Immunity in humans to Necator americanus: IgE, parasite weight and fecundity

Human IgE mAbs identify major antigens of parasitic worm infection

BACKGROUND

Much of our understanding of the targets of IgE comes from studies of allergy, though little is known about the natural immunogenic targets seen after parasitic worm infections.

OBJECTIVE

We used human monoclonal antibodies (mAbs) for an unbiased and comprehensive characterization of the immunodominant antigens targeted by IgE in conditions like allergy or helminth infection that are associated with elevated levels of IgE.

METHODS

Using human hybridoma technology to immortalize IgE encoding B-cells from peripheral blood of subjects with filarial infections and elevated IgE, we generated naturally occurring human IgE mAbs. B-cell cultures were screened in an unbiased manner for IgE production without regard to specificity. Isolated IgE mAbs were then tested for binding to Brugia malayi somatic extracts using ImmunoCAP, immunoblot, and ELISA. Immunoprecipitation followed by mass spectrometry proteomics was used to identify helminth antigens that were then expressed in Escherichia coli for IgE binding characterization.

RESULTS

We isolated 56 discrete IgE mAbs from 7 individuals with filarial infections. From these mAbs, we were able to definitively identify 19 filarial antigens. All IgE mAbs targeted filarial excreted/secretory proteins, including a family of previously uncharacterized proteins. Interestingly, the transthyretin-related antigens acted as the dominant inducer of the filaria-specific IgE antibody response. These filaria-specific IgE mAbs were potent inducers of anaphylaxis when passively administered to human FcεRI-expressing mice.

CONCLUSIONS

We generated human hybridomas secreting naturally occurring helminth-specific IgE mAbs from filarial-infected subjects. This work provides much-needed insight into the ontogeny of helminth-induced immune response and IgE antibody response.

Hookworm infections: Reappraising the evidence for a role of Neutrophils in light of NETosis

In Hookworm infection, neutrophils have long had the image of the villain, being recruited to the site of larval migration because of damage but participating themselves in tissue injury. With recent developments in neutrophil biology, there is an increasing body of evidence for the role of neutrophils as effector cells in hookworm immunity. In particular, their ability to release extracellular traps, or neutrophil extracellular traps (NETs), confer neutrophils a larvicidal activity. Here, we review recent evidence in this nascent field and discuss the avenue for future research on NETs/hookworm interactions.

Kinetics of Parasite-Specific Antibody and B-Cell-Associated Gene Expression in Brown Trout, Salmo trutta during Proliferative Kidney Disease

Tetracapsuloides bryosalmonae, a myxozoan endoparasite often causes chronic infection in brown trout. Antiparasite immunity mediated by antibodies and B cells is known as an important determinant of host survival and parasite proliferation during chronic infections. Accordingly, studying their time course during proliferative kidney disease (PKD) might be helpful in improving our understanding of its chronic nature. Therefore, we conducted this study to examine parasite specific serum antibody and B-cell-mediated response in laboratory-infected brown trout at different time points. Brown trout were exposed to the spores of T. bryosalmonae, derived from infected bryozoans. Samples were collected at different time points and processed for indirect ELISA, histopathology, and qRT-PCR. T. bryosalmonae specific antibody was detected at 4 weeks post exposure (wpe) and it persisted until 17 wpe. Additionally, the expressions of C4A, CD34, CD79A, BLNK, CD74, BCL7, and CD22 were differentially regulated in the important immune organs, kidney and spleen. To our knowledge, this is the first study addressing anti-T. bryosalmonae antibody response in brown trout at different time points. The results from this study provide valuable insights into the processes leading to changes in B cell development, inflammation and antibody production during the course of PKD in brown trout.

The evolution of IgE-mediated type I hypersensitivity and its immunological value

The allergic phenotype manifests itself in a spectrum of troublesome to life-threatening diseases, from seasonal hay fever, through the food allergies, atopic eczema, asthma, to anaphylaxis. Allergy, that is an overreaction to allergen in hypersensitive individuals, results from the production of IgE, mast cell and basophil sensitisation and degranulation, requiring a range of medications to manage the conditions. Yet it is highly likely that allergy evolved for a purpose and that allergic diseases are accidental consequences of an insufficiently regulated immune response. This article presents a viewpoint from which to restore the immunological reputation of the allergic phenotype. We consider the evolutionary origins of potential allergens, toxins and parasites, and how they might have influenced early-mammal species in existence when IgE first developed. We conclude that the allergic phenotype has likely saved the lives of many more mammals than have ever died from allergy, so justifying the positive role of IgE in our evolution.

Diagnostic Potential of an IgE-ELISA in Detecting Strongyloidiasis

Strongyloides stercoralis infection is prevalent worldwide and can cause lifelong infection in immunocompetent individuals, and potentially death in immunosuppressed patients. The diagnosis is hindered by the low sensitivity of microscopic examination, thus making serology an important complementary test to improve the detection rate. However, there were reports that some Strongyloides-infected individuals were negative with specific IgG and IgG4 assays, and other helminth infections were positive with commercial Strongyloides IgG-ELISAs. Thus, there is a need to develop better serodiagnostic methods for strongyloidiasis. We investigated the diagnostic potential of IgE-ELISAs using Strongyloides larval lysate. Sera from two groups infected with Strongyloides served as the positive reference, that is, 1) positive by commercial IgG-ELISAs and IgG4 rapid test, and stool samples positive by microscopy and/or PCR (group IA; n = 20); and 2) negative by IgG-ELISAs and IgG4 rapid test, but stool samples were PCR positive (group IB sera; n = 11). Sera from another two groups served as negative reference (controls), that is, 1) infected with other parasites (group II; n = 73) and 2) healthy donors (group III; n = 22). Results showed a 100% diagnostic sensitivity in detecting sera from groups IA and IB. The latter group of individuals probably had early infection because their IgG and IgG4 assays were negative. The optical density values of group IB sera were also significantly lower than those of group IA (P < 0.003). The IgE-ELISA was 100% specific when tested against sera from groups II and III. This study highlights the diagnostic potential of IgE-ELISA using larval lysate to detect strongyloidiasis, especially those with probable early infection.

