Anti-C1q autoantibodies specific against the globular domain of the C1qB-chain from patient with lupus nephritis inhibit C1q binding to IgG and CRP

Lupus nephritis is one of the most severe manifestations of systemic lupus erythematosus. Higher titers of serum anti-C1q autoantibodies correlate with disease activity in patients with lupus nephritis. Anti-C1q autoantibodies have been shown to bind neo-epitopes within the collagen region of human C1q. In a preliminary study, we recently reported that the anti-C1q autoantibodies could also recognize epitopes within the globular domain (gC1q) of the C1q molecule. Here, 38 sera from patients with renal biopsy-proven lupus nephritis were screened for the presence of anti-gC1q autoantibodies, using recombinant globular head regions of individual A (ghA), B (ghB) and C (ghC) chains of human C1q. We isolated anti-gC1q autoantibodies from three selected patients. Human C1q was pre-incubated with increasing concentrations of the isolated anti-ghA, anti-ghB or anti-ghC autoantibodies and its binding to different C1q target molecules such as IgG and CRP was then evaluated. Anti-ghB, but not anti-ghA and anti-ghC autoantibodies, markedly inhibited C1q interaction with IgG as well as CRP. These results appear to suggest that the anti-ghB autoantibodies may partially induce acquired functional C1q deficiency and thus may interfere with the biological function of C1q.