Role of T-lymphocyte subsets in Rhodococcus equi infection.

Rhodococcus equi, a facultative intracellular gram-positive bacterium, can induce life-threatening infections in immunocompromised patients, especially those with AIDS. We have studied the mechanism of acquired immunity to this pathogen in a murine model. Protective immunity was induced by live but not killed bacteria. Adoptive transfer of resistance was obtained with spleen cells but not immune serum from mice immunized intravenously 30 days earlier with live bacteria. In normal mice, an intravenous challenge of 5 x 10(6) CFU of R. equi was cleared from the spleen, liver, and lungs within 3 weeks, whereas athymic nude mice were unable to clear the bacteria. In vivo depletion with monoclonal antibodies showed that both CD4+ and CD8+ T-cell subsets participate in the clearance of bacteria and that CD8+ T cells play the major role.