient, one or the other may predominate, or both may be present. On histologic examination, cellular rejection is marked by an interstitial mononuclear cell infiltrate with associated edema and parenchymal injury, whereas humoral rejection is associated with vasculitis.

Acute cellular rejection most commonly is seen within the first months after transplantation and typically is accompanied by clinical signs of renal failure. Histologic examination usually shows extensive interstitial CD4+ and CD8+ T cell infiltration with edema and mild interstitial hemorrhage (Fig. 4–24, B). Glomerular and peritubular capillaries contain large numbers of mononuclear cells, which also may invade the tubules, leading to focal tubular necrosis. In addition to tubular injury, CD8+ T cells also may injure the endothelium, causing an endothelitis. Cyclosporine (a widely used immunosuppressive agent) is also nephrotoxic and induces so-called arteriolar hyaline deposits. Renal biopsy is used to distinguish rejection from drug toxicity. Accurate recognition of cellular rejection is important, because patients typically respond promptly to increased immunosuppressive therapy.

Acute humoral rejection (rejection vasculitis) caused by antidonor antibodies also may participate in acute graft rejection. The histologic lesions may take the form of necrotizing vasculitis with endothelial cell necrosis; neutrophilic infiltration; deposition of antibody, complement, and fibrin; and thrombosis. Such lesions may be associated with ischemic necrosis of the renal parenchyma. Somewhat older subacute lesions are characterized by marked thickening of the intima by proliferating fibroblasts, myocytes, and foamy macrophages (Fig. 4–24, C). 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 stimulate proliferation of vascular smooth muscle cells. Local deposition of complement breakdown products (specifically C4d) is used to detect antibody-mediated rejection of kidney allografts.

Chronic Rejection

Patients present with chronic rejection late after transplantation (months to years) with a progressive rise in serum creatinine levels (an index of renal function) over a period of 4 to 6 months. Chronic rejection is dominated by vascular changes, interstitial fibrosis, and loss of renal parenchyma; there are typically only mild or no ongoing cellular parenchymal infiltrates. The vascular changes occur predominantly in the arteries and arterioles, which exhibit intimal smooth muscle cell proliferation and extracellular matrix synthesis (Fig. 4–24, D). These lesions ultimately compromise vascular perfusion and result in renal ischemia manifested by loss or hyalinization of glomeruli, interstitial fibrosis, and tubular atrophy. The vascular lesion may be caused by cytokines released by activated T cells that act on the cells of the vascular wall, and it may be the end stage of the proliferative arteritis described earlier.

Summary

Recognition and Rejection of Organ Transplants (Allografts)

• The graft rejection response is initiated mainly by host T cells that recognize the foreign HLA antigens of the graft, either directly (on APCs in the graft) or indirectly (after uptake and presentation by host APCs).

• Types and mechanisms of rejection comprise the following:

Hyperacute rejection: Pre-formed antidonor antibodies bind to graft endothelium immediately after transplantation, leading to thrombosis, ischemic damage, and rapid graft failure.

Acute cellular rejection: T cells destroy graft parenchyma (and vessels) by cytotoxicity and inflammatory reactions.

Acute humoral rejection: Antibodies damage graft vasculature.

Chronic rejection: Dominated by arteriosclerosis, this type is probably caused by T cell reaction and secretion of cytokines that induce proliferation of vascular smooth muscle cells, associated with parenchymal fibrosis.

X-Linked Agammaglobulinemia: Bruton Disease

X-linked agammaglobulinemia (XLA), or Bruton disease, is characterized by the failure of pre-B cells to differentiate into B cells and, as the name implies, there is a resultant absence of antibodies (gamma globulin) in the blood. It occurs at a frequency of about 1 in 100,000 male infants. During normal B cell maturation, immunoglobulin heavy chain genes are rearranged first, followed by light chain rearrangement. At each stage, signals are received from the expressed components of the antigen receptor that drive maturation to the next stage; these signals act as quality controls, to ensure that the correct receptor proteins are being produced. In XLA, B cell maturation stops after the initial heavy chain gene rearrangement because of mutations in a tyrosine kinase that is associated with the pre-B cell receptor and is involved in pre-B cell signal transduction. This kinase is called Bruton tyrosine kinase (BTK). When it is nonfunctional, the pre-B cell receptor cannot signal the cells to proceed along the maturation pathway. As a result, immunoglobulin light chains are not produced, and the complete immunoglobulin molecule containing heavy and light chains cannot be assembled and transported to the cell membrane, although free heavy chains can be found in the cytoplasm. Because the BTK gene is on the X chromosome, the disorder is seen in males.

Classically, this disease is characterized by the following:

XLA does not become apparent until the affected infant attains the age of approximately 6 months, when the transplacental supply of maternal antibodies is depleted. In most cases, recurrent bacterial infections such as acute and chronic pharyngitis, sinusitis, otitis media, bronchitis, and pneumonia suggest an underlying immune defect. The causal organisms typically are those bacterial pathogens that are cleared by antibody-mediated opsonization and phagocytosis (e.g., Haemophilus influenzae, Streptococcus pneumoniae, and Staphylococcus aureus). Because antibodies are important for neutralizing viruses, patients with XLA also are susceptible to certain viral infections, especially those caused by enteroviruses. Similarly, Giardia lamblia, an intestinal protozoan usually neutralized by secreted IgA, cannot be efficiently cleared and causes persistent infections. Fortunately, replacement therapy with intravenous immunoglobulin (IVIG) from pooled human serum allows most patients to adequately combat bacterial infections. Patients with XLA clear some viral, fungal, and protozoal infections, because their T cell–mediated immunity is intact. For unclear reasons, autoimmune diseases (such as RA and dermatomyositis) occur in as many as 20% of patients with this disease.

