Release of prostaglandin E2 by microfilariae of Wuchereria bancrofti and Brugia malayi

Techniques, Databases and Software Used for Studying Polar Metabolites and Lipids of Gastrointestinal Parasites

Gastrointestinal parasites (GIPs) are organisms known to have coevolved for millennia with their mammalian hosts. These parasites produce small molecules, peptides, and proteins to evade or fight their hosts' immune systems and also to protect their host for their own survival/coexistence. The small molecules include polar compounds, amino acids, lipids, and carbohydrates. Metabolomics and lipidomics are emerging fields of research that have recently been applied to study helminth infections, host-parasite interactions and biochemicals of GIPs. This review comprehensively discusses metabolomics and lipidomics studies of the small molecules of GIPs, providing insights into the available tools and techniques, databases, and analytical software. Most metabolomics and lipidomics investigations employed LC-MS, MS or MS/MS, NMR, or a combination thereof. Recent advancements in artificial intelligence (AI)-assisted software tools and databases have propelled parasitomics forward, offering new avenues to explore host-parasite interactions, immunomodulation, and the intricacies of parasitism. As our understanding of AI technologies and their utilisation continue to expand, it promises to unveil novel perspectives and enrich the knowledge of these complex host-parasite relationships.

The Immune Response to Nematode Infection

Nematode infection is a major threat to the health of humans, domestic animals and wildlife. Nematodes vary in their effect on the host and in the mechanisms underlying immunity but the general features are becoming clear. There is considerable variation among individuals in resistance to infection and much of this variation is due to genetic variation in the immune response. The major histocompatibility complex has a strong influence on resistance to infection but other genes are collectively more important. Resistant individuals produce more IgA, eosinophils, IgE and mast cells than susceptible individuals and this is a consequence of stronger type 2 (Th2) immune responses. A variety of factors promote Th2 responses including genetic background, diet, molecules produced by the parasite and the location of the infection. A variety of cells and molecules including proteins, glycolipids and RNA act in concert to promote responses and to regulate the response. Nematodes themselves also modulate the host response and over 20 parasite-derived immunomodulatory molecules have been identified. Different species of nematodes modulate the immune response in different ways and probably use multiple molecules. The reasons for this are unclear and the interactions among immunomodulators have still to be investigated.

The Immunological Role of Vascular and Lymphatic Endothelial Cells in Filarial Infections

Simple Summary

The endothelium is a monolayer of cells forming a thin membrane that lines the inside of blood vessels. These cells release molecules that regulate vascular relaxation, contraction, and can control blood clotting and the immune response. During infections with filarial nematodes, common parasites of humans and animals, the endothelium is believed to play a key role in the communication between the host and the parasite, since the embryonic stage of filaroids is distributed in the bloodstream. Therefore, this review aims to gather research from different scientists in order to better understand the host immune response in infections with filarial nematodes.

Abstract

The embryonic stage of filarial nematodes, or microfilariae (Mf), shows daily and seasonal periodicity that requires their migration through blood vessels into the lungs, where they are sequestered when not circulating in the peripheral blood. Therefore, Mf and the host endothelium are likely in a permanent state of hide and seek. Interestingly, filarial nematodes co-cultured in media with a murine endothelial cell line survive eight times longer than those cultured in media alone. This suggests that the endothelium is an important element of the immune response in filarial nematodes, perversely promoting their survival in the host. In this review, we will focus on potential pathways involved in the relationship between filarial nematodes and the host endothelium, including the role of endothelial ICAM/VCAM/PECAM adhesion molecules, surface markers involved in the passage of Mf through host tissue, anti-thrombolic effects caused by the presence of filarial nematodes (including plasmins), endothelial cell proliferation (VEGF), and other aspects of the immune activation of the endothelium. The aim of this review is to merge the knowledge about the cross-talk between Mf of different filarial nematode species and endothelial cells (EC), thus allowing a better understanding of the mechanism of these parasitic infections.

IL-33: A central cytokine in helminth infections

IL-33 is an alarmin cytokine which has been implicated in allergy, fibrosis, inflammation, tumorigenesis, metabolism, and homeostasis. However, amongst its strongest roles are in helminth infections, where IL-33 usually (but not always) is central to induction of an effective anti-parasitic immune response. In this review, we will summarise the literature around this fascinating cytokine, its activity on immune and non-immune cells, the unique (and sometimes counterintuitive) responses it induces, and how it can coordinate the immune response during infections by parasitic helminths. Finally, we will summarise some of the ways that parasites have developed to modulate the IL-33 pathway for their own benefit.

Prostaglandin regulation of type 2 inflammation: From basic biology to therapeutic interventions

Type 2 immunity is critical for the protective and repair responses that mediate resistance to parasitic helminth infection. This immune response also drives aberrant inflammation during atopic diseases. Prostaglandins are a class of critical lipid mediators that are released during type 2 inflammation and are integral in controlling the initiation, activation, maintenance, effector functions, and resolution of Type 2 inflammation. In this review, we explore the roles of the different prostaglandin family members and the receptors they bind to during allergen‐ and helminth‐induced Type 2 inflammation and the mechanism through which prostaglandins promote or suppress Type 2 inflammation. Furthermore, we discuss the potential role of prostaglandins produced by helminth parasites in the regulation of host–pathogen interactions, and how prostaglandins may regulate the inverse relationship between helminth infection and allergy. Finally, we discuss opportunities to capitalize on our understanding of prostaglandin pathways to develop new therapeutic options for humans experiencing Type 2 inflammatory disorders that have a significant prostaglandin‐driven component including allergic rhinitis and asthma.

