Modulation of intestinal epithelial permeability by chronic small intestinal helminth infections

Increased permeability of the intestinal epithelial layer is linked to the pathogenesis and perpetuation of a wide range of intestinal and extra-intestinal diseases. Infecting humans with controlled doses of helminths, such as human hookworm (termed hookworm therapy), is proposed as a treatment for many of the same diseases. Helminths induce immunoregulatory changes in their host which could decrease epithelial permeability, which is highlighted as a potential mechanism through which helminths treat disease. Despite this, the influence of a chronic helminth infection on epithelial permeability remains unclear. This study uses the chronically infecting intestinal helminth Heligmosomoides polygyrus to reveal alterations in the expression of intestinal tight junction proteins and epithelial permeability during the infection course. In the acute infection phase (1 week postinfection), an increase in intestinal epithelial permeability is observed. Consistent with this finding, jejunal claudin-2 is upregulated and tricellulin is downregulated. By contrast, in the chronic infection phase (6 weeks postinfection), colonic claudin-1 is upregulated and epithelial permeability decreases. Importantly, this study also investigates changes in epithelial permeability in a small human cohort experimentally challenged with the human hookworm, Necator americanus. It demonstrates a trend toward small intestinal permeability increasing in the acute infection phase (8 weeks postinfection), and colonic and whole gut permeability decreasing in the chronic infection phase (24 weeks postinfection), suggesting a conserved epithelial response between humans and mice. In summary, our findings demonstrate dynamic changes in epithelial permeability during a chronic helminth infection and provide another plausible mechanism by which chronic helminth infections could be utilized to treat disease.

Safety and tolerability of experimental hookworm infection in humans with metabolic disease: study protocol for a phase 1b randomised controlled clinical trial

BACKGROUND

Abdominal obesity and presence of the metabolic syndrome (MetS) significantly increase the risk of developing diseases such as Type 2 diabetes mellitus (T2DM) with escalating emergence of MetS and T2DM constituting a significant public health crisis worldwide. Lower prevalence of inflammatory and metabolic diseases such as T2DM in countries with higher incidences of helminth infections suggested a potential role for these parasites in the prevention and management of certain diseases. Recent studies confirmed the potential protective nature of helminth infection against MetS and T2DM via immunomodulation or, potentially, alteration of the intestinal microbiota. This Phase 1b safety and tolerability trial aims to assess the effect of inoculation with helminths on physical and metabolic parameters, immune responses, and the microbiome in otherwise healthy women and men.

METHODS

Participants eligible for inclusion are adults aged 18-50 with central obesity and a minimum of one additional feature of MetS recruited from the local community with a recruitment target of 54. In a randomised, double-blind, placebo-controlled design, three groups will receive either 20 or 40 stage three larvae of the human hookworm Necator americanus or a placebo. Eligible participants will provide blood and faecal samples at their baseline and 6-monthly assessment visits for a total of 24 months with an optional extension to 36 months. During each scheduled visit, participants will also undergo a full physical examination and complete diet (PREDIMED), physical activity, and patient health (PHQ-9) questionnaires. Outcome measurements include tolerability and safety of infection with Necator americanus, changes in metabolic and immunological parameters, and changes in the composition of the faecal microbiome.

DISCUSSION

Rising cost of healthcare associated with obesity-induced metabolic diseases urgently calls for new approaches in disease prevention. Findings from this trial will provide valuable information regarding the potential mechanisms by which hookworms, potentially via alterations in the microbiota, may positively influence metabolic health.

TRIAL REGISTRATION

The protocol was registered on ANZCTR.org.au on 05 June 2017 with identifier ACTRN12617000818336. Alternatively, a Google search using the above trial registration number will yield a direct link to the trial protocol within the ANZCTR website.

Immune polarization by hookworms: taking cues from T helper type 2, type 2 innate lymphoid cells and alternatively activated macrophages

Cellular and molecular investigation of parasitic helminth infections has greatly accelerated the understanding of type 2 immune responses. However, there remains considerable debate regarding the specific leucocytes that kill parasites and whether these mechanisms are distinct from those responsible for tissue repair. Herein, we chronicle discoveries over the past decade highlighting current paradigms in type 2 immunity with a particular emphasis upon how CD4(+) T helper type 2 cells, type 2 innate lymphoid cells and alternatively activated macrophages coordinately control helminth-induced parasitism. Primarily, this review will draw from studies of the murine nematode parasite Nippostrongylus brasiliensis, which bears important similarities to the human hookworms Ancylostoma duodenale and Necator americanus. Given that one or more hookworm species currently infect millions of individuals across the globe, we propose that vaccine and/or pharmaceutical-based cure strategies targeting these affected human populations should incorporate the conceptual advances outlined herein.

Impact of experimental hookworm infection on the human gut microbiota

The interactions between gastrointestinal parasitic helminths and commensal bacteria are likely to play a pivotal role in the establishment of host-parasite cross-talk, ultimately shaping the development of the intestinal immune system. However, little information is available on the impact of infections by gastrointestinal helminths on the bacterial communities inhabiting the human gut. We used 16S rRNA gene amplification and pyrosequencing to characterize, for the first time to our knowledge, the differences in composition and relative abundance of fecal microbial communities in human subjects prior to and following experimental infection with the blood-feeding intestinal hookworm, Necator americanus. Our data show that, although hookworm infection leads to a minor increase in microbial species richness, no detectable effect is observed on community structure, diversity or relative abundance of individual bacterial species.

