Rheumatoid arthritis is characterized by synovial joint infiltration of activated CD4+ T cells and MHC class II+ APC, and is linked to specific HLA-DR alleles. Candidate autoantigens in synovial fluid and cartilage include type II collagen (CII) and cartilage gp39 (HCgp39). Using preparations of native Ag and T cells derived from Ag-immunized DR4-transgenic mice, we determined that human ex vivo differentiated DR4+ dendritic cells (DC) and macrophages (Mφ) can mediate MHC class II presentation of CII or HCgp39 epitopes. The form of the Ag (soluble, partially degraded, or particulate) delivered to the APC influenced its presentation by DC and Mφ. DC efficiently presented partially degraded, but not native CII α-chains, while Mφ presentation was most efficient after phagocytosis of bead-conjugated CII. Both DC and Mφ presented soluble HCgp39, and activated Mφ from some donors presented epitopes derived from endogenously synthesized HCgp39. When synovial fluid from rheumatoid arthritis patients was used as a source of Ag, DC presentation of HCgp39 and CII epitopes was efficient, indicating that synovial fluid contains soluble forms of CII and HCgp39 amenable to internalization, processing, and presentation. These data support the hypothesis that CII and HCgp39 are autoantigens and that their class II-mediated presentation by DC and Mφ to T cells in vivo has a critical role in the pathogenesis of human rheumatoid arthritis.