AntiDNA Antibodies

Antibodies to DNA were first described in 1957 [6-9]. They constitute a subgroup of ANAs that bind single-stranded and/or double-stranded DNA. They may be IgM antibodies or any IgG antibody subclass. In general, tests for IgG complement-fixing antibodies to DNA, especially those that bind double-stranded DNA, have the greatest diagnostic value in patients in whom SLE is suspected. Therefore, anti-dsDNA antibodies were included in the American College of Rheumatology classification criteria [22, 23]. In addition to serving as a laboratory marker for SLE, these antibodies may contribute to the development of associated diseases, such as nephritis. The underlying stimulus for anti-DNA antibody production in SLE patients remains unknown [24].

Antibodies that bind exclusively to single-stranded DNA can bind its component bases, nucleosides, nucleotides, oligonucleotides, or ribose-phosphate backbone, all of which are exposed in single-stranded DNA. In contrast, anti-double-stranded DNA antibodies bind to the ribose-phosphate backbone, base pairs, or specific conformations of the double helix. Double-stranded DNA exists primarily in a right-handed helical form called B DNA. There is also a left-handed helical form called Z DNA. Most widely available tests for measuring anti-DNA antibodies are based on reactivity with B DNA. Most anti-dsDNA antibodies bind both double-stranded and single-stranded DNA [24].

There is a high potency of anti-DNA antibodies to cross-react with non-DNA antigens such as laminin, heparan sulfate, type IV collagen, and a-actinin, which are located in the kidney. The cross-reactivity with renal antigens may contribute to the pathogenesis of lupus nephritis in which anti-DNA antibodies clearly play a central role [24, 25]. Recently, it was demonstrated that the penta-peptide Asp/Glu-Trp-Asp/Glu-Tyr-Ser/Gly is a molecular mimic of dsDNA. The sequence that is also present in the extracellular ligand-binding domain of mur-ine and human N-methyl-D-aspartate (NMDA) receptor subunits NR2a and NR2b is recognized by a subset of both murine and human anti-DNA antibodies. These antibodies can signal neuronal death and can be detected in the cerebrospinal fluid of SLE patients [26]. Very recently, an association between neuropsychiatric disturbances in SLE and antibodies against a decapeptide containing this sequence motif present in the extracellular NMDA receptor was shown [27].

Enzyme-linked immunosorbent assays (ELISA) and indirect immunofluorescence using the substrate Crithidia luciliae are currently the most widely used techniques for the detection of anti-DNA antibodies. Radioimmunoassays such as the Farr assay are still available, but their use has decreased sharply. There are important differences between these techniques. The Farr assay measures the precipitation of radiolabeled dsDNA using anti-dsDNA antibodies under stringent conditions (high saline concentrations) to ensure that only high-affinity antibodies are detected. However, this assay may also detect other proteins capable of precipitating dsDNA; furthermore, it may occasionally be contaminated by ssDNA in the test preparation, and the test does not distinguish between isotypes. The Crithidia test detects binding of anti-dsDNA to the kineto-plast of the organism, which contains circular dsDNA unrelated to histone proteins. This test can be used to measure IgG, IgM, or all isotypes of anti-dsDNA [28]. In ELISA, the wells of a test plate are coated with dsDNA, the test serum is added as a source of anti-dsDNA, and the target anti-DNA antibody is detected by a second antibody. Although the ELISA can be used to detect various antibody isotypes, IgG anti-dsDNA detection normally suffices for clinical purposes. The ELISA detects both low- and high-affinity antibodies, which could make it less specific than other assays [29]. Therefore, when interpreting the results of anti-DNA antibody tests, the clinician should consider the technique used, the type of laboratory in which the test was performed, and the labora-tor/s ranges for that test.

After a review of all relevant available literature, guidelines for anti-DNA antibody testing were recently published with the conclusions that a positive anti-dsDNA test offers strong evidence for the diagnosis of SLE but that a negative test does not exclude the diagnosis. Anti-DNA testing should be reserved for patients who tested ANA positive. Anti-DNA antibodies correlate with overall disease activity in SLE. However, as the correlations are modest at best, test results must be interpreted in the overall clinical context. Similarly, anti-DNA antibodies correlate with the activity of renal disease in SLE, but to a limited extent. Higher titers of anti-DNA antibodies have a stronger correlation with disease ac tivity. Concerning longitudinal assessment, a positive anti-DNA test does not predict a flare-up of disease. Increasing titers of anti-DNA may precede or accompany flare-ups of disease activity. However, the number of high-quality studies addressing this issue is limited, and a number of important questions concerning the optimal use of anti-DNA testing longitudinally remain to be answered [30].

0 0

Post a comment