Elucidating the role of pre-existing immunity in mucosal lymphoid architectural dynamics following live attenuated influenza vaccination.

Mucosal immune responses in the upper respiratory tract are central to preventing influenza infection. The live attenuated influenza vaccine (LAIV) is delivered intranasally, making it a valuable model for investigating mucosal immunity in humans. In this study, we examined structural and cellular changes in palatine tonsils following LAIV vaccination in cohorts of children and adults undergoing tonsillectomy. Using quantitative immunofluorescence microscopy, we assessed germinal centre and mantle zone architecture over time and quantified the distribution of IgG- and IgA-positive cells within these compartments. We also developed an in-situ staining approach to detect hemagglutinin-binding cells in paraffin-embedded tonsillar tissue. Age-dependent differences in lymphoid architecture following LAIV vaccination were observed 3 days after vaccination in children; germinal centre sizes appear smaller and IgG and IgA frequency peaks. In contrast, in adults, germinal centre sizes and IgG and IgA frequency gradually increase with increasing time post-vaccination. Mantle zone areas in children and adults increased following LAIV. In addition, hemagglutinin-binding cells were detected in tonsillar tissue, and in some instances co-localized with IgA. Pre-existing immunity influences immune kinetics; our results suggest children rapidly generate de novo germinal centres and class-switched antibodies, in contrast to adults who may expand preexisting germinal centres and trend toward gradual increases over time.