Serum and mucosal antibody isotype responses to M-like protein (SeM) of Streptococcus equi in convalescent and vaccinated horses

Equine strangles, caused by the clonal pathogen Streptococcus equi, is a source of serious economic loss despite the widespread use of commercial vaccines. The anti-phagocytic 58 kDa M-like protein (SeM) is an important protective antigen. The objective of this study was to define differences, if any, between SeM-specific convalescent serum and mucosal IgA and IgG subisotypes and those induced by vaccination with commercial strangles vaccine. SeM-specific opsonophagocytic IgGb was the predominant serum antibody in horses intramuscularly vaccinated or recently recovered from infection. Infection also induced high levels of specific opsonophagocytic serum IgGa during and shortly after S. equi infection whereas vaccination stimulated only low levels of serum IgGa. Specific serum IgGc and opsonophagocytic IgA were present at very low levels following infection or vaccination. A strong specific mucosal antibody response occurred during the acute and convalescent phases of infection whereas vaccinated horses made no mucosal response. Specific IgGb was generally predominant in nasopharyngeal washings during the acute phase but was replaced by specific IgA during convalescence. SeM-specific mucosal IgGa and IgG(T) but not IgGc were detected only during the acute and early convalescent phase. The results therefore indicate that vaccination, although inducing SeM-specific serum isotype responses qualitatively and quantitatively similar to those seen in convalescence, did not induce mucosal responses. This suggests that mucosal immunity may be important in acquired resistance to strangles.