Treatment of Antigen-Presenting Cells with Histone Deacetylase Inhibitors Represents a Novel Strategy To Overcome CD4+ T-Cell Tolerance: Divergent Effects on the IL10 and IL-12 Promoter.

Tumor-antigen-specific T-cell tolerance imposes a significant barrier to the development of effective therapeutic cancer vaccines. Bone marrow-derived antigen presenting cells (APCs) are critical in the induction of this unresponsive state. The requirement for APCs in tolerance induction, together with their well-known role in priming T-cell antitumor responses place APCs at the crossroads of immune activation versus immune tolerance and points to manipulation of these cells as an enticing strategy to modulate T-cell responses against tumors. Identification of the intracellular mechanisms by which APCs induces either T-cell outcome represents therefore a critical step to better understand and overcome tumor-induced immune tolerance. Histones tail plays an important role in modulation of gene transcription. Emerging evidence suggest that inhibition of hystone deacetylases (HDAC) increases the expression of inflammatory genes. Given that the inflammatory status of the APC at the time of antigen presentation is central in determining T-cell priming versus T-cell tolerance, we evaluated the effects of the HDAC inhibitor LAQ842 (Novartis Pharmaceutical Inc.) on APC function and regulation of antigen-specific CD4+ T-cell responses. First, treatment of peritoneal elicited macrophages (PEM) or bone marrow derided dendritic cells (DCs) with increasing concentrations of LAQ842 resulted in enhanced acetylation of hystones H-2A, H-2B, H3 and H4. Analysis of the expression of MHC class molecules and co-stimulatory molecules revealed a significant increase in B7.2 and CD40 in LAQ842-treated APCs as compared to untreated APCs. Utilizing multi-template RNA probes and ELISA we found that LAQ842-treated APCs produce enhanced levels of several inflammatory mediators such as IL-1a, IL-1b, IL-6, TNF-a and RANTES relative to untreated APCs. Similarly, in response to LPS-stimulation, LAQ842-treated APCs produce significant higher levels of the pro-inflammatory cytokine IL-12 but reduce production of the anti-inflammatory cytokine IL-10 as determined by RT-PCR and ELISA. Furthermore, by chromatin immune precipitation (CHIP) assays we found that LAQ842-treated APCs display an increased acetylation of histones associated with the IL-12 promoter but a diminished acetylation of histones at the IL-10 promoter in response to LPS stimulation. Next, we evaluated whether the inflammatory APCs induced by LAQ842 were capable of effectively present antigen and prime productive antigen-specific T-cell responses. In vitro treatment of PEM or DCs with increasing concentrations of LAQ842 resulted in an enhanced presentation of HA-peptide to naïve CD4+ T cells specific for a MHC class II restricted epitope of influenza hemagglutinin (HA). Indeed, these clonotypic T cells display an enhanced HA-specific proliferation, IL-2 and IFN-gamma production relative to clonotypic T cells that encountered HA-antigen on untreated APCs. More importantly, LAQ842-treated APCs were able to restore the responsiveness of tolerant CD4+ T-cells isolated from lymphoma bearing hosts.

By demonstrating that HDAC inhibitor induces inflammatory APCs capable of restoring the responsiveness of tolerant T-cells, our studies have unveiled a previously unknown immunological effect of these agents and have broadened their clinical scope as promising adjuvants in cancer immunotherapy.