Abstract 4723: Combinatorial efficacy of anti-PD1 treatment and selective histone deacetylase 6 (HDAC6) inhibition in chronic lymphocytic leukemia (CLL)

A suppressive microenvironment allows CLL B cells to avoid immune surveillance. Coinhibitory antigens, anti-inflammatory cytokines and skewed immune populations support malignancy and contribute to poorer immune responses in CLL patients. Novel approaches to counter immunosuppression are therefore being explored. The HDAC family of proteins epigenetically regulate immune-related pathways in cancer and may be noteworthy immunomodulatory targets. In addition to the recognized role of HDACs in cell survival, HDACi may alter inflammatory status of immune and tumor cells. We have previously reported that HDAC6 regulates IL-10 transcription, and is responsible for T cell anergy in murine and human APCs. IL-10 is secreted by CLL B cells and is thought to influence immunosuppression. Therefore, we hypothesized that HDAC6i could reverse immunosuppression in CLL for antitumor benefit. The purpose of this study was to determine 1) the effects of selective HDAC6i on CLL immune biology and 2) whether combination with HDAC6i could improve efficacy of anti-PD1 for CLL therapy. To accomplish these aims, the euTCL1 and euTCL1/HDAC6KO adoptive transfer models of murine CLL were utilized. ACY738 (selective HDAC6i) was administered orally in chow at 25mpk daily. Anti-PD1 was administered intraperitoneally 3x per week at 3mpk. CLL surface antigens were analyzed via flow cytometry. Functional mixed lymphocyte assays were performed ex vivo, measuring IFNg production, and Th1/Th2 factors were examined via qRT-PCR in CLL T cells. Early PD-L1 expression on B cells correlated to CLL burden in euTCL1 mice. Post ACY738 treatment, euTCL1 B cells downregulated expression of PD-L1 and plasma IL-10. In addition, PD-1 and CTLA4 was decreased on CD4+, CD8+, Tregs and effector memory T cells. Splenic T cells from ACY738 mice upregulated TBET and downregulated GATA3, indicative of a less exhausted phenotype. Subsequently, euTCL1 B cells were treated ex vivo with ACY738 and cocultured with T cells. T cell activation (IFNg production) was increased after coculture with treated B cells compared to control B cells. We then hypothesized that ACY738 could be rationally be combined with anti-PD1 mAb for CLL treatment. Mice treated with anti-PD1 accumulated fewer CD5+ B cells, confirming that PD-1/PD-L1 blockade demonstrated anti-CLL benefit. Strikingly, combination of ACY738 and anti-PD1 further delayed the growth of CLL cells compared to either single agent. Consistently, anti-PD1 treatment in euTCL1/HDAC6KO mice further delayed CLL development. In conclusion, we report here that HDAC6i can reinvigorate CLL T cells, and combination of HDAC6i with anti-PD1 increased anti-CLL efficacy in vivo. This work provides evidence that this combination could be clinically investigated to simultaneously reduce tumor burden and improve immune function in CLL patients. Citation Format: Kamira Maharaj, John Powers, Alex Achille, Eva Sahakian, Javier Pinilla-Ibarz. Combinatorial efficacy of anti-PD1 treatment and selective histone deacetylase 6 (HDAC6) inhibition in chronic lymphocytic leukemia (CLL) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4723.