An Absence of Epstein-Barr Virus Reactivation and Associations with Disease Activity in People with Multiple Sclerosis Undergoing Therapeutic Hookworm Vaccination

Background: Epstein–Barr virus (EBV) infection is strongly associated with multiple sclerosis (MS). Helminth infection can downregulate antiviral immune responses, potentially protecting against MS, but with a theoretical risk for reactivating latent EBV infection. Objective: To investigate parameters of EBV infection and their relationship with disease activity in people with MS (PwMS) therapeutically vaccinated with Necator americanus (hookworm). Methods: Sequential serum samples from 51 PwMS; 26 therapeutically infected (25 larvae) with N. americanus and 25 controls were tested for EBV virus capsid antigen (VCA) IgG and IgM, EBV nuclear antigen-1 (EBNA-1) IgG, and EBV early antigen (EA) IgG. Disease activity was assessed by periodic MRI. Significance was set at p < 0.05. Results: All PwMS were EBV VCA IgG and EBNA-1 IgG positive, and 35.2% were EBV EA IgG positive. EBV antibody levels were generally stable, and EBV reactivation in PwMS was not demonstrated by significant increases in IgG titre over 12 months. Disease activity was most frequent in PwMS possessing high levels of EBV VCA IgG (>600 units/mL) or EBNA-1 IgG (>150 units/mL); however, there was no association with hookworm treatment. Interpretation: Therapeutic hookworm vaccination was not associated with EBV reactivation. Multiple sclerosis disease activity was associated with high levels of EBV VCA IgG or EBNA-1 IgG.

A Randomized Controlled Trial to Investigate Safety and Variability of Egg Excretion After Repeated Controlled Human Hookworm Infection

BACKGROUND

Controlled human hookworm infections could significantly contribute to the development of a hookworm vaccine. However, current models are hampered by low and unstable egg output, reducing generalizability and increasing sample sizes. This study aims to investigate the safety, tolerability, and egg output of repeated exposure to hookworm larvae.

METHODS

Twenty-four healthy volunteers were randomized, double-blindly, to 1, 2, or 3 doses of 50 Necator americanus L3 larvae at 2-week intervals. Volunteers were monitored weekly and were treated with albendazole at week 20.

RESULTS

There was no association between larval dose and number or severity of adverse events. Geometric mean egg loads stabilized at 697, 1668, and 1914 eggs per gram feces for the 1 × 50L3, 2 × 50L3, and 3 × 50L3 group, respectively. Bayesian statistical modeling showed that egg count variability relative to the mean was reduced with a second infectious dose; however, the third dose did not increase egg load or decrease variability. We therefore suggest 2 × 50L3 as an improved challenge dose. Model-based simulations indicates increased frequency of stool sampling optimizes the power of hypothetical vaccine trials.

CONCLUSIONS

Repeated infection with hookworm larvae increased egg counts to levels comparable to the field and reduced relative variability in egg output without aggravating adverse events.

CLINICAL TRIALS REGISTRATION

NCT03257072.

Pityriasis alba: Possible associations with intestinal helminths and pathogenic protozoa

BACKGROUND

The aetiology of pityriasis alba (PA) remains uncertain, and children are at risk for PA and intestinal parasites.

AIM

To determine the prevalence of intestinal parasites in PA patients and to evaluate their possible role in PA pathogenesis.

METHODS

Overall, 180 PA patients and 100 healthy individuals were enrolled. Intestinal parasites were diagnosed by triple coproscopy, and the total serum Immunoglobulin E (IgE) levels were determined by ELISA. PA patients with intestinal parasites were treated with conventional antiparasitic drugs. Patient response to antiparasitic therapy was evaluated after 6 weeks.

RESULTS

The prevalence of intestinal parasites in PA patients and controls was 60 ± 3.6% and 32 ± 4.6%, respectively (P < .0001). Regardless of the parasite species among the PA patients and control individuals, the total IgE levels were significantly higher in PA patients (P ≤ .05). The highest values of IgE were found in PA patients with Hymenolepis nana (641.7 ± 46.3 IU/mL). The total IgE level in PA patients with parasites decreased after antiparasitic therapy, but the reduction was only significant in patients with H. nana (P < .05). Complete disappearance of hypopigmented patches was observed after the elimination of H. nana, Giardia lamblia and Enterobius vermicularis in 65 ± 10.6%, 48.7 ± 8.0% and 33.3 ± 8.2% of cases, respectively. In total, a positive clinical response to antiparasitic therapy was achieved in 60 ± 4.7% of infected PA patients.

CONCLUSION

A positive clinical response to antiparasitic therapy indicates the role of intestinal parasites in PA pathogenesis. Parasitological examination is justified by the recovery of 60 ± 4.7% of infected PA patients after the elimination of intestinal parasites.

The physicochemical fingerprint of Necator americanus

Necator americanus, a haematophagous hookworm parasite, infects ~10% of the world's population and is considered to be a significant public health risk. Its lifecycle has distinct stages, permitting its successful transit from the skin via the lungs (L3) to the intestinal tract (L4 maturing to adult). It has been hypothesised that the L3 larval sheath, which is shed during percutaneous infection (exsheathment), diverts the immune system to allow successful infection and reinfection in endemic areas. However, the physicochemical properties of the L3 larval cuticle and sheath, which are in direct contact with the skin and its immune defences, are unknown. In the present study, we controlled exsheathment, to characterise the sheath and underlying cuticle surfaces in situ, using atomic force microscopy (AFM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). AFM revealed previously unseen surface area enhancing nano-annuli exclusive to the sheath surface and confirmed greater adhesion forces exist between cationic surfaces and the sheath, when compared to the emergent L3 cuticle. Furthermore, ToF-SIMS elucidated different chemistries between the surfaces of the cuticle and sheath which could be of biological significance. For example, the phosphatidylglycerol rich cuticle surface may support the onward migration of a lubricated infective stage, while the anionic and potentially immunologically active heparan sulphate rich deposited sheath could result in the diversion of immune defences to an inanimate antigenic nidus. We propose that our initial studies into the surface analysis of this hookworm provides a timely insight into the physicochemical properties of a globally important human pathogen at its infective stage and anticipate that the development and application of this analytical methodology will support translation of these findings into a biological context.

