Date Published: July 29, 2019
Publisher: Public Library of Science
Author(s): Guillermo Arango Duque, Armando Jardim, Étienne Gagnon, Mitsunori Fukuda, Albert Descoteaux, Dario S. Zamboni.
To colonize phagocytes, Leishmania subverts microbicidal processes through components of its surface coat that include lipophosphoglycan and the GP63 metalloprotease. How these virulence glycoconjugates are shed, exit the parasitophorous vacuole (PV), and traffic within host cells is poorly understood. Here, we show that lipophosphoglycan and GP63 are released from the parasite surface following phagocytosis and redistribute to the endoplasmic reticulum (ER) of macrophages. Pharmacological disruption of the trafficking between the ER and the Golgi hindered the exit of these molecules from the PV and dampened the cleavage of host proteins by GP63. Silencing by RNA interference of the soluble N-ethylmaleimide-sensitive-factor attachment protein receptors Sec22b and syntaxin-5, which regulate ER-Golgi trafficking, identified these host proteins as components of the machinery that mediates the spreading of Leishmania effectors within host cells. Our findings unveil a mechanism whereby a vacuolar pathogen takes advantage of the host cell’s secretory pathway to promote egress of virulence factors beyond the PV.
The protozoan parasite Leishmania causes a spectrum of human diseases known as the leishmaniases . Infectious insect-stage promastigotes are inoculated in mammals upon the blood meal of the vector and are internalized by phagocytes where they differentiate into mammalian-stage amastigotes . In contrast to amastigotes, promastigotes exist only transiently within mammals. To avoid destruction within phagocytes, promastigotes derail phagolysosome biogenesis and function, shut down microbicidal pathways, and sabotage immune processes [3–10]. Subversion of these host defence processes is achieved to a large extent through the action of the two major components of the Leishmania promastigote surface coat: the metalloprotease GP63 and lipophosphoglycan (LPG), a polymer of repeating Galβ1,4Manα1-PO4 units attached to the promastigote surface via a glycosylphosphatidylinositol anchor [11, 12]. Whereas GP63 acts by cleaving a set of host cell molecules that includes components of signaling cascades, transcription factors, and regulators of membrane fusion [13, 14], LPG inserts into lipid microdomains of host cell membranes, causing the disorganization of these structures [15, 16]. One direct consequence of LPG-mediated lipid microdomain disorganization is the inhibition of phagolysosome biogenesis , which is characterized by the exclusion of the v-ATPase and of the NADPH oxidase from the phagosome membrane [18, 19]. As LPG and GP63 are highly down-modulated or absent in mammalian-stage amastigotes [20, 21], their role is to create an environment permissive for the establishment of infection and propitious to promastigote-to-amastigote differentiation within phagocytes.
To manipulate host cell processes and pathways, intracellular pathogens translocate effector proteins in the cytosol of infected cells through the action of specialized secretion systems [25, 26]. The fact that Leishmania has no known such dedicated secretion apparatus prompted us to investigate the strategy used by these parasites to transfer virulence factors from their surface coat into their host cells. In the present study, we report that Leishmania promastigotes take advantage of the trafficking pathway between the PV and the ER/ERGIC to deliver LPG and GP63 beyond the PV where they can alter host cell processes (S9 Fig).