CHAPTER 72 Assessment
Primary immunodeficiency diseases result from gene defects that affect one or more components of the immune system. The hallmarks of immunodeficiency include increased susceptibility and frequent, chronic, or unusually severe infections. The major components of host defenses include an anatomic barrier, innate immunity, and adaptive immunity. Integrity of the anatomic-mucociliary barrier at the interface between the body and its environment (skin and mucous membranes) is essential for protection against infection (Table 72-1). The innate immune system initiates antigen-nonspecific mechanisms as a first line of defense against pathogens, before the development of more versatile adaptive immune responses involving antigen-specific T and B lymphocytes. The innate immune system responds rapidly to pathogens. Complement proteins activate sequentially, killing pathogens by facilitating uptake by phagocytic cells or by antibody-independent lysis of pathogens. Other important soluble factors of innate immunity are acute-phase proteins, cytokines, and chemokines (Table 72-2). Proinflammatory acute-phase proteins, including C-reactive protein and mannose-binding lectin, are involved in recognition of damaged cells and pathogens, and they activate complement and induce production of inflammatory cytokines. The pattern of chemokines produced determines the type and location of inflammatory infiltrate.
TABLE 72-1 Anatomic and Mucociliary Defects That Result in Recurrent or Opportunistic Infections
ANATOMIC DEFECTS IN UPPER AIRWAYS
ANATOMIC DEFECTS IN THE TRACHEOBRONCHIAL TREE
PHYSIOLOGIC DEFECTS IN UPPER AND LOWER AIRWAYS
OTHER DEFECTS
CSF, cerebrospinal fluid.
TABLE 72-2 Cytokines and Chemotactic Cytokines and Their Functions
| Factor | Source | Function |
|---|---|---|
| IL-1 | Macrophages | Costimulatory effect on T cells enhances antigen presentation |
| IL-2 | T cells | Primary T-cell growth factor; B-cell and NK-cell growth factor |
| IL-3 | T cells | Mast cell growth factor; multicolony-stimulating factor |
| IL-4 | T cells | T-cell growth factor; enhances IgE synthesis; enhances B-cell differentiation; mast cell growth |
| IL-5 | T cells | Enhances immunoglobulin synthesis; enhances IgA synthesis; enhances eosinophil differentiation |
| IL-6 | T cells, macrophages, fibroblasts, endothelium | Enhances immunoglobulin synthesis, antiviral activity, and hepatocyte-stimulating factor |
| IL-7 | Stromal cells | Enhances growth of pre-B cells and pre-T cells |
| IL-8 | T cells, macrophages, epithelium | Neutrophil-activating protein; T lymphocyte, neutrophil chemotactic factor |
| IL-9 | T cells | Acts in synergy with IL-4 to induce IgE production, mast cell growth |
| IL-10 | T cells, including regulatory T cells, macrophages | Cytokine synthesis inhibitory factor; suppresses macrophage function; enhances B-cell growth; inhibits IL-12 production |
| IL-12 | Macrophages, neutrophils | NK cell stimulatory factor; cytotoxic lymphocyte maturation factor; enhances IFN-γ synthesis; inhibits IL-4 synthesis |
| IL-13 | T cells | Enhances IgE synthesis; enhances B-cell growth; inhibits macrophage activation; causes airway hyperreactivity |
| IL-17 | T cells | Induces IL-1β and IL-6 synthesis, involved in autoimmunity |
| IL-18 | Macrophages | Enhances IFN-γ synthesis |
| IFN-γ | T cells | Macrophage activation; inhibits IgE synthesis; antiviral activity |
| TGF-β | T cells including regulatory T cells, many other cells | Inhibits T-cell and B-cell proliferation and activation |
| RANTES | T cells, endothelium | Chemokine for monocytes, T cells, eosinophils |
| MIP-1α | Mononuclear cells, endothelium | Chemokine for T cells; enhances differentiation of CD4+ T cells |
| Eotaxin 1, 2, and 3 | Epithelium, endothelium, eosinophils, fibroblasts, macrophages | Chemokine for eosinophils, basophils, and Th2 cells |
| IP-10 | Monocytes, macrophages, endothelium | Chemokine for activated T cells, monocytes, and NK cells; T cells activate NK cells |
IFN, interferon; NK, natural killer; RANTES, regulated on activation, normal T expressed and secreted; Th2, T helper 2.
