A hookworm allergen which strongly resembles calreticulin

In vitro immunoregulatory role of recombinant Ancylostoma ceylanicum calreticulin

Ancylostoma ceylanicum is a zoonotic soil-derived nematode that parasitizes the intestines of humans and animals (dogs and cats), leading to malnutrition and iron-deficiency anemia. Helminth parasites secrete calreticulin (CRT), which regulates or blocks the host's immune response. However, no data on A. ceylanicum calreticulin (Ace-CRT) are available. We investigated the biological function of recombinant Ace-CRT (rAce-CRT). rAce-CRT showed reliable antigenicity and stimulated the proliferation of mouse splenocytes and canine peripheral blood mononuclear cells. Quantitative reverse-transcription PCR assays revealed that rAce-CRT primarily promoted the expression of T helper 2 cytokines, particularly IL-13, in canine peripheral blood lymphocytes. rAce-CRT inhibited complement-mediated sheep erythrocyte hemolysis in vitro. Our findings indicate that Ace-CRT plays an immunomodulatory role and may be a promising candidate molecule for a hookworm vaccine.

The Relationship of Parasite Allergens to Allergic Diseases

PURPOSE OF REVIEW

Helminth infections modify the natural history of allergic diseases, by either decreasing or increasing their symptoms. Several helminth components are involved in the increasing of the allergic response and symptoms, overcoming the concomitant immunosuppression of helminthiases. However, the role of individual IgE-binding molecules in this process remains to be defined.

RECENT FINDINGS

We updated the list of helminth allergens and IgE-binding molecules, their effects on asthma presentation, and their impact on allergy diagnosis. Data from genetic and epigenetic studies of ascariasis are analyzed. A new species-specific A. lumbricoides allergen has been discovered, with potential use in molecular diagnosis. Most helminth IgE-binding components are not officially classified as allergens in the WHO/IUIS database, although there is evidence of their influence increasing allergic manifestations. Further immunological characterization of these components is needed to better understand their mechanisms of action and evaluate the ways in which they can influence the diagnosis of allergy.

Hidden in plain sight: How helminths manage to thrive in host blood

Parasitic helminths have evolved a plethora of elegant stratagems to regulate and evade the host immune system, contributing to their considerable persistence and longevity in their vertebrate hosts. Various mechanisms to achieve this state have been described, ranging from interfering with or actively modulating host immune responses to hiding from immune recognition. Because they damage surrounding vessels and disturb blood flow, blood-borne and blood-feeding parasites in particular must deal with much more than immune effector cells. Management of the host complement system and coagulation cascade, as well as the development of processes of hiding and masking, represent hallmarks of life in blood. Here we review recent findings on putative evasion strategies employed by blood-borne parasitic helminths, focusing on the interaction with and utilisation of host serum components by nematodes and trematodes.

The therapeutic mavericks: Potent immunomodulating chaperones capable of treating human diseases

Two major chaperones, calreticulin (CRT) and binding immunoglobulin protein (GRP78/BiP) dependent on their location, have immunoregulatory or anti-inflammatory functions respectively. CRT induces pro-inflammatory cytokines, dendritic cell (DC) maturation and activates cytotoxic T cells against tumours. By contrast, GRP78/BiP induces anti-inflammatory cytokines, inhibits DC maturation and heightens T-regulatory cell responses. These latter functions rebalance immune homeostasis in inflammatory diseases, such as rheumatoid arthritis. Both chaperones are therapeutically relevant agents acting primarily on monocytes/DCs. Endogenous exposure of CRT on cancer cell surfaces acts as an 'eat-me' signal and facilitates improved elimination of stressed and dying tumour cells by DCs. Therefore, therapeutics that promote endogenous CRT translocation to the cell surface can improve the removal of cancer cells. However, infused recombinant CRT dampens this cancer cell eradication by binding directly to the DCs. Low levels of endogenous BiP appear as a surface biomarker of endoplasmic reticulum (ER) stress in some types of tumour cells, a reflection of cells undergoing proliferation, in which resulting hypoxia and nutrient deprivation perturb ER homeostasis triggering the unfolded protein response, leading to increased expression of GRP78/BiP and altered cellular location. Conversely, infusion of an analogue of GRP78/BiP (IRL201805) can lead to long-term immune resetting and restoration of immune homeostasis. The therapeutic potential of both chaperones relies on them being relocated from their intracellular ER environment. Ongoing clinical trials are employing therapeutic interventions to either enhance endogenous cell surface CRT or infuse IRL201805, thereby triggering several disease-relevant immune responses leading to a beneficial clinical outcome.

Eukaryotic expression and immunogenicity of Ancylostoma ceylanicum calreticulin

Ancylostoma ceylanicum is a zoonotic soil-derived nematode that parasitizes human and animal intestines, causing malnutrition and iron-deficiency anemia. Calreticulin is a multifunctional protein involved in all stages of parasitic infection. Studies have found that parasites can secret calreticulin to regulate the host's immune response. To explore the immunogenicity of the eukaryotic expression plasmid of Ancylostoma ceylanicum calreticulin (Ace-CRT), we constructed a recombinant Ace-CRT eukaryotic expression plasmid (pEGFP-N3-Ace-CRT). Successful expression of the target protein in Human Embryonic Kidney (HEK) 293 T cells was confirmed by indirect immunofluorescence and Western blot analysis. BALB/c mice were immunized with pEGFP-N3-Ace-CRT plasmid. Measuring IgG antibody levels in immunized mice sera by ELISA showed that the recombinant plasmid stimulated IgG antibody production in mice. Spleen lymphocytes were collected from vaccinated mice to determine the proportion of T cell subsets and the expression levels of cytokines. Flow cytometry revealed that the percentage of CD3 + CD4+ and CD3 + CD8+ T cells in mice spleen in the immunization group was significantly higher than that in the control group. Recombinant plasmid immunization increased IL-4, IL-10, IL-12, and IL-13 expression while decreasing IL-5, IL-6, and INF-γ in mice spleens. These results indicate that the eukaryotic plasmid constructed in this study had good immunogenicity and mainly induced a T helper 2 response in the host, laying a foundation for screening candidate molecules for anti-hookworm vaccines.