A possible relationship between Thromboxane B2 and Leukotriene B4 and the encapsulation of Dirofilaria repens worms in human subcutaneous dirofilariasis

Human subcutaneous dirofilariosis has several clinical presentations. Many cases present as subcutaneous nodules, as a consequence of a local inflammatory reaction that encapsulates and destroys the worms. In addition, there are cases in which migrating worms located in the ocular area remain unencapsulated. In the present work, the levels of two pro-inflammatory eicosanoids, thromboxane B2 (TxB2) and leukotriene B4 (LTB4) are analysed by commercial Enzime-Linked immunosorbent assay (ELISA) in serum samples from 43 individuals, 28 diagnosed as having subcutaneous dirofilariasis presenting a subcutaneous nodule, five diagnosed as having dirofilariasis, in which the worms remained unencapsulated in the periphery of the eye, and ten healthy individuals living in a non-endemic area, used as controls. The worms were surgically removed, identifying Dirofilaria repens as the causative agent in all cases, by Polymerase Chain Reaction (PCR). Individuals with nodules showed significantly higher levels of TxB2 and LTB4 than healthy controls, whereas significant differences in LTB4 levels were observed between individuals with unencapsulated worms and healthy controls. It is speculated that the absence of LTB4 may contribute to the fact that worms remain unencapsulated as a part of immune evasion mechanisms.

Modulation of Host Immunity by Helminths: The Expanding Repertoire of Parasite Effector Molecules

Helminths are extraordinarily successful parasites due to their ability to modulate the host immune response. They have evolved a spectrum of immunomodulatory molecules that are now beginning to be defined, heralding a molecular revolution in parasite immunology. These discoveries have the potential both to transform our understanding of parasite adaptation to the host and to develop possible therapies for immune-mediated disease. In this review we will summarize the current state of the art in parasite immunomodulation and discuss perspectives on future areas for research and discovery.

Infective Larvae of Brugia malayi Induce Polarization of Host Macrophages that Helps in Immune Evasion

Filarial parasites suppress, divert, or polarize the host immune response to aid their survival. However, mechanisms that govern the polarization of host MΦs during early filarial infection are not completely understood. In this study, we infected BALB/c mice with infective larvae stage-3 of Brugia malayi (Bm-L3) and studied their effect on the polarization of splenic MΦs. Results showed that MΦs displayed M2-phenotype by day 3 p.i. characterized by upregulated IL-4, but reduced IL-12 and Prostaglandin-D2 secretion. Increased arginase activity, higher arginase-1 but reduced NOS2 expression and poor phagocytic and antigen processing capacity was also observed. M2 MΦs supported T-cell proliferation and characteristically upregulated p-ERK but downregulated NF-κB-p65 and NF-κB-p50/105. Notably, Bm-L3 synergized with host regulatory T-cells (Tregs) and polarized M2 MΦs to regulatory MΦs (Mregs) by day 7 p.i., which secreted copious amounts of IL-10 and prostaglandin-E2. Mregs also showed upregulated expression levels of MHC-II, CD80, and CD86 and exhibited increased antigen-processing capacity but displayed impaired activation of NF-κB-p65 and NF-κB-p50/105. Neutralization of Tregs by anti-GITR + anti-CD25 antibodies checked the polarization of M2 MΦs to Mregs, decreased accumulation of regulatory B cells and inflammatory monocytes, and reduced secretion of IL-10, but enhanced IL-4 production and percentages of eosinophils, which led to Bm-L3 killing. In summary, we report hitherto undocumented effects of early Bm-L3 infection on the polarization of splenic MΦs and show how infective larvae deftly utilize the functional plasticity of host MΦs to establish themselves inside the host.

Brugia malayi microfilariae adhere to human vascular endothelial cells in a C3-dependent manner

Brugia malayi causes the human tropical disease, lymphatic filariasis. Microfilariae (Mf) of this nematode live in the bloodstream and are ingested by a feeding mosquito vector. Interestingly, in a remarkable co-evolutionary adaptation, Mf appearance in the peripheral blood follows a circadian periodicity and reaches a peak when the mosquito is most likely to feed. For the remaining hours, the majority of Mf sequester in the lung capillaries. This circadian phenomenon has been widely reported and is likely to maximise parasite fitness and optimise transmission potential. However, the mechanism of Mf sequestration in the lungs remains largely unresolved. In this study, we demonstrate that B. malayi Mf can, directly adhere to vascular endothelial cells under static conditions and under flow conditions, they can bind at high (but not low) flow rates. High flow rates are more likely to be experienced diurnally. Furthermore, a non-periodic nematode adheres less efficiently to endothelial cells. Strikingly C3, the central component of complement, plays a crucial role in the adherence interaction. These novel results show that microfilariae have the ability to bind to endothelial cells, which may explain their sequestration in the lungs, and this binding is increased in the presence of inflammatory mediators.

EicosaCell: An Imaging-Based Assay to Identify Spatiotemporal Eicosanoid Synthesis

Eicosanoids are bioactive lipids derived from enzymatic metabolism of arachidonic acid via the cyclooxygenase (COX) and lipoxygenase (LOX) pathways. These lipids are newly formed and nonstorable molecules that have important roles in physiological and pathological processes. The particular interest to determine intracellular compartmentalization of eicosanoid-synthetic machinery has emerged as a key component in the regulation of eicosanoid synthesis and in delineating functional intracellular and extracellular actions of eicosanoids. In this chapter, we discuss the EicosaCell protocol, an assay that enables the intracellular detection and localization of eicosanoid lipid mediator-synthesizing compartments by means of a strategy to covalently cross-link and immobilize eicosanoids at their sites of synthesis followed by immunofluorescent-based localization of the targeted eicosanoid. EicosaCell assays have been successfully used to identify different intracellular compartments of synthesis of prostaglandins and leukotrienes upon cellular activation. This chapter covers basics of EicosaCell assay including its selection of reagents, immunodetection design as well as some troubleshooting recommendations.