Helminthic infections in returning travelers and migrants with eosinophilia: Diagnostic value of medical history, eosinophil count and IgE

OBJECTIVES

Eosinophilia in travelers and migrants returning from the tropics is often associated with invasive helminthic infections. Total IgE is considered a useful additional diagnostic parameter; however, both parameters are also increased in various other non-helminthic diseases.

METHODS

We retrospectively evaluated travelers and migrants seen at our department between September 2007 and May 2014. Patients with an absolute eosinophil count ≥500 cells/μl were considered for further analyses.

RESULTS

Among 6618 returned travelers and migrants, 154 (2.3%) had a total eosinophil count ≥500 cells/μL. Of these, 71 patients (46%) were diagnosed with helminthic infection. In an additional 62 patients (40%) with eosinophilia a final diagnosis was found, including non-helminthic infections in 34 patients and non-infectious causes in 28 patients, while in 21 patients (14%) no diagnosis was made. Patients with helminthic infections had higher eosinophil counts than travelers and migrants with other diagnoses (median 981 vs. 710 cells/μl; p = 0.001), while total IgE levels (n = 70; 172 vs. 152 kU/l; p = 0.731) were similar in both groups.

CONCLUSION

Eosinophil count but not total IgE levels are associated with helminthic infections in returning travelers and migrants with eosinophilia. Our results do not support the use of total IgE to differentiate helminthic infections from other causes of eosinophilia in this population.

Regulatory monocytes in helminth infections: insights from the modulation during human hookworm infection

Background

While the macrophage polarization is well characterized in helminth infections, the natural heterogeneity of monocytes with multiple cell phenotypes might influence the outcome of neglected diseases, such hookworm infection. Here, we report the profile of monocytes in human hookworm infections as a model to study the regulatory subpopulation of monocytes in helminth infections.

Methods

Blood samples were collected from 19 Necator americanus-infected individuals and 13 healthy individuals. Peripheral blood mononuclear cells (PBMCs) were isolated, and immunophenotyping was conducted by flow cytometry. The expressions of genes encoding human nitric oxide synthase (iNOS), interleukin 4 (IL-4), arginase-1 (Arg-1) and glyceraldehyde 3-phosphate dehydrogenase were quantified by qPCR. Plasma levels of IL-4 were determined by sandwich ELISA. Unpaired t-tests or Mann-Whitney tests were used depending on the data distribution.

Results

Hookworm infected individuals (HWI) showed a significant increase in the number of monocytes/mm³ (555.2 ± 191.0) compared to that of the non-infected (NI) individuals (120.4 ± 44.7) (p < 0.0001). While the frequencies of CD14⁺IL-10⁺ and CD14⁺IL-12⁺ cells were significantly reduced in the HWI compared to NI group (p = 0.0289 and p < 0.0001, respectively), the ratio between IL-10/IL-12 producing monocytes was significantly elevated in HWI (p = 0.0004), indicating the potential regulatory activity of these cells. Measurement of IL-4 levels and gene expression of IL-4 and Arg-1 (highly expressed in alternatively activated macrophages) revealed no significant differences between the NI and HWI groups. Interestingly, individuals from the HWI group had higher expression of the iNOS gene (associated with a regulatory profile) (20.27 ± 2.97) compared to the NI group (11.28 ± 1.18, p = 0.0409). Finally, individuals from the HWI group had a significantly higher frequency of CD206⁺CD23⁺IL-10⁺ (7.57 ± 1.96) cells compared to individuals from the NI group (0.35 ± 0.09) (p < 0.001), suggesting that activated monocytes are a potential source of regulatory cytokines during hookworm infection.

Conclusions

Natural hookworm infection induces a high frequency of circulating monocytes that present a regulatory profile and promote the downmodulation of the proinflammatory response, which may contribute to prolonged survival of the parasite in the host.

Allergic Sensitization Underlies Hyperreactive Antigen-Specific CD4+ T Cell Responses in Coincident Filarial Infection

Among the various hypotheses put forward to explain the modulatory influence of helminth infection on allergic effector responses in humans, the IL-10-induced suppression of Th2-associated responses has been the leading candidate. To explore this helminth/allergy interaction more fully, parasite- and allergen-specific CD4(+) T cell responses in 12 subjects with filarial infections, and coincident allergic sensitization (filarial [Fil](+)allergy [A](+)) were compared with the responses to three appropriate control groups (Fil(-)A(-) [n = 13], Fil(-)A(+) [n = 12], Fil(+)A(-) [n = 11]). The most important findings revealed that Fil(+)A(+) had marked (p < 0.0001 for all cytokines) increases in parasite Ag-driven Th2 (IL-4, IL-5, IL-13), Th9 (IL-9), and the regulatory (IL-10) cytokines when compared with Fil(+)A(-) Moreover, using multiparameter flow cytometry, filarial parasite Ag induced a marked increase in not only the frequency of CD4(+) T cells producing IL-4, IL-5, IL-2, and TNF-α in Fil(+)A(+) when compared with Fil(+)A(-) patients, but also in the frequencies of polyfunctional Th2-like (CD4(+)IL-4(+)IL-5(+) and CD4(+)IL-2(+)IL-4(+)IL-5(+)TNF-α(+)) cells. The Th2-associated responses seen in the Fil(+)A(+) group were correlated with serum IgE levels (p < 0.01, r = 0.5165 for IL-4; p < 0.001, r = 0.5544 for IL-5; and p < 0.001, r = 0.4901 for IL-13) and levels of circulating eosinophils (p < 0.0116, r = 0.5656) and their degranulation/activation products (major basic protein [p < 0.001, r = 0.7353] and eosinophil-derived neurotoxin [p < 0.01, r = 0.7059]). CD4(+) responses to allergen were not different (to a large extent) among the groups. Taken together, our data suggest that allergic sensitization coincident with filarial infection drives parasite Ag-specific T cell hyperresponsiveness, which is characterized largely by an augmented Th2-dominated immune response.