The cellular arm of innate immunity includes phagocytic cells that engulf and digest foreign antigens and microorganisms. Polymorphonuclear neutrophils ingest pyogenic bacteria and some fungi, especially Aspergillus. Macrophages, which develop from circulating monocytes, are effective in killing facultative intracellular organisms such as Mycobacterium, Toxoplasma, and Legionella. In addition, natural killer (NK) lymphocytes mediate cytotoxic activity against virus-infected cells and cancer cells. Recognition of pathogens by the innate immune system is facilitated by receptors on macrophages, NK cells, and neutrophils that recognize conserved pathogen motifs called pathogen-associated molecular patterns, including lipopolysaccharide of gram-negative bacteria, lipoteichoic acid of gram-positive bacteria, mannans of yeast, and specific nucleotide sequences of bacterial and viral DNA.
The key features of the adaptive immune system are antigen specificity and the development of immunologic memory, produced by expansion and maturation of antigen-specific T cells and B cells. Antibodies (immunoglobulin) produced by B cells neutralize toxins released by pathogens, facilitating opsonization of the pathogen by phagocytic cells, activating complement causing cytolysis of the pathogen, and directing NK cells to kill infected cells through antibody-mediated cytotoxicity. T cells kill virus-infected cells and cancer cells, delivering the necessary signals for B-cell antibody synthesis, memory B-cell formation, and activating macrophages to kill intracellular pathogens.
Immunodeficiency can result from defects in one or more components of innate or adaptive immunity, leading to recurrent, opportunistic, or life-threatening infections. Primary immunodeficiency diseases are relatively rare individually, but together cause significant chronic disease, morbidity, and mortality. To distinguish true immunodeficiency from an immunologically normal child who has recurrent infections, evaluation must be based on a comprehensive understanding of the immune system, the patient’s age, the site, and the pathogens involved (Table 72-3).
TABLE 72-3 Clinical Characteristics of Primary Immunodeficiencies
B-CELL DEFECTS
COMPLEMENT DEFECTS
T-CELL DEFECTS
NEUTROPHIL DEFECTS
HISTORY
A family history of primary immunodeficiency disease, or of infants dying from infection, warrants an immunologic evaluation, especially in the presence of recurrent infections. The frequency, severity, and location of the infection and the pathogens involved can help differentiate infections in a normal host from infections in an immunodeficient patient (see Table 72-3). Patients with primary immunodeficiency acquire infection with opportunistic organisms that do not ordinarily cause disease. Antibody deficiency diseases initially manifest with common infections, such as otitis media and sinusitis, but at a higher frequency than in immunocompetent children. Eight or more episodes of otitis media, two or more serious sinus infections, or two or more episodes of pneumonia in 1 year suggest an antibody deficiency, especially if the infections are difficult to treat. Recurrent sinopulmonary infections with encapsulated bacteria suggest antibody-mediated immunity because these pathogens evade phagocytosis. Failure to thrive, diarrhea, malabsorption, and fungal infections suggest T-cell immunodeficiency. Recurrent viral infections can result from T-cell or NK-cell deficiency. Opportunistic infections with organisms, such as Pneumocystis jiroveci (carinii), suggest a T-cell disorder, such as severe combined immunodeficiency (SCID), or T-cell dysfunction, as in X-linked hyper-IgM. Deep-seated abscesses and infections with Staphylococcus aureus, Serratia marcescens, and Aspergillus suggest a disorder of neutrophil function, such as chronic granulomatous disease (CGD). Delayed separation of the umbilical cord, especially in the presence of omphalitis, and periodontal disease, in addition to abscesses, indicate leukocyte adhesion deficiency, whereas hyper-IgE syndrome is associated with cold abscesses, eczema, and frequent fractures. The presence of deep-seated infections can help differentiate hyper-IgE syndrome from atopic dermatitis, which can be associated with extremely elevated levels of IgE. Onset of symptoms in adolescence or young adulthood suggests common variable immunodeficiency (CVID) rather than agammaglobulinemia, although milder phenotypes of primary immunodeficiency disease may not present until later in life. The presence of associated problems, such as congenital heart disease and hypocalcemia, suggests DiGeorge syndrome. Ataxia-telangiectasia is associated with abnormal gait and telangiectasia on the skin. Atopic dermatitis is present in patients with hyper-IgE syndrome and is associated with easy bruising or a bleeding disorder in patients with Wiskott-Aldrich syndrome.