Functional Diversity of the Excretory/Secretory Proteins of Nematode Parasites

INTRODUCTION

Parasites release a wide array of protein as excretory and secretory products (ESPs). Irrespective of their mode of propagation, ESPs are found to be secreted or excreted by both naturally occurring and laboratory-cultivated parasites. Mass spectrometry-based approaches have been extensively used to identify and characterize the ESP constituents. ESPs are involved in various cellular activities such as immune modulation, proteolysis, inhibition of proteases and protection of cells against oxidants. Specifically, their role in host immune evasion by down-regulation of pro-inflammatory cytokines and up-regulation of anti-inflammatory cytokines attracts scientific attention. A thorough investigation of functional diversity of ESPs may be helpful in planning control strategies against many parasites.

METHODS

This review focuses on diversity of ES proteins, various approaches to identify them and discusses about the biochemical and functional aspects of such proteins.

RESULTS

The diverse array of proteins secreted or excreted (a, GST-1, acetylcholinesterase, GAPDH) by the parasites are also described emphasizing their role in cellular physiology.

CONCLUSION

Finally, it concludes by citing some of these proteins as potential therapeutic agents against helminth challenge.

Immunopathology and modulation induced by hookworms: From understanding to intervention

Hookworm infection is considered the most prevalent human soil-transmitted helminth infection affecting approximately 500 million people and accounting for 3.2 million disability-adjusted life years lost annually. As with many other neglected tropical diseases, no international surveillance mechanisms that show accurate data on the prevalence of hookworm infection are in place, thus hindering strategies to control parasite transmission. In this review, we unravel the current knowledge in immunopathology and immunoregulation of hookworm infection and present discoveries in drug therapies based on the capability of hookworms to regulate inflammation to treat allergic, inflammatory and metabolic diseases. Additionally, we highlight potential vaccine development and treatments and propose avenues for further inquiry.

The Interactions of Parasite Calreticulin With Initial Complement Components: Consequences in Immunity and Virulence

Because of its capacity to increase a physiologic inflammatory response, to stimulate phagocytosis, to promote cell lysis and to enhance pathogen immunogenicity, the complement system is a crucial component of both the innate and adaptive immune responses. However, many infectious agents resist the activation of this system by expressing or secreting proteins with a role as complement regulatory, mainly inhibitory, proteins. Trypanosoma cruzi, the causal agent of Chagas disease, a reemerging microbial ailment, possesses several virulence factors with capacity to inhibit complement at different stages of activation. T. cruzi calreticulin (TcCalr) is a highly-conserved, endoplasmic reticulum-resident chaperone that the parasite translocates to the extracellular environment, where it exerts a variety of functions. Among these functions, TcCalr binds C1, MBL and ficolins, thus inhibiting the classical and lectin pathways of complement at their earliest stages of activation. Moreover, the TcCalr/C1 interaction also mediates infectivity by mimicking a strategy used by apoptotic cells for their removal. More recently, it has been determined that these Calr strategies are also used by a variety of other parasites. In addition, as reviewed elsewhere, TcCalr inhibits angiogenesis, promotes wound healing and reduces tumor growth. Complement C1 is also involved in some of these properties. Knowledge on the role of virulence factors, such as TcCalr, and their interactions with complement components in host-parasite interactions, may lead toward the description of new anti-parasite therapies and prophylaxis.

Twenty-five-year research progress in hookworm excretory/secretory products

Hookworm infection is a major public health problem that threatens about 500 million people throughout tropical areas of the world. Adult hookworms survive for many years in the host intestine, where they suck blood, causing iron deficiency anemia and malnutrition. Numerous molecules, named excretory/secretory (ES) products, are secreted by hookworm adults and/or larvae to aid in parasite survival and pathobiology. Although the molecular cloning and characterization of hookworm ES products began 25 years ago, the biological role and molecular nature of many of them are still unclear. Hookworm ES products, with distinct structures and functions, have been linked to many essential events in the disease pathogenesis. These events include host invasion and tissue migration, parasite nourishment and reproduction, and immune modulation. Several of these products represent promising vaccine targets for controlling hookworm disease and therapeutic targets for many inflammatory diseases. This review aims to summarize our present knowledge about hookworm ES products, including their role in parasite biology, host-parasite interactions, and as vaccine and pharmaceutical targets and to identify research gaps and future research directions in this field.

Trypanosoma cruzi Calreticulin: Immune Evasion, Infectivity, and Tumorigenesis

To successfully infect, Trypanosoma cruzi evades and modulates the host immune response. T. cruzi calreticulin (TcCalr) is a multifunctional, endoplasmic reticulum (ER)-resident chaperone that, translocated to the external microenvironment, mediates crucial host-parasite interactions. TcCalr binds and inactivates C1 and mannose-binding lectin (MBL)/ficolins, important pattern- recognition receptors (PRRs) of the complement system. Using an apoptotic mimicry strategy, the C1-TcCalr association facilitates the infection of target cells. T. cruzi infection also seems to confer protection against tumorigenesis. Thus, recombinant TcCalr has important antiangiogenic properties, detected in vitro, ex vivo, and in ovum, most likely contributing at least in part, to its antitumor properties. Consequently, TcCalr is useful for investigating key issues of host-parasite interactions and possible new immunological/pharmacological interventions in the areas of Chagas' disease and experimental cancer.

