The High Expression of PD-1 Defines A Subpopulation of Tfh Cells Responding to COVID-19 Vaccine in Humans.

Inactivated coronavirus disease 2019 (COVID-19) vaccines and receptor-binding domain subunit (RBD-subunit) booster vaccination can induce effective humoral immune responses. CD4+ T helper cells are essential for helping B cells and antibody responses. However, the response of CD4+ T cells to booster vaccination, especially the virus-induced T follicular helper (Tfh) cells, needs to be better characterized. In this study, we investigated this response using single-cell sequencing and flow cytometry. Additionally, we employed a customized algorithm to identify virus-induced T cell receptors (VI-TCRs), enabling further exploration of the activation and persistence of virus-induced CD4+ T cell responses. We identified a subset of classic Tfh (cTfh) cells with high expression of PD-1 and IFN-γ. These cells were notably activated following booster vaccination, and their proportion was correlated with antibody titers. Trajectory analysis of activated cTfh cells revealed a subset of virus-induced cTfh cells that might maintain immune responses beyond 90 days post-vaccination. In summary, we identified a group of PD-1high cTfh cells induced by COVID-19 vaccination, which can enhance humoral responses and exhibit the long-term persistence against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We also developed a method for single-cell immune data analysis to understand virus-induced immune responses. Understanding how cTfh cells help antibody production will provide essential insights into the rational design of new vaccine strategies to optimize long-term immunity.