Background: The components of the classical complement pathway play an important role in the pathogenesis of systemic lupus erythematosus (SLE) and are reportedly useful biomarkers of disease activity. In this study, we evaluate disease activity, complement function (total hemolytic complement, CH50) and complement protein levels (C3, C4, C3d, C4d, SC5b-9), comparing the results of patients with active disease versus those with inactive disease.
Methods: This cross-sectional study included 78 hospitalized women with SLE, 24 of whom were in the active group, with SLE disease activity indexes (SLEDAI.2K) of >6, and 54 in the inactive group, with SLEDAI.2K of ≤6. Serum CH50 was measured using a red blood cell hemolytic assay. C3 and C4 levels were determined by nephlometry and plasma levels of C3d, C4d, SC5b-9 by ELISA. The data were statistically analyzed using SPSS.
Results: The mean (±standard error) C4d levels of the inactive group were significantly higher than those of the active group (23.39±1.1µg/ml and 16.9±1.6µg/ml, respectively; p=0.003). There was also a significant correlation between C3 and C4 levels (p=0.807). The mean values of the other proteins (C3, C4, CH50, SC5b-9, and C3d circulating immune complex concentrations) were not significantly different between the inactive group vs. the active group: 89.35±6.8 vs. 85.54±7.6mg/dl, 18.33±2.3 vs. 20.45±2.4mg/dl, 149.03±4.3 vs. 157±4.3U, 1414.4±114.94 vs. 1471.1±216.9ng/ml, 9.43±0.96 vs. 13.31±3.16µgEq/ml, respectively (p>0.05).
Conclusions: According to our results, C4d levels may be used as a biomarker of disease activity. The significant correlation between C3 and C4 may confirm the activity of the classical pathway in SLE patients.