Histone Deacetylase 6 (HDAC6) As a Regulator of Immune Check-Point Molecules in Chronic Lymphocytic Leukemia (CLL)

Co-inhibitory receptors and ligands such as PD-1, PD-L1, TIM3 and LAG3 are commonly over expressed in malignancies resulting in immune-surveillance evasion and generation of a tumorigenic microenvironment. Targeting of these molecules has become a new therapeutic strategy demonstrating promising and favorable clinical outcomes in a variety of human cancers. Although, inhibitory antibody therapy has been successful for many patients, other subsets of patients receive no clinical benefit. This is potentially due to the activation of redundant untargeted co-inhibitory pathways. In CLL, an immunosuppressive phenotype enables the malignant B-cell to evade immune detection and obtain a vast array of immune regulatory mechanisms, causing T-cell dysfunction and ultimately immunosuppression. In fact CD8+ effector and terminal memory subsets are skewed and over-express PD-1, LAG3 and TIM3 in a Eu-TCL1 CLL mice model. Epigenetic changes triggered by proteins such as histone deacetylases are gaining special attention particularly because of their active role in the regulation of pathogenesis and immune-related pathways in CLL; however the exact mechanism of action is yet to be elucidated. Recently, we have identified one member of the HDAC family, HDAC6 which plays a role in the regulation of immunogenicity in CLL. Utilizing CLL murine models, CLL patient samples, and CLL cell lines we have found the following: First, HDAC6 is overexpressed in human CLL Second, using selective HDAC6 inhibitor (ricolinostat) we have been able to modulate the expression of co-inhibitory molecules in CLL. Indeed, treatment of MEC1 cells with varying doses of ricolinostat for 48hrs resulted in decreased expression of PD-L1 Interestingly in primary B-cells isolated from CLL patient samples; PD-1 expression was decreased when exposed to various concentrations of ricolinostat for 48hrs. Third, silencing of HDAC6 in the Eu-TCL1 mice (Eu-TCL1/HDAC6KO mouse model) results in 1) significantly lower levels of PD-L1 expression and 2) a restoration of the T-cell CD4:CD8 ratio, from 8:10 (Eu-TCL1) to 1:1 (Eu-TCL1/HDAC6KO). Although the overall absolute count of T-cells was observed to be higher than the wild-type, the memory compartment was returned to mostly naive T-cells, doubling in numbers from 4% of total lymphocytes to 9%. Consequently, a total reduction in surface expression of co-inhibitory molecules was observed on all T-cell memory subsets, but with substantial reduction in the CD8+ effector and terminal memory phenotype. In conclusion, selective inhibition of HDAC6 in both B and T-cell compartment of CLL results in the reduction of co-inhibitory molecules, which in combination with subsequent therapeutic regimen could provide a successful immunotherapeutic strategy for this disease.