Histones play a critical role in transcriptional regulation, cell cycle progression and developmental events. Histone acetylation/deacetylation alters chromatin structure and affects transcription factor access to DNA. Histone deacetylase inhibitors (HDI) induce growth arrest, cellular differentiation, and apoptosis and are being pursued as anticancer drugs. Interestingly, in addition to their antitumor properties, HDI have also been shown to modulate inflammatory responses.
Recently, we have found that HDAC6 is required for the production of the immunosuppressive cytokine IL-10 by APCs. Given the role of this cytokine in T-cell tolerance induction we asked whether inhibition of HDAC6 with Tubastatin A, a potent and selective HDAC6 inhibitor would influence the inflammatory status of APCs and their ability to determine activation versus tolerance of antigen-specific CD4+ T cells in vitro. First, treatment of peritoneal elicited macrophages (PEM) with increasing concentration of Tubastatin A resulted in enhanced tubulin acetylation which was accompanied by enhanced expression of co-stimulatory molecules in treated cells. In addition, Tubastatin-A treated PEM were unable to produce IL-10 and TNF-a in response to LPS stimulation. In sharp contrast, Tubastatin-A treated PEM produce higher level of the pro-inflammatory cytokines IL-12 and IL-6. Next, we evaluated the ability of Tubastatin A-treated APCs to present cognate antigen to naïve and tolerant CD4+ T-cells specific for a MHC class II restricted epitope of influenza hemagglutinin (HA). We found that treatment of PEM with Tubastatin A significantly enhanced their antigen-presenting capabilities leading to effective priming of naïve CD4+ T-cells confirmed by their increased production of IL-2 and IFN-g in response to cognate antigen. More importantly, Tubastatin-A treated APCs were able to restore the responsiveness of tolerant CD4+ T cells isolated from lymphoma bearing mice.
Taken together, selective HDAC6 inhibition with Tubastatin A provides a novel therapeutic approach to induce inflammatory APCs and overcome the significant barrier that T-cell tolerance has imposed to effective lymphoma immunotherapy.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.