Cytokine-mediated regulation of intestinal helminth infections: the Trichuris muris model

Rodent Models for the Study of Soil-Transmitted Helminths: A Proteomics Approach

Soil-transmitted helminths (STH) affect hundreds of millions worldwide and are some of the most important neglected tropical diseases in terms of morbidity. Due to the difficulty in studying STH human infections, rodent models have become increasingly used, mainly because of their similarities in life cycle. Ascaris suum and Trichuris muris have been proven appropriate and low maintenance models for the study of ascariasis and trichuriasis. In the case of hookworms, despite most of the murine models do not fully reproduce the life cycle of Necator americanus, their proteomic similarity makes them highly suitable for the development of novel vaccine candidates and for the study of hookworm biological features. Furthermore, these models have been helpful in elucidating some basic aspects of our immune system, and are currently being used by numerous researchers to develop novel molecules with immunomodulatory proteins. Herein we review the similarities in the proteomic composition between Nippostrongylus brasiliensis, Heligmosomoides polygyrus bakeri and Trichuris muris and their respective human counterpart with a focus on the vaccine candidates and immunomodulatory proteins being currently studied.

Identifying the immunomodulatory components of helminths

Immunomodulatory components of helminths offer great promise as an entirely new class of biologics for the treatment of inflammatory diseases. Here, we discuss the emerging themes in helminth-driven immunomodulation in the context of therapeutic drug discovery. We broadly define the approaches that are currently applied by researchers to identify these helminth molecules, highlighting key areas of potential exploitation that have been mostly neglected thus far, notably small molecules. Finally, we propose that the investigation of immunomodulatory compounds will enable the translation of current and future research efforts into potential treatments for autoimmune and allergic diseases, while at the same time yielding new insights into the molecular interface of host-parasite biology.

Blocking peripheral serotonin synthesis by telotristat etiprate (LX1032/LX1606) reduces severity of both chemical- and infection-induced intestinal inflammation

Mucosal inflammation is accompanied by an alteration in 5-HT. Intestinal 5-HT synthesis is catalyzed by tryptophan hydroxylase 1 (Tph1) and we have shown that mice deficient in this rate-limiting enzyme have reduced severity of intestinal inflammation in models of chemical-induced experimental colitis. Here, we investigated the effect of blocking peripheral 5-HT synthesis in generation of intestinal inflammation by a using peripheral Tph inhibitor, telotristat etiprate (LX1606), in models of intestinal inflammation. LX1606 was given orally either prophylactically or therapeutically to mice with dextran sulfate sodium (DSS)-induced colitis or with infection with Trichuris muris. Severity of intestinal inflammation was measured by assessment of disease activity scores, histological damage, and MPO and inflammatory cytokine levels. LX1606 significantly reduced intestinal 5-HT levels and delayed onset and severity of DSS-induced acute and chronic colitis. This was associated with decreased MPO and proinflammatory cytokine levels compared with vehicle-treated controls. In the infection-induced inflammation model, treatment with LX1606 enhanced worm expulsion as well as increased IL-10 production and goblet cell numbers. LX1606-treated mice had significantly lower MPO and IL-1β levels compared with controls postinfection. Our results demonstrate that peripheral 5-HT plays an important role in intestinal inflammation and in the generation of immune responses. Pharmacological reduction of peripheral 5-HT may serve as a potential strategy for modulating various intestinal inflammatory disorders.

Lactobacillus rhamnosus ingestion promotes innate host defense in an enteric parasitic infection

Enteric parasite infections around the world are a huge economic burden and decrease the quality of life for many people. The use of beneficial bacteria has attracted attention for their potential therapeutic applications in various diseases. However, the effects of beneficial bacteria in enteric parasitic infections remain largely unexplored. We investigated the effects of ingestion of Lactobacillus rhamnosus (JB-1) in a model of enteric nematode (Trichuris muris) infection. C57BL/6 (resistant to infection), AKR (susceptible to infection), interleukin 10 (IL-10) knockout (KO), and mucin Muc2 KO mice were infected with T. muris and treated orally with probiotic JB-1 or medium. The mice were sacrificed on various days postinfection to examine goblet cells, epithelial cell proliferation, cytokines, and worm burdens. Treatment with JB-1 significantly enhanced worm expulsion in resistant C57BL/6 mice, and this was associated with increases in IL-10 levels, goblet cell numbers, and epithelial cell proliferation. Beneficial effects of JB-1 were absent in IL-10 KO and resistant mice treated with γ-irradiated bacteria. Live JB-1 treatment also expedited worm expulsion in Muc2 KO mice and, more importantly, in AKR mice (susceptible to infection). Injection of IL-10 directly into the colonic tissue of uninfected mice induced goblet cell hyperplasia. These findings demonstrate that JB-1 modulates goblet cell biology and promotes parasite expulsion via an IL-10-mediated pathway and provide novel insights into probiotic effects on innate defense in nematode infection.