Known Allergen Structures Predict Schistosoma mansoni IgE-Binding Antigens in Human Infection

The IgE response has been associated with both allergic reactions and immunity to metazoan parasites. Recently, we hypothesized that all environmental allergens bear structural homology to IgE-binding antigens from metazoan parasites and that this homology defines the relatively small number of protein families containing allergenic targets. In this study, known allergen structures (Pfam domains) from major environmental allergen families were used to predict allergen-like (SmProfilin, SmVAL-6, SmLipocalin, SmHSP20, Sm triosephosphate isomerase, SmThioredoxin, Sm superoxide dismutase, SmCyclophilin, and Sm phosphoglycerate kinase) and non-allergen-like [Sm dynein light chain (SmDLC), SmAldolase SmAK, SmUbiquitin, and Sm14-3-3] proteins in Schistosoma mansoni. Recombinant antigens were produced in Escherichia coli and IgG1, IgG4, and IgE responses against them measured in a cohort of people (n = 222) infected with S. mansoni. All allergen-like antigens were targeted by IgE responses in infected subjects, whilst IgE responses to the non-allergen-like antigens, SmAK, SmUbiquitin, and Sm14-3-3 were essentially absent being of both low prevalence and magnitude. Two new IgE-binding Pfam domain families, not previously described in allergen family databases, were also found, with prevalent IgE responses against SmDLC (PF01221) and SmAldolase (PF00274). Finally, it was demonstrated that immunoregulatory serological processes typically associated with allergens also occurred in responses to allergen-like proteins in S. mansoni infections, including the production of IgG4 in people responding with IgE and the down-regulation of IgE in response to increased antigen exposure from S. mansoni eggs. This study establishes that structures of known allergens can be used to predict IgE responses against homologous parasite allergen-like molecules (parallergens) and that serological responses with IgE/IgG4 to parallergens mirror those seen against allergens, supporting our hypothesis that allergenicity is rooted in expression of certain protein domain families in metazoan parasites.

Immunology of experimental and natural human hookworm infection

Human hookworm infection is one amongst the most prevalent of the neglected tropical diseases. An informative experimental animal model, that is, one that parallels a human infection, is not available for the study of human hookworm infection. Much of our current understanding of the human immune response during hookworm infection relies on the studies from experimental infection of hookworm-naïve individuals or the natural infections from individuals residing in hookworm-endemic areas. The experimental human infections tend to be acute, dose-controlled infections, often with a low larval inoculum so that they are well tolerated by human volunteers. Natural hookworm infections usually occur in areas where hookworm transmission is constant and infection is chronic. In cases where there has been drug administration in an endemic area, re-infection often occurs quickly even amongst those who were treated. Hence, although many of the characteristics of experimental and natural hookworm infection differ, both models have elements in common: mainly an intense Th2 response with the production of total and specific IgE as well as elevated levels of eosinophilia, IL-5, IL-10 and TNF. While hookworm infection affects millions of individuals worldwide, much of the human immunology of this infection still needs to be studied and understood.

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.

Immunological characteristics of patients infected with common intestinal helminths: results of a study based on reverse-transcriptase PCR

To determine whether common helminth infections could modify the intestinal immunopathological status of the host, the expression in the human duodenal mucosa of cytokines, eosinophil- and mast-cell-specific molecules and monosaccharide transporters of the glucose-transporter (GLUT) family was explored. The 31 subjects were all patients at the gastro-intestinal disease unit of Nongkhai Hospital, Thailand. Four of the 10 patients who presented with eosinophilia (> or = 6.0% of their leucocytes were eosinophils), and five of the other 21 patients, had intestinal infections with helminths when they presented or within the previous 3 months. Studies based on semi-quantitative, reverse-transcriptase PCR revealed that the interleukin-5/interferon-gamma ratio was significantly higher in the noneosinophilic, helminth-infected patients than in the non-eosinophilic, uninfected patients, whereas the IgE receptor type I (Fc epsilon RI)/mast-cell tryptase ratio was significantly higher in the eosinophilic, helminth-infected patients than in the eosinophilic, uninfected patients. Expression of Charcot-Leyden-crystal protein, GLUT-1 and GLUT-5, however, showed no significant inter-group differences. Principal-components analysis of the data on eosinophils, interleukin-5, interferon-gamma, Fc epsilon RI and mast-cell tryptase revealed that one principal component could discriminate the patients who had helminth infection from the non-eosinophilic, uninfected patients, but not from the eosinophilic, uninfected patients. These results indicate that, whatever the intestinal pathology, patients infected with common intestinal helminths tend to develop a mucosal immunological response of the Th2 type.

Changed gluten immunity in celiac disease by Necator americanus provides new insights into autoimmunity

We recently completed clinical trials in people with diet-treated celiac disease who were purposefully infected with the ubiquitous human hookworm, Necator americanus. Hookworm infection elicited not only parasite-specific immunity but also modified the host's immune response to gluten. After infection, mucosal IL-1β and IL-22 responses were enhanced, but IFNγ and IL-17A levels and circulating regulatory T cells following gluten challenge were suppressed, and the adaptive response to gluten acquired a helper T cell type-2 profile. In this review, we briefly, (i) highlight the utility celiac disease offers autoimmune research, (ii) discuss safety and personal experience with N. americanus, (iii) summarise the direct and bystander impact that hookworm infection has on mucosal immunity to the parasite and gluten, respectively, and (iv) speculate why this hookworm's success depends on healing its host and how this might impact on a propensity to autoimmunity.

