Histone Methyltransferase Ezh2 Controls T-Cell Immunity by Regulating Bioenergetic Metabolism

Abstract 953

Adoptive T cell therapy has the potential to enhance antitumor immunity and improve vaccine efficacy, of which a key challenge is to generate sufficient numbers of T cells that can persist in vivo after transfer. Cellular metabolism plays important roles in regulating T cell proliferation and survival. T cells responding to antigen activation dramatically upregulate both glycolysis and oxidative phosphorylation (OXPHOS), leading to increased production of adenosine triphosphate (ATP) and metabolic intermediates that are required for cell growth and proliferation. Without sufficient support for their demands, activated T cells may be deleted or become quiescent. Thus, better understanding of the mechanism that regulates cellular metabolism in T cell response will lead to new strategies to improve the efficacy of adoptive T cell therapy. Here we explore the functional impact of an epigenetic pathway in cellular metabolism in antigen-driven T cells and tumor immunity. Using genetic approaches and experimental mouse models, we demonstrate that Ezh2, which is a histone methyltransferase that represses the transcription of cohorts of developmental regulators, promotes the survival and expansion of antigen-driven T cells through regulating bioenergetic metabolism. Conditional deletion of Ezh2 caused selective apoptosis in T cells upon activation with alloantigens in vivo and in vitro or with T cell receptor (TCR)-ligation in vitro. Ezh2 deficiency resulted in markedly increased expression of proapoptotic gene Bim, but had no significant impact on the expression of other Bcl-2 family members (e.g., anti-apoptotic genes Bcl-2 and Bcl-xL,). Genetic inactivation of Bim only slightly improved the survival of alloantigen-activated Ezh2-deficient T cells, suggesting that Ezh2 may control T-cell immunity largely through a Bim-independent mechanism. This differs from our recent observations showing that Bim is required for increased apoptosis in activated T cells treated with a pharmacologic inhibitor of Ezh2 and histone methylation 3-Deazaneplanocin A (Blood, 2012). Our prior studies and others suggest that impaired cellular metabolism may lead to increased apoptosis of antigen-activated T cells. We observed that upon TCR-ligation Ezh2 null T cells were incapable to upregulate OXPHOS as compared to wild-type (WT) T cells, which was accompanied with reduced ATP levels and increased reactive oxygen species (ROS). Neutralization of ROS by N-acetylcysteine significantly improved the survival of TCR-activated Ezh2 null T cells. Interestingly, overexpression of WT Ezh2 in TCR-activated Ezh2 null T cells, but not enzymatically inactive H689A Ezh2 mutant or nuclear localization-inactive Ezh2 mutant, restored the ability of Ezh2-deficient T cells to upregulate OXPHOS, reduced ROS levels, and rescued their survival capability in vitro. These results suggest that Ezh2 is important for regulating bioenergetic metabolism in activated T cells. Furthermore, the nuclear but not cytoplasmic Ezh2 is required to regulate bioenergetic metabolism in activated T cells during clonal expansion phase, although Ezh2 in the cell cytoplasm could be involved in regulating actin polymerization. In mouse models of graft-versus-host disease (GVHD) and leukemia, transfer of donor T cells lacking Ezh2 failed to mediate GVHD and anti-leukemia activity in mice receiving allogeneic bone marrow transplantation. In addition, Ezh2 deficiency also ablated the ability of adoptively transferred antigen-specific CD8 T cells to control tumor growth in mice with established melanoma. Importantly, the absence of Ezh2 did not impair the development of effector T cells producing IFN-γ, granzyme B, Fas ligand and Trail, ruling out the possibility that impaired T-cell immunity of Ezh2 null T cells results from defective effector differentiation. Our findings identify the critical role of Ezh2 in regulating bioenergetic metabolism in antigen-driven T cells, therefore for the first time linking the epigenetic pathway to cellular metabolism in T cell response. Thus, Ezh2 and its-regulated bioenergetic metabolism may represent novel targets to improve the efficacy of adoptive T-cell immunotherapy. Modulation of Ezh2 and its activity may have broad implications in the treatment of many other inflammatory disorders, such as graft rejection after organ transplantation, GVHD and autoimmune diseases.