Entamoeba histolytica (Eh) is a protozoan parasite that infects 10% of the world's population and results in 100,000 deaths/year from amebic dysentery and/or liver abscess. In most cases, this extracellular parasite colonizes the colon by high affinity binding to MUC2 mucin without disease symptoms, whereas in some cases, Eh triggers an aggressive inflammatory response upon invasion of the colonic mucosa. The specific host-parasite factors critical for disease pathogenesis are still not well characterized. From the parasite, the signature events that lead to disease progression are cysteine protease cleavage of the C-terminus of MUC2 that dissolves the mucus layer followed by Eh binding and cytotoxicity of the mucosal epithelium. The host mounts an ineffective excessive host pro-inflammatory response following contact with host cells that causes tissue damage and participates in disease pathogenesis as Eh escapes host immune clearance by mechanisms that are not completely understood. Ameba can modulate or destroy effector immune cells by inducing neutrophil apoptosis and suppressing respiratory burst or nitric oxide (NO) production from macrophages. Eh adherence to the host cells also induce multiple cytotoxic effects that can promote cell death through phagocytosis, apoptosis or by trogocytosis (ingestion of living cells) that might play critical roles in immune evasion. This review focuses on the immune evasion mechanisms that Eh uses to survive and induce disease manifestation in the host.
During invasion, Entamoeba histolytica (Eh) encounter macrophages and activate them to elicit tissue damaging pro-inflammatory responses. When Eh binds macrophages via the Gal-lectin, surface EhCP-A5 RGD sequence ligates α 5 β 1 integrin to activate caspase-1 in a complex known as the NLRP3 inflammasome. In this study, we investigated Eh requirements underlying macrophage caspase-4 and -1 activation and the role caspase-4 and gasdermin D (GSDMD) play in augmenting pro-inflammatory cytokine responses. Caspase-4 activation was similar to caspase-1 requiring live Eh attachment via the Gal-lectin and EhCP-A5. However, unlike caspase-1, caspase-4 activation was independent of ASC and NLRP3. Using CRISPR/Cas9 gene editing of caspase-4 and -1 and GSDMD, we determined that caspase-1 and bioactive IL-1β release was highly dependent on caspase-4 activation and cleavage of GSDMD in response to Eh. Formaldehyde cross-linking to stabilize protein-protein interactions in transfected COS-7 cells stimulated with Eh revealed that caspase-4 specifically interacted with caspase-1 in a protein complex that enhanced the cleavage of caspase-1 CARD domains to augment IL-1β release. Activated caspase-4 and -1 cleaved GSDMD liberating the N-terminal p30 pore-forming fragment that caused the secretion of IL-1β. These findings reveal a novel role for caspase-4 as a sensor molecule to amplify proinflammatory responses when macrophage encounters Eh.
Entamoeba histolytica (Eh) is the causative agent of amebiasis, one of the major causes of dysentery-related morbidity worldwide. Recent studies have underlined the importance of the intercellular junction between Eh and host cells as a determinant in the pathogenesis of amebiasis. Despite the fact that direct contact and ligation between Eh surface Gal-lectin and EhCP-A5 with macrophage α5β1 integrin are absolute requirements for NLRP3 inflammasome activation and IL-1β release, many other undefined molecular events and downstream signaling occur at the interface of Eh and macrophage. In this study, we investigated the molecular events at the intercellular junction that lead to recognition of Eh through modulation of the macrophage cytoskeleton. Upon Eh contact with macrophages key cytoskeletal-associated proteins were rapidly post-translationally modified only with live Eh but not with soluble Eh proteins or fragments. Eh ligation with macrophages rapidly activated caspase-6 dependent cleavage of the cytoskeletal proteins talin, Pyk2 and paxillin and caused robust release of the pro-inflammatory cytokine, IL-1β. Macrophage cytoskeletal cleavages were dependent on Eh cysteine proteinases EhCP-A1 and EhCP-A4 but not EhCP-A5 based on pharmacological blockade of Eh enzyme inhibitors and EhCP-A5 deficient parasites. These results unravel a model where the intercellular junction between macrophages and Eh form an area of highly interacting proteins that implicate the macrophage cytoskeleton as a sensor for Eh contact that leads downstream to subsequent inflammatory immune responses.
A hallmark of amebiasis is acute intestinal inflammation dominated by secretions of pro‐inflammatory cytokines from macrophages. Entamoeba histolytica (Eh) in contact with macrophages activates caspase‐1 by the inflammasome complex resulting in the maturation of IL‐1β and ‐18. The role of inflammatory caspase‐4, ‐5, ‐12 in humans, and caspase‐11, ‐12 in mice are less understood. Caspase‐11, the murine ortholog of caspase‐4, mediates non‐canonical inflammasome activation, suggesting that caspase‐4 may have similar roles. As caspase‐4 might be an immune sensor in Eh infection the aim of this study was to identify the requirements for Eh‐induced caspase‐4/11 activation. Macrophages were treated with live Eh, soluble Eh proteins and secreted components derived from viable Eh. Only live Eh activated caspase‐1, ‐4, and ‐11 in a contact‐dependent manner. Blockade of Eh Gal‐lectin adhesin inhibited binding to macrophages and caspase‐1, ‐4 and ‐11 secretions. Interestingly, cysteine protease 5, another protein critical for Eh virulence, was required for caspase‐1 inflammasome activation but not for caspase‐4/11 activation. To summarize, caspase‐4/11 activation occurred in a contact‐dependent manner and the requirements for their activation are different from caspase‐1, arguing for distinct, non‐redundant roles. Determining how Eh activates these caspases may lead to new therapeutics to treat amebiasis. Grant Funding Source: Supported by NSERC
Entamoeba histolytica (Eh) is a protozoan parasite that colonizes in/on the mucus layer and in 10% of individuals, Eh invades the colonic mucosa to cause amebic colitis. A hallmark of amebiasis is acute intestinal inflammation dominated by the secretion of pro‐inflammatory cytokines, interleukin (IL)‐1β and tumour necrosis factor (TNF)‐α. Eh in contact with macrophages activates caspase‐1 by the inflammasome complex resulting in the maturation of IL‐1β and ‐18. The role of inflammatory caspase‐4, ‐5, and ‐12 in humans are less understood. Caspase‐11, the murine homolog of caspase‐4, mediates non‐canonical inflammasome activation in response to particular pathogens, suggesting that caspase‐4 may have similar roles. As caspase‐4/11 might be an immune sensor in Eh infection the aim of this study was to identify a signature role for caspase‐4/11 in Eh‐induced macrophage responses. To determine if caspase‐4/11 was involved in inflammasome signalling, the gene was silenced with siRNA. Knockdown of caspase‐4/11 did not affect caspase‐1 activation or IL‐1β secretion in response to Eh. However, multiplex cytokine arrays showed marked up regulation of TNF‐α, MIP‐1β, and KC suggesting that caspase‐4/11 may have regulatory roles in the secretion of distinct cytokines/chemokines. Although there was no evidence for crosstalk between caspase‐4/11 and caspase‐1, these inflammatory caspases seem to play a critical role in regulating other cytokines/chemokines that shape the overall macrophage response towards Eh infection. Elucidating the role these inflammatory caspases play in Eh disease pathogenesis may shed light on new therapeutics to treat amebiasis.Grant Support: NSERC
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