High numbers of tumor infiltrating regulatory T (Treg) cells are indicative of poor outcome in several malignancies, including ovarian cancer, colorectal cancer, and melanoma. Thus, selectively abrogating intratumoral Treg cell function while maintaining systemic immune tolerance remains an attractive, albeit elusive, strategy for cancer immunotherapy. We have identified the epigenetic enzyme Ezh2, an H3K27 methyltransferase, to be a critical mediator of lineage stability and function in activated Treg cells. Consequently, deficiency of Ezh2 in Treg cells strongly impairs their function in non-lymphoid tissues. We hypothesized that this phenomenon might translate to a similar defect that is restricted to the tumor microenvironment. Here we tested this hypothesis in three transplantable syngeneic tumor models in mice (MC38 colon carcinoma, TRAMP-C2 prostate cancer, and B16F10 melanoma). We observed that constitutive deletion of Ezh2 in Treg cells resulted in potent anti-tumor activity, significantly impaired tumor outgrowth, and in many instances, complete tumor rejection. These results were also recapitulated when Ezh2 was temporally deleted in the vast majority of Treg cells at the time of tumor inoculation. Functional analyses at early time points after Ezh2 deletion revealed enhanced functionality of effector CD4 and CD8 populations, as evidenced by increased IFN-γ production. These effects were evident without reduction of intratumoral Treg cell frequencies and despite presence of residual wild type Treg cells. Mechanistic studies showed that Ezh2-deficient Treg cells were destabilized and produced pro-inflammatory cytokines IL-2 and IFN-γ selectively within the tumor microenvironment, suggesting a direct contribution of Ezh2-deficient Treg cells to tumor clearance. To more directly address this hypothesis, we generated mice that harbored both Ezh2-deficient and wild type Treg cells and observed anti-tumor activity similar to mice that exclusively harbored Ezh2-deficient Treg cells. In stark contrast to Ezh2 deficiency, mice whose Treg cells were completely depleted, using a Foxp3-driven diphtheria toxin receptor allele, were unable to reject tumors, further supporting a dominant role of Ezh2-deficient Treg cells in driving anti-tumor immunity. Finally, tumor protection in the presence of Ezh2-deficient Treg cells occurred without significant morbidity due to autoimmune pathologies, which were prevalent in mice with systemic Treg cell depletion. These results suggest that targeted deletion of Ezh2 in Treg cells drives a unique phenotype among tumor infiltrating Treg cells, reprogramming the tumor microenvironment and selectively augmenting the anti-tumor immune response. Cancer therapies that pharmacologically inhibit Ezh2 to target tumor cells are under investigation, going forward it will also be important to assess the potential for beneficial effects via modulating the immune response.

Citation Format: David Q. Wang, Jason R. Quiros, Chien-Chun S. Pai, Lawrence H. Fong, Jeffrey A. Bluestone, Michel J. DuPage. Selective impairment of intratumoral regulatory T cells by targeting Ezh2 enhances cancer immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1014. doi:10.1158/1538-7445.AM2017-1014