AUTOIMMUNITY DISORDERS

Autoimmunity Autoimmune diseases cannot be explained by a solitary cause or mechanism. Small amounts of autoantibodies are normally produced and may have physiologic roles in cellular interactions. Positive serologic findings may be found years before the development of pathogenic autoimmunity or clinical illness, and in some cases, they represent normal immunity or "benign autoimmunity" without disease. The major theories regarding the development of autoimmune disease are (1) release of normally sequestered antigens; (2) escape from anergy or defective apoptosis (programmed cell death) leading to abnormal autoreactive cellular clones; (3) shared antigens between the host and microorganisms, ie, "molecular mimicry"; and (4) defects in helper or suppressor T cell function. A genetic susceptibility is also a likely determinant of autoimmune disease. In nearly all autoimmune diseases, multiple mechanisms of autoimmunity are operative. Cell-Mediated Autoimmunity Certain autoimmune diseases are mediated by T cells that have become specifically immunized to autologous tissues. Cytotoxic or killer T cells generated by this aberrant immune response injure specific organs in the absence of serum autoantibodies. Diminished suppressor T cell activity or loss of clonal anergy results in disordered regulation of immune function and consequent autoreactivity. The immune damage in systemic (non-organ-specific) diseases such as systemic lupus erythematosus may be due to such a mechanism. Antibody-Mediated Autoimmunity Several autoimmune diseases have been shown to be caused by autoantibodies in the absence of cell-mediated autoimmunity. The autoimmune hemolytic anemias, idiopathic thrombocytopenia, and Goodpasture's syndrome appear to be mediated solely by autoantibodies directed against autologous cell membrane constituents. In these diseases, antibody attaches to cell membranes and fixes complement; the ensuing inflammatory reaction injures the cells. Anti-receptor antibodies that compete with or mimic physiologic agonists for cellular receptors cause several diseases. In Graves' disease, antibodies are present that bind to thyroid cells' thyroid-stimulating hormone and thereby stimulate thyroid hormone production. In rare instances of type 1 diabetes mellitus, anti-insulin receptor antibodies cause insulin resistance in peripheral target tissues. Antibodies to acetylcholine receptors of the myoneural junction in myasthenia gravis block neuromuscular transmission and produce muscle weakness. Immune Complex Disease In this group of diseases (systemic lupus erythematosus, lupus nephritis, rheumatoid arthritis, some drug-induced hemolytic anemias, and thrombocytopenias), autologous tissues are injured as "innocent bystanders." Autoantibodies are not directed against cellular components of the target organ but rather against autologous or heterologous antigens in the serum. The resultant antigen-antibody complexes bind nonspecifically to autologous membranes (eg, glomerular basement membrane) and fix complement. Fixation and subsequent activation of complement components produce a local inflammatory response resulting in tissue injury. Autoimmune Diseases The diagnosis and treatment of specific autoimmune diseases are described elsewhere in this book. Autoantibodies associated with certain autoimmune diseases may not be pathogenetic but are thought to be markers or by-products of the injury (eg, autoimmune thyroiditis and antithyroglobulin antibody). See Table 19–2 for autoantibody patterns in connective tissue diseases. (See also Musculoskeletal Disorders.) Table 19–2. Autoantibodies: Associations with connective tissue diseases. Suspected Disease State Test Primary Disease Association (Sensitivity, Specificity) Other Disease Associations Comments CREST syndrome Anticentromere antibody CREST (70–90%, high) Scleroderma (10–15%), Raynaud's disease (10–30%). Predictive value of a positive test is > 95% for scleroderma or related disease (CREST, Raynaud's). Diagnosis of CREST is made clinically. Systemic lupus erythematosus (SLE) Antinuclear antibody (ANA) SLE (> 95%, low) Rheumatoid arthritis (30–50%), discoid lupus, scleroderma (60%), drug-induced lupus (100%), Sjögren's syndrome (80%), miscellaneous inflammatory disorders. Often used as a screening test; a negative test virtually excludes SLE; a positive test, while nonspecific, increases posttest probability of SLE. Titer