Antibody response of horses to Rhodococcus equi antigens

The antigens extracted from strains belonging to seven capsular serotypes of Rhodococcus equi, as well as from two wild strains isolated from pneumonic foals, were examined. Whole-cell antigens and soluble products present in broth culture supernatants were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, electroblotted onto nitrocellulose, and stained with serum from hyperimmunized rabbits or foals. Foal sera used included sera from pneumonic animals with known titer to equi factors; from animals bled monthly on a farm with enzootic pneumonia, and from animals bled monthly on a farm with no history of R. equi pneumonia. The humoral response of foals to somatic antigen preparations was negligible, with few differences noted between sera from healthy, subclinically affected, and sick foals. The humoral response to R. equi broth culture supernatant products appeared more marked and was related to equi factor antibody titer. These findings suggest that the humoral response to R. equi whole-cell antigens is unimportant in protection against disease, which is consistent with the behavior of the organism as a facultative intracellular pathogen.