Suppression of inflammatory immune responses in celiac disease by experimental hookworm infection

We present immunological data from two clinical trials where the effect of experimental human hookworm (Necator americanus) infection on the pathology of celiac disease was evaluated. We found that basal production of Interferon- (IFN-)γ and Interleukin- (IL-)17A from duodenal biopsy culture was suppressed in hookworm-infected participants compared to uninfected controls. Increased levels of CD4+CD25+Foxp3+ cells in the circulation and mucosa are associated with active celiac disease. We show that this accumulation also occurs during a short-term (1 week) oral gluten challenge, and that hookworm infection suppressed the increase of circulating CD4+CD25+Foxp3+ cells during this challenge period. When duodenal biopsies from hookworm-infected participants were restimulated with the immunodominant gliadin peptide QE65, robust production of IL-2, IFN-γ and IL-17A was detected, even prior to gluten challenge while participants were strictly adhering to a gluten-free diet. Intriguingly, IL-5 was produced only after hookworm infection in response to QE65. Thus we hypothesise that hookworm-induced TH2 and IL-10 cross-regulation of the TH1/TH17 inflammatory response may be responsible for the suppression of these responses during experimental hookworm infection.

Necator americanus infection: a possible cause of altered dendritic cell differentiation and eosinophil profile in chronically infected individuals

BACKGROUND

Hookworms survive for several years (5 to 7 years) in the host lumen, inducing a robust but largely ineffective immune response. Among the most striking aspects of the immune response to hookworm (as with many other helminths) is the ablation of parasite-specific T cell proliferative response (hyporesponsiveness). While the role of the adaptive immune response in human helminth infection has been well investigated, the role of the innate immune responses (e.g., dendritic cells and eosinophils) has received less attention and remains to be clearly elucidated.

METHODOLOGY/PRINCIPAL FINDINGS

We report on the differentiation/maturation of host dendritic cells in vitro and the eosinophil activation/function associated with human hookworm infection. Mature DCs (mDCs) from Necator americanus (Necator)-infected individuals showed an impaired differentiation process compared to the mDCs of non-infected individuals, as evidenced by the differential expression of CD11c and CD14. These same hookworm-infected individuals also presented significantly down-regulated expression of CD86, CD1a, HLA-ABC, and HLA-DR. The lower expression of co-stimulatory and antigen presentation molecules by hookworm-infected-derived mDCs was further evidenced by their reduced ability to induce cell proliferation. We also showed that this alternative DC differentiation is partially induced by excreted-secreted hookworm products. Conversely, eosinophils from the same individuals showed a highly activated status, with an upregulation of major cell surface markers. Antigen-pulsed eosinophils from N. americanus-infected individuals induced significant cell proliferation of autologous PBMCs, when compared to non-infected individuals.

CONCLUSION

Chronic N. americanus infection alters the host's innate immune response, resulting in a possible modulation of the maturation process of DCs, a functional change that may diminish their ability for antigen presentation and thus contribute to the ablation of the parasite-specific T cell proliferative response. Interestingly, a concomitant upregulation of the major cell surface markers of eosinophils was observed in hookworm-infected individuals, indicative of antigen-specific immune responses, especially antigen presentation. We showed that in addition to the postulated role of the eosinophils as effector cells against helminth infection, activated cells may also be recruited to sites of inflammation and contribute to the immune response acting as antigen presenting cells.

[Relevance of helminths in the prevention and healing of immune diseases]

The incidence of immune diseases has had a steady significant increase in the industrialized countries since the end of the last century. Epidemiological data suggest that prevention of exposure to helminths may be one of the factors promoting the rise in these diseases. Animal models of immune diseases support this concept by showing that these parasites can prevent and reverse developed diseases. Helminths strongly modulate the host's immune system inducing regulatory immune cells and pathways that afford protection from these illnesses. Helminthic therapy has emerged as a promising treatment for several immune diseases; the potential benefits are startling and new therapeutic strategies for the control of these conditions may result.

Basophil competence during hookworm (Necator americanus) infection

A popular hypothesis to explain parasite survival in the presence of a pronounced T helper 2 phenotype in helminth-parasitized populations has been Fc epsilonRI blockade by parasite-induced polyclonal IgE. To begin to test the hypothesis that Fc epsilonRI-bearing cells would be refractory to activation in parasitized populations, we investigated basophil function in 43 individuals from a hookworm endemic area. Study individuals had high levels of total IgE and eosinophilia and a mean hookworm burden of 2,257 epg. Basophils from all members of this parasitized population were shown to release histamine to a number of agonists, including anti IgE and a hookworm allergen, calreticulin. These data would indicate that Fc epsilonRI blockade at the level of the basophil did not occur in this parasitized population despite the presence of possible immunologic blocking agents. This would suggest that this effector arm of the T helper 2 phenotype remains operative in infected populations.

Stage-specific immune responses in human Necator americanus infection

We describe how hookworms interact with their human hosts by comparing lymphocyte phenotyping, proliferative responses, and cytokine and chemokine secretion patterns in adults who are either mono-infected with Necator americanus or egg-negative controls resident in an area of high transmission in Brazil. Cellular immune responses against crude hookworm antigen extracts from different developmental stages were evaluated simultaneously. Principal component analysis (PCA) was used to reduce the standardized immune responses. Random effects multivariate regression was then used to investigate whether principal components (PC) differ between the two groups once potential confounders and effect modifiers have been accounted for. Although hookworm patients had reduced percentages of T and B cells, they had higher levels of activated CD4⁺ T and CD19⁺ B cells. This state of ‘immune activation’ coincided with lower proliferative responses, especially to third-stage larval antigen. Cytokine levels in mono-infected adults were also lower and characterized by a mixed Th1/Th2-type profile. Excretory/secretory antigen from adult worms was a potent modulator of the immune response, resulting in diminished TNF-α and IL-10 secretion in peripheral blood mononuclear cells (PBMC) from hookworm infected patients. We propose that the longevity of hookworms in their human hosts results from a stage-specific, down-modulation of the immune response.