A new role for mucins in immunity: insights from gastrointestinal nematode infection

The body's mucosal surfaces are protected from pathogens and physical and chemical attack by the gel-like extracellular matrix, mucus. The framework of this barrier is provided by polymeric, gel-forming mucins. These enormous O-linked glycoproteins are synthesised, stored and secreted by goblet cells that are also the source of other protective factors. Immune regulation of goblet cells during the course of infection impacts on mucin production and properties and ultimately upon barrier function. The barrier function of mucins in protection of the host is well accepted as an important aspect of innate defence. However, it is becoming increasingly clear that mucins have a much more direct role in combating pathogens and parasites and are an important part of the coordinated immune response to infection. Of particular relevance to this review is the finding that mucins are essential anti-parasitic effector molecules. The current understanding of the roles of these multifunctional glycoproteins, and other goblet cell products, in mucosal defence against intestinal dwelling nematodes is discussed.

Muc5ac: a critical component mediating the rejection of enteric nematodes

The mucin Muc5ac is essential for the expulsion of Trichuris muris and other gut-dwelling nematodes.

De novo expression of Muc5ac, a mucin not normally expressed in the intestinal tract, is induced in the cecum of mice resistant to Trichuris muris infection. In this study, we investigated the role of Muc5ac, which is detected shortly before worm expulsion and is associated with the production of interleukin-13 (IL-13), in resistance to this nematode. Muc5ac-deficient mice were incapable of expelling T. muris from the intestine and harbored long-term chronic infections, despite developing strong T_H2 responses. Muc5ac-deficient mice had elevated levels of IL-13 and, surprisingly, an increase in the T_H1 cytokine IFN-γ. Because T_H1 inflammation is thought to favor chronic nematode infection, IFN-γ was neutralized in vivo, resulting in an even stronger T_H2-type immune response. Nevertheless, despite a more robust T_H2 effector response, the Muc5ac-deficient mice remained highly susceptible to chronic T. muris infection. Importantly, human MUC5AC had a direct detrimental effect on nematode vitality. Moreover, the absence of Muc5ac caused a significant delay in the expulsion of two other gut-dwelling nematodes (Trichinella spiralis and Nippostrongylus brasiliensis). Thus, for the first time, we identify a single mucin, Muc5ac, as a direct and critical mediator of resistance during intestinal nematode infection.

B7RP-1-ICOS interactions are required for optimal infection-induced expansion of CD4+ Th1 and Th2 responses

Although initial reports linked the costimulatory molecule ICOS preferentially with the development of Th2 cells, there is evidence that it is not required for protective type 2 immunity to helminths and that it contributes to Th1 and Th2 responses to other parasites. To address the role of ICOS in the development of infection-induced polarized Th cells, ICOS(-/-) mice were infected with Trichuris muris or Toxoplasma gondii. Wild-type mice challenged with T. muris developed Th2 responses and expelled these helminths by day 18 postinfection, whereas ICOS(-/-) mice failed to clear worms and produced reduced levels of type 2 cytokines. However, by day 35 postinfection, ICOS(-/-) mice were able to mount an effective Th2 response and worms were expelled. This delay in protective immunity was associated with a defect in infection-induced increases in the number of activated and proliferating CD4+ T cells. Similarly, following challenge with T. gondii ICOS was required for optimal proliferation by CD4+ T cells. However, the reduced number of activated CD4+ T cells and associated defect in the production of IFN-gamma did not result in increased susceptibility to T. gondii, but rather resulted in decreased CNS pathology during the chronic phase of this infection. Taken together, these data are consistent with a model in which ICOS is not involved in dictating polarity of the Th response but rather regulates the expansion of these subsets.

The Trichuris muris system: a paradigm of resistance and susceptibility to intestinal nematode infection

Gastrointestinal helminths infect over 1 billion people worldwide. Although rarely causing death, such diseases are associated with high levels of morbidity and furthermore bear a large economic burden within areas where infections are endemic. Trichuris muris, a natural intestinal parasite of mice has been extensively utilised as a laboratory model for the study of human whipworm Trichuris trichiura. This has proven to be an invaluable tool in dissecting the different components involved in immunity to trichuris infection. Moreover, it has become a paradigm of cytokine mediated immunity to gastrointestinal nematodes in general. It is well established that resistance and susceptibility to T. muris infection are tightly associated with the generation of a T helper 2 (TH2) or a T helper 1 (TH1) immune response, respectively. This review gives a detailed account of the experimental work which has provided us with this knowledge, and further builds upon this, by focusing upon the most recent developments and important findings from this host-parasite relationship.

