T-CELL RESPONSE DURING RHODOCOCCUS-EQUI INFECTION IN A MURINE EXPERIMENTAL-MODEL

Rhodococcus equi is a facultative intracellular bacterium that can cau se pneumonia in both young horses and immunocompromised humans. In thi s study, we have tried to determine the T-cell populations that recogn ize this pathogen during murine infection, as well as the bacterial an tigens recognized by these cells. When BALB/c mice were hyperimmunized with a virulent R. equi strain, we did not observe preferential expan sion 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 differe nt controls were included in these experiments: (i) T-cell proliferati on and expression of the IL2R in the presence of the major membrane co nstituent of Bacillus megaterium were studied comparatively with the p resence of the R. equi bacterial antigen, and (ii) T-cell proliferatio n and expression of the IL2R from naive, non-infected mice in the pres ence of bacterial antigens were compared to those observed in hyperimm unized mice. In our study, the T cells from hyperimmunized mice did no t 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 pr oliferation and activation of T cells, we found that they were constit uents of the bacterial cell wall and the cytoplasm, but they were not excreted in the culture medium. For these experiments, T-cell recognit ion of the bacterial antigens in hyperimmunized mice was compared to t hat of naive mice. With T-cell immunoblotting, we found that T-cell pr oliferation and activation were obtained with proteins having molecula r masses of approximately 65, 43, 30, 22-27 and 15-17 kDa. It is notew orthy that among the recognized bacterial antigens, some have been des cribed as being associated with virulence.