Onchocerca volvulus-specific antibody and cytokine responses in onchocerciasis patients after 16 years of repeated ivermectin therapy

The recommended control option against onchocerciasis is repeated ivermectin treatment, which will need to be implemented for decades, and it remains unknown how repeated ivermectin therapy might affect immunity against Onchocerca volvulus in the long term. O. volvulus-specific antibody reactivity and cellular cytokine production were investigated in onchocerciasis patients receiving ivermectin (150 microg/kg) annually for 16 years. In treated patients, the T helper type 2 (Th2) cytokine interleukin (IL)-5 and T regulatory IL-10 in response to O. volvulus antigen (OvAg) and bacteria-derived Streptolysin O (SL-O) diminished to levels found in infection-free endemic controls; also, cellular release of Th1-type interferon (IFN)-gamma at 16 years post initial ivermectin treatment (p.i.t.) approached control levels. In ivermectin-treated onchocerciasis patients, IL-5 production in responses to the mitogen phytohaemagglutinin (PHA) decreased, but IL-10 in response PHA increased, and neither attained the cytokine production levels of endemic controls. At 16 years p.i.t., O. volvulus-specific IgG1 and IgG4 subclass reactivity still persisted at higher levels in onchocerciasis patients than in O. volvulus exposed but microfilariae-free endemic controls. In addition, cytokine responses remained depressed in onchocerciasis patients infected concurrently with Mansonella perstans and Necator americanus or Entamoeba histolytica/dispar. Thus, long-term ivermectin therapy of onchocerciasis may not suffice to re-establish fully a balanced Th1 and Th2 immune responsiveness in O. volvulus microfilariae-negative individuals. Such deficient reconstitution of immune competence may be due to an as yet continuing and uncontrolled reinfection with O. volvulus, but parasite co-infections can also bias and may prevent the development of such immunity.

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.

Protective immunity elicited by ultraviolet-irradiated third-stage infective hookworm (Necator americanus and Ancylostoma caninum) larvae in mice and hamsters

The protective immunity elicited by ultraviolet-irradiated third-stage infective larvae of Necator americanus (UV-NaL3) and Ancylostoma caninum (UV-AcL3) was evaluated in laboratory mice (a non-permissive model) and hamsters (a permissive model). After optimizing the time of exposure to UV-irradiation, both oral and subcutaneous vaccination routes with UV-AcL3 in mice were explored. Oral vaccination was more effective at reducing the number of challenge AcL3 entering the lungs, whereas subcutaneous vaccination was more effective at blocking muscle entry. When UV-irradiated NaL3 and non-irradiated AcL3 were used as vaccines in hamsters, both of them were effective at reducing adult hookworm burdens. However, the length of protection afforded by UV-irradiated L3 was substantially greater than that resulting from immunization with non-irradiated L3. A single dose was less effective than multiple doses. The protective immunity elicited by UV-irradiated NaL3 given once every other week for a total of three immunizations was similar to that elicited by non-irradiated AcL3 given during the same schedule. Protection was not significantly affected by administering the L3 on a weekly basis for a total of three immunizations, even though the antibody titers were reduced using this schedule. These studies will facilitate the elucidation of the mechanisms underlying larval protection.

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.

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.

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.

X-ray structure of Na-ASP-2, a pathogenesis-related-1 protein from the nematode parasite, Necator americanus, and a vaccine antigen for human hookworm infection

Human hookworm infection is a major cause of anemia and malnutrition of adults and children in the developing world. As part of on-going efforts to control hookworm infection, The Human Hookworm Vaccine Initiative has identified candidate vaccine antigens from the infective L3 larval stages of the parasite, including a family of pathogenesis-related (PR) proteins known as the Ancylostoma-secreted proteins (ASPs). A novel crystal structure of Na-ASP-2, a PR-1 protein secreted by infective larvae of the human hookworm Necator americanus, has been solved to resolution limits of 1.68 A and to an R-factor of 17% using the recombinant protein expressed in and secreted by Pichia pastoris. The overall fold of Na-ASP-2 is a three-layer alphabetaalpha sandwich flanked by an N-terminal loop and a short, cysteine-rich C terminus. Our structure reveals a large central cavity that is flanked by His129 and Glu106, two residues that are well conserved in all parasitic nematode L3 ASPs. Na-ASP-2 has structural and charge similarities to chemokines, which suggests that Na-ASP-2 may be an extra-cellular ligand of an unknown receptor. Na-ASP-2 is a useful homology model for NIF, a natural antagonistic ligand of CR3 receptor. From these modeling studies, possible binding modes were predicted. In addition, this first structure of a PR-1 protein from parasitic helminths may shed light on the molecular basis of host-parasite interactions.

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.

Immune Responses in Human Necatoriasis: Association between Interleukin‐5 Responses and Resistance to Reinfection

Cytokine and proliferative responses to Necator americanus infection were measured in a treatment-reinfection study of infected subjects from an area of Papua New Guinea where N. americanus is highly endemic. Before treatment, most subjects produced detectable interleukin (IL)-4 (97%), IL-5 (86%), and interferon (IFN)- gamma (64%) in response to adult N. americanus antigen. Pretreatment IFN- gamma responses were negatively associated with hookworm burden, decreasing by 18 pg/mL for each increase of 1000 eggs/gram (epg) (n=75; P<.01). Mean IFN- gamma responses increased significantly after anthelmintic treatment, from 166 to 322 pg/mL (n=42; P<.01). The intensity of reinfection was significantly negatively correlated with pretreatment IL-5 responses, decreasing by 551 epg for each 100 pg/mL increase in production of IL-5 (n=51; P<.01). These data indicate that there is a mixed cytokine response in necatoriasis, with worm burden-associated suppression of IFN- gamma responses to adult N. americanus antigen. Resistance to reinfection is associated with the parasite-specific IL-5 response.