The role of CD8+ cells in the establishment and maintenance of a Trichuris muris infection

Chronic infection by the caecal-dwelling intestinal murine nematode Trichuris muris occurs if given as a high-dose infection to 'susceptible' AKR mice or as a low-dose infection to the normally 'resistant' C57BL/6 mouse strain. Both regimes result in a type 1 cytokine response, i.e. high levels of IFN-gamma and IL-12. Here we show this susceptible response is associated with a large population of CD8(+) IFN-gamma(+) cells within the mesenteric lymph nodes and numerous CD8(+) cells infiltrating the caecal mucosa. Despite this, the in vivo abolition of CD8(+) cells within AKR and C57BL/6 mice, either prior to infection or once infection has become established, failed to affect chronicity, implying that CD8(+) T cells are not essential for the initiation or maintenance of the susceptible response to T. muris. Interestingly, the percentage of IFN-gamma(+) CD4(+) cells increased in treated groups, perhaps in a compensatory role. The majority of antigen-specific cytokine responses were comparable in both treated and control groups, although IL-5 was fivefold higher in animals receiving anti-CD8 mAbs and IFN-gamma was also raised in treated mice. Mastocytosis was unaltered by CD8 depletion, however, paradoxically, eosinophilia within the caecum was reduced in treated mice. Together these data clearly demonstrate that CD8(+) T cells are associated with chronic T. muris infection; however, these cells are dispensable for both the early and late phases of this response, but do appear to play a role in the regulation of certain cytokines and caecal eosinophilia.

Effects of ageing on the immunoregulation of parasitic infection

The plethora of changes associated with immunosenescence radically alters virtually all aspects of immune responsiveness. How this transformation effects resolution of an infectious challenge is addressed in this study. A well-established infection model was used; Trichuris muris, a cecum-dwelling helminth, is natural to mice, and infection in different strains results in clearly polarized responses. A dominating T helper 2 (Th2) response orchestrates immunity, whereas a Th1 response will result in susceptibility. Mice between 19 and 28 months old were more susceptible to infection, whereas 3-month-old mice of the same strain demonstrated the resistant phenotype. The cytokine response made by these aged mice was clearly altered at the site of infection, and within the local draining lymph nodes higher Th1 and lower Th2 cytokine levels were found, both at the protein and RNA level. Confirming these changes, aged mice also showed a delayed parasite-specific immunoglobulin G1 response and intestinal mastocytosis, both of which are driven by Th2 cytokines. To address possible causes of the observed immune deviation, purified CD4 cells from both young and aged mice were stimulated in vitro. Cells from aged mice did not respond to stimulation via CD28 and in vitro were less able to proliferate and polarize into Th2 cells; Th1 polarization was found to be normal. Together these data suggest that changes in cytokine phenotype, particularly CD4 cells, contribute to the observed age-associated switch from T. muris resistance to susceptibility.

Mice lacking T and B lymphocytes develop transient colitis and crypt hyperplasia yet suffer impaired bacterial clearance during Citrobacter rodentium infection

The bacterial pathogen Citrobacter rodentium belongs to a family of gastrointestinal pathogens that includes enteropathogenic and enterohemorrhagic Escherichia coli and is the causative agent of transmissible colonic hyperplasia in mice. The molecular mechanisms used by these pathogens to colonize host epithelial surfaces and form attaching and effacing (A/E) lesions have undergone intense study. In contrast, little is known about the host's immune response to these infections and its importance in tissue pathology and bacterial clearance. To address these issues, wild-type mice and mice lacking T and B lymphocytes (RAG1 knockout [KO]) were infected with C. rodentium. By day 10 postinfection (p.i.), both wild-type and RAG1 KO mice developed colitis and crypt hyperplasia, and these responses became more exaggerated in wild-type mice over the next 2 weeks, as they cleared the infection. By day 24 p.i., bacterial clearance was complete, and the colitis had subsided; however, crypt heights remained increased. In contrast, inflammatory and crypt hyperplastic responses in the RAG1 KO mice were transient, subsiding after 2 weeks. By day 24 p.i., RAG1 KO mice showed no signs of bacterial clearance and infection was often fatal. Surprisingly, despite remaining heavily infected, tissues from RAG1 KO mice surviving the acute colitis showed few signs of disease. These results thus emphasize the important contribution of the host immune response during infection by A/E bacterial pathogens. While T and/or B lymphocytes are essential for host defense against C. rodentium, they also mediate much of the tissue pathology and disease symptoms that occur during infection.

