Federica Sallusto

T follicular helper (T FH ) cells play an essential role in promoting B cell responses and antibody affinity maturation in germinal centers (GC). A subset of memory CD4 + T cells expressing the chemokine receptor CXCR5 has been described in human blood as phenotypically and clonally related to GC T FH cells. However, the antigen specificity and relationship of these circulating T FH (cT FH ) cells with other memory CD4 + T cells remain poorly defined. Combining antigenic stimulation and T cell receptor (TCR) Vβ sequencing, we found T cells specific to tetanus toxoid (TT), influenza vaccine (Flu), or Candida albicans ( C.alb ) in both cT FH and non‐cT FH subsets, although with different frequencies and effector functions. Interestingly, cT FH and non‐cT FH cells specific for C.alb or TT had a largely overlapping TCR Vβ repertoire while the repertoire of Flu‐specific cT FH and non‐cT FH cells was distinct. Furthermore, Flu‐specific but not C.alb ‐specific PD‐1 + cT FH cells had a “GC T FH ‐like” phenotype, with overexpression of IL21, CXCL13 , and BCL6 . Longitudinal analysis of serial blood donations showed that Flu‐specific cT FH and non‐cT FH cells persisted as stable repertoires for years. Collectively, our study provides insights on the relationship of cT FH with non‐cT FH cells and on the heterogeneity and persistence of antigen‐specific human cT FH cells.

Numerous safe and effective coronavirus disease 2019 vaccines have been developed worldwide that use various delivery technologies and engineering strategies. We show here that vaccines containing prefusion-stabilizing S mutations elicit antibody responses in humans with enhanced recognition of S and the S 1 subunit relative to postfusion S as compared with vaccines lacking these mutations or natural infection. Prefusion S and S 1 antibody binding titers positively and equivalently correlated with neutralizing activity, and depletion of S 1 -directed antibodies completely abrogated plasma neutralizing activity. We show that neutralizing activity is almost entirely directed to the S 1 subunit and that variant cross-neutralization is mediated solely by receptor binding domain–specific antibodies. Our data provide a quantitative framework for guiding future S engineering efforts to develop vaccines with higher resilience to the emergence of variants than current technologies.