Investigation into the migration of Leishmania within Phlebotomine sand flies

Abstract

Protozoan parasites of the genus Leishmania are the causative agents of a wide spectrum of diseases from self-healing cutaneous leishmaniasis to visceral leishmaniasis. The parasites undergo a complex life cycle including motile and nonmotile cell types within the insect vector and vertebrate host. Within the insect vector, promastigotes generally migrate anteriorly along the gut as they undergo morphological changes from procyclic to nectomonad and later to metacyclic form of promastigotes. In order for the insect vector to transmit infective stage Leishmania promastigotes to the mammalian host via a blood feed, metacyclic promastigotes need to be located within the foregut. The study of the elicitors of migration within the sand fly alimentary canal have to date been fragmentary with no exploration of the different promastigote forms and the effects of the vast array of potential chemoeffectors present. Two Leishmania species were selected based on their migration properties in the sand fly gut. This study focussed on understanding the chemotaxis of different morphotypes of posterior migrating reptilian- pathogenic Leishmania tarentolae compared to the anterior migrating human pathogenic Leishmania mexicana within the biochemical gradients of the sand fly alimentary canal. This study explored the movement of both L. mexicana and L. tarentolae promastigotes towards a gradient of urea that may be found emitting from Malpighian tubules in the hindgut, the novel morphologies of L. tarentolae, the migration of procyclics, neptomonads, leptomonads and metacyclics, and the development of a novel microfluidic device for the study of chemotaxis in Leishmania. The results from the chemotaxic assays suggested that the migration of promastigotes occurred through the attraction towards cues such as the urea gradient from the Malpighian tubules and hindgut, and the sugars gradient from the diverticulum. These assays showed that L. tarentolae had a significantly higher attraction to urea and L.mexicana to sugars; confirming the species-specific differences between suprapylarian and hypopylarian parasites. Using different populations of L. mexicana and L.tarentolae promastigotes, a significant difference in migration between population based on age was observed. The results also suggested that a population rich in leptomonads and nectomonads had a higher migration and therefore a higher attraction towards the chemical cues. The results shed light on parasite migration that is dependent on the developmental stage of promastigotes as well as the speciesspecific cues. The role that the cues play in determining which Leishmania species can be transmitted via the bite of a sandfly are discussed.