Antibodies targeting the immune check point inhibitor programmed death receptor 1 (PD-1) and its ligand (PD-L1) have been associated with relatively high durable response rates in most patients. 38 to 53% of melanomas express PD-L1 mostly associated with tumor infiltrating lymphocytes (TIL) which is believed to be induced by IFN-γ from T cells. Relatively few melanomas had (constitutive) PD-L1 expression in the absence of TILs.
The present study examines the signal pathways involved in inducible and constitutive expression of PD-L1 on cultured melanoma cells, including those derived from patients pre and post vemurafenib treatment. Our results suggest that nuclear factor kappa B (NF-kB) has a role in IFN-γ inducible expression, as shown by inhibition of PD-L1 expression with the inhibitor, BMS 345541 which blocks phosphorylation of the inhibitory IkB kinase, resulting in cytoplasmic retention of NF-kB p65 and p50 subunits. Promoter reporter assays confirmed an association between NF-kB activity and PD-L1 expression levels. Si RNA mediated knockdown of the subunits and transfection with the super repressor, Inhibitor of kappa B alpha, mutated at S32 and S36, to correlate with PD-L1 levels are being carried out to confirm the involvement of NF-kB in PD-L1 expression. We are also dissecting the signaling pathways involved in constitutive expression of PD-L1.
NF-kB is an important protein complex involved in adaptive and innate immune response and regulates cytokine and chemokine production in melanoma. It also contributes to induction of proteins involved in cell proliferation and survival, playing a role in resistance to targeted therapy. The present results highlight another important function of this crucial transcription factor which our recent studies show may be targetable by inhibitors of the bromo domain and terminal repeat (BET) reader proteins.