Mucosal nasopharyngeal immune responses of horses to protein antigens of Streptococcus equi.

Mucosal nasopharyngeal immunoglobulin A (IgA) and IgG responses to proteins of Streptococcus equi were studied in horses after the experimental production of strangles. S. equi-specific IgA and IgG titers in nasopharyngeal mucus were much higher in samples from animals 1 to 2 weeks after challenge than in samples from control animals. Although IgA was the major immunoglobulin in nasal mucus, there was more antibody activity associated with IgG as measured by radioimmunoassay. Great differences between the specificities of antibodies in nasal mucus and in serum were detected. IgA and IgG of mucus origin recognized only two major proteins with molecular weights of about 41,000 and 46,000 in acid extracts of S. equi and gave no detectable reaction with culture supernatant proteins. Only one protein of about 62,000 molecular weight was recognized in acid extracts of an equine strain of S. zooepidemicus. In contrast, immunoglobulins in serum recognized a great variety of proteins in culture supernatants and acid extracts of S. equi and S. zooepidemicus which did not include those proteins recognized by immunoglobulins in mucus. These findings provide good evidence for the independence of the local and systemic immune responses of the horse to S. equi. Horses rechallenged shortly after recovery from the first infection were resistant to challenge with an inoculum of S. equi 10 times greater than that to which they were originally susceptible. This resistance appeared to be independent of the levels of bactericidal antibody in serum. We therefore suggest that immunity to S. equi infection is mediated by locally produced nasopharyngeal antibodies.