Common epitopes in Clq and collagen type II.

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