Complement Evasion: An Effective Strategy That Parasites Utilize to Survive in the Host

Parasitic infections induce host immune responses that eliminate the invading parasites. However, parasites have evolved to develop many strategies to evade host immune attacks and survive in a hostile environment. The complement system acts as the first line of immune defense to eliminate the invading parasites by forming the membrane attack complex (MAC) and promoting an inflammatory reaction on the surface of invading parasites. To date, the complement activation pathway has been precisely delineated; however, the manner in which parasites escape complement attack, as a survival strategy in the host, is not well understood. Increasing evidence has shown that parasites develop sophisticated strategies to escape complement-mediated killing, including (i) recruitment of host complement regulatory proteins on the surface of the parasites to inhibit complement activation; (ii) expression of orthologs of host RCA to inhibit complement activation; and (iii) expression of parasite-encoded proteins, specifically targeting different complement components, to inhibit complement function and formation of the MAC. In this review, we compiled information regarding parasitic abilities to escape host complement attack as a survival strategy in the hostile environment of the host and the mechanisms underlying complement evasion. Effective escape of host complement attack is a crucial step for the survival of parasites within the host. Therefore, those proteins expressed by parasites and involved in the regulation of the complement system have become important targets for the development of drugs and vaccines against parasitic infections.

Evaluation of Opisthorchis viverrini calreticulin for potential host modulation

The multifunctional calreticulin (CALR) was identified as a major calcium-binding protein of the endoplasmic reticulum before being recognized as a chaperone in the same place. Only later were activities of calreticulin outside the endoplasmic reticulum described that for example affect cell proliferation and the innate immune system. In the present work we have investigated those extracellular activities of CALR from the cancerogenic human liver fluke Opisthorchis viverrini (OvCALR), as they might be important in host/parasite interaction. We first demonstrate that OvCALR is released from the parasite and stimulates a specific humoral immune response. Recombinant OvCALR is then shown to suppress proliferation of primary endothelial cells, their motility and sprouting activities. The potential of OvCALR to interfere with the complement system is established, firstly by demonstrating its direct binding to C1q and, secondly by suppression of hemolysis of sensitized red blood cells. These findings suggest that OvCALR is an important parasite antigen that could modulate diverse host functions and support parasite survival.

Parasite allergens

Human IgE against helminths is a normal component of the whole protective response elicitesd during infection, when specific IgE to a great number of antigens is produced; however, few of those IgE binding components are actually allergens. In general, considering the strong Th2/IgE responses during helminth infections is intriguing that they are not usually associated with allergic symptoms, which probably (but not exclusively) depends on parasite-induced immunomodulation. However, allergic manifestations have been described during some helminth infections such as ascariasis, strongyloidiasis, anisakiasis and hydatidosis. In addition, there is evidence that helminthiases (e.g. ascariasis) can increase symptoms in allergic patients. Furthermore, allergic reactions during anti-helminth vaccination have been observed, a problem that also could be associated to the future use of parasite derived immunomodulators. Therefore, identification and characterization of helminth allergens is a matter of increasing research and a great number of IgE binding antigens have been found (www.allergen.org and www.allergome.org). Here we describe only a small group of them, for which allergenic activity (the ability to induce IgE mediated inflammation) have been clinical or experimentally demonstrated. Ascaris lumbricoides tropomyosin (Asc l 3) has strong allergenic activity; in the Tropics it has been associated with asthma and asthma severity, suggesting clinical relevance. In addition, due to its cross reactivity with mite tropomyosins this allergen could influence house dust mite (HDM) allergy diagnosis. Characterized Ascaris allergens also include the polyprotein As s 1 (ABA-1) and the Glutathione transferase As l 13. Other helminth allergens include Anisakis simplex Ani s 1, Ani s 4, Ani s 7 and Ani s 9; Necator americanus NaASP2q and Nacal1 and Schistosoma mansoni SmVAL4 and Sm22.6. Future work on helminth IgE binding antigens will help to understand several aspects of allergenicity and allergenic activity, among them the increasing finding of IgE binding molecules that not induce allergic symptoms.

Particularities of allergy in the Tropics

Allergic diseases are distributed worldwide and their risk factors and triggers vary according to geographical and socioeconomic conditions. Allergies are frequent in the Tropics but aspects of their prevalence, natural history, risk factors, sensitizers and triggers are not well defined and some are expected to be different from those in temperate zone countries. The aim of this review is to investigate if allergic diseases in the Tropics have particularities that deserve special attention for research and clinical practice. Such information will help to form a better understanding of the pathogenesis, diagnosis and management of allergic diseases in the Tropics. As expected, we found particularities in the Tropics that merit further study because they strongly affect the natural history of common allergic diseases; most of them related to climate conditions that favor permanent exposure to mite allergens, helminth infections and stinging insects. In addition, we detected several unmet needs in important areas which should be investigated and solved by collaborative efforts led by the emergent research groups on allergy from tropical countries.

Helminth Allergens, Parasite-Specific IgE, and Its Protective Role in Human Immunity

The Th2 immune response, culminating in eosinophilia and IgE production, is not only characteristic of allergy but also of infection by parasitic worms (helminths). Anti-parasite IgE has been associated with immunity against a range of helminth infections and many believe that IgE and its receptors evolved to help counter metazoan parasites. Allergens (IgE-antigens) are present in only a small minority of protein families and known IgE targets in helminths belong to these same families (e.g., EF-hand proteins, tropomyosin, and PR-1 proteins). During some helminth infection, especially with the well adapted hookworm, the Th2 response is moderated by parasite-expressed molecules. This has been associated with reduced allergy in helminth endemic areas and worm infection or products have been proposed as treatments for allergic conditions. However, some infections (especially Ascaris) are associated with increased allergy and this has been linked to cross-reactivity between worm proteins (e.g., tropomyosins) and highly similar molecules in dust-mites and insects. The overlap between allergy and helminth infection is best illustrated in Anisakis simplex, a nematode that when consumed in under-cooked fish can be both an infective helminth and a food allergen. Nearly 20 molecular allergens have been isolated from this species, including tropomyosin (Ani s 3) and the EF-hand protein, Ani s troponin. In this review, we highlight aspects of the biology and biochemistry of helminths that may have influenced the evolution of the IgE response. We compare dominant IgE-antigens in worms with clinically important environmental allergens and suggest that arrays of such molecules will provide important information on anti-worm immunity as well as allergy.