Does the failure to acquire helminthic parasites predispose to Crohn's disease?

Two polarized patterns (Th1 and Th2) of cytokines regulate inflammatory responses. Each cytokine pattern inhibits production of the opposing pattern. Lymphocytes from inflamed intestine due to Crohn's disease secrete a Th1 pattern of cytokines. Crohn's disease is most prevalent in highly industrialized countries with temperate climates. It occurs rarely in tropical third world countries with poor sanitation. We propose that exposure to an environmental agent predisposes individuals to Crohn's disease. Parasitic worms (helminths) are common in tropical climates and in populations subject to crowding and poor sanitation. Children are most subject to helminthic colonization. Many helminths live within or migrate through the human gut where they interact with the mucosal immune system. The host mounts a mucosal response that includes Th2 cytokine production limiting helminthic colonization. Helminths and their eggs probably are the most potent stimulators of mucosal Th2 responses. The Th2 response provoked by parasitic worms can modulate immune reactions to unrelated parasitic, bacterial, and viral infections. Many people in developed countries now live in increasingly hygienic environments, avoiding exposure to helminths. Perhaps failure to acquire these parasites and experience mucosal Th2 conditioning predisposes to Crohn's disease, which is an overly active Th1 inflammation.

Pathology of pyogenic liver abscess in children

Little is known about preexisting lesions in livers of children with pyogenic liver abscess (PLA). Study of these lesions may elucidate possible predisposing factors for the disease. In Vitória, state of Espirito Santo, Brazil, PLA in children is frequently associated with helminthic infections and eosinophilia. We hypothesize that nematode infection with larvae migrating through the liver is a predisposing factor for PLA, because the infection induces immunomodulation and likely trapping of bacteria in liver granulomas. In this report, we describe observations of 22 cases of PLA in children studied at autopsy (16 cases) or in surgical biopsies (6 cases), including 17 boys and 5 girls ranging in age from 1 to 13 years (mean 4.6 years, median 3.5 years). Multiple abscesses in both lobes were found in 13 cases and a single abscess was found in the right lobe in 10 cases. All cases showed histologically classical pyogenic inflammation without morphological evidence of amoebiasis. In six cases there were granulomas similar to those caused by larva migrans visceralis (from Toxocara or other nematodes) in liver tissue not affected by the abscess. Nematode antigens in central areas of necrosis of granuloma in all six cases and fragments of a larva, possibly of Toxocara, were found on samples immunohistochemically stained with polyclonal anti-Toxocara antibodies. There were numerous eosinophils in abscesses with Charcot-Leyden crystals. Eosinophils were found frequently in portal triads far from the abscess wall. In four cases, in which bile duct ascariasis was found, worms were noted in the bile ducts, and eggs were found in liver parenchyma surrounding the abscess in two cases. Foreign-body granulomas were found in one case in which penetrating trauma was the cause of abscess. In one case there was one histiocytic granuloma whose origin was not determined. The observation of six cases of granuloma similar to larva migrans visceralis (or produced by other nematode larva) in liver tissue not directly affected by the abscess supports the hypothesis that helminth infections with larva migrating through the liver are a predisposing factor for pyogenic hepatic abscess in children.

Fasciola hepatica suppresses a protective Th1 response against Bordetella pertussis

Fasciolosis, like other helminth infections, is associated with the induction of T-cell responses polarized to the Th2 subtype. Respiratory infection with Bordetella pertussis or immunization with a pertussis whole-cell vaccine (Pw) induces a potent Th1 response, which confers a high level of protection against bacterial challenge. We have used these two pathogens to examine bystander cross-regulation of Th1 and Th2 cells in vivo and provide evidence of immunomodulation of host T-cell responses to B. pertussis by a concomitant infection with Fasciola hepatica. Mice with a coinfection of F. hepatica and B. pertussis exhibited a Th2 cytokine profile in response to F. hepatica antigens, similar to those infected with F. hepatica alone. By contrast, the Th1 response to B. pertussis antigens was markedly suppressed and the bacterial infection was exacerbated following infection with F. hepatica. Furthermore, an established Th1 response induced in mice by infection with B. pertussis or by parenteral immunization with Pw was also suppressed following infection with F. hepatica. This immunomodulatory effect of B. pertussis-induced responses by F. hepatica infection is significantly reduced, but not completely abrogated, in IL-4 knockout mice. Our findings demonstrate that Th2-inducing parasites can exert bystander suppression of protective Th1 responses to infection or vaccination with a bacterial pathogen and that the modulation is mediated in part by IL-4 and, significantly, is effective at both the induction and effector stages of the Th1 response.

