Helminth products bypass the need for TSLP in Th2 immune responses by directly modulating dendritic cell function

Cooperation of ILC2s and TH2 cells in the expulsion of intestinal helminth parasites

Type 2 immune responses form a critical defence against enteric worm infections. In recent years, mouse models have revealed shared and unique functions for group 2 innate lymphoid cells and T helper 2 cells in type 2 immune response to intestinal helminths. Both cell types use similar innate effector functions at the site of infection, whereas each population has distinct roles during different stages of infection. In this Perspective, we review the underlying mechanisms used by group 2 innate lymphoid cells and T helper 2 cells to cooperate with each other and suggest an overarching model of the interplay between these cell types over the course of a helminth infection.

Epithelial regulation of microbiota-immune cell dynamics

The mammalian gastrointestinal tract hosts a diverse community of trillions of microorganisms, collectively termed the microbiota, which play a fundamental role in regulating tissue physiology and immunity. Recent studies have sought to dissect the cellular and molecular mechanisms mediating communication between the microbiota and host immune system. Epithelial cells line the intestine and form an initial barrier separating the microbiota from underlying immune cells, and disruption of epithelial function has been associated with various conditions ranging from infection to inflammatory bowel diseases and cancer. From several studies, it is now clear that epithelial cells integrate signals from commensal microbes. Importantly, these non-hematopoietic cells also direct regulatory mechanisms that instruct the recruitment and function of microbiota-sensitive immune cells. In this review, we discuss the central role that has emerged for epithelial cells in orchestrating intestinal immunity and highlight epithelial pathways through which the microbiota can calibrate tissue-intrinsic immune responses.

High-fat diet-induced resistance to helminth infection via alternative induction of type 2 immunity

Gastrointestinal nematode infections cause morbidity and socioeconomic loss in the most deprived communities. The shift in the context of obesity has led to spatial overlap with endemic gastrointestinal nematode regions resulting in the emergence of a novel comorbidity. Despite this, the impact of a high-fat diet (HFD) on immune-regulated protection against gastrointestinal infections remains largely unknown. We employed the murine model of nematode infection, Trichuris muris, to investigate the effect of an HFD on the immune response against chronic infection. Surprisingly, diet-induced obesity drove parasite expulsion in both single and repeated trickle low doses of T. muris eggs. Mechanistically, an HFD increased the expression of the ST2 receptor on CD4+ T cells, priming an enhanced type 2 helper T (Th2) cell cytokine production following interleukin (IL)-33 stimulation ex vivo. Despite IL-33-/- mice demonstrating that IL-33 is not critical for host protective immunity to T. muris under a conventional diet, HFD-fed T-cell deplete mice adoptively transferred with ST2-/- CD4 T cells were unable to expel a T. muris infection unlike those transferred with ST2-sufficient cells. Collectively, this study demonstrates that an HFD primes CD4+ T cells to utilize the IL-33-ST2 axis in a novel induction of type 2 immunity, providing insights into the emerging comorbidities of obesity and nematode infection.

Role of thymic stromal lymphopoietin in allergy and beyond

Thymic stromal lymphopoietin (TSLP) is a pleiotropic cytokine that acts on multiple cell lineages, including dendritic cells, T cells, B cells, neutrophils, mast cells, eosinophils and innate lymphoid cells, affecting their maturation, survival and recruitment. It is best known for its role in promoting type 2 immune responses such as in allergic diseases and, in 2021, a monoclonal antibody targeting TSLP was approved for the treatment of severe asthma. However, it is now clear that TSLP has many other important roles in a variety of settings. Indeed, several genetic variants for TSLP are linked to disease severity, and chromosomal alterations in TSLP are common in certain cancers, indicating important roles of TSLP in disease. In this Review, we discuss recent advances in TSLP biology, highlighting how it regulates the tissue environment not only in allergic disease but also in infectious diseases, inflammatory diseases and cancer. Encouragingly, therapies targeting the TSLP pathway are being actively pursued for several diseases.

TSLP, IL-33, and IL-25: Not just for allergy and helminth infection

The release of cytokines from epithelial and stromal cells is critical for the initiation and maintenance of tissue immunity. Three such cytokines, thymic stromal lymphopoietin, IL-33, and IL-25, are important regulators of type 2 immune responses triggered by parasitic worms and allergens. In particular, these cytokines activate group 2 innate lymphoid cells, TH2 cells, and myeloid cells, which drive hallmarks of type 2 immunity. However, emerging data indicate that these tissue-associated cytokines are not only involved in canonical type 2 responses but are also important in the context of viral infections, cancer, and even homeostasis. Here, we provide a brief review of the roles of thymic stromal lymphopoietin, IL-33, and IL-25 in diverse immune contexts, while highlighting their relative contributions in tissue-specific responses. We also emphasize a biologically motivated framework for thinking about the integration of multiple immune signals, including the 3 featured in this review.

