Characterization of novel human Tfh cell subsets

Abstract

T follicular helper cells (Tfh) are critical for class switched affinity matured antibody production. Their developmental trajectory involves serial reciprocal interactions with antigen presenting cells, initially being dendritic cells, then B cells, with distinct developmental and functional outcomes characterizing each interaction. Thus, each step along the developmental trajectory represents a super-positioned precursor and functional state. However, it remains unclear if all cells harbor both precursor and functional capacities or these properties exist only within limited and discrete subsets. Evidence suggests that there is underappreciated heterogeneity with the Tfh compartment. To explore this, we conducted single cell RNA sequencing on human tonsil Tfh cells. We Identified extensive transcriptional heterogeneity within both extrafollicular and germinal center Tfh cells. While extrafollicular Tfh cells, commonly called preTfh, are thought to be precursors of GCTfh, they harbored additional subsets suggesting they may be precursors to multiple other lineages, including circulating Tfh (cTfh), cytotoxic CD4 T cells (CD4 CTLs), and regulatory follicular helper cells (Tfrs). Surprisingly, the vast majority of FOXP3+ Tfr were found within the extrafollicular compartment (as defined by flow cytometry, not location), raising the possibility that Tfr cells may regulate both extrafollicular and germinal center T:B collaborations. CXCL13 was found expressed almost only in a subset of GCTfh. Together, these data defined several novel subsets of Tfh cells and support a model whereby developmental and functional capacities are asymmetrically distributed within the Tfh compartment. Experimental analyses are required to validate the developmental relationship between these subsets and their functional properties.