Vaccination against the gastrointestinal nematode, Haemonchus contortus, using a purified larval surface antigen

Sheep were immunized with a purified antigen (Hc-sL3) expressed on the surface of L3 larvae of the gastro-intestinal parasite, Haemonchus contortus, using different adjuvant and immunization routes. In the first experiment, intradermal immunization of sheep with Hc-sL3 and QuilA did not result in reductions in faecal egg counts after subsequent challenge infection while significant reductions were obtained when aluminium hydroxide (AH) was used as the adjuvant. Significant protection with Hc-sL3 absorbed on AH was confirmed in a second experiment and this protection was maintained when dextran sulphate was added to the Hc-sL3/AH mixture while the addition of pertussis toxin abrogated the protective effect. Significant levels of protection, as determined by reductions in both faecal egg counts and worm burdens, were also obtained when the Hc-sL3/AH mixture was injected into the rectal mucosa or the Hc-sL3 antigen was deposited on the surface of the rectal mucosa with cholera toxin. No correlations with antibody levels or isotype and protection were observed.

Progress and expectations for helminth vaccines

The large amount of scientific progress made in the last 5 years has allowed a more rational approach to the design of nematode vaccines to develop. Successful experimental trials have been published using two different approaches, one aiming to boost acquired host immunity through vaccination with natural immunogens, the other affecting parasite viability by targeting parasite molecules crucial for nutrition or survival in the host. The individual or combined action of these two vaccination procedures will need to be evaluated with respect to their potential effects on animal health and productivity in the field. To this effect, more data are required concerning the level and duration of immunity of the vaccine-induced protection using acceptable adjuvant systems. In addition, the age at which vaccination is effective and the effect of vaccination on highly susceptible or temporarily immunosuppressed individuals will need to be considered. In the case of gastrointestinal nematodes, the level of pasture contamination with infective larvae is dependent on the worm burdens in the host animal and, in turn, affects the buildup of natural resistance in the host. An appreciation of these complex interactive factors is best achieved through computer simulation models using the powerful simulation software that has recently become available. Further animal trials will need to be performed to establish the necessary data to incorporate into the models and to adapt the model outcomes to the trial results. These epidemiologic and simulation studies should be pursued in parallel with vaccine development so that a better appreciation is gained of the requirements of a successful commercial vaccine.

Enterobius, trichuris, capillaria, and hookworm including ancylostoma caninum

Infections by the nematodes Enterobius, Trichuris, Capillaria, and hookworm are some of the most widespread gastrointestinal infections worldwide. This article is designed not only to provide an overview of the different parasites in terms of epidemiology, clinical manifestation, and disease but also to highlight particular areas of intensive research activity that are providing new insights into both the biology of the host-parasite relationship and possible new approaches to infection management for the future.

Rational design of nematode vaccines; natural antigens

Bacterial and viral vaccines have been used for several decades and have made an invaluable impact on the control of both human and animal diseases. As these vaccines are based on the induction of the "natural" protective immune response of the host, there is ample proof that this approach is effective and there is no a priori reason why a similar approach should not work for other infectious organisms, including helminth parasites. Two main stumbling blocks have so far precluded the development of effective anti-nematode vaccines: (i) the identification and isolation of the protective parasite antigens; and (ii) the induction of the appropriate protective immune effector mechanisms through vaccination. This paper will discuss the unique characteristics of nematode vaccine development and the significant progress that has been made in recent years in this area.

The role of CD4+ and CD8+ T cells in protective immunity to the murine nematode parasite Trichuris muris

The role of CD4+ and CD8+ T cells in protective immunity to Trichuris muris was studied in CD4+ or CD8+ or both CD4+ and CD8+ T cell-depleted BALB/c mice. To assess in vivo depletion of T-cell subsets, CD4+ and CD8+ T cells in the Peyer's patches, the mesenteric lymph nodes (MLN), and the spleens of mice treated with T cell-specific monoclonal antibodies (MoAbs) were analysed by FACS. CD4+ T cells were selectively depleted in mice injected with anti-CD4 MoAb i.p. and injection of anti-CD8 MoAb resulted in selective depletion of CD8+ T cells. The expulsion of T. muris was inhibited in CD4+ T cell-depleted mice and numerous worms were detected in the large intestine on days 14 and 21 after infection, although no suppression of protective immunity to T. muris was observed in CD8+ T cell-depleted mice. Moreover, there was no difference in suppression of protective immunity to T. muris between CD4+ T cell-depleted and both CD4+ and CD8+ T cells play a central role in protective immunity to T. muris infection.