Stepwise differentiation of follicular helper T cells reveals distinct developmental and functional states [scRNA-seq]
Follicular helper T (Tfh) cells are essential for the formation of high affinity antibodies. However, the signals that promote Tfh development after initial differentiation are unknown. Here we use fate mapping and reporter strategies to show that, after initial differentiation, Tfh cells transition from a progenitor-like Tfh (Tfh-Prog) stage to an IL-21-expressing fully developed Tfh (Tfh-Full) stage which is regulated by follicular regulatory T (Tfr) cells. Using single cell RNA sequencing and epigenomic analysis, we uncover additional developmental stages including an early Tfh transitory (Tfh-Trans) stage prior to full development, as well as an Tfh-Ex stage that occurs after full development marked by loss of IL-21 expression. The transcriptional and epigenetic landscape is formed in the Tfh-Transitory stage and is maintained throughout the Tfh-Ex state, suggesting terminal development. We also find the transcription factor Foxp1 regulates the transitions between these stages in Tfh cells to balance the ratio of stem-like progenitor and effector Tfh cells. Through selective in vivo deletion of Tfh-Full and Tfh-Ex cells, we show that these cells control antigen-specific germinal center formation, maintenance and somatic hypermutation at distinct stages of responses to a SARS-CoV-2 vaccine. Together, these studies demonstrate the transitionary phases of Tfh development and how control of the progression through these stages regulates humoral immunity. Overall design: Lymph node T follicular helper cells/duplicates or triplicates/Lymph node conventional CD4 T cells as controls