Ascaris suum excretory/secretory products differentially modulate porcine dendritic cell subsets

Helminths produce excretory/secretory products (E/S) which can modulate the immune responses of their hosts. Dendritic cells (DC) are essential for initiating the host T cell response and are thus potential targets for modulation by helminth E/S. Here we study immunomodulation of porcine peripheral blood DC subsets following ex vivo stimulation with E/S from Ascaris suum, a common helminth of pigs with considerable public health and economic importance. Our data showed that the relative frequencies of DC subsets in porcine blood differ, with plasmacytoid DC (pDC) being the most prominent in healthy 6-month-old pigs. pDC are an important cytokine source, and we found that A. suum E/S suppressed production of the type 1 cytokines IL-12p40 and TNF-α by this subset following toll-like receptor (TLR) ligation. In contrast, conventional DC (cDC) are more efficient antigen presenters, and the expression of CD80/86, costimulatory molecules essential for efficient antigen presentation, were modulated differentially by A. suum E/S between cDC subsets. CD80/86 expression by type 1 cDC (cDC1) following TLR ligation was greatly suppressed by the addition of A. suum E/S, while CD80/86 expression by type 2 cDC (cDC2) was upregulated by A. suum E/S. Further, we found that IFN-γ production by natural killer (NK) cells following IL-12 and IL-18 stimulation was suppressed by A. suum E/S. Finally, in the presence of E/S, IFN-γ production by CD4+ T cells co-cultured with autologous blood-derived DC was significantly impaired. Together, these data provide a coherent picture regarding the regulation of type 1 responses by A. suum E/S. Responsiveness of pDC and cDC1 to microbial ligands is reduced in the presence of E/S, effector functions of Th1 cells are impaired, and cytokine-driven IFN-γ release by NK cells is limited.

Association between anti-Anisakis simplex antibodies and interleukin-7 levels

IL-7 is a crucial factor for the development of lymphocytes, and it is absolutely necessary for γδ T cells. Mice deficient in L-7 have a deficit of B and αβ T lymphocytes, and an absence of mature γδ TCR cells. IL-7 is essential for the survival, development and maturation of Schistosoma sp., although its production is associated with protection against intestinal helminths. The presence of anti-Anisakis simplex antibodies, especially IgA, is related to a lower frequency in CD3 + CD56 + αβ + lymphocytes and all subpopulations of γδ T cells. In this work, the relationship of IL-7 with humoral and cellular responses against A. simplex in 100 healthy subjects was studied. We have found significantly higher IL-7 levels in anti-A. simplex IgA-positive subjects (p < 0.001). The positivity of anti-A. simplex IgA was associated with a significant reduction in the frequency of CD3 + αβ+ (p < 0.01), CD3 + CD4 + αβ+, CD3 + CD8 + αβ+, CD3 + CD56 + αβ+, CD3 + γδ+, CD3 + CD4-CD8-γδ+ and CD3 + CD56 + γδ+ (p < 0.05) cells. In the case of NKT cells, this same phenomenon was also associated with IgE positivity. There was a weak inverse correlation (Spearman) of IL-7 levels with the frequencies of CD3 + CD4 + αβ+ (-0.125, p = 0.047), CD3 + CD8 + αβ+ (-0.204, p = 0.032), CD3 + CD56 + αβ+ (-0.247, p = 0.007), CD3 + γδ+ (-0.267, p = 0.007), CD3 + CD4-CD8-γδ+ (-0.266, p = 0.003), and CD3 + CD8 + γδ + (-0.302, p = 0.002) cells. The role of NKT cells in the anti-A. simplex response was confirmed and an association between IL and 7 levels and specific antibodies, especially IgA, was demonstrated. The higher production of IL-7 would represent a compensatory mechanism in response to the reduction in lymphocyte populations associated with the response against this parasite.

Acetate, a metabolic product of Heligmosomoides polygyrus, facilitates intestinal epithelial barrier breakdown in a FFAR2-dependent manner

Approximately 2 billion people worldwide and a significant part of the domestic livestock are infected with soil-transmitted helminths, of which many establish chronic infections causing substantial economic and welfare burdens. Beside intensive research on helminth-triggered mucosal and systemic immune responses, the local mechanism that enables infective larvae to cross the intestinal epithelial barrier and invade mucosal tissue remains poorly addressed. Here, we show that Heligmosomoides polygyrus infective L3s secrete acetate and that acetate potentially facilitates paracellular epithelial tissue invasion by changed epithelial tight junction claudin expression. In vitro, impedance-based real-time epithelial cell line barrier measurements together with ex vivo functional permeability assays in intestinal organoid cultures revealed that acetate decreased intercellular barrier function via the G-protein coupled free fatty acid receptor 2 (FFAR2, GPR43). In vivo validation experiments in FFAR2^-/- mice showed lower H. polygyrus burdens, whereas oral acetate-treated C57BL/6 wild type mice showed higher burdens. These data suggest that locally secreted acetate - as a metabolic product of the energy metabolism of H. polygyrus L3s - provides a significant advantage to the parasite in crossing the intestinal epithelial barrier and invading mucosal tissues. This is the first and a rate-limiting step for helminths to establish chronic infections in their hosts and if modulated could have profound consequences for their life cycle.

