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MD Consult: Books: Goldman: Cecil Medicine: LABORATORY EVALUATION OF SYSTEMIC INFLAMMATORY DISEASE

Goldman: Cecil Medicine, 23rd ed.

Copyright © 2007 Saunders, An Imprint of Elsevier

LABORATORY EVALUATION OF SYSTEMIC INFLAMMATORY DISEASE

 

Among rheumatic diseases, some are characterized by severe systemic inflammation that can cause organ-threatening and life-threatening manifestations. These diseases can have arthritis as a component and presenting complaint, although the prominence of extra-articular manifestations, especially as they develop over time and involve organs such as the kidney, points to their systemic nature. These diseases can be categorized on the basis of clinical, serologic, and pathologic findings, with the presence of vasculitis, irrespective of blood vessel size, providing a unifying feature in disease classification.

The terms connective tissue disease (CTD) and collagen vascular disease are both used to denote a group of diseases that includes RA, SLE, Sjögren’s syndrome, polymyositis, dermatomyositis, and progressive systemic sclerosis. Diseases in this group can share common or overlapping clinical features, especially early in their course, when their presentations may be similar. In this stage of disease, the condition may be called undifferentiated CTD, with serologic markers sometimes predictive of the eventual diagnosis.

Antinuclear Antibodies

 

The expression of antibodies to components of the cell nucleus (antinuclear antibodies, or ANAs) is characteristic of CTD and is essentially invariable in patients with SLE. These antibodies target a host of nuclear macromolecules, including DNA, RNA, and proteins as well as complexes of proteins with nucleic acid. These antigens are ubiquitously expressed in cells and subserve critical processes related to chromosomal structure, cell division, transcription, and translation. The basis for the antigenicity of these molecules is unknown, although DNA and RNA both have intrinsic immunologic activities and can stimulate cytokine production and inflammation when outside the cell.

ANAs are measured primarily by immunofluorescence assays in which sera are incubated with tissue culture cells (e.g., Hep2 cells) fixed to a glass slide. Antibody binding is revealed by fluorescence microscopy after incubation of the slide with a fluoresceinated anti-immunoglobulin reagent. Results are reported in terms of the pattern of fluorescence as well as the end-point titer of sera at which fluorescence can be observed. The patterns of binding differ depending on the location of the particular macromolecular target, although a few patterns predominate. These patterns include homogeneous, rim, nucleolar, and speckled; in addition, ANA tests can detect antibodies to cytoplasmic antigens. Despite some disease associations, these patterns do not have diagnostic significance. Table 278-1 presents a list of major ANAs with their pattern and disease associations.
TABLE 278-1   — SELECTED ANTINUCLEAR ANTIBODIES AND RHEUMATIC DISEASES

Pattern Antibody Antigen Disease Association
Homogeneous Anti-histone Histones H1, H2A, H2B, H3, H4 Drug-induced lupus (>95%)
Rim Anti-double-stranded DNA Double-stranded DNA snRNP proteins SLE (50%)
Speckled Anti-Sm snRNP proteins SLE (30%)
Anti-U1-RNP U1 snRNP proteins SLE (30%); MCTD (>95%)
Anti-Ro (SS-A) Two proteins complexed to small RNAs Y1–Y5 SLE (30%); Sjögren’s syndrome (70–80%)
Anti-La (SS-B) Single protein plus RNA polymerase III transcript SLE (15%); Sjögren’s syndrome (50–70%)
Anti-Ku DNA binding protein SLE (10%)
Anti-SCL-70 DNA topoisomerase I PSS (40–70%); CREST (10–20%)
Nucleolar Anti-PM-Scl Nucleolar protein complex PSS (3%); PM (8%)
Anti-Mi-2 Nuclear protein complex DM (15–20%)
Anti-RNA polymerase Subunits of RNA polymerase I PSS (4%)
Dividing cell Anticentromere Centromere/kinetocore protein CREST (80%); PSS (30%)
Anti-proliferating cell nuclear antigen Auxiliary protein of DNA polymerase δ SLE (3%)
Cytoplasmic Anti-Jo-1 Histidyl tRNA synthetase PM/DM (18–25%)
Anti-PL-7 Threonyl tRNA synthetase PM/DM (3%)
Anti-PL-12 Alanyl tRNA synthetase PM (4%)
Anti-SRP Signal recognition particle SLE (10%)
Anti-ribosomal P Large ribosomal subunit PM/DM (3%)
CREST = calcinosis, Raynaud’s phenomenon, esophageal dysmotility, sclerodactyly, and telangiectasia; DM = dermatomyositis; MCTD = mixed connective tissue disease; PM = polymyositis; PSS = progressive systemic sclerosis (diffuse scleroderma); SLE = systemic lupus erythematosus; snRNP = small nuclear ribonucleoprotein.

A major limitation in the assays of ANA concerns the frequency of positive reactivity in the sera of otherwise normal individuals who lack evidence of a CTD. The sera of as many as 5% of normal individuals express reactivity in the ANA test. The basis of this reactivity is not fully understood, although it may reflect a predisposition to autoimmunity that is manifest in ANA production in the absence of other immunopathologic disturbances for the complete development of a CTD. Because ANA testing is often performed to evaluate nonspecific complaints such as arthralgias, fatigue, and fever, a positive test must be interpreted with caution and not used as proof of a CTD in the absence of correlative clinical or laboratory findings.