PHYSICAL EXAMINATION
Recurrent infection in immunologically deficient children is associated with incomplete recovery at sites of infection, (scarring of skin or tympanic membranes, abnormal hearing, persistent perforation of the tympanic membrane, persistent ear drainage, chronic lung disease, persistent cough, sputum production, failure to thrive, digital clubbing, or anemia). Substantial morbidity from repeated infections suggests the presence of significant immunologic disease. Height and weight percentiles, nutritional status, and presence of subcutaneous fat should be assessed. Oral thrush, purulent nasal or otic discharge, and chronic rales may be evidence of repeated or persistent infections. Lymphoid tissue (tonsils and lymph nodes) should be examined. Absence of tonsils suggests agammaglobulinemia or SCID, whereas increased size of lymphoid tissue suggests CVID, CGD, autosomal recessive hyper-IgM syndrome, or human immunodeficiency virus (HIV) infection. Cerebellar ataxia and telangiectasia indicate ataxia-telangiectasia. Eczema and petechiae or bruises suggest Wiskott-Aldrich syndrome.
DIFFERENTIAL DIAGNOSIS
Separate episodes of infection must be differentiated from a recrudescence or relapse of a single episode, which may occur when infections are treated inadequately. Recurrent episodes of severe infection, such as meningitis or sepsis, are much more worrisome, but antibody deficiency states initially manifest with common infections, such as otitis media and sinusitis. In patients with antibody deficiency states, ciliary dyskinesis, T-cell deficiencies, or neutrophil disorders, infections develop at multiple sites (ears, sinuses, lungs, skin), whereas in individuals with anatomic problems (sequestered pulmonary lobe, ureteral reflux), infections are confined to a single anatomic site. Asplenia is associated with recurrent and severe infections, even in the presence of protective antibody titers. Infection with HIV should be considered in any patient presenting with a history suggesting a T-cell immunodeficiency. There are many secondary causes of immunodeficiency (Table 72-4).
DIAGNOSTIC EVALUATION
The diagnosis of patients with primary immunodeficiency diseases depends on early recognition of signs and symptoms, followed by laboratory tests to evaluate immune function. Recognizing the patient who may have an immunodeficiency disease prompts evaluation and referral to an immunologist (see Table 72-3).
Laboratory Tests
A diagnosis of primary immunodeficiency disease cannot be established without the use of laboratory tests. The choice of test and the extent of testing depend on the clinical history. Several tests are used in the diagnosis of primary immunodeficiency disease (Table 72-5).
TABLE 72-5 Tests for Suspected Immune Deficiency
GENERAL
ANTIBODY-MEDIATED IMMUNITY
CELL-MEDIATED IMMUNITY
PHAGOCYTOSIS
COMPLEMENT
NK, natural killer.
A complete blood count with differential should always be obtained to identify patients with neutropenia or lymphopenia (SCID) as well as the presence of eosinophils (allergic disease) and anemia (chronic disease).
Serum immunoglobulin levels vary with age, with normal adult values of IgG at full-term birth from transplacental transfer of maternal IgG, a physiologic nadir occurring at 6 to 8 months of age, and a gradual increase to adult values over several years. IgA can be absent at birth. IgA and IgM levels increase gradually over several years, with IgA taking the longest to reach normal adult values. Low albumin levels with low immunoglobulin levels suggest low synthetic rates for all proteins or increased loss of proteins, as in protein-losing enteropathy. High immunoglobulin levels suggest intact B-cell immunity and can be found in diseases with recurrent infections, such as CGD, immotile cilia syndrome, cystic fibrosis, and HIV infection. Elevated IgE levels can be found in hyper-IgE syndrome and in atopic dermatitis.
