Project Summary T follicular helper (Tfh) cells are essential for germinal center (GC) responses and long-term humoral immunity. However, the complex regulation that determines the differentiation of Tfh cells, in particular, the initial CXCR5– versus CXCR5+ CD4+ T cell differentiation, the developmental progression of CXCR5+CD4+ T cells to become GC-Tfh cells, and the generation of follicular helper-like memory CD4+ T cells expressing CXCR5, are still not fully understood. Our proposal aims to fill in these knowledge gaps with long-term goals to identify novel genes/pathways underlying the CD4+ T cell differentiation. In our preliminary studies, we have discovered a novel network engaging various factors/pathways that fine-tunes CXCR5+ versus CXCR5– CD4+ T cell differentiation and regulates the generation of cytotoxic CD4+ T cells in the early stage of CD4+ T cell response. By combining RNA-seq, ATAC-seq, and single cell RNA-seq, our preliminary data also suggest that the PD-1+CXCR5+ pre-Tfh cells undergo substantial further differentiation to become PD-1hiCXCR5hi GC-Tfh cells. Additionally, we have generated novel “fate-mapping” reporter mice that will allow us to track the varied CXCR5– and CXCR5+ memory CD4+ T cells. Thus, in this application, we aim to dissect the molecular underpinning of the early stage CXCR5+ versus CXCR5– CD4+ T cell differentiation as well as to elucidate the mechanisms underlying the pre- to GC-Tfh differentiation and the generation of diversified CD4+ T cell memory. Our work will have a profound impact on the field of CD4+ T cell differentiation. The research will not only shed new light on our understanding of the mechanisms underlying the multiple steps of Tfh cell differentiation but also establish new model systems for memory CD4+ T cell studies. This proposal has the potential to provide important knowledge on how to control both the humoral and the cellular arms of the CD4+ T cell response to aid vaccine development for new pandemic threats and help the treatment of infectious diseases and autoimmune disorders.