Transmission of cutaneous leishmaniasis by sand flies is enhanced by regurgitation of fPPG

Abstract

Sand flies are the exclusive vectors of the protozoan parasite Leishmania1, but the mechanism of transmission by fly bite has not been determined nor incorporated into experimental models of infection. In sand flies with mature Leishmania infections the anterior midgut is blocked by a gel of parasite origin, the promastigote secretory gel (PSG)2,3. Here, we analyse for the first time the inocula from Leishmania mexicana infected Lutzomyia longipalpis sand flies. This revealed the size of the infectious dose, the underlying mechanism of parasite delivery by regurgitation, and the novel contribution made to infection by filamentous proteophosphoglycan (fPPG), a component of PSG found to accompany the parasites during transmission. Collectively, these results have important implications for understanding the relationship between parasite and its vector, the pathology of cutaneous leishmaniasis in humans and also the development of effective vaccines and drugs. These findings emphasise that to fully understand transmission of vector-borne diseases the interaction between all three participants must be considered.Leishmaniasis is a parasitic disease that currently infects some 12 million people worldwide, causing severe morbidity and mortality4. Infection is initiated by distinct life cycle stages, metacyclic promastigotes, that are introduced into the skin by fly bite along with sand fly saliva5-7. Leishmania are known to express various "virulence factors" in the sand fly, which may facilitate transmission to and infection of the mammalian host8-12. However, despite these discoveries our knowledge of parasite molecules that facilitate sand fly transmission is still limited. Furthermore, a number of key issues of transmission remain unresolved, such as the true infective dose, the mechanism of parasite delivery and the biological consequences of these upon infection. Significantly, in all Leishmania-vector combinations examined to date a gel-like plug, the parasite-derived PSG, blocks the anterior parts of the midgut coincident with the accumulation of metacyclic promastigotes2,3. An important structural component of PSG is fPPG, an unusual mucin-like glycoprotein unique to Leishmania13,14. Here we address these issues regarding transmission and reveal a novel contribution made by L. mexicana PSG to the infection process.Correspondence and requests for materials should be addressed to P. A.B. (pbates@liv.ac.uk To begin to understand the nature of the infective inoculum, the number and composition of L. mexicana parasites delivered during transmission was determined. A membrane feeding system was adapted to collect parasites egested by infected sand flies, revealing an average of 1086 parasites delivered per bite, highly enriched in metacyclic promastigotes (86-98%) ( Table 1). The only previous investigations quantitating egested parasites have been made using microcapillary forced feeding15,16. When this method was employed on L. mexicanainfected sand flies an average of 105 promastigotes were collected per...

Olfaction in vector-host interactions

“…Transmission occurs when these promastigotes are regurgitated within a gel-like material that fills the cardia and stomodeal valve, causing a blockage. This promastigote secretory gel (PSG) is formed by the parasites and has been shown to enhance cutaneous infections when deposited onto the skin with sand fly saliva (Rogers et al 2004, Titus 1998. A major component of PGS is a filamentous glycoprotein, proteophosphoglycan (fPPG) (Ilg et al 1996), which forms the 3D matrix of the plug that blocks the gut (Stierhof et al 1999) and keeps the valve open.…”

Two Functionally Divergent UDP-Gal Nucleotide Sugar Transporters Participate in Phosphoglycan Synthesis in Leishmania major

“…PPGs occur in different forms, including membrane-bound PPG, filamentous PPG, and secreted PPG (9). These PG-containing molecules play roles in parasite transmission and virulence following sand fly biting, such as modulating macrophage immune functions (11)(12)(13). The role(s) of secreted acid phosphatases in parasite virulence are not well understood, since their levels are relatively low in L. major (14), and secreted acid phosphatase null mutants in Leishmania mexicana are virulent (15).…”

Immunity to a salivary protein of a sand fly vector protects against the fatal outcome of visceral leishmaniasis in a hamster model

“…These components have been shown to enhance cutaneous leishmaniasis (CL) in mice (1)(2)(3). Saliva contains a variety of potent and pharmacologically active components that favorably change the environment at the feeding site (4-7).…”