Roles of Endogenous Gamma Interferon and Macrophage Microbicidal Mechanisms in Host Response to Chemotherapy in Experimental Visceral Leishmaniasis

Abstract

In experimental visceral leishmaniasis, in which the tissue macrophage is the target, in vivo responsiveness to conventional chemotherapy (pentavalent antimony [Sb]) requires a T-cell-dependent mechanism. To determine if this mechanism involves gamma interferon (IFN-␥)-induced activation and/or specific IFN-␥-regulated macrophage leishmanicidal mechanisms (generation of reactive nitrogen or oxygen intermediates, we treated gene-deficient mice infected with Leishmania donovani. In IFN-␥ gene knockout (GKO) mice, Sb inhibited but did not kill intracellular L. donovani (2% killing versus 76% in controls). Sb was active (>94% killing), however, in both inducible nitric oxide synthase (iNOS) knockout (KO) and respiratory burst (phagocyte oxidase)-deficient chronic granulomatous disease (X-CGD) mice. Sb's efficacy was also maintained in doubly deficient animals (X-CGD mice treated with an iNOS inhibitor). In contrast to Sb, amphotericin B (AmB) induced high-level killing in GKO mice; AmB was also fully active in iNOS KO and X-CGD animals. Although resolution of L. donovani infection requires iNOS, residual visceral infection remained largely suppressed in iNOS KO mice treated with Sb or AmB. These results indicate that endogenous IFN-␥ regulates the leishmanicidal response to Sb and achieves this effect via a pathway unrelated to the macrophage's primary microbicidal mechanisms. The role of IFN-␥ is selective, since it is not a cofactor in the response to AmB. Treatment with either Sb or AmB permits an iNOS-independent mechanism to emerge and control residual intracellular L. donovani infection.

Inhibition of receptor tyrosine kinases restores immunocompetence and improves immune-dependent chemotherapy against experimental leishmaniasis in mice

“…Of note, it has been long recognized from studies in experimental VL that the efficacy of the antimonial drugs is exquisitely dependent upon host immune function, being modulated by a number of key T cell-derived cytokines (11,14,44). Although amphotericin B is less immune dependent in its mode of action in mice (11), clinical experience in the treatment of VL in HIV-infected patients, using this drug (72) as well as miltefosine (73), suggest that in humans, all antileishmanial drugs in current use probably have some degree of dependency on host immune effector mechanisms, particularly when measured in terms of the frequency of relapse. Hence, we believe that the restoration of immune competence described here could prove beneficial in combination with a variety of established therapeutic modalities.…”

“…The remaining mice received a single suboptimal (50 mg/kg) or optimal (500 mg/kg) dose of Sb v i.p. or saline, and at 35 dpi, parasite burden and percent inhibition relative to appropriate saline controls were determined (11). Data (mean ± SEM) were pooled from 2 independent experiments (n = 10 per group).…”

“…Conventional chemotherapy for leishmaniasis involves the use of pentavalent antimonial drugs (sodium stibogluconate; Sb v ), and treatment of leishmaniasis with Sb v represents a paradigm for cytokine-dependent chemotherapy (11,44). IFN-γ and TNF have been shown in vitro, in experimental models, and (in the case of IFN-γ) in humans to synergize with antimonial drugs (45)(46)(47), and blockade of IL-10 is associated with improved chemotherapy (14).…”

“…Drug toxicity, increasing drug resistance, and a paucity of new drugs on the horizon (8) have focused attention on the need to develop new combined approaches to therapy, including therapeutic vaccination, and on the development of dose-sparing regimens (9,10). The recognition that many antileishmanial drugs operate in synergy with host immune mechanisms (11) and some success with immunochemotherapy in humans (9,12) have fueled further interest in defining molecular targets for immunotherapy, with CD40, CTLA4, OX40L, and IL-10R (13)(14)(15)(16) all being validated in experimental models as candidates to boost T cell responses and enhance macrophage leishmanicidal activity.…”

Dual Action of Antimonial Drugs on Thiol Redox Metabolism in the Human Pathogen Leishmania donovani

“…In laboratory-derived trivalent antimony-resistant cell lines, these rate-limiting activities are increased severalfold, thereby increasing trypanothione concentrations (19 -21). Interferon-␥ (IFN-␥) induces reactive nitrogen intermediates via inducible nitric oxide synthase and reactive oxygen intermediates via the NADPH oxidase complex (phox) as well as increasing accumulation of antimony, but the relative contributions of these effects to the efficacy of Sb-killing remain unclear (51,52). Other abbreviations: Orn, ornithine; Spd, spermidine; TryS, trypanothione synthetase.…”