The role of leishmania proteophosphoglycans in sand fly transmission and infection of the Mammalian host

Abstract

Leishmania are transmitted by the bite of their sand fly vector and this has a significant influence on the virulence of the resulting infection. From our studies into the interaction between parasite, vector, and host we have uncovered an important missing ingredient during Leishmania transmission. Leishmania actively adapt their sand fly hosts into efficient vectors by secreting Promastigote Secretory Gel (PSG), a proteophosphoglycan (PPG)-rich, mucin-like gel which accumulates in sand fly gut and mouthparts. This has the effect of blocking the fly, such that during bloodfeeding both parasites and gel are co-transmitted in an act of regurgitation. We are discovering that this has further implications for the mammalian infection, again, in favor of the parasite. Experimentally, PSG exacerbates cutaneous and visceral leishmaniasis and can promote the chronicity of Leishmania infection, even in mouse strains normally capable of controlling leishmaniasis. The underlying mechanism of PSG's action is a major focus of our ongoing work. This review aims to synthesize what is known about the role and action of PSG and its constituent proteophosphoglycans, for parasite colonization of the sand fly, transmission, and mammalian infection. Lastly, we discuss potential exploitation of this important vector-transmitted product and future avenues of research.

Keywords: Leishmania; macrophage; promastigote secretory gel; proteophosphoglycan; saliva; sand fly; transmission; wound healing.

Figure 1

The known components of Leishmania transmission.

Figure 2

Structure of Leishmania filamentous proteophosphoglycan.

Figure 3

The PSG plug. (A) The typical position of Leishmania PSG in the gut of heavily infected sand flies (FG, foregut; TMG, thoracic midgut; AMG, abdominal midgut; HG, hindgut; CV/SV, cardiac or stomodeal valve; SG, salivary gland; scale bar: 1 mm). (B) Freshly dissected PSG from L. mexicana-infected Lu. longipalpis (scale bar: 100 μm). (C) Sagittal section through the anterior thoracic midgut of a heavy L. mexicana-Lu. longipalpis infection, showing numerous attached and unattached parasites (Pa) and occlusion to the stomodeal valve (Scale bar: 10 μm).

Figure 4

The blocked fly hypothesis and regurgitative model of Leishmania transmission. Female sand flies (Lutzomyia longipalpis is shown, scale bar: 1 mm) feed from a pools of blood (in red) in the skin. (A–D) Represent cross-sections through the sand fly head as it feeds and show how PSG promotes parasite regurgitation: During feeding, uninfected sand flies (A,B) draw blood into the pharynx which is diverted into the midgut through the one-way cibarial or stomodeal valve where it is digested. In heavily infected sand flies (C,D) Leishmania parasites attach to the valve and multiply in the anterior midgut. At the same time the parasites secrete filamentous proteophosphoglycan (fPPG) which condenses to form the promastigote secretory gel (PSG – in blue) obstructing the midgut and pharynx. Gel blockages have been observed in a large number of Leishmania-sand fly combinations described to date and characterized in both experimental (L. mexicana in Lu. longipalpis) and natural (L. major in P. papatasi, L. infantum in Lu. longipalpis) Leishmania-sand fly combinations. The PSG blockage forces open the cibarial valve allowing parasites and gel to reach the pharynx and foregut, allowing blood to mix with PSG when it is taken into the pharynx (C). Since PSG is highly soluble, when blood is eventually drawn through the parasite-gel obstruction a proportion of midgut parasites and PSG are regurgitated by backflow into the skin of the vertebrate host (D). Sand fly photo is courtesy of Prof. R.W. Ashford and the diagram is after Schlein et al. (1992).

Figure 5

Arginine and Leishmania survival. (A) l-arginine can have a direct influence on Leishmania intra-macrophage survival. l-arginine can influence the macrophage’s ability to host or kill Leishmania, depending on the cytokine milieu they are exposed to. In the classically activated state, macrophages can metabolize l-arginine into nitric oxide (NO) when they perceive Th1 cytokines or pro-inflammatory mediators. Nitric oxide is potently toxic to Leishmania amastigotes. In direct contrast, alternatively activated macrophages can catabolise l-arginine into polyamines when they experience Th2 cytokines/anti-inflammatory mediators. Polyamines are an essential food source for intracellular amastigotes. Promastigote secretory gel appears to promote the alternative activation of macrophages, corrupting them into “superfeeders” for the parasites they host (NOS2, nitric oxide synthase 2; OH-arg, naturally occurring endogenous inhibitor of arginase; nor-NOHA, selective competitive arginase inhibitor). (B) l-arginine can also influence the adaptive immune response indirectly by affecting T cell function. T cell proliferation, T cell receptor signaling, and cytokine secretion are acutely responsive to the levels of extracellular l-arginine. Within cutaneous Leishmania lesions of mice the accumulation of alternatively activated macrophages leads to the depletion of extracellular l-arginine. As a consequence, T cell function is severely impaired, and the long-term survival of the parasites enhanced. A role for PSG in this model of immunopathology for leishmaniasis has yet to be determined but is speculated to occur. The figures are after Kropf et al. (2005) and Müller et al. (2009).