Cellular responses and cytokine production in post-treatment hookworm patients from an endemic area in Brazil

Human hookworm infections are distributed widely in tropical areas and have a significant impact on host morbidity and human health. In the present study, we investigated the cellular responsiveness and cytokine production in peripheral blood mononuclear cells (PBMC) from Necator americanus-infected schoolchildren who had recently received chemotherapy, and compared them with non-infected endemic controls. Hookworm patients and treated, egg-negative individuals showed a lower cellular reactivity against phytohaemagglutinin (PHA) and hookworm antigen when compared with egg-negative endemic controls. The baseline production of proinflammatory tumour necrosis factor-alpha (TNF-alpha) in PBMC from infected patients and treated, egg-negative individuals was elevated. On the other hand, PHA- or hookworm antigen-induced interleukin (IL)-12 and interferon (IFN)-gamma secretion was higher in endemic controls than in hookworm patients, who either continued egg-positive or were egg-negative after treatment. Also, PBMC from endemic controls secreted more IL-5 and IL-13 than the other patient groups. Opposite to that, the spontaneous as well as the antigen-driven IL-10 secretion was lower in endemic controls when compared with the other groups. In summary, patently hookworm-infected as well as egg-negative treated patients disclosed an elevated spontaneous cellular secretion of proinflammatory TNF-alpha, a prominent secretion of regulatory Th2-type IL-10 and an impaired production of IL-12, IFN-gamma, IL-5 and IL-13.

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.

Successful vaccination of BALB/c mice against human hookworm (Necator americanus): the immunological phenotype of the protective response

In this murine (BALB/c) model of necatoriasis, high levels of protection against challenge infection by Necator americanus larvae (n=300) were afforded by successive vaccinations at 14-day intervals, either subcutaneously or percutaneously, with gamma-irradiated N. americanus larvae (n=300). Percutaneous vaccination was significantly more effective than the subcutaneous route, with pulmonary larval burdens at 3 days post-infection being reduced by 97.8 vs. 89.3%, respectively, after three immunisations (P<0.05). No worms were recovered from the intestines of thrice vaccinated mice. Two percutaneous vaccinations also reduced worm burdens, by 57% in the lungs and 98% in the intestines; P<0.05. In vaccinated animals, lung pathology (mainly haemorrhage) following infection was greatly reduced compared with non-vaccinated animals. In vaccinated mice (but not in non-vaccinated mice) mast cells accumulated in the skin and were degranulated. RT-PCR analyses of mRNAs in the skin of vaccinated animals indicated increased expression of interleukin (IL)-4 relative to gamma-interferon (gamma-IFN). Lymphocytes from the axillary (skin-draining) lymph nodes of vaccinated mice, stimulated in vitro with concanavalin A, exhibited enhanced secretion of IL-4 protein and a higher IL-4/gamma-IFN protein ratio than lymphocytes from non-vaccinated animals. In vaccinated mice, levels of IgG1 and IgG3 (directed against larval excretory/secretory products) were elevated for the most part compared with those in non-vaccinated animals. These data demonstrate the successful vaccination of BALB/c mice against human hookworm infection and suggest that a localised Th2 response may be important for conferring protection against necatoriasis.

Innate and cognate mechanisms of pulmonary eosinophilia in helminth infection

Passage of helminth larvae through the lungs can cause pulmonary eosinophilia that may have evolved as a means of parasite attrition. If allergic responses represent a misdirected activation of this arm of the immune system, then mechanisms governing eosinophil recruitment during infection would be expected to be closely related to those seen in allergy. We studied primary Necator americanus infection and compared this to multiply-infected or vaccinated mice. The arrival of larvae in the lungs triggered rapid eosinophil recruitment, which was greatly enhanced in previously sensitized mice. Interestingly, the presence of larvae in the lung was sufficient to trigger eosinophil chemoattractant production, including the chemokines eotaxin and MIP-1alpha, and was not enhanced by prior exposure to the parasites. Infection stimulated IL-5 production in all groups; however, this and IgE production were greatly enhanced in sensitized animals. Elevated IL-5 increased bone marrow production of eosinophils, and eosinophilia was abrogated by treatment with anti-IL-5 antibody. Therefore, trapping of larvae in the pulmonary vasculature is sufficient to trigger eosinophil recruitment, by induction of chemokines and IL-5. Primed cognate Th2 immunity does not increase local chemokine production, but does increase IL-5 production, which greatly enhances the availability of eosinophils for recruitment to the lung.

T-lymphocyte subsets in patients with hookworm infection in Zaria, Nigeria

Peripheral lymphocyte subsets CD3, CD4, CD8 were studied using monoclonal antibodies to determine the mechanism of immunosuppression observed in an earlier study with total T-cells using the sheep erythrocyte rosetting technique. The study was carried out in 37 Nigerian patients with hookworm infection (Necator americanus and Ancylostoma duodenale and 17 healthy Nigerians as controls. The leucocyte migration inhibition (LMI) test was also carried out to assess the functional integrity of the lymphocytes, while the hookworm status of the patients was assessed by the Stoll technique for measuring egg counts. The results of the T-cell studied showed that CD3 and CD4 cell percentages were significantly depressed in hookworm patients compared to controls (P<0.05). The difference between mean CD8 percentages and absolutes in hookworms patients and controls were not significant. Absolute CD3 and CD4 numbers were not significantly lowered compared to controls in the study and CD4/CD8 ratios were insignificantly reduced. Patients with polyparasitism including hookworm infection showed significant reduction in CD3 and CD4 cells when compared with controls and with patients with hookworm infection alone. The leucocyte migration inhibition response to purified protein derivative of M. tuberculosis (PPD) was significantly decreased in hookworm patients compared to controls, confirming that functionally cell mediated immunity is depressed in hookworm infection.

