Common epitopes in C1q and collagen type II

Abstract

An epitope common for collagen type II and C1q was demonstrated by specific binding of a monoclonal anti-collagen type II antibody, MAb B1, to purified C1q. This was further substantiated by the affinity shown between F(ab') 2 fragments of anti-C1q antibodies and rat chondrosarcoma collagen type II. The interaction between MAb B1 and C1q was demonstrated in hemolytic assays, in an enzyme-linked biotin-avidin assay and by the binding of C1q to MAb B1 immobilized on Sepharose 4B beads. MAb B1 recognized only purified C1q and not the macromolecular C1 complex, indicating that the epitope for MAb B1 was situated in the collagen-like region in C1q, where C1q and C1s are anchored. The binding of the purified collagen-like fragment of C1q to radiolabelled MAb B1 confirmed these findings. The affinity between MAb B1 and C1q was significantly increased if C1q was first reacted with heat aggregated IgG, indicating a demasking of the reactive epitope on binding to the aggregated IgG. The present findings raise the question of the pathogenetic significance of the presence of anti-collagen type II antibodies and free C1q, both of which are frequently seen in high amounts in rheumatoid arthritis.