Bromodomain and Extra-Terminal (BET) proteins are a highly conserved family of epigenetic ‘readers’ that recognise and bind acetylated lysine residues on histones and other proteins to modulate gene expression. BET proteins are enriched at enhancer regions regulating oncogenic transcription and inhibitors (BETi) displace BET proteins from chromatin leading to suppression of oncogenes. BETi have potent anti-inflammatory properties, including chromatin-independent downregulation of NF-kB signalling. However, broader mechanisms of immunomodulation by BETi in the context of anti-tumour responses remain poorly defined. Therefore we sought to evaluate the immunomodulatory activity of the prototypical BETi, JQ1.
Utilising the syngeneic model of transplanted Eμ-Myc aggressive B-cell lymphoma we first compared the efficacy of JQ1 in wild-type (immunocompetent) and immunodeficient RAG1-/- or RAG2-/-cγ-/- mice. We observed a 50% reduction in the survival advantage conveyed by JQ1 in mice deficient in T- and/or B-lymphocytes compared to immunocompetent controls. Notably, tumour-infiltrating lymphocytes from wild-type mice failing JQ1 therapy expressed high levels of PD-1, suggesting suppression of an endogenous anti-lymphoma immune response during disease progression. Having previously identified recurrent copy number amplification of PD-L1 (universally juxtaposed to the Eμ-Myc transgene), we hypothesised this endogenous host response may be dampened by the PDL1/PD1 axis and further modulated by JQ1.
Genome-wide analysis of the BETi induced transcriptional response identified the immune checkpoint ligand Cd274 (Pd-l1) as a MYC-independent, BETi target-gene. BETi were found to directly suppress both constitutively expressed and IFN-γ induced CD274 expression across a number of human and mouse tumor cell lines and primary patient samples. Mechanistically, BETi act by decreasing Brd4 occupancy at the Cd274 locus, resulting in transcriptional pausing and rapid loss of Cd274 mRNA production in both the constitutive and IFN-γ/IRF1-induced context. Importantly, we demonstrate that Cd274 regulation is MYC-independent and that in most human cancers MYC-levels do not correlate with CD274 expression.
We suggest that oncogenic PD-L1 transcription (including IFNγ-induced expression) is directly regulated by BRD4 and can be suppressed for therapeutic gain by BETi leading to augmented anti-tumour immunity, particularly in the context of immune checkpoint inhibitors.