Interleukin-12 regulates the response to chemotherapy in experimental visceral Leishmaniasis.

In experimental visceral leishmaniasis, interleukin (IL)-12 initiates control over Leishmania donovani via Th1 cell activation, interferon (IFN)-gamma secretion, and granuloma formation. Because the leishmanicidal effect of conventional therapy, pentavalent antimony (Sb), also requires T cells and endogenous IFN-gamma, we tested IL-12 as a determinant of host responsiveness to chemotherapy. L. donovani-challenged IL-12p35 gene knockout (KO) mice permitted uncontrolled hepatic infection and failed to respond to Sb. In contrast, 96% of liver parasites in KO mice were killed by amphotericin B, which acts independently of immune responses. Exogenous IL-12 combined with Sb was tested in normal mice: low-dose Sb was converted from weakly to strongly leishmanicidal, and a no-effect Sb dose was converted to approximately 100% leishmanistatic. IL-12 plus Sb synergism in normal mice was IFN-gamma dependent; however, IL-12 also increased responsiveness to Sb in IFN-gamma KO mice. Thus, IL-12 regulates host IFN-gamma-dependent and -independent responses that permit and/or enhance the leishmanicidal activity of Sb.

Visceral leishmaniasis in mice devoid of tumor necrosis factor and response to treatment.

Tumor necrosis factor (TNF)-deficient mice were challenged with Leishmania donovani to characterize TNF in the response of visceral intracellular infection to antileishmanial chemotherapy. In wild-type controls (i) liver infection peaked at week 2 and resolved, (ii) discrete liver granulomas developed at weeks 2 to 4 and involuted, and (iii) leishmanicidal responses to antimony (Sb), amphotericin B (AmB), and miltefosine were intact. In TNF knockout (KO) mice (i) initial liver infection was unrestrained, plateaued, and then declined somewhat by week 6, (ii) an absent early granulomatous reaction abruptly accelerated with striking tissue inflammation, widespread hepatic necrosis, and 100% mortality by week 10, and (iii) while the initial response to AmB and miltefosine was intact, killing induced by Sb therapy was reduced by >50%. Although initial AmB treatment during weeks 2 to 3 killed 98% of liver parasites, 75% of AmB-treated KO mice subsequently relapsed and died by week 12; however, additional maintenance AmB preserved long-term survival. These results for a model of visceral infection indicate that endogenous TNF is required early on to control intracellular L. donovani, support granuloma development, and mediate optimal initial effects of Sb and prevent relapse after ordinarily curative AmB treatment. A compensatory, TNF-independent antileishmanial mechanism developed in TNF KO mice; however, its effect was uncontrolled fatal inflammation. Chemotherapeutic elimination of the parasite stimulus reversed the hyperinflammatory response and preserved survival.