Common Variable Immunodeficiency

Common variable immunodeficiency is an umbrella term for a heterogeneous group of disorders characterized by hypogammaglobulinemia, impaired antibody responses to infection (or vaccination), and increased susceptibility to infections. The clinical manifestations are superficially similar to those of XLA, but in common variable immunodeficiency, males and females are affected equally and the onset of symptoms is much later, in the second or third decade of life. The diagnosis usually is one of exclusion (after other causes of immune deficiency are ruled out). The estimated prevalence of the disease is about 1 in 50,000. Although most patients have normal numbers of mature B cells, plasma cells are absent, suggesting a block in antigen-stimulated B cell differentiation. The defective antibody production has been variably attributed to intrinsic B cell defects, deficient T cell help, or excessive T cell suppressive activity. Paradoxically, these patients are prone to develop a variety of autoimmune disorders (hemolytic anemia, pernicious anemia), as well as lymphoid tumors. The underlying mechanism of the antibody deficiency is variable (hence the name). Some patients with this disease have mutations in B cell receptors for certain growth factors, or in molecules involved in T cell–B cell interactions. However, the genetic basis of most cases of the disease is not known.

Isolated IgA Deficiency

The most common of all the primary immune deficiency diseases, IgA deficiency affects about 1 in 700 whites. As noted previously, IgA is the major immunoglobulin in mucosal secretions and is thus involved in defending the airways and the gastrointestinal tract. Although most people with this condition are asymptomatic, weakened mucosal defenses predispose patients to recurrent sinopulmonary infections and diarrhea. There is also a significant (but unexplained) association with autoimmune diseases. The pathogenesis of IgA deficiency seems to involve a block in the terminal differentiation of IgA-secreting B cells to plasma cells; IgM and IgG subclasses of antibodies are present in normal or even supranormal levels. The molecular basis for this defect is not understood.

Hyper-IgM Syndrome

In a normal immune response to protein antigen, IgM antibodies are produced first, followed by the sequential elaboration of IgG, IgA, and IgE antibodies. As discussed earlier in this chapter, the orderly appearance of different antibody types is called heavy-chain class (isotype) switching and is important for generating classes of antibody that can effectively activate complement and/or opsonize bacterial pathogens. The ability of IgM-producing B cells to turn on the transcription of genes that encode other immunoglobulin isotypes depends on certain cytokines, as well as on contact-mediated signals from CD4+ helper T cells. The contact-dependent signals are provided by interaction between CD40 molecules on B cells and CD40L (also known as CD154), expressed on activated helper T cells. Patients with the hyper-IgM syndrome produce normal (or even supranormal) levels of IgM antibodies to antigens but lack the ability to produce the IgG, IgA, and IgE isotypes; the underlying defect is an inability of T cells to induce B cell isotype switching. The most common genetic abnormality is mutation of the gene encoding CD40L. This gene is located on the X chromosome; consequently, in approximately 70% of the cases, hyper-IgM syndrome is X-linked. In the remaining patients, the mutations affect CD40 or other molecules involved in class switching, notably an enzyme called activation-induced deaminase. In addition to defective class switching, in those with CD40 or CD40L mutations there is also often a defect in the production of high-affinity antibodies, because the same mechanism is responsible for affinity maturation of the antibody response.

Although the disease is diagnosed and named because of the antibody abnormality, in patients with CD40 or CD40L mutations there is also a defect in cell-mediated immunity because the CD40–CD40L interaction is critical for helper T cell–mediated activation of macrophages, the central reaction of cell-mediated immunity. Male patients with the X-linked form of hyper-IgM syndrome present with recurrent pyogenic infections owing to low levels of opsonizing IgG antibodies. These patients also are susceptible to infections with a variety of intracellular pathogens that normally are combated by cell-mediated immunity, including Pneumocystis jiroveci (formerly called Pneumocystis carinii).

Thymic Hypoplasia: DiGeorge Syndrome

DiGeorge syndrome results from a congenital defect in thymic development with deficient T cell maturation. T cells are absent in the lymph nodes, spleen, and peripheral blood, and infants with this defect are extremely vulnerable to viral, fungal, and protozoal infections. Patients are also susceptible to infection with intracellular bacteria, because of defective T cell–mediated immunity. B cells and serum immunoglobulins are generally unaffected.

The disorder is a consequence of a developmental malformation affecting the third and fourth pharyngeal pouches, structures that give rise to the thymus, parathyroid glands, and portions of the face and aortic arch. Thus, in addition to the thymic and T cell defects, there may be parathyroid gland hypoplasia, resulting in hypocalcemic tetany, as well as additional midline developmental abnormalities. In 90% of cases of DiGeorge syndrome there is a deletion affecting chromosomal region 22q11, as discussed in Chapter 6. Transplantation of thymic tissue has successfully treated some affected infants. In patients with partial defects, immunity may improve spontaneously with age.

Severe Combined Immunodeficiency

Severe combined immunodeficiency (SCID) represents a constellation of genetically distinct syndromes with the common feature of defects in both humoral and cell-mediated immune responses. Affected infants are susceptible to severe recurrent infections by a wide array of pathogens, including bacteria, viruses, fungi, and protozoans, and opportunistic infections by Candida, Pneumocystis, CMV, and Pseudomonas. These pathogens cause serious (and occasionally lethal) disease. The prevalence of the disease is approximately 1 in 65,000 to 1 in 100,000, and the frequency is 20 to 30 times higher in some Native American populations.

Despite the common clinical features, the underlying defects in individual patients are quite diverse. Some forms of SCID are caused by a single defect affecting both T and B cells, and others may result from a primary T cell deficit with secondary impairment of humoral immunity. Approximately half of the cases are X-linked; these are caused by mutations in the gene encoding the common γ chain shared by the receptors for the cytokines IL-2, IL-4, IL-7, IL-9, and IL-15. Of these cytokines, IL-7 is the most important in this disease because it is the growth factor responsible for stimulating the survival and expansion of immature B and T cell precursors in the generative lymphoid organs. Another 40% to 50% of SCID cases are inherited in an autosomal recessive fashion, with approximately half of these caused by mutations in adenosine deaminase (ADA), an enzyme involved in purine metabolism. ADA deficiency results in accumulation of adenosine and deoxyadenosine triphosphate metabolites, which inhibit DNA synthesis and are toxic to lymphocytes. The other autosomal recessive cases of SCID are attributed to defects in another purine metabolic pathway, primary failure of class II MHC expression, or mutations in genes encoding the recombinase responsible for the rearrangement of lymphocyte antigen-receptor genes.

