Anti-dsDNA antibody subtypes and anti-C1q antibodies: toward a more reliable diagnosis and monitoring of systemic lupus erythematosus and lupus nephritis

The putative distinct diagnostic and pathogenic potential of aDNA-Ab subtypes, differing in their affinity or epitope specificity, was subject of several studies with controversial results. Comparing five assays, characterized by different reaction conditions and nature/source of dsDNA, we investigated the abovementioned problem in a retrospective study on 100 systemic lupus erythematosus (SLE) patients and 100 controls (other CTD, autoimmune hepatopathies). As demonstrated, only assay 3 (Farrzyme, TBS, UK) and 5 (Farr-RIA, Trinity Biotech, Ireland) are really suitable to detect primarily high avidity aDNA-Ab. Both were significantly linked to lupus nephritis (specificity 84%) and highly specific for SLE (95 and 96%). Thereby, assay 3 was found to be the first solid phase ELISA probably suitable to replace the Farr-RIA. Classical ELISAs (assay 1, Orgentec, Germany, and 2, Bindazyme, TBS, UK), detecting aDNA-Ab more or less independent from their avidity, or tests with only intermediate specificity for high avidity Ab (assay 4, ELIAdn, Sweden Diagnostics, Germany), were less specific for SLE (83, 79, 91%, respectively) and not associated with renal involvement (specificity 54-57%). At least in the patients studied here, obvious antigen-related differences could not be observed. With slight differences, all assays were suitable to monitor disease activity and therapy in SLE, agreeing with the ECLAM score in about 70-80% of cases. For lupus nephritis, aC1q-Ab are as specific as high avidity aDNA-Ab and capable to close a diagnostic gap in some cases. Thus, to enhance the specificity ( up to 98%) and to consider the distinct diagnostic/pathogenic potential of aDNA-Ab subtypes in SLE, under routine clinical laboratory conditions it should be recommended to combine a sensitive screening test with a more specific second assay.