Lipid Body Organelles within the Parasite Trypanosoma cruzi: A Role for Intracellular Arachidonic Acid Metabolism

Most eukaryotic cells contain varying amounts of cytosolic lipidic inclusions termed lipid bodies (LBs) or lipid droplets (LDs). In mammalian cells, such as macrophages, these lipid-rich organelles are formed in response to host-pathogen interaction during infectious diseases and are sites for biosynthesis of arachidonic acid (AA)-derived inflammatory mediators (eicosanoids). Less clear are the functions of LBs in pathogenic lower eukaryotes. In this study, we demonstrated that LBs, visualized by light microscopy with different probes and transmission electron microscopy (TEM), are produced in trypomastigote forms of the parasite Trypanosoma cruzi, the causal agent of Chagas' disease, after both host interaction and exogenous AA stimulation. Quantitative TEM revealed that LBs from amastigotes, the intracellular forms of the parasite, growing in vivo have increased size and electron-density compared to LBs from amastigotes living in vitro. AA-stimulated trypomastigotes released high amounts of prostaglandin E2 (PGE2) and showed PGE2 synthase expression. Raman spectroscopy demonstrated increased unsaturated lipid content and AA incorporation in stimulated parasites. Moreover, both Raman and MALDI mass spectroscopy revealed increased AA content in LBs purified from AA-stimulated parasites compared to LBs from unstimulated group. By using a specific technique for eicosanoid detection, we immunolocalized PGE2 within LBs from AA-stimulated trypomastigotes. Altogether, our findings demonstrate that LBs from the parasite Trypanosoma cruzi are not just lipid storage inclusions but dynamic organelles, able to respond to host interaction and inflammatory events and involved in the AA metabolism. Acting as sources of PGE2, a potent immunomodulatory lipid mediator that inhibits many aspects of innate and adaptive immunity, newly-formed parasite LBs may be implicated with the pathogen survival in its host.

Host parasite communications-Messages from helminths for the immune system: Parasite communication and cell-cell interactions

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Helminth parasites release a spectrum of mediators to dampen host immunity.

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Secreted proteins can act on host receptors and intracellular signalling.

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Parasites also produce exosome-like extracellular vesicles containing microRNAs.

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Exosomes can enter host cells and modulate host gene expression.

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Extracellular vesicles may be a more general mode of host-parasite interaction.

Helminths are metazoan organisms many of which have evolved parasitic life styles dependent on sophisticated manipulation of the host environment. Most notably, they down-regulate host immune responses to ensure their own survival, by exporting a range of immuno-modulatory mediators that interact with host cells and tissues. While a number of secreted immunoregulatory parasite proteins have been defined, new work also points to the release of extracellular vesicles, or exosomes, that interact with and manipulate host gene expression. These recent results are discussed in the overall context of how helminths communicate effectively with the host organism.

Unraveling the complexity of lipid body organelles in human eosinophils

Lipid-rich organelles are common in many cell types. In cells, such as adipocytes, these organelles are termed LDs, whereas in other cells, such as leukocytes, they are called LBs. The study of leukocyte LBs has attracted attention as a result of their association with human diseases. In leukocytes, such as eosinophils, LB accumulation has been documented extensively during inflammatory conditions. In these cells, LBs are linked to the regulation of immune responses by compartmentalization of several proteins and lipids involved in the control and biosynthesis of inflammatory mediators (eicosanoids). However, it has been unclear how diverse proteins, including membrane-associated enzymes involved in eicosanoid formation, incorporate into LBs, especially if the internal content of LBs is assumed to consist solely of stores of neutral lipids, as present within adipocyte LDs. Studies of the formation, function, and ultrastructure of LBs in eosinophils have been providing insights pertinent to LBs in other leukocytes. Here, we review current knowledge of the composition and function of leukocyte LBs as provided by studies of human eosinophil LBs, including recognitions of the internal architecture of eosinophil LBs based on 3D electron tomographic analyses.

Effect of Diethylcarbamazine (DEC) on prostaglandin levels in Wuchereria bancrofti infected microfilaraemics

Diethylcarbamazine (DEC) interferes with arachidonic acid metabolism for the clearance of microfilariae in Wuchereria bancrofti infected individuals. In this study, we have quantified the plasma concentrations of prostaglandin E2 (PGE2) and 6-keto-PGF1α, the end products of arachidonic acid metabolic pathway in microfilaraemics (DEC treated and untreated), and normal healthy individuals at pre- and 3,9,12,36, and 72 h of post-DEC treatment. We have also determined the microfilariae counts at pre and post day 2 (36 h) and day 3 (72 h) of DEC treatment by membrane filtration technique. Significant reduction in PGE2 and 6-keto-PGF1α concentrations was found at 12 h of DEC treatment. Rapid reduction in microfilarial counts was observed at 36 h of post-DEC treatment. Higher levels of prostaglandins were found at pre-treatment hours in microfilaraemics compared to normal healthy individuals (P < 0.05). Our findings indicate that DEC inhibits prostaglandins for the clearance of microfilariae, and increased levels of prostaglandins in microfilaraemics may be contributed by the parasite or host upon stimulation.

Host cell lipid bodies triggered by Trypanosoma cruzi infection and enhanced by the uptake of apoptotic cells are associated with prostaglandin E₂ generation and increased parasite growth

Lipid bodies (lipid droplets) are lipid-rich organelles with functions in cell metabolism and signaling. Here, we investigate the mechanisms of Trypanosoma cruzi-induced lipid body formation and their contributions to host-parasite interplay. We demonstrate that T. cruzi-induced lipid body formation in macrophages occurs in a Toll-like receptor 2-dependent mechanism and is potentiated by apoptotic cell uptake. Lipid body biogenesis and prostaglandin E₂ (PGE₂) production triggered by apoptotic cell uptake was largely dependent of α(v)β₃ and transforming growth factor-β signaling. T. cruzi-induced lipid bodies act as sites of increased PGE synthesis. Inhibition of lipid body biogenesis by the fatty acid synthase inhibitor C75 reversed the effects of apoptotic cells on lipid body formation, eicosanoid synthesis, and parasite replication. Our findings indicate that lipid bodies are highly regulated organelles during T. cruzi infection with roles in lipid mediator generation by macrophages and are potentially involved in T. cruzi-triggered escape mechanisms.

