OBJECTIVE: Cartilage oligomeric matrix protein (COMP) is a major noncollagenous component of cartilage and is used as a biomarker in rheumatoid arthritis and experimental arthritis. Injection of COMP leads to severe inflammatory joint disease, and antibodies play a critical role in mediating arthritis. The arthritogenicity of COMP might be due to the lack of self tolerance. This study was undertaken to determine the status of COMP-specific B cell tolerance using COMP-deficient mice. METHODS: Arthritis development and antibody responses were compared between COMP-sufficient and COMP-deficient littermates after immunization with rat COMP. Serum anti-COMP antibody levels were measured using a panel of recombinant mouse COMP proteins, and antibody-secreting cells were enumerated by enzyme-linked immunospot assays. A novel sandwich enzyme-linked immunosorbent assay was developed to assess COMP molecules in serum. RESULTS: COMP-sufficient mice, but not COMP-deficient mice, developed severe arthritis following immunization with rat COMP. However, anti-COMP antibody titers to native COMP and recombinant protein domains covering the entire mouse COMP sequence, except the less immunodominant type 3 repeat domains, were decreased in COMP-sufficient mice compared to COMP-deficient mice. In addition, COMP-sufficient mice had fewer B cells secreting COMP-reactive antibodies. Detectable levels of full-length COMP in arthritic COMP-sufficient B10.Q NCF-1(*/*) and healthy mice suggested systemic availability of COMP to the immune system. CONCLUSION: The lack of arthritis, together with high levels of COMP-specific antibodies, in COMP-deficient mice indicates that susceptibility to arthritis is COMP specific and that endogenous expression of COMP in wild-type mice tolerizes B cells in vivo.