Pleiotropic effects of BET inhibition broadly boost tumor immunogenicity to CD8+ T cells

BET inhibitors (BETi) have shown potential to augment tumor immunogenicity in melanoma. However, conflicting evidence exists regarding their precise mechanism of action, and their overall impact on melanoma immunogenicity and antitumoral T cell responses remains unclear. To address this, human melanoma cell lines treated with JQ1 and/or IFNγ were investigated for gene and protein expression changes in key pathways governing immunogenicity and cocultured with autologous tumor-infiltrating lymphocytes (TIL) with known antigen-specificity. JQ1-induced proteome-wide alterations were examined using mass spectrometry-based cellular thermal shift assay (MS-CETSA), which revealed that JQ1 broadly impacts melanoma immunogenicity by regulating IFN signaling, antigen processing and presentation, and innate immune signaling pathways. More specifically, JQ1 enhanced JAK1/STAT1 signaling and upregulated components of the HLA class I (HLA-I) antigen processing and presentation machinery (APM), increased MART-1 expression while concomitantly dampening tumoral expression of PD-L1, IDO1, and HLA class II (HLA-II). Functionally, JQ1 markedly improved tumor recognition by autologous MART-1- and neoantigen-specific CD8+ TIL, while dampening CD4+ TIL activation through the downregulation of Cathepsin S (CTSS). Preliminary results using JQ1-treated melanoma cells in a mixed lymphocyte-tumor cell culture (MLTC) markedly enhanced TIL proliferation and resulted in a T cell product enriched for CD8+ T cells. These findings reveal how the pleiotropic effects of BETi on melanoma cells broadly boost their immunogenicity towards CD8+ T cells and uncover novel pathways that might be therapeutically exploited to enhance CD8+ T cell-mediated anti-tumor immunity in ex vivo and in vivo approaches to cancer immunotherapy.