Vaccination establishes clonal relatives of germinal center T cells in the blood of humans

Germinal center T follicular helper cells (GCTfh) in lymphatic tissue are critical for B cell differentiation and protective antibody induction, but whether GCTfh establish clonal derivatives as circulating memory T cells is less understood. Here, we used markers expressed on GCTfh, CXCR5, PD1, and ICOS, to identify potential circulating CXCR5(+)CD4(+) Tfh-like cells (cTfh) in humans, and investigated their functional phenotypes, diversity, and ontogeny in paired donor blood and tonsils, and in blood after vaccination. Based on T cell receptor repertoire analysis, we found that PD-1–expressing cTfh and tonsillar GCTfh cells were clonally related. Furthermore, an activated, antigen-specific PD1(+)ICOS(+) cTfh subset clonally expanded after booster immunization whose frequencies correlated with vaccine-specific serum IgG; these phenotypically resembled GCTfh, and were clonally related to a resting PD1(+)ICOS(−) CD4(+) memory T cell subset. Thus, we postulate that vaccination establishes clonal relatives of GCTfh within the circulating memory CD4(+)CXCR5(+)PD1(+) T cell pool that expand upon reencounter of their cognate antigen.