In the two most common forms of SCID (cytokine receptor common γ chain mutation and ADA deficiency), the thymus is hypoplastic. Lymph nodes and lymphoid tissues (e.g., in the tonsils, gut, and appendix) are atrophic and lack germinal centers as well as paracortical T cells. Affected patients may have marked lymphopenia, with both T and B cell deficiency; others may have increased numbers of immature T cells and/or large numbers of B cells that are nonfunctional as a consequence of a lack of T cell help. Patients with SCID are currently treated by bone marrow transplantation. X-SCID is the first disease in which gene therapy has been used to successfully replace the mutated gene, but the approach is being reevaluated because some of the treated patients have developed T cell leukemias, presumably because the introduced gene was inserted close to a cellular oncogene.

Defects in Lymphocyte Activation

Rare patients with mutations in genes required for T cell activation have been identified that manifest with defective cell-mediated immunity or a phenotype resembling SCID. One of the interesting ones is a mutation in a transcription factor that is required for the T_H17 response. The resulting disease manifestations include fungal (and occasionally bacterial) skin infections and chronic mucocutaneous candidiasis. By contrast, mutations in genes involved in T_H1 responses result in susceptibility to infections with atypical mycobacteria. These observations emphasize the importance of T_H17 cells for defense against fungal infections and of T_H1 cells for combating intracellular bacterial infections. Mutations in genes encoding calcium channel proteins, and other components of T cell signaling, also have been described.

Immune Deficiency 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; the only treatment is bone marrow transplantation. This is a curious syndrome in that the clinical presentation and immunologic deficits are difficult to explain on the basis of the known underlying genetic defect. The thymus is initially normal, but there is progressive age-related depletion of T lymphocytes in the peripheral blood and lymph nodes, with concurrent loss of cellular immunity. Additionally, patients do not make effective antibody responses to polysaccharide antigens, and are therefore particularly susceptible to infections with encapsulated, pyogenic bacteria. Affected patients also are prone to the development of malignant lymphomas. The responsible gene encodes a protein (Wiskott-Aldrich syndrome protein) that links several membrane receptors to the cytoskeleton. Although the mechanism is not known, a defect in this protein could result in abnormal cellular morphology (including platelet shape changes) or defective cytoskeleton-dependent activation signals in lymphocytes and other leukocytes, with abnormal cell–cell adhesions and leukocyte migration.

Genetic Deficiencies of Components of Innate Immunity

Several genetic defects have been shown to affect molecules or cells that are important in the early innate immune response to microbes.

Other Genetic Disorders of Innate Immunity

Mutations in TLRs and their signaling pathways are quite rare, but study of the associated disorders has been informative. One of the surprises that has emerged from these diseases is that the immune deficiency typically is very restricted. For instance, patients with mutations affecting TLR3, which recognizes viral RNA, develop recurrent herpes simplex encephalitis, and those with mutations affecting MyD88, the signaling protein downstream of many TLRs, are susceptible to bacterial infections, especially severe pneumococcal lung disease, but neither suffers from multiple disseminated infections.

Summary

Primary (Congenital) Immune Deficiency Diseases

• Caused by mutations in genes involved in lymphocyte maturation or function, or in innate immunity

• Some of the common disorders:

XLA: failure of B cell maturation, absence of antibodies; caused by mutations in BTK, which encodes a tyrosine kinase required for maturation signals from the pre-B cell and B cell receptors

Common variable immunodeficiency: defects in antibody production; cause unknown in most cases

Selective IgA deficiency: failure of IgA production; cause unknown

X-SCID: failure of T cell and B cell maturation; mutation in the common γ chain of a cytokine receptor, leading to failure of IL-7 signaling and defective lymphopoiesis

Autosomal SCID: failure of T cell development, secondary defect in antibody responses; approximately 50% of cases caused by mutation in the gene encoding ADA, leading to accumulation of toxic metabolites during lymphocyte maturation and proliferation

X-linked hyper-IgM syndrome: failure to produce isotype-switched high-affinity antibodies (IgG, IgA, IgE); mutation in gene encoding CD40L

• Clinical presentation: increased susceptibility to infections in early life

Epidemiology

Epidemiologic studies in the United States have identified five groups at risk for developing AIDS, and these are similar in other countries, except as noted in the following list. Transmission of HIV occurs under conditions that facilitate the exchange of blood or body fluids that contain the virus or virus-infected cells. Thus, the major routes of HIV infection are sexual contact, parenteral inoculation, and passage of the virus from infected mothers to their newborns. In about 10% of cases, the risk factors are unknown or not reported. The case distribution data cited are for the United States.

Etiology and Pathogenesis

AIDS is caused by HIV, a human retrovirus belonging to the lentivirus family (which also includes feline immunodeficiency virus, simian immunodeficiency virus, visna virus of sheep, and the equine infectious anemia virus). Two genetically different but antigenically related forms of HIV, called HIV-1 and HIV-2, have been isolated from patients with AIDS. HIV-1 is the more common type associated with AIDS in the United States, Europe, and Central Africa, whereas HIV-2 causes a similar disease principally in West Africa. Specific tests for HIV-2 are now available, and blood collected for transfusion also is routinely screened for HIV-2 seropositivity. The ensuing discussion relates primarily to HIV-1 and diseases caused by it, but it is generally applicable to HIV-2 as well.

Structure of HIV

Like 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. 4–26). The virus core contains (1) major capsid protein p24, (2) nucleocapsid protein p7/p9, (3) two copies of genomic RNA, and (4) three viral enzymes—protease, reverse transcriptase, and integrase. The p24 protein is the most readily detected viral antigen and is therefore the target for the antibodies used to diagnose HIV infection in blood screening. The viral core is surrounded by a matrix protein called p17, lying beneath the virion envelope. The viral envelope itself is studded with two viral glycoproteins (gp120 and gp41), critical for HIV infection of cells. The HIV-1 proviral genome contains the gag, pol, and env genes, which code for various viral proteins. The products of the gag and pol genes are translated initially into large precursor proteins that must be 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.

Figure 4–26 The structure of human immunodeficiency virus (HIV). The 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.

