Mucosal unadjuvanted booster vaccines elicit local IgA responses by conversion of pre-existing immunity in mice
Abstract
Mucosal delivery of vaccine boosters induces robust local protective immune responses even
without any adjuvants. Yet, the mechanisms by which antigen alone induces mucosal immunity in
the respiratory tract remain unclear. Here we show that an intranasal booster with an unadjuvanted
recombinant SARS-CoV-2 spike protein, after intramuscular immunization with 1 μg of mRNA–
LNP vaccine encoding the full-length SARS-CoV-2 spike protein (Pfizer/BioNTech BNT162b2),
elicits protective mucosal immunity by retooling the lymph node-resident immune cells. On
intranasal boosting, peripheral lymph node-primed B cells rapidly migrated to the lung through
CXCR3–CXCL9 and CXCR3–CXCL10 signaling and differentiated into antigen-specific IgA-
secreting plasma cells. Memory CD4+ T cells in the lung served as a natural adjuvant for
developing mucosal IgA by inducing the expression of chemokines CXCL9 and CXCL10 for
memory B cell recruitment. Furthermore, CD40 and TGFβ signaling had important roles in
mucosal IgA development. Repeated mucosal boosting with an unadjuvanted protein amplified
anamnestic IgA responses in both the upper and the lower respiratory tracts. These findings help
explain why nasal boosters do not require an adjuvant to induce robust mucosal immunity at the
respiratory mucosa and can be used to design safe and effective vaccines against respiratory
pathogens.