Metacyclics, molecules and manipulation: Transmission of Leishmania by sand flies

Transmission of Leishmania by phlebotomine sand flies is a complex process that requires the parasite to overcome a variety of challenges. Parasite secretory molecules play an important role in transmission, and recent work in our laboratory has focussed on two of these: a chitinase and promastigote secretory gel (PSG). Leishmania mexicana episomally transfected with LmexCht1 (the L. mexicana chitinase gene) were generated and their ability to survive and grow within the permissive sand fly vector, Lutzomyia longipalpis was examined. Compared to control plasmid transfectants, the over-expression of chitinase was found to increase the average number of parasites per sand fly, accelerate the escape of parasites from the peritrophic matrix-enclosed bloodmeal, and increased damage to the structure of the stomodeal valve. When exposed to mice, flies infected with parasites expressing higher levels of chitinase parasites spent ~2.5 times longer in contact with their host during feeding and transmitted higher numbers of parasites, compared to flies with control infections. Another key feature of parasite development in the sand fly gut is the secretion of PSG, composed of a high molecular weight filamentous proteophosphoglycan. Using both experimental and natural parasite-sand fly combinations we showed that PSG secretion correlates with differentiation of mammal-infective transmission stages (metacyclic promastigotes). Further, Leishmania infection specifically caused an increase in vector biting persistence on mice (re-feeding after interruption), and promoted feeding on multiple hosts. Both of these aspects of vector behaviour were found to be finely tuned to the differentiation of parasite transmission stages in the sand fly gut. This crucial adaptive manipulation resulted in enhanced infection of experimental hosts thereby providing a selective advantage to the parasite.