Parasite-specific antibody and cellular immune responses in human infected with Necator americanus and Oesophagostomum bifurcum

In this study parasite-specific antibody, cellular reactivity and Thl-type or Th2-type cytokine responses were investigated in humans concurrently infected with Necator americanus and Oesophagostomum bifurcum. The prospects for O. bifurcum-specific serodiagnosis based on IgG4 and IgE were evaluated. IgG4 showed low specificity for O. bifurcum due to antigen cross-reactivity with N. americanus, while IgE specifically distinguished between hookworm and O. bifurcum, and, in doubly infected patients, levels of O. bifurcum-specific as well as N. americanus-specific IgE were significantly elevated compared to those with N. americanus mono-infections. Cellular immunity was not strictly dominated by a Thl- or Th2- type reactivity. In co-infected patients cellular unresponsiveness to parasite antigens was observed, while cellular production of tumour necrosis factor alpha (TNF-alpha) and gamma-interferon (IFN-gamma) was greater in those doubly infected. Th2-type cytokines (interleukin-5 and interleukin-10) were produced in equal amounts by peripheral blood mononuclear cells from individuals with mono- and coinfections. Such mixed Thl-type and Th2-type immune responsiveness associated with persisting gastrointestinal parasitic nematodes may reflect a state of infection at which parasite-induced inflammatory and enteropathogenic responses co-exist, and furthermore, helminth coinfection will not only suppress parasite-specific cellular responsiveness but may also direct cytokine production towards a "permissive Th1-type cytokine profile" that favours parasite persistence.

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 hookworm allergen which strongly resembles calreticulin

Immmoglobulin E-rich plasma from patients from Papua New Guinea infected with Necator americanus has been used to probe an adult N. americanus cDNA library for the presence of hookworm allergens. Using this approach, one hookworm allergen has been identified as calreticulin, which was subsequently expressed in Escherichia coli. Little serological cross reactivity was seen between the recombinant calreticulins of this hookworm and its host. Prospective roles for hookworm calreticulin in the host-parasite relationship are discussed in depth.

Necator americanus in inbred mice: evidence in support of genetically determined differences in the cellular immune response to a primary infection

The blood and broncho-alveolar leucocyte (BAL) responses, to a primary Necator americanus infection, were studied in male BALB/c and NIH mice. Following percutaneous infection, a significant blood leucocytosis occurred in both the BALB/c and NIH strains. The peak response occurred, in both strains, on day 10 p.i. and this was reflected in the eosinophil response which peaked at the same time-point. The change in blood eosinophil numbers, as a result of infection, was found to be the greatest recorded for any cell type. In BALB/c mice, however, infection elicited a significantly more intense total leucocyte, lymphocyte and eosinophil response than in NIH mice. In contrast, the BAL response was more intense in the NIH strain. Peak BAL responses were observed between days 12 and 16 p.i., in both strains, and the change in broncho-alveolar eosinophil numbers, as a result of infection, was found to be the greatest recorded for any cell type. The relationship between the observed leucocyte responses and resistance to the migrating larvae of a primary infection is discussed.

IgG4 responses to antigens of adult Necator americanus: potential for use in large-scale epidemiological studies

Described is an epidemiological investigation of hookworm infections in a rural community in Zimbabwe, where Necator americanus is the only human helminth species present. Among a cohort of 120 individuals the overall prevalence of infection was 78%. Intensity of infection was quantified both as egg counts (range: 0-2563 eggs per g of stool) and worm burden (range: 0-100 worms). Although both these measures provide useful quantitative data, they are tedious to determine in large-scale epidemiological studies and may present social and logistic difficulties. As an alternative screening method, we therefore investigated isotype-specific responses to adult worm antigens of N. americanus. The results show that specific IgG4 responses correlate positively and significantly with both measures of intensity and may be a useful marker of hookworm infection.

Gastrointestinal nematodes: the Karkar experience

This paper reviews our research on the hookworm Necator americanus over several years. Our field site for the research was on Karkar Island, Papua New Guinea, where we found a prevalence of N. americanus infection among adults of nearly 100%. The intensity of infection was related to host age and to the development of iron deficiency anaemia, which occurred at a much lower infection intensity than had been previously reported. We studied the immune response to infection and our results initially suggested that antibody responses and eosinophilia do not protect the host against infection. However, we have more recently found a negative correlation of both IgE and eosinophilia with the weight and fecundity of N. americanus which suggests that the immune response does have some effect on N. americanus and that this immunity is dependent on the Th2 subset of T lymphocytes. Following treatment for hookworm, the prevalence of N. americanus returned almost to pretreatment levels within 2 years, with the rate of acquisition of adult worms independent of host age. A significant predisposition to hookworm infection was demonstrated by individuals. Prevention will result from measures to reduce the transmission and intensity of infection, and can be achieved through improved sanitation or by vaccination. However, vaccination is not yet a viable option because of our limited knowledge about protective immunity.

The specificity of the human IgE response to Necator americanus

Human IgE responses to helminth infections have been described as both problematic and beneficial, in that type 1 hypersensitivity to parasite infections can reputedly cause pathology and/or parasite expulsion, (Pritchard, Quinnell & Walsh 1995). In the present communication, we can report that the IgE response to helminth infections may be beneficial in a diagnostic sense, in that antibodies of this isotype showed minimal cross-reactivity against antigens from the parasites tested. This information may be of benefit to workers attempting to establish ELISA assays for the specific diagnosis of zoonotic hookworm infections, for example, Ancylostoma caninum (Croese 1988), particularly in regions where human hookworm infection is endemic.

