Date Published: June 9, 2016
Publisher: Public Library of Science
Author(s): Christine Matte, Pierre-André Casgrain, Olivier Séguin, Neda Moradin, Wan Jin Hong, Albert Descoteaux, Ingrid Müller.
The protozoan Leishmania parasitizes macrophages and evades the microbicidal consequences of phagocytosis through the inhibition of phagolysosome biogenesis. In this study, we investigated the impact of this parasite on LC3-associated phagocytosis, a non-canonical autophagic process that enhances phagosome maturation and functions. We show that whereas internalization of L. major promastigotes by macrophages promoted LC3 lipidation, recruitment of LC3 to phagosomes was inhibited through the action of the parasite surface metalloprotease GP63. Reactive oxygen species generated by the NOX2 NADPH oxidase are necessary for LC3-associated phagocytosis. We found that L. major promastigotes prevented, in a GP63-dependent manner, the recruitment of NOX2 to phagosomes through a mechanism that does not involve NOX2 cleavage. Moreover, we found that the SNARE protein VAMP8, which regulates phagosomal assembly of the NADPH oxidase NOX2, was down-modulated by GP63. In the absence of VAMP8, recruitment of LC3 to phagosomes containing GP63-deficient parasites was inhibited, indicating that VAMP8 is involved in the phagosomal recruitment of LC3. These findings reveal a role for VAMP8 in LC3-associated phagocytosis and highlight a novel mechanism exploited by L. major promastigotes to interfere with the host antimicrobial machinery.
Phagocytosis plays a central role in linking innate and adaptive immunity [1, 2]. During this process, pathogens are internalized in a vacuole, the phagosome, which engages in a maturation program involving sequential interactions with various cellular compartments . These interactions lead to the acidification of the phagosome and the acquisition of an array of hydrolases, culminating in the generation of a microbicidal phagolysosome. Peptides generated through the degradation of microbial antigens are processed in the phagolysosome, loaded on MHC molecules, and transported to the cell surface to initiate an adaptive immune response [1, 3, 4]. Phagolysosome biogenesis thus represents an important means of controlling infections, but several pathogenic microorganisms have evolved mechanisms to subvert this process and cause disease .
Establishment of an intracellular infection by Leishmania promastigotes is characterized by an inhibition of phagosome maturation and function [7, 8, 34, 35]. Whilst phagosome remodeling is mostly achieved through LPG-mediated disruption of lipid rafts [10, 36], recent evidence indicates that the Leishmania metalloprotease GP63 contributes to this process through cleavage of membrane fusion regulators [7, 11, 15]. In the present study, we provide evidence that L. major promastigotes evade LC3-associated phagocytosis, a non-canonical autophagic process normally involved in the elimination of pathogens through the enhancement of phagosome maturation [18, 23]. We showed that recruitment of the autophagy protein LC3 to phagosomes containing L. major promastigotes is impaired through GP63-mediated cleavage of VAMP8. These findings are consistent with the notion that targeting components of the host cell membrane fusion machinery contributes to the ability of Leishmania promastigotes to alter phagosomal properties and modulate host immune responses during the establishment of infection [7, 15].