Revisiting the Mechanisms of Immune Evasion Employed by Human Parasites

For the establishment of a successful infection, i.e., long-term parasitism and a complete life cycle, parasites use various diverse mechanisms and factors, which they may be inherently bestowed with, or may acquire from the natural vector biting the host at the infection prelude, or may take over from the infecting host, to outmaneuver, evade, overcome, and/or suppress the host immunity, both innately and adaptively. This narrative review summarizes the up-to-date strategies exploited by a number of representative human parasites (protozoa and helminths) to counteract the target host immune defense. The revisited information should be useful for designing diagnostics and therapeutics as well as vaccines against the respective parasitic infections.

Immune responses against protozoan parasites: a focus on the emerging role of Nod-like receptors

Nod-like receptors (NLRs) have gained attention in recent years because of the ability of some family members to assemble into a multimeric protein complex known as the inflammasome. The role of NLRs and the inflammasome in regulating innate immunity against bacterial pathogens has been well studied. However, recent studies show that NLRs and inflammasomes also play a role during infections caused by protozoan parasites, which pose a significant global health burden. Herein, we review the diseases caused by the most common protozoan parasites in the world and discuss the roles of NLRs and inflammasomes in host immunity against these parasites.

A review of the proposed role of neutrophils in rodent amebic liver abscess models

Host invasion by Entamoeba histolytica, the pathogenic agent of amebiasis, can lead to the development of amebic liver abscess (ALA). Due to the difficulty of exploring host and amebic factors involved in the pathogenesis of ALA in humans, most studies have been conducted with animal models (e.g., mice, gerbils, and hamsters). Histopathological findings reveal that the chronic phase of ALA in humans corresponds to lytic or liquefactive necrosis, whereas in rodent models there is granulomatous inflammation. However, the use of animal models has provided important information on molecules and mechanisms of the host/parasite interaction. Hence, the present review discusses the possible role of neutrophils in the effector immune response in ALA in rodents. Properly activated neutrophils are probably successful in eliminating amebas through oxidative and non-oxidative mechanisms, including neutrophil degranulation, the generation of free radicals (O₂⁻, H₂O₂, HOCl) and peroxynitrite, the activation of NADPH-oxidase and myeloperoxidase (MPO) enzymes, and the formation of neutrophil extracellular traps (NETs). On the other hand, if amebas are not eliminated in the early stages of infection, they trigger a prolonged and exaggerated inflammatory response that apparently causes ALAs. Genetic differences in animals and humans are likely to be key to a successful host immune response.

Cytokine and chemokine responses to helminth and protozoan parasites and to fungus and mite allergens in neonates, children, adults, and the elderly

Background

In rural sub-Saharan Africa, endemic populations are often infected concurrently with several intestinal and intravascular helminth and protozoan parasites. A specific, balanced and, to an extent, protective immunity will develop over time in response to repeated parasite encounters, with immune responses initially being poorly adapted and non-protective. The cellular production of pro-inflammatory and regulatory cytokines and chemokines in response to helminth, protozoan antigens and ubiquitous allergens were studied in neonates, children, adults and the elderly.

Results

In children schistosomiasis prevailed (33%) while hookworm and Entamoeba histolytica/E. dispar was found in up to half of adults and the elderly. Mansonella perstans filariasis was only present in adults (24%) and the elderly (25%). Two or more parasite infections were diagnosed in 41% of children, while such polyparasitism was present in 34% and 38% of adults and the elderly. Cytokine and chemokine production was distinctively inducible by parasite antigens; pro-inflammatory Th2-type cytokine IL-19 was activated by Entamoeba and Ascaris antigens, being low in neonates and children while IL-19 production enhanced “stepwise” in adults and elderly. In contrast, highest production of MIP-1delta/CCL15 was present in neonates and children and inducible by Entamoeba-specific antigens only. Adults and the elderly had enhanced regulatory IL-27 cytokine responses, with Th2-type chemokines (MCP-4/CCL13, Eotaxin-2/CCL24) and cytokines (IL-33) being notably inducible by helminth- and Entamoeba-specific antigens and fungus-derived allergens. The lower cellular responsiveness in neonates and children highlighted the development of a parasite-specific cellular response profile in response to repeated episodes of exposure and re-infection.

