Harness RNA‐binding protein HuR to boost Anti‐PD‐1 Immunotherapy in immunologically cold tumors

Cancer immunotherapy, especially those targeting the immune checkpoint PD-1 and PD-L1 has emerged as a revolutionizing treatment modality. Immune checkpoint inhibitors (ICIs)-mediated anti-tumor response depends on T cell capability of recognizing and killing tumor cells. 'Hot' tumors, which are characterized by the T cell infiltration, show a better response rate to immune checkpoint blockade (ICB) treatment. In contrast, 'Cold' tumors have a particularly low objective response rate (ORR) to ICB. In consideration of the durable and impressive responses observed in 'hot' tumors, it will be valuable to make an endeavor to turn immunologically cold tumors into hot tumors. The RNA-binding protein HuR is a member of the embryonic lethal abnormal vision (ELAV) family that is overexpressed in a variety of cancers and promotes tumorigenesis by interacting with a subset of oncogenic mRNAs. In this study, we show the direct binding of HuR protein to PD-L1 mRNA in multiple types of cells by Ribonucleoprotein immunoprecipitation (RNP-IP) and further validated by RNA pulldown assay. We also show that PD-L1 mRNA decay is greatly expedited in HuR-knockout cells, which underlines the stabilization function of HuR on PD-L1 transcripts. Immunoblotting results demonstrate that PD-L1 protein expression level is decreased in HuR-knockout cells while up-regulated in HuR-proficient cells in comparison with control. These data together clarify HuR is engaged in PD-L1 post-transcription regulation. Based on this mechanism, we develop a combination strategy using HuR inhibitors and anti-PD1 antibody to overcome the limitation by regulating the tumor immune microenvironment. KH-200 is a compound that disrupts the transportation of HuR from nuclear to cytoplasm. To test the combination strategy in vivo, we established the EMT6 syngeneic breast cancer model. After 4-week treatment, the combination group shows great effects in inhibiting tumor growth and prolonging survival. The similar combination effect was further observed in Lewis lung syngeneic lung cancer model. In conclusion, we preliminarily explored the role of HuR in tumor immune evasion. The method using HuR inhibitors as a combination in immunotherapy shows an effective in immunologically cold tumor treatment and may have reference significance for clinical cancer therapy.