Enhancing regulation

Currently under clinical investigation as anticancer therapies, histone deacetylase inhibitors (HDACis) also have anti-inflammatory effects, but the underlying mechanisms of action are not clear. Hancock and colleagues now show that HDACi therapy increases the number and suppressor ability of regulatory T cells (T_reg cells), which are key mediators of immune tolerance. They also provide evidence that HDACis have therapeutic potential for autoimmune diseases and preventing transplant rejection.

Using three different mouse models, the authors showed that HDACi treatment enhances the production of T_reg cells by either increasing thymic output of T_reg cells or peripheral conversion of conventional T cells (non-T_reg cells) into T_reg cells. HDACis are known to increase histone acetylation, resulting in chromatin remodelling and modulation of gene transcription. HDACi therapy also increases expression of T_reg-associated genes such as Foxp3 and cytotoxic T-lymphocyte-associated protein 4 (Ctla4). In vitro, HDACi-treated T_reg cells were two- to three-fold more effective than wild-type T_reg cells at suppressing proliferation of T effector (T_eff ) cells, which produce cytokines associated with inflammation.

Specifically investigating HDAC expression in T_reg cells, the authors showed that HDAC9 expression was higher in these cells than in non-T_reg cells. Following T-cell receptor stimulation, expression of HDAC9 in non-T_reg cells decreased as they became T_eff cells, whereas expression in T_reg cells was 30-fold higher. Furthermore, Hdac9^−/− mice had significantly more T_reg cells compared with wild-type mice with enhanced expression of Foxp3, Ctla4 and Gitr, and increased acetylation of Foxp3, indicating that HDAC9 is a regulator of T_reg-associated genes.

To further investigate Foxp3 acetylation, western blotting and immunoprecipitations were performed on HDACi-treated T_reg and non-T_reg cells, revealing increased Foxp3 acetylation as well as increased binding of Foxp3 to promoter regions of target genes in the HDACi-treated T_reg cells. The importance of Foxp3 acetylation for the suppressor activity of T_reg cells was also demonstrated by transduction of non-T_reg cells with a vector encoding wild-type Foxp3, which resulted in non-T_reg cells with suppressive ability equivalent to T_reg cells. Mutagenesis studies showed that FOXP3 protein acetylation was required for avid DNA binding and Foxp3-dependent suppression and modulation of T_reg gene expression. Taken together, these data show that HDACi therapy increases histone and Foxp3 acetylation, as well as FOXP3 protein expression and DNA binding.

Suppressor activity of HDACi-enhanced T_reg cells in vivo was explored using three models. First, co-transfer of HDACi-treated T_reg and non-T_reg cells into immunodeficient mice resulted in substantially decreased T-cell proliferation compared with non-T_reg cells alone. Second, in a model of colitis, HDACi treatment increased the number of T_reg cells in lymphoid tissues and decreased disease severity. Last, HDACi treatment was shown to modestly prolong the survival of cardiac and islet transplants, with addition of low-dose rapamycin inducing permanent transplant survival. Overall, inhibiting HDACs has a broad therapeutic potential for immunomodulation.