Background: The development of vaccines is a promising and cost-effective strategy to prevent emerging multidrug-resistant (MDR) Acinetobacter baumannii  (A. baumannii ) infections. The purpose of this study was to prepare a multiepitope peptide nanovaccine and evaluate its immunogenicity and protective effect in BALB/c mice.
Methods: The B-cell and T-cell epitopes of Omp22 from A. baumannii  were predicted using bioinformatics methods and identified by immunological experiments. The optimal epitopes were conjugated in series by 6-aminocaproic acid and chemically synthesized multiepitope polypeptide rOmp22. Then, rOmp22 was encapsulated by chitosan (CS) and poly (lactic-co-glycolic) acid (PLGA) to prepare CS-PLGA-rOmp22 nanoparticles (NPs). The immunogenicity and immunoprotective efficacy of the vaccine were evaluated in BALB/c mice.
Results: CS-PLGA-rOmp22 NPs were small (mean size of 272.83 nm) with apparently spherical structures, positively charged (4.39 mV) and nontoxic to A549 cells. A high encapsulation efficiency (54.94%) and a continuous slow release pattern were achieved. Compared with nonencapsulated rOmp22, CS-PLGA-rOmp22 immunized BALB/c mice induced higher levels of rOmp22-specific IgG in serum and IFN-γ in splenocyte supernatant. Additionally, lung injury and bacterial burdens in the lung and blood were suppressed, and potent protection (57.14%-83.3%) against acute lethal intratracheal A. baumannii  challenge was observed in BALB/c mice vaccinated with CS-PLGA-rOmp22.
Conclusion: CS-PLGA-rOmp22 NPs elicited specific IgG antibodies, Th1 cellular immunity and protection against acute lethal intratracheal A. baumannii  challenge. Our results indicate that this nanovaccine is a desirable candidate for preventing A. baumannii  infection.
Keywords: A. baumannii , epitope vaccine, polymeric nanoparticles, PLGA, chitosan