Invade and Survival Strategy of Leishmania

Abstract

Leishmania parasite has a digenic life cycle with phlebotomine sand flies (Phlebotomus spp., Lutzomyia spp. and Psychodopygus spp.) as secondary invertebrate vector host. In the sand fly, parasites initially reside within the peritrophic membrane of the midgut after ingestion of an infected blood meal. After release from macrophages, differentiation of amastigote to promastigote occurs with concurrent synthesis of a thick glycocalyx coat composed of a variety of glycophospatidylinositol (GPI) anchor compounds. A layer of low molecular weight glycoinositol phospholipids (GIPLs) with barrier function is found below the glycocalyx. A lipophosphoglycan (LPG), containing a repeated polymer of disaccharide phosphate units, is the most abundant glycoconjugate (McConville and Ralton 1997). After 2 days, the peritrophic membrane is ruptured and the promastigotes attach to the mid gut wall through specific binding of LPG and rapidly divide. The division ceases after 4–7 days and parasite undergoes metacyclogenesis into infective metacyclic promastigote, which is unable of binding to the midgut wall due to structural alteration of LPG (Sacks et al. 1995). Then they migrate to the foregut and oesophagus, being suspended in saliva and ready to be inoculated. The insect cardiac valve is enzymatically damaged during the process that normally prevents reflux from gut to the pharynx (Schlein et al. 1992). The insect saliva promotes survival and development of promastigotes (Ghosh et al. 1995).