B7-H1 limits the entry of effector CD8⁺ T cells to the memory pool by upregulating bim

Protective T-cell immunity against cancer and infections is dependent on the generation of a durable effector and memory T-cell pool. Studies from cancer and chronic infections reveal that B7-H 1 (PD-L1) engagement with its receptor PD-1 promotes apoptosis of effector T cells. It is not clear how B7-H 1 regulates T-cell apoptosis and the subsequent impact of B7-H 1 on the generation of memory T cells. In immunized B7-H 1-deficient mice, we detected an increased expansion of effector CD8⁺ T cells and a delayed T-cell contraction followed by the emergence of a protective CD8⁺ T-cell memory capable of completely rejecting tumor metastases in the lung. Intracellular staining revealed that antigen-primed CD8⁺ T cells in B7-H 1-deficient mice express lower levels of the pro-apoptotic molecule Bim. The engagement of activated CD8⁺ T cells by a plate-bound B7-H 1 fusion protein led to the upregulation of Bim and increased cell death. Assays based on blocking antibodies determined that both PD-1 and CD80 are involved in the B7-H 1-mediated regulation of Bim in activated CD8⁺ T cells. Our results suggest that B7-H 1 may negatively regulate CD8⁺ T-cell memory by enhancing the depletion of effector CD8⁺ T cells through the upregulation of Bim. Our findings may provide a new strategy for targeting B7-H 1 signaling in effector CD8⁺ T cells to achieve protective antitumor memory responses.