Immunogenicity of synthetic Rhodococcus equi virulence-associated protein peptides in neonate foals

Rhodococcus equi infection is considered the most common cause of pneumonia in foals less than 6 months of age. Immunization of foals and/or mares may become a procedure of choice for prevention. The present work documents the antibody response of neonate foals to R. equi virulence-associated protein (Vap) vaccine candidate peptides. A mixture of 4 R. equi (ATCC 33701) Vap peptides was selected based on their hydrophilicity and recognition by naturally acquired IgG antibodies from 13 adult horses and 33 neonate foals from France and Japan. They were combined with a water-based nanoparticular adjuvant to promote a protective immune response including both Th1 cytokine pattern and antibody response. A single intramuscular injection resulted in an IgG antibody response 30 days later, although inconsistently. In responding animals, no bias in IgG subclass distribution was observed, and antibody response was associated with enhanced serum opsonic activity. In conclusion, data indicate that synthetic Vap peptides combined with nanoparticular adjuvant were immunogenic and resulted in a significant increase in IgG antibodies against the corresponding virulent R. equi strain in a majority of foals.