Effects of treatment on IgE responses against parasite allergen-like proteins and immunity to reinfection in childhood schistosome and hookworm coinfections

Naturally occurring human immunity to both schistosomiasis and hookworm infection has been associated with IgE responses against parasite allergen-like proteins. Since the two helminths frequently coinfect the same individuals, there is growing advocacy for their concurrent treatment. However, both helminths are known to exert strong immunomodulatory effects; therefore, coinfected individuals could have immune responses different from those characteristically seen in monoinfected individuals. In this study, we measured changes in IgE, IgG1, and IgG4 responses to schistosome and hookworm antigens, including the allergen-like proteins Schistosoma mansoni tegumental-allergen-like 1 protein (SmTAL1), SmTAL2, and Necator americanus Ancylostoma-secreted protein-2 (Na-ASP-2), following concurrent treatment of schoolchildren coinfected with Schistosoma mansoni and hookworm. Antibody responses to schistosome egg (soluble egg antigen and SmTAL2) or somatic adult hookworm (AHW) antigens either decreased after treatment or were unchanged, whereas those to schistosome worm antigens (soluble worm antigen and SmTAL1) increased. The observed different effects of treatment likely reflect the different modes of drug action and sites of infection for these two helminths. Importantly, there was no evidence that the simultaneous treatment of coinfected children with praziquantel and albendazole affected schistosome- and hookworm-specific humoral responses differently from those characteristic of populations in which only one organism is endemic; schistosome- and hookworm-specific responses were not associated, and there was no evidence for cross-regulation. Posttreatment increases in the levels of IgE to schistosome worm antigens were associated with lower Schistosoma mansoni reinfection intensity, while no associations between humoral responses to AHW antigen and protection from hookworm reinfection were observed in this sample of school-aged children.

Immune modulation and modulators in Heligmosomoides polygyrus infection

The intestinal nematode parasite Heligmosomoides polygyrus bakeri exerts widespread immunomodulatory effects on both the innate and adaptive immune system of the host. Infected mice adopt an immunoregulated phenotype, with abated allergic and autoimmune reactions. At the cellular level, infection is accompanied by expanded regulatory T cell populations, skewed dendritic cell and macrophage phenotypes, B cell hyperstimulation and multiple localised changes within the intestinal environment. In most mouse strains, these act to block protective Th2 immunity. The molecular basis of parasite interactions with the host immune system centres upon secreted products termed HES (H. polygyrus excretory-secretory antigen), which include a TGF-β-like ligand that induces de novo regulatory T cells, factors that modify innate inflammatory responses, and molecules that block allergy in vivo. Proteomic and transcriptomic definition of parasite proteins, combined with biochemical identification of immunogenic molecules in resistant mice, will provide new candidate immunomodulators and vaccine antigens for future research.

Immunity-mediated regulation of fecundity in the nematode Heligmosomoides polygyrus--the potential role of mast cells

Previous studies have shown that host immunity regulates the fecundity of nematodes. The present study was aimed at clarifying the reversible nature of fecundity in response to changes of immunological status and to determine which effector cells are responsible for compromising fecundity in Heligmosomoides polygyrus. Enhanced fecundity was observed in immunocompromised SCID and nu/nu mice compared to those in the corresponding wild-type mice, with significantly fewer numbers of intrauterine eggs produced in the wild-type than in the immunodeficient mice. When 14-day-old adult worms from BALB/c mice were transplanted into naïve BALB/c mice, their fecundity increased significantly as early as 24 h post-transplantation, but not when they were transferred into immune mice, suggesting the plastic and reversible nature of fecundity in response to changes in host immunological status. In mast cell-deficient W/W(v) mice, nematode fecundity was significantly higher than in mast cell-reconstituted W/W(v) or +/+ mice. The serum levels of the mast-cell protease mMCP1 were markedly increased in the wild-type as well as the mast cell-reconstituted W/W(v), but not in the W/W(v), SCID, or nu/nu mice during infection. These findings raise the interesting possibility that certain activities of mast cells, either directly or indirectly, regulate parasite fecundity during infection.

Experimental hookworm infection: a randomized placebo-controlled trial in asthma

BACKGROUND

Epidemiological studies suggest that hookworm infection protects against asthma, and therefore that hookworm infection may have a direct or an indirect therapeutic potential in this disease. We now report the first clinical trial of experimental hookworm infection in people with allergic asthma.

OBJECTIVES

To determine the effects of experimental hookworm infection in asthma.

METHODS

Thirty-two individuals with asthma and measurable airway responsiveness to adenosine monophosphate (AMP) were randomized and double blinded to cutaneous administration of either ten Necator americanus larvae, or histamine solution (placebo), and followed for 16 weeks. The primary outcome was the change in provocation dose of inhaled AMP required to reduce forced expiratory volume in 1 s by 20% (PD(20)AMP) from baseline to week 16. Secondary outcomes included change in several measures of asthma control and allergen skin sensitivity and the occurrence of adverse effects.

RESULTS

Mean PD(20)AMP improved in both groups, more in the hookworm [1.49 doubling doses (DD)] than the placebo group (0.98 DD), but the difference between groups was not significant (0.51 DD; 95% confidence interval: -1.79 to 2.80; P=0.65). There were no significant differences between the two groups for other measures of asthma control or allergen skin sensitization. Infection was generally well tolerated.

CONCLUSIONS

Experimental infection with ten hookworm larvae in asthma did not result in significant improvement in bronchial responsiveness or other measures of asthma control in this study. However, infection was well tolerated and resulted in a non-significant improvement in airway responsiveness, indicating that further studies that mimic more closely natural infection are feasible and should be undertaken.

Depleted intestinal goblet cells and severe pathological changes in SCID mice infected with Heligmosomoides polygyrus

To determine the role of T cells and mast cells in intestinal pathology and immune expulsion of intestinal nematodes, worm burdens, goblet cell responses and villus structures were analysed in T- and B-cell-deficient severe combined immunodeficiency (SCID) mice, athymic nu/nu mice and mast cell deficient W/W(v) mice after infection with the nematode Heligmosomoides polygyrus. SCID and nu/nu mice showed significantly higher worm burdens at week 9 post-infection compared with the wild-type controls. SCID and nu/nu mice showed compromised goblet cell hyperplasia and/or Muc 2 expression, indicating that both events are T-cell dependant. On the other hand, the SCID mice showed increased pathology (villus atrophy and crypt hyperplasia) and increased numbers of proliferating cell nuclear antigen positive cells compared to the wild-type controls. W/W(v) mice, conversely, were able to expel the worms normally, had normal goblet cell hyperplasia, and did not demonstrate the changes in mucosal architecture seen in SCID mice, confirming that a normal mast cell response is not necessarily required for these changes. These results suggest that a functional T-cell response, but not a mast cell response, is necessary for anti-parasite responses, goblet cell function, and maintaining normal mucosal architecture.

