B7-H1 signaling is integrated during CD8(+) T cell priming and restrains effector differentiation.

A promising strategy in tumor immunotherapy is the use of activated dendritic cells as vehicles for tumor vaccines with the goal of activating anti-tumor T cell responses. Current formulations for dendritic cell-based immunotherapies have limited effects on patient survival, providing motivation for further investigation of ways to enhance dendritic cell priming of anti-tumor T cell responses. Using a brief in vitro priming model, we have found that B7-H1 expressed by activated dendritic cells is integrated during priming of naïve CD8(+) T cells and functions to limit the differentiation of effector T cell responses. CD8(+) T cells primed by B7-H1-deficient dendritic cells exhibit increased production of IFN-γ, enhanced target cell killing, and improved anti-tumor activity. Additionally, enhanced memory populations arise from CD8(+) T cells primed by B7-H1-deficient dendritic cells. Based on these findings, we suggest that early blockade of B7-H1 signaling should be investigated as a strategy to improve dendritic cell-based anti-tumor immunotherapy.

B7-h1 expressed by activated CD8 T cells is essential for their survival.

An immunoinhibitory role of B7 homologue 1 (B7-H1) expressed by non-T cells has been established; however, the function of B7-H1 expressed by T cells is not clear. Peak expression of B7-H1 on Ag-primed CD8 T cells was observed during the contraction phase of an immune response. Unexpectedly, B7-H1 blockade at this stage reduced the numbers of effector CD8 T cells, suggesting B7-H1 blocking Ab may disturb an unknown function of B7-H1 expressed by CD8 T cells. To exclusively examine the role of B7-H1 expressed by T cells, we introduced B7-H1 deficiency into TCR transgenic (OT-1) mice. Naive B7-H1-deficient CD8 T cells proliferated normally following Ag stimulation; however, once activated, they underwent more robust contraction in vivo and more apoptosis in vitro. In addition, B7-H1-deficient CD8 T cells were more sensitive to Ca-dependent and Fas ligand-dependent killing by cytotoxic T lymphocytes. Activation-induced Bcl-x(L) expression was lower in activated B7-H1-deficient CD8 T cells, whereas Bcl-2 and Bim expression were comparable to the wild type. Transfer of effector B7-H1-deficient CD8 T cells failed to suppress tumor growth in vivo. Thus, upregulation of B7-H1 on primed T cells helps effector T cells survive the contraction phase and consequently generate optimal protective immunity.

Endogenous tumor-reactive CD8+ T cells are differentiated effector cells expressing high levels of CD11a and PD-1 but are unable to control tumor growth.

Immunotherapies aimed at enhancing natural or endogenous antitumor T-cell immunity in patients affected by advanced malignancies are currently being implemented in the clinic with promising results. In order to optimize therapeutic protocols and monitor the effectiveness of such therapies, reliable biomarkers are needed. We used CD11a, an integrin that is upregulated on the surface of effector and memory CD8+ T cells, and PD-1, an immunoregulatory receptor expressed by activated T cells, as biomarkers to identify, quantify and monitor endogenous tumor-reactive cytotoxic T lymphocytes (CTLs) in two mouse tumor models and in the peripheral blood of 12 patients affected by Stage IV melanoma. High expression levels of CD11a and PD-1 were detected among CD8+ T cells residing within primary and metastatic murine tumor sites, as well as in spontaneous murine breast cancer tissues. In the peripheral blood of melanoma patients, tumor antigen-specific CD8+ T cells were associated with a population of CD11ahigh CD8+ T cells that co-expressed high levels of PD-1. Healthy donors exhibited a comparatively much lower frequency of such PD-1+CD11ahighCD8+ T cells. Phenotypic analyses demonstrated that CD11ahighCD8+ T cells are proliferating (Ki67+) and activated (CD62L-CD69+). Increased CD11ahighCD8+ T cells and delayed tumor growth were observed in PD-1 deficient mice, suggesting that the antitumor effector functions of CD8+ T cells is compromised by an elevated expression of PD-1. The CD11ahighCD8+ T-cell population expresses high levels of PD-1 and presumably constitutes the cellular target of PD-1 blockade therapy. The expression level of CD11a and PD-1 by CD8+ T cells may therefore represent a novel biomarker to identify and monitor endogenous tumor-reactive CTLs. This may not only provide an immunological readout for evaluating the efficacy of immunotherapy but also contribute to the selection of cancer patients who are likely to benefit from anti-PD-1 therapy.

B7-H1 limits the entry of effector CD8(+) T cells to the memory pool by upregulating Bim.

Protective T­cell immunity against cancer and infections is dependent on the generation of a durable effector and memory T­cell pool. Studies from cancer and chronic infections reveal that B7-H1 (PD-L1) engagement with its receptor PD-1 promotes apoptosis of effector T cells. It is not clear how B7-H1 regulates T­cell apoptosis and the subsequent impact of B7-H1 on the generation of memory T cells. In immunized B7-H1-deficient mice, we detected an increased expansion of effector CD8(+) T cells and a delayed T­cell contraction followed by the emergence of a protective CD8(+) T­cell memory capable of completely rejecting tumor metastases in the lung. Intracellular staining revealed that antigen-primed CD8(+) T cells in B7-H1-deficient mice express lower levels of the pro-apoptotic molecule Bim. The engagement of activated CD8(+) T cells by a plate-bound B7-H1 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-H1-mediated regulation of Bim in activated CD8(+) T cells. Our results suggest that B7-H1 may negatively regulate CD8(+) T­cell 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-H1 signaling in effector CD8(+) T cells to achieve protective antitumor memory responses.