Infection with Arginase-DeficientLeishmania majorReveals a Parasite Number-Dependent and Cytokine-Independent Regulation of Host Cellular Arginase Activity and Disease Pathogenesis

Abstract

The balance between the products of l-arginine metabolism in macrophages regulates the outcome of Leishmania major infection. l-arginine can be oxidized by host inducible NO synthase to produce NO, which contributes to parasite killing. In contrast, l-arginine hydrolysis by host arginase blocks NO generation and provides polyamines, which can support parasite proliferation. Additionally, Leishmania encode their own arginase which has considerable potential to modulate infectivity and disease pathogenesis. In this study, we compared the infectivity and impact on host cellular immune response in vitro and in vivo of wild-type (WT) L. major with that of a parasite arginase null mutant (arg−) L. major. We found that arg− L. major are impaired in their macrophage infectivity in vitro independent of host inducible NO synthase activities. As with in vitro results, the proliferation of arg− L. major in animal infections was also significantly impaired in vivo, resulting in delayed onset of lesion development, attenuated pathology, and low parasite burden. Despite this attenuated pathology, the production of cytokines by cells from the draining lymph node of mice infected with WT and arg− L. major was similar at all times tested. Interestingly, in vitro and in vivo arginase levels were significantly lower in arg− than in WT-infected cases and were directly correlated with parasite numbers inside infected cells. These results suggest that Leishmania-encoded arginase enhances disease pathogenesis by augmenting host cellular arginase activities and that contrary to previous in vitro studies, the host cytokine response does not influence host arginase activity.

Continual renewal and replication of persistentLeishmania majorparasites in concomitantly immune hosts

“…S4B). Lastly, although L. major also express arginase, promastigotes do not express enough to affect NO production by activated MΦs in vitro (41). Our studies revealed that arginase levels were reduced twofold in PIPs relative to cultured promastigotes (Fig.…”

Spermidine Synthase Is Required for Virulence of Leishmania donovani

“…Whether all of these enzymes are also essential for the amastigote to maintain an infection is less clear, because virulence data with knockout strains from several species have not offered a consistent conclusion. Parasite burdens and lesion sizes of mice infected with ⌬arg L. mexicana (23) or ⌬arg L. major (43,48) were only somewhat lower than those of mice infected with the corresponding wildtype line from which these mutants were derived. In contrast, parasite loads in mice infected with ⌬odc L. donovani (12) were dramatically reduced by many orders of magnitude compared to those in mice inoculated with the wild-type progenitor.…”

“…The genes encoding all four enzymes have been cloned, and conditionally lethal gene knockouts have been created in L. mexicana and L. major (⌬arg mutant only) (23,43,48,52) and in L. donovani (⌬odc, ⌬spdsyn, and ⌬adometdc mutants) (12,34,50,51) via double-targeted gene replacement. Growth studies with the mutant promastigotes revealed that each knockout was auxotrophic for polyamines as a consequence of the gene deletion events and that this nutritional deficiency could be circumvented by propagation in medium supplemented with an appropriate source of polyamine or polyamine precursor.…”

Genetically Modified Live Attenuated Leishmania donovani Parasites Induce Innate Immunity through Classical Activation of Macrophages That Direct the Th1 Response in Mice

“…Further, Th2 cytokine-induced enhanced host arginase activity during Leishmania infection is associated with depletion of L-arginine availability, thereby resulting in reduced levels of NO and enhanced production of polyamines for parasite survival (10,12,63). In contrast, another study group has shown that Leishmania major encodes arginase, which itself enhances disease pathogenesis by augmenting host cellular arginase activities in a parasite number-dependent manner (64). In our study, we found that live attenuated parasites were unable to replicate and suppressed antiinflammatory or Th2 cytokine production in BMDM, thereby downregulating the arginase-1 activity compared to results after WT infection in response to either LPS or rIFN-␥ stimulation.…”