The Study of Host Immune Responses Elicited by the Model Murine HookwormsNippostrongylus brasiliensisandHeligmosomoides polygyrus

IL-4 induces CD22 expression to restrain the effector program of virtual memory T cells

Parasitic helminths induce the production of interleukin-4 (IL-4), which causes the expansion of virtual memory CD8+ T cells (TVM cells), a cell subset that contributes to the control of coinfection with intracellular pathogens. However, the mechanisms regulating IL-4-dependent TVM cell activation and expansion remain ill defined. Here, we used single-cell RNA sequencing of CD8+ T cells to identify pathways that control IL-4-dependent TVM cell responses. Gene signature analysis of CD8+ T cells identified a cell cluster marked by CD22, a canonical regulator of B cell activation, as a selective surface marker of IL-4-induced TVM cells. CD22+ TVM cells were enriched for interferon-γ and granzyme A and retained a diverse TCR repertoire while enriched in self-reactive CDR3 sequences. CD22 intrinsically regulated the IL-4-induced CD8+ T cell effector program, resulting in reduced responsiveness of CD22+ TVM cells and regulatory functions to infection and inflammation. Thus, helminth-induced IL-4 drives the expansion and activation of TVM cells that is counterinhibited by CD22.

New data on the life cycle of Nippostrongylus brasiliensis (Travassos, 1914) (Nematoda: Heligmosomidae): development of eggs and larval stages in the intestine of naturally infected Rattus norvegicus (Berkenhout, 1769)

Nippostrongylus brasiliensis is an intestinal parasite of rodents with a worldwide distribution. Due to the similarities between its life cycle and that of the human hookworms Ancylostoma duodenale and Necator americanus, N. brasiliensis has been widely used as a model in experimental research. We provide new data on the embryonation and hatching processes of N. brasiliensis eggs, not in the soil, but in the large intestine of naturally infected Rattus norvegicus. The parasite was investigated in 109 rats trapped in urban and periurban areas of Valencia, Spain. Rats were preserved at - 20 °C. After thawing, all the rat organs were examined. The contents of the large intestine were analysed by the Midi-Parasep® technique. N. brasiliensis (adult prevalence 69.72%) was identified by morphological and molecular techniques. Parasite eggs were found in 88.16% of the sediments. In addition to unembryonated eggs, eggs in different stages of embryonation were encountered in 67 of the 76 contents (88.16%). Free larval stages (L1 and more developed larvae) were also found in 47 of the 76 (61.84%) sediments studied. Filariform larval stages of N. brasiliensis were also sporadically detected in the lungs of the infected rats. According to our results, the development of eggs and larvae in the intestine of naturally infected rats opens up the possibility of autoinfection in the natural life cycle of N. brasiliensis, which could explain the high burdens of adult stages frequently found in the small intestine of infected rats.

Influence and distinctions of particulate matter exposure across varying etiotypes in chronic obstructive pulmonary disease (COPD) mouse model

BACKGROUND

Air pollution, notably particulate matter (PM), significantly impacts chronic respiratory disease such chronic obstructive pulmonary disease (COPD). Although asthma-COPD overlap (ACO), considered one of the COPD etiotype, is associated with greater severity in both symptoms and outcomes, effects of PM exposure remain unclear. Thus, this study aimed to evaluate impact of PM on chronic airway disease animal models.

METHODS

We established two distinct COPD etiotypes, cigarette smoking-related COPD (COPD-C) and COPD with asthma (COPD-A), using porcine pancreatic elastase (PPE) for COPD-C and a combination of PPE with ovalbumin for COPD-A. To reflect smoking influence, cigarette smoking extract was administered to both disease models. To assess impact of PM exposure, bronchoalveolar lavage fluid (BALF), proinflammatory cytokines, lung histology, and cellular damage mechanisms were analyzed.

RESULTS

In the COPD-A model, cell counts and type 2 cytokines were elevated in BALF independent of PM exposure. All models exhibited increased lung inflammation and emphysema due to PM exposure. Expression levels of apoptosis-related protein B-cell lymphoma protein 2 (Bcl-2) associated X (Bax) showed an inclination to increase with PM exposure. In the COPD-A model, decreased expression of basal nuclear factor erythroid-derived 2-like 2 (Nrf-2) and increased production of reactive oxygen species (ROS) due to PM exposure were noted.

