Biomarkers and mediators in systemic lupus erythematosus IFNα versus the CRP response, and evaluation of suPAR and anti-dsDNA antibody assays

Abstract: Systemic lupus erythematosus (SLE) is a heterogeneous autoimmune disease which may affect multiple organ systems. Interferon alpha (IFNα) and autoantibodies that form immune complexes with nuclear antigens (ANA) are hallmarks believed to drive the disease into a vicious circle of inflammation, tissue damage, autoantigen exposure and autoantibody production.In SLE, the disease course is characterized by episodes of exacerbations alternating with remissions. In order to best treat the patient it is important to closely monitor symptoms and signs of disease activity. Because of the disease heterogeneity, no single biomarker has yet been found to reflect SLE disease activity in general, although antidouble stranded DNA (anti-dsDNA) antibodies sometimes indicate activity, primarily with renal involvement, and constitutes an item of the SLE disease activity score SLEDAI-2K. However, the method of anti-dsDNA measurement is not standardized and therefore varies between different laboratories. In many other inflammatory conditions, such as rheumatoid arthritis and during bacterial infections, the C-reactive protein (CRP) level is a good indicator of ongoing inflammation, but in SLE and during viral infections, CRP commonly fails to reflect the degree of inflammation. Both viral infections and SLE are characterized by IFNα, and we thus aimed to elucidate whether IFNα can inhibit CRP production. Further, four assays for anti-dsDNA antibody measurements were evaluated with regard to SLE disease specificity and activity, and a new potential biomarker of inflammation, the soluble urokinase plasminogen activator receptor (suPAR), was assessed in relation to disease activity and organ damage.An in vitro inhibitory effect of IFNα on CRP transcription and production was found in hepatocytes, and this was consolidated by in vivo studies of CRP and IFNα in sera from well-characterized SLE patients (KLURING; Kliniskt lupusregister i nordöstra Götaland). Here, CRP and disease activity were associated among patients without IFNα and without a CRP lowering gene variant (SNP rs1205). The poor disease activity compliance of CRP could therefore be explained, at least in part, by polymorphisms in the CRP gene and increased levels of IFNα. Critical differences between the methods measuring anti-dsDNA were found regarding disease specificity and ability to reflect disease activity and the results suggests the Crithidia luciliae immunofluorescence test (CLIFT) for diagnostic purposes and a bead-based multiplex assay (FIDIS) for monitoring of disease activity. Evaluation of suPAR in SLE revealed no association of suPAR with disease activity, but interestingly instead with accumulated organ damage. suPAR could therefore possibly be used to advert patients at high risk of organ damage.A detailed biological and clinical characterization of established and emerging SLE biomarkers is of importance since it may improve the clinical management as well as increase the knowledge about disease mechanisms.