Glutathione S-transferase (GST) expression in the human hookworm Necator americanus: potential roles for excretory-secretory forms of GST

The difficulty in demonstrating protective immunity to human gastro-intestinal nematodes is thought to be a consequence of the expression of defences by the parasites directed against the toxic metabolites of leukocytes produced during inflammation (Brophy and Pritchard, 1992a). Parasite glutathione S-transferases (GSTs) may provide part of this defence by detoxifying the secondary products of lipid peroxidation produced via immune initiated free-radical attack on host or parasite membranes (Brophy and Pritchard, 1994; Taylor et al., 1988). Neutralisation of parasite immune defence components could tip the molecular balance in favour of the immune response during chronic infections. For example, GSTs have been extensively investigated from the digenean parasites Schistosoma and Fasciola hepatica and provide protection in animal-model systems (Mitchell, 1988; Wijffels et al., 1991). In contrast, although GSTs have been initially characterised in filarial nematodes (Salinas et al., 1994; Leibau et al., 1994; Jaffe and Lambert, 1986), there is limited information on GSTs from human gastro-intestinal nematode parasites. We were particularly interested in analysing the products of hookworms for evidence of the presence of excretory-secretory forms of this putative immune defence protein.

Immunoepidemiology of human necatoriasis: correlations between antibody responses and parasite burdens

In this paper we describe the relationship between antibodies to Necator americanus stage-specific antigens and parasite burden in an endemically-infected population in Papua New Guinea. Using an age-structured analysis, we show that the correlation coefficient between levels of IgG against adult worm excretory-secretory (ES) antigen and parasite burden declined significantly with host age from positive in younger hosts to significantly negative in older hosts. A trend towards similar patterns was present for anti-larval IgG both pretreatment and after reinfection, and for anti-ES IgM and anti-ES IgE pretreatment. These patterns are consistent with a role for these isotypes in a protective immune response, although parasite-induced immunosuppression may provide an alternate explanation. This is another demonstration of possibly protective responses to N. americanus infection.

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

The effect of the humoral immune response on the weight and fecundity of the hookworm Necator americanus was examined in an endemically-infected human population. There was a highly significant negative correlation between total IgE levels and parasite weight and fecundity, after controlling for any effects of host age and hookworm burden. This correlation was present both at initial treatment and after 2 years' reinfection. There was a similar negative correlation between the number of eosinophils and hookworm weight and fecundity at initial treatment. Correlations with levels of specific antibodies to N. americanus excretory-secretory products were weaker and not significant, although there was a trend towards negative correlations with anti-ES IgE. This is the first field evidence for an effective human immune response to N. americanus. Although the mechanism of this effect is not clear, we suggest that total IgE levels reflected the level of Th2 cell activation.

Immunosuppression in Nigerians with hookworm infection

T rosette cells results in patients with (Necator americanus and Ancylostoma duodenale) hookworm infection, divided into groups on the basis of the degree of anaemia (Group I had marked anaemia Hb 1-7, hypochromia ), Group II had moderate anaemia (Hb 8-11, hypochromia ++) and Group III had no signs of anaemia despite the underlying hookworm infection), were compared with results in 28 healthy controls who had no signs of other infection and excreting no ova of hookworm or other parasites. The level of sheep erythrocyte rosette-forming T-lymphocytes (SE-TL) in the patients (Group I:42 +/- 8%; Group II:45 +/- 11%; Group III:47 +/- 6%) was markedly reduced compared to controls (58 +/- 13%) (P < 0.001). The leucocyte migratory responses to purified protein derivative (P.P.D); were significantly (P < 0.05) reduced in all the subgroups as compared with the controls. The results support the hypothesis of the existence of immunosuppression in patients with hookworm infection.

Resistance of the hookworms Ancylostoma ceylanicum and Necator americanus to intestinal inflammatory responses induced by heterologous infection

Experiments were carried out to ascertain whether the acute inflammatory phase of the intestinal response of hamsters to infection with Trichinella spiralis would adversely affect hookworms in concurrently infected animals. The survival and growth of hookworms were unaffected. However, the presence of hookworms reduced the establishment of T. spiralis, the initial growth of female worms and their fecundity. The expulsion of T. spiralis was also significantly slower in concurrently infected animals and there was significant depression of the serum IgG antibody response to muscle stage and adult worm antigens of T. spiralis in concurrently infected animals. These results are discussed in relation to the chronicity of human hookworm infections.

Necator americanus in inbred mice: a re-evaluation of primary infection kinetics

The course of a primary Necator americanus infection was studied in the lungs and small intestines of syngeneic mice. Following percutaneous infection no difference in initial larval establishment in the lungs was found between male BALB/c, NIH or B10.G mice. However, significant differences in the subsequent kinetics of infection were demonstrated between the BALB/c and NIH strains. Lung worm burdens declined more slowly in NIH mice than in BALB/c strain. Surprisingly, however, a greater proportion of larvae remaining in the lungs of BALB/c mice, 9 days p.i., were trapped than in NIH mice. Nevertheless, establishment in the small intestines of the BALB/c strain was consistently greater than in NIH mice. Host immunosuppression resulted in increased larval retention in the lungs of both the BALB/c and NIH strains as well as in the small intestines of BALB/c mice. Treatment with hydrocortisone acetate did not increase intestinal worm burdens in NIH mice. The data presented suggest that, in this complex, dynamic model system, designation of 'susceptible' and 'resistant' strains is inappropriate. The factors underlying the observed strain differences in resistance to infection are discussed.

