Alloimmunization to transfused platelets requires priming of CD4+ T cells in the splenic microenvironment in a murine model

BACKGROUND: Alloantibodies are a clinically significant sequelae of platelet (PLT) transfusion, potentially rendering patients refractory to ongoing PLT transfusion support. These antibodies are often IgG class switched, suggesting the involvement of CD4+ T‐cell help; however, PLT‐specific CD4+ T cells have not been visualized in vivo, and specifics of their stimulation are not completely understood. STUDY DESIGN AND METHODS: A murine model of alloimmunization to transfused PLTs was developed to allow in vivo assessment and characterization of CD4+ T cells specific for PLT major histocompatibility complex (MHC) alloantigen. PLTs were harvested from BALB/c mice, filter leukoreduced, and transfused into C57BL/6 recipients. PLT‐specific CD4+ T‐cell responses were visualized by using a T‐cell receptor transgenic mouse that detects peptide from donor MHC I presented on recipient MHC II. Antibody responses were determined by indirect immunofluorescence using BALB/c donor targets. RESULTS: C57BL/6 recipients of BALB/c leukoreduced PLT transfusions produced BALB/c antibodies, with proliferation of antigen‐specific CD4+ T cells seen in the spleen but not lymph nodes or liver. Depletion of recipient CD4+ cells or splenectomy independently abrogated the alloantibody response. CONCLUSION: We report a novel model to study antigen‐specific CD4+ T cells during alloimmunization to PLT transfusion. The presented data support a critical role for CD4+ T‐cell help in the humoral response to PLT transfusion and establish the spleen as a required microenvironment for effective CD4+ T‐cell priming against donor PLT–derived MHC I.