Matrix metalloproteinase 13 (MMP-13) processing of type II collagen is altered by antibodies and citrullination found in the early stages of rheumatoid arthritis.

A subset of rheumatoid arthritis (RA) is the production of autoantibodies, including antibodies to citrullinated proteins (ACPA) and antibodies to type II collagen (AC2A). Type II collagen (COL2) is the major protein in joint cartilage and is a target of arthritogenic autoantibodies. We could confirm that sera from RA patients react with both citrullinated and native triple-helical COL2 epitopes. We examined the modulation of COL2 processing by matrix metalloproteinase 13 (MMP-13), the main collagenase responsible for degradation of articular cartilage. Anti-COL2 antibodies (AC2A) targeting the C1 epitope (residues 359-363) partially inhibited intact COL2 and fragment hydrolysis, resulting in two distinct fragments in the 25-30 kDa range. The AC2A targeting the E10 epitope (residues 777-783, the region where MMP-13 initially cleaves COL2) partially inhibited intact COL2 and fragment hydrolysis, resulting in a distinct fragment of ∼30 kDa. The AC2As targeting the F4 epitope (residues 932-936) partially inhibited collagen fragment hydrolysis, resulting in four distinct fragments in the 20-30 kDa range. Sequencing of isolated fragments revealed 14 terminated cleavage sites. Citrullination of the COL2 cleavage site reduced MMP-13 efficiency, which should further exacerbate fragment production rather than complete digestion. The results indicated that, under normal maintenance, MMP-13 cleaves COL2 initially at the 775-776 bond, followed by further digestion of COL2 fragments. Citrullination slows the initial processing of COL2 by MMP-13. In concert, AC2As inhibit the action of MMP-13 at different stages, resulting in production of collagen fragments differing in composition encountered under normal circumstances. The abnormal COL2 fragments could activate the immune system to be more pathogenic or regulatory as well as modify chondrocyte functions, and thereby play a role in the initiation of RA.