New insights into follicular regulatory T cells in the intestinal and tumor microenvironments

Follicular regulatory T (Tfr) cells are a novel and unique subset of effector regulatory T (Treg) cells that are located in germinal centers (GCs). Tfr cells express transcription profiles that are characteristic of both follicular helper T (Tfh) cells and Treg cells and negatively regulate GC reactions, including Tfh cell activation and cytokine production, class switch recombination and B cell activation. Evidence also shows that Tfr cells have specific characteristics in different local immune microenvironments. This review focuses on the regulation of Tfr cell differentiation and function in unique local immune microenvironments, including the intestine and tumor.

The hippo kinase MST1 negatively regulates the differentiation of follicular helper T cells

Follicular helper T (T_FH ) cells are essential for inducing germinal centre (GC) reactions to mediate humoral adaptive immunity and antiviral effects, but the mechanisms of T_FH cell differentiation remain unclear. Here, we found that the hippo kinase MST1 is critical for T_FH cell differentiation, GC formation, and antibody production under steady-state conditions and viral infection. MST1 deficiency intrinsically enhanced T_FH cell differentiation and GC reactions in vivo and in vitro. Mechanistically, mTOR and HIF1α signalling is involved in glucose metabolism and increased glycolysis and decreased OXPHOS, which are critically required for MST1 deficiency-directed T_FH cell differentiation. Moreover, upregulated Foxo3 expression is critically responsible for T_FH cell differentiation induced by Mst1^-/- . Thus, our findings identify a previously unrecognized relationship between hippo kinase MST1 signalling and mTOR-HIF1α-metabolic reprogramming coupled with Foxo3 signalling in reprogramming T_FH cell differentiation.