ICOS costimulation is indispensable for the differentiation of T follicular regulatory cells

Abstract

The inducible costimulator (ICOS) is a T cell costimulatory receptor critical for humoral immunity. In mice and humans, ICOS deficiency leads to a lack of T follicular helper (Tfh) cells and protective antibodies. However, the role of ICOS in T follicular regulatory (Tfr) cell generation and function remains poorly understood. Using Foxp3-cre-mediated ICOS knockout (ICOS FC) mice, we show that T regulatory (Treg)-specific ICOS deletion drastically reduces the number of Tfr cells without altering Treg cell numbers. Further, we observed a lowered ratio of antigen-specific germinal center B (GC B) cells and increased anti-nuclear antibodies in ICOS FC mice, suggesting a rise of autoreactive GC B cells. Single-cell transcriptome analysis revealed shifts in transitory Tfr precursor populations in immunized ICOS FC mice. Mechanistically, our data suggest that ICOS promotes the Treg-to-Tfr transition by regulating KLF2 and NFAT2 with downstream impacts on Bcl6 and CXCR5 expression. Importantly, we discovered a Tfr subset originating from Foxp3- precursors by analyzing flow cytometry data of immunized mice in which ICOS expression in Foxp3+ cells was ablated at different stages of GC reactions. Consistently, single-cell transcriptome analysis of global follicular T cells revealed a subset of Tfh-like cells beginning to express Foxp3 at day 6 post-immunization. These Tfh-like Tfr cells appear to promote GC B cell expansion and plasma cell differentiation in an ICOS-dependent manner. Taken together, we propose that Tfr cells derived from natural Tregs suppress the expansion of autoreactive GC B cells whereas Tfh-like Tfr cells promote early-stage GC output. Further, these Tfr subsets depend on ICOS costimulation for their differentiation or function.