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.

Gamma interferon-activated human macrophages and Toxoplasma gondii, Chlamydia psittaci, and Leishmania donovani: antimicrobial role of limiting intracellular iron.

Iron-saturated transferrin did not reverse the intracellular killing or inhibition of Toxoplasma gondii, Chlamydia psittaci, or Leishmania donovani by gamma interferon-activated human macrophages. Deferoxamine, an iron chelator, also did not impair replication within unstimulated macrophages. Limiting the availability of intracellular iron is an unlikely mechanism in human macrophage activity against these three diverse pathogens.

Response to chemotherapy in experimental visceral leishmaniasis: T cell-dependent but interferon-gamma- and interleukin-2-independent.

The capacity of Leishmania donovani-infected BALB/c mice to respond to conventional chemotherapy with pentavalent antimony (Sb) is T cell dependent and, in nude mice, can be restored in part by treatment with the T cell lymphokines, interferon-gamma (IFN-gamma) or interleukin-2 (IL-2). To document the presumed role of endogenous IFN-gamma and IL-2 in responsiveness to antileishmanial chemotherapy in the T cell-intact host, L. donovani-infected euthymic BALB/c mice were treated with anti-IFN-gamma or anti-IL-2 monoclonal antibodies (MAbs) before and after Sb administration. Treatment with MAbs exacerbated visceral infection but did not inhibit the in vivo efficacy of Sb. Thus, while combination therapy of Sb plus IFN-gamma or IL-2 may prove beneficial in T cell-deficient hosts with visceral leishmaniasis, T cell activities other than or in addition to IFN-gamma or IL-2 production may mediate in vivo responsiveness to antileishmanial chemotherapy in the euthymic host.

Requirement for T cells and effect of lymphokines in successful chemotherapy for an intracellular infection. Experimental visceral leishmaniasis.

Although directly microbicidal, pentavalent antimony has failed as treatment for visceral leishmaniasis in patients who also have AIDS or are receiving immunosuppressive therapy. To define the role of T cells in the successful host response to chemotherapy, we examined the efficacy of pentavalent antimony (sodium stibogluconate, Pentostam) in normal and T cell-deficient BALB/c mice infected with Leishmania donovani. In euthymic (nu/+) mice, single injections of 250 and 500 mg/kg of Pentostam induced the killing of 67% and 89% of intracellular liver amastigotes, respectively. In contrast, in athymic nude (nu/nu) mice, up to three injections of 500 mg/kg achieved no L. donovani killing and did not retard visceral parasite replication. Once nude mice were reconstituted with nu/+ spleen cells, however, Pentostam exerted strong leishmanicidal activity, an effect that appeared to be transferred by either L3T4+ or Lyt-2+ cells. Responsiveness to chemotherapy could also be induced by providing nude mice with either interferon-gamma or interleukin 2 alone. The absence of this T cell- and probably lymphokine-dependent mechanism is a likely explanation for treatment failures in immunocompromised patients infected with L. donovani and perhaps other systemic intracellular pathogens as well.

Role of tryptophan degradation in respiratory burst-independent antimicrobial activity of gamma interferon-stimulated human macrophages.

To determine whether extracellular tryptophan degradation represents an oxygen-independent antimicrobial mechanism, we examined the effect of exogenous tryptophan on the intracellular antimicrobial activity of gamma interferon (IFN-gamma)-stimulated human macrophages. IFN-gamma readily induced normal monocyte-derived macrophages (MDM) to express indoleamine 2,3-dioxygenase (IDO) activity and stimulated MDM, alveolar macrophages, and oxidatively deficient chronic granulomatous disease MDM to degrade tryptophan. All IFN-gamma-activated, tryptophan-degrading macrophages killed or inhibited Toxoplasma gondii, Chlamydia psittaci, and Leishmania donovani. Although exogenous tryptophan partially reversed this activity, the increases in intracellular replication were variable for normal MDM (T. gondii [5-fold], C. psittaci [3-fold], L. donovani [2-fold]), chronic granulomatous disease MDM (T. gondii [2.5-fold], C. psittaci [5-fold]), and alveolar macrophages (T. gondii [1.5-fold], C. psittaci [1.5-fold]). In addition, IFN-alpha and IFN-beta also stimulated normal MDM to express IDO and degrade tryptophan but failed to induce antimicrobial activity, and IFN-gamma-treated mouse macrophages showed neither IDO activity nor tryptophan degradation but killed T. gondii and L. donovani. These results suggest that while tryptophan depletion contributes to the oxygen-independent antimicrobial effects of the activated human macrophage, in certain cytokine-stimulated cells, tryptophan degradation may be neither sufficient nor required for antimicrobial activity.