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

Abstract 829

APCs are critical in T-cell activation and in the induction of T-cell tolerance. Epigenetic modifications of specific genes in the APC play a key role in this process, and among them, histone deacetylases (HDACs) have emerged as key participants.

HDAC6 is a 131 KDa protein with preferential cytoplasmic localization where it regulates the acetylation of proteins involved in cytoskeleton, cell-cell interaction and cell migration. Emerging evidence also implicates HDAC6 in regulation of immune responses, in particular at the level of the APC/T cell immune synapse¹ and in the suppressive function of regulatory T-cells². Expanding upon these immunoregulatory properties, here we show for the first time that HDAC6 physically interacts with STAT3, a transcriptional activator of IL-10 gene expression. By co-immunoprecipitation studies and confocal studies we found that HDAC6 co-localize with STAT3 in the cytoplasm and nuclei of macrophages. Furthermore, by using several HDAC6 and STAT3 mutants we have identified that the aminoacids 503–840 of HDAC6 and the STAT3 domain comprising aminoacids 465–585 are required for this interaction. Functionally, knocking down HDAC6 in a macrophage cell line (RAW264.7-HDAC6KD) resulted in inhibition of STAT3 phosphorylation, decreased recruitment of STAT3 to the IL-10 gene promoter and abrogation of IL-10 production by these cells in response to either LPS or IL-10. Similar results were observed in dendritic cells (DCs) or macrophages isolated from HDAC6 knock-out (KO) mice. Furthermore, HDAC6KD clones or APCs from HDAC6 KO mice displayed an enhanced expression of the co-stimulatory molecule B7.2 and are better activators of antigen-specific CD4⁺ T-cell responses in vitro. More importantly, these APCs are able of restoring the responsiveness of anergic T-cells from lymphoma-bearing mice. Pharmacologic inhibition of HDAC6 in APCs with Tubastatin A, an isotype-selective HDAC6 inhibitor, yielded similar enhancement of APC and T-cell function in vitro. Further support for HDAC6 as an appealing target in cancer immunotherapy has been recently provided by the significant delay in tumor growth observed in either HDAC6 KO mice or in wild type mice treated with Tubastatin A.

In summary, we have shown for the first time that HDAC6 interacts physically with STAT3 and such an interaction is necessary for STAT3 phosphorylation and IL-10 gene expression in APCs. Disrupting the HDAC6/STAT3/IL-10 axis in APCs with selective HDAC6 inhibitors represents a novel approach to overcome tolerogenic pathways in these cells and tip the balance towards effective antitumor immune responses.