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

Abstract

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

Protective immune responses against Rhodococcus equi : an innate immunity-focused review

“…In a study in which mice were vaccinated orally with Salmonellaserving as a carrier for VapG, vaccination resulted in increased INF-γ, IL-12, and TNF-α. 249 Upon challenge, vaccinated mice had reduced bacterial burden and TNF-α, and increased IL-12 and IFN-γ. 249 B-cell knock-out (KO) mice were also vaccinated and challenged; however, B-cell KO vaccinated mice did not have reduced bacterial burden.…”

“…249 Upon challenge, vaccinated mice had reduced bacterial burden and TNF-α, and increased IL-12 and IFN-γ. 249 B-cell knock-out (KO) mice were also vaccinated and challenged; however, B-cell KO vaccinated mice did not have reduced bacterial burden. 249 These findings suggested that humoral immunity was protective; however, B-cells also have roles other than antibody production, such as antigen presentation and T-cell activation.…”

“…249 B-cell knock-out (KO) mice were also vaccinated and challenged; however, B-cell KO vaccinated mice did not have reduced bacterial burden. 249 These findings suggested that humoral immunity was protective; however, B-cells also have roles other than antibody production, such as antigen presentation and T-cell activation. Unfortunately, this study did not evaluate innate immune cells.…”

Evaluation of vaccine candidates against Rhodococcus equi in BALB/c mice infection model: cellular and humoral immune responses

“…However, only a limited number of R. equi antigens have been reported and validated, among which virulence-associated proteins (Vaps) have been widely investigated in vaccine development to prevent R. equi infections. Vaps-based recombinant protein subunit vaccines, recombinant DNA vaccines, vector vaccines, and other engineered vaccines can induce specific humoral and cellular immune responses in the host to varying degrees, but they provide inadequate protection for foals [ 21 – 23 ]. Bacteria have a complete cellular structure, and the complexity of their composition makes it theoretically impossible for a single antigen to be better than inactivated or live attenuated vaccines.…”

Identification of vaccine candidates against rhodococcus equi by combining pangenome analysis with a reverse vaccinology approach

“…Traditional types of vaccines, including live, killed and attenuated (physical and chemical) vaccines, have proven to be ineffective [ 14 , 15 ]. In addition, second-generation molecular-based vaccines, including DNA, genetically attenuated and subunit vaccines, have provided inadequate protection of foals [ 16 , 17 ]. The development of an effective universal vaccine depends on identifying and selecting common protective antigens and epitopes [ 18 ].…”