Il-21 From Cd4 T Cells Drives Differentiation Of Brain-Resident Cd8 T Cells During Polyomavirus Cns Infection

Abstract CD4 T cells help CD8 T cells differentiate into competent effector and memory cells. We recently showed that CD4 T cells are essential for CD8 T cell differentiation into brain-resident memory (bTRM) during polyomavirus (PyV) infection. Here we identify IL-21 as the CD4 T cell help. Using MHC-II tetramers for two epitopes in mouse PyV, we show brain PyV-specific CD4 T cells express PD-1 and CXCR5. IL21-VFP reporter mice revealed CD4 T cells as the only cellular source of IL-21 in the brain. 2D micropipette adhesion assays, measuring TCR affinity, showed brain IL-21-producing CD4s have higher affinity TCRs than IL-21− cells. RNAseq analysis of IL-21+ vs. IL-21− brain CD4s corroborates that IL-21 production is positively associated with TCR affinity and that IL-21 producers are positively enriched for TFH signature genes. We further found that IL-21 receptor (IL21R) is required for CD8 TRM differentiation. Brain CD8 T cells upregulated IL21R at 15 days post-infection, coincident with expression of the TRM marker CD103. Few CD8 T cells in brains of IL-21R−/− mice expressed CD103, a finding confirmed using donor anti-PyV CD8 T cells lacking IL-21R in infected wild type (WT) recipients. Unlike IL21R-sufficient CD8s, IL21R−/− CD8 T cells are not maintained in the brain upon systemic CD8 antibody depletion. IL21R−/− CD8s also have a diminished response to a rechallenge in the brain than what we observed with WT CD8 T cells. RNAseq analysis revealed IL21R−/− CD8 T cells were negatively enriched for TRM core signature genes and oxidative metabolism genes compared to IL-21R-sufficient CD8 T cells. Thus, we conclude that IL-21 produced by high-affinity, PyV-specific CD4 T cells in the brain is required for differentiation of CD8 bTRM during PyV CNS infection.