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

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 . Nevertheless, other proteins, such as VapG, present in most virulent strains, are also encoded by vap genes located on the bacterial virulence plasmid. In the present study, we evaluated the effect of VapG immunization on the survival of -challenged mice. We used attenuated as a carrier for VapG (-+), a procedure previously adopted to develop a VapA-based vaccine. We observed that vaccination with -+ induced both an increased IFN-γ, IL-12, and TNF-α production, and a decreased bacterial burden in organs of the -challenged mice. Nevertheless, -+ vaccination protected only 50% of the mice challenged with a lethal dose of . Interestingly, we observed an increased frequency of B cells in the spleen of -+-vaccinated mice and showed that -+-vaccinated -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 -+ vaccination in conferring protection against infection, as well as the employment of VapG antigen for obtaining hyperimmune plasma to prevent rhodoccocosis in young foals.