The emerging clinical success of checkpoint blockade immunotherapy in human cancer patients has highlighted the critical importance of PD-L1, a ligand for the T cell inhibitory receptor PD-1, as a molecular target in cancer immunotherapy. PD-L1 is constitutively expressed in tumor cells and is also inducible by inflammatory cytokines such as IFNgamma. It has been proposed that tumor cells may sense the elevated IFNgamma from activated host T cells as a “threat” in the tumor microenvironment and adapt it by up-regulating PD-L1. The aim of this study is to elucidate the molecular mechanism underlying IFNgamma regulation of PD-L1 expression in tumor cells. We observed that PD-L1 is constitutively expressed, albeit at low level, in multiple types of cancer cells, including pancreatic, colon, breast, and sarcoma cells. IFNgamma treatment dramatically increased PD-L1 expression level and IFNgamma up-regulates PD-L1 expression through the Jak-STAT1 signaling pathway in vitro and in vivo. Chromatin immunoprecipitation assay did not identify IFNgamma-activated pSTAT1 binding to the pd-l1 promoter. Instead, we observed that IFNgamma activates IRF1 transcription and IRF1 is required for IFNgamma-induced PD-L1 expression. Chromatin immunoprecipitation analysis shows that pSTAT1 is associated with the irf1 but not the pd-l1 promoter. Analysis of the irf1 promoter DNA sequence revealed a pSTAT1-binding consensus sequence, and electrophoretic mobility shift assay indicates that pSTAT1 directly binds to this DNA element of the irf1 promoter. Furthermore, we demonstrated that IRF1 is associated with the pd-l1 promoter chromatin near the irf1 transcription initiation site. The pd-l1 promoter region contains a putative IRF1-binding consensus sequence and electrophoretic mobility shift assay shows that IRF1 binds to this DNA element of the pd-l1 promoter region. Taken together, our data indicate that IFNgamma activates pSTAT1 that binds to the irf1 promoter to activate irf1 transcription. IFNgamma-induced IRF1 then binds to the pd-l1 promoter to activate pd-l1 transcription in tumor cells.

Citation Format: Chunwan Lu, Amy V. Paschall, Priscilla S. Redd, Iryna Lebedyeva, Kebin Liu. IFNgamma regulates PD-L1 expression through activating IRF1 transcription in tumor cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2327.