A common feature of both osteoarthritis (OA) and rheumatoid arthritis (RA) is the degradation and loss of matrix components from joint articular cartilage (AC).These cartilage-derived antigens (CDA) can initiate and sustain a synovitis and a systemic autoimmune response.Such events are signalled by the appearance of T cell populations,which specifically recognise CDA presented by antigen-presenting cells within the synovium of OA and RAjoints.Although type Ⅱ collagen (Coll-Ⅱ) has been the most widely studied CDA,our earlier studies had shown that the non-collagenous domain,NC4,of the much less abundant Type Ⅸ collagen was more susceptible than Co(II)-Ⅱ to proteolytic cleavage during the early stages of cartilage degradation.Moreover,a mixture of fragments of NC4 and other polypeptides released from cartilage,stimulated hyaluronan (HA) and proteoglycan (PG) synthesis by OA synovial fibroblasts and chondrocytes in culture.The objective of the present study was to confirm and extend these earlier observations using authenticated recombinant NC4.