In addition to these three standard retroviral genes, HIV contains several other genes (given three-letter names such as tat, rev, vif, nef, vpr, and vpu) that regulate the synthesis and assembly of infectious viral particles. The product of the tat (transactivator) gene, for example, is critical for virus replication, causing a 1000-fold increase in the transcription of viral genes. The nef protein activates intracellular kinase activity (affecting T cell activation, viral replication, and viral infectivity) and reduces surface expression of CD4 and MHC molecules on infected cells. The progression of HIV infection in vivo is dependent on nef; strains of simian immunodeficiency virus with mutated nef genes cause AIDS in monkeys at a markedly decreased rate, and humans infected with a nef-defective HIV-1 strain display low viral burden, with AIDS onset at a substantially slower pace than for nonmutant strains. The products of various regulatory genes are important for HIV pathogenicity, and therapeutic approaches are being developed to block their actions.

Nucleic acid sequencing of different viral isolates reveals considerable variability in many parts of the HIV genome. This high variability is due to the relatively low fidelity of the viral polymerase, with estimates of one mistake for each 10⁵ replicated nucleotides. Most sequence variants cluster in parts of the genome encoding the envelope glycoproteins. Because the immune response against HIV-1 is targeted against its envelope, such extreme variability in antigen structure poses a formidable barrier to vaccine development.

On the basis of genomic analyses, HIV-1 can be divided into two groups, designated M (major) and O (outlier). Group M viruses, the more common form worldwide, are further divided into subtypes, or clades, designated A through J. The clades differ in their geographic distribution, with B being the most common form in Western Europe and the United States and E being the most common in Thailand. Beyond molecular homologies, the clades also show differences in modes of transmission. Thus, E clade is spread predominantly by heterosexual contact (male-to-female), presumably because of its ability to infect vaginal subepithelial DCs. By contrast, B clade virus grows poorly in DCs and may be transmitted by monocytes and lymphocytes.

Life Cycle of HIV

The two major targets of HIV infection are the immune system and the CNS. The life cycle of the virus is best understood in terms of its interactions with the immune system.

The entry of HIV into cells requires the CD4 molecule, which acts as a high-affinity receptor for the virus (Fig. 4–27). This requirement explains the tropism of the virus for CD4+ T cells and its ability to infect other CD4+ cells, particularly macrophages and DCs. However, binding to CD4 is not sufficient for infection; the HIV envelope gp120 must also bind to other cell surface molecules (coreceptors) to facilitate cell entry. Two cell surface chemokine receptors, CCR5 and CXCR4, serve this role. HIV envelope gp120 (noncovalently attached to transmembrane gp41) binds initially to CD4 molecules (Fig. 4–27). This binding leads to a conformational change that exposes a new recognition site on gp120 for the coreceptors CXCR4 (mostly on T cells) or CCR5 (mostly on macrophages). The gp41 then undergoes a conformational change that allows it to insert into the target membrane, and this process facilitates fusion of the virus with the cell. After fusion, the virus core containing the HIV genome enters the cytoplasm of the cell.

Figure 4–27 Molecular basis of entry of human immunodeficiency virus (HIV) into host cells. Interactions with CD4 and a chemokine receptor (“coreceptor”).

(Adapted by permission from Macmillan Publishers Ltd, from Wain-Hobson S: HIV. One on one meets two. Nature 384:117, copyright 1996.)

The coreceptors are critical components of the HIV infection process, and their discovery resolved some previously unexplained observations regarding HIV tropism. It had been known that HIV strains could be classified according to their relative ability to infect macrophages and/or CD4+ T cells. Macrophage-tropic (R5 virus) strains infect both monocytes/macrophages and freshly isolated peripheral blood T cells, whereas T cell–tropic (X4 virus) strains infect only activated T cell lines. This selectivity is now explained by selective coreceptor usage. R5 strains use CCR5 as their coreceptor, and, because CCR5 is expressed on both monocytes and T cells, these cells are susceptible to infection by R5 strains. Conversely, X4 strains bind to CXCR4, which is expressed on T cell lines (and not on monocytes/macrophages), so that only activated T cells are susceptible. Of interest, approximately 90% of HIV infections initially are transmitted by R5 strains. Over the course of infection, however, X4 viruses gradually accumulate; these are especially virulent and are responsible for T cell depletion in the final, rapid phase of disease progression. It is thought that during the course of HIV infection, R5 strains evolve into X4 strains, as a result of mutations in genes that encode gp120. Persons with defective CCR5 receptors (of U.S. whites, 20% are heterozygous and 1% are homozygous for the mutant CCR5) are relatively resistant to developing AIDS, despite repeated HIV exposure in vivo. Because of the significance of HIV–coreceptor interaction in the pathogenesis of AIDS, preventing this interaction may be of significant therapeutic value.

Once internalized, the viral genome undergoes reverse transcription, leading to formation of complementary DNA (cDNA). In quiescent T cells, HIV proviral cDNA may remain in the cytoplasm in a linear episomal form. However, in dividing T cells, the cDNA enters the nucleus and becomes integrated into the host genome. After integration, the provirus may remain nontranscribed for months or years, and the infection becomes latent; alternatively, proviral DNA may be transcribed to form complete viral particles that bud from the cell membrane. Such productive infections, associated with extensive viral budding, lead to cell death. It is important to note that although HIV-1 can infect resting T cells, the initiation of proviral DNA transcription (and hence productive infection) occurs only when the infected cell is activated by exposure to antigens or cytokines. Thus, in a cruel twist, physiologic responses to infections and other stimuli promote the death of HIV-infected T cells.

Progression of HIV Infection

HIV disease begins with acute infection, which is only partly controlled by the host immune response, and advances to chronic progressive infection of peripheral lymphoid tissues (Fig. 4–28). The first cell types to be infected may be memory CD4+ T cells (which express CCR5) in mucosal lymphoid tissues. Because the mucosal tissues are the largest reservoir of T cells in the body and a major site of residence of memory T cells, the death of these cells results in considerable depletion of lymphocytes.

Figure 4–28 Pathogenesis of human immunodeficiency virus (HIV) infection. Initially, HIV infects T cells and macrophages directly or is carried to these cells by Langerhans cells. Viral replication in the regional lymph nodes 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 by productive infection (or other mechanisms, not shown). Ultimately, CD4+ cell numbers decline and the patient develops clinical symptoms of full-blown AIDS. Macrophages are also parasitized by the virus early; they are not lysed by HIV and they transport the virus to tissues, particularly the brain.

