Abstract 4485: Regulation of chronic lymphocytic leukemia (CLL) immunobiology by histone deacetylase 6 (HDAC6)
Authors: Sahakian Eva , Maharaj Kamira , Powers John , Fonesca Renee M. , Deng Susan , Pinilla-Ibraz Javier , Quayle Steven N. , Jones Simon S. 
Published: July 2016

In CLL, an immunosuppressive phenotype enables the malignant B cell to evade immune detection leading to immune suppression. In recent years, epigenetic changes prompted by proteins known as histone deacetylases (HDACs) have gained special attention predominantly because of their active role in the regulation of pathogenesis and immune-related pathways in CLL; though the precise mechanism by which these regulatory events take place has yet to be elucidated. In addition to the well-recognized role of histone deacetylase inhibitors (HDACi) in the control of cell cycle and apoptosis, HDACi have a potential role in modulating the immunobiology of CLL. Remarkably, HDACi alter the inflammatory status of immune cells and tumor cells themselves. Among the family of HDACs, we have recently demonstrated that HDAC6 has a regulatory role in transcriptional regulation of IL-10, and is responsible for T cell anergy in murine and human antigen presenting cells.

Activation of T-cells is predominantly dependent on both co-stimulatory and co-inhibitory molecules, including PD-1/PD-L1 as well as CTLA4-B7, OX40-OX40L, CD40-CD154, GAL9-TIM3, and 41BBL/41BB. Furthermore, most of these molecules have been identified to have epigenetically induced changes in expression.

Previously we had reported that expression of HDAC6 is increased in CLL patient samples. Recent studies from our lab show that selective HDAC6 inhibitors modify the expression of immunomodulatory molecules, which may ultimately lead to increases in the immunogenicity of CLL. Thus far, findings from our lab reveal that CLL cells treated with HDAC6i show 1) a dose dependent cell kill, 2) a reduction of IL-10—an important cytokine in the regulation of cell proliferation in CLL, and 3) synergistic reduction of viability when combined with the BTK inhibitor ibrutinib. Subsequently, we have also demonstrated that MEC2-HDAC6KD cells exhibit an increase in MHCII and a decrease in PD-L1 expression. Interestingly, we also observed a decrease in the expression of PD-L1 and other immune checkpoint markers in CLL cell lines treated with low-doses of HDAC6i. Additionally, malignant B cells isolated from euTCL1 mice and treated ex vivo with HDAC6i become more immunogenic and elicit greater type I allogeneic T cell immune response. Lastly, utilizing our in vivo CLL murine models (euTCL1, and euTCL1-HDAC6KO), we have been able to demonstrate a reduction in circulating lymphocytes in euTCL1-HDAC6KO, as well as euTCL1 mice receiving systemic administration of HDAC6i.

In conclusion, selective inhibition of HDAC6 in CLL results in the reduction of co-inhibitory molecules, dose dependent increases in cell killing as single treatment as well as strong synergy when combined with ibrutinib. These findings could provide a successful combination immunotherapeutic strategy for the treatment of CLL.

Citation Format: Eva Sahakian, Kamira Maharaj, John Powers, Renee M. Fonesca, Susan Deng, Javier Pinilla-Ibraz, Steven N. Quayle, Simon S. Jones. Regulation of chronic lymphocytic leukemia (CLL) immunobiology by histone deacetylase 6 (HDAC6). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4485.