CONCLUSION

We developed two distinct models for the etiotypes of COPD and found increased vulnerability to cell damage in COPD-A after PM exposure. Moreover, the control group displayed escalated airway inflammation and emphysema due to PM exposure, substantiating the risk of respiratory diseases.

Helminth infection induces a distinct subset of CD101hi lung tissue-infiltrating eosinophils that are differentially regulated by type 2 cytokines

Eosinophils play divergent roles in health and disease, contributing to both immunoregulatory and proinflammatory responses. Helminth infection is strongly associated with eosinophilia and the induction of the type 2 cytokines interleukin (IL)-5, IL-4 and IL-13. This study aimed to elucidate the heterogeneity of pulmonary eosinophils in response to helminth infection and the roles of IL-5, IL-4 and IL-13 in driving pulmonary eosinophil responses. Using the murine helminth model Nippostrongylus brasiliensis (Nb), we characterize a subtype of eosinophils, defined by high expression of CD101, that is induced in the lungs of Nb-infected mice and are phenotypically distinct from lung eosinophils that express low levels of CD101. Strikingly, we show that the two eosinophil subtypes have distinct anatomical localization within the lung: CD101low eosinophils are predominantly localized in the lung vasculature, whereas Nb-induced CD101hi eosinophils are predominantly localized in the extravascular lung niche. We show that CD101hi eosinophils are also induced across other models of pulmonary infection and inflammation, including a nonlung-migrating helminth infection, house dust mite-induced allergic inflammation and influenza infection. Furthermore, we demonstrate that the induction of CD101hi tissue eosinophils is independent of IL-5 and IL-4 signaling, but is dependent on intact IL-13 signaling. These results suggest that IL-13 produced during helminth infection and other disease states promotes a pulmonary tissue-infiltrating program in eosinophils defined by high expression of CD101.

Single-cell proteomics and transcriptomics capture eosinophil development and identify the role of IL-5 in their lineage transit amplification

The activities, ontogeny, and mechanisms of lineage expansion of eosinophils are less well resolved than those of other immune cells, despite the use of biological therapies targeting the eosinophilia-promoting cytokine interleukin (IL)-5 or its receptor, IL-5Rα. We combined single-cell proteomics and transcriptomics and generated transgenic IL-5Rα reporter mice to revisit eosinophilopoiesis. We reconciled human and murine eosinophilopoiesis and provided extensive cell-surface immunophenotyping and transcriptomes at different stages along the continuum of eosinophil maturation. We used these resources to show that IL-5 promoted eosinophil-lineage expansion via transit amplification, while its deletion or neutralization did not compromise eosinophil maturation. Informed from our resources, we also showed that interferon response factor-8, considered an essential promoter of myelopoiesis, was not intrinsically required for eosinophilopoiesis. This work hence provides resources, methods, and insights for understanding eosinophil ontogeny, the effects of current precision therapeutics, and the regulation of eosinophil development and numbers in health and disease.

Recruitment and Maintenance of CX3CR1+CD4+ T Cells during Helminth Infection

Distinct subsets of T lymphocytes express CX3CR1 under inflammatory conditions, but little is known about CX3CR1+CD4+ T cells during type 2 inflammation in helminth infections. In this study, we used a fate-mapping mouse model to characterize CX3CR1+CD4+ T cells during both acute Nippostrongylus brasiliensis and chronic Schistosoma mansoni murine models of helminth infections, revealing CX3CR1+CD4+ T cells to be an activated tissue-homing subset with varying capacity for cytokine production. Tracking these cells over time revealed that maintenance of CX3CR1 itself along with a TH2 phenotype conferred a survival advantage in the inflamed tissue. Single-cell RNA sequencing analysis of fate-mapped CX3CR1+CD4+ T cells from both the peripheral tissue and the spleen revealed a considerable level of diversity and identified a distinct population of BCL6+TCF-1+PD1+CD4+ T cells in the spleen during helminth infections. Conditional deletion of BCL6 in CX3CR1+ cells resulted in fewer CX3CR1+CD4+ T cells during infection, indicating a role in sustaining CD4+ T cell responses to helminth infections. Overall, our studies revealed the behavior and heterogeneity of CX3CR1+CD4+ T cells during type 2 inflammation in helminth infections and identified BCL6 to be important in their maintenance.

