Abstract LB566: Class 1 HDAC inhibition induces antitumor immunity by NF-kB-mediated enhanced metabolic fitness and generation of unique effector function enriched memory CD8 T cell subtype

Histone/protein deacetylases (HDACs) are a class of enzymes that regulate gene expression and are involved in regulating the transcription process as well as regulating fundamental cellular processes such as proliferation, differentiation and development. Class I HDACs (which includes HDAC1, 2, 3, and 8) contribute in epigenetic regulations of T cell tolerance. Under anergic T cell condition, HDAC1 and HDAC2 are involved in the histone deacetylation. However, the role of HDACs in CD8 T cell function and differentiation is not well understood. Therefore, we aimed to understand the effect of HDAC inhibition (HDACi) on CD8 T cells phenotype and functionality by using Entinostat (ENT), a class I HDAC (HDAC1 and 3) inhibitor. We found that ENT induces strong immune-mediated anti-tumor effects in TC-1 and B16 tumor-bearing mice. Mechanistically, ENT modulated the tumor microenvironment through an increased infiltration of CD8 T cells, enhanced CD8 T cell antigen-specificity and CD8 T cell functionality represented by interferon gamma (IFN-γ) and granzyme B (GrB) secretion. Our in vitro and in vivo data also indicate that ENT enhances central memory phenotype (CD62L+, CD44+) of CD8 T cells. We also found that ENT-treated CD8 T cells have distinct metabolic phenotype compared to activated CD8 T cells. Specifically, these cells relied more on amino acid metabolism while, unexpectedly, they had lower fatty acid uptake with enhanced mitochondrial potential (TMRM). Interestingly, ENT-induced central memory CD8 T cells also showed increased mitochondrial potential as well as reliance on amino acid metabolism. Therefore, ENT treatment induced memory cells with a unique phenotype different from the classical memory CD8 T cells with enhanced effector functions and reliance on distinct metabolic pathways. Furthermore, ENT-treated CD8 T cells have higher peaks at H3K27Ac site compared to the control. Since these peaks are mostly associated with genes such as IFN-γ, GrB, Pim1, EOMES, and IL-2, the known canonical target genes of NF-κB in CD8 T cells, we further dissected the role of NF-κB in these ENT-mediated effects. First, motif analysis showed that NF-κB binding motif is enriched in differential H3K27Ac sites associated with HDACi. Next, flow cytometry and immunofluorescence analysis showed that although ENT does not increase total NF-κB levels in CD8 T cells, it enhances its binding to the chromatin due to enhanced gene accessibility. This research highlights the signif