Intramuscular mRNA prime plus intranasal adenoviral vector booster elicit robust respiratory mucosal IgA responses against SARS-CoV-2

Current SARS-CoV-2 mRNA vaccines induce robust humoral and cellular immunity in the circulation, but its ability in eliciting respiratory mucosal immunity is less characterized. Here, we demonstrated that systemic mRNA expressing ancestral spike (mRNA-S) vaccination alone induced weak respiratory mucosal neutralizing antibody and cellular immunity, particularly against SARS-CoV-2 Omicron strain. In contrast, an immunization strategy combining systemic mRNA-S administration plus mucosal adenoviral vector expressing ancestral spike (Ad5-S) booster induced strong T cell, B cell, and antibody responses in the respiratory tract, which can last for a long time. Furthermore, we found that Ad5-S mucosal booster promoted robust IgA-producing B cells in the respiratory tract, which were correlated with the levels of mucosal S-specific IgA levels. Strikingly, these local IgA-producing B cells were more cross-reactive to the Delta and Omicron Spike proteins compared to those IgG-producing B cells. We further showed that CD4 T cell help was required for the development of IgA-producing B cell in the respiratory mucosa. Taken together, our study identified a vaccination strategy and its associated mechanisms for the induction of strong cross-reactive IgA responses, which were shown to correlate with optimal protection against breakthrough infection, especially by Omicron sub-lineages. Hence, our data provide insights into the rational design of next-generation SARS-CoV-2 mucosal vaccines required for the protection against infection by SARS-CoV-2 Omicron sub-lineages or future variants.