Oral immunization of mice with gamma-irradiated Brucella neotomae induces protection against intraperitoneal and intranasal challenge with virulent B. abortus 2308.

Neha Dabral,Nammalwar Sriranganathan,Ramesh Vemulapalli,Martha-Moreno-Lafont Martha-Moreno-Lafont,Roy Martin Roop

doi:10.1371/journal.pone.0107180

Abstract

Brucella spp. are Gram-negative, facultative intracellular coccobacilli that cause one of the most frequently encountered zoonosis worldwide. Humans naturally acquire infection through consumption of contaminated dairy and meat products and through direct exposure to aborted animal tissues and fluids. No vaccine against brucellosis is available for use in humans. In this study, we tested the ability of orally inoculated gamma-irradiated B. neotomae and B. abortus RB51 in a prime-boost immunization approach to induce antigen-specific humoral and cell mediated immunity and protection against challenge with virulent B. abortus 2308. Heterologous prime-boost vaccination with B. abortus RB51 and B. neotomae and homologous prime-boost vaccination of mice with B. neotomae led to the production of serum and mucosal antibodies specific to the smooth LPS. The elicited serum antibodies included the isotypes of IgM, IgG1, IgG2a, IgG2b and IgG3. All oral vaccination regimens induced antigen-specific CD4+ and CD8+ T cells capable of secreting IFN-γ and TNF-α. Upon intra-peritoneal challenge, mice vaccinated with B. neotomae showed the highest level of resistance against virulent B. abortus 2308 colonization in spleen and liver. Experiments with different doses of B. neotomae showed that all tested doses of 109, 1010 and 1011 CFU-equivalent conferred significant protection against the intra-peritoneal challenge. However, a dose of 1011 CFU-equivalent of B. neotomae was required for affording protection against intranasal challenge as shown by the reduced bacterial colonization in spleens and lungs. Taken together, these results demonstrate the feasibility of using gamma-irradiated B. neotomae as an effective and safe oral vaccine to induce protection against respiratory and systemic infections with virulent Brucella.

Highlights

Brucella species, Gram-negative, facultative intracellular coccobacilli, are the causative agents of brucellosis, a chronic bacterial infection in a variety of mammals, including humans
As parenteral route of vaccination is seldom favored due to its invasive delivery and is unlikely to be a preferred route of immunization against brucellosis in humans, in this study we examined the feasibility of oral vaccination with gamma-irradiated B. neotomae and B. abortus RB51 in a primeboost approach to induce protection against systemic and mucosal challenge infections
Immunization of mice with IRBn, IRBn/RB51 and IRRB51/Bn resulted in the induction of significantly higher levels of serum IgG and IgM antibodies specific to LPS when compared with the saline inoculated mice and the group of mice vaccinated with IRRB51 (Fig. 1A)

Summary

Introduction

Gram-negative, facultative intracellular coccobacilli, are the causative agents of brucellosis, a chronic bacterial infection in a variety of mammals, including humans. B. melitensis, B. abortus and B. suis are the most frequent causes of human infections [1]. These Brucella species are categorized in the class B list of select agents by the CDC due to their highly infectious nature and their potential use in bio-warfare. Infected animals shed the bacteria in milk. Few human infections are documented to occur through accidental exposure to live bacteria in the laboratory or inoculation of live vaccine strains used for controlling animal brucellosis in the field. There is no vaccine available for use in humans against brucellosis

Methods

Results

Conclusion