Activation of TGF-β by Leishmania chagasi: Importance for Parasite Survival in Macrophages

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Abstract
TGF-β is a potent regulatory cytokine that suppresses expression of inducible NO synthase and IFN-γ, and suppresses Th1 and Th2 cell development. We examined whether functionally active TGF-β is present in the local environment surrounding the invading protozoan Leishmania chagasi. Our prior data showed that TGF-β levels are significantly increased in L. chagasi-infected mice. In the current study, we found TGF-β was also abundant in bone marrows of humans with acute visceral leishmaniasis but not in those of uninfected controls. Furthermore, L. chagasi infection caused an increase in biologically active TGF-β in human macrophage cultures without changing the total TGF-β. Therefore, we investigated the means through which leishmania could augment activated but not total TGF-β. Incubation of latent TGF-β with Leishmania sp. promastigotes caused active TGF-β to be released from the latent complex. In contrast, the nonpathogenic protozoan Crithidia fasciculata could not activate TGF-β. TGF-β activation by leishmania was prevented by inhibitors of cysteine proteases and by the specific cathepsin B inhibitor CA074. Physiologic concentrations of TGF-β inhibited killing of intracellular L. chagasi in macrophages, although the phagocytosis-induced respiratory burst remained intact. In contrast, supraphysiologic concentrations of TGF-β had no effect on parasite survival. We hypothesize that the combined effect of abundant TGF-β stores at extracellular sites during infection, and the ability of the parasite to activate TGF-β in its local environment, leads to high levels of active TGF-β in the vicinity of the infected macrophage. Locally activated TGF-β could, in turn, enhance parasite survival through its effects on innate and adaptive immune responses.