Myeloid- and epithelial-derived RELMα contribute to tissue repair following lung helminth infection

Soil-transmitted helminth (STH) infections impact billions of individuals globally; however, there is a need to clarify the long-term impacts of these infections on pulmonary health owing to their transient migration and subsequent damage to the lungs. In mouse models of these infections using Nippostrongylus brasiliensis, lung pathology persists at later time points post single infection. These studies also indicate the persistent transcriptional expression of resistin-like molecule α (RELMα), an immunomodulatory protein induced in type 2 immunity and alternatively activated macrophages. Using constitutive and tamoxifen-inducible cell-specific RELMα knockout mouse strains, we identified that epithelial- and myeloid-derived RELMα protein remained elevated at 30 days post infection and altered the immune cell signature and gene expression in lung compartments. Histopathological assessment of alveolar damage revealed a role for RELMα in tissue repair, suggesting the importance of sustained RELMα expression for lung recovery from helminth infection. Acellular three-dimensional (3D) lung scaffolds were prepared from the lungs of wild-type (WT), RELMα KO-naive, or 30 days post N. brasiliensis-infected mice to assess their ability to support epithelial cell growth. N. brasiliensis infection significantly altered the scaffold and impaired epithelial cell growth and metabolic activity, especially in the RELMα KO scaffolds. These findings underscore a need to identify the long-term impacts of helminth infection on human pulmonary disease, particularly as alveolar destruction can develop into chronic obstructive pulmonary disease (COPD), which remains among the top global causes of death. Translation of these findings to human protein resistin, with sequence homology to RELMα therapeutic opportunities in lung repair.

Choline metabolism underpins macrophage IL-4 polarization and RELMα up-regulation in helminth infection

Type 2 cytokines like IL-4 are hallmarks of helminth infection and activate macrophages to limit immunopathology and mediate helminth clearance. In addition to cytokines, nutrients and metabolites critically influence macrophage polarization. Choline is an essential nutrient known to support normal macrophage responses to lipopolysaccharide; however, its function in macrophages polarized by type 2 cytokines is unknown. Using murine IL-4-polarized macrophages, targeted lipidomics revealed significantly elevated levels of phosphatidylcholine, with select changes to other choline-containing lipid species. These changes were supported by the coordinated up-regulation of choline transport compared to naïve macrophages. Pharmacological inhibition of choline metabolism significantly suppressed several mitochondrial transcripts and dramatically inhibited select IL-4-responsive transcripts, most notably, Retnla. We further confirmed that blocking choline metabolism diminished IL-4-induced RELMα (encoded by Retnla) protein content and secretion and caused a dramatic reprogramming toward glycolytic metabolism. To better understand the physiological implications of these observations, naïve or mice infected with the intestinal helminth Heligmosomoides polygyrus were treated with the choline kinase α inhibitor, RSM-932A, to limit choline metabolism in vivo. Pharmacological inhibition of choline metabolism lowered RELMα expression across cell-types and tissues and led to the disappearance of peritoneal macrophages and B-1 lymphocytes and an influx of infiltrating monocytes. The impaired macrophage activation was associated with some loss in optimal immunity to H. polygyrus, with increased egg burden. Together, these data demonstrate that choline metabolism is required for macrophage RELMα induction, metabolic programming, and peritoneal immune homeostasis, which could have important implications in the context of other models of infection or cancer immunity.

Exposure to lung-migrating helminth protects against murine SARS-CoV-2 infection through macrophage-dependent T cell activation

Helminth endemic regions report lower COVID-19 morbidity and mortality. Here, we show that lung remodeling from a prior infection with a lung-migrating helminth, Nippostrongylus brasiliensis, enhances viral clearance and survival of human-ACE2 transgenic mice challenged with SARS-CoV-2 (SCV2). This protection is associated with a lymphocytic infiltrate, including increased accumulation of pulmonary SCV2-specific CD8+ T cells, and anti-CD8 antibody depletion abrogated the N. brasiliensis-mediated reduction in viral loads. Pulmonary macrophages with a type 2 transcriptional and epigenetic signature persist in the lungs of N. brasiliensis-exposed mice after clearance of the parasite and establish a primed environment for increased CD8+ T cell recruitment and activation. Accordingly, depletion of macrophages ablated the augmented viral clearance and accumulation of CD8+ T cells driven by prior N. brasiliensis infection. Together, these findings support the concept that lung-migrating helminths can limit disease severity during SCV2 infection through macrophage-dependent enhancement of antiviral CD8+ T cell responses.

