Background/Purpose: Antibody titers to double stranded DNA (anti-dsDNA) and complement C3 and C4 proteins have clinical utility in the routine monitoring of systemic lupus erythematosus (SLE). We evaluated the performance characteristics of antibody titers to C1q (anti-C1q), and C4d bound to erythrocytes (EC4d) as additional biomarker candidates in the monitoring of SLE disease activity.

Methods: SLE patients (total 124 patients, mean age 42 years, 97% females) were enrolled from three different cohorts. The first cohort enrolled 37 subjects, all selected for active disease in the presence of complement activation. The second and third cohorts enrolled 64 and 24 SLE subjects, respectively. Disease activity was assessed longitudinally using the Safety of Estrogens in Lupus Erythematosus National Assessment (SELENA) SLE disease activity index (SLEDAI) without anti-dsDNA and complement components; this modification is referred to as the clinical SELENA-SLEDAI. Also, the Physician’s Global Assessment of disease activity (PGA: 0-3-point scale) was collected. Antibody titers, serum C3 and C4 levels were determined using standard immunoassays (low C3/C4 was defined as either C3 or C4 below normal range). EC4d levels were determined using quantitative flow cytometry and expressed as net mean fluorescence intensity (MFI). The relationships between fluctuations in SLE disease activity and biomarkers were analyzed using Spearman rank test and linear mixed effect models with the clinical SELENA-SLEDAI and PGA as the dependent variables and biomarkers as independent predictor of disease activity fluctuations (autoantibody titers and EC4d levels were log normalized for the analysis). Marginal R² were calculated to evaluate the proportion of variance explained by independent variables.

Results: Overall, a total of 624 study visits were collected in the 124 patients. At baseline, both clinical SELENA-SLEDAI (average 6.0 points) and PGA scores (average 1.0 point) correlated with EC4d levels C3/C4 status, anti-dsDNA and anti-C1q titers (p<0.01). Linear mixed effect models revealed that changes in EC4d, C3/C4 anti-dsDNA and anti-C1q titers were all significantly associated with fluctuations in disease activity (p<0.01)(Table I). Of the 124 subjects, 97 of them presented with either chronically low C3/C4 (n=40) or normal C3/C4 status at all visits (n=57). In this subset, changes in EC4d levels were associated with fluctuations in clinical SELENA-SLEDAI (estimate 1.2±0.3, marginal R²=8%) and PGA (estimate 0.2±0.1, marginal R²=9%).

Conclusion: These data indicate that anti-dsDNA, antiC1q, low complement and EC4d are associated with disease fluctuations in SLE. EC4d correlates with disease activity among subjects with chronically low or normal complement.