Recombinant outer membrane protein 25c from Brucella abortus induces Th1 and Th2 mediated protection against Brucella abortus infection in mouse model

Development of a safe and efficacious vaccine for brucellosis is a long standing challenge for scientists. Recognizing potential antigens towards developing vaccine candidate is crucial. Omp25c, a porin protein of Brucella, is a paralog of two previously identified promising vaccine candidates namely, Omp25 and Omp31, with notable sequence identity. Also, Omp25c is conserved in all major Brucella species. This highlights the possibility of employing this protein in multivalent subunit vaccine based approach of Brucella management. In this study, we were interested in examining the immunogenicity and protective efficacy of Omp25c against Brucella infections. Recombinant unlipidated form of this antigen (rOmp25c) produced, upon intraperitoneal immunization in BALB/c mice along with Freund's adjuvant, was confirmed to be highly immunogenic; leading to high IgG antibody titers during the study duration. The IgG2a/IgG2b ratio of anti-rOmp25c antibodies revealed elicitation of Th2 based humoral immunity. Lymphocyte proliferation study divulged induction of specific memory response and secretion of both Th1-type (IFN-γ, GM-CSF and TNF-α) and Th2-type cytokine (IL-5) from restimulated splenocytes of rOmp25c immunized mice. CD4 T-cell subpopulation was comparatively increased than total B cell subpopulation in case of immunized mice, indicating the induction of strong cell-mediated (Th1 biased) immunity than humoral (Th2) immunity. The collective Th1 plus Th2 immune response specific to rOmp25c could be the reason for protection against Brucella challenge observed in mice groups that was comparable with S19 vaccine strain. Thus, the study encourages rOmp25c as a potent candidate vaccine against brucellosis.