Parasitic worms affect virus coinfection: a mechanistic overview

Helminths are parasitic worms that coevolve with their host, usually resulting in long-term persistence through modulating host immunity. The multifarious mechanisms altering the immune system induced by helminths have significant implications on the control of coinfecting pathogens such as viruses. Here, we explore the recent literature to highlight the main immune alterations and mechanisms that affect the control of viral coinfection. Insights from these mechanisms are valuable in the understanding of clinical observations in helminth-prevalent areas and in the design of new therapeutic and vaccination strategies to control viral diseases.

Evaluation of asthma-chronic obstructive pulmonary disease overlap using a mouse model of pulmonary disease

BACKGROUND

Features of asthma and chronic obstructive pulmonary disease (COPD) can coexist in the same patient, in a condition termed asthma- chronic obstructive pulmonary disease overlap (ACO). ACO is heterogeneous condition exhibiting various combinations of asthma and COPD features. No clinically acceptable experimental model of ACO has been established. We aimed to establish an animal model of ACO.

METHODS

We generated two phenotypes of ACO by administering ovalbumin and porcine pancreatic elastase in combination, and papain. The proinflammatory cytokines and cell types in bronchoalveolar lavage fluid (BALF) were investigated, and lung function parameters were measured using the FlexiVent system.

RESULTS

Greater airway inflammation was observed in the asthma and both ACO models, and emphysema was found in the COPD and both ACO models. The proportion of eosinophils in BALF was elevated in the asthma and ACO-a model. Type 2 inflammatory cytokine levels were highest in the ACO-a model, and the neutrophil gelatinase-associated lipocalin level was elevated in the asthma and ACO-a model. Of lung function parameters, compliance was greater in the COPD and ACO-b model, in which elastance was lower than in the asthma model. Airway resistance increased with the methacholine concentration in the asthma and both ACO models, but not in the control or COPD model.

CONCLUSION

We established two murine models of ACO that exhibit features of asthma and COPD. We validated the clinical relevance of the ACO models based on changes in cytokine profiles and lung function. These models will be useful in further studies of the pathogenesis of, and therapeutic targets for ACO.

Helminth exposure protects against murine SARS-CoV-2 infection through macrophage dependent T cell activation

Helminth endemic regions report lower COVID-19 morbidity and mortality. Here, we show that lung remodeling from a prior infection with a lung migrating helminth, Nippostrongylus brasiliensis , enhances viral clearance and survival of human-ACE2 transgenic mice challenged with SARS-CoV-2 (SCV2). This protection is associated with a lymphocytic infiltrate including an increased accumulation of pulmonary SCV2-specific CD8+ T cells and anti-CD8 antibody depletion abrogated the N. brasiliensis -mediated reduction in viral loads. Pulmonary macrophages with a type-2 transcriptional signature persist in the lungs of N. brasiliensis exposed mice after clearance of the parasite and establish a primed environment for increased antigen presentation. Accordingly, depletion of macrophages ablated the augmented viral clearance and accumulation of CD8+ T cells driven by prior N. brasiliensis infection. Together, these findings support the concept that lung migrating helminths can limit disease severity during SCV2 infection through macrophage-dependent enhancement of anti-viral CD8+ T cell responses.

Abstract Figure

Organoids as tools to investigate gastrointestinal nematode development and host interactions

Gastrointestinal nematodes are a diverse class of pathogens that colonise a quarter of the world's human population and nearly all grazing livestock. These macroparasites establish, and some migrate, within host gastrointestinal niches during their life cycles and release molecules that condition the host mucosa to enable chronic infections. Understanding how helminths do this, and defining the molecules and mechanisms involved in host modulation, holds promise for novel strategies of anthelmintics and vaccines, as well as new knowledge of immune regulation and tissue repair. Yet the size and complexity of these multicellular parasites, coupled with the reliance on hosts to maintain their life cycles, present obstacles to interrogate how they interact with the gastric and intestinal epithelium, stroma and immune cells during infection, and also to develop protocols to genetically modify these parasites. Gastrointestinal organoids have transformed research on gastric and gut physiology during homeostasis and disease, including investigations on host-pathogen interactions with viruses, bacteria, protozoa and more recently, parasitic nematodes. Here we outline applications and important considerations for the best use of organoids to study gastrointestinal nematode development and interactions with their hosts. The careful use of different organoid culture configurations in order to achieve a closer replication of the in vivo infection context will lead not only to new knowledge on gastrointestinal nematode infection biology, but also towards the replication of their life cycles in vitro, and the development of valuable experimental tools such as genetically modified parasites.

