Follicular CD4 T cells tutor CD8 early memory precursors: An initiatory journey to the frontier of B cell territory

The early events of CD8 memory generation remain largely unknown. Here we report that as early as 2 days after antigen priming, very earlymemory precursors can be identified by their expression of the chemokine receptor CXCR5. These early precursors, which have an effector phenotype, expand and temporarily migrate to the T-B cell zone junction where they interact with follicular CD4(+) T cells (Tfh). Remarkably, this interaction with Tfh, hitherto considered as exclusive B cell helpers, is required for CD8 memory precursors to become highly competent memory cells. CD40 and interleukin-21 signaling are involved in the help provided to CXCR5(+)CD8memory precursors. This study thus unveils critical early steps in the generation of CD8memory, identifies CXCR5 as the earliest known marker of CD8 memory precursors, suggests a major helper role for Tfh, and points to possible coordination between the pathways of CD8 and B cell memory generation at the T-B-cell zone junction.