Abstract
All natural Leishmania infections start in the skin; however, little is known of the contribution made by the sand fly vector to the earliest events in mammalian infection, especially in inflamed skin that can rapidly kill invading parasites. During transmission sand flies regurgitate a proteophosphoglycan gel synthesized by the parasites inside the fly midgut, termed promastigote secretory gel (PSG). Regurgitated PSG can exacerbate cutaneous leishmaniasis. Here, we show that the amount of Leishmania mexicana PSG regurgitated by Lutzomyia longipalpis sand flies is proportional to the size of its original midgut infection and the number of parasites transmitted. Furthermore, PSG could exacerbate cutaneous L. mexicana infection for a wide range of doses (10–10,000 parasites) and enhance infection by as early as 48 hours in inflamed dermal air pouches. This early exacerbation was attributed to two fundamental properties of PSG: Firstly, PSG powerfully recruited macrophages to the dermal site of infection within 24 hours. Secondly, PSG enhanced alternative activation and arginase activity of host macrophages, thereby increasing L-arginine catabolism and the synthesis of polyamines essential for intracellular parasite growth. The increase in arginase activity promoted the intracellular growth of L. mexicana within classically activated macrophages, and inhibition of macrophage arginase completely ablated the early exacerbatory properties of PSG in vitro and in vivo. Thus, PSG is an essential component of the infectious sand fly bite for the early establishment of Leishmania in skin, which should be considered when designing and screening therapies against leishmaniasis.
Highlights
Leishmaniasis is a disfiguring and potentially fatal parasitic infection that affect some 350 million people worldwide [1]
We have shown that the quantity of promastigote secretory gel (PSG) in a sand fly midgut is dictated by the number of leptomonad promastigotes in the infection [2,5]
The findings of the present report reveal a crucial role for PSG delivered by infected sand flies on macrophages and the early establishment of leishmaniasis in the skin
Summary
Leishmaniasis is a disfiguring and potentially fatal parasitic infection that affect some 350 million people worldwide [1]. Leishmania are obligate intracellular protozoan parasites of macrophages that are transmitted between hosts by the bite of female phlebotomine sand flies and are delivered to a host as mammal-infective metacyclic promastigotes along with the saliva of the sand fly and a mucin-rich gel produced by the parasites in the sand fly midgut [2,3]. Leishmania major infected sand flies were found to inoculate a median of 5,000 to 10,000 parasites; with a minor proportion of flies able to deliver much higher doses of up to 100,000 parasites [4] Most of these parasites are likely to originate from a gel-like blockage which occludes the anterior midgut of the sand fly [3,7]. The active component of PSG was found to be its major constituent, filamentous proteophosphoglycan (fPPG), the immune mechanisms by which PSG promote Leishmania infection remain unknown
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