Role of circulating T follicular helper subsets following Ty21a immunization and oral challenge with wild type S. Typhi in humans

Despite decades of intense research, our understanding of the correlates of protection against <i>Salmonella</i> Typhi (<i>S</i>. Typhi) infection and disease remains incomplete. T follicular helper cells (T_FH), an important link between cellular and humoral immunity, play an important role in the development and production of high affinity antibodies. While traditional T_FH cells reside in germinal centers, circulating T_FH (cT_FH) (a memory subset of T_FH) are present in blood. We used specimens from a typhoid controlled human infection model whereby participants were immunized with Ty21a live attenuated <i>S</i>. Typhi vaccine and then challenged with virulent <i>S</i>. Typhi. Some participants developed typhoid disease (TD) and some did not (NoTD), which allowed us to assess the association of cT_FH subsets in the development and prevention of typhoid disease. Of note, the frequencies of cT_FH were higher in NoTD than in TD participants, particularly 7 days after challenge. Furthermore, the frequencies of cT_FH2 and cT_FH17, but not cT_FH1 subsets were higher in NoTD than TD participants. However, we observed that ex-vivo expression of activation and homing markers were higher in TD than in NoTD participants, particularly after challenge. Moreover, cT_FH subsets produced higher levels of <i>S</i>. Typhi-specific responses (cytokines/chemokines) in both the immunization and challenge phases. Interestingly, unsupervised analysis revealed unique clusters with distinct signatures for each cT_FH subset that may play a role in either the development or prevention of typhoid disease. Importantly, we observed associations between frequencies of defined cT_FH subsets and anti-<i>S.</i> Typhi antibodies. Taken together, our results suggest that circulating T_FH2 and T_FH17 subsets might play an important role in the development or prevention of typhoid disease. The contribution of these clusters was found to be distinct in the immunization and/or challenge phases. These results have important implications for vaccines aimed at inducing long-lived protective T cell and antibody responses.