The measurement of immunological stress in nematodes

No article involving the nematode cuticle can ignore the work of Donald Lee. Since the publication of his book Physiology of nematodes (Lee, 1965), he has provided us with an extensive knowledge of the structure, composition, maintenance and growth of a variety of nematode cuticles and has undoubtedly contributed a great deal to our understanding of this unique and diverse structure.

T Cell and cytokine basis of host variability in response to intestinal nematode infections

Infection by a variety of species of intestinal nematode infection gives rise to a wide variation in parasite load within a host population. There has been much investigation into the basis of this variation and is thought to involve several factors. Studies of infections of gut dwelling nematodes in laboratory rodents has clearly demonstrated that this variation may be due to the production of cytokines produced as part of the host immune response to infection. More specifically, activation of distinct T helper cell subsets leads to the generation of effective or ineffective responses resulting in clearance of the parasite load or maintenance of chronic infection. The induction of differential responses remains to be determined but is likely to be influenced at a number of levels including involvement of accessory cells and activation of co-stimulatory molecules on antigen presenting cells. Moreover, it appears that these parasites may actively interfere with the host cytokine response to promote their own survival. This review concentrates on recent findings of cytokine mediated control of intestinal nematodes highlighting a central role for the immune system in regulating both acute and chronic infection by these parasites.

Density-dependent effects on the weight of female Ascaris lumbricoides infections of humans and its impact on patterns of egg production

BACKGROUND

Ascaris lumbricoides exhibits density-dependent egg production, a process which has a marked impact on both the transmission dynamics and the stability of the parasite population. Evidence suggests that the egg production of female Ascaris is also associated with the size of the worm. If worm size is mediated by density-dependent processes then the size of female worms may have a causal impact upon patterns of Ascaris egg production.

RESULTS

We analyse data collected from a cohort of human hosts, and demonstrate that the per host mean weight (a proxy for size) of female Ascaris is dependent on the number of infecting females (worm burden) following a pattern of initial facilitation followed by limitation. Applying a negative binomial (NB) generalized linear model (GLM) and a zero-inflated negative binomial (ZINB) model we confirm that the per host female mean weight is significantly associated with per host egg production. Despite these associations, the mean weight of female Ascaris has little causal impact on patterns of density-dependent egg output. The ZINB model is able to account for the disproportionately large number of zero egg counts within the data and is shown to be a consistently better fit than the NB model. The probability of observing a zero egg count is demonstrated as being negatively associated with both female worm burden and female mean weight.

CONCLUSION

The mean weight of female Ascaris is statistically significantly associated with egg output, and follows a consistent pattern of facilitation preceding limitation with increasing female worm burden. Despite these relationships, incorporation of female Ascaris mean weight into models of egg output has little effect on patterns of density dependence. The ZINB model is a superior fit to the data than the NB model and provides additional information regarding the mechanisms that result in a zero egg count. The ZINB model is shown to be a useful tool for the analysis of individual-based egg output data.

Heritability of human hookworm infection in Papua New Guinea

Hookworms infect approximately 740 million humans worldwide and are an important cause of morbidity. The present study examines the role of additive genetic effects in determining the intensity of hookworm infection in humans, and whether these effects vary according to the sex of the host. Parasitological and epidemiological data for a population of 704 subjects in Papua New Guinea were used in variance components analysis. The ‘narrow-sense’ heritability of hookworm infection was estimated as 0·15±0·04 (P<0·001), and remained significant when controlling for shared environmental (household) effects. Allowing the variance components to vary between the sexes of the human host consistently revealed larger additive genetic effects in females than in males, reflected by heritabilities of 0·18 in females and 0·08 in males in a conservative model. Household effects were also higher in females than males, although the overall household effect was not significant. The results indicate that additive genetic effects are an important determinant of the intensity of human hookworm infection in this population. However, despite similar mean and variance of intensity in each sex, the factors responsible for generating variation in intensity differ markedly between males and females.

Geohelminth infections: a review of the role of IgE and assessment of potential risks of anti-IgE treatment

Geohelminth infections are major parasitic infections with a worldwide distribution. Immunoglobulin E (IgE) is considered to play a central role in protective immunity against these parasites although the evidence from experimental animal models infected with helminth parasites and treated with anti-IgE antibodies and from observational studies in human populations of the immunologic correlates of protective immunity against helminths do not support a critical role for IgE in mediating protection against helminths. Anti-IgE treatment of human allergic disorders using a humanized monoclonal IgE antibody (omalizumab, Xolair) has been approved for clinical use in the USA and Europe and there is concern that this treatment may be associated with increased morbidity in populations exposed to helminth infections. A recently published randomized controlled trial investigating the risk of geohelminth infections in allergic patients receiving omalizumab in Brazil has provided some evidence that omalizumab may not be associated with increased morbidity attributable to these parasites. This review examines the evidence for a role of IgE in protective immunity against helminth parasites, discusses the findings of the randomized controlled trial, assesses the potential risks and provides recommendations for anti-IgE treatment in groups of allergic patients with different exposure risks for helminth infections.

