Gibbons, Rachel

Abstract A promising strategy in tumor immunotherapy is the use of activated dendritic cells as vehicles for tumor vaccines. Clinical trials of dendritic cell-based immunotherapies have proven successful, resulting in recent FDA approval for their use in the treatment of late-stage prostate cancer. Current formulations for dendritic cell-based immunotherapies are capable of prolonging patient survival by only four months, providing motivation for further investigation of ways to enhance dendritic cell priming of anti-tumor T cell responses. Since the discovery of B7-H1 by our laboratory, we have been investigating the regulatory function of B7-H1 expressed by immune cells. In preliminary studies investigating the use of B7-H1 knockout dendritic cells as tumor vaccines, we found that B7-H1 knockout dendritic cells prime enhanced anti-tumor CD8 T cell responses. Further investigations revealed that the enhanced CD8 T cell response raised by B7-H1 knockout dendritic cells is not due to a lack of signaling through the receptors for B7-H1 on CD8 T cells. We observed that B7-H1 knockout dendritic cells produced increased levels of IL-12, a key cytokine that is centrally involved in both innate and adaptive immunity. Our results suggest that B7-H1 expressed by dendritic cells has a role in negatively regulating IL-12 production, and as a result B7-H1 deficient dendritic cells are able to prime enhanced anti-tumor CD8 T cell responses.

Abstract We utilized CD11a, an integrin up-regulated on effector and memory CD8+ T cells, as a biomarker to identify, measure, and monitor endogenous tumor-reactive cytotoxic lymphocytes (CTLs) in two mouse tumor models and in melanoma patients. CD11ahigh CD8+ T cells were detected in primary and metastatic tumor sites after tumor cell injection, as well as in spontaneous murine breast cancer tissues. In the peripheral blood of melanoma patients, tumor antigen-specific T cells were detected within the population of CD11ahigh CD8+ T cells that express high levels of PD-1. Infusion of lethally irradiated tumor cells failed to stimulate the expansion of CD11ahigh CD8+ T cells, suggesting that the access of tumor cells to target tissues is required to induce CD11ahigh CD8+ T cell responses. Phenotypic analysis revealed that the CD11ahigh CD8+ T cells detected in our studies were proliferating (Ki67+) activated T cells (CD62L-, CD69+). Although CD11ahigh CD8+ T cells were differentiated effector cells with CTL effector functions, they were typically unable to control tumor growth in our mouse models, possibly due to high expression of PD-1. Our results demonstrate that CD11a can be used as a biomarker to identify and track endogenous tumor-reactive CTLs.

Abstract Spontaneous antitumor immunity remains challenging to detect and monitor largely due to the absence of established biomarkers. We selected the integrin CD11a as a biomarker to define antitumor T cell responses. CD11a is up-regulated on effector and memory T cells and is involved in the killing of tumor cells by cytotoxic T lymphocytes. In primary (subcutaneously inoculated) and metastatic (intravenously injected) tumors, CD11a high CD8 T cells were detected in the primary (skin) or metastatic (lung) tumor sites, but not in secondary lymphoid organs. Once tumors were established, CD11a high CD8 T cells were identified in tumor draining lymph nodes and peripheral blood. Phenotype analysis revealed that CD11a high CD8 T cells were proliferating (Ki67+) activated T cells (CD62L low, CD69 high, PD-1 high). CD11a high CD8 T cells expressed the transcription factor T-bet but not Foxp3. Live, but not dead, tumor cells, induced expansion of CD11a high CD8 T cells in the lung, suggesting that the access of tumor cells to target tissues is required to induce CD11a high CD8 T cell responses. Our results suggest that CD11a may provide a new T cell biomarker to measure ongoing natural tumor-reactive T cell responses and to predicate the efficacy of tumor immunotherapy.

Abstract 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-xL 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.