Acquired resistance and granuloma formation in experimental visceral leishmaniasis. Differential T cell and lymphokine roles in initial versus established immunity.

In naive BALB/c mice, acquisition of resistance to Leishmania donovani and formation of antileishmanial tissue granulomas are linked expressions that require both L3T4+ and Lyt 2+ cells as well as both IL-2 and IFN-gamma. To determine the mechanisms of established resistance to L. donovani, rechallenged immune BALB/c mice were treated with T cell- and lymphokine-depleting mAb or cyclosporin A. In the liver, resistance to rechallenge was inhibited by treatment with anti-Lyt 2 but not anti-L3T4 mAb. Resistance was also impaired by anti-IL-2 treatment but not by anti-IFN-gamma mAb. The hepatic granulomatous response to rechallenge, however, was not impaired by either anti-Lyt 2 or anti-IL-2 mAb nor by anti-L3T4 or anti-IFN-gamma treatment. In contrast, cyclosporin A suppressed granuloma formation but not antileishmanial activity. These results indicate a particularly important antileishmanial host defense role for Lyt 2+ cells and IL-2 in sensitized animals, and when compared to prior observations in L. donovani-infected naive mice, suggest that 1) discrete T cell- and lymphokine-dependent mechanisms are involved in initial acquisition of resistance vs established immunity, 2) more than one mechanism can mediate the development of tissue granulomas, and 3) granuloma formation by itself may not be required nor necessarily sufficient to confer antimicrobial activity.