Synovitis in rheumatoid arthritis is characterized by infiltration of the synovium by T and B lymphocytes and monocytes, as well as by the proliferation of synovial lining cells, fibroblasts, and endothelial cells. To study synovial cell-T cell interactions in vitro, we established cultures of fibroblast-like synovial cells, and used these cells in a synovial cell-T cell binding assay. Using T cells at various stages of differentiation and activation, we found that human thymocytes and mitogen-activated peripheral blood T cells bound to fibroblast-like synovial cells, whereas fresh peripheral blood T cells did not. Moreover, activated T cells from inflammatory synovial tissue or from synovial fluid also bound to fibroblast-like synovial cells cultured in vivo. Antibodies against certain epitopes of the T cell CD2 (35.1) and synovial cell lymphocyte function-associated antigen-3 (LFA-3) (TS2/9) molecules inhibited synovial cell-thymocyte binding. However, these same anti-CD2 and anti-LFA-3 antibodies only partially inhibited synovial cell binding to activated normal peripheral blood T cells. Moreover, T cells from inflammatory synovium from rheumatoid arthritis and psoriatic arthritis patients also From the