Soluble (s)CD23 levels in a parasitized population from Papua New Guinea

Levels of sCD23, total and specific IgE were found to be considerably elevated in the plasma of a helminth infected population from Papua New Guinea. Using age-corrected data, a significant negative correlation was seen between sCD23 and total IgE in the young, at a time when hookworms are being rapidly acquired, and total IgE levels are rising most dramatically. It is suggested on the basis of this data that this inverse correlation is due to the stabilizing effect of IgE on the Fc epsilon RII receptor in vivo, and that the high levels of sCD23 seen in hookworm infected individuals are partly responsible for the high levels of IgE recorded in these patients.

The immunogenicity of hookworm (Necator americanus) acetylcholinesterase (AChE) in man

A number of different but complementary approaches have been used to demonstrate the immunogenicity of Necator americanus acetylcholinesterase to infected individuals. Western blotting of parasite somatic extracts with human post-infection sera and a specific rabbit antiserum to AChE resulted in the development of almost identical antigen-recognition profiles. AChE-containing fractions produced by preparative iso-electric focusing were subsequently shown to be antigenic in ELISA using post-infection sera. This preliminary data was reinforced by the affinity purification of AChE by immobilized post-infection IgG, and the immunoprecipitation of AChE activity from ES by post-infection IgG. Finally, AChE purified by affinity chromatography on edrophonium chloride was shown to be antigenic by Western blotting, and in ELISA, against post-infection sera, although a degree of re-activity was also seen with normal human sera. This data is discussed in the context of the host-parasite relationship.

The detection of autoantibodies to IgE in plasma of individuals infected with hookworm (Necator americanus) and the demonstration of a predominant IgG1 anti-IgE autoantibody response

In this study we have demonstrated significantly elevated levels of circulating IgG autoanti-IgE antibody in hookworm infected individuals from Kebasob village on Karkar Island, Papua New Guinea. Although anti-IgE activity was demonstrable in IgG1, IgG3 and IgG4, IgG1 was by far the most important subclass of IgG anti-IgE in terms of frequency of detection (34/39; 87.2%) and magnitude of increase (P = 0.0000); with IgG3 (16/39; 41.0%) and IgG4 (15/39; 38.5%) antibodies being considerably less prevalent. Plasma levels of IgG1 anti-IgE (P = 0.0019) and IgG3 anti-IgE (P = 0.0034) showed significant correlations with total IgE concentrations, but not with IgE specific to excretory-secretory worm products; thus suggesting that anti-IgE synthesis is more related to polyclonal hyper IgE production than to antigen-specific IgE stimulation. No correlation was seen between IgG subclass anti-IgE levels and faecal egg counts or worm burden. Given that our data failed to show a negative or a positive correlation between anti-IgE and the degree of infection with hookworm, it is tempting to speculate that the main role of autoanti-IgE is to provide the host with protection against immune complex- and IgE-mediated hypersensitivity reactions to parasitic antigens.

Isotypic variation in antibody responses in a community in Papua New Guinea to larval and adult antigens during infection, and following reinfection, with the hookworm Necator americanus

The natural infection of a community with the hookworm Necator americanus induces a vigorous humoral response to both larval and adult parasite antigens. This response occurs in all five human antibody isotypes, and data are presented to show that, at the population level, isotypes respond differently, following chemotherapy and during reinfection, to changes in antigen stimulation. The differential response probably reflects the fact that the parasite, during the course of its life cycle, presents different amounts of antigens at different anatomical locations. It is suggested that IgG and IgM responses against adult excretory-secretory (ES) products most accurately reflect the efficacy of chemotherapy, and the load of resident adult infection, while IgG responses against larval somatic antigens reflect continuous exposure to infection. These hypotheses should now be tested, at the level of the individual, in a longitudinal manner using more closely spaced sampling intervals. This repetitive sampling, and the inclusion of a measure of the exposure of the population to infective stages, will allow more definitive conclusions to be made about the role of the immune response in controlling worm burdens.

Antigenic cross-reactivity between Necator americanus and Ascaris lumbricoides in a community in Papua New Guinea infected predominantly with hookworm

Sero-epidemiological data are presented in which antigenic cross-reactivity between Necator americanus and Ascaris lumbricoides has been investigated in a community in Papua New Guinea infected predominantly with N. americanus. It is our contention that the antigenic cross-reactivity which undoubtedly exists between these species accounted for (i) a peak in antibody levels against N. americanus in 10-13 years old children (driven by infection with A. lumbricoides), and (ii) the maintenance of apparent antibody levels against A. lumbricoides in older age groups (driven by infection with N. americanus in the absence of overt infection with A. lumbricoides). Cross-reactivity was analysed further, and apparently N. americanus-specific epitopes identified, by immunoblotting. These observations could have considerable bearing on the interpretation of data from sero-epidemiological studies which failed to take account of concurrent infection with these parasites.

Epidemiology and immunology of Necator americanus infection in a community in Papua New Guinea: humoral responses to excretory-secretory and cuticular collagen antigens

Baseline data from an immuno-epidemiological study of hookworm infection in a rural village in Madang Province, Papua New Guinea are reported. Necator americanus was found to be the commonest helminth infection, with a prevalence of near 100% and intensity of 40 worms per host in adults. Enterobius vermicularis, Ascaris lumbricoides and Trichuris trichiura were also present, at prevalences of 53, 10 and 3% respectively; Ancylostoma duodenale was absent. The frequency distribution of N. americanus was highly over-dispersed, and was well described by a negative binomial distribution with aggregation parameter, k, of 0.370. Intensity of infection was significantly related to host age, but did not differ between the sexes. Haemoglobin levels and haematocrit values were indicative of anaemia in the community, but were unrelated to hookworm infection. Levels of antibodies (IgG, IgA and IgM combined) against adult Necator cuticular collagen and excretory-secretory (ES) products were determined. Serum concentrations of the two types of antibody were significantly correlated with each other. Significant positive correlations were found between anti-ES antibody levels and hookworm egg production, and between anti-collagen antibody levels and host age. It is suggested that the level of anti-collagen antibodies may reflect cumulative exposure to infection, whereas levels of anti-ES antibodies may be more dependent on current worm burden. No evidence was found to suggest that either antibody response is important in regulating parasite population growth. Similarly, the presence of a positive correlation between eosinophil concentration and infection intensity in adults indicates that eosinophilia reflects, rather than determines, the host's worm burden.

