The infiltration of suppressive myeloid cells into the tumor microenvironment restrains anti-tumor immunity. However, cytokines may alter the function of myeloid lineage cells to support tumor rejection, regulating the balance between pro- and anti-tumor immunity. In this study, it is shown that effector cytokines secreted by adoptively transferred T cells expressing a chimeric antigen receptor (CAR) shape the function of myeloid cells to promote endogenous immunity and tumor destruction. Mice bearing the ovarian ID8 tumor were treated with T cells transduced with a chimeric NKG2D receptor. GM-CSF secreted by the adoptively transferred T cells recruited peripheral F4/80loLy-6C+ myeloid cells to the tumor microenvironment in a CCR2-dependent fashion. T cell IFN-γ and GM-CSF activated local, tumor-associated macrophages, decreased expression of regulatory factors, increased IL-12p40 production, and augmented antigen processing and presentation by host macrophages to antigen-specific T cells. In addition, T cell-derived IFN-γ, but not GM-CSF, induced the production of nitric oxide by F4/80hi macrophages and enhanced their lysis of tumor cells. The ability of CAR T cell therapy to eliminate tumor was moderately impaired when inducible nitric oxide synthase was inhibited, and greatly impaired in the absence of peritoneal macrophages after depletion with clodronate encapsulated liposomes. This study demonstrates that the activation of host macrophages by CAR T cell-derived cytokines transformed the tumor microenvironment from immunosuppressive to immunostimulatory and contributed to inhibition of ovarian tumor growth.

Tumor heterogeneity presents a substantial barrier to increasing clinical responses mediated by targeted therapies. Broadening the immune response elicited by treatments that target a single antigen is necessary for the elimination of tumor variants that fail to express the targeted antigen. In this study, it is shown that adoptive transfer of T cells bearing a chimeric antigen receptor (CAR) inhibited the growth of target-expressing and –deficient tumor cells within ovarian and lymphoma tumors. Mice bearing the ID8 ovarian or RMA lymphoma tumors were treated with T cells transduced with a NKG2D-based CAR (chNKG2D). NKG2D CAR T cell therapy protected mice from heterogeneous RMA tumors. Moreover, adoptive transfer of chNKG2D T cells mediated tumor protection against highly heterogeneous ovarian tumors in which 50%, 20%, or only 7% of tumor cells expressed significant amounts of NKG2D ligands. CAR T cells did not mediate an in vivo response against tumor cells that did not express sufficient amounts of NKG2D ligands, and the number of ligand-expressing tumor cells correlated with therapeutic efficacy. In addition, tumor-free surviving mice were protected against a tumor re-challenge with NKG2D ligand-negative ovarian tumor cells. These data indicate that NKG2D CAR T cell treatment can be an effective therapy against heterogeneous tumors and induce tumor-specific immunity against ligand-deficient tumor cells.

Conditioning strategies that deplete host lymphocytes have been shown to enhance clinical responses to some adoptive T-cell therapies. However, host T cells are capable of eliminating tumor cells upon the relief of immunosuppression, indicating that lymphodepletion prior to T-cell transfer may reduce optimal tumor protection elicited by cell treatments that are capable of shaping host immunity. In this study, we show that adoptively transferred T cells bearing a chimeric antigen receptor (CAR) harness endogenous T cells for optimal tumor elimination and the development of a tumor-specific memory T cell response. Mice bearing ID8 ovarian cancer cells were treated with T cells transduced with a NKG2D-based CAR. CAR-expressing T cells increased the number of host CD4+ and CD8+ T cells at the tumor site in a CXCR3-dependent manner and increased the number of antigen-specific host CD4+ T cells in the tumor and draining lymph nodes. In addition, the administration of CAR-expressing T cells increased antigen presentation to CD4+ T cells, and this increase was dependent on interferon γ and granulocyte-macrophage colony-stimulating factor produced by the former. Host CD4+ T cells were sufficient for optimal tumor protection mediated by NKG2D CAR-expressing T cells, but they were not necessary if CD4+ T cells were adoptively co-transferred. However, host CD4+ T cells were essential for the development of an antigen-specific memory T-cell response to tumor cells. Moreover, optimal tumor elimination as orchestrated by NKG2D CAR-expressing T cells was dependent on host CD8+ T cells. These results demonstrate that adoptively transferred T cells recruit and activate endogenous T-cell immunity to enhance the elimination of tumor cells and the development of tumor-specific memory responses.

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