Conclusions

Following repeated exposure to parasites, and as a consequence of host inability to prevent or eliminate intestinal helminth or protozoa infections, a repertoire of immune responses will evolve with lessened pro-inflammatory and pronounced regulatory cytokines and chemokines; this is required for partial parasite control as well as for preventing inadequate and excessive host tissue and organ damage.

The dynamic interdependence of amebiasis, innate immunity, and undernutrition

Entamoeba histolytica, the protozoan parasite that causes amebic dysentery, greatly contributes to disease burden in the developing world. Efforts to exhaustively characterize the pathogenesis of amebiasis have increased our understanding of the dynamic host-parasite interaction and the process by which E. histolytica trophozoites transition from gut commensals to invaders of the intestinal epithelium. Mouse models of disease continue to be instrumental in this area. At the same time, large-scale studies in human populations have identified genetic and environmental factors that influence susceptibility to amebiasis. Nutritional status has long been known to globally influence immune function. So it is not surprising that undernutrition has emerged as a critical risk factor. A better understanding of how nutritional status affects immunity to E. histolytica will have dramatic implications in the development of novel treatments. Future work should continue to characterize the fascinating host-parasite arms race that occurs at each stage of infection.

Entamoeba lysyl-tRNA synthetase contains a cytokine-like domain with chemokine activity towards human endothelial cells

Immunological pressure encountered by protozoan parasites drives the selection of strategies to modulate or avoid the immune responses of their hosts. Here we show that the parasite Entamoeba histolytica has evolved a chemokine that mimics the sequence, structure, and function of the human cytokine HsEMAPII (Homo sapiens endothelial monocyte activating polypeptide II). This Entamoeba EMAPII-like polypeptide (EELP) is translated as a domain attached to two different aminoacyl-tRNA synthetases (aaRS) that are overexpressed when parasites are exposed to inflammatory signals. EELP is dispensable for the tRNA aminoacylation activity of the enzymes that harbor it, and it is cleaved from them by Entamoeba proteases to generate a standalone cytokine. Isolated EELP acts as a chemoattractant for human cells, but its cell specificity is different from that of HsEMAPII. We show that cell specificity differences between HsEMAPII and EELP can be swapped by site directed mutagenesis of only two residues in the cytokines' signal sequence. Thus, Entamoeba has evolved a functional mimic of an aaRS-associated human cytokine with modified cell specificity.

Th-1/Th-2 cytokine pattern in human amoebic colitis

Amoebiasis, caused by Entamoeba histolytica, is still considered a major health problem in developing countries. Since the immune response during human amoebiasis has not been clearly defined, we chose to evaluate cytokine production in patients suffering from amoebic colitis. A case-control association study was carried out on 62 subjects, including 31 patients with amoebic colitis and 31 healthy controls (age, sex and geographic region-matched). Serum levels of IL-12, IFN-gamma, IL-13 and IL-5 were measured by solid-phase sandwich enzyme linked immunosorbant assay. Serum levels of IFN-gamma, IL-12, IL-13 and IL-5 were higher in the patients with amoebic colitis than in healthy controls, but were only statistically increased for IL-5 (p = 0.04) and IL-13 (p = 0.014). Stratification of patients according to gender revealed that IL-13 was significantly elevated in men as compared to levels measured in women (p = 0.04). These findings suggest that E. histolytica induce a mixed Th-1/Th-2 response with a polarization toward Th-2 during the early stage of amoebiasis, which may aide in developing a clinical illness.

Crosstalk at the initial encounter: interplay between host defense and ameba survival strategies

The host-parasite relationship is based on a series of interplays between host defense mechanisms and parasite survival strategies. Progress has been made in understanding the role of host immune response in amebiasis. While host cells elaborate diverse mechanisms for pathogen expulsion, amebae have also developed complex strategies to modulate host immune response and facilitate their own survival. This paper will give an overview of current research on the mutual interactions between host and Entamoeba histolytica in human and experimental amebiasis. Understanding this crosstalk is crucial for the effective design and implementation of new vaccines and drugs for this leading parasitic disease.

