Entamoeba histolytica stimulates the alarmin molecule HMGB1 from macrophages to amplify innate host defenses

Extracellular Vesicles Derived From Entamoeba histolytica Have an Immunomodulatory Effect on THP-1 Macrophages

Recent studies have shown that extracellular vesicles (EVs) secreted by various parasites are capable of modulating the host's innate immune responses, such as by altering macrophage (Mϕ) phenotypes and functions. Studies have shown that Mϕ promote early host responses to amoebic infection by releasing proinflammatory cytokines that are crucial to combating amoebiasis. Here, we are reporting for the first time the effect of EVs released by Entamoeba histolytica (EhEVs) on human THP-1 differentiated Mϕ (THP-1 Mϕ). We show that the EhEVs are internalized by THP-1 Mϕ which leads to differential regulation of various cytokines associated with both M1 and M2 Mϕ. We also saw that EhEV treatment thwarted Type 2 immune-response-related transcriptome pSTAT6 in the THP-1 Mϕ. Furthermore, EhEVs stimulated Mϕ to reduce their energy demand by suppressing oxidative phosphorylation (OXPHOS) and adenosine triphosphate (ATP) production. Hence, the human parasite E. histolytica-derived EVs are capable of eliciting an immune response from Mϕ that may contribute to overall infection status.

The macrophage polarization in Entamoeba histolytica infection modulation by the C fragment of the intermediate subunit of Gal/GalNAc-inhibitable lectin

The protozoan parasite Entamoeba histolytica is the causative agent of amebiasis, with clinical outcomes ranging from asymptomatic infections to severe invasive diseases. The innate immune system, particularly macrophages, is of paramount importance in resisting the invasion of host tissues and organs by the trophozoites of E. histolytica. Parasite-derived pathogenic factors, such as lectins, play a pivotal role in the promotion of macrophage polarization phenotypes that have undergone alteration. Nevertheless, the precise mechanisms by which E. histolytica modulates immune polarization remain largely unknown. The current study focused on the immunomodulatory effects of the Igl-C fragment of E. histolytica Gal/GalNAc lectin on macrophage polarization. These results demonstrated that Igl-C could induce the secretion of IL-1β, IL-6, and other cytokines, activating a mixed M1/M2 polarization state. M1 polarization of macrophages occurs in the early stages and gradually transitions to M2 polarization in the later stages, which may contribute to the persistence of the infection. Igl-C induces the macrophage M1 phenotype and causes the release of immune effector molecules, including iNOS and cytokines, by activating the NF-κB p65 and JAK-STAT1 transcription factor signaling pathways. Furthermore, Igl-C supports the macrophage M2 phenotype via JAK-STAT3 and IL-4-STAT6 pathways, which activate arginase expression in later stages, contributing to the tissue regeneration and persistence of the parasite. The involvement of distinct signaling pathways in mediating this response highlights the complex interplay between the parasite and the host immune system. These findings enhance our understanding of the Igl-C-mediated pathogenic mechanisms during E. histolytica infection.

Pathogenicity and virulence of Entamoeba histolytica, the agent of amoebiasis

The amoeba parasite Entamoeba histolytica is the causative agent of human amebiasis, an enteropathic disease affecting millions of people worldwide. This ancient protozoan is an elementary example of how parasites evolve with humans, e.g. taking advantage of multiple mechanisms to evade immune responses, interacting with microbiota for nutritional and protective needs, utilizing host resources for growth, division, and encystation. These skills of E. histolytica perpetuate the species and incidence of infection. However, in 10% of infected cases, the parasite turns into a pathogen; the host-parasite equilibrium is then disorganized, and the simple lifecycle based on two cell forms, trophozoites and cysts, becomes unbalanced. Trophozoites acquire a virulent phenotype which, when non-controlled, leads to intestinal invasion with the onset of amoebiasis symptoms. Virulent E. histolytica must cross mucus, epithelium, connective tissue and possibly blood. This highly mobile parasite faces various stresses and a powerful host immune response, with oxidative stress being a challenge for its survival. New emerging research avenues and omics technologies target gene regulation to determine human or parasitic factors activated upon infection, their role in virulence activation, and in pathogenesis; this research bears in mind that E. histolytica is a resident of the complex intestinal ecosystem. The goal is to eradicate amoebiasis from the planet, but the parasitic life of E. histolytica is ancient and complex and will likely continue to evolve with humans. Advances in these topics are summarized here.

Sex Difference in Amebiasis

Infection with the protozoan parasite Entamoeba histolytica is much more likely to cause severe, focal liver damage in males than females, although the infection rate is the same in both sexes. The differences in disease susceptibility may be due to modulation of key mechanisms of the innate immune response by sex hormones. Complement-mediated mechanisms and estrogen-dependent activated natural killer T cells lead to early elimination of the parasite in females, whereas a pathological immune axis is triggered in males. Testosterone, which is generally thought to have more immunosuppressive properties on cells of the immune response, leads to overwhelming activation of monocytes and host-dependent destruction of liver tissue in males resulting in worse outcomes.