EicosaCell - an immunofluorescent-based assay to localize newly synthesized eicosanoid lipid mediators at intracellular sites

Eicosanoids (prostaglandins, leukotrienes and lipoxins) are a family of signaling lipids derived from arachidonic acid that have important roles in physiological and pathological processes. Over the past years, it has been established that successful eicosanoid production is not merely determined by arachidonic acid and eicosanoid-forming enzymes availability, but requires sequential interactions between specific biosynthetic proteins acting in cascade and may involve very unique spatial interactions. Direct assessment of specific subcellular locales of eicosanoid synthesis has been elusive, as those lipid mediators are newly formed, not stored and often rapidly released upon cell stimulation. In this chapter, we discuss the EicosaCell protocol for intracellular detection of eicosanoid-synthesizing compartments by means of a strategy to covalently cross-link and immobilize the lipid mediators at their sites of synthesis followed by immunofluorescent-based localization of the targeted eicosanoid.

Identifying intracellular sites of eicosanoid lipid mediator synthesis with EicosaCell assays

Eicosanoids, arachidonic acid-derived signaling lipid mediators, are newly formed and nonstorable molecules that have important roles in physiological and pathological processes. EicosaCell is a microscopic assay that enables the intracellular detection and localization of eicosanoid lipid mediator-synthesizing compartments by means of a strategy to covalently cross-link and immobilize eicosanoids at their sites of synthesis followed by immunofluorescent-based localization of the targeted eicosanoid. EicosaCell is a versatile assay which allows analyses of different types of cell preparations, such as cells isolated from humans or harvested cells from in vivo models of inflammation and adherent or suspension cells stimulated in vitro. EicosaCell assays have been successfully used to identify different intracellular compartments of synthesis of prostaglandins and leukotrienes upon cellular activation. This is of particular interest given that over the past decade intracellular compartmentalization of eicosanoid-synthetic machinery has emerged both as a key component in the regulation of eicosanoid synthesis and in delineating functional intracellular and extracellular actions of eicosanoids. This review covers basics of EicosaCell assay including its selection of reagents, immunodetection design as well as some troubleshooting recommendations.

Identification of the lipid mediator prostaglandin E2 in tissue immune cells of humans infected with the filaria Onchocerca volvulus

Prostaglandins generated by multiple tissue and immune cells exhibit regulatory effects on the vascular and immune systems. Prostaglandin E(2) (PGE(2)), in particular, affects innate as well as adaptive immune mechanisms. We identified PGE(2) in host immune cells adjacent to Onchocerca volvulus in subcutaneous onchocercomas and the affected skin. Using immunohistology, PGE(2) was predominantly detected in infiltrating macrophages but also in plasma cells. Consecutive sections revealed concomitant presence of PGE(2) and transforming growth factor-beta (TGF-beta), representing a second immunoregulative mediator in macrophages and plasma cells. TGF-beta was preferentially observed in the infiltrating macrophages in patients with a generalized hyporeactive onchocerciasis and less in patients with the hyperreactive form. The presence of PGE(2) and TGF-beta in adjoining host cells infiltrating in the onchocercoma and dermis may indicate containment of inflammatory responses that could favour survival of the filarial parasite.

Evidence for the presence of prostaglandin H synthase like enzyme in female Setaria cervi and its inhibition by diethylcarbamazine

Experimental evidence has shown that Setaria cervi a bovine filarial parasite contains significant amount of prostaglandin H synthase like activity in the somatic extract of its different life stages. A protein with characteristics of prostaglandin H synthase was purified to homogeneity from female somatic extract using a combination of affinity and gel filtration chromatography. Molecular weight of purified enzyme was 70kDa as determined by SDS-PAGE. Purified enzyme showed high activity with arachidonic acid and TMPD substrates suggests the presence of both cyclooxygenase and peroxidase activity in enzyme. Fluorescence spectroscopy and hemin-associated peroxidase activity confirmed presence of heme in purified enzyme. The K(m) and V(max) values using arachidonic acid were determined to be 79+/-1.5microM and 0.165+/-0.2U/ml, respectively. Further, indomethacin and aspirin, specific inhibitors for PGHS, significantly inhibited the enzyme activity. Diethylcarbamazine, an antifilarial drug inhibited the microfilarial PGHS like activity as well as their motility. Here we are reporting for the first time PGHS like activity in filarial parasite and its inhibition with DEC which provide that this enzyme could be used as a drug target.

T(H)2 adjuvants: implications for food allergy

A persistent question for immunologists studying allergic disease has been to define the characteristics of a molecule that make it allergenic. There has been substantial progress elucidating mechanisms of innate priming of T(H)2 immunity in the past several years. These accumulating data demonstrate that T(H)2 immunity is actively induced by an array of molecules, many of which were first discovered in the context of antihelminthic immune responses. Similar intrinsic or associated activities are now known to account for the T(H)2 immunogenicity of some allergens, and may prove to play a role for many more. In this review, we discuss what has been discovered regarding molecules that induce innate immune activation and the pathways that promote T(H)2-polarized immune responses generally, and specifically what role these mechanisms may play in food allergy from models of food allergy and the study of T(H)2 gastrointestinal adjuvants.