Effects of helminths on the human immune response and the microbiome

Helminths have evolved sophisticated immune regulating mechanisms to prevent rejection by their mammalian host. Our understanding of how the human immune system responds to these parasites remains poor compared to mouse models of infection and this limits our ability to develop vaccines as well as harness their unique properties as therapeutic strategies against inflammatory disorders. Here, we review how recent studies on human challenge infections, self-infected individuals, travelers, and endemic populations have improved our understanding of human type 2 immunity and its effects on the microbiome. The heterogeneity of responses between individuals and the limited access to tissue samples beyond the peripheral blood are challenges that limit human studies on helminths, but also provide opportunities to transform our understanding of human immunology. Organoids and single-cell sequencing are exciting new tools for immunological analysis that may aid this pursuit. Learning about the genetic and immunological basis of resistance, tolerance, and pathogenesis to helminth infections may thus uncover mechanisms that can be utilized for therapeutic purposes.

Inhalant Mammal-Derived Lipocalin Allergens and the Innate Immunity

A major part of important mammalian respiratory allergens belongs to the lipocalin family of proteins. By this time, 19 respiratory mammalian lipocalin allergens have been registered in the WHO/IUIS Allergen Nomenclature Database. Originally, lipocalins, small extracellular proteins (molecular mass ca. 20 kDa), were characterized as transport proteins but they are currently known to exert a variety of biological functions. The three-dimensional structure of lipocalins is well-preserved, and lipocalin allergens can exhibit high amino acid identities, in several cases more than 50%. Lipocalins contain an internal ligand-binding site where they can harbor small principally hydrophobic molecules. Another characteristic feature is their capacity to bind to specific cell-surface receptors. In all, the physicochemical properties of lipocalin allergens do not offer any straightforward explanations for their allergenicity. Allergic sensitization begins at epithelial barriers where diverse insults through pattern recognition receptors awaken innate immunity. This front-line response is manifested by epithelial barrier-associated cytokines which together with other components of immunity can initiate the sensitization process. In the following, the crucial factor in allergic sensitization is interleukin (IL)-4 which is needed for stabilizing and promoting the type 2 immune response. The source for IL-4 has been searched widely. Candidates for it may be non-professional antigen-presenting cells, such as basophils or mast cells, as well as CD4+ T cells. The synthesis of IL-4 by CD4+ T cells requires T cell receptor engagement, i.e., the recognition of allergen peptides, which also provides the specificity for sensitization. Lipocalin and innate immunity-associated cell-surface receptors are implicated in facilitating the access of lipocalin allergens into the immune system. However, the significance of this for allergic sensitization is unclear, as the recognition by these receptors has been found to produce conflicting results. As to potential adjuvants associated with mammalian lipocalin allergens, the hydrophobic ligands transported by lipocalins have not been reported to enhance sensitization while it is justified to suppose that lipopolysaccharide plays a role in it. Taken together, type 2 immunity to lipocalin allergens appears to be a harmful immune response resulting from a combination of signals involving both the innate and adaptive immunities.

Host phenotype and microbiome vary with infection status, parasite genotype, and parasite microbiome composition

A growing literature demonstrates the impact of helminths on their host gut microbiome. We investigated whether the stickleback host microbiome depends on eco-evolutionary variables by testing the impact of exposure to the cestode parasite Schistocephalus solidus with respect to infection success, host genotype, parasite genotype, and parasite microbiome composition. We observed constitutive differences in the microbiome of sticklebacks of different origin, and those differences increased when sticklebacks exposed to the parasite resisted infection. In contrast, the microbiome of successfully infected sticklebacks varied with parasite genotype. More specifically, we revealed that the association between microbiome and immune gene expression increased in infected individuals and varied with parasite genotype. In addition, we showed that S. solidus hosts a complex endo- microbiome and that bacterial abundance in the parasite correlates with expression of host immune genes. Within this comprehensive analysis we demonstrated that (i) parasites contribute to modulating the host microbiome through both successful and unsuccessful infection, (ii) when infection is successful, the host microbiome varies with parasite genotype due to genotype-dependent variation in parasite immunomodulation, and (iii) the parasite-associated microbiome is distinct from its host's and impacts the host immune response to infection.