Modulation of dendritic cell responses by parasites: a common strategy to survive

Parasitic infections are one of the most important causes of morbidity and mortality in our planet and the immune responses triggered by these organisms are critical to determine their outcome. Dendritic cells are key elements for the development of immunity against parasites; they control the responses required to eliminate these pathogens while maintaining host homeostasis. However, there is evidence showing that parasites can influence and regulate dendritic cell function in order to promote a more permissive environment for their survival. In this review we will focus on the strategies protozoan and helminth parasites have developed to interfere with dendritic cell activities as well as in the possible mechanisms involved.

Subversion of complement by hematophagous parasites

The complement system is a crucial part of innate and adaptive immunity which exerts a significant evolutionary pressure on pathogens. It has selected for those pathogens, mainly microorganisms but also parasites, that have evolved countermeasures. The characterization of how pathogens evade complement attack is a rapidly developing field of current research. In recent years, multiple complement evasion strategies have been characterized. In this review, we focus on complement escape mechanisms expressed by hematophagous parasites, a heterogeneous group of metazoan parasites that share the property of ingesting the whole blood of their host. Complement inhibition is crucial for parasite survival within the host tissue or to facilitate blood feeding. Finally, complement inhibition by hematophagous parasites may also contribute to their success as pathogen vectors.

Calreticulin from the intestinal nematode Heligmosomoides polygyrus is a Th2-skewing protein and interacts with murine scavenger receptor-A

Helminth infections are commonly associated with a Th2 immune response, yet only a few parasite molecules involved in triggering such immune responses have been identified. Here, we describe the Th2-skewing property of calreticulin of Heligmosomoides polygyrus (HpCRT). HpCRT is a secreted protein most abundantly expressed by tissue invasive larvae (L4). Native HpCRT purified from adult worm extract (nHpCRT) stimulated robust IL-4 release from CD4(+) T cells of H. polygyrus infected mice. Interestingly, CD4(+) T cells also produced significant amounts of IL-10 while IFN-gamma was not detectable. Likewise, immunization with recombinant HpCRT (rHpCRT) without extrinsic adjuvant led predominantly to a specific IL-4 production implying the innate ability of HpCRT to drive Th2 responses. The triggering of a Th2-skewed immune response to rHpCRT is corroborated by the induction of HpCRT-specific IgG1 and IgE antibodies. Furthermore, rHpCRT bound to scavenger receptor type A (SR-A) on dendritic cells, and interaction of HpCRT with SR-A led to internalization of HpCRT that could be partially blocked by competition with SR-A ligands as well as with an anti-SR-A monoclonal antibody. Hence, our data imply that nematode calreticulin interacts with a mammalian scavenger receptor and at the same time induces a Th2 response.

Screening for immunomodulatory proteins of the intestinal parasitic nematode Heligmosomoides polygyrus

Parasitic nematodes are constantly exposed to the immune effector mechanisms of their hosts. One strategy of the worms to cope with these defence reactions is the secretion of modulatory proteins that down-regulate cell-mediated immune responses. We analysed the proliferation of mesenteric lymph node cells of mice infected with Heligmosomoides polygyrus and showed that cellular proliferation was strongly suppressed in the chronic phase of infection. To identify proteins of H. polygyrus that are involved in parasite-induced immunomodulation, worm extract and culture supernatant of adult H. polygyrus were fractionated by gel chromatography and activity of each fraction was determined. One of the fractions (fraction 9) of worm extract as well as worm secretory products inhibited the antigen-specific cellular proliferation by about 40%. This reduced cellular reactivity coincided with a down-regulation of inducible nitric oxide production of mouse macrophages by 57%. Furthermore, fraction 9 contained antigens that were recognized by IgE antibodies of H. polygyrus-infected mice and induced degranulation of an IgE-sensitized basophil cell line. Single proteins of fraction 9 were analysed by mass spectrometry. These data suggest that antigens that are recognised by IgE antibodies might play an important role in immunomodulation exerted by nematode infections.

DIFFERENTIAL EXPRESSION OF CALRETICULIN IN DEVELOPMENTAL STAGES OF TAENIA SOLIUM

Taenia solium, a cestode that causes neurocysticercosis and taeniasis in humans, has a complex life cycle. The adult tapeworm develops in the intestine of human beings and is also responsible for neurocysticercosis, which is caused by the metacestode or cysticercus that develops in the brain. Recently, we have cloned the coding region for T. solium calreticulin (TsCRT) as a functional Ca(2+)-binding protein. Calreticulin is a ubiquitous protein involved in cellular Ca2+ homeostasis and protein folding. These important functions affect several aspects of cell physiology. To explore the expression of TsCRT during the T. solium life cycle, we used a specific polyclonal antibody raised against recombinant TsCRT to localize this protein by immunolabeling techniques. In sections of cysticerci obtained from swine muscle, as well as of adult tapeworms obtained after infection of hamsters with cysticerci, TsCRT was preferentially localized in tegumentary and muscle cytons of the suckers and rostellum. In mature proglottids obtained from infected humans, positive staining was observed in spermatogonia, ovogonia, uterine epithelium, and cells of the vas deferens. In the gravid uterus, the morula and early stage embryos were highly positive to TsCRT. However, expression diminished as embryonic development progressed and was absent in fully developed oncospheres that were surrounded by an embryophore. A similar down regulation was observed during spermatogenesis. Although early spermatocytes showed a high expression of TsCRT, mature spermatozoa present in the vas deferens were completely negative. These data indicate that calreticulin expression is spatially and temporally regulated during development of T. solium, especially during germ cell development and embryogenesis. In addition, these original images illustrate, for the first time, these processes at a histological level.