Changes in the levels of eicosanoids in cats naturally and experimentally infected with Dirofilaria immitis

Feline heartworm (Dirofilaria immitis) infection is a severe, life-threatening disease. The eicosanoids are lipid mediators derived from the metabolism of the arachidonic acid, involved in the regulation of the immune response and of inflammatory reactions. In this study, naturally infected cats showed significant higher levels of prostaglandin E(2) (PGE2), thromboxane B(2) (TXB(2)) and leukotriene B(4) (LTB4) than uninfected cats. Changes in the levels of eicosanoids during the infection were observed in experimentally infected cats. PGE2 increased significantly during the first 60 days post-infection, then progressively decreased until day 180 post-infection. At this time, PGE2 values are still significantly higher than those observed before the infection. TxB2 and LTB4 increased progressively from the beginning of infection and reached their maximum levels 180 days post-infection. In experimentally infected, ivermectin-treated cats, 15 days after treatment (45 days after infection) both PGE2 and LTB4 levels were similar to those observed in experimentally infected, untreated cats. No significant differences of PGE2 levels were found before the infection and at the end of the experiment (165 days post-treatment, 195 days post-infection). Increased levels of LTB4 were found 15 days post-treatment, afterward they progressively decreased. These data show that D. immitis infection influences the production of intravascular eicosanoids in cats. The high levels of PGE2 observed in the early phase of infection could be related to the survival of the worms, while those of TxB2 and LTB4 detected at the end of the study could mediate the inflammatory reactions and thrombi formation during the feline dirofilariosis.

High levels of serum thromboxane B2 are generated during human pulmonary dirofilariosis

The canine parasite Dirofilaria immitis can infect humans. Patients with pulmonary dirofilariosis develop significantly higher thromboxane B2 levels than healthy individuals living in areas where dirofilariosis is endemic and in areas where dirofilariosis is not endemic. The possible role of Wolbachia bacteria in the appearance of this eicosanoid is discussed.

The filarial parasite Onchocerca volvulus generates the lipid mediator prostaglandin E2

Prostaglandins exhibit regulatory effects on the vascular and immune system. Prostaglandin E(2) (PGE(2)) modulates T helper (Th) cell and effector cell functional reactivity, thereby promoting Th2 responses. We found significant expression of PGE(2) in male and female Onchocerca volvulus. Using immunohistology, PGE(2) was predominantly detected in the hypodermis of adult O. volvulus, the metabolically most active tissue of the filaria. In contrast, the muscles were PGE(2)-negative and the epithelia of intestine and uterus and male genital tract showed only weak staining. Oocytes were well labeled whereas embryos and sperms did not react. Less pronounced PGE(2) staining was observed in some dermal microfilariae. The expression of PGE(2) was found independent of antifilarial (ivermectin) as well as anti-endobacterial (doxycycline) treatment of O. volvulus-infected patients. PGE(2) was also demonstrated in extracts of adult worms by high-performance liquid chromatography-mass spectrometry. Release of PGE(2) from live or moribund filariae can affect the host s metabolism and immune response in favor of the filarial parasite.

Mycobacterium bovisBacillus Calmette-Guérin Induces TLR2-Mediated Formation of Lipid Bodies: Intracellular Domains for Eicosanoid Synthesis In Vivo

Differentiation of macrophages into foamy (lipid-laden) macrophages is a common pathological observation in tuberculous granulomas both in experimental settings as well as in clinical conditions; however, the mechanisms that regulate intracellular lipid accumulation in the course of mycobacterial infection and their significance to pathophysiology of tuberculosis are not well understood. In this study, we investigated the mechanisms of formation and function of lipid-laden macrophages in a murine model of tuberculosis. Mycobacterium bovis bacillus Calmette-Guérin (BCG), but not Mycobacterium smegmatis, induced a dose- and time-dependent increase in lipid body-inducible nonmembrane-bound cytoplasmic lipid domain size and numbers. Lipid body formation was drastically inhibited in TLR2-, but not in TLR4-deficient mice, indicating a role for TLR2 in BCG recognition and signaling to form lipid bodies. Increase in lipid bodies during infection correlated with increased generation of PGE2 and localization of cyclooxygenase-2 within lipid bodies. Moreover, we demonstrated by intracellular immunofluorescent localization of newly formed eicosanoid that lipid bodies were the predominant sites of PGE2 synthesis in activated macrophages. Our findings demonstrated that BCG-induced lipid body formation is TLR2 mediated and these structures function as signaling platforms in inflammatory mediator production, because compartmentalization of substrate and key enzymes within lipid bodies has impact on the capacity of activated leukocytes to generate increased amounts of eicosanoids during experimental infection by BCG.

Diethylcarbamazine activity against Brugia malayi microfilariae is dependent on inducible nitric-oxide synthase and the cyclooxygenase pathway

Background

Diethylcarbamazine (DEC) has been used for many years in the treatment of human lymphatic filariasis. Its mode of action is not well understood, but it is known to interact with the arachidonic acid pathway. Here we have investigated the contribution of the nitric oxide and cyclooxygenase (COX) pathways to the activity of DEC against B. malayi microfilariae in mice.

Methods

B. malayi microfilariae were injected intravenously into mice and parasitaemia was measured 24 hours later. DEC was then administered to BALB/c mice with and without pre-treatment with indomethacin or dexamethasone and the parasitaemia monitored. To investigate a role for inducible nitric oxide in DEC's activity, DEC and ivermectin were administered to microfilaraemic iNOS^-/- mice and their background strain (129/SV). Western blot analysis was used to determine any effect of DEC on the production of COX and inducible nitric-oxide synthase (iNOS) proteins.

Results

DEC administered alone to BALB/c mice resulted in a rapid and profound reduction in circulating microfilariae within five minutes of treatment. Microfilarial levels began to recover after 24 hours and returned to near pre-treatment levels two weeks later, suggesting that the sequestration of microfilariae occurs independently of parasite killing. Pre-treatment of animals with dexamethasone or indomethacin reduced DEC's efficacy by almost 90% or 56%, respectively, supporting a role for the arachidonic acid and cyclooxygenase pathways in vivo. Furthermore, experiments showed that treatment with DEC results in a reduction in the amount of COX-1 protein in peritoneal exudate cells. Additionally, in iNOS^-/- mice infected with B. malayi microfilariae, DEC showed no activity, whereas the efficacy of another antifilarial drug, ivermectin, was unaffected.

