Comparison of the efficacies of Rhodococcus equi recombinant vaccine in mice

Rhodococcus equi is an important bacterial pathogen and causes severe chronic granulomatous pneumonia in foals below 6 months of age. It has also become an opportunistic and emerging pathogen in immunocompromised humans. Vaccination is the most cost-effective strategy for controlling and preventing this infection. Although several potential virulence genes and candidate immunogens have been identified over the years, no effective vaccine is currently available to prevent R. equi disease in horses. Recently, bacterial vector vaccines have been shown to be promising for R. equi. In this study, the virulence-associated protein A (VapA) gene of R. equi was cloned into Protein Expression System small ubiquitin-related modifier (pET-SUMO) expression vectors and transferred into Escherichia coli BL21 (DE3). Also, adjuvant significantly affects the efficacy of recombinant vaccines. Therefore, native VapA and recombinant VapA were formulated with Immunostimuling Microparticle System (IMS 3012) or PetGel A (recommended for horses) and subcutaneously administered to mice. The immunization effect of four different vaccines was determined by assaying antibody titers and survival rates. The antibody response was slightly higher in the PetGel A formulations than IMS 3012. Survival rates were lower in the PetGel A formulations than IMS 3012. Given these results, recombinant VapA adjuvanted with PetGel A represents a promising formulation for developing new-generation R. equi vaccines.