Intradermal immunization with outer membrane protein 25 protects Balb/c mice from virulent B. abortus 544.

Brucella abortus is a causative agent of brucellosis, a zoonosis affecting the endemic areas, which infects domestic animals as well as humans, thus, posing a potential bioterror threat. Outer membrane protein 25 is conserved among the Brucella species. Omp25 mutant strain of Brucella is shown to be attenuated in mice emphasizing on the role of Omp25 in Brucella virulence. Moreover, Omp25 has been shown to inhibit TNF-α production in human macrophages, thereby, abrogating cell mediated immunity. In this study, we evaluated the immunogenic potential of recombinant Omp25 and its protective efficacy against virulent B. abortus challenge in Balb/c mice. Recombinant Omp25 was administered via two routes of immunization: intraperitoneal and intradermal. Dosage reduction was observed with intradermal immunization when compared with intraperitoneal immunization. A higher IgG1:IgG2b ratio suggested a strong Th2 bias of immune response in both the routes of immunization. In vitro stimulation of splenocytes from immunized mice resulted in high level of IL-4 along with increasing levels of IL-12 and IFN-γ indicating a mixed Th1 and Th2 type of immune response. Immunized mice were challenged with virulent B. abortus and splenic colonization of B. abortus reduced significantly in intradermally immunized mice. Intradermal immunization gave protection comparable to that of B. abortus S-19 strain. Cytokine levels in spleen homogenate after challenge revealed a cell mediated immune response with elevated levels of IL-12 and IFN-γ but no detectable amount of IL-4. This can be a possible reason behind the protection observed in mice after rOmp25 immunization. Thus, our study proposes recombinant Omp25 to be a potential subunit vaccine candidate against brucellosis.