Background Systemic lupus erythematosus (SLE) is a complex autoimmune disease characterized by diverse pathogenic autoantibodies and clinical phenotype. Laboratory monitoring of complement activation products (CAP; C3a, C5a, C3d, C4d) is commonly used in diagnosis and follow up. CAP bound to cellular surfaces (CB-CAP) analysed by flow cytometry offer a relatively novel approach to study complement turnover in SLE.
Objectives We evaluated the significance and usability of cell bound CAP in monitoring complement turnover in a whole blood staining approach, comparing cell bound CAPs on different leucocyte subpopulations with conventional complement and disease activity parameters.
Methods CAP bound to lymphocyte subpopulations were analysed by flow cytometry, compared to serological complement and other SLE activity parameters determined by nephelometry, rocket electrophoresis and ELISA.
Results We have analysed CB-CAPs bound to lymphocytes in 50 adult patients with SLE. B-cells and CD4+ T-cells of SLE patients showed significantly higher levels of cell-surface-bound C4d compared to controls. C4d bound to B-lymphocytes was inversely correlated with serum levels of C4 (r = -0.309, p=0.013) and C3 (r= - 0.356, p=0.004) reflecting complement decay. In addition, B-cell C4d showed a significant positive correlation with anti-dsDNA antibody levels (r =0.574, p<0.001) and plasma C3d (r =0.319, p=0.009) thereby corresponding to disease activity. CB-CAPs were stable over 24 hours after blood drawing thus offering more flexibility in sample handling than conventional complement turnover assays.
Conclusions Our data show that CB-CAP measurement correlates with established disease activity markers, and given its easier handling, is of possible utility in routine diagnostic and monitoring of patients with SLE. A multicentre study for evaluation of this method in a cohort of paediatric patients is planned.
Disclosure of Interest None declared