Interrogating the Small Intestine Tuft Cell-ILC2 Circuit Using In Vivo Manipulations

Recent findings position tuft cells as key mediators of intestinal immunity through their production of the cytokine interleukin (IL)-25 and activation of group 2 innate lymphoid cells (ILC2s). Though tuft cells are found in numerous epithelial tissues, their phenotype and function have been best characterized in the small intestine, where robust in vivo techniques have enabled the dissection of their cellular function, ontogeny, and key signaling pathways. We describe methods for the identification, quantification, and manipulation of tuft cells, focusing on analysis of ILC2s as a readout of tuft cell function. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Ex vivo analysis of small intestinal tuft cells and ILC2 by flow cytometry Alternate Protocol: Ex vivo analysis of small intestinal tuft cells and ILC2 by flow cytometry in the context of type 2 inflammation Basic Protocol 2: Ex vivo analysis of small intestinal tuft cells by imaging of intestinal Swiss roll Basic Protocol 3: Tuft-ILC2 circuit activation by oral gavage of adult Nippostrongylus brasiliensis worms Basic Protocol 4: Circuit activation by colonization with Tritrichomonas spp. Basic Protocol 5: Circuit activation by treatment with succinate in drinking water Basic Protocol 6: Circuit activation by treatment with recombinant IL-25.

Trichuris muris and comorbidities - within a mouse model context

Trichuris muris is a mouse intestinal parasitic nematode that inhabits the large intestine of its host and induces a strong immune response. The effects of this strong anti-parasite response can be found locally within the intestinal niche and also systemically, having effects on multiple organs. Additionally, the anti-parasite response can have multiple effects on infectious organisms and on microbiota that the host is harbouring. It has been shown that Th1 responses induced by T. muris can affect progression of bowel inflammation, cause colitic-like intestinal inflammation, reduce barrier function and intestinal mucosal responses. In the brain, T. muris can exacerbate stroke outcome and other neurological conditions. In the lung, T. muris can suppress airway inflammation and alter immune responses to other parasites. Additionally, T. muris induced responses can inhibit anti-tumour immunity. Although this parasite maintains a localized niche in the large intestine, its effects can be far-reaching and substantially impact other infections through modulation of bystander immune responses.

Comparative Transcriptomics of IBD Patients Indicates Induction of Type 2 Immunity Irrespective of the Disease Ideotype

Inflammatory cytokines initiate and sustain the perpetuation of processes leading to chronic inflammatory conditions such as inflammatory bowel diseases (IBD). The nature of the trigger causing an inflammatory reaction decides whether type 1, type 17, or type 2 immune responses, typically characterized by the respective T- helper cell subsets, come into effect. In the intestine, Type 2 responses have been linked with mucosal healing and resolution upon an immune challenge involving parasitic infections. However, type 2 cytokines are frequently elevated in certain types of IBD in particular ulcerative colitis (UC) leading to the assumption that Th2 cells might critically support the pathogenesis of UC raising the question of whether such elevated type 2 responses in IBD are beneficial or detrimental. In line with this, previous studies showed that suppression of IL-13 and other type 2 related molecules in murine models could improve the outcomes of intestinal inflammation. However, therapeutic attempts of neutralizing IL-13 in ulcerative colitis patients have yielded no benefits. Thus, a better understanding of the role of type 2 cytokines in regulating intestinal inflammation is required. Here, we took a comparative transcriptomic approach to address how Th2 responses evolve in different mouse models of colitis and human IBD datasets. Our data show that type 2 immune-related transcripts are induced in the inflamed gut of IBD patients in both Crohn's disease and UC and across widely used mouse models of IBD. Collectively our data implicate that the presence of a type 2 signature rather defines a distinct state of intestinal inflammation than a disease-specific pathomechanism.

Enteric helminth coinfection enhances host susceptibility to neurotropic flaviviruses via a tuft cell-IL-4 receptor signaling axis

Although enteric helminth infections modulate immunity to mucosal pathogens, their effects on systemic microbes remain less established. Here, we observe increased mortality in mice coinfected with the enteric helminth Heligmosomoides polygyrus bakeri (Hpb) and West Nile virus (WNV). This enhanced susceptibility is associated with altered gut morphology and transit, translocation of commensal bacteria, impaired WNV-specific T cell responses, and increased virus infection in the gastrointestinal tract and central nervous system. These outcomes were due to type 2 immune skewing, because coinfection in Stat6-/- mice rescues mortality, treatment of helminth-free WNV-infected mice with interleukin (IL)-4 mirrors coinfection, and IL-4 receptor signaling in intestinal epithelial cells mediates the susceptibility phenotypes. Moreover, tuft cell-deficient mice show improved outcomes with coinfection, whereas treatment of helminth-free mice with tuft cell-derived cytokine IL-25 or ligand succinate worsens WNV disease. Thus, helminth activation of tuft cell-IL-4-receptor circuits in the gut exacerbates infection and disease of a neurotropic flavivirus.