A calreticulin-like protein from endoparasitoid venom fluid is involved in host hemocyte inactivation

During oviposition, most endoparasitoid wasps inject maternal factors into their hosts to interfere with host immune reactions and ensure successful development of their progeny. Since encapsulation is a major cellular defensive response of insects against intruding parasites, parasitoids have developed numerous mechanisms to suppress the host encapsulation capability by interfering with every step in the process, including recognition, adherence and spreading. In previous studies, components of Cotesia rubecula venom were shown to inhibit melanization of host hemolymph by interfering with the prophenoloxidase activation cascade and facilitate expression of polydnavirus genes. Here we report the isolation and characterization of another venom protein with similarity to calreticulin. Results indicate that C. rubecula calreticulin (CrCRT) inhibits hemocyte spreading behavior, thus preventing encapsulation of the developing parasitoid. It is possible that the protein might function as an antagonist competing for binding sites with the host hemocyte calreticulin, which mediates early-encapsulation reactions.

The innate allergenicity of helminth parasites

Helminth parasites are well known to induce an immune response in their hosts characterised by elevated IgE, peripheral blood or local tissue eosinophilia, and in some cases, intestinal mastocytosis. This immunological response has a strong T-helper 2 (Th2) cytokine bias and is reminiscent of the immunological constellation found in allergic diseases. However, the molecular forces driving the Th2 response to helminth parasites are still not understood. By using the human hookworm parasite Necator americanus as an example, the authors of the current article propose that in the course of its life cycle, this parasite becomes innately allergenic through the secretion of a molecular array designed to promote tissue migration and homing, feeding and survival against immunological attack. This complex array comprises proteases, lectins and other classes of molecules. Subsequent immunological and physiological events seemingly protect the host from both the allergic sequelae of exposure to environmental allergens and, moreover, from the parasite itself.

The assessment of hookworm calreticulin as a potential vaccine for necatoriasis

A vaccine against the human hookworm Necator americanus is urgently required to reduce hookworm-induced morbidity in endemic areas. In the present study, recombinant hookworm calreticulin, a nominated vaccine candidate, has been tested in mice. Mice given calreticulin had 43-49% fewer worms in their lungs, compared to non-vaccinated controls, following challenge infection with infective hookworm larvae. These levels of protection were achieved in the absence of adjuvant following intraperitoneal administration of three doses of 15 microg antigen. Antigen was also encapsulated in PLG microparticles. Encapsulated calreticulin elicited higher levels of anti-calreticulin IgG1 than free antigen but failed to induce protective immunity. The protection induced by free calreticulin was associated with low levels of serum IgE and moderate lung eosinophilia whilst administration of calreticulin-loaded microparticles was associated with high levels of serum IgE and higher lung eosinophil activity, suggesting that the classical Th2 phenotype may not always be associated with protective immunity, albeit in experimental necatoriasis.

Characterization of Haemonchus contortus calreticulin suggests its role in feeding and immune evasion by the parasite

Haemonchus contortus, a gastrointestinal parasite of sheep and goat feeds on the blood of its host and causes bleeding at the biting site. In this report, we demonstrate that the Ca2+ binding protein, calreticulin (CalR), is present in excretory/secretory products of adult worms. The secreted CalR enhanced plasma coagulation time. Using recombinant fragments, this property has been mapped to C-terminal part of the molecule which has binding sites for Ca2+ as well as clotting factors. Complement protein C1q bound to immobilized CalR and C1q dependent lysis of sensitized sheep erythrocytes was inhibited by CalR, a function mapped to N-domain of the protein. Factor X and a 24 kDa polypeptide derived from prothrombin but not prothrombin bound to immobilized CalR. The binding site for 24 kDa polypeptide in the CalR molecule has been localized in the P-domain. Our results suggest at least two functions for secreted CalR: first, to prevent blood clotting by binding to Ca2+ and clotting factors thus enabling parasite to feed on the host blood and second to modulate the host immune response by binding to complement C1q thereby facilitating parasite's survival within the host.

The immunoepidemiology of human hookworm infection

Advances in hookworm immunoepidemiology are reviewed. Recent studies demonstrate a mixed Th1/Th2 response in human hookworm infection, with immunosuppression of specific and nonspecific IFN-gamma responses. There is increasing evidence for protective immunity in human hookworm infection, including anti-larval IL-5- and IgE-dependent mechanisms, and for immunological interactions between hookworm infection and other diseases.

Cloning and sequencing of putative calreticulin complementary DNAs from four hard tick species

Calreticulin (CRT) is a calcium-binding protein and has many functions in eukaryotic cells. CRT is possibly involved in parasite host immune system evasion. To better understand the molecular basis of CRT in ticks, we cloned and sequenced 4 full-length complementary DNAs (cDNAs) from the hard tick species, Dermacentor variabilis, Haemaphysalis longicornis, Ixodes scapularis, and Rhipicephalus sanguineus, using the technique of rapid amplification of cDNA ends. The deduced amino acid sequences share high identities (between 77 and 98%) with 3 known tick CRT sequences. The major characteristics of known CRTs are observed in all 4 of our deduced tick CRTs. These include 3 major domains, a signal peptide sequence at the beginning of the coding region, 2 triplets of conserved regions, cysteine sites providing disulfide bridges for N-terminal folding, and a nuclear localization signal. Remarkably, the replacement of the endoplasmic reticulum retention signal KDEL by HEEL, which is believed to be associated with secretion of CRT into the host during feeding and was previously recorded only in 2 ticks and a hookworm, is also present in all 4 of our tick putative CRTs. In addition, the CRT gene is potentially useful for tick phylogenetic reconstruction.

