Generation of protective immune responses to plague by mucosal administration of microsphere coencapsulated recombinant subunits

We have investigated noninvasive immunization to plague. Recombinant subunit antigens, F1 and V from Yersinia pestis, were coencapsulated in biodegradable poly(L-lactide) microspheres and intranasally administered to mice over a range of dose levels. Proteins retained antigenicity during, and postmicroencapsulation as evidenced by immunoblotting and capture enzyme- linked immunosorbent assay protocols. Supporting the rationale that a subunit antigen based vaccine for plague should contain both the F1 and V antigens, we observed that systemic antibody titres to V were improved by concomitant nasal immunization with F1. Conversely, serum titres to F1 were unaffected by the presence of V in the nasal inoculum. Interestingly, intramuscular injection of F1 augmented humoral immunity to nasally applied V antigen, suggesting that F1 adjuvantizes nasally instilled V even when introduced at a spatially distinct location. Although the magnitude of the specific serum response to nasally applied microspheres and equivalent doses of soluble subunits was not always directly proportional to administered dose and frequency of dosing, generally coencapsulated antigens evoked higher titred serum antibody responses. Also, when T-cell recall indices were measured they were found to be maximum in microsphere vaccinees. As few as two appropriately timed nasal inoculations of coencapsulated F1 and V afforded complete protection from >100 LD(50's) inhalational challenge with virulent Y. pestis. These data expand on previous findings from our laboratories, providing further insight into the mechanics of safeguarding mice from plague through nasal immunization. Further, these results demonstrate that in a murine model, solid protection from pneumonic plague can be engendered by two intranasal administrations of appropriately formulated recombinant proteins.