Developmental Biology Of Leishmania Promastigotes

Niaid Extramural Activities

Investigators

Linked publications & trials

Abstract

We have developed the first reproducible model of Leishmania infection transmitted by actual sand fly bites, and have anlalyzed the host response in the skin at the site of an infective bite. Compared to naive mice, mice pre-exposed to the bites of uninfected flies showed a reduction in lesion pathology, a reduction in parasite load in the skin and a reduction in the ability to transmit Leishmania back to uninfected flies. The protection was associated not with the neutralization of an infection enhancing component, but with a strong saliva-specific DTH response at the site of the bite. This host response appears to reproduce the strong DTH reaction to sand fly bites that is well known amongst humans living in endemic regions. The protection in mice was also associated with a strong up-regulation of INF-g and IL-12 at the site of bite, suggesting that within this inflammatory setting, infected macrophages might be activated for early killing of the parasites. A salivary protein extracted from an SDS gel, having an apparent mol wt of 15kD (named SP15), was able to protect vaccinated mice challenged with parasites plus salivary gland homogenate. The aminoterminal sequence of the SP15 protein was determined by Edman degradation, and used to screen a 3-frame translation of a cDNA database originating from randomly picked clones from a unidirectionally cloned salivary gland cDNA library. The SP15 gene coding for the mature protein was inserted in the VR1020 vector, and the plasmid was used to vaccinate mice. Vaccinated mice developed a strong DTH reaction to sand fly bites, and were protected against challenge using L. major plus SGH. Leishmania promastigotes synthesize an abundance of phosphoglycans which are either present on the cell surface anchored by PI (lipophosphoglycan, LPG) or secreted as protein-containing glycoconjugates. The relative contributions of different phosphoglycan-containing molecules in Leishmania/sand fly interactions have been tested using mutants specifically deficient in either total phosphoglycans or LPG alone. These mutants have revealed dual roles for phosphoglycans as virulence molecules in the sand fly vector: the sereted molecules protect the parasite from proteolytic digestion in the bloodfed midgut, and the surface LPG mediates attachment of the parasite to the gut wall so as to prevent loss of infection during bloodmeal excretion. The role of LPG in mediating attachment to the midgut suggests that gut-associated lectins or lectin-like molecules, which have been described for sand flies, serve as parasite attachment sites, and that these molecules can vary between phlebotomine species. We have established cDNA libraries of sand fly midguts from newly emergent sand flies of different species, including P. papatasi, P. argentipes, P. sergenti, and Lutzomyia Longipaplpis. The libraries were constructed using the SMART cDNA library construction kit. The library of P. papatasi has been completed. About 500 randomly picked genes were sequenced from the 5' end of the gene followed by sequence similarity searches using the BLAST program. This was followed by blasting each sequence within the library with the remaining genes, thus grouping them in clusters. An important finding was the discovery of the gene encoding for galectin, a galactose-binding lectin. This gene was only found in the midgut of P. papatasi when compared to first round libraries of P. sergenti, P. argentipes and Lu. Longipalpis.