T cell-dependent and -independent expression of intestinal epithelial cell-related molecules in rats infected with the nematode Nippostrongylus brasiliensis

To determine how T cells of thymic origin regulate the intestinal mucous response induced by nematode infection, mucin production and goblet cell-specific secretory peptide expression were examined in euthymic rnu/+ and athymic rnu/rnu rats infected with the nematode Nippostrongylus brasiliensis. Euthymic rats showed transient goblet cell hyperplasia and upregulation of mucin production, which returned to preinfection levels by 21 days postinfection, when nematodes had been rejected from the intestine. In athymic rats, which failed to reject nematodes, goblet cell hyperplasia and accelerated mucin production continued at least until 21 days postinfection. Gene transcription of mucin-core peptide (MUC)-2 and -3 and trefoil factor (TFF)-2 and -3 in the jejunal epithelium was upregulated parallel to the levels of goblet cell hyperplasia in both euthymic and athymic rats. On the other hand, resistin-like molecule (Relm)beta, sialyltransferase Siat4c and sulfotransferase 3ST1 showed significantly higher transcription levels in euthymic than in athymic rats at 7 and/or 10 days postinfection. These results suggest that the induction of intestinal mucin production occurs without the activation of thymus-derived T cells, while the expression of Relmbeta, Siat4c and 3ST1 in the intestinal epithelial cells seems to be regulated at least partly by thymus-dependent mechanisms.

Vaccines against blood-feeding nematodes of humans and livestock

This paper summarises the progress towards vaccine development against the major blood-feeding nematodes of man and livestock, the hookworms and Haemonchus contortus, respectively. The impact of the diseases and the drivers for vaccine development are summarized as well as the anticipated impact of the host immune response on vaccine design. The performance requirements are discussed and progress towards these objectives using defined larval and adult antigens, many of these being shared between species. Specific examples include the Ancylostoma secreted proteins and homologues in Haemonchus as well as proteases used for digestion of the blood meal. This discussion shows that many of the major vaccine candidates are shared between these blood-feeding species, not only those from the blood-feeding stages but also those expressed by infective L3s in the early stages of infection. Challenges for the future include: exploiting the expanding genome information for antigen discovery, use of different recombinant protein expression systems, formulation with new adjuvants, and novel methods of field testing vaccine efficacy.

Intestinal allergy expels hookworms: seeing is believing

It is unclear how immunity limits hookworm infection. Australian researchers, using capsule and conventional gastrointestinal endoscopy in volunteers inoculated with Necator americanus, have reported that virtually all larvae reach the intestine within six weeks. Unlike the neutral response surrounding resident hookworms, newly arrived adults provoke an eosinophilic enteropathy. This allergic reaction curtails the attachment of hookworms and accompanies the passage of additional worms as they are expelled from the proximal small intestine.

Allergy controls the population density of Necator americanus in the small intestine

BACKGROUND & AIMS

Nearly 700 million people remain infected with hookworms. Although allergy is intuitively linked to immunity against helminths, few positive examples have been characterized. Larval migration through the lungs has been considered the likely interface at which hookworm attrition occurs. As part of a study evaluating a potential role for hookworms in the modulation of human autoimmunity, we examined parasite migration and intestinal colonization.

METHODS

Capsule and conventional endoscopies supplemented the evaluation of healthy volunteers and Crohn's disease patients recently inoculated with larvae of the human hookworm Necator americanus. Two healthy volunteers with a previously established and stable hookworm infection were inoculated with 50 larvae and had serial capsule endoscopies performed.

RESULTS

Eosinophilic enteritis developed in all subjects after the initial inoculation. Newly inoculated larvae in the 2 subjects with an established infection reliably reached the intestine within 4 weeks. Thereafter, the colony diminished to the host's constitutive status quo because mostly immature worms failed to attach. The intensity of the eosinophilic response correlated negatively with the time available for hookworms to feed and positively with hookworm attrition.

CONCLUSIONS

Necator larval migration to the intestine is uncontested. We propose that allergic inflammation purposefully degrades the hookworm's bite, causing premature detachment, restricted feeding, and expulsion. This novel biological dynamic suggests a new paradigm of hookworm resistance.

Comparative immunology of human and animal models of hookworm infection

Hookworm infection is a major cause of disease burden for animals and humans. Over the past years, the use of animal models in hookworm infections has been driven by the search of new anthelminthic therapies and, especially, vaccine development. These studies also contributed to the advance of knowledge on immunity to hookworms, offering new insights to understand the nature of this parasitic infection. In this article, we will summarize the essential features of the immune response in the two major animal models of hookworm infections (dog and hamster) and then consider its implication for the human immune response.

Immune responses following experimental human hookworm infection

To characterize the immune response following primary human hookworm infection, an adult volunteer was infected with 50 L3 larvae of Necator americanus, reinfected 27 months later and followed for a further 6 months. Clinical signs, blood picture, ex-vivo peripheral blood cytokine production (IFN-gamma, IL-5, IL-13, IL-10 to mitogen and hookworm antigen), acute phase proteins (APP) (C-reactive protein, CRP and alpha1-antitrypsin, alpha1-AT) and antibody levels were determined. Dermatitis, oedema, mild nausea and abdominal discomfort followed the primary infection. Eosinophil counts peaked early during both infections but remained elevated ( approximately 18%) throughout. Transient production of IL-5, IL-13 and APP also followed infection but there were negligible levels of IFN-gamma or IL-10. The onset of nausea, oedema and the initial rise in CRP, alpha1-AT, eosinophilia and IL-5 coincided (days 13-27) with the late larval migration and early establishment of the preadult worms in the intestine. Apart from the eosinophilia these responses declined to baseline levels within 4 months and were less pronounced on re-infection.

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.

Parasite role reversal: worms on trial

Asthma has reached epidemic proportions globally. This has been attributed by many to improved hygiene. The frequent failure of conventional pharmaceuticals to manage the disease has led to the introduction of parasites as a potential alternative therapy for asthma and other immunological diseases. In this article, we briefly review the immunological rationale underpinning therapeutic parasitic infection, describe recently initiated trials, and highlight potential risks and benefits of introducing parasites into patient cohorts.

Immunobiology of hookworm infection

Hookworms infect almost one billion people and are a major cause of iron-deficiency anaemia in developing countries of the tropics. Despite their prevalence and the morbidity they cause, little is known about the immune response to this complex eukaryotic parasite. Recent publications have shed light on the human cellular immune responses to hookworms, as well as mechanisms that hookworms utilize to skew the immune response in its favour. Unlike most other human helminth infections, neither age- nor exposure-related immunity develops in the majority of infected people. A vaccine is therefore a highly desirable goal. To this end, gene sequencing efforts have resulted in the deposition of more than 10,000 hookworm cDNA sequences in the public domain, providing a molecular snapshot of this intriguing parasite and providing novel tools for the development of new control strategies. Significant progress has been made in the development of anti-hookworm recombinant vaccines, and clinical trials are expected to begin in the near future.

