Rhodococcus equi is a facultative intracellular bacterium that can cause
pneumonia in both young horses and immunocompromised humans. In this study, we have tried to determine the T-cell populations that recognize this pathogen during murine
infection, as well as the
bacterial antigens recognized by these cells. When BALB/c mice were hyperimmunized with a virulent R. equi strain, we did not observe preferential expansion of a particular T-cell subset in their spleens. However, when the splenic T lymphocytes of the hyperimmunized BALB/c mice were cultured in the presence of killed bacteria, we found that alpha/beta CD4+ T cells proliferated and exhibited increased levels of the
interleukin-2 receptor (IL2R). In order to ensure
antigen specificity, two different controls were included in these experiments: (i) T-cell proliferation and expression of the IL2R in the presence of the major membrane constituent of Bacillus megaterium were studied comparatively with the presence of the R. equi
bacterial antigen, and (ii) T-cell proliferation and expression of the IL2R from naive, non-infected mice in the presence of
bacterial antigens were compared to those observed in hyperimmunized mice. In our study, the T cells from hyperimmunized mice did not significantly proliferate nor were they activated in the presence of non-related
bacterial antigens, and T cells from naive mice were not found to significantly recognize R. equi
antigens. When we studied the localization of R. equi
antigens that could stimulate the in vitro proliferation and activation of T cells, we found that they were constituents of the bacterial cell wall and the cytoplasm, but they were not excreted in the culture medium. For these experiments, T-cell recognition of the
bacterial antigens in hyperimmunized mice was compared to that of naive mice. With T-cell immunoblotting, we found that T-cell proliferation and activation were obtained with
proteins having molecular masses of approximately 65, 43, 30, 22-27 and 15-17 kDa. It is noteworthy that among the recognized
bacterial antigens, some have been described as being associated with virulence.