Protection studies following bronchopulmonary and intramuscular immunisation with yersinia pestis F1 and V subunit vaccines coencapsulated in biodegradable microspheres: a comparison of efficacy

We have compared the ability of intramuscularly and intratracheally administered recombinant F1 and V subunit antigens to safeguard mice from a lethal systemic challenge with plague. The combined subunits (1 microg V plus 5 microg F1) were inoculated either in the 'free' state as a solution, or entrapped within microspheres composed of a biodegradable polyester (Poly-L-lactide), on day 1 and 60 of the experiment. In comparison to the other regimens, introduction of microsphere suspensions into the respiratory tract resulted in statistically elevated levels of specific immunoglobulins in day 82 lung wash samples. A subcutaneous challenge with virulent Yersinia pestis bacteria on day 137, equivalent to more than 10(5) mouse LD(50)s, was comparatively well tolerated by all subunit treatment groups (with survival rates between 66 and 90%). In contrast, 80% of the mice injected intramuscularly with soluble F1 and V were defeated by a 10(7) MLD(50) subcutaneous challenge, whereas the group immunised intramuscularly with microparticles were significantly better protected (p<0.1) with 50% survival. Similarly, mice immunised intratracheally with microparticles were significantly better safeguarded (56% survival) compared with the group immunised with soluble subunits intramuscularly (p<0.01). Soluble sub-units delivered intratracheally afforded 33% protection against 10(7) MLD(50)s. These data indicate that bronchopulmonary administration of microsphere co-encapsulated recombinant F1 and V antigens elicits a similar level of protective immunity against systemic plague infection as that evoked by injecting co-encapsulated subunits into the muscle. Such findings corroborate the thesis that introduction of appropriately formulated F1 and V subunits into the respiratory tract may be an alternative to parenteral immunisation schedules for protecting individuals from plague.