Because of detailed biochemical studies, the molecular identity of many ANAs is now known, allowing for the development of specific immunochemical assays using a variety of technologies. These tests can be performed individually, although multiplex assays provide simultaneous assessment of multiple specificities. Among those assays, only a few are performed routinely because of their value for diagnosis and prognosis. For several CTDs, diagnosis can be readily determined from clinical findings or other laboratory tests. In these instances, the ANA determination provides confirmatory information as well as clues for the occurrence of certain clinical manifestations.

Antibodies to DNA

 

Antibodies to DNA (anti-DNA) are serologic markers of SLE and represent a criterion in the classification of patients with this disease. These antibodies bind sites on both single-stranded (ss) and double-stranded (ds) DNA, although anti-dsDNA are more specific for SLE and therefore routinely measured. Although these antibodies can bind free DNA, DNA in the cell occurs in association with histones to form a structure called the nucleosome, with DNA wrapped around a histone core. Anti-DNA may therefore be considered a subset of antibodies to nucleosomes, with nucleosomes probably being the driving antigen for this response.

In clinical practice, the measurement of anti-DNA is an important element in the evaluation of patients with a broad array of clinical complaints, given the heterogeneity and multisystemic nature of SLE. Anti-DNA determinations, in addition to their value in diagnosis, also convey prognostic information and an index of disease activity. The association with disease activity appears strongest with glomerulonephritis, most likely because of the role of DNA–anti-DNA immune complexes in immunopathogenesis. The association of anti-DNA with other disease manifestations is less certain, limiting the use of this marker as a measure of overall disease activity.

Several different immunochemical approaches can be used to detect anti-DNA antibodies, although solid-phase ELISA assays are convenient and sensitive and eliminate the need for radioactivity. The assays vary in regard to the spectrum of anti-DNA detected, and results between assays may not correlate. Nevertheless, for each assay, the dynamic range for testing is very large. With treatment and disease quiescence, anti-DNA antibodies may essentially disappear; with flare, levels may increase dramatically. This property distinguishes anti-DNA from other ANAs in SLE, levels of which tend to be more consistent over time.

As is the case for other ANAs, the appearance of anti-DNA in the serum may precede other manifestations of SLE, suggesting vigilance if these antibodies are present in patients who have symptoms that suggest a CTD but lack other evidence to establish a firm diagnosis.

Other Antinuclear Antibodies

 

Anti-Sm and anti-RNP are related specificities that commonly occur together in the sera of patients with SLE, a phenomenon called linkage. These antibodies bind proteins on subcellular particles called snRNPs (small nuclear ribonucleoproteins) that are composed of a set of proteins and uridine-rich RNAs. Anti-Sm and anti-RNP differ in protein specificity and in the ability to cause immunoprecipitation of the bound RNA molecules.

Anti-Sm antibodies occur only in patients with SLE and represent a serologic marker in disease classification. In contrast, anti-RNP can appear in the sera of patients with other clinical presentations and, in the absence of anti-Sm, may characterize patients with overlapping CTD features, so-called mixed CTD. In SLE, the frequencies of anti-Sm and anti-RNP vary among racial and ethnic groups, although a clear association with particular clinical manifestations has not been established.

Anti-Ro and anti-La, another set of linked ANAs, are directed to protein–RNA complexes that are involved in cellular metabolism of RNA. These antibodies are expressed more widely in patients with CTD and appear in the sera of patients with SLE, RA, and Sjögren’s syndrome, among others. Assessment of these antibodies is important because of their association with the neonatal lupus syndrome, which results from the transplacental passage of antibodies and causes congenital heart block as well as rash in the neonate.

Although ANAs are directed to ubiquitous antigens, they nevertheless are expressed in disease-specific patterns and may show association with particular organ-specific manifestations. These associations include anti-ribosomal P with central nervous systemic involvement in SLE, antibodies to DNA topoisomerase 1 (anti-SCL-70) with progressive systemic sclerosis (diffuse scleroderma), antibodies to centromeres with CREST syndrome, and antibodies to histidyl tRNA synthetase (anti-Jo-1) with interstitial lung disease in scleroderma. The basis for the associations is unclear, although they may reflect cross-reactivity to a tissue-specific antigen or unexpected expression of a nuclear antigen on a differentiated cell to promote tissue-specific injury.

Antibodies to Phospholipids

 

Originally defined by their effects on in vitro clotting tests, antibodies to phospholipids (APLs) are associated with in vivo thrombosis. Patients with these antibodies display a clinical condition, termed the antiphospholipid antibody syndrome, that is characterized by arterial or venous thrombosis, thrombocytopenia, and first-trimester spontaneous abortions. This syndrome may occur by itself or in the context of SLE, where it may contribute to the acceleration of atherosclerosis, premature stroke, and myocardial infarction.

