The Epigenetic Regulator Ezh2 Is An Effective Target For Controlling Th1 Cell-Mediated Acquired Aplastic Anemia In Mice

Severe acquired aplastic anemia (AA) is a fatal disorder characterized by immune-mediated destruction of hematopoietic stem and progenitor cells. Evidence in most AA patients indicates that IFN-g-producing T helper (Th)1 effector CD4⁺ T cells are important for mediating bone marrow (BM) failure in AA. However, the efficacy of standard therapies that typically include antithymocyte globulin and cyclosporine A is limited, and novel approaches are urgently needed. Ezh2 is a histone methyltransferase that specifically catalyzes trimethylation of histone H3 at lysine 27 (H3K27me3) and acts primarily as a gene silencer. We investigated whether Ezh2 regulatory control governs Th1 immune-mediated cytopenias in AA. We tested this hypothesis in a mouse model of AA using genetic approaches of Ezh2 inhibition. In naïve CD4⁺ T cells, high levels of H3K27me3 are correlated with repressed expression of IFNG and TBX21, the gene that encodes T-bet, which is essential for inducing IFN-g expression. Upon Th1 cell differentiation, the regulatory regions of both IFNG and TBX21 gene loci show a marked reduction of H3K27me3. We found that Ezh2 is required to induce Th1 cell differentiation and T cell-mediated AA in mice. Conditionally deleting Ezh2 in mature T cells had the effect of dramatically reducing the production of Th1 cells secreting high levels of IFN-g in vivo, decreasing BM-infiltrating Th1 cells during active disease, and rescuing mice from BM failure. In vitro culture assays confirmed that Ezh2-deficient T cells showed significantly reduced production of IFN-g under Th1-skewing conditions compared to wild-type (WT) T cells. This effect of Ezh2 deficiency on Th1 cell differentiation was accompanied by a marked decrease in the expression of both IFNG and TBX21 genes. These results stand in sharp contrast to the conventional view that Ezh2 and its catalyzed H3K27me3 may repress gene expression, and the corollary that loss of Ezh2 may result in increased production of IFN-g and T-bet. Using chromatin immunoprecipitation assay, we found that upon Th1 cell differentiation in vitro, naïve WT CD4⁺ T cells showed a significant reduction of H3K27me3 at the regulatory region of both IFNG and TBX21 gene loci, in agreement with previous reports. In contrast, high levels of Ezh2 were detected at the regulatory region of the TBX21 gene in activated WT CD4⁺ T cells, suggesting that Ezh2 may be required to promote TBX21 transcription during Th1 cell development. To test this possibility, we infected Ezh2-deficient CD4⁺ T cells with a retrovirus construct encoding T-bet. Ectopic expression of T-bet rescued Th1 cell differentiation of Ezh2-deficient T cells in vitro. Collectively, our findings identify a critical role for Ezh2 in regulating Th1 responses and AA. Given the availability of Ezh2-specific inhibitors newly developed for cancer therapy in clinical trials, we propose that targeting Ezh2 should be investigated as a new strategy for treating AA in patients.