To further investigate the mechanisms by which the antineoplastic agent cisplatin interferes with immune function, we studied its effect on the biosynthesis of interleukin-2 (IL-2) and its alpha-chain receptor (IL-2Ralpha). Normal human peripheral blood lymphocytes (PBL) were activated in vitro with phytohemagglutinin (PHA), and anti-CD3 antibody in the presence of various concentrations of cisplatin. Purified T cells were also cultured with anti-CD3 antibody and costimulated by CD80 (B7-1, B7/BB1)-transfected P815 mastocytoma cells in the presence of cisplatin. Tritiated thymidine incorporation assays, an enzyme-linked immunosorbent assay for soluble IL-2Ralpha determination, and RNA dot-blot analysis and hybridization with IL-2- and IL-2Ralpha-specific probes were used. PHA-induced and anti-CD3 antibody-induced proliferation of PBL were significantly inhibited by cisplatin at concentrations attainable in vivo. This inhibition was not due to direct cell death as shown by the absence of trypan blue uptake in the presence of high concentrations of cisplatin. Therapeutic concentrations of cisplatin (1 microgram/ml) also inhibited the IL-2-dependent proliferation of purified T cells, mediated via the CD28-CD80 costimulatory pathway. In addition, the amount of soluble IL-2Ralpha released in the T cell culture supernatants was decreased by cisplatin in a dose-dependent fashion, suggesting that inhibition of cell proliferation was associated with a parallel decrease in IL-2Ralpha production. These effects correlated with a specific cisplatin-induced downregulation of both IL-2 and IL-2Ralpha messenger RNA accumulation in PHA-stimulated PBL that was dependent on the concentration of the drug. These findings suggest that the immunomodulatory effects of cisplatin may result in part from its capacity to directly downregulate the IL-2/IL-2R system in activated lymphocytes.