does not correlate with disease activity. Anti-double-stranded-DNA (anti-ds-DNA) SLE (60–70%, high) Lupus nephritis, rarely rheumatoid arthritis, other connective tissue disease, usually in low titer. Predictive value of a positive test is > 90% for SLE if present in high titer; a decreasing titer may correlate with worsening renal disease. Titer generally correlates with disease activity. Anti-Smith antibody (anti-Sm) SLE (30–40%, high)   SLE-specific. A positive test substantially increases posttest probability of SLE. Test rarely indicated. Mixed connective tissue disease (MCTD) Anti-ribonucleoprotein antibody (RNP) Scleroderma (20–30%, low), MCTD (95–100%, low) SLE (30%), Sjögren's syndrome, rheumatoid arthritis (10%), discoid lupus (20–30%). A negative test essentially excludes MCTD; a positive test in high titer, while nonspecific, increases post-test probability of MCTD. Rheumatoid arthritis Rheumatoid factor (RF) Rheumatoid arthritis (50–90%) Other rheumatic diseases, chronic infections, some malignancies, some healthy individuals, elderly patients. Titer does not correlate with disease activity. Scleroderma Anti-Scl-70 antibody Scleroderma (15–20%, low)   Predictive value of a positive test is > 95% for scleroderma. Sjögren's syndrome Anti-SS-A/Ro antibody Sjögren's (60–70%, low) SLE (30–40%), rheumatoid arthritis (10%), subacute cutaneous lupus, vasculitis. Useful in counseling women of child-bearing age with known connective tissue disease, since a positive test is associated with a small but real risk of neonatal SLE and congenital heart block. Wegener's granulomatosis Antineutrophil cytoplasmic antibody (ANCA) Wegener's granulomatosis (systemic necrotizing vasculitis) (56–96%, high) Crescentic glomerulonephritis or other systemic vasculitis (eg, polyarteritis nodosa). Ability of this assay to reflect disease activity remains unclear. CREST, calcinosis, Raynaud's phenomenon, esophageal dysmotility, sclerodactyly, and telangiectasia. Modified, with permission, from Harvey AM et al (editors). The Principles and Practice of Medicine, 22nd ed. Appleton & Lange, 1988; White RH et al. Clinical significance and interpretation of antinuclear antibodies. West J Med. 1987 Aug;147(2):210–3; and Tan EM. Autoantibodies to nuclear antigens (ANA): their immunobiology and medicine. Adv Immunol. 1982;33:167–240. Tests for Autoantibodies Associated with Autoimmune Disease Agglutination Assays Red cells are incubated with purified specific antigen (eg, thyroglobulin), which is adsorbed to the cell surface. The antigen-coated cells are suspended in the patient's serum, and antibody is detected by red cell agglutination. Antigen-coated latex particles are substituted for red cells in latex fixation tests. Enzyme-Linked Immunosorbent Assay Antibodies to various tissue antigens can be readily detected by these tests. Extracted and purified antigens are fixed to a plastic microtiter well or beads. The patient's serum is added, and excess proteins are removed by washing and centrifugation. Adherent immunoglobulin is then detected when a second antibody coupled to an enzyme (eg, alkaline phosphatase) is added. Finally, the enzyme's substrate is added; color forms and is measured in a spectrophotometer. This test can also be adapted for antigen detection by placing the antibody on the plastic surface. ELISA is very sensitive and less cumbersome than radioimmunoassay techniques. Immunofluorescence Microscopy This technique is most frequently used for detection of antinuclear antibody (ANA). Frozen sections of mouse liver or other substrates are cut and placed on glass slides or, alternatively, monolayers of cultured cell lines may be used. A patient's serum is placed over the sections and incubated. Fluorescein-conjugated rabbit anti-human immunoglobulin is then applied and washed. ANA specifically binds to the nucleus, and the fluorescein conjugate binds to the human antibody. Fluorescence of the cell nucleus on microscopy indicates a positive test. Complement Fixation Specific antigen, unknown serum, and complement are combined. Sheep red blood cells coated with anti-sheep cell antibody are added for 30 minutes at 37 °C. If antigen-specific antibody is present in the patient's serum, complement is bound and consumed, preventing lysis of sheep red cells. Arbuckle MR et al. Development of autoantibodies before the clinical onset of systemic lupus erythematosus. N Engl J Med. 2003 Oct 16;349(16):1526–33. [PMID: 14561795] Colglazier CL et al. Laboratory testing in rheumatic diseases: a practical review. South Med J. 2005 Feb;98(2):185–91. [PMID: 15759949]