The action of a mast cell protease on the cuticular collagens of Necator americanus

Collagen-like proteins, thought to be responsible for maintaining the structural integrity of the nematode cuticle, were isolated from adult Necator americanus and shown to be susceptible to digestion by purified mast cell proteases. Although these collagens would appear normally to be masked by superficially expressed (surface) antigens, it is suggested that a sufficiently avid and specific immune response could remove this potentially protective coat, rendering the structurally important underlying layers open to immune attack.

Acquired resistance to the human hookworm Necator americanus in mice

BALB/c mice were exposed to primary or secondary infection with the hamster-adapted strain of Necator americanus, and the course of infection was monitored through worm recovery and immunological assays. Significantly fewer viable larvae were recovered from the skin site of reinfected mice on day 2 post-infection, and fewer larvae resided in the lungs of challenged mice 3-5 days after infection, suggesting that the skin was involved in resistance to secondary infection. The serum antibody response to L3 antigen was enhanced during secondary infection, peaking on day 9, and the bronchoalveolar leucocyte (BAL) response was more intense at this stage. Thus the secondary BAL response was initiated more promptly than the primary response, peaking on day 13 at twice the intensity of the primary response and five times above the resting level. Differential counts revealed that by far the most significant changes in cell populations were those observed for eosinophils in lavage fluid. At the peak of the response a 925-fold increase over control levels was detected in mice undergoing a challenge infection. Some cellular and serological components of the secondary response were defined in the present work and it was concluded that reinfected mice have the capacity to trap parasites during their passage through the skin and development in the lungs.

Necator americanus in neonatally infected hamsters. The time-course of infection and antibody response to the surface antigens of L4 and adult worms

Neonatal hamsters were infected with a hamster-adapted strain of Necator americanus, and the time-course of infection was followed by worm and faecal egg counts. Parasite eggs were first recorded during the 6th week of infection, increasing rapidly thereafter to peak in weeks 7-10. Male hamsters excreted more eggs than females, but both sexes were equally susceptible to infection and harboured comparable worm burdens. Faecal egg counts declined from week 10 onwards and this was associated with a loss of worms from animals with heavy infections. Low level infections were stable over the first 114 d of infection but worm fecundity nevertheless still declined over this period. Both hamster sexes responded similarly to surface antigens on adult worms, the antibody levels rising from week 5 onwards to reach a plateau in weeks 6-7, which persisted until the experiments were terminated. The major antigens recognised on the surface of adult worms had molecular masses corresponding to 25 kDa, 32 kDa, a doublet with the heaviest polypeptide resolving at 46 kDa, and a triplet with the heaviest at 67 and 93kDa. In contrast L4 had only 2 major cuticular antigens resolving at 41 and 93kDa. The 93kDa molecule on L4 and adult worms may be antigenically related.

The clinical and immunologic responses of normal human volunteers to low dose hookworm (Necator americanus) infection

Five normal human volunteers were exposed to approximately 50 infective larvae of Necator americanus and were observed for the development of clinical signs or symptoms and for changes in blood eosinophil levels, IgG antibody titers, total and parasite-specific IgE, and lymphocyte blastogenic responses for 6-10 weeks. Bronchoalveolar lavage was performed on four subjects prior to infection and at times when larval migration through the pulmonary tree was likely. Eggs were demonstrated in the stools of four volunteers who remained untreated for more than 6 weeks; one volunteer had to be treated at day 40 because of severe gastrointestinal symptoms. All others also complained of abdominal pain and flatulence between days 35-40. All volunteers developed marked blood eosinophilia which peaked between days 38-64 and ranged from 1,350-3,828 eosinophils/mm3. Small increases in total and parasite-specific IgE and IgG were noted in some volunteers. One volunteer showed a significant lymphocyte blastogenic response. With the exception of mucosal erythema, bronchoalveolar lavage results were unremarkable. Our data indicate that a single small inoculum of hookworm larvae is capable of producing significant transient gastrointestinal morbidity and marked blood eosinophilia but does not induce other prominent T cell- and B cell-dependent immune responses.

Adherence of human eosinophils to infective filariform larvae of Necator americanus in vitro

Eosinophilia is common in hookworm infection but the interaction between eosinophils and the larval stage of the parasite is poorly understood. The present study was conducted to test the ability of the eosinophils to adhere to infective filariform larvae of Necator americanus in vitro. Adherence of eosinophils to the larvae was found to be serum dependent. Antibody facilitated eosinophil adherence but this was maximal in the presence of complement. The adherence was greatly diminished by EGTA treated normal human serum (NHS) and was completely abolished when NHS was treated with either EDTA or heat-inactivation, suggesting that the process can be facilitated through complement activation via the alternative pathway. As with other nematodes, the surface of hookworm larvae appeared to be both antigenic and complement-activating. Although it is not known whether eosinophil adherence has any larvicidal effect, the present study demonstrated for the first time a definite interaction between human eosinophils and hookworm filariform larvae.