Conclusion

These results confirm the important role of the arachidonic acid metabolic pathway in DEC's mechanism of action in vivo and show that in addition to its effects on the 5-lipoxygenase pathway, it targets the cyclooxygenase pathway and COX-1. Moreover, we show for the first time that inducible nitric oxide is essential for the rapid sequestration of microfilariae by DEC.

Prostaglandins in non-insectan invertebrates: recent insights and unsolved problems

Prostaglandins (PG) are oxygenated derivatives of C20 polyunsaturated fatty acids including arachidonic and eicosapentaenoic acids. In mammals, these compounds have been shown to play key roles in haemostasis, sleep-wake regulation, smooth muscle tone, and vaso-, temperature and immune regulation. In invertebrates, PGs have been reported to perform similar roles and are involved in the control of oogenesis and spermatogenesis, ion transport and defence. Although there is often a detailed understanding of the actions of these compounds in invertebrates such as insects, knowledge of their mechanism of biosynthesis is often lacking. This account provides a critical review of our current knowledge on the structure and modes of biosynthesis of PGs in invertebrates, with particular reference to aquatic invertebrates. It emphasises some of the most recent findings, which suggest that some PGs have been misidentified.

Prostaglandins in invertebrates can be categorised into two main types; the classical forms, such as PGE2 and PGD2 that are found in mammals, and novel forms including clavulones, bromo- and iodo-vulones and various PGA2 and PGE2 esters. A significant number of reports of PG identification in invertebrates have relied upon methods such as enzyme immunoassay that do not have the necessary specificity to ensure the validity of the identification. For example, in the barnacle Balanus amphitrite, although there are PG-like compounds that bind to antibodies raised against PGE2, mass spectrometric analysis failed to confirm the presence of this and other classical PGs. Therefore, care should be taken in drawing conclusions about what PGs are formed in invertebrates without employing appropriate analytical methods. Finally, the recent publication of the Ciona genome should facilitate studies on the nature and mode of biosynthesis of PGs in this advanced deuterostomate invertebrate.

Immune regulation by helminth parasites: cellular and molecular mechanisms

Immunology was founded by studying the body's response to infectious microorganisms, and yet microbial prokaryotes only tell half the story of the immune system. Eukaryotic pathogens--protozoa, helminths, fungi and ectoparasites--have all been powerful selective forces for immune evolution. Often, as with lethal protozoal parasites, the focus has been on acute infections and the inflammatory responses they evoke. Long-lived parasites such as the helminths, however, are more remarkable for their ability to downregulate host immunity, protecting themselves from elimination and minimizing severe pathology in the host.

Production of eicosanoids and other oxylipins by pathogenic eukaryotic microbes

Oxylipins are oxygenated metabolites of fatty acids. Eicosanoids are a subset of oxylipins and include the prostaglandins and leukotrienes, which are potent regulators of host immune responses. Host cells are one source of eicosanoids and oxylipins during infection; however, another potential source of eicosanoids is the pathogen itself. A broad range of pathogenic fungi, protozoa, and helminths produce eicosanoids and other oxylipins by novel synthesis pathways. Why do these organisms produce oxylipins? Accumulating data suggest that phase change and differentiation in these organisms are controlled by oxylipins, including prostaglandins and lipoxygenase products. The precise role of pathogen-derived eicosanoids in pathogenesis remains to be determined, but the potential link between pathogen eicosanoids and the development of TH2 responses in the host is intriguing. Mammalian prostaglandins and leukotrienes have been studied extensively, and these molecules can modulate Th1 versus Th2 immune responses, chemokine production, phagocytosis, lymphocyte proliferation, and leukocyte chemotaxis. Thus, eicosanoids and oxylipins (host or microbe) may be mediators of a direct host-pathogen "cross-talk" that promotes chronic infection and hypersensitivity disease, common features of infection by eukaryotic pathogens.

Filarial antigens impair the function of human dendritic cells during differentiation

The antigen-specific T-cell unresponsiveness seen in lymphatic filariasis is mediated, in part, by diminished antigen-presenting cell function and is most specific for microfilariae (MF), the parasite stage found in large numbers in the peripheral circulation. We investigated the effect of MF antigen (MFAg) on dendritic cells (DC) in both their differentiation process from monocyte precursors and also after they have developed into DC. When MFAg was added to cultures of monocytes during their differentiation process to immature DC, the production of interleukin 12 (IL-12) p40, p70 protein, and IL-10 was significantly (P < 0.03) inhibited in response to Staphylococcus aureus Cowan (SAC) and SAC-gamma interferon (IFN-gamma) (60% to 80% inhibition). IL-10 was also inhibited (P = 0.04) in response to CD40 ligand-IFN-gamma. Moreover, MFAg inhibited the mRNA expression of IL-12 p40 and IL-10 as assessed by RNA protection assays. This effect was antigen specific, as another parasite antigen (soluble Toxoplasma gondii antigen) did not inhibit the production of these cytokines. This effect was also not a result of diminished cell viability nor of an alteration in surface expression of most costimulatory surface molecules, including major histocompatibility complex class I and class II. In contrast to exposure throughout the differentiation process, MFAg added to immature DC had no effect on DC cytokine expression. Although MF-differentiated DC were capable of inducing an allogeneic mixed lymphocyte reaction, they did so to a significantly lesser degree than DC without antigen exposure. These data collectively suggest that once DC are differentiated from their precursor cells, they become resistant to changes by MFAg.