Gasdermins in Innate Host Defense Against Entamoeba histolytica and Other Protozoan Parasites

Gasdermins (GSDMs) are a group of proteins that are cleaved by inflammatory caspases to induce pore formation in the plasma membrane to cause membrane permeabilization and lytic cell death or pyroptosis. All GSDMs share a conserved structure, containing a cytotoxic N-terminal (NT) pore-forming domain and a C-terminal (CT) repressor domain. Entamoeba histolytica (Eh) in contact with macrophages, triggers outside-in signaling to activate inflammatory caspase-4/1 via the noncanonical and canonical pathway to promote cleavage of gasdermin D (GSDMD). Cleavage of GSDMD removes the auto-inhibition that masks the active pore-forming NT domain in the full-length protein by interactions with GSDM-CT. The cleaved NT-GSDMD monomers then oligomerize to form pores in the plasma membrane to facilitate the release of IL-1β and IL-18 with a measured amount of pyroptosis. Pyroptosis is an effective way to counteract intracellular parasites, which exploit replicative niche to avoid killing. To date, most GSDMs have been verified to perform pore-forming activity and GSDMD-induced pyroptosis is rapidly emerging as a mechanism of anti-microbial host defence. Here, we review our comprehensive and current knowledge on the expression, activation, biological functions, and regulation of GSDMD cleavage with emphases on physiological scenario and related dysfunctions of each GSDM member as executioner of cell death, cytokine secretion and inflammation against Eh and other protozoan parasitic infections.

The NF-κB Pathway: Modulation by Entamoeba histolytica and Other Protozoan Parasites

Protozoan parasites have led to worldwide devastation because of their ability to cause infectious diseases. They have evolved as successful pathogens in part because of their remarkable and sophisticated ways to evade innate host defenses. This holds true for both intracellular and extracellular parasites that deploy multiple strategies to circumvent innate host defenses for their survival. The different strategies protozoan parasites use include hijacking the host cellular signaling pathways and transcription factors. In particular, the nuclear factor-κB (NF-κB) pathway seems to be an attractive target for different pathogens owing to their central role in regulating prompt innate immune responses in host defense. NF-κB is a ubiquitous transcription factor that plays an indispensable role not only in regulating immediate immune responses against invading pathogens but is also a critical regulator of cell proliferation and survival. The major immunomodulatory components include parasite surface and secreted proteins/enzymes and stimulation of host cells intracellular pathways and inflammatory caspases that directly or indirectly interfere with the NF-κB pathway to thwart immune responses that are directed for containment and/or elimination of the pathogen. To showcase how protozoan parasites exploits the NF-κB signaling pathway, this review highlights recent advances from Entamoeba histolytica and other protozoan parasites in contact with host cells that induce outside-in and inside-out signaling to modulate NF-κB in disease pathogenesis and survival in the host.

Entamoeba histolytica exploits the autophagy pathway in macrophages to trigger inflammation in disease pathogenesis

The mechanism whereby Entamoeba histolytica (Eh) binding with macrophages at the intercellular junction triggers aggressive pro-inflammatory responses in disease pathogenesis is not well understood. The host intracellular protein degradation process autophagy and its regulatory proteins are involved in maintenance of cellular homeostasis and excessive inflammatory responses. In this study we unraveled how Eh hijacks the autophagy process in macrophages to dysregulate pro-inflammatory responses. Direct contact of live Eh with macrophages activated caspase-6 that induced rapid proteolytic degradation of the autophagy ATG16L1 protein complex independent of NLRP3 inflammasome and caspase-3/8 activation. Crohn's disease susceptible ATG16L1 T300A variant was highly susceptible to Eh-mediated degradation that augmented pro-inflammatory cytokines in mice. Quantitative proteomics revealed downregulation of autophagy and vesicle-mediated transport and upregulation of cysteine-type endopeptidase pathways in response to Eh. We conclude during Eh-macrophage outside-in signaling, ATG16L1 protein complex plays an overlooked regulatory role in shaping the pro-inflammatory landscape in amebiasis.

Host Protective Mechanisms to Intestinal Amebiasis

The protozoan parasite Entamoeba histolytica is the causative agent of amebiasis, an infection that manifests as colitis and, in some cases, liver abscess. A better understanding of host protective factors is key to developing an effective remedy. Recently, significant advances have been made in understanding the mechanisms of MUC2 production by goblet cells upon amebic infection, regulation of antimicrobial peptide production by Paneth cells, the interaction of commensal microbiota with immune stimulation, and host genetics in conferring protection from amebiasis. In addition to host pathways that may serve as potential therapeutic targets, significant progress has also been made with respect to development of a vaccine against amebiasis. Here, we aim to highlight the current understanding and knowledge gaps critically.