The serology of APLs, also known as lupus anticoagulants, is complicated related to their antigenic targets as well as heterogeneity. Of phospholipids, cardiolipin is a common antigen for these assessments. Although they are designated as antibodies to cardiolipin or phospholipids, these antibodies may be directed to the protein β2-glycoprotein 1; in the format of an ELISA, this protein may bind to phospholipids to create antigenic sites. Importantly, patients with this syndrome may lack antibodies to phospholipids when measured in these assays, although levels of these antibodies can vary over time.

Adjunctive tests involve tests directed at inhibition of in vitro clotting (e.g., activated partial thromboplastin time, dilute Russell’s viper venom time), recognizing the discordancy between in vivo thrombosis and in vitro anticoagulation. The mechanism by which antibodies to phospholipids and other clotting factors induce thrombosis in vivo is unknown, although they may interact with the surface of cells (e.g., endothelium) and with soluble clotting factors to promote a prothrombotic state.

Complement

 

Assessment of the complement system provides valuable information in diseases in which immune complex deposition may promote inflammation and tissue injury (see Chapter 47 ). This system involves a large number of proteins which function in enzyme cascades to generate degradation products that amplify immunologic reactions and promote the destruction or removal of foreign organisms as well as damaged cells. In the setting of SLE and in certain forms of vasculitis and glomerulonephritis, immune complexes activate complement to promote local inflammation. This activation can be measured in terms of the total complement level in the blood by functional assays of hemolytic activity; by measurement of individual complement components such as C3 and C4, whose levels are reduced by cleavage during activation; by measurement of split products of cleaved complement components; and by measurement of complement fragments bound to red blood cells during complement activation.

Proteins of the complement system are acute phase reactants and can increase with inflammation, including active disease. Correspondingly, low levels may reflect inherited complement deficiency rather than consumption; genetic deficiency of C1q, for example, is highly associated with SLE.

Anti-neutrophil Cytoplasmic Antibodies

 

Anti-neutrophil cytoplasmic antibodies (ANCAs) are autoantibodies that react to determinants in the neutrophil and occur prominently in patients with certain forms of necrotizing vasculitis or rapidly progressive glomerulonephritis. Two main forms of ANCA have been distinguished on the basis of the target antigens and pattern of immunofluorescence staining of fixed neutrophils: PR3-ANCA (C-ANCA), which reacts with proteinase-3 (PR3), and MPO-ANCA (P-ANCA), which reacts with myeloperoxidase (MPO). By immunofluorescence, PR3-ANCA shows staining in the cytoplasm; staining by MPO-ANCA localizes in the perinuclear area. ANCAs to other proteins have also been identified, but these may also occur in conditions other than vasculitis.

In the evaluation of severe, multisystem inflammatory disease, ANCA testing is necessary to distinguish possible etiologies. PR3-ANCA occurs commonly in patients with Wegener’s granulomatosis, whereas MPO-ANCA marks the course of vasculitis caused by microscopic polyangiitis, polyarteritis nodosum, and Churg-Strauss disease, among others. In patients with ANCA-associated glomerulonephritis, the kidney lacks evidence of immune deposits, as indicated by the lack of staining for immunoglobulins or complement. Kidney disease of this kind is termed pauci-immune glomerulonephritis. Although ANCA testing is useful in initial diagnosis, its role for assessing disease activity is less certain. Occasionally, in patients who are desperately ill and cannot tolerate a lung or kidney biopsy, the presence of an ANCA can be used as preliminary evidence for diagnosis to allow the initiation of immunosuppressive therapy.

Cryoglobulins

 

Cryoglobulins are serum immunoglobulins that precipitate in the cold and promote the pathogenesis of systemic inflammatory disease through tissue deposition. The presence of a cryoglobulin is detected by allowing blood, collected warm, to remain cool at 2° to 4° C for one or more days. After centrifugation, the amount of cryoprecipitate is measured and expressed as a cryocrit. Subsequent analysis of the cryoprecipitate by immunochemical assays allows determination of its components. Cryoglobulins can be classified into three main types on the basis of their composition: (1) single or type I; (2) mixed, type II; and (3) mixed, type III. A type I cryoglobulin consists of a monoclonal immunoglobulin that precipitates in the cold. A mixed-type cryoglobulin contains RFs bound to polyclonal IgG to form an immune complex. In type II cryoglobulins, the IgM RF is monoclonal, and in type III the IgM RF is polyclonal.

Type I cryoglobulins occur in patients with lymphoproliferative disorders such as Waldenström’s macroglobulinemia, multiple myeloma, or chronic lymphocytic lymphoma. In contrast, patients with mixed cryoglobulins can present with a wide range of signs and symptoms resulting from vasculitis. These manifestations include purpura (a sign of leukocytoclastic vasculitis), weakness, arthritis, and neuropathy and comprise a syndrome known as essential mixed cryoglobulinemia. Most patients with this condition have infection with hepatitis C virus, with viral components present in the complexes. These patients have serologic evidence of this infection as well as manifestations attributable to the underlying liver disease. As in the case of other CTDs and systemic inflammatory diseases, the evaluation of patients with essential mixed cryoglobulinemia demands attention to the entire patient and the impact of disease on multiple organs.

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