Epigenetic Modulation of STAT3 by Histone Deacetylase 6 (HDAC6) Regulates IL-10 Gene Expression and Immune Tolerance Mediated by Antigen-Presenting Cells (APCs)

Abstract 519

We have previously shown that the pan-HDAC inhibitors LAQ824 and LBH589 inhibit IL-10 production in APCs, rendering these cells more inflammatory and capable of effectively priming naïve antigen-specific CD4⁺ T-cells and restoring the responsiveness of tolerant T-cells¹ . These findings led us to explore which HDAC(s) might be involved in the regulation of IL-10 gene transcription and be the putative target(s) of HDI-mediated IL-10 inhibition. To answer these questions we subjected the macrophage cell line RAW264.7 to shRNA screening using specific shRNAs to knockdown each known HDAC. We found that among all the HDACs, knocking down HDAC6 (HDAC6KD) was associated with a significant decrease in IL-10 mRNA and protein in response to LPS stimulation. Furthermore, HDAC6KD clones display an enhanced expression of the co-stimulatory molecule B7.2. Functionally, HDAC6KD cells were better activators of anti-HA (hemagglutinin-influenza) transgenic CD4⁺ T cells, leading to significantly enhanced production of IL-2 and IFN-g in response to cognate antigen. More importantly, anti-HA CD4⁺ anergic T cells isolated from animals bearing HA-expressing A20 B-cell lymphoma regained their ability to produce IL-2 and IFN-g when cultured in vitro with HDAC6KD cells. These results have been confirmed in APCs isolated from HDAC6 knock-out mice and in wild type APCs treated in vitro with isotype-selective HDAC6 inhibitors.

Given that HDACs do not bind to DNA and they need to interact with transcription factors to regulate gene expression, we investigate next which transcription factor(s) HDAC6 might be associated with, to regulate IL-10 transcriptional activity. One likely candidate was Stat3, a well-known transcriptional activator of IL-10 gene expression that we have previously shown to play a central role in tolerance induction by APCs² . By co-immunoprecipitation studies we found that HDAC6 indeed interacts physically with Stat3. Of note, knocking down HDAC6 in APCs resulted in absence of Stat3 phosphorylation and decreased recruitment of Stat3 to the IL-10 gene promoter which might explain the inability of HDAC6KD cells to produce IL-10. The additional findings that IL-10 production by HDAC6KD cells was restored when these cells were transfected with a constitutively active mutant version of Stat3 (Stat3c) provides additional support for the important role of HDAC6 upon Stat3 activation. Further confirmation for a concerted regulatory mechanism involving HDAC6 and Stat3 in IL-10 gene regulation was provided by studies using CPA-7, a specific Stat3 inhibitor that disrupts Stat3 recruitment and binding to gene promoters. As expected, a complete abrogation of Stat3 recruitment to the IL-10 gene promoter was observed in CPA-7 treated APCs. Interestingly, such an effect was accompanied by a parallel decrease in HDAC6 recruitment to the IL-10 promoter and inhibition of IL-10 gene transcriptional activity.

Taken together, we have shown for the first time that HDAC6 interacts physically with Stat3 and is required for its phosphorylation. Since Stat3 phosphorylation is absolutely necessary for activation of Stat3 target genes, HDAC6 inhibition is an enticing molecular approach to disrupt the Stat3/IL-10 axis and overcome tolerogenic mechanisms in APCs.