LAUSR

Background: The chronic lymphocytic leukemia (CLL) tumor microenvironment (TME) is laden with hyporesponsive T-cells that permit disease persistence. Yet, redundant TME immunosuppressive mechanisms and epigenetic maintenance of T-cell exhaustion limit the efficacy of T-cell targeted therapies in CLL. Bromodomain and extra-terminal (BET) proteins regulate key pathways contributing to CLL pathogenesis and TME interactions, including T-cell function and differentiation. We hypothesize that blocking BET protein function can reverse T-cell exhaustion to yield durable tumor elimination in CLL. Methods: WT C57BL/6 mice were engrafted with Eμ-TCL1 spleen-derived lymphocytes, then treated daily with the novel pan-BET inhibitor, OPN-51107 (OPN5; 20mg/kg PO) for up to 4 weeks. Splenic gene expression was evaluated with the NanoString PanCancer iO360 panel. T-cell differentiation status, immune inhibitory receptor (IR) expression, proliferation (72 h ex vivo α-CD3/α-CD28 stimulation), and cytokine production (6 h ex vivo PMA/ionomycin stimulation) was measured via flow cytometry. CLL patient and healthy donor PBMCs were used for validation studies and to assess T-cell transcription factor (TF) expression via flow cytometry. Evaluation of BRD4 occupancy at select T-cell TFs via ChIP qPCR is ongoing. Results: OPN5 significantly increased cytotoxic cell signatures and reduced exhaustion-associated cell signatures in leukemic mice through inhibition of T-cell exhaustion signaling, as well as activation of Th1, natural killer cell, and IL-7 signaling pathways. Correspondingly, T-cells from OPN5-treated mice demonstrated greater ex vivo proliferative capacity and effector response to stimuli. A greater proportion of CD8+ T-cells from OPN5-treated mice were classified as naïve, and OPN5 significantly reduced KLRG1 expression on antigen-experienced CD8+ T-cells. Importantly, OPN5 curtailed IR co-expression (PD-1, PD-L1, VISTA, CD244, CD160, and LAG3) on splenic T-cells. These findings were confirmed with primary CLL cells ex vivo. OPN5 also impaired expression of terminal differentiation-associated TFs in CLL patient-derived T-cells, enriching for a TCF1+ progenitor T-cell population. While BTK inhibitors are known to similarly improve T-cell function in CLL, ibrutinib treatment was inadequate to revert CLL T-cell terminal differentiation. Future ATAC-sequencing analysis will inform how BET inhibition alleviates exhaustion-associated chromatin organization in CLL T-cells. Conclusion: BET inhibition dismantles immunosuppressive mechanisms in the CLL TME, alleviating CLL-induced T-cell dysfunction and terminal differentiation. These findings suggest that BET inhibition may be a useful component of combination strategies for the treatment of CLL to yield lasting anti-cancer immune memory and prevent relapsed/refractory disease. Citation Format: Audrey L. Smith, Alexandria P. Eiken, Sydney A. Skupa, Christopher R. D'Angelo, Avyakta Kallam, Matthew A. Lunning, Gregory Bociek, Julie M. Vose, Ben Powell, Gideon Bollag, Dalia El-Gamal. BET inhibition alleviates T-cell dysfunction in chronic lymphocytic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6396.