Purification and molecular cloning of and immunization with Ancylostoma ceylanicum excretory-secretory protein 2, an immunoreactive protein produced by adult hookworms

Hookworms remain major agents of global morbidity, and vaccination against these bloodfeeding parasites may be an attractive complement to conventional control methods. Here we describe the cloning of Ancylostoma ceylanicum excretory-secretory protein 2 (AceES-2), a novel immunoreactive protein produced by adult worms. Native AceES-2 was purified from excretory-secretory (ES) products by reverse-phase high-pressure liquid chromatography, subjected to amino-terminal sequencing, and cloned from adult worm RNA by using reverse transcription-PCR. The translated AceES-2 cDNA predicts that the mature protein consists of 102 amino acids and has a molecular mass of 11.66 kDa. Western immunoblot and enzyme-linked immunosorbent assay analyses demonstrated that recombinant AceES-2 (rAceES-2) reacted strongly with antibodies from A. ceylanicum-infected hamsters. Immunization of hamsters with native ES products adsorbed to alum induced antibodies that recognized rAceES-2, while rAceES-2-alum vaccination resulted in antibodies that reacted with a single protein band in ES products that closely approximated the size predicted for the native molecule. Infected hamsters that were passively immunized with hyperimmune rabbit anti-rAceES-2 serum exhibited more rapid and complete recovery from anemia than controls that received normal serum. Oral immunization with rAceES-2 was associated with significantly reduced anemia upon challenge, an outcome similar to the outcome observed in hamsters that were orally vaccinated with soluble hookworm extract (the latter animals were also resistant to weight loss). These data suggest that AceES-2 plays an important role in the host-parasite interaction and that vaccination against this protein may represent a useful strategy for controlling hookworm anemia.

Comparative genomics of gene expression in the parasitic and free-living nematodes Strongyloides stercoralis and Caenorhabditis elegans

Although developmental timing of gene expression is used to infer potential gene function, studies have yet to correlate this information between species. We analyzed 10,921 ESTs in 3311 clusters from first- and infective third-stage larva (L1, L3i) of the parasitic nematode Strongyloides stercoralis and compared the results to Caenorhabditis elegans, a species that has an L3i-like dauer stage. In the comparison of S. stercoralis clusters with stage-specific expression to C. elegans homologs expressed in either dauer or nondauer stages, matches between S. stercoralis L1 and C. elegans nondauer-expressed genes dominated, suggesting conservation in the repertoire of genes expressed during growth in nutrient-rich conditions. For example, S. stercoralis collagen transcripts were abundant in L1 but not L3i, a pattern consistent with C. elegans collagens. Although a greater proportion of S. stercoralis L3i than L1 genes have homologs among the C. elegans dauer-specific transcripts, we did not uncover evidence of a robust conserved L3i/dauer 'expression signature.' Strikingly, in comparisons of S. stercoralis clusters to C. elegans homologs with RNAi knockouts, those with significant L1-specific expression were more than twice as likely as L3i-specific clusters to match genes with phenotypes. We also provide functional classifications of S. stercoralis clusters.

Wheezing, allergy, and parasite infection in children in urban and rural Ethiopia

Epidemiological studies in developing countries suggest that intestinal parasite infection may reduce the risk of asthma. Because this evidence is all derived from adults and older children, we have investigated the relation between parasite infection, wheezing, and allergen skin sensitization in nested case-control studies drawn from a survey of 7,155 children aged 1 to 4 years living in urban and rural areas of Jimma, Ethiopia. Infection with parasites was common, predominantly with Trichuris (54%), Ascaris (38%), and hookworm (10%). Wheezing in the past year was significantly more prevalent in urban (4.4%) than rural children (2.0%), and was less prevalent in those infected with Ascaris (age, sex, and urban/rural adjusted odds ratio, 0.5; 95% confidence interval, 0.3 to 0.9), particularly in relation to high-intensity infection. Similar, although nonsignificant, associations were found for hookworm (adjusted odds ratio, 0.6; 95% confidence interval, 0.2 to 1.8), but there was no suggestion of any relation to Trichuris infection. Dermatophagoides pteronyssinus and cockroach (Blattella germanica) skin sensitization was more prevalent in rural than urban children, and was unrelated to wheeze. We conclude that Ascaris and possibly hookworm infection protects against wheeze in young Ethiopian children, and that this effect is not mediated by inhibition of allergen sensitization.

Polydnavirus particle proteins with similarities to molecular chaperones, heat-shock protein 70 and calreticulin

Multipartite nucleic acid-containing virus-like particles, known as polydnaviruses, are special structures produced by female parasitoid wasps to deliver wasp components into the body of their host at oviposition. The particles confer protection for the developing parasitoid by passive and active means. Although several genes expressed from the circular DNA of these particles have been identified from various host-parasitoid systems, there is not much known about the structural proteins of these particles. Here we report on two genes encoding Cotesia rubecula particle proteins with similarities to molecular chaperones, calreticulin and heat-shock protein 70.

Modulation of a heterologous immune response by the products of Ascaris suum

Helminth infections are among the most potent stimulators of Th2-type immune responses and have been widely demonstrated to modify responsiveness to both nonparasite antigens and other infectious agents in a nonspecific manner in infected animals. We investigated the immunomodulatory properties of pseudocoelomic body fluid from adult Ascaris suum gastrointestinal helminths (ABF) and its defined allergen (ABA-1) by examining their effects on the immune response to a heterologous antigen, ovalbumin. Our results indicate that ABF has potent immunomodulatory activity and that the effects observed are consistent with skewing towards a Th2-type response rather than induction of anergy. Our findings show that the immunomodulatory activities of ABF are associated with components other than the major constituent and putative allergen, ABA-1. Furthermore, the allergic responses to ABA-1 are not a result of an intrinsic allergenicity of the protein but are more a reflection of the wider induction of a Th2 response by the infection. Importantly, the induction of interleukin-10 by ABF also suggests that T regulatory cells may play a role in immunomodulation of immune responses by parasitic helminths.

