Intra-tracheal, intra-nasal and intra-muscular immunisation with admixed Y. pestis sub-units (3 micrograms V, 0.47 microgram F1) or equivalent doses of
poly-L-lactide microsphere co-encapsulated
antigens was done. Systemic and mucosal responses to F1 and V differed according to immunisation route, and encapsulated status of the sub-units. Irrespective of immunisation site, particulated sub-units stimulated statistically superior primary systemic reactions, with intra-tracheal and nasal
microsphere immunisations eliciting superior serum anti-V
IgG titres in comparison to intra-muscular injection of free
vaccines (p < 0.001 beyond day 8). Pulmonary and nasal delivery of
microspheres induced primary serum anti-V
IgG titres which were greater (p < 0.039) or equal to (p > 0.056) those after intra-muscular injection of spheres. In terms of serum anti-F1 titres, mice responded best to intra-muscular, and comparatively poorly to intra-nasal immunisations. Intra-tracheal administration of
microspheres induced strongest responses in the respiratory tract, dominated by the
IgG rather than
IgA isotype. An intra-nasal booster immunisation on day 63 potentiated strong local and circulating anti-V
IgG titres in
microsphere vaccinees. Priming and boosting with free
vaccines induced significantly depressed secondary serum anti-F1 titres relative to
microsphere immunisations (p < 0.024 at days 78 and 120). In contrast to other priming sites, intra-tracheal instillation of encapsulated
vaccines facilitated the induction of
IgG antibody to both F1 and V in day 146 broncho-alveolal washings. With the exception of primary responses to F1 in mice immunised intra-tracheally with
microspheres,
IgG1 was the dominant subclass of anti-F1/V
IgG in serum. We conclude that introduction of biodegradable
microspheres containing the F1 and V sub-units into to the upper or lower respiratory tract engenders immune responses of a magnitude comparable with that induced by parenteral immunisation, and may present a means of protecting individuals from
plague.