Extranuclear lipid bodies, elicited by CCR3-mediated signaling pathways, are the sites of chemokine-enhanced leukotriene C4 production in eosinophils and basophils

Eosinophils and basophils, when activated, become major sources of cysteinyl leukotrienes, eicosanoid mediators pertinent to allergic inflammation. We show that the C-C chemokines, eotaxin and RANTES (regulated upon activation normal T cell expressed and secreted), activate eosinophils and basophils for enhanced leukotriene C(4) (LTC(4)) generation by distinct signaling and compartmentalization mechanisms involving the induced formation of new cytoplasmic lipid body organelles. Chemokine-induced lipid body formation and enhanced LTC(4) release were both mediated by CCR3 receptor G protein-linked downstream signaling involving activation of phosphoinositide 3-kinase, extracellular signal-regulated kinases 1 and 2, and p38 mitogen-activated protein kinases. Chemokine-elicited lipid body numbers correlated with increased calcium ionophore-stimulated LTC(4) production; and as demonstrated by intracellular immunofluorescent localization of newly formed eicosanoid, lipid bodies were the predominant sites of LTC(4) synthesis in both chemokine-stimulated eosinophils and chemokine-primed and ionophore-activated eosinophils. Eotaxin and RANTES initiated signaling via phosphoinositide 3-kinase and mitogen-activated protein kinases both elicits the formation of lipid body domains and promotes LTC(4) formation at these specific extranuclear sites.

The nematode polyprotein allergens/antigens

Interest in the nematode polyprotein allergens/antigens (NPAs) originally arose because they were often found to be immunodominant antigens of nematode parasites, and in some cases also potent allergens. Quite separately, they attracted attention as the 'ladder' proteins found close to the surface of filarial nematodes. Screening of cDNA expression libraries with antibody from infected humans or domestic animals continues to reveal more NPAs. The search for the biochemical function(s) of NPAs was originally hampered by the lack of amino acid sequence similarities between NPAs and proteins of known function, but much is now known about their biochemical activity, the highly unusual means of their biosynthesis and their gene structure. Here, Malcolm Kennedy provides an update on these intriguing proteins.

The polyprotein lipid binding proteins of nematodes

The nematode polyprotein allergens/antigens (NPAs) are specific to nematodes, and are synthesised as tandemly repetitive polypeptides comprising 10 or more repeated units. The polyproteins are post-translationally cleaved at consensus sites to yield multiple copies of the approximately 15-kDa NPA units. These units can be highly diverse in their amino acid sequences, but absolutely conserved signature amino acid positions are identifiable. NPA units are helix-rich and possibly fold as four helix bundle proteins. The NPA units have relatively non-specific lipid binding activities, binding fatty acids and retinoids, with dissociation constants similar to those of lipid transport proteins of vertebrates. Fluorescence-based analysis has indicated that, like most lipid transport proteins, the ligand is taken into the binding site in its entirety, but the binding site environment is unusual. NPAs are synthesised in the gut of nematodes, and presumably act to distribute small lipids from the gut, via the pseudocoelomic fluid, to consuming tissues (muscles, gonads, etc.). In some species, one of the units has a histidine-rich extension peptide which binds haems and certain divalent metal ions. NPAs appear to be released by parasitic nematodes, and may thereby be involved in modification of the local inflammatory and immunological environment of the tissues they inhabit by delivering or sequestering pharmacologically active lipids - they are known to bind arachidonic acids and some of its metabolites, lysophospholipids, and retinoids. NPAs are the only known lipid binding protein made as polyproteins, and are exceptions to the rule that repetitive polyproteins are only produced by cells undergoing programmed cell death and producing specialist products.

The secretory Onchocerca volvulus protein OvS1/Ov20 exhibits the capacity to compete with serum albumin for the host's long-chain fatty acids

The mechanism by which filarial parasites derive fatty acids bound to the host's carrier protein is poorly understood. The capacity of a secretory protein of Onchocerca volvulus (OvS1/Ov20) to compete with serum albumin for arachidonic and other fatty acids was investigated in this study. Binding affinities of the two proteins for the long-chain fatty acids were determined using displacement assays. The fluorescent probes used included 11-((5-dimethylaminonaphthalene-1-sulfonyl)amino) undecanoic acid (DAUDA) and cis-parinaric acid. OvS1 protein bound arachidonic acid with an affinity five-fold greater than the affinity exhibited by serum albumin. Oleic acid was bound by the parasite protein with an affinity two-fold greater than the affinity shown by serum albumin. Furthermore, the affinities exhibited by OvS1 protein in binding arachidonic and linoleic acid were about two times higher than the affinity for oleic acid. The results suggest that the OvS1 protein has the capacity to compete with the main host's fatty acid carrier protein for the long-chain fatty acids, in particular arachidonic acid, the precursor for eicosanoids.

Interleukin-10 and antigen-presenting cells actively suppress Th1 cells in BALB/c mice infected with the filarial parasite Brugia pahangi

Infection with the third-stage larvae (L3) of the filarial nematode Brugia results in a Th2-biased immune response in mice and humans. Previously we have shown that the production of interleukin 4 (IL-4) is critical for down-regulating polyclonal Th1 responses in L3-infected mice. However, the in vitro neutralization of IL-4 did not fully recover the defective polyclonal Th1 responses, nor did it result in the production of any antigen (Ag)-specific Th1 cytokines, suggesting that perhaps infection with L3 does not result in priming of Th1 cells in vivo. In this study, we analyzed the role of IL-10 and Ag-presenting cells (APCs) in the spleen as additional factors controlling the Th2 bias in infected mice. Our data show that IL-10 and APCs also contribute to the suppression of mitogen-driven Th1 responses of spleen cells from infected mice. In addition, the neutralization of IL-10 or the replacement of the resident APC population from spleen cell cultures resulted in the production of Ag-specific Th1 cytokines. Irradiated spleen cells from either L3-infected or uninfected mice were able to restore Ag-specific Th1 responses in vitro. Therefore, it appears that Brugia-reactive Th1 cells are primed following infection with L3, but are actively suppressed in vivo by a mechanism that involves IL-10 and the resident APC population, but not IL-4. These results indicate that a complex interplay of cytokines and cell populations underscores the Th2-polarized response in L3-infected mice.

