Single-cell multiomic analysis revealed the differentiation, localization, and heterogeneity of IL10+ Foxp3– follicular T cells in humans

Germinal center (GC) reactions are tightly regulated to generate high-affinity antibodies. Although IL10+ Foxp3- follicular T cells have recently been described as contributing to the suppression of GC reactions, their differentiation, localization, and heterogeneity remain incompletely understood. Additionally, it remains unclear whether IL10+ Foxp3- follicular T cells represent a transient status or an independent subset. To address these gaps, we performed integrative single-cell analysis of transcriptomes, epigenomes, surface proteomes, and TCR repertoires in human tonsillar CD4+ T cells. Unbiased clustering revealed IL10+ Foxp3- follicular T cells as a transcriptionally and epigenetically unique subset. This subset exhibited features of both T follicular helper (Tfh) and T regulatory type 1 (Tr1) cells, and accordingly, hereafter we call them T follicular regulatory type 1 (Tfr1) cells. Analysis using imaging mass cytometry and spatial RNA-TCR sequencing demonstrated their presence within GCs in humans. Bioinformatic analysis suggested that Tfr1 cells differentiate from GC-Tfh cells upon strong TCR stimulation, a finding corroborated by mouse in vivo experiments and time-series single-cell RNA-TCR sequencing of human in vivo CD4+ T cells. Of note, our bioinformatic analysis suggested that Tfr1 cells receive strong TCR signals from ICOS-Lhigh GC-B cells, likely representing high-affinity GC-B cells. Finally, we show that Tfr1 cells acquire a resident memory phenotype following an effector phase. Together, our findings suggest that high-affinity ICOS-Lhigh GC-B cells transform follicular T cells from GC-Tfh cells to Tfr1 cells, which likely become memory cells and reside in the lymphoid organ to support effective antibody production.