The transition from the acute phase to a chronic phase of infection is characterized by dissemination of the virus, viremia, and the development of host immune responses. DCs in epithelia at sites of virus entry capture the virus and then migrate into the lymph nodes. Once in lymphoid tissues, DCs 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, accompanied by an acute HIV syndrome that includes a variety of nonspecific signs and symptoms typical of many viral diseases. The virus disseminates throughout the body and infects helper T cells, macrophages, and DCs in peripheral lymphoid tissues. As the infection spreads, the immune system mounts both humoral and cell-mediated immune responses directed at viral antigens. 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.

In the next, chronic phase of the disease, lymph nodes and the spleen are sites of continuous HIV replication and cell destruction (Fig. 4–28). During this period of the disease, the immune system remains competent at handling most infections with opportunistic microbes, and few or no clinical manifestations of the HIV infection are present. Therefore, this phase of HIV disease is called the clinical latency period. Although a majority of peripheral blood T cells do not harbor the virus, destruction of CD4+ T cells within lymphoid tissues steadily progresses during the latent period, 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 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, so 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 number of circulating CD4+ T cells that are infected at any one time may be less than 0.1% of the total CD4+ T cells in a given case. Eventually, over a period of years, the continuous cycle of virus infection and T cell death leads to a steady decline in the number of CD4+ T cells in the lymphoid tissues and the circulation.

In addition to T cell depletion, abnormalities have been described in many components of the immune system, summarized in Table 4–11. Discussed next are the major defects in immune cells during the course of HIV infection.

Table 4–11 Major Abnormalities of Immune Function in AIDS

Lymphopenia

Predominantly caused by selective loss of the CD4+ helper T cell subset; reduced CD4+/CD8+ ratio

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 interleukin-2 and interferon-γ production

Polyclonal B Cell Activation

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

Altered Monocyte or Macrophage Functions

Decreased chemotaxis and phagocytosis Decreased HLA class II antigen expression Diminished capacity to present antigen to T cells Increased spontaneous secretion of interleukin-1, tumor necrosis factor, interleukin-6

HLA, human leukocyte antigen.

Mechanisms of T Cell Depletion in HIV Infection

The major mechanism of loss of CD4+ T cells is lytic HIV infection of the cells, and cell death during viral replication and production of virions (Fig. 4–29). Like other cytopathic viruses, HIV disrupts cellular functions sufficiently to cause death of infected cells. In addition to direct cell lysis, other mechanisms may cause T cell loss:

• Loss of immature precursors of CD4+ T cells, either by direct infection of thymic progenitor cells or by infection of accessory cells that secrete cytokines essential for CD4+ T cell maturation. The result is decreased production of mature CD4+ T cells.

• Chronic activation of uninfected cells by HIV antigens or by other concurrent infectious microbes may lead to apoptosis of the T cells. Because of this “activation-induced death” of uninfected cells, the numbers of T cells that die may be much greater than the number of HIV-infected cells.

• Infection of various cells in lymphoid tissues may disrupt the normal architecture, leading to impaired immune responses.

• Fusion of infected and uninfected cells causes formation of syncytia (giant cells). In tissue culture, the gp120 expressed on productively infected cells binds to CD4 molecules on uninfected T cells, followed by cell fusion, ballooning, and death within a few hours. This property of syncytia formation is confined to the X4 strain of HIV.

• Uninfected CD4+ T cells may bind soluble gp120 to the CD4 molecule, leading to aberrant signaling and apoptosis.

• Infected CD4+ T cells may be killed by HIV-specific CD8+ CTLs.

Figure 4–29 Mechanisms of CD4+ T cell loss in human immunodeficiency virus (HIV) infection. Some of the principal known and postulated mechanisms of T cell depletion after HIV infection are shown.

The loss of CD4+ cells leads to an inversion of the CD4+/CD8+ ratio in the peripheral blood. Thus, while the normal CD4+/CD8+ ratio is 1 to 2, patients with AIDS have a ratio of 0.5 or less. Such inversion is a common finding in AIDS, but it also may occur in other viral infections and is therefore not diagnostic.

Although marked reduction in CD4+ T cells is a hallmark of AIDS and can account for much of the immune deficiency late in the course of HIV infection, there is also compelling evidence for qualitative defects in T cell function that can be detected even in asymptomatic HIV-infected persons. Such defects include reduced antigen-induced T cell proliferation, impaired T_H1 cytokine production, and abnormal intracellular signaling. There is also a selective loss of memory CD4+ T cells early in the course of the disease, possibly related to the abundance of these cells in mucosal tissues and higher level of CCR5 expression in this T cell subset.

Low-level chronic or latent infection of T cells (and macrophages) is an important feature of HIV infection. Although only rare CD4+ T cells express infectious virus early in the course of infection, up to 10% of lymph node T cells can be demonstrated to actually harbor the HIV genome. It is widely believed that integrated provirus, without virus production (latent infection), can persist within cells for months or years. Even with highly active antiretroviral therapy (which can eliminate most of the virus in the blood), latent virus lurks in lymph node CD4+ cells (as many as 0.05% of resting, long-lived CD4+ T cells are infected). Completion of the viral life cycle in latently infected cells requires cell activation. Thus, if latently infected CD4+ cells are activated by environmental antigens, an unfortunate consequence is increased HIV proviral DNA transcription. This increased transcription leads to virion production and, in the case of T cells, also results in cell lysis. In addition, TNF, IL-1, and IL-6 produced by activated macrophages during normal immune responses can also lead to increased HIV gene transcription (Fig. 4–28). Thus, it seems that HIV thrives when the host macrophages and T cells are physiologically activated (e.g., through intercurrent infection by other microbial agents). The life styles of most HIV-infected patients in the United States place them at increased risk for recurrent exposure to other sexually transmitted diseases; in Africa, socioeconomic conditions probably impose a higher burden of chronic microbial infections. It is easy to understand how patients with AIDS enter a vicious circle of T cell destruction; infections to which these patients are prone because of diminished helper T cell function lead to increased production of pro-inflammatory cytokines, which in turn stimulate more HIV production, followed by infection and loss of additional CD4+ T cells.

