Th1 effector cells and macrophages facilitate respiratory IgA after mucosal vaccination
Mucosal immunization and mucosal IgA offer significant promise in protecting against airborne pathogens, including SARS-CoV-2. However, the conditions and mechanisms that lead to the robust induction of mucosal IgA responses following mucosal vaccination remain poorly understood. It is also currently debatable whether mucosal vaccination is still warranted given that most individuals in developed countries have established a hybrid immunity from vaccination and infection. Here we characterized respiratory mucosal immune responses after SARS-CoV-2 infection, vaccination or both in humans. We found that hybrid immunity resulted in moderately increased mucosal IgA and neutralizing antibody responses compared to infection or vaccination alone. However, a direct comparison of hybrid immunity and a mucosal adenovirus-based booster vaccination in animal models revealed that respiratory booster immunization elicited markedly stronger and more durable mucosal IgA, T cell response, and protective immunity against SARS-CoV-2, supporting the promise of respiratory mucosal vaccination. Mechanistically, we found that mucosal booster immunization induced local IgA-secreting cells in the respiratory mucosa, aided by pulmonary CD4+ T cells in situ. Strikingly, local IL-21-producing Blimp-1+ Th1 effector cells were critical in mediating the CD4+ T cell help for IgA production. Furthermore, lung macrophages were important for this mucosal IgA response via the production of TGF-ß. Consequently, Spike mRNA coated with pulmonary surfactant (PS) containing liposomes, rationally designed to target lung macrophages, elicits a stronger mucosal IgA response. Collectively, our results uncover a local cellular network supporting enhanced mucosal IgA responses, with implications for the development of optimal mucosal immunization strategies against SARS-CoV-2 and other respiratory pathogens. Overall design: After mRNA-S plus Ad5-S immunization, CD4+ T cells in the BAL, lung parenchyma, and draining lymph nodes in IL-21-VFP reporter mice were enriched with negative selection by using biotin-labeled CD8a, TER-119, and B220 antibody and streptavidin magnetic beads (Miltenyi Biotec) before surface markers staining. Live CD4+CD44+VFP+ and CD4+CD44+VFP- cells were sorted by Sony MA900 Cell Sorter and then resuspended in Trizol for RNA extraction. Two replicates of each cell subset were obtained. RNA quality was assessed and sequenced using Illumina 2 x 150 platform provided by Admera Health. cDNA libraries construction, sequencing alignment to the mouse reference genome through services were provided by Admera Health. Differentially expressed genes between each group were identified using DESeq2. For functional analysis, Gene Set Enrichment Analysis was performed to identify enriched gene sets from MSigDB.