Assessment of the safety and immunogenicity of Rhodococcus equi-secreted proteins combined with either a liquid nanoparticle (IMS 3012) or a polymeric (PET GEL A) water-based adjuvant in adult horses and foals–identification of promising new candidate antigens.

This research aims to investigate the safety and effectiveness of using Rhodococcus equi-secreted proteins (ReSP) in combination with specific adjuvants as a potential vaccine against Rhodococcus equi pneumonia in horses and foals. The study revealed that using ReSP with the adjuvant Pet Gel A caused increased local inflammation, but also led to higher immune responses. In addition, cholesterol oxidase, mycolyl transferase 3, and PSP were identified as the most immunogenic compounds, indicating their potential as key components in developing an effective vaccine.

Study Objectives and Approach

• The primary aim of this study was to explore the safety and immunogenicity (ability to provoke an immune response) of R. equi-secreted proteins (ReSP) used along with Montanide™ IMS 3012 VG (a liquid nanoparticular adjuvant) or Montanide™ PET GEL A (a new polymeric adjuvant).
• Another goal was to identify the proteins that trigger the strongest immune response for use in future vaccines against Rhodoccocal pneumonia. For this purpose, they examined the safety of the vaccination in both adult horses and 3-week-old foals.
• A novel method based on ultrasound measurements was introduced for the safety assessment in adult horses. With this method, the researchers intended to evaluate possible changes at the injection site, skin and underlying muscle spot.

Important Findings

• The research suggests that combining ReSP and the Pet Gel A adjuvant increased local inflammation, indicating 4- to 7-fold higher levels of anti-R. equi IgGa, IgGb and IgGT, as compared with using ReSP alongside the IMS 3012 adjuvant. Despite the local inflammation, it didn’t have any effect on the behavior of the animals, suggesting its safety.
• In foals, the study has found a lower incidence of adverse tissue reactions at the injection site, with moderate and temporary swelling in the group that was given ReSP combined with Pet Gel A. This group showed immunization results similar to adults in IgGa and IgGT levels, but a lesser response in IgGb levels, indicating a possible bias towards resistance to R. equi.
• Using dot-blot screening, the researchers discovered cholesterol oxidase, mycolyl transferase 3, and PSP as the most immunogenic component from the crude extract of secreted proteins. These findings suggest that these proteins could be key in creating an effective vaccine against Rhodoccocal pneumonia.

Implications of the Study

• The results from this research are crucial in the development of a vaccine to combat Rhodococcus equi pneumonia in foals, a common cause of mortality.
• This study substantiates the use of R. equi-secreted proteins along with specific adjuvants to provoke immune responses in horses and foals.
• The observation of promising immunogenic properties in identified proteins provides new avenues for the development of effective vaccines against Rhodococcus equi pneumonia, thus addressing the rise in antibiotic-resistant strains.