Nasal immunization with Streptococcus mutans membrane vesicles and eggshell-derived hydroxyapatites elicits enhanced antigen-specific mucosal IgA responses

Nasal vaccines characterized by secretory IgA (SIgA) induction are effective in preventing mucosal infection. SIgA induction is potentiated by mucosal adjuvants, and the exploration of new safe and effective candidates is important. Nanosized hydroxyapatites (HAs) have the potential to enhance immune responses. The membrane vesicles (MVs) released from Streptococcus mutans UA159, a cariogenic bacterium, contain immunogenic glucosyltransferases (GTFs). We evaluated HAs made from eggshell waste, including nanosized particles, as a mucosal adjuvant against nasally administered MVs. Scanning electron microscopy revealed that the MVs were adsorbed to the eggshell-derived HA surface, and these complexes formed various sizes of aggregates. The coadministration of eggshell-derived HAs reinforced the MV-specific mucosal IgA and systemic IgG response, with levels nearly to those induced by the coadministration of polyriboinosinic polyribocytidylic acid, known as a mucosal adjuvant. MV-only immunization also induced MV-specific antibody responses, although the levels were lower. Immunization with MVs with or without eggshell-derived HAs induced IFN-γ/IL-17 secretion. Biofilm formation by the S. mutans UA159 gtfBgtfC double-deficient strain with wild-type-derived MVs containing GTF-B and GTF-C was inhibited by the induced IgGs via MV neutralization. These results suggest that eggshell-derived HAs are potentially effective mucosal adjuvants against S. mutans MVs coadministered nasally.