Cloning and partial characterization of a Boophilus microplus (Acari: Ixodidae) calreticulin

We report the cloning, sequence characterization and expression analysis of a calreticulin (CRT) coding cDNA of Boophilus microplus. CRT is a calcium-binding protein involved in multiple cell functions and possibly implicated in parasites host immune system evasion. The CRT cDNA sequence and its molecular characterization are described. Sequence similarity and phylogenetic analyses indicate a close relationship to other arthropod CRT sequences. The CRT cDNA was also expressed in a procariotic system and the recombinant protein (rBmCRT) was used to raise antibodies in a rabbit. Expression analyses of the corresponding gene in different developmental stages and tissues were performed by RT-PCR and Western-blot, which indicated a ubiquitous expression of the B. microplus calreticulin gene and demonstrated its presence in saliva. Sera of tick-infested bovines suggested that this protein may not be able to induce an IgG-based humoral response in its natural host.

Direct identification of stylet secreted proteins from root-knot nematodes by a proteomic approach

The stylet secretions produced by plant parasitic root-knot nematodes are thought to be pathogenicity factors involved in the invasion of the root tissue and in the induction and maintenance of feeding cells. A new procedure was established that allowed the direct qualitative analysis of proteins secreted by Meloidogyne incognita infective juveniles. Purified proteins whose isoelectric point (pI) ranged from 5.0 to 7.5 were separated by two-dimensional (2D) electrophoresis and the seven most abundant proteins were identified by micro-sequencing. A calreticulin (CRT) was isolated and transcription of its gene in infective juveniles and adults was demonstrated. Moreover, evidence for expression of the CRT in the subventral oesophageal glands of infective juveniles was obtained. The potential roles of this secreted protein in pathogenesis and the advantages of developing this strategy to obtain new insights into plant-nematode interactions are discussed.

Independent effects of intestinal parasite infection and domestic allergen exposure on risk of wheeze in Ethiopia: a nested case-control study

BACKGROUND

Why asthma is rare in rural subsistence societies is not clear. We tested the hypotheses that the risk of asthma is reduced by intestinal parasites or hepatitis A infection, and increased by exposure to dust-mite allergen or organophosphorus insecticides in urban and rural areas of Jimma, Ethiopia.

METHODS

From 12876 individuals who took part in a study of asthma and atopy in urban and rural Jimma in 1996, we identified all who reported wheeze in the previous 12 months, and a random subsample of controls. In 1999, we assessed parasites in faecal samples, Der p 1 levels in bedding, hepatitis A antibodies, serum cholinesterase (a marker of organophosphorus exposure), total and specific serum IgE, and skin sensitisation to Dermatophagoides pteronyssinus in 205 cases and 399 controls aged over 16 years. The effects of parasitosis, Der p 1 level, hepatitis A seropositivity, and cholinesterase concentration on risk of wheeze, and the role of IgE and skin sensitisation in these associations, were analysed by multiple logistic regression.

FINDINGS

The risk of wheeze was independently reduced by hookworm infection by an odds ratio of 0.48 (95% CI 0.24-0.93, p=0.03), increased in relation to Der p 1 level (odds ratio per quartile 1.26 [1.00-1.59], p=0.05), and was unrelated to hepatitis A seropositivity or cholinesterase concentration. In the urban population, D pteronyssinus skin sensitisation was more strongly related to wheeze (9.45 [5.03-17.75]) than in the rural areas (1.95 [0.58-6.61], p for interaction=0.017), where D pteronyssinus sensitisation was common, but unrelated to wheeze in the presence of high-intensity parasite infection.

INTERPRETATION

High degrees of parasite infection might prevent asthma symptoms in atopic individuals.

Immune responses in hookworm infections

Hookworms infect perhaps one-fifth of the entire human population, yet little is known about their interaction with our immune system. The two major species are Necator americanus, which is adapted to tropical conditions, and Ancylostoma duodenale, which predominates in more temperate zones. While having many common features, they also differ in several key aspects of their biology. Host immune responses are triggered by larval invasion of the skin, larval migration through the circulation and lungs, and worm establishment in the intestine, where adult worms feed on blood and mucosa while injecting various molecules that facilitate feeding and modulate host protective responses. Despite repeated exposure, protective immunity does not seem to develop in humans, so that infections occur in all age groups (depending on exposure patterns) and tend to be prolonged. Responses to both larval and adult worms have a characteristic T-helper type 2 profile, with activated mast cells in the gut mucosa, elevated levels of circulating immunoglobulin E, and eosinophilia in the peripheral blood and local tissues, features also characteristic of type I hypersensitivity reactions. The longevity of adult hookworms is determined probably more by parasite genetics than by host immunity. However, many of the proteins released by the parasites seem to have immunomodulatory activity, presumably for self-protection. Advances in molecular biotechnology enable the identification and characterization of increasing numbers of these parasite molecules and should enhance our detailed understanding of the protective and pathogenetic mechanisms in hookworm infections.

