Functional and epigenetic studies reveal multistep differentiation and plasticity of in vitro and in vivo follicular T helper cells

Follicular T helper cells (Tfh) provide critical help to B cells for germinal center (GC) formation. Mutations affecting SLAM-associated Protein (SAP) prevent GC formation due to defective T-B cell interactions, yet effects on Tfh cell differentiation remain unclear. We describe the in vitro differentiation of functionally-competent “Tfh-like” cells that expressed Interleukin-21, Tfh markers, and Bcl6, and rescued GC formation in SAP-deficient hosts better than other T helper (Th) cells. SAP-deficient Tfh-like cells appeared virtually indistinguishable from wildtype, yet failed to support GCs in vivo. Interestingly, both Tfh-like and in vivo-derived Tfh cells could produce effector cytokines in response to polarizing conditions. Moreover, Th1, Th2 and Th17 cells could be reprogrammed to obtain Tfh characteristics. ChIP-Seq analyses revealed positive epigenetic markings on Tbx21, Gata3 and Rorc in Tfh-like and ex vivo Tfh cells, and Bcl6 in other Th cell populations, supporting the concept of plasticity between Tfh and other Th cells. Epigenetic modifications were evaluated in ChIP-Seq studies by profiling histone H3K4 and H3K27 trimethylation marks between Th1, Th2, Th17, Tfh-like, and CXCR5+PD-1+ sorted Tfh-like in vitro generated T helper cell populations and ex vivo derived Tfh cells.