Pathogenesis of CNS Involvement

The pathogenesis of the neurologic manifestations in AIDS deserves special mention because, in addition to the lymphoid system, the nervous system is a major target of HIV infection. Macrophages and cells belonging to the monocyte–macrophage lineage (microglial cells) are the predominant cell types in the brain that are infected with HIV. The virus is most likely carried into the brain by infected monocytes (thus, brain HIV isolates are almost exclusively of the R5 type). The mechanism of HIV-induced damage of the brain, however, remains obscure. Because neurons are not infected by HIV, and the extent of neuropathologic changes is often less than might be expected from the severity of neurologic symptoms, most experts believe that the neurologic deficit is caused indirectly by viral products and soluble factors (e.g., cytokines such as TNF) produced by macrophages and microglial cells. In addition, injury from nitric oxide induced in neuronal cells by gp41 and direct damage of neurons by soluble HIV gp120 have been postulated.

Summary

Human Immunodeficiency Virus Life Cycle and the Pathogenesis of AIDS

• Virus entry into cells: requires CD4 and co-receptors, which are receptors for chemokines; involves binding of viral gp120 and fusion with the cell mediated by viral gp41 protein; main cellular targets: CD4+ helper T cells, macrophages, DCs

• Viral replication: integration of provirus genome into host cell DNA; triggering of viral gene expression by stimuli that activate infected cells (e.g., infectious microbes, cytokines produced during normal immune responses)

• Progression of infection: acute infection of mucosal T cells and DCs; viremia with dissemination of virus; latent infection of cells in lymphoid tissue; continuing viral replication and progressive loss of CD4+ T cells

• Mechanisms of immune deficiency:

Loss of CD4+ T cells: T cell death during viral replication and budding (similar to other cytopathic infections); apoptosis occurring as a result of chronic stimulation; decreased thymic output; functional defects

Defective macrophage and DC functions

Destruction of architecture of lymphoid tissues (late)

Natural History and Clinical Course

The clinical course of HIV infection can best be understood in terms of an interplay between HIV and the immune system. Three phases reflecting the dynamics of virus–host interaction can be recognized: (1) an early acute phase, (2) a middle chronic phase, and (3) a final crisis phase (Fig. 4–30).

Figure 4–30 Clinical and immune response to human immunodeficiency virus (HIV) infection. A, Clinical course. The early period after primary infection is characterized by 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 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.

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

In the absence of treatment, most patients with HIV infection develop AIDS after a chronic phase lasting 7 to 10 years. Exceptions to this time frame are seen in so-called rapid progressors and long-term nonprogressors. In rapid progressors, the middle, chronic phase is telescoped to 2 to 3 years after primary infection. Nonprogressors (less than 5% of infected persons) are defined as HIV-infected patients who remain asymptomatic for 10 years or more, with stable CD4+ counts and low levels of plasma viremia; notably, AIDS eventually develops in a majority of these patients, albeit after a much-prolonged clinical latency. Despite much study, the reason for nonprogression is not known.

Because the loss of immune containment is associated with declining numbers of CD4+ T cells, the CDC classification of HIV infection stratifies patients into three categories on the basis of CD4+ T cell counts: more than 500 cells/µL, between 200 and 500 cells/µL, and less than 200 cells/µL. Patients in the first group are generally asymptomatic; counts below 500 cells/µL are associated with early symptoms, and a decline of CD4+ T cell levels below 200 cells/µL is associated with severe immunosuppression. For clinical management, CD4+ cell counts are an important adjunct to HIV viral load measurements. The significance of these two measurements, however, is slightly different: Whereas CD4+ cell counts indicate the status of the patient’s disease at the time of measurement, the viral load provides information about the direction in which the disease is progressing.

Although this summary of the clinical course is true for untreated or refractory cases, recently developed anti-retroviral therapy has changed the course of the disease and greatly reduced the incidence of severe opportunistic infections (such as Pneumocystis pneumonia) and tumors (such as Kaposi sarcoma). The available therapy does not eliminate all of the virus, however, and the disease can recur if treatment is stopped. Also not known is whether drug-resistant viral strains will become widespread.

Primary Amyloidosis: Immunocyte Dyscrasias with Amyloidosis

Amyloid in this category usually is systemic in distribution and is of the AL type. With approximately 3000 new cases each year in the United States, this is the most common form of amyloidosis. In some of these cases, there is a readily identifiable monoclonal plasma cell proliferation; best defined is the occurrence of systemic amyloidosis in 5% to 15% of patients with multiple myeloma, a plasma cell tumor characterized by multiple osteolytic lesions throughout the skeletal system (Chapter 11). The malignant plasma cells characteristically synthesize abnormal amounts of a single specific immunoglobulin (monoclonal gammopathy), producing an M (myeloma) protein spike on serum electrophoresis. In addition to the synthesis of whole immunoglobulin molecules, plasma cells also may synthesize and secrete either the λ or κ light chain, also known as Bence Jones proteins. By virtue of their small molecular size, these proteins frequently are also excreted in the urine. Almost all patients with myeloma who develop amyloidosis have Bence Jones proteins in the serum or urine, or both. However, amyloidosis develops in only 6% to 15% of patients with myeloma who have free light chains. Clearly, the presence of Bence Jones proteins, although necessary, is by itself not sufficient to produce amyloidosis. Other variables, such as the type of light chain produced and its catabolism, contribute to the “amyloidogenic potential” and influence the deposition of Bence Jones proteins.

The great majority of patients with AL amyloid do not have classic multiple myeloma or any other overt B cell neoplasm; such cases are nevertheless classified as primary amyloidosis because their clinical features derive from the effects of amyloid deposition without any other associated disease. In virtually all such cases, patients have a modest increase in the number of plasma cells in the bone marrow, and monoclonal immunoglobulins or free light chains can be found in the serum or urine. Clearly, these patients have an underlying monoclonal plasma cell proliferation in which production of an abnormal protein, rather than production of tumor masses, is the predominant manifestation.

