Histone Deacetylase 11 (HDAC11) Is a Regulatory Checkpoint of T-Cell Function: Implications for T-Cell Adoptive Immunotherapy

Abstract 359^FN2

T-cells are an essential component of immune mediated tumor rejection. Adoptive transfer of T-cells has resulted in durable antitumor responses in some patients with hematologic malignancies. Further improvement in the efficacy of this treatment modality will require a better understanding of the regulatory checkpoint(s) limiting T-cell expansion and/or reactivity in vivo. Our group has been especially interested in the epigenetic regulatory mechanisms affecting T-cells, particularly those involving histone deacetylases (HDACs). HDACs are enzymes recruited to gene promoters where they regulate transcription through histone modifications. The role of HDACs in cell biology, initially limited to their effects upon histones, now encompasses more complex regulatory functions that are dependent on their tissue expression, subcellular compartment distribution and the stage of cellular differentiation. For instance, HDAC11, the most recently discovered member of the HDAC family, has been found to be predominantly expressed in the brain, kidney, testis as well as in T-cells. Recently, HDAC11 knock out (KO) mice have been generated by targeted deletion of floxed exon 3 of the HDAC11 gene. Although analysis of the T-cell compartment in these mice revealed no significant differences in the absolute number and/or percent of T-cells in lymphoid organs as compared to wildtype mice, striking functional differences were observed in HDAC11KO T cells. First, in response to in vitro stimulation with anti-CD3 plus anti-CD28 antibodies, HDAC11KO T-cells display an enhanced proliferation, produce significantly higher levels of the pro-inflammatory cytokines IL-2, IFN-gamma and TNF-alpha and are less susceptible to Treg-mediated suppression. Studies performed in HDAC11KO T-cells expressing a transgenic receptor (TCR) specific for ovalbumin (OTII;HDAC11KO mice) confirm that these T-cells are hyperreactive in an antigen-specific manner. In vivo, HDAC11KO T cells induced significantly more severe graft-versus-host disease (GVHD) than wildtype T-cells after allogeneic bone marrow transplantation. Enhanced GVHD mediated by HDAC11KO T cells was associated with increased levels of T-cell expansion and secreted Th1-cytokines such as IFN-gamma. Further demonstration of the intrinsic potency of HDAC11KO T-cells was provided by the finding that when the number of adoptively transferred T-cells was titrated down such that wildtype T-cells no longer induced GVHD, T-cells from HDAC11KO still potently did so. HDAC11KO T-cells are also endowed with a stronger antitumor effect given the prolonged survival observed in mice challenged with C1498 leukemic cells. Mechanistically, using ChIP analysis, we have found that HDAC11 is recruited to the T-bet promoter, which may explain, at least in part, the Th1 phenotype displayed by T-cells in the absence of the repressive effect of HDAC11. Taken together, we have unveiled for the first time that HDAC11 is a regulatory checkpoint in T-cells and represents a novel molecular target to improve the efficacy of T-cell adoptive immunotherapy for the treatment of hematologic malignancies.