Interference of antimodified C-reactive protein autoantibodies from lupus nephritis in the biofunctions of modified C-reactive protein

Autoantibodies against modified C-reactive protein (mCRP) are frequently found in patients with lupus nephritis and are associated with disease activity, suggesting their pathogenic role in lupus nephritis. The aim of this study was to investigate the influence of anti-mCRP autoantibodies from patients with lupus nephritis on the biofunctions of mCRP. mCRP plays important roles in the clearance of apoptotic cells and immune complex–mediated injuries via binding to C1q and factor H and acting as an opsonin. In this study, we confirmed that mCRP could bind to C1q and factor H, and the binding between mCRP and C1q was mainly via the collagen-like region of C1q by enzyme-linked immunosorbent assay and surface plasmon resonance. mCRP could significantly enhance the phagocytosis of late apoptotic cells in the presence of normal human serum. Autoantibodies against mCRP, purified from immunoglobulin G fractions of 3 patients with lupus nephritis by affinity chromatography, could significantly inhibit the binding between mCRP and C1q or factor H and reduce the clearance of late apoptotic cells enhanced by mCRP. Our observations suggest that anti-mCRP autoantibodies from patients with lupus nephritis might be pathogenic in systemic lupus erythematosus and lupus nephritis through interfering with the biofunctions of mCRP.