Project Summary CD4+ T follicular helper (Tfh) cells are essential for germinal center (GC) responses and long- term humoral immunity. It has been proposed that Tfh cell differentiation is a multi-stage process that includes steps of PD-1+CXCR5+ pre-Tfh and PD-1hiCXCR5hi germinal center Tfh (GC-Tfh) cells. However, most studies have been using the broadly defined “CXCR5+ Tfh” cells, which starts to become too simplistic and may lead to confusion—particularly regarding the GC-Tfh cell differentiation and memory. Dendritic cells activate naïve CD4+ T cells and initiate Tfh cell differentiation including the induction of CXCR5 expression. The further GC-Tfh cell differentiation requires interactions with B cells. Our preliminary results suggest that instead of simple maintenance, pre-Tfh cells undergo substantial further differentiation to become GC-Tfh cells. To a large extent, however, little is known about the mechanism underlying this pre-Tfh to GC-Tfh transition. In this application, we aim to identify novel regulators in pre-Tfh to GC-Tfh cell differentiation. Meanwhile, we will also establish new model systems to study GC-Tfh memory. Our proposal has the potential to provide important knowledge on how to control the humoral immune response for the treatment of infectious diseases, autoimmune disorders, and to aid vaccine development for new pandemic threats.