Ac-SAA-1, an immunodominant 16 kDa surface-associated antigen of infective larvae and adults of Ancylostoma caninum

A cDNA encoding a surface-associated antigen was cloned from an Ancylostoma caninum infective larvae (L(3)) cDNA library by immunoscreening with pooled human immune sera. The sera were obtained from individuals living in an Ancylostoma duodenale hookworm-endemic region of China, who had light intensity infections and high antibody titers against A. caninum L(3). Ancylostoma caninum surface-associated antigen-1 is encoded by an 843 bp mRNA with a predicted open reading frame of 162 amino acids. Recombinant Ancylostoma caninum surface-associated antigen-1 was expressed in Escherichia coli and used to prepare a specific antiserum. A Western blot with anti-Ancylostoma caninum surface-associated antigen-1 specific antiserum showed that native Ancylostoma caninum surface-associated antigen-1 protein is expressed by both L(3) and adult hookworms; RT-PCR confirmed that the mRNA is transcribed in both stages. In adult hookworms, the protein localised to the basal layer of the cuticle and hypodermis of adult worms. Serological analysis determined that recombinant Ancylostoma caninum surface-associated antigen-1 protein is recognised by 61% of human sera from a Necator americanus hookworm endemic area in China, indicating the antigen is immunodominant. Anti-Ancylostoma caninum surface-associated antigen-1 antiserum partially inhibited (46.7%) invasion of hookworm L(3) into dog skin in vitro. Together these results suggest that Ancylostoma caninum surface-associated antigen-1 offers promise as a protective vaccine antigen.

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.

A secreted protein from the human hookworm necator americanus binds selectively to NK cells and induces IFN-gamma production

Parasitic helminths induce chronic infections in their hosts although, with most human helminthiases, protective immunity gradually develops with age or exposure of the host. One exception is infection with the human hookworm, Necator americanus, where virtually no protection ensues over time. Such observations suggest these parasites have developed unique mechanisms to evade host immunity, leading us to investigate the role of the excretory/secretory (ES) products of adult N. americanus in manipulating host immune responses. Specifically, we found that a protein(s) from ES products of adult N. americanus bound selectively to mouse and human NK cells. Moreover, incubation of purified NK cells with N. americanus ES products stimulated the production of augmented (4- to 30-fold) levels of IFN-gamma. This augmentation was dependent on the presence of both IL-2 and IL-12 and was endotoxin-independent. This is the first report of a pathogen protein that binds exclusively to NK cells and the first report of a nematode-derived product that induces abundant levels of cytokines from NK cells. Such an interaction could provide a means of cross-regulating deleterious Th2 immune responses in the host, thereby contributing to the long-term survival of N. americanus.

Progress in the development of a recombinant vaccine for human hookworm disease: the Human Hookworm Vaccine Initiative

Hookworm infection is one of the most important parasitic infections of humans, possibly outranked only by malaria as a cause of misery and suffering. An estimated 1.2 billion people are infected with hookworm in areas of rural poverty in the tropics and subtropics. Epidemiological data collected in China, Southeast Asia and Brazil indicate that, unlike other soil-transmitted helminth infections, the highest hookworm burdens typically occur in adult populations, including the elderly. Emerging data on the host cellular immune responses of chronically infected populations suggest that hookworms induce a state of host anergy and immune hyporesponsiveness. These features account for the high rates of hookworm reinfection following treatment with anthelminthic drugs and therefore, the failure of anthelminthics to control hookworm. Despite the inability of the human host to develop naturally acquired immune responses to hookworm, there is evidence for the feasibility of developing a vaccine based on the successes of immunising laboratory animals with either attenuated larval vaccines or antigens extracted from the alimentary canal of adult blood-feeding stages. The major antigens associated with each of these larval and adult hookworm vaccines have been cloned and expressed in prokaryotic and eukaryotic systems. However, only eukaryotic expression systems (e.g., yeast, baculovirus, and insect cells) produce recombinant proteins that immunologically resemble the corresponding native antigens. A challenge for vaccinologists is to formulate selected eukaryotic antigens with appropriate adjuvants in order to elicit high antibody titres. In some cases, antigen-specific IgE responses are required to mediate protection. Another challenge will be to produce anti-hookworm vaccine antigens at high yield low cost suitable for immunising large impoverished populations living in the developing nations of the tropics.

Human hookworm infection in the 21st century

The scientific study of human hookworm infection began at the dawn of the twentieth century. In recent years, there have been dramatic improvements in our understanding of many aspects of this globally widespread parasite. This chapter reviews recent advances in our understanding in the biology, immunology, epidemiology, public health significance and control of hookworm, and to look forward to the study of this important parasite in the 21st century. Advances in molecular biology has lead to the identification of a variety of new molecules from hookworms, which have importance either in the molecular pathogenesis of hookworm infection or in the host-parasite relationship; some are also promising vaccine targets. At present, relatively little is known about the immune responses to hookworm infection, although it has recently been speculated that hookworm and other helminths may modulate specific immune responses to other pathogens and vaccines. Our epidemiological understanding of hookworm has improved through the development of mathematical models of transmission dynamics, which coupled with decades of field research across multiple epidemiological settings, have shown that certain population characteristics can now be recognised as common to the epidemiology, population biology and control of hookworm and other helminth species. Recent recognition of the subtle, but significant, impact of hookworm on health and education, together with the simplicity, safety, low cost and efficacy of chemotherapy has spurred international efforts to control the morbidity due to infection. Large-scale treatment programmes are currently underway, ideally supported by health education and integrated with the provision of improved water and sanitation. There are also on-going efforts to develop novel anthelmintic drugs and anti-hookworm vaccines.