Reactive Systemic Amyloidosis

The amyloid deposits in this pattern are systemic in distribution and are composed of AA protein. This category was previously referred to as secondary amyloidosis, because it is secondary to an associated inflammatory condition. In fact, the feature common to most cases of reactive systemic amyloidosis is chronic inflammation. Classically, tuberculosis, bronchiectasis, and chronic osteomyelitis were the most common causes; with the advent of effective antimicrobial therapies, reactive systemic amyloidosis is seen most frequently in the setting of chronic inflammation caused by autoimmune states (e.g., RA, ankylosing spondylitis, inflammatory bowel disease). Patients with RA are particularly prone to develop amyloidosis, with amyloid deposition seen in as many as 3% of RA cases. Chronic skin infections caused by “skin-popping” of narcotics are also associated with amyloid deposition. Finally, reactive systemic amyloidosis may also occur in association with tumors not derived from immune cells, the two most common being renal cell carcinoma and Hodgkin lymphoma.

Familial (Hereditary) Amyloidosis

A variety of familial forms of amyloidosis have been described; most are rare and occur in limited geographic areas. The best-characterized is an autosomal recessive condition called familial Mediterranean fever. This is a febrile disorder characterized by attacks of fever accompanied by inflammation of serosal surfaces, including peritoneum, pleura, and synovial membrane. This disorder is encountered largely in persons of Armenian, Sephardic Jewish, and Arabic origins. It is associated with widespread tissue involvement indistinguishable from reactive systemic amyloidosis. The amyloid fibril proteins are made up of AA proteins, suggesting that this form of amyloidosis is related to the recurrent bouts of inflammation that characterize this disease. The gene for familial Mediterranean fever is called pyrin and encodes a protein that is a component of the inflammasome (Chapter 2). Patients have gain-of-function mutations in pyrin that result in constitutive overproduction of the pro-inflammatory cytokine IL-1 and persistent inflammation.

In contrast with familial Mediterranean fever, a group of autosomal dominant familial disorders is characterized by deposition of amyloid predominantly in the peripheral and autonomic nerves. These familial amyloidotic polyneuropathies have been described in kindreds in different parts of the world—for example, in Portugal, Japan, Sweden, and the United States. As mentioned previously, the fibrils in these familial polyneuropathies are made up of mutant forms of transthyretin (ATTRs).

Localized Amyloidosis

Sometimes deposition of amyloid is limited to a single organ or tissue without involvement of any other site in the body. The deposits may produce grossly detectable nodular masses or be evident only on microscopic examination. Nodular (tumor-forming) deposits of amyloid are most often encountered in the lung, larynx, skin, urinary bladder, tongue, and the region about the eye. Frequently, there are infiltrates of lymphocytes and plasma cells in the periphery of these amyloid masses, raising the question of whether the mononuclear infiltrate is a response to the deposition of amyloid or instead is responsible for it. At least in some cases, the amyloid consists of AL protein and may therefore represent a localized form of plasma cell–derived amyloid.

Endocrine Amyloid

Microscopic deposits of localized amyloid may be found in certain endocrine tumors, such as medullary carcinoma of the thyroid gland, islet tumors of the pancreas, pheochromocytomas, and undifferentiated carcinomas of the stomach, as well as in the islets of Langerhans in patients with type 2 diabetes mellitus. In these settings, the amyloidogenic proteins seem to be derived either from polypeptide hormones (medullary carcinoma) or from unique proteins (e.g., islet amyloid polypeptide).

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 persons (usually in their 70s and 80s). Because of the dominant involvement and related dysfunction of the heart (typically manifesting as a restrictive cardiomyopathy and arrhythmias), this form also is called senile cardiac amyloidosis. The amyloid in this form is composed of normal transthyretin. In addition, another form typically affecting only the heart results from the deposition of a mutant form of TTR. Approximately 4% of the black population in the United States are carriers of the mutant allele, and cardiomyopathy has been identified in both homozygous and heterozygous patients.

Morphology

There are no consistent or distinctive patterns of organ or tissue distribution of amyloid deposits in any of the categories cited. Nonetheless, a few generalizations can be made. In amyloidosis secondary to chronic inflammatory disorders, kidneys, liver, spleen, lymph nodes, adrenals, and thyroid, as well as many other tissues, typically are affected. Although primary (AL) amyloidosis cannot reliably be distinguished from the secondary form by its organ distribution, it more often involves the heart, gastrointestinal tract, respiratory tract, peripheral nerves, skin, and tongue. However, the same organs affected by reactive systemic amyloidosis (secondary amyloidosis), including kidneys, liver, and spleen, also may contain deposits in the immunocyte-associated form of the disease. The localization of amyloid deposits in the hereditary syndromes is varied. In familial Mediterranean fever, the amyloidosis may be widespread, involving the kidneys, blood vessels, spleen, respiratory tract, and (rarely) liver. The localization of amyloid in the remaining hereditary syndromes can be inferred from the designation of these entities.

Whatever the clinical disorder, the amyloidosis may or may not be apparent grossly. Often small amounts are not recognized until the surface of the cut organ is painted with iodine and sulfuric acid. This yields mahogany brown staining of the amyloid deposits. When amyloid accumulates in larger amounts, the organ frequently is enlarged and the tissue typically appears gray with a waxy, firm consistency. On histologic examination, the amyloid deposition is always extracellular and begins between cells, often closely adjacent to basement membranes. As the amyloid accumulates, it encroaches on the cells, in time surrounding and destroying them. In the AL form, perivascular and vascular localizations are common.

The histologic diagnosis of amyloid is based almost entirely 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 so-called apple-green birefringence (Fig. 4–33). This reaction is shared by all forms of amyloid and is caused by the crossed β-pleated configuration of amyloid fibrils. Confirmation can be obtained by electron microscopy, which reveals amorphous nonoriented thin fibrils. AA, AL, and ATTR types of amyloid also can be distinguished from one another by specific immunohistochemical staining.

Figure 4–33 Amyloidosis: hepatic involvement. A, Staining of a section of the liver with Congo red reveals pink-red deposits of amyloid in the walls of blood vessels and along sinusoids. B, Note the yellow-green birefringence of the deposits when observed under the polarizing microscope.

(Courtesy of Dr. Trace Worrell and Sandy Hinton, Department of Pathology, University of Texas Southwestern Medical School, Dallas, Texas.)

