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.
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