RecombinantLeishmania majorSecreting Biologically Active Granulocyte-Macrophage Colony-Stimulating Factor Survives Poorly in Macrophages In Vitro and Delays Disease Development in Mice

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Abstract
Leishmaniais an intracellular pathogen that replicates inside macrophages. Activated macrophages produce a specific subset of cytokines that play an important role in the control ofLeishmaniainfections. As part of our interest in developing suicide parasites that produce abortive infections for the purposes of vaccination, we engineered recombinantLeishmania majorstrains producing biologically active granulocyte-macrophage colony-stimulating factor (GM-CSF). We showed that GM-CSF is being produced in the phagosomes of infected macrophages and that it can be detected in the culture supernatants of both infected macrophages and extracellular parasites. Our data support the notion that GM-CSF secreted by both developmental forms of recombinantL. majorcan activate macrophages to produce high levels of proinflammatory cytokines such as interleukin-1β (IL-1β), IL-6, and IL-18 and various chemokines including RANTES/CCL5, MIP-1α/CCL3, MIP-1β/CCL4, MIP-2/CXCL2, and MCP-1/CCL2, which enhance parasite killing. Indeed, GM-CSF-expressing parasites survive poorly in macrophages in vitro and produce delayed lesion development in susceptible BALB/c mice in vivo. Selective killing of intracellularLeishmaniaexpressing cytokine genes capable of activating cellular responses may constitute a promising strategy to control and/or prevent parasitic infections.

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