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 most serious feature of the disease. Grossly, the kidney may appear unchanged, or it may be abnormally large, pale, gray, and firm; in long-standing cases, the kidney may be reduced in size. Microscopically, the amyloid deposits are found principally in the glomeruli, but they also are present in the interstitial peritubular tissue as well as in the walls of the blood vessels. The glomerulus first develops focal deposits within the mesangial matrix and diffuse or nodular thickenings of the basement membranes of the capillary loops. With progression, the deposition encroaches on the capillary lumina and eventually leads to total obliteration of the vascular tuft (Fig. 4–34, A). The interstitial peritubular deposits frequently are associated with the appearance of amorphous pink casts within the tubular lumens, presumably of a proteinaceous nature. Amyloid deposits may develop in the walls of blood vessels of all sizes, often causing marked vascular narrowing.

Figure 4–34 Amyloidosis: renal and cardiac involvement. A, Amyloidosis of the kidney. The glomerular architecture is almost totally obliterated by the massive accumulation of amyloid. B, Cardiac amyloidosis. The atrophic myocardial fibers are separated by structureless, pink-staining amyloid.

Spleen

Amyloidosis of the spleen often causes moderate or even marked enlargement (200 to 800 gm). For obscure reasons, either of two patterns may develop. The deposits may be virtually limited to the splenic follicles, producing tapioca-like granules on gross examination (“sago spleen”), or the amyloidosis may principally involve the splenic sinuses, eventually extending to the splenic pulp, with formation of large, sheetlike deposits (“lardaceous spleen”). In both patterns, the spleen is firm in consistency. The presence of blood in splenic sinuses usually imparts a reddish color to the waxy, friable deposits.

Liver

Amyloidosis of the liver may cause massive enlargement (as much as 9000 gm). In such advanced cases, the liver is extremely pale, grayish, and waxy on both the external surface and the cut section. Histologic analysis shows that amyloid deposits first appear in the space of Disse and then progressively enlarge to encroach on the adjacent hepatic parenchyma and sinusoids (Fig. 4–33). The trapped liver cells undergo compression atrophy and are eventually replaced by sheets of amyloid; remarkably, normal liver function may be preserved even in the setting of severe involvement.

Heart

Amyloidosis of the heart may occur either as isolated organ involvement or as part of a systemic distribution. When accompanied by systemic involvement, it is usually of the AL form. The isolated form (senile amyloidosis) usually is confined to older persons. The deposits may not be evident on gross examination, or they may cause minimal to moderate cardiac enlargement. The most characteristic gross findings are gray-pink, dewdrop-like subendocardial elevations, particularly evident in the atrial chambers. On histologic examination, deposits typically are found throughout the myocardium, beginning between myocardial fibers and eventually causing their pressure atrophy (Fig. 4–34, B).

Other Organs

Amyloidosis of other organs generally is encountered in systemic disease. The adrenals, thyroid, and pituitary are common sites of involvement. In such cases as well, the amyloid deposition begins in relation to stromal and endothelial cells and progressively encroaches on the parenchymal cells. Surprisingly large amounts of amyloid may be present in any of these endocrine glands without apparent disturbance of function. In the gastrointestinal tract, a relatively favored site for deposition, amyloid may be found at all levels, sometimes producing tumorous masses that must be distinguished from neoplasms. Nodular depositions in the tongue may produce macroglossia. On the basis of the frequent involvement of the gastrointestinal tract in systemic cases, gingival, intestinal, and rectal biopsies serve in the diagnosis of suspected cases. Deposition of β₂-microglobulin amyloid in patients receiving long-term dialysis occurs most commonly in the carpal ligaments of the wrist, resulting in compression of the median nerve (leading to carpal tunnel syndrome).

Clinical Course

Amyloidosis may be an unsuspected finding at autopsy in a patient who has no apparent related clinical manifestations, or it may be responsible for serious clinical dysfunction and even death. The clinical course depends on the particular sites or organs affected and the severity of the involvement. Nonspecific complaints such as weakness, fatigue, and weight loss are the most common presenting manifestations. Later in the course, amyloidosis tends to manifest in one of several ways: by renal disease, hepatomegaly, splenomegaly, or cardiac abnormalities. Renal involvement giving rise to severe proteinuria (nephrotic syndrome) (Chapter 13) often is the major cause of symptoms in reactive systemic amyloidosis. Progression of the renal disease may lead to renal failure, which is an important cause of death in amyloidosis. The hepatosplenomegaly rarely causes significant clinical dysfunction, but it may be the presenting finding. Cardiac amyloidosis may manifest as conduction disturbances or as restrictive cardiomyopathy (Chapter 10). Cardiac arrhythmias are an important cause of death in cardiac amyloidosis. In one large series, 40% of the patients with AL amyloid died of cardiac disease.

The diagnosis of amyloidosis may be suspected from the clinical signs and symptoms and from some of the findings mentioned; however, more specific tests must often be done for definitive diagnosis. Biopsy and subsequent Congo red staining is the most important tool in the diagnosis of amyloidosis. In general, biopsy is taken from the organ suspected to be involved. For example, renal biopsy is useful in the presence of urinary abnormalities. Rectal and gingival biopsy specimens contain amyloid in as many as 75% of cases with generalized amyloidosis. Examination of abdominal fat aspirates stained with Congo red is a simple, low-risk method. In suspected cases of AL amyloidosis, serum and urinary protein electrophoresis and immunoelectrophoresis should be performed. Bone marrow examination in such cases usually shows plasmacytosis, even if skeletal lesions of multiple myeloma are not present. Proteomic analysis of affected tissue is now being widely used for detection of small amounts of amyloid (from fat aspirates) and for definitive identification of the type of amyloid.

The outlook for patients with generalized amyloidosis is poor, with the mean survival time after diagnosis ranging from 1 to 3 years. In AA amyloidosis, the prognosis depends to some extent on the control of the underlying condition. Patients with myeloma-associated amyloidosis have a poorer prognosis, although they may respond to cytotoxic drugs used to treat the underlying disorder. Resorption of amyloid after treatment of the associated condition has been reported, but this is a rare occurrence.