Entamoeba histolytica: an update

PURPOSE OF REVIEW

Over the past decade, since it was formally recognized that Entamoeba histolytica and Entamoeba dispar were two distinct species, studies in this field have made dramatic in-roads into the understanding of E. histolytica and the pathogenesis of invasive amoebiasis. Over the same period it has also become clear that the true incidence of E. histolytica infection, particularly in vulnerable populations such as low socioeconomic children, is exceedingly high. Understanding the epidemiology, pathophysiology, and the molecular and genetic biology of the organism will not only lead to improved diagnostic and treatment options but, ultimately, to the development of a safe and efficacious vaccine.

RECENT FINDINGS

The recent advances in the genetic and molecular sciences have increased our understanding of the mechanisms that make E. histolytica unique among enteric protozoa in causing invasive disease. In addition, host factors, which predispose individuals or populations to infection or disease, are beginning to be elucidated. New diagnostic tools specific to E. histolytica are being exploited by clinicians and researchers to identify and treat patients as well as to add to the knowledge of the epidemiology and natural history of this infection. The ultimate goal - eradication of disease - is theoretically feasible since humans and primates are the only reservoirs of E. histolytica. Many talented and dedicated individuals are pursuing the development of an effective and safe amoebiasis vaccine.

SUMMARY

E. histolytica remains an important pathogen in many populations of the world and although there has been substantial progress into understanding the disease major challenges still exist.

Entamoeba histolytica: immunohistochemical study of hepatic amoebiasis in mouse. Neutrophils and nitric oxide as possible factors of resistance

Studies in mice have not rendered conclusive data on cell and humoral factors to support the resistance of this rodent to Entamoeba histolytica infection. In Balb/c and C3H/HeJ mice inoculated with live or fixed trophozoites, we studied the evolution of the hepatic lesion, the kinetics of inflammatory cells, and the participation of some humoral factors in the development of the hepatic amoebic lesion. From the first hour, amoebae were surrounded by neutrophils containing inducible nitric oxide synthase (iNOS); macrophages also expressing iNOS appeared lately, whereas NK cells were not part of the inflammatory infiltrates. On the fourth day, neutrophils, macrophages, T and B lymphocytes, plasma cells, and some NK cells limited the lesions and anti-amoeba antibodies appeared when most parasites had been eliminated. Therefore, the resistance of the mice to E. histolytica probably lies in non-specific immune responses, among which the activation of neutrophils and the production of nitric oxide (NO) may be important amoebicide factors.

[Interleukin-8 gene expression in the human colon epithelial cell line, HT-29, exposed to Entamoeba histolytica]

The protozoan parasite, Entamoeba histolytica, is one of major causative agents of intestinal disease all over the world. In acute experimental infection, the early host response to E. histolytica is characterized by an infiltration of neutrophils. However, the chemotactic signal for this response is not well known. Based on the finding that human epithelial cells produce the potent neutrophil chemoattractant and activator, interleukin-8 (IL-8), IL-8 gene expression was examined thoroughly in human colon epithelial cells exposed to E. histolytica trophozoites. Cellular RNAs were extracted from HT-29 or Caco-2 human colon epithelial cells exposed to E. histolytica trophozoites for 30 minutes, 1 and 3 hours. IL-8 mRNA transcripts were measured by reverse transcriptional polymerase chain reaction (RT-PCR) using synthetic standard RNA. The number of IL-8 mRNA molecules increased from 30 minutes to 3 hours of exposure period, reaching 3.1 x 10(7) molecules/micrograms of total RNA. Expression pattern of IL-8 mRNA transcripts was parallel to the amounts of IL-8 protein measured by enzyme-linked immunosorbent assay (ELISA). Lysates of E. histolytica also induced expression of mRNA for IL-8 in colon epithelial cells. These results suggest that acute inflammatory reaction by E. histolytica may be initially triggered by proinflammatory cytokines such as IL-8 secreted from epithelial cells of the colon.