Antigen-presenting cells (APC) can induce T-cell activation as well as T-cell tolerance. The induction of such a divergent outcomes is determined by the inflammatory status of the APC at the time of encounter with antigen specific T-cells. The molecular basis by which the APC regulate this critical decision of the immune system remain not well understood. Chromatin modification induced by acetylation/deacetylation of histones plays an important role in regulation of gene transcription, including genes involved in the inflammatory response. Histone deacetylases, a set of enzymes involved in histone modification are molecular targets for histone deacetylase inhibitors (HDI), novel compounds being evaluated as anticancer drugs. Interestingly, in addition to their antitumor properties, HDI have been also shown to modulate inflammatory responses. We evaluated therefore whether treatment with the hydroxamic acid analogue pan-HDAC inhibitor LAQ824 could influence the inflammatory status of the APC and their ability to determine CD4+ T-cell priming versus tolerance. In vitro treatment of APCs with LAQ824 resulted in enhanced acetylation of histones H-2A, H-2B, H3 and H4, increased expression of the co-stimulatory molecule B7.2 and enhanced production of pro-inflammatory mediators such as IL-1a, IL-1-b, IL-6, IL-12, TNF-a and RANTES in response to LPS stimulation. To our surprise, a dose-dependent inhibition of IL-10 mRNA and protein was observed in APCs treated with LPS and LAQ824. Chromatin immune precipitation (CHIP) assays indicate that this particular effect of LAQ824 involves histone modifications at the IL-10-promoter level. Given this inhibitory effect of LAQ824 and the central role of IL-10 in immune tolerance, we asked next whether a specific histone deacetylase(s) could predominantly influence IL-10 gene expression. By utilizing a reporter gene carrying the IL10 promoter fused to a luciferase gene, plasmids coding for Flag-tagged versions of all HDACs and plasmids carrying siRNA for specific silencing of HDACs, we found that among all the HDACs evaluated, HDAC11 negatively regulates the production of IL-10 in APCs. Importantly, treatment of APCs with LAQ824 resulted in increased expression of HDAC 11, diminished IL-10 production and the generation of APCs that effectively prime naive CD4+ T-cells and restore the responsiveness of tolerized antigen-specific T-cells from lymphoma bearing hosts. Taken together, we have demonstrated for the first time that HDAC11, a member of the HDAC family with no prior defined physiological role, is involved in regulation of IL-10 gene expression. Furthermore, our findings that HDAC11 expression in APCs can be manipulated by treatment of these cells with LAQ824, points to HDAC11 as a novel therapeutic target to influence immune activation versus immune tolerance, a critical decision with significant implications in autoimmunity, transplantation and cancer immunotherapy.
Disclosure: No relevant conflicts of interest to declare.