Epigenetic Regulation of Regulatory T-Cells: Impact on Autoimmunity and Graft Rejection

Abstract SCI-23

Mutations of Foxp3, a transcription factor characteristic of T regulatory (Treg) cells, often cause lethal autoimmunity, leading to much research into how Foxp3+ Tregs control inflammatory and immune responses. This presentation will emphasize therapeutic aspects of our ongoing studies showing that the functions of Foxp3 are regulated by histone/protein deacetylases (HDAC), histone acetyltransferases (HAT), and DNA methyltransferases (DNMT), and for the sake of time will focus on the effects of HDAC inhibitors (HDACi). We have found that Foxp3 acetylation promotes DNA binding and can induce or suppress expression of multiple Foxp3-dependent genes in Tregs. Acetylation is catalyzed by specific HATs whose neutralization can diminish Treg function, with relevance to cancer and HIV, whereas use of selected HDAC inhibitors (HDACi) can increase acetylation and Treg suppression, with relevance to control of autoimmunity and transplant rejection. Many HDAC inhibitors (HDACi) were evaluated for their effects on Treg function. As class I-selective HDACi agents (e.g. benzamides) had little or no effect on Treg function, but pan-HDACi (e.g. hydroxymates) enhanced Treg function, we focused on class II HDACs. There are 2 class II subfamilies; class IIa members are thought to largely function in a tissue-specific manner through recruitment of other proteins since they display only weak catalytic activity, whereas class IIb family members display bona fide catalytic activity. We therefore analyzed class IIb members, of which HDAC6 is the best established and for which selective HDACi are available. HDAC6 exists in the cytoplasm and regulates acetylation of alpha-tubulin and other proteins, including HSP90. Blocking HDAC6 through the use of an HDAC6-specific inhibitor promotes HSP90 acetylation and release of HSP90 client proteins. Use of HDAC6 or HSP90 inhibitors increased Foxp3 expression and enhanced Treg function in vitro and in vivo, and could prevent, or treat pre-existing, autoimmunity in a Treg-dependent manner. We have also investigated the various class IIa family members, of which HDAC9 is of particular interest since its expression is increased 30-fold in Treg vs. regular T cells. HDAC9 decreases Foxp3 expression and function, and its neutralization promotes Treg survival by regulating expression of HSP70 and related HSPs. Our ongoing studies show that HDAC6 neutralization leads to acetylation of HSP90, release of HSF-1 and induction of HSP70, and also suggest that HDAC9 may regulate the acetylation and stabilization of HSF-1. Once produced, HSP70 can chaperone and promote Foxp3 nuclear translocation and function, such that the HDAC6 and HDAC9 pathways are closely intertwined with regard to control of Treg biology. In summary, acetylation, methylation and other epigenetic mechanisms in Tregs are being probed using genetic and pharmacologic approaches. Various currently approved drugs influence Foxp3-dependent Treg functions by affecting epigenetic mechanisms, and while additional HDAC-specific regulators are needed, a rationale is now in place for use of HDAC inhibitors as powerful tools to promote the development and functions of Foxp3+ Tregs in vitro and in vivo.