Induction of humoral immune response and protective immunity in chickens against Salmonella enteritidis after a single dose of killed bacterium-loaded microspheres

Formalin-inactivated Salmonella enteritidis phage type 4 strain 119/95 (SE) was encapsulated in biodegradable poly (DL-lactide co-glycolic acid) PLGA; (65:35) microspheres by a modified water-in-oil-in-water (w/o/w) double-emulsion solvent extraction/evaporation technique. These SE-loaded microspheres (SE-MS) were porous and spherical in shape with diameters of 0.4-10 μm and 20-80 μm in two preparations. SE-MS were subsequently used to vaccinate specific-pathogen-free chickens in a single dose in order to investigate the potency of a single-dose vaccination in inducing immune responses and protective immunity. In Experiment 1, 4-wk-old chickens that were vaccinated intramuscularly with 20-80-μm SE-MS generated long lasting (over 6 mo) and persistently high serum anti-SE immunoglobulin (Ig)G antibody response. In Experiment 2, 2-wk-old chickens were vaccinated orally with 0.4-10-μm or intramuscularly with 20-80-μm SE-MS and challenged with 10⁹ colony-forming units of homologous SE strain at 6 wk postvaccination. When challenged intramuscularly, one each of the orally vaccinated (n = 10) and the intramuscularly vaccinated birds (n = 10) showed clinical signs and death, whereas all of the nonvaccinated control birds (n = 12) were sick and 11 of them were killed. When challenge was via oral route, 26.1% of cloacal swabs and 24.0% of organs (liver, spleen, and cecum) collected from orally vaccinated birds (n = 35) were positive for SE, comparable to 27.9% of feces and 18.7% of organs from the intramuscularly vaccinated birds (n = 35). These figures were significantly lower than those for nonvaccinated birds (n = 30) from which 59.3% of feces and 44.0% of organs tested SE positive (P < 0.05). The humoral immune response was also determined after vaccination with a single dose. The intramuscular vaccination elicited higher serum IgG response than oral administration, but the latter elicited a significant intestinal mucosal IgA antibody response. This is the first evidence that chickens vaccinated with killed SE-loaded PLGA microspheres, intramuscularly and orally in a single dose, developed systematic and local immune responses, thereby conferring protective immunity.