Intranasal immunization with liposomes induces strong mucosal immune responses in mice

BALB/c mice were immunized intranasally with either soluble ovalbumin (OVA) or OVA entrapped in liposomes. The effect of adding Sigma cholera toxin B subunit (sCT-B), which contained low amounts of cholera holotoxin (CT), or recombinant CT-B (rCT-B) which was free from CT, as mucosal adjuvants was also investigated. The mucosal [lung enzyme-linked immunospot assay (ELISPOT), lung washing] and systemic (serum antibody and spleen ELISPOT) responses of immunized mice to OVA and CT-B were determined. Results showed that soluble OVA and liposome-entrapped OVA were poor inducers of mucosal or systemic responses unless CT-B was added as adjuvant. The types of responses augmented by sCT-B and rCT-B were different. CT-B containing low levels of CT (i.e. sCT-B) boosted both mucosal and systemic IgA and IgG responses, whereas rCT-B only increased IgG responses, unless antigen was entrapped in liposomes. Although rCT-B was unable to adjuvant IgA responses against soluble OVA, it was able to induce IgA responses against itself. These data show that mucosal responses can be increased by addition of CT-B containing low levels of CT to antigen preparations given intranasally, suggesting a direct role for CT-A in isotype switching. Furthermore, the ability of CT-B to adjuvant IgA responses against added antigens and its ability to induce responses against itself appear to be separate phenomena. The results from this study should assist the rational formulation of mucosal vaccines which induce potent mucosal and systemic immune responses.