RB tumor suppressor promotes cancer immunity through downregulating PD-L1 expression

Aberrant expression of immune checkpoint protein programmed death ligand-1 (PD-L1) promotes immune tolerance in cancer. RB is a tumor suppressor known to regulate the cell cycle, DNA damage response, and differentiation. Here, we demonstrate transient knockdown or homozygous deletion of RB markedly induces PD-L1 mRNA expression. RB binds to NFκB protein p65 and serine-249/threonine-252 (S249/T252) phosphorylation of RB is important for its interaction with p65 and suppression of PD-L1 expression. RNA-seq analysis identifies a subset of NFκB pathway genes including PD-L1 are selectively upregulated by RB knockdown. S249/T252-phosphorylated RB inversely correlates with PD-L1 expression in patient samples. Expression of a RB-derived S249/T252 phospho-mimicking peptide blocks radiation-induced PD-L1 expression and increases the anti-cancer efficacy of radiation in mice. Our findings reveal a previously unappreciated tumor suppressor function of hyperphosphorylated RB in inhibition of NFκB activity and PD-L1 expression, suggesting this regulatory module can be exploited to overcome cancer immune evasion. Examination of RNA expression in PC-3 cells knocked down endogenous RB or treated cells with the CDK4/6 inhibitor palbociclib