Immunogenetics & Transplantation Associations Between HLA Antigens & Specific Diseases In humans, very striking associations are observed between particular HLA antigens and specific diseases. Some of these are listed in Table 19–3. In some diseases, the HLA molecule may be implicated in the pathogenesis. In others, the specific HLA allele may be linked to a gene determining immune responsiveness to a particular antigen. Table 19–3. Association between the presence of various HLA markers and selected autoimmune diseases. Disease Associated HLA Marker1   Relative Risk of Disease2   Ankylosing spondylitis B27 87.4 Reactive arthropathy, including Reiter's syndrome B27 37.0 Rheumatoid arthritis DR4 4.2 Behçet's syndrome B51 3.8 Systemic lupus erythematosus DR3 5.8 Insulin-dependent (type 1) diabetes mellitus DR3 3.3 DQB1*0201 2.4 DR4 6.4 DQB1*0302 9.5 DR2 0.19 DRB*15013     DRB*01013     DQB1*06023   0.15 Idiopathic Addison's disease DR3 6.3 Graves' disease DR3 3.7 Hashimoto's disease DR11 3.2 Postpartum thyroiditis DR4 5.3 Celiac disease DR3 10.8 DQB1*02013     DQA1*05013     DR7,11 6.0–10.00 DR7, DQB1*02013     DR11, DQA1*05013     Dermatitis herpetiformis DR3 15.9 Sicca syndrome DR3 9.7 Myasthenia gravis DR3 2.5 B8 3.4 Idiopathic membranous glomerulonephritis DR3 12.0 Goodpasture's syndrome DR2 15.9 Multiple sclerosis DR2 4.1 DRB1*15013     DRB5*01013     DQB1*06023     Pemphigus vulgaris (among Ashkenazi Jews) DR4 14.4 Psoriasis vulgaris CW6  13.3 Birdshot retinochoroidopathy A29 109.0 1Symbols with asterisks indicate alleles, and symbols without asterisks indicate serologically defined antigens. For each disease, the marker or markers with the strongest associations are given. In many cases in which it is difficult to decide whether HLA-DR or -DQ markers are responsible for association, both markers are given. 2The relative risk indicates the frequency of a disease in persons with the HLA marker as compared with persons without the marker. A positive association (ie, when the HLA marker is more frequent in persons with the disease than in those without it) is indicated by a relative risk of more than 1.0, a negative association by a relative risk of less than 1.0, and no association by a relative risk of 1.0. 3The risk has not been assessed separately for this allele. Reproduced, with permission, from Klein J et al: The HLA system. First of two parts. N Engl J Med 2000;343:782. The standard method for detecting HLA-A, -B, and -C antigens is that of lymphocyte microcytotoxicity. Lymphocytes isolated from peripheral blood or lymph nodes are added to each well of a typing tray filled with sera containing the appropriate cytotoxic alloantibody. When complement is added, cells to which antibody has been specifically bound will have complement activated at the cell surface, resulting in cell death or lysis. It is thus possible to type for all of the known HLA-A, -B, and -C specificities. An appreciable majority of typing serum samples are obtained from multiparous women since they form antibodies to fetal alloantigens. Typing for the class II antigens HLA-DR and -DQ by serologic methods is technically more difficult. Antigens of the HLA-D, -DR, -DQ, and -DP series may also be detected by in vitro mixed lymphocyte culture. Lymphocytes of one individual (responder cells) will undergo proliferation upon encountering lymphocytes from another individual possessing foreign HLA-DR and -DQ antigens (stimulator cells). Lymphocyte proliferation can be readily measured by DNA incorporation of tritiated thymidine. Responders possessing matching -DR and -DQ antigens will remain nonreactive. Increasingly, HLA class II typing is being performed by molecular technology. The DNA sequences for the HLA genes and their flanking sequences are known. Selected primers that amplify the gene of interest using the polymerase chain reaction technique are known as sequence-specific primers. HLA typing by PCR provides better resolution than serologic identification because typing is done at the genetic level. Reveille JD et al. Spondyloarthritis: update on pathogenesis and management. Am J Med. 2005 Jun;118(6):592–603. [PMID: 15922688] Turesson C et al. Genetics of rheumatoid arthritis. Mayo Clin Proc. 2006 Jan;81(1):94–101. [PMID: 16438485]