Preparation of Nippostrongylus brasiliensis Larvae for the Study of Host Skin Response

Hookworms are skin penetrating parasites, however in the laboratory the hookworm model Nippostrongylus brasiliensis, the parasite is traditionally administered subcutaneously bypassing the skin (epidermis and dermis). Here, we describe two complementary approaches for infecting mice with N. brasiliensis in order to study the skin immune responses. The first approach employs a skin percutaneous injection that is poorly efficient with the laboratory strain of the parasite in mice, but represents a natural infection. The second approach employs an intradermal injection of the parasite, allowing the controlled delivery of the parasitic larvae and leads to an infection that closely mimics the natural kinetics of parasite migration and development. Both of those infection models allow the investigator to study the skin immune response mounted against the parasite, in addition to detailed investigations of the early immunomodulatory strategies employed by the parasite during skin invasion.

Tissue-specific pathways extrude activated ILC2s to disseminate type 2 immunity

Group 2 innate lymphoid cells (ILC2s) are tissue-resident cells prominent at barrier sites. Although precursors are found in blood, mature ILC2s can enter the circulation after small intestinal perturbation by migratory helminths and move to distant tissues to influence the local reparative response. Using fate-mapping and methods to bypass the lung or intestinal phases of Nippostrongylus brasiliensis infection, we show that blood ILC2s comprise heterogeneous populations derived from distinct tissues that are dependent on alarmins matched to the receptor profile of the specific tissue ILC2s. Activation of local ILC2s by tissue-specific alarmins induced their proliferation, lymph node migration, and blood dissemination, thus systemically distributing type 2 cytokines. These studies uncover a possible mechanism by which local innate responses transition to systemic type 2 responses by extrusion of activated sentinel ILC2s from tissue into the circulation.

Immunomodulation and Immune Escape Strategies of Gastrointestinal Helminths and Schistosomes

Parasitic worms (helminths) developed various immunoregulatory mechanisms to counteract the immune system of their host. The increasing identification and characterization of helminth-derived factors with strong immune modulatory activity provides novel insights into immune escape strategies of helminths. Such factors might be good targets to enhance anti-helminthic immune responses. In addition, immunosuppressive helminth-derived factors could be useful to develop new therapeutic strategies for treatment of chronic inflammatory conditions. This review will take an in depth look at the effects of immunomodulatory molecules produced by different helminths with a focus on schistosomes and mouse models of hookworm infections.

Basophils prime group 2 innate lymphoid cells for neuropeptide-mediated inhibition

Type 2 cytokine responses promote parasitic immunity and initiate tissue repair but, can also result in immunopathologies when not properly restricted. Basophilia is recognized as a common feature of type 2 inflammation, however, the roles basophils play in regulating these responses remain unknown. Here, we demonstrate that helminth-induced ILC2 responses are exaggerated in the absence of basophils, resulting in increased inflammation and diminished lung function. Additionally, we show that ILC2s from basophil-depleted mice express reduced amounts of the receptor for the neuropeptide, neuromedin B (NMB). Critically, NMB stimulation inhibited ILC2 responses from control but not basophil-depleted mice, and basophils were sufficient to directly enhance NMB receptor (NMBR) expression on ILC2s. These studies suggest that basophils prime ILC2s to respond to neuron-derived signals necessary to maintain tissue integrity. Further, these data provide mechanistic insight into the functions of basophils and identify NMB as a potent inhibitor of type 2 inflammation.

