Foxp3⁺ Tregs are key to immune homeostasis, but the contributions of various large, multiprotein complexes that regulate gene expression remain unexplored. We analyzed the role in Tregs of the evolutionarily conserved CoREST complex, consisting of a scaffolding protein, Rcor1 or Rcor2, plus Hdac1 or Hdac2 and Lsd1 enzymes. Rcor1, Rcor2, and Lsd1 were physically associated with Foxp3, and mice with conditional deletion of Rcor1 in Foxp3⁺ Tregs had decreased proportions of Tregs in peripheral lymphoid tissues and increased Treg expression of IL-2 and IFN-γ compared with what was found in WT cells. Mice with conditional deletion of the gene encoding Rcor1 in their Tregs had reduced suppression of homeostatic proliferation, inability to maintain long-term allograft survival despite costimulation blockade, and enhanced antitumor immunity in syngeneic models. Comparable findings were seen in WT mice treated with CoREST complex bivalent inhibitors, which also altered the phenotype of human Tregs and impaired their suppressive function. Our data point to the potential for therapeutic modulation of Treg functions by pharmacologic targeting of enzymatic components of the CoREST complex and contribute to an understanding of the biochemical and molecular mechanisms by which Foxp3 represses large gene sets and maintains the unique properties of this key immune cell.

The transcription factor MEF2D is important in the regulation of differentiation and adaptive responses in many cell types. We found that among T cells, MEF2D gained new functions in Foxp3⁺ T regulatory (Treg) cells due to its interactions with the transcription factor Foxp3 and its release from canonical partners, like histone/protein deacetylases. Though not necessary for the generation and maintenance of Tregs, MEF2D was required for the expression of IL-10, CTLA4, and Icos, and for the acquisition of an effector Treg phenotype. At these loci, MEF2D acted both synergistically and additively to Foxp3, and downstream of Blimp1. Mice with the conditional deletion in Tregs of the gene encoding MEF2D were unable to maintain long-term allograft survival despite costimulation blockade, had enhanced antitumor immunity in syngeneic models, but displayed only minor evidence of autoimmunity when maintained under normal conditions. The role played by MEF2D in sustaining effector Foxp3⁺ Treg functions without abrogating their basal actions suggests its suitability for drug discovery efforts in cancer therapy.

HDAC8, an evolutionarily distinct, X-linked, zinc-dependent class I histone/protein deacetylase, is implicated in developmental disorders, parasitic infections, myopathy, and cancers. Our study demonstrates the important role of HDAC8 in immune cells by conditional targeting of HDAC8 in murine T cells and application of selective HDAC8 inhibitors. Using flow cytometry, RNA-seq, and ChIP-seq analyses, we demonstrate that knocking down or inhibiting HDAC8 impaired murine regulatory T cell (Treg) suppressive function in vitro and in vivo, but promoted conventional host T cell responses, thereby limiting syngeneic tumor growth. Mechanistically, HDAC8 knockout downregulated Foxp3 expression, enhanced H3K27 acetylation levels, and promoted IL-2, IL-6, Fas, and FasL expression in both Treg and conventional effector T cells. Thus, our combined genetic and pharmacologic studies establish the central importance of HDAC8 in T cell responses and suggest that selective HDAC8 inhibitors represent a potential therapeutic approach in immuno-oncology.