Posttranscriptional regulation of PD-1 by PRMT5/WDR77 complex shapes T cell effector function and antitumor immunity.

The regulation of the programmed cell death protein 1 (PD-1) gene, PDCD1, has been widely explored at transcription and posttranslational levels in T cell function and tumor immune evasion. However, the mechanism for PDCD1 dysregulation at the posttranscriptional level remains largely unknown. Here, we identify protein arginine methyltransferase 5 (PRMT5) as a RNA binding protein in a methyltransferase activity-independent manner, which promotes PDCD1 decay with WD repeat domain 77 protein (WDR77) and Argonaute2. Furthermore, the type-I IFN/STAT1 pathway transcriptionally activates PRMT5 and WDR77, thus enhancing PRMT5/WDR77 binding on a conserved AU-rich element of PDCD1 3' UTR. Functionally, conditional knockout of either PRMT5 or WDR77 in T cells disrupts T cell effector function and sensitizes the tumors to anti-PD-1 therapy. Clinically, PRMT5 and WDR77 expression in tumor-infiltrating T cells are negatively correlated with PDCD1 expression and renders tumors resistant to PD-1-targeted immunotherapy. Moreover, fludarabine targeting STAT1 in combination with anti-PD-1 has a synergetic effect on suppressing tumor growth in mice. Overall, this study reveals that the RNA binding-dependent function of PRMT5 regulates PDCD1 and T cell effector function with WDR77 and identifies potential combinatorial therapeutic strategies for enhancing antitumor efficacy.