Specific antibody titers after childhood vaccination (tetanus, diphtheria, Haemophilus influenzae type b, or Streptococcus pneumoniae vaccines) reflect the capacity of the immune system to synthesize antibodies and to develop memory B cells. If titers are low, immunization with a specific vaccine and titers obtained 4 to 6 weeks later confirm response to the immunization. Poor response to bacterial polysaccharide antigens is normal before 24 months of age, but is also associated with IgG subclass deficiency or specific antibody deficiency with normal immunoglobulins in older children. The development of protein-conjugate polysaccharide vaccines has prevented infections with these organisms in early childhood. This situation has made it difficult, however, to assess antibody responses to polysaccharide vaccines in children older than 2 years. Antibody responses to the S. pneumoniae serotypes found in the 23-valent polysaccharide vaccine, but not in the conjugate vaccine, can be used to test antibody responses to polysaccharide antigens. Low or absent specific antibody responses confirm the diagnosis of antibody deficiency even if B cells are present and serum immunoglobulin levels are in the normal range.
Lymphocyte phenotyping by flow cytometry enumerates the percentage and absolute numbers of T-cell, B-cell, and NK-cell subsets and the presence of surface proteins that are necessary for normal immunity, such as major histocompatibility complex molecules or adhesion molecules. Absent T cells in the presence or absence of B cells indicates SCID, whereas isolated deficiency of B cells characterizes agammaglobulinemia.
Delayed-type hypersensitivity skin tests to antigens such as tetanus, diphtheria, Candida, or mumps demonstrate the presence of antigen-specific memory T cells and functioning antigen-presenting cells. T cells recognize peptide fragments derived from antigens and presented by major histocompatibility complex molecules on the surface of antigen-presenting cells. If delayed-type hypersensitivity skin test results are negative, patients should receive a booster vaccination and be retested 4 weeks later. If the results remain negative, in vitro T-cell proliferation assays should be performed to confirm or exclude the lack of T-cell responsiveness.
T-cell proliferation assays to mitogens (phytohemagglutinin, concanavalin A, or pokeweed mitogen) or antigens (tetanus toxoid or Candida) are in vitro assays that confirm the capacity of T cells to proliferate in response to a nonspecific stimulus (mitogens) or the presence of antigen-specific memory T cells (antigens). Such presence requires prior vaccination (tetanus) or exposure (Candida) to the antigen.
Tests for cytokine synthesis or expression of activation markers by T cells may be performed in specialized research laboratories and can help identify defects in T-cell function when, despite present T cells, the clinical history suggests a T-cell disorder.
Complement assays include the total hemolytic activity of serum (CH50), which identifies presence of normal levels of the major components of complement. Activation of the alternative pathway of complement is performed by the AP50 test. If the CH50 or AP50 level is abnormal, tests for individual complement components must be analyzed in specialized laboratories. Tests for C1-inhibitor antigen and function are used to diagnose hereditary or acquired angioneurotic edema. False-negative results for the commercially available test of C1-inhibitor function used by most clinical laboratories require testing by specialized laboratories if the diagnosis of angioneurotic edema is suspected. Tests for neutrophil function include the nitroblue tetrazolium test for CGD, in which a soluble yellow dye turns into an insoluble blue dye inside activated neutrophils that have generated oxygen radicals to kill bacteria. Patients with CGD have no blue-staining neutrophils, whereas, on average, half of the neutrophils turn blue in carriers. A more sensitive test for CGD is a flow cytometry–based assay using dihydrorhodamine 123. In vitro tests for evaluation of neutrophil phagocytosis, chemotaxis, and bacterial killing and for the presence of myeloperoxidase activity are available in some laboratories, and tests for the expression of adhesion molecules such as CD18 (leukocyte function–associated antigen type 1, LFA-1) can be performed by flow cytometry.
Genetic testing to confirm the diagnosis of a primary immunodeficiency disease can be performed in specialized laboratories and may be helpful for deciding on a course of treatment, determining the natural history and prognosis of the disease, genetic counseling, and prenatal diagnosis. In patients in whom DiGeorge syndrome is suspected, fluorescent in situ hybridization studies for deletions of chromosome 22 can be helpful. In patients in whom ataxia-telangiectasia is suspected, chromosomal studies for breakage in chromosomes 7 and 14 are useful.
Diagnostic Imaging
The absence of a thymus on chest x-ray suggests DiGeorge syndrome. Abnormalities in the cerebellum are found in patients with ataxia-telangiectasia. Otherwise, the use of diagnostic imaging in the evaluation of immunodeficiency diseases is essentially limited to the diagnosis of infectious diseases.