Activation and induction of antigen-specific T follicular helper cells (TFH) play a critical role in LAIV-induced human mucosal anti-influenza antibody response.

There is increasing interest recently in developing intranasal vaccines against respiratory tract infections. Antibody response is critical in vaccine-induced protection and TFHis considered important in mediating antibody response. Most data supporting the role for TFHin antibody response are from animal studies, and direct evidence from humans is limited, apart from TFH-like cells in blood. We studied activation and induction of TFHand its role on anti-influenza antibody response by live-attenuated influenza vaccine(LAIV) in human nasopharynx-associated lymphoid tissue(NALT). TFHactivation in adenotonsillar tissues were analysed by flow-cytometry, and anti-hemagglutinin(HA) antibodies examined following LAIV stimulation of tonsillar mononuclear cells(MNC). Induction of antigen-specific TFHby LAIV was studied by flow-cytometry for induced TFHand CD154 expression. LAIV induced TFHproliferation which correlated with anti-HA antibody production, and TFHwas shown critical for antibody response. Induction of TFHfrom naïve T cells by LAIV was shown in newly induced TFHexpressing BCL6 and CD21, which was followed by the detection of anti-HA antibodies. Antigen specificity of LAIV-induced TFHwas demonstrated by the expression of antigen-specific T cell activation marker CD154 upon challenge by H1N1 virus antigen or HA. LAIV-induced TFHdifferentiation was inhibited by BCL6, IL21, ICOS and CD40 signalling blocking respectively, and that diminished anti-HA antibody production. CONCLUSION: We demonstrate for the first time the induction of antigen-specific TFHby LAIV in human NALT that provide critical support for anti-influenza antibody response. Promoting antigen-specific TFHin NALT by intranasal vaccines may provide an effective vaccination strategy against respiratory infections in humans.IMPORTANCE.Airway infection such as influenza is common in humans. Intranasal vaccination has been considered a more biologically relevant and effective way of immunization against airway infection. Vaccine-induced antibody response is crucial for protection against infection. Recent data from animal studies suggest one type of T cells, named TFHis important for the antibody response. However, data on whether this TFH-mediated help for antibody production operates in humans is limited, due to the lack of access to human immune tissue containing the TFHIn this study, we demonstrated the induction of TFHcells by an intranasal influenza vaccine in human immune tissue that provide critical support for anti-influenza antibody response. Our findings provide direct evidence that TFHcells play a critical role in vaccine-induced immunity in humans, and suggest a novel strategy to promote such cells by intranasal vaccines against respiratory infections