RNA-binding Protein PCBP1/hnRNP E1 is an Intracellular Checkpoint for Balancing Effector versus Regulatory T Cells

Distinct lineages of T cells can act in response to various environmental cues to either drive or restrict immune-mediated pathology. Immunotherapy involving the block of cell surface immune checkpoints CTLA-4 and PD-1/PD-L1 has revolutionized cancer treatment across a wide spectrum of malignancies. However, additional immune regulatory mechanisms remain to be discovered, and will facilitate development of next generation agents to overcome resistance to current therapies. We now report that the RNA-binding protein, PCBP1, is critical for stabilizing effector T cell functions by limiting expression of T effector cell-intrinsic Treg-commitment programs and subverting expression of immune-suppressive signals. Indeed, T cell-specific deletion of Pcbp1 increased Treg development, enlisted multiple inhibitory checkpoint molecules including PD-1, TIGIT and VISTA on TILs, and thwarted anti-tumor immunity. Our results demonstrate a critical role for PCBP1 as an intracellular checkpoint for balancing effector versus Treg cells and also suggest PCBP1 as a novel target for cancer immunotherapy. Overall design: Spleen mRNA profiles of 6-8 weeks old wild type (WT) and Pcbp1f/fCD4-Cre male mice were generated by deep sequencing, in quadruplicate, using Illumina HiSeq3000.