To study the cellular immunology of platelet-induced alloimmunization, a murine transfusion model was developed. BALB/c (H-2d) recipient mice were transfused weekly with 2 x 10(8) platelets or 10(3) leukocytes from C57BL/6 (H-2b) donor mice. Recipient antidonor major histocompatibility complex (MHC) class I alloantibodies could be detected in flow cytometric assays by the fifth platelet transfusion. In contrast, when leukocytes only were transfused, alloantibodies were not detected. In vitro assays demonstrated that murine H-2b platelets were positive for MHC class I expression but lacked MHC class II molecules on their membranes and were unable to stimulate proliferation or cytokine production when incubated with naive H-2d spleen cells. In vivo, however, platelet transfusions induced two distinct patterns of cell-mediated reactivity. First, during the initial transfusions and before alloantibody formation, there was induction of T-cell anergy, characterized by the inability of recipient T cells to respond to Concanavalin A (ConA) or to proliferate in an antidonor mixed lymphocyte reaction (MLR), together with suppressed natural killer (NK) cell activity. This unresponsiveness was associated with a transient increase in nitric oxide (NO)-dependent cytotoxicity and interleukin-1 (IL-1) production. Second, once alloantibodies developed, significantly increased antidonor CD8+ cytotoxic T lymphocyte (CTL) and NK cell responses were observed. At this time, when recipient spleen cells were depleted of CD8+ T cells and incubated with only donor platelets in 7- day antigen-presenting cell (APC) assays, enhanced proliferation and IL- 2 production occurred. These cellular responses were not seen when 10(3) allogeneic leukocytes were transfused. Thus, the results suggest that leukoreduced platelet transfusions induce antidonor MHC antibodies and CD8+ CTL responses in recipient mice. At the same time, the transfusions induced recipient CD4+ T-cell activation when incubated with donor platelets in the presence of syngeneic APCs, an indirect recognition pathway that correlates with the time of alloantibody production.