Infection with a small intestinal helminth, Heligmosomoides polygyrus bakeri, consistently alters microbial communities throughout the murine small and large intestine

Increasing evidence suggests that intestinal helminth infection can alter intestinal microbial communities with important impacts on the mammalian host. However, all of the studies to date utilize different techniques to study the microbiome and access different sites of the intestine with little consistency noted between studies. In the present study, we set out to perform a comprehensive analysis of the impact of intestinal helminth infection on the mammalian intestinal bacterial microbiome. For this purpose, we investigated the impact of experimental infection using the natural murine small intestinal helminth, Heligmosomoides polygyrus bakeri and examined possible alterations in both the mucous and luminal bacterial communities along the entire small and large intestine. We also explored the impact of common experimental variables including the parasite batch and pre-infection microbiome, on the outcome of helminth-bacterial interactions. This work provides evidence that helminth infection reproducibly alters intestinal microbial communities, with an impact of infection noted along the entire length of the intestine. Although the exact nature of helminth-induced alterations to the intestinal microbiome differed depending on the microbiome community structure present prior to infection, changes extended well beyond the introduction of new bacterial species by the infecting larvae. Moreover, striking similarities between different experiments were noted, including the consistent outgrowth of a bacterium belonging to the Peptostreptococcaceae family throughout the intestine.

Eosinophils and Macrophages within the Th2-Induced Granuloma: Balancing Killing and Healing in a Tight Space

Granuloma formation is a key host immune response generated to confine invading pathogens and limit extensive host damage. It consists of an accumulation of host immune cells around a pathogen. This host response has been extensively studied in the context of inflammatory diseases. However, there is much less known about Th2-type granulomas generated in response to parasitic worms. Based on in vitro data, innate immune cells within the granuloma are thought to immobilize and kill parasites but also act to repair damaged tissue. Understanding this dual function is key. The two billion people and many livestock/wild animals infected with helminths demonstrate that granulomas are not effective at clearing infection. However, the lack of high mortality highlights their importance in ensuring that parasite migration/tissue damage is restricted and wound healing is effective. In this review, we define two key cellular players (macrophages and eosinophils) and their associated molecular players involved in Th2 granuloma function. To date, the underlying mechanisms remain poorly understood, which is in part due to a lack of conclusive studies. Most have been performed in vitro rather than in vivo, using cells that have not been obtained from granulomas. Experiments using genetically modified mouse strains and/or antibody/chemical-mediated cell depletion have also generated conflicting results depending on the model. We discuss the caveats of previous studies and the new tools available that will help fill the gaps in our knowledge and allow a better understanding of the balance between immune killing and healing.

CRISPR/Cas9 Mutagenesis and Expression of Dominant Mutant Transgenes as Functional Genomic Approaches in Parasitic Nematodes

DNA transformation of parasitic nematodes enables novel approaches to validating predictions from genomic and transcriptomic studies of these important pathogens. Notably, proof of principle for CRISPR/Cas9 mutagenesis has been achieved in Strongyloides spp., allowing identification of molecules essential to the functions of sensory neurons that mediate behaviors comprising host finding, invasion, and location of predilection sites by parasitic nematodes. Likewise, CRISPR/Cas9 knockout of the developmental regulatory transcription factor Ss-daf-16 has validated its function in regulating morphogenesis of infective third-stage larvae in Strongyloides stercoralis. While encouraging, these studies underscore challenges that remain in achieving straightforward validation of essential intervention targets in parasitic nematodes. Chief among these is the likelihood that knockout of multifunctional regulators like Ss-DAF-16 or its downstream mediator, the nuclear receptor Ss-DAF-12, will produce phenotypes so complex as to defy interpretation and will render affected worms incapable of infecting their hosts, thus preventing establishment of stable mutant lines. Approaches to overcoming these impediments could involve refinements to current CRISPR/Cas9 methods in Strongyloides including regulatable Cas9 expression from integrated transgenes and CRISPR/Cas9 editing to ablate specific functional motifs in regulatory molecules without complete knockout. Another approach would express transgenes encoding regulatory molecules of interest with mutations designed to similarly ablate or degrade specific functional motifs such as the ligand binding domain of Ss-DAF-12 while preserving core functions such as DNA binding. Such mutant transgenes would be expected to exert a dominant interfering effect on their endogenous counterparts. Published reports validate the utility of such dominant-negative approaches in Strongyloides.

Th2 cell-derived IL-4/IL-13 promote ILC2 accumulation in the lung by ILC2-intrinsic STAT6 signaling in mice

Infection of mice with the gastrointestinal helminth Nippostrongylus brasiliensis elicits profound local proliferation and accumulation of type 2 innate lymphoid cells (ILC2s) in the lung. The regulation of ILC2 proliferation and accumulation in the lung is poorly understood. Using T cell-specific IL-4/IL-13-deficient mice, we demonstrate that IL-4/IL-13 secretion from Th2 cells promotes proliferation and expansion of the ILC2 population in the lung of N. brasiliensis-infected mice. Competitive mixed BM chimeras containing normal and STAT6-deficient ILC2s further indicated that ILC2s have to respond directly to IL-4/IL-13 for this effect while STAT6 is not required for IL-13 production in ILC2s. In addition, expression of a constitutively active form of STAT6 in ILC2s was sufficient to promote their proliferation in uninfected mice. The expression of MHC class II in ILC2s appeared to be enhanced by STAT6 signaling supporting the concept that Th2 cells and ILC2s can communicate in an antigen-dependent manner resulting in a Th2-regulated accumulation of ILC2s in the lung during an acute type 2 immune response. Based on our observations, targeting the STAT6 pathway in ILC2s could help to develop new treatments to dampen ILC2 proliferation in the lung and thereby ameliorate ILC2-mediated allergic inflammation.

ILC2s-Trailblazers in the Host Response Against Intestinal Helminths

Group 2 innate lymphoid cells (ILC2s) were first discovered in experimental studies of intestinal helminth infection—and much of our current knowledge of ILC2 activation and function is based on the use of these models. It is perhaps not surprising therefore that these cells have also been found to play a key role in mediating protection against these large multicellular parasites. ILC2s have been intensively studied over the last decade, and are known to respond quickly and robustly to the presence of helminths—both by increasing in number and producing type 2 cytokines. These mediators function to activate and repair epithelial barriers, to recruit other innate cells such as eosinophils, and to help activate T helper 2 cells. More recent investigations have focused on the mechanisms by which the host senses helminth parasites to activate ILC2s. Such studies have identified novel stromal cell types as being involved in this process—including intestinal tuft cells and enteric neurons, which respond to the presence of helminths and activate ILC2s by producing IL-25 and Neuromedin, respectively. In the current review, we will outline the latest insights into ILC2 activation and discuss the requirement for—or redundancy of—ILC2s in providing protective immunity against intestinal helminth parasites.

Infection by the parasitic helminth Trichinella spiralis activates a Tas2r-mediated signaling pathway in intestinal tuft cells

Intestinal tuft cells are sentinels monitoring the luminal contents and play a critical role in type 2 immunity. In this work, Trichinella spiralis excretion–secretion and extract were shown to directly induce interleukin 25 (IL-25) release from the intestinal villi, evoke calcium responses in tuft cells, and activate Tas2r bitter-taste receptors, whereas the bitter compound salicin was shown to activate and induce tuft cells to release IL-25. Gα-gustducin/Gβ1γ13 and/or Gαo/Gβ1γ13, Plcβ2, Ip₃r2, and Trpm5 comprise the signal transduction pathways that tuft cells utilize to initiate type 2 immune responses. Potentiation of Trpm5 by a natural sweet compound, stevioside, can enhance the tuft cell–ILC2 circuit’s activity, indicating that modulating these signaling components can help devise new means of combating parasites.

The parasitic helminth Trichinella spiralis, which poses a serious health risk to animals and humans, can be found worldwide. Recent findings indicate that a rare type of gut epithelial cell, tuft cells, can detect the helminth, triggering type 2 immune responses. However, the underlying molecular mechanisms remain to be fully understood. Here we show that both excretory–secretory products (E–S) and extract of T. spiralis can stimulate the release of the cytokine interleukin 25 (IL-25) from the mouse small intestinal villi and evoke calcium responses from tuft cells in the intestinal organoids, which can be blocked by a bitter-taste receptor inhibitor, allyl isothiocyanate. Heterologously expressed mouse Tas2r bitter-taste receptors, the expression of which is augmented during tuft-cell hyperplasia, can respond to the E–S and extract as well as to the bitter compound salicin whereas salicin in turn can induce IL-25 release from tuft cells. Furthermore, abolishment of the G-protein γ13 subunit, application of the inhibitors for G-protein αo/i, Gβγ subunits, and phospholipase Cβ2 dramatically reduces the IL-25 release. Finally, tuft cells are found to utilize the inositol triphosphate receptor type 2 (Ip₃r2) to regulate cytosolic calcium and thus Trpm5 activity, while potentiation of Trpm5 by a sweet-tasting compound, stevioside, enhances tuft cell IL-25 release and hyperplasia in vivo. Taken together, T. spiralis infection activates a signaling pathway in intestinal tuft cells similar to that of taste-bud cells, but with some key differences, to initiate type 2 immunity.