Transitional cell carcinomas (TCC) of the urinary bladder are known to express proteins which can yield potentially immunogenic peptide epitopes for expression in the context of cell surface class I or class II MHC antigens. However, additional costimulatory ligands must also be expressed before such a cell might directly induce full activation and proliferation of resting, antigen-specific T lymphocytes. Intravesical therapy might be used to manipulate T cell costimulation in order to promote specific rejection of TCC cells. This in vitro study examined the potential of such a strategy by transfection of the prototypical TCC line J82 with the important costimulatory molecules CD80 (B7-1) and CD86 (B7-2). Untransfected J82 cells expressed class I and II MHC antigens, a range of cell adhesion molecules, though did not induce T cell proliferation in a robust, allogeneic co-culture system. Transfected J82 cells expressed CD80 or CD86 at levels comparable to an antigen-presenting B cell line. Furthermore, functional surface expression of CD80 and CD86 was demonstrated in a mitogen-dependent assay of costimulation. However, neither CD80+ nor CD86+ transfectant J82 cells could induce significant proliferation of antigen-specific CD4+ T cells. Further analysis showed that bystander J82 cells could inhibit independent T cell activation in an effect dependent on direct cell contact. This inhibitory effect was associated with increased cell death in the responding lymphocyte population and is concordant with surface expression of CD95L by the J82 cell line.