BET inhibition sensitizes immunologically-cold Rb-deficient prostate cancer to immune checkpoint blockade.

Non-T cell-inflamed immunologically "cold" tumor microenvironments (TME) are associated with poor responsiveness to immune checkpoint blockade (ICB), and can be sculpted by tumor cell genomics. Here we evaluated how Retinoblastoma (Rb) tumor suppressor loss of function (LOF), one of the most frequent alterations in human cancer and associated with lineage plasticity, poor prognosis and therapeutic outcomes, alters the TME, and whether therapeutic strategies targeting the molecular consequences of Rb loss enhance ICB efficacy. We performed bioinformatics analysis to elucidate the impact of endogenous Rb LOF on the immune TME in human primary and metastatic tumors. Next, we utilized isogenic murine models of Rb-deficient prostate cancer (PC) for in vitro and in vivo mechanistic studies to examine how Rb loss and bromodomain and extraterminal (BET) domain inhibition (BETi) reprograms the immune landscape, and evaluated in vivo therapeutic efficacy of BETi, singly and in combination with ICB and androgen deprivation therapy. Rb loss was enriched in non-T cell-inflamed tumors, and Rb-deficient murine tumors demonstrated decreased immune infiltration in vivo. The BETi JQ1 increased immune infiltration into the TME through enhanced tumor cell STING/NF-κB activation and type I interferon (IFN) signaling within tumor cells, resulting in differential macrophage and T cell-mediated tumor growth inhibition and sensitization of Rb-deficient PC to ICB. BETi can reprogram the immunologically cold Rb-deficient TME via STING/NF-κB/IFN signaling to sensitize Rb-deficient PC to ICB. These data provide the mechanistic rationale to test combinations of BETi and ICB in clinical trials of Rb-deficient PC.