Intravacuolar Pathogens Hijack Host Extracellular Vesicle Biogenesis to Secrete Virulence Factors

Abstract

Extracellular vesicles (EVs) have garnered significant interest in recent years due to their contributions to cell-to-cell communication and disease processes. EVs are composed of a complex profile of bioactive molecules, which include lipids, nucleic acids, metabolites, and proteins. Although the biogenesis of EVs released by cells under various normal and abnormal conditions has been well-studied, there is incomplete knowledge about how infection influences EV biogenesis. EVs from infected cells contain specific molecules of both host and pathogen origin that may contribute to pathogenesis and the elicitation of the host immune response. Intracellular pathogens exhibit diverse lifestyles that undoubtedly dictate the mechanisms by which their molecules enter the cell's exosome biogenesis schemes. We will discuss the current understanding of the mechanisms used during infection to traffic molecules from their vacuolar niche to host EVs by selected intravacuolar pathogens. We initially review general exosome biogenesis schemes and then discuss what is known about EV biogenesis in Mycobacterium, Plasmodium, Toxoplasma, and Leishmania infections, which are pathogens that reside within membrane delimited compartments in phagocytes at some time in their life cycle within mammalian hosts. The review includes discussion of the need for further studies into the biogenesis of EVs to better understand the contributions of these vesicles to host-pathogen interactions, and to uncover potential therapeutic targets to control these pathogens.

Keywords: Leishmania; Mycobacterium; Plasmodium; Toxoplasma; exosomes; extracellular vesicles; infections.

Figure 1

Biogenesis of extracellular vesicles in intravacuolar pathogen infections. (A) Mammalian cell-derived extracellular vesicles include exosomes and microvesicles, which can be characterized by their biogenesis, size, and composition. While microvesicles are generated by the direct outward budding of the cell’s outer membrane, exosomes are derived from an endocytic origin. First, early endosomes (EE) undergo inward budding. This forms intralumenal vesicles (ILVs) inside what is then called the late endosomal vesicle (LE) or multivesicular body (MVB). Depending on molecular signals, the MVB may then be destined for degradation by fusing with lysosomes or will fuse with the plasma membrane, releasing the ILVs to the extracellular space where they are then called exosomes. In the context of intravacuolar pathogens, exosomes are a possible mechanism for the release of pathogen molecules from infected cells. The exact mechanisms for how these pathogen molecules escape their respective vacuoles and are trafficked to host exosomes is not fully understood, but it is likely to be unique for each pathogen as discussed in this review. The composition of pathogen-containing vacuoles is diverse and requires unique methods for the exchange of molecules between host and pathogen. Mycobacteria utilize secA, type VII secretion system (represented by blue channel), and possibly other secretion systems to secrete exosome-bound proteins from the Mycobacterium containing vacuole (MCV). Plasmodium containing vacuoles (PL-PVs) conversely contain pores (represented by dashed vacuole boarder) through which small molecules may pass freely between the PV lumen and host cytosol, presenting easy access of pathogen molecules to host cytosol and exosome processes. Leishmania parasitophorous vacuoles (LPVs) are dynamic compartments that interact with the host’s endocytic and secretory pathways. Multimembranous structures within LPVs whose cellular origin is not known, may transport molecules from LPVs to host cell organelles including MVBs. Dashed arrows represent speculative pathways for trafficking of pathogen molecules into host ILVs. (B, C) Exosome biogenesis can occur by two general mechanisms- either ESCRT-dependent or ESCRT-independent. (B) ESCRT, or endosomal sorting complexes required for transport, is a specialized multi-subunit complex which allows for the recruitment of ubiquitinated proteins and the inward budding and scission of ILVs. ESCRT-0 recruits ubiquitinated proteins while ESCRT-I, ESCRT-II, and ESCRT-III facilitate ILV budding, and finally Vsp4 facilitates membrane scission. Accessory proteins such as ALIX are also involved which perform deubiquitylation for cargo loading. (C) Less is currently known about the mechanisms of ESCRT-independent exosome biogenesis, however it is proposed that ceramide and other lipids, such as sphingomyelin and cholesterol, as well as tetraspanins play a part in the trafficking of proteins and inward budding. Created with BioRender.com.