Abstract

The fact that live attenuated vaccine but not killed Brucella cells could induce protective immune response against Brucellosis implied that survival capability in host is important for the immune response induction. Member of the Omp25 protein family are shown to be important for Brucella virulence and immunological regulation. Omp25 mutants of virulent strains shows reduced virulence and could provide protection against virulent strain challenge, indicating that Omp25 is important for intracellular survival but dispensable for immunological protection. In the present study, to further define roles of Omp25 and clarify the possible protection mechanism of live vaccine strains, mutant strain M5Δomp25 of a live attenuated strain M5 was constructed and analyzed. The mutant was rapidly cleared from macrophage and mice when compared with wild type strain, indicating their reduced intracellular survival and virulence. The mutant induced decreased antibody responses and elicited lower level of IFN-γ and IL-10 in mice than their parent strain M5, implying reduced both humoral and cellular immunity induced by the mutant. Protection efficiency assay showed that M5Δomp25 could provide protection against M5 challenge but not the virulent strain 16M challenge. This indicated that sufficient survival capability is essential for induction of protective immune response for the live attenuated vaccine M5. This finding is helpful for screening and development of live attenuated or genetically marked vaccines.

Key words: Brucella, Omp25, intracellular survival, protective immune response.