Down regulation of macrophage activation in Brugia pahangi-infected jirds (Meriones unguiculatus)

The macrophage is a major component of the inflammatory response induced by lymphatic tissue-dwelling filariae. Intraperitoneal (i.p.) infections with Brugia pahangi in Mongolian gerbils, or jirds (Meriones unguiculatus), induce a peritoneal inflammatory response characterized by accumulation of numerous macrophages and fewer eosinophils. This inflammatory response is associated with the release of microfilariae by female worms. The aim of this study was to investigate the activation state of the peritoneal macrophages during the course of i.p. infections with either male or female worms. Activation was determined by a toxoplasmacidal assay and assays which measured the production of tumor necrosis factor (TNF)-like activity and nitric oxide (NO) production. The development of these assays with jirds was initially conducted in parallel with the mouse system, which served as a positive control. Jird macrophages became activated to kill Toxoplasma gondii by in vivo immunization with Mycobacterium bovis BCG in a pattern similar to that of mouse macrophages. However, unlike the mouse system, supernatants from purified protein derivative- or concanavalin A-stimulated jird splenocytes plus lipopolysaccharide failed to activate jird macrophages in vitro or induce NO production. These results indicate that factors involved in jird macrophage activation may differ from those demonstrated in the mouse system and other systems. i.p. infections of 15 days in duration with either male or female worms induced macrophage activation as measured by Toxoplasma killing and TNF production. These responses decreased as the infection progressed to the chronic period on a time course that parallels the down regulation of experimental B. pahangi granulomas. There was no evidence of NO production by activated jird macrophages. These data indicate that macrophage function is down modulated during filarial infection and suggest that mechanisms involved in macrophage deactivation are related to those that induce down modulation of the systemic granulomatous inflammatory response in the jird. This response is not dependent on the microfilarial stage of the parasite and is also independent of mechanisms which induce peritoneal accumulations of macrophages.

Purification and characterization of prostaglandin-H E-isomerase, a sigma-class glutathione S-transferase, from Ascaridia galli

Comparison of partial primary sequences of sigma-class glutathione S-transferases (GSH) of parasitic helminths and a GSH-dependent prostaglandin (PG)-H D-isomerase of rat immune accessory cells suggested that some of the helminth enzymes may also be involved in PG biosynthesis [Meyer and Thomas (1995) Biochem. J. 311, 739-742]. A soluble GSH transferase of the parasitic nematode Ascaridia galli has now been purified which shows high activity and specificity in the GSH-dependent isomerization of PGH to PGE, comparable to that of the rat spleen enzyme in its isomerization of PGH to PGD, and similarly stimulates the activity of prostaglandin H synthase. The enzyme subunit is structurally related to the rat spleen enzyme and sigma-class GSH transferases of helminths according to the partial primary sequence. The data support the hypothesis that some sigma-class GSH transferases of helminth parasites are involved in PG biosynthesis which, in the case of PGE, is likely to be associated with the subversion or suppression of host immunity. A PG-H E-isomerase of comparable specificity and activity has not previously been isolated.

Characterization of rat spleen prostaglandin H D-isomerase as a sigma-class GSH transferase

The prostaglandin H D-isomerase of rat immune accessory cells has been purified from spleen by a simple procedure, and its high specificity and activity [Urade, Fujimoto, Ujihara and Hayaishi (1987) J. Biol. Chem. 262, 3820-3825] have been confirmed in an assay coupled to prostaglandin H synthase. The enzyme also decreases the formation of 12[S]-hydroxy-5,8,10-heptadecatrienoic acid formed by the synthase in the presence of GSH and increases the overall rate of arachidonate oxidation. A partial amino acid sequence shows a strong relationship to GSH transferases of parasitic helminths and molluscs, indicating that it is the first example of a vertebrate sigma-class GSH transferase, and suggesting that certain helminth GSH transferases may be involved in prostaglandin synthesis.

Specific T cell unresponsiveness in human filariasis: diversity in underlying mechanisms

In an attempt to overcome T cell unresponsiveness to filarial antigens, 65 individuals belonging to the three clinical groups of elephantiasis patients, microfilaraemics, and asymptomatic amicrofilaraemics who exhibited unresponsiveness to Brugia malayi adult worm antigen (BmA) were studied. Peripheral blood mononuclear cells were cocultured with antigen and one of the following reagents that have been reported to be effective in reconstituting T cell proliferation: interleukin-2 (IL-2), interleukin-7 (IL-7), anti-interleukin-4, anti-interleukin-10, anti-CD2, anti-CD27, anti-CD28, indomethacin, phorbol myristate acetate (PMA), or calcium ionophore (A23187). We were able to overcome antigen-specific unresponsiveness in only a minority of the individuals studied. Co-culture with IL-2, IL-7, indomethacin and PMA were the only conditions which resulted in enhanced proliferation to BmA in these individuals. In general, unresponsiveness in elephantiasis patients was easier to reverse than in other clinical groups: in 50% of elephantiasis patients, in 12.5% of microfilaraemics and in 20% of asymptomatic amicrofilaraemics. The results indicate that more than one distinct immunological mechanism may account for the antigen-specific unresponsiveness in individuals exposed to and infected with brugian filariasis.

Eicosanoid production by parasites:

In this review, Adam Belley and Kris Chadee discuss eicosanoid production by various parasites and propose roles they may play in pathogenesis and immunomodulation. The commonality between parasites is prostaglandin production and, therefore, special attention is given to the cyclooxygenase pathway, highlighting the enzymes and functions of prostaglandins.