Identification of a collagen-binding protein from Necator americanus by using a cDNA-expression phage display library

A phage display library was made starting from a cDNA library from the hematophagous human parasite Necator americanus. The cDNA library was transferred by polymerase chain reaction (PCR) cloning into phage display vectors (phagemids), using specially designed primers such that proteins would be expressed as fusions with the C-terminal part of the phage coat protein pVI. The vectors used are multicloning site variants of the original pDONG vectors described by Jespers et al. (1995). Electroporation of the ligation mixtures into electrocompetent Escherichia coli TGI cells yielded 3 x 10(8) pG6A, 1.9 x 10(8) pG6B, and 1 x 10(8) pG6C transfectants for N. americanus. The final libraries consisted of a mix of equal numbers of insert-containing phages from the A, B, and C libraries. Selection of phages for binding to human collagen was performed. Four rounds of panning on human collagens I and III resulted in a significant enrichment of collagen-binding phages from the N. americanus libraries. PCR analysis revealed various insert lengths; however, sequence determination indicated that all phages contained the same protein, albeit with different poly-A tail lengths. The encoded protein itself is a 135-amino acid protein (15 kDa), with no apparent homology to any other known protein. Next the protein was recloned into E. coli using the pET-15b-vector. Upon isopropyl-1-thio-beta-D-galactopyranoside induction, the recombinant protein, rNecH1, could be recovered by urea treatment from inclusion bodies. The rNecH1 protein binds to different collagens: human I > rat I > human III = calf skin I in a specific, dose-dependent, and saturable manner.

Is Necator americanus approaching a mutualistic symbiotic relationship with humans?

The hookworm Necator americanus establishes infections of impressive longevity in the immunologically hostile environment of its human host. In the process, it promotes pronounced T-helper 2 (Th2) cell activity, which in turn seemingly affords the host at least a degree of protection. Given the relatively asymptomatic nature of infection, we argue here that Necator americanus might be approaching a mutualistic symbiotic relationship with humans. In our view, infection is controlled by the immune system while being supported by a subtle immune-evasion strategy that is tolerated and possibly beneficial to the host in certain immunological circumstances, such as in counterbalancing potentially damaging Th1 responses.

A calreticulin-like molecule from the human hookworm Necator americanus interacts with C1q and the cytoplasmic signalling domains of some integrins

Calreticulin was recently identified as a hookworm (Necator americanus) allergen, implying secretion, and contact with cells of the immune system, or significant worm attrition in the tissues of the host. As human calreticulin has been shown to bind to and neutralize the haemolytic activity of the complement component C1q, and to be putatively involved in integrin-mediated intracellular signalling events in platelets, it was of interest to determine whether a calreticulin from a successful nematode parasite of humans, with known immune modulatory and antihaemostatic properties, exhibited a capacity to interfere with complement activation and to interact with integrin domains associated with cell signalling in platelets and other leucocytes. We can now report that recombinant calreticulin failed to demonstrate significant calcium binding capacity, which is a hallmark of calreticulins in general and may indicate inappropriate folding following expression in a prokaryote. Nevertheless, recombinant calreticulin retained sufficient molecular architecture to bind to, and inhibit the haemolytic capacity of, human C1q. Furthermore, recombinant calreticulin reacted in surface plasmon resonance analysis (SPR) with peptides corresponding to cytoplasmic signalling domains of the integrins alphaIIb and alpha5, in a calcium independent manner. SPR was also used to ratify the specificity of a polyclonal antibody to hookworm calreticulin, which was then used to assess the stage specificity of expression of the native molecule (in comparison with reverse transcriptase-polymerase chain reaction), to indicate its apparent secretion, and to purify native calreticulin from worm extracts by affinity chromatography. This development will allow the functional tests described above to be repeated for native calreticulin, to ascertain its role in the host-parasite relationship.

Do basophils play a role in immunity against parasites?

The link between parasites and eosinophilia has been known for more than a century, although the role of eosinophils in host protection is still an open issue. Much less appreciated, however, is the concurrent systemic induction of a related cell type, the basophil, in parasitized hosts. To date, little is known about the role of basophils in immunity against parasites, but recent evidence points to a possible crucial role in the initiation of T-helper type 2 responses in the host. In this article, we review the current understanding of parasitic infections and basophils and discuss their putative role in immunity.

Caenorhabditis elegans as a biomonitor for immunological stress in nematodes

An experimental system has been developed using Caenorhabditis elegans (Secernentea: Rhabditida), to monitor immunological stress in nematodes. The transgenic C. elegans strain PC72 carries a lacZ reporter gene fused to a C. elegans hsp16-1 gene, which is inducible for beta-galactosidase activity at the heat stress temperature of 26 degrees C. The investigate the possibility of using PC72 to monitor immunological stress, its surface coat was targeted, to mimic immune attack, by raising immune sera against surface coat components selectively removed by the cationic detergent cetyltrimethylammoniunm bromide. Initially, a highly significant induction of beta-galactosidase activity was seen in PC72 incubated in either surface-reactive or naïve rabbit serum. Complement (C3) was detected over the entire surface of adult PC72 and was thought to be responsible for stress-induction with naïve sera. When the immunoglobulin (Ig)G fraction of naïve sera was used in isolation, no stress-induction was seen. In contrast, a two-fold increase in beta-galactosidase activity was seen in the presence of surface-reactive IgG (SR-IgG) which recognised surface components of between 6 and 40 kDa in western blot. The belief that surface reactive IgG could induce a stress response was reinforced by analysis of hsp-16 protein expression. Cationised ferritin was then used to assess whether stress-induction was truly a surface reactive event; binding of cationised ferritin to the nematode surface also resulted in two-fold induction of beta-galactosidase activity. To investigate the downstream biological effects of stress induction, worm growth and fecundity were measured in the presence of IgG preparations. A significant reduction was seen in both worm length and fecundity only when larvae were incubated in surface-reactive IgG, compared to both naïve IgG and K-medium controls. In conclusion, it would appear that C. elegans is a suitable model to monitor the induction of immunological stress at the level of the nematode surface coat. Given the ability of nematode surface antigens to protect the vaccinated host in animal model systems, and the close phylogenetic relationships which exist between C. elegans and nematodes of medical and veterinary importance, it is conceivable that the immunological targets in or on the surface of C. elegans warrant rapid identification.