AUTHOR=Trevisani Marcel M. , Hanna Ebert S. , Oliveira Aline F. , Cardoso Silvia A. , Roque-Barreira Maria C. , Soares Sandro G. 

TITLE=Vaccination of Mice with Virulence-Associated Protein G (VapG) Antigen Confers Partial Protection against Rhodococcus equi Infection through Induced Humoral Immunity

JOURNAL=Frontiers in Microbiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.00857

DOI=10.3389/fmicb.2017.00857

ISSN=1664-302X

ABSTRACT=<p><italic>Rhodococcus equi</italic> is a facultative intracellular bacterium causing severe pyogranulomatous pneumonia, ulcerative enterocolitis, and mesenteric lymphadenopathy in foals aged less than 6 months. Less frequently, this pathogen affects various other species, such as pigs, cattle, cats, and even humans. Although rhodococcosis is treated with a combination of antimicrobial agents, resistance is developed in some cases, and thus, antimicrobial susceptibility must be monitored and managed. Considering these limitations of the current therapy and unavailability of a vaccine to prevent the disease, research is particularly focused on the development of an effective vaccine against rhodococcosis. Most vaccines undergoing development utilize the virulence-associated protein (Vap) A antigen, which was identified previously as a key virulence factor of <italic>R. equi</italic>. Nevertheless, other proteins, such as VapG, present in most virulent <italic>R. equi</italic> strains, are also encoded by vap genes located on the <italic>R. equi</italic> bacterial virulence plasmid. In the present study, we evaluated the effect of VapG immunization on the survival of <italic>R. equi</italic>-challenged mice. We used attenuated <italic>Salmonella</italic> as a carrier for VapG (<italic>Salmonella</italic>-<italic>vapG</italic>+), a procedure previously adopted to develop a VapA-based vaccine. We observed that vaccination with <italic>Salmonella</italic>-<italic>vapG</italic>+ induced both an increased IFN-γ, IL-12, and TNF-α production, and a decreased bacterial burden in organs of the <italic>R. equi</italic>-challenged mice. Nevertheless, <italic>Salmonella</italic>-<italic>vapG</italic>+ vaccination protected only 50% of the mice challenged with a lethal dose of <italic>R. equi</italic>. Interestingly, we observed an increased frequency of B cells in the spleen of <italic>Salmonella</italic>-<italic>vapG</italic>+-vaccinated mice and showed that <italic>Salmonella</italic>-<italic>vapG</italic>+-vaccinated <italic>R. equi</italic>-challenged B-cell-knockout mice did not reduce the bacterial burden. Given these results, we discussed the potential role of the humoral immune response induced by <italic>Salmonella</italic>-<italic>vapG</italic>+ vaccination in conferring protection against <italic>R. equi</italic> infection, as well as the employment of VapG antigen for obtaining hyperimmune plasma to prevent rhodoccocosis in young foals.</p>