Monocytes/macrophages and synovial fibroblast-like cells are in intimate contact in the synovium and are believed to play a critical role in the development of rheumatoid arthritis. We investigated the effects of monocyte-synoviocyte interactions in vitro on cytokine release and the expression of adhesion molecules. Using a sensitive Western blot assay, we found that VCAM-1 and ICAM-1 expression were up-regulated in synoviocytes following coculture. The interaction also resulted in the accumulation of TNF and IL-6, but not IFN-gamma in the culture medium. Culture supernatant from monocyte-synoviocyte samples effectively induced adhesion molecules in synoviocytes. Anti-TNF partially inhibited the increase in VCAM-1 and ICAM-1 expression, indicating that TNF in part mediates VCAM-1 and ICAM-1 expression. Interestingly, the induction of cytokines and adhesion molecules did not require cell contact between monocytes and synoviocytes, suggesting cell communication via soluble factors. T cell cytokines enhanced the induction of adhesion molecules induced by the monocyte-synoviocyte interaction. IFN-gamma and IL-4, which are produced by distinct T helper subsets, had differential effects on monocyte-synoviocyte interactions. IFN-gamma had a minimal effect on VCAM-1 expression by synovial fibroblasts, but synergized with monocytes to dramatically up-regulate ICAM-1 expression. IL-4 had no effect on ICAM-1 expression but enhanced monocyte-induced expression of VCAM-1. Our results demonstrate that the up-regulation of adhesion molecules following monocyte-synoviocyte interactions is mediated by soluble factors and can be regulated by specific T cell cytokines.