Avirulent strains of Toxoplasma gondii infect macrophages by active invasion from the phagosome

Abstract

Unlike most intracellular pathogens that gain access into host cells through endocytic pathways, Toxoplasma gondii initiates infection at the cell surface by active penetration through a moving junction and subsequent formation of a parasitophorous vacuole. Here, we describe a noncanonical pathway for T. gondii infection of macrophages, in which parasites are initially internalized through phagocytosis, and then actively invade from within a phagosomal compartment to form a parasitophorous vacuole. This phagosome to vacuole invasion (PTVI) pathway may represent an intermediary link between the endocytic and the penetrative routes for host cell entry by intracellular pathogens. The PTVI pathway is preferentially used by avirulent strains of T. gondii and confers an infectious advantage over virulent strains for macrophage tropism.virulence | Trojan horse | apicomplexa | phagocytes P hagocytosis is one of the most ancient defense mechanisms for the host to destroy invasive pathogens. However, most intracellular pathogens exploit this very pathway for internalization and survival in phagocytes (1). A notable exception to this paradigm is the infection pathway used by apicomplexan parasites-exemplified by Toxoplasma gondii. The current consensus model of T. gondii infection suggests that the parasite actively invades host cells by forming a moving junction (MJ) at the host cell surface (2). Penetration of T. gondii through this junction is largely driven by its own actin motor complex (3). The parasitophorous vacuoles formed by this pathway are nonfusogenic with the host endocytic system, thus evading lysosome-mediated destruction (4-6). However, recent reports including the findings that host F-actin participates in entry by T. gondii (7), and that the parasite is still able to infect cells, albeit much less efficiently, even without some key components of the invasion machinery (8), suggest the existence of alternative infection pathways for Toxoplasma. The active penetration model was defined largely by using nonphagocytic host cells and hypervirulent strains of the parasite. Unlike their virulent counterparts, the interaction of avirulent Toxoplasma strains with macrophage and dendritic cell results in heightened innate cytokine and chemokine production (9) and the development of a "hypermotile" host cellular phenotype (10), which promotes the control of acute infection and mediates dissemination into sites of parasite latency (11,12). Here, we investigated whether avirulent parasites interact with phagocytic host cells in a fundamentally different way from the outset. We found that the avirulent Toxoplasma strains infect macrophages initially via phagocytosis and subsequent active penetration from within the phagosome to form a parasitophorous vacuole. This hybrid invasion pathway may represent an intermediary link between the endocytic and the penetrative routes for host cell entry by intracellular pathogens. ResultsTo investigate whether avirulent Toxoplasma (PTG, type II strain) uses the active penetration p...

Genetic impairment of parasite myosin motors uncovers the contribution of host cell membrane dynamics to Toxoplasma invasion forces

“…Since uptake of tachyzoites through a transient phagosome has been reported [5, 22], it was pertinent to analyze in detail the fate of the Δ MyoA parasites once enwrapped within PM ruffles. Monitoring host cell PM dynamics allowed us to distinguish the outward collar of ruffles from a single PM invagination that hinted at a nascent PV membrane.…”

“…However, it has not been investigated so far whether a contribution from the host cell could potentially account for the residual invasiveness of MyoA -deficient Toxoplasma . Yet, phagocytosis-mediated uptake of live zoites into phagocytes followed within a few minutes by egress from the early phagosome into a second vacuole, namely the PV, has already been reported for motor-competent virulent [5] and avirulent [22] tachyzoites. We therefore decided to re-evaluate the role of T. gondii zoite motors during invasion by applying high resolution live and fixed imaging in conjunction with functional assays to compare how motor-competent and MyoA - or MyoB/C -deficient tachyzoites access a growth-compatible PV in non-phagocytic cells.…”

ToxoplasmaGRA15 and GRA24 are important activators of the host innate immune response in the absence of TLR11

“…T cells constitute the largest fraction of PBMCs (roughly ¾ th), and IFNγ production by these cells is primarily determined by IL12 and IL18-mediated activation of STAT4, p38 MAPK, NFκB, and activator protein 1 (AP-1) family of transcription factors, respectively (70). It has been shown that within human PBMCs Toxoplasma preferentially infects monocytes but lymphocytes were also infected at a lower level (71, 72). Based on these facts and the results observed in our study, we hypothesize that in human PBMCs the parasite-infected monocytes produce IL12 through cREL activation and IL18 by inflammasome activation (73), which together activate T cells to produce anti-parasitic IFNγ which could destroy some PVs.…”

Toxoplasmicidalin vitroeffect of dehydroepiandrosterone on Toxoplasma gondii extracellular tachizoytes

“…In comparison with DHEA treatment, neither the conventional treatment with S-P, nor the combined treatment DHEA / S-P present significant differences, even when combined treatment reduced the extracellular viability. It has been proposed that tachyzoites can transform the phagocytic vacuole into a parasitoforous vacuole by two different processes; the first includes the formation of the moving junction at the same time that the parasite is phagocytated, and the second proposal is that once the parasite has been phagocytated, is able to invade the phagolysosomal vacuole [13, 39]. Both proposals imply the fusion of the tachyzoite plasma membrane with the macrophage plasma membrane or with the phagolysosomal membrane; this mechanism involves the secretion of proteins from secretory organelles, such as MIC2 and RON4 [18].…”