O ur previous work showed that the proteoglycan aggrecan can induce erosive polyarthritis and spondylitis in BALB/c mice, and that the G1 domain of the proteoglycan aggrecan (G1) is the arthritogenic region. In this study, two T cell epitopes residing on G1 within residues 70-84 (peptide G5) and 150-169 (peptide G9) were identified using synthetic peptides and aggrecan-specific T cell lines. Two G1-specific T cell hybridomas exclusively responded to peptide G5. When the G5-specific T cell line was injected intraperitoneally into BALB/c mice, it induced acute inflammatory arthritis in joints, but only in those that had been injected with the epitope recognized by these T cells. Furthermore, we also demonstrate that the keratan sulfate chain(s) (KS) on G1 possess immunosuppressive properties with respect to T and B cell epitope recognition. T cell lines that recognize both G1 and peptide G5 show an increased response to G1 after KS is removed. Antibodies in hyperimmune sera of mice immunized with G1 show increased epitope recognition (quantitative and qualitative) after KS removal before immunization. These studies reveal that a T cell line specific to an epitope on the G1 domain of aggrecan, also recognizing a corresponding mouse G1 epitope, can induce arthritis by adoptive transfer and homing to the intraarticular epitope, thereby implicating T cells in arthritis development caused by immunity to the G1 domain of aggrecan. Moreover, the presence of KS on G1 can inhibit arthritis development by suppressing T and B cell epitope recognition.