Food for thought - ILC metabolism in the context of helminth infections

Helminths are multicellular ancient organisms residing as parasites at mucosal surfaces of their host. Through adaptation and co-evolution with their hosts, helminths have been able to develop tolerance mechanisms to limit inflammation and avoid expulsion. The study of helminth infections as an integral part of tissue immunology allowed us to understand fundamental aspects of mucosal and barrier immunology, which led to the discovery of a new group of tissue-resident immune cells, innate lymphoid cells (ILC), over a decade ago. Here, we review the intricate interplay between helminth infections and type 2 ILC (ILC2) biology, discuss the host metabolic adaptation to helminth infections and the metabolic pathways fueling ILC2 responses. We hypothesize that nutrient competition between host and helminths may have prevented chronic inflammation in the past and argue that a detailed understanding of the metabolic restraints imposed by helminth infections may offer new therapeutic avenues in the future.

Trichuris muris Model: Role in Understanding Intestinal Immune Response, Inflammation and Host Defense

Several parasites have evolved to survive in the human intestinal tract and over 1 billion people around the world, specifically in developing countries, are infected with enteric helminths. Trichuris trichiura is one of the world’s most common intestinal parasites that causes human parasitic infections. Trichuris muris, as an immunologically well-defined mouse model of T. trichiura, is extensively used to study different aspects of the innate and adaptive components of the immune system. Studies on T. muris model offer insights into understanding host immunity, since this parasite generates two distinct immune responses in resistant and susceptible strains of mouse. Apart from the immune cells, T. muris infection also influences various components of the intestinal tract, especially the gut microbiota, mucus layer, epithelial cells and smooth muscle cells. Here, we reviewed the different immune responses generated by innate and adaptive immune components during acute and chronic T. muris infections. Furthermore, we discussed the importance of studying T. muris model in understanding host–parasite interaction in the context of alteration in the host’s microbiota, intestinal barrier, inflammation, and host defense, and in parasite infection-mediated modulation of other immune and inflammatory diseases.

Acceleration of Trichinella spiralis worm expulsion by leukotriene B4 receptor binding inhibition

AIMS

Helminth infection typically induces a Th2 inflammatory response that is characterized by eosinophilia, high levels of IgE and mast cells. LTB4 is generated from innate immune cells, such as neutrophils, macrophages and mast cells, in response to a range of stimuli. It mainly acts on myeloid leukocytes, inducing the activation of integrins, adhesion to endothelium walls, and chemotaxis.

METHODS AND RESULTS

The objective of the present study was to determine the role of the LTB4 receptor in Trichinella spiralis expulsion. We treated mice with the LTB4 receptor antagonist before infection with T. spiralis. We observed that the number of mast cells and worm infection decreased following treatment with the BLT antagonist during the intestinal phase. We also demonstrated that blocking the LTB4 receptor inhibited neutrophil and eosinophil infiltration.

CONCLUSIONS

Further studies are required to investigate the specific mechanism of mast cell number decrease and worm infection and the in vitro interactions between LTB4 and worm expulsion.

Thymic stromal lymphopoietin contributes to protection of mice from Strongyloides venezuelensis infection by CD4+ T cell-dependent and -independent pathways

When animals are infected with helminthic parasites, resistant hosts mount type II helper T (Th2) immune responses to expel worms. Recent studies have clearly shown that epithelial cell-derived cytokines contribute to the induction of Th2 immune responses. Here we demonstrate the role of endogenous thymic stromal lymphopoietin (TSLP) for protection against Strongyloides venezuelensis (S. venezuelensis) infection, utilizing TSLP receptor-deficient Crlf2-/- mice. The number of eggs per gram of feces (EPG) and worm burden were significantly higher in Crlf2-/- mice than in wild type (WT) mice. S. venezuelensis infection induced Tslp mRNA expression in the skin, lung, and intestine and also facilitated the accumulation of mast cells in the intestine in a TSLP-dependent manner. Furthermore, CD4+ T cells from S. venezuelensis-infected Crlf2-/- mice showed diminished capacity to produce Th2 cytokines in the early stage of infection. Finally, CD4+ cell-depleted Crlf2-/- mice still showed higher EPG counts and worm burden than CD4+ cell-depleted WT mice, indicating that TSLP contributes to protecting mice against S. venezuelensis infection in both CD4+ T cell-dependent and -independent manners.

The IL-33/ST2 pathway is not essential to Th2 stimulation but is key for modulation and survival during chronic infection with Schistosoma mansoni in mice

Morbidity during chronic schistosomiasis has been associated with the induction and modulation of type-2 granulomatous inflammatory response induced by antigens secreted by the eggs, which become trapped in capillary venules of the host tissues, especially in the liver and intestines. IL-33, an alarmin released after cell damage, binds to its ST2 (suppressor of tumorigenicity 2) receptor, expressed in an variety of immune cells, including ILC2 and macrophages, and stimulates the early production of IL-5 and IL-13, which leads to eosinophil infiltration and activation of a Th2 response. However, the role of IL-33/ST2 activation on Schistosoma-induced granuloma formation and modulation is mostly unknown. In the current work, we comparatively evaluated the immune response and granuloma formation in wild-type BALB/c (WT) and BALB/c mice genetically deficient in the IL-33 receptor (ST2^-/-) experimentally infected with Schistosoma mansoni. Mice were infected with 25 or 50 S. mansoni cercariae and followed for up to 14 weeks to assess mortality. Mice from each experimental group were comparatively evaluated for parasite burden, liver immune response, and granuloma appearance during acute and chronic schistosomiasis. Our data showed that the number of circulating worms and eggs retained in the liver and eliminated in the feces was similar in WT and ST2^-/- infected mice, but infected ST2^-/- mice presented an enhanced rate of mortality. Interestingly, the production of type-2 cytokines by soluble egg antigens (SEA)-stimulated spleen cells, the serum concentrations of IL-5 and Immunoglobulin (Ig)-E, and the level of parasite-reactive IgG1 were similar in infected mice of both experimental groups. The concentrations of IL-4, IL-5, IL-13, and IFN-γ in liver homogenate of infected mice also did not differ between the strains at acute schistosomiasis, but there was a significant increase in IL-17 levels in ST2^-/- infected mice at this phase. On the other hand, IL-4, IL-13, IL-10, IL-17, and IFN-γ concentrations were reduced and the ratios of IL-4/IFN-γ and IL-17/IFN-γ were higher in liver homogenate of chronically infected ST2^-/- mice, suggesting unbalanced Th2 and Th17 responses. Moreover, liver granulomas of ST2^-/- mice were larger and disorganized, showing an intense cellular infiltrate, rich in eosinophils and neutrophils. Our results suggest that the absence of the IL-33/ST2 pathway is not essential for the Schistosoma-induced Th2 response, but is necessary to prevent host mortality by modulating granuloma-mediated pathology.

Heterogeneity in the initiation, development and function of type 2 immunity

Type 2 immune responses operate under varying conditions in distinct tissue environments and are crucial for protection against helminth infections and for the maintenance of tissue homeostasis. Here we explore how different layers of heterogeneity influence type 2 immunity. Distinct insults, such as allergens or infections, can induce type 2 immune responses through diverse mechanisms, and this can have heterogeneous consequences, ranging from acute or chronic inflammation to deficits in immune regulation and tissue repair. Technological advances have provided new insights into the molecular heterogeneity of different developmental lineages of type 2 immune cells. Genetic and environmental heterogeneity also contributes to the varying magnitude and quality of the type 2 immune response during infection, which is an important determinant of the balance between pathology and disease resolution. Hence, understanding the mechanisms underlying the heterogeneity of type 2 immune responses between individuals and between different tissues will be crucial for treating diseases in which type 2 immunity is an important component.

Cytokines and beyond: Regulation of innate immune responses during helminth infection

Parasitic helminth infection elicits a type 2 cytokine-mediated inflammatory response. During type 2 inflammation, damaged or stimulated epithelial cells exposed to helminths and their products produce alarmins and cytokines including IL-25, IL-33, and thymic stromal lymphopoietin. These factors promote innate immune cell activation that supports the polarization of CD4+ T helper type 2 (Th2) cells. Activated innate and Th2 cells produce the cytokines IL-4, -5, -9, and -13 that perpetuate immune activation and act back on the epithelium to cause goblet cell hyperplasia and increased epithelial cell turnover. Together, these events facilitate worm expulsion and wound healing processes. While the role of Th2 cells in this context has been heavily studied, recent work has revealed that epithelial cell-derived cytokines are drivers of key innate immune responses that are critical for type 2 anti-helminth responses. Cutting-edge studies have begun to fully assess how other factors and pathways, including lipid mediators, chemokines, Fc receptor signaling, danger-associated molecular pattern molecules, and direct cell-cell interactions, also participate in shaping innate cell-mediated type 2 inflammation. In this review, we discuss how these pathways intersect and synergize with pathways controlled by epithelial cell-derived cytokines to coordinate innate immune responses that drive helminth-induced type 2 inflammation.

The Intestinal Epithelium at the Forefront of Host-Helminth Interactions

Gastrointestinal helminth infection still constitutes a major public health issue, particularly in the developing world. As these parasites can undergo a large part of their lifecycle within the intestinal tract the host has developed various structural and cellular specializations at the epithelial barrier to contend with infection. Detailed characterization of these cells will provide important insights about their contributions to the protective responses mediated against helminths. Here, we discuss how key components of the intestinal epithelium may function to limit the initial establishment of helminths, and how these cells are altered during an active response to infection.

Inflammatory bowel diseases, the hygiene hypothesis and the other side of the microbiota: Parasites and fungi

This review tackles the concept of the evolutionary mismatch, in relation with the reduction of the prevalence of the so-called "dirty old friends". These formed the variegated community of parasites and microorganisms, either prokaryotic or eukaryotic, that, over long evolutionary times, co-evolved with humans and their ancestors, inhabiting their digestive tracts, and other body districts. This community of microbial symbionts and metazoan parasites is thought to have evolved a complex network of inter-independence with the host, in particular in relation with their immune stimulating capacity, and with the consequent adaptation of the host immune response to this chronic stimulation. Strictly related to this evolutionary mismatch, the hygiene hypothesis, proposed by David Strachan in 1989, foresees that the increase in the incidence of inflammatory and autoimmune disorders during the twentieth century has been caused by the reduced exposure to parasites and microorganisms, especially in industrialized countries. Among these pathologies, inflammatory bowel diseases (IBDs) occupy a prominent role. From these premises, this review summarizes current knowledge on how variations in the composition of the gut bacterial microbiota, as well as its interactions with fungal communities, influence the overall immune balance, favouring or counteracting gut inflammation in IBDs. Additionally, the effect of worm parasites, either directly on the immune balance, or indirectly, through the modulation of bacterial and fungal microbiota, will be addressed. Finally, we will review a series of studies related to the use of molecules derived from parasitic worms and fungi, which hold the potential to be developed as postbiotics for the treatment of IBDs.

BHLHE40 Promotes TH2 Cell-Mediated Antihelminth Immunity and Reveals Cooperative CSF2RB Family Cytokines

The transcription factor BHLHE40 is an emerging regulator of the immune system. Recent studies suggest that BHLHE40 regulates type 2 immunity, but this has not been demonstrated in vivo. We found that BHLHE40 is required in T cells for a protective T_H2 cell response in mice infected with the helminth Heligmosomoides polygyrus bakeri H. polygyrus elicited changes in gene and cytokine expression by lamina propria CD4⁺ T cells, many of which were BHLHE40 dependent, including production of the common β (CSF2RB) chain family cytokines GM-CSF and IL-5. In contrast to deficiency in GM-CSF or IL-5 alone, loss of both GM-CSF and IL-5 signaling impaired protection against H. polygyrus Overall, we show that BHLHE40 regulates the T_H2 cell transcriptional program during helminth infection to support normal expression of Csf2, Il5, and other genes required for protection and reveal unexpected redundancy of common β chain-dependent cytokines previously thought to possess substantially divergent functions.

Shaping the diversity of Th2 cell responses in epithelial tissues and its potential for allergy treatment

Th2 cells have evolved to protect from large helminth infections and to exert tissue protective functions in response to nonmicrobial noxious stimuli. The initiation, maintenance, and execution of these functions depend on the integration of diverse polarizing cues by cellular sensors and molecular programs as well as the collaboration with cells that are coopted for signal exchange. The complexity of input signals and cellular collaboration generates tissue specific Th2 cell heterogeneity and specialization. In this review, we aim to discuss the advances and recent breakthroughs in our understanding of Th2 cell responses and highlight developmental and functional differences among T cells within the diversifying field of type 2 immunity. We will focus on factors provided by the tissue microenvironment and highlight factors with potential implications for the pathogenesis of allergic skin and lung diseases. Especially new insights into the role of immunometabolism, the microbiota and ionic signals enhance the complexity of Th2 cell regulation and warrant a critical evaluation. Finally, we will discuss how this ensemble of established knowledge and recent breakthroughs about Th2 immunobiology advance our understanding of the pathogenesis of allergic diseases and how this could be exploited for future immunotherapies.

Dendritic Cell-Mediated Th2 Immunity and Immune Disorders

Dendritic cells (DCs) are the professional antigen-presenting cells that recognize and present antigens to naïve T cells to induce antigen-specific adaptive immunity. Among the T-cell subsets, T helper type 2 (Th2) cells produce the humoral immune responses required for protection against helminthic disease by activating B cells. DCs induce a Th2 immune response at a certain immune environment. Basophil, eosinophil, mast cells, and type 2 innate lymphoid cells also induce Th2 immunity. However, in the case of DCs, controversy remains regarding which subsets of DCs induce Th2 immunity, which genes in DCs are directly or indirectly involved in inducing Th2 immunity, and the detailed mechanisms underlying induction, regulation, or maintenance of the DC-mediated Th2 immunity against allergic environments and parasite infection. A recent study has shown that a genetic defect in DCs causes an enhanced Th2 immunity leading to severe atopic dermatitis. We summarize the Th2 immune-inducing DC subsets, the genetic and environmental factors involved in DC-mediated Th2 immunity, and current therapeutic approaches for Th2-mediated immune disorders. This review is to provide an improved understanding of DC-mediated Th2 immunity and Th1/Th2 immune balancing, leading to control over their adverse consequences.

Host responses to intestinal nematodes

Helminth infection remains common in developing countries, where residents who suffer from the consequences of such infections can develop serious physical and mental disorders and often persist in the face of serious economic problems. Intestinal nematode infection induces the development of Th2-type immune responses including the B-cell IgE response; additionally, this infection induces an increase in the numbers and activation of various types of effector cells, such as mast cells, eosinophils and basophils, as well as the induction of goblet cell hyperplasia, anti-microbial peptide production and smooth-muscle contraction, all of which contribute to expel nematodes. Innate immunity is important in efforts to eliminate helminth infection; cytokines, including IL-25, IL-33 and thymic stromal lymphopoietin, which are products of epithelial cells and mast cells, induce Th2 cells and group 2 innate lymphoid cells to proliferate and produce Th2 cytokines. Nematodes also facilitate chronic infection by suppression of immune reactions through an increased number of Treg cells. Immunosuppression by parasite infection may ultimately be beneficial for the host animals; indeed, a negative correlation has been found between parasite infection and the prevalence of inflammatory disease in humans.

TSLP Protects Corneas From Pseudomonas aeruginosa Infection by Regulating Dendritic Cells and IL-23-IL-17 Pathway

Purpose

We sought to determine the role of epithelium-produced thymic stromal lymphopoietin (TSLP) and its underlying mechanisms in corneal innate immune defense against Pseudomonas (P.) aeruginosa keratitis.

Methods

The expression of TSLP and TSLPR in cultured human corneal epithelial cells (HCECs) and mouse corneas was determined by PCR, Western, and/or ELISA. Cellular localization of TSLP receptor (TSLPR) was determined by whole mount confocal microscopy. TSLP-TSLPR signaling was downregulated by neutralizing antibodies and/or small interfering (si)RNA; their effects on the severity of P. aeruginosa–keratitis and cytokine expression were assessed using clinical scoring, bacterial counting, PMN infiltration, and real-time PCR. The role of dendritic cells (DCs) in corneal innate immunity was determined by local DC depletion using CD11c-DTR mice.

Results

P. aeruginosa–infection induced the expression of TSLP and TSLPR in both cultured primary HCECs and in C57BL/6 mouse corneas. While TSLP was mostly expressed by epithelial cells, CD11c-positive cells were positive for TSLPR. Targeting TSLP or TSLPR with neutralizing antibodies or TSLPR with siRNA resulted in more severe keratitis, attributable to an increase in bacterial burden and PMN infiltration. TSLPR neutralization significantly suppressed infection-induced TSLP and interleukin (IL)-17C expression and augmented the expression of IL-23 and IL-17A. Local depletion of DCs markedly increased the severity of keratitis and exhibited no effects on TSLP and IL-23 expression while suppressing IL-17A and C expression in P. aeruginosa–infected corneas.

Conclusions

The epithelium-expressed TSLP plays a protective role in P. aeruginosa keratitis through targeting of DCs and in an IL-23/IL-17 signaling pathway-related manner.

Helminth therapy for autism under gut-brain axis- hypothesis

Autism is a neurodevelopmental disease included within Autism Syndrome Disorder (ASD) spectrum. ASD has been linked to a series of genes that play a role in immune response function and patients with autism, commonly suffer from immune-related comorbidities. Despite the complex pathophysiology of autism, Gut-brain axis is gaining strength in the understanding of several neurological disorders. In addition, recent publications have shown the correlation between immune dysfunctions, gut microbiota and brain with the behavioral alterations and comorbid symptoms found in autism. Gut-brain axis acts as the "second brain", in a communication network established between neural, endocrine and the immunological systems. On the other hand, Hygiene Hypothesis suggests that the increase in the incidence of autoimmune diseases in the modern world can be attributed to the decrease of exposure to infectious agents, as parasitic nematodes. Helminths induce modulatory and protective effects against several inflammatory disorders, maintaining gastrointestinal homeostasis and modulating brain functions. Helminthic therapy has been previously performed in diseases such as ulcerative colitis, Crohn's disease, diabetes, multiple sclerosis, asthma, rheumatoid arthritis, and food allergies. Considering gut-brain axis, Hygiene Hypothesis, and the modulatory effects of helminths I hypothesized that a treatment with Trichuris suis soluble products represents a feasible holistic treatment for autism, and the key for the development of novel treatments. Preclinical studies are required to test this hypothesis.

Professional and 'Amateur' Antigen-Presenting Cells In Type 2 Immunity

Dendritic cells (DCs) are critical for the activation of naïve CD4⁺ T cells and are considered professional antigen-presenting cells (APCs), as are macrophages and B cells. Recently, several innate type 2 immune cells, such as basophils, mast cells (MCs), eosinophils, and innate type 2 lymphocytes (ILC2), have also emerged as harboring APC behavior. Through surface expression or transfer of peptide-loaded MHCII, expression of costimulatory and co-inhibitory molecules, as well as the secretion of polarizing cytokines, these innate cells can extensively communicate with effector and regulatory CD4⁺ T cells. An exciting new concept is that the complementary tasks of these 'amateur' APCs contribute to shaping and regulating adaptive immunity to allergens and helminths, often in collaboration with professional APCs.

Importance of EMT Factor ZEB1 in cDC1 "MutuDC Line" Mediated Induction of Th1 Immune Response

The role of Epithelial to Mesenchymal Transition (EMT) factor Zeb1 is well defined in metastasis and cancer progression but it's importance in dendritic cells (DCs) is unexplored until now. For the first time we report here that Zeb1 controls immunogenic responses of CD8α⁺ conventional Type-I (cDC1) DCs. We found that ZEB1 expression increases significantly after TLR9 stimulation and its depletion impairs activation, co-stimulation and secretion of important cytokines like IL-6, IL-10 and IL-12 in cDC1 MutuDC line. We further confirmed our findings in primary cDC1 DCs derived from bone marrow. Co-culture of these Zeb1 knock down (KD) DCs with OT-II CD4⁺ T helper cells skewed their differentiation toward Th2 subtype. Moreover, adoptive transfer of activated Zeb1 KD DCs cleared intestinal worms in helminth infected mice by increasing Th2 responses in vivo. Integrative genomic analysis showed Zeb1 as an activator of immune response genes in cDC1 MutuDCs as compared to other pathway genes. In addition, differentially regulated genes in Zeb1 KD RNA-seq showed significant enrichment of Th2 activation pathways supporting our in vitro findings. Mechanistically, we showed that decreased IL-12 secreted by Zeb1 KD DCs is the plausible mechanism for increased Th2 differentiation. Collectively our data demonstrate that Zeb1 could be targeted in DCs to modulate T-cell mediated adaptive immune responses.

Analysis of the Trichuris suis excretory/secretory proteins as a function of life cycle stage and their immunomodulatory properties

Parasitic worms have a remarkable ability to modulate host immune responses through several mechanisms including excreted/secreted proteins (ESP), yet the exact nature of these proteins and their targets often remains elusive. Here, we performed mass spectrometry analyses of ESP (TsESP) from larval and adult stages of the pig whipworm Trichuris suis (Ts) and identified ~350 proteins. Transcriptomic analyses revealed large subsets of differentially expressed genes in the various life cycle stages of the parasite. Exposure of bone marrow-derived macrophages and dendritic cells to TsESP markedly diminished secretion of the pro-inflammatory cytokines TNFα and IL-12p70. Conversely, TsESP exposure strongly induced release of the anti-inflammatory cytokine IL-10, and also induced high levels of nitric oxide (NO) and upregulated arginase activity in macrophages. Interestingly, TsESP failed to directly induce CD4+ CD25+ FoxP3+ regulatory T cells (Treg cells), while OVA-pulsed TsESP-treated dendritic cells suppressed antigen-specific OT-II CD4+ T cell proliferation. Fractionation of TsESP identified a subset of proteins that promoted anti-inflammatory functions, an activity that was recapitulated using recombinant T. suis triosephosphate isomerase (TPI) and nucleoside diphosphate kinase (NDK). Our study helps illuminate the intricate balance that is characteristic of parasite-host interactions at the immunological interface, and further establishes the principle that specific parasite-derived proteins can modulate immune cell functions.

Helminth Infections Induce Tissue Tolerance Mitigating Immunopathology but Enhancing Microbial Pathogen Susceptibility

Helminths are ubiquitous and have chronically infected vertebrates throughout their evolution. As such helminths have likely exerted considerable selection pressure on our immune systems. The large size of multicellular helminths and their limited replicative capacity in the host necessarily elicits different host protective mechanisms than the immune response evoked by microbial pathogens such as bacteria, viruses and intracellular parasites. The cellular damage resulting from helminth migration through tissues is a major trigger of the type 2 and regulatory immune responses, which activates wound repair mechanisms that increases tissue tolerance to injury and resistance mechanisms that enhance resistance to further colonization with larval stages. While these wound healing and anti-inflammatory responses may be beneficial to the helminth infected host, they may also compromise the host's ability to mount protective immune responses to microbial pathogens. In this review we will first describe helminth-induced tolerance mechanisms that develop in specific organs including the lung and the intestine, and how adaptive immunity may contribute to these responses through differential activation of T cells in the secondary lymphoid organs. We will then integrate studies that have examined how the immune response is modulated in these specific tissues during coinfection of helminths with viruses, protozoa, and bacteria.

First Responders: Innate Immunity to Helminths

Helminth infections represent a significant public health concern resulting in devastating morbidity and economic consequences across the globe. Helminths migrate through mucosal sites causing tissue damage and the induction of type 2 immune responses. Antihelminth protection relies on the mobilization and activation of multiple immune cells, including type 2 innate lymphocytes (ILC2s), basophils, mast cells, macrophages, and hematopoietic stem/progenitor cells. Further, epithelial cells and neurons have been recognized as important regulators of type 2 immunity. Collectively, these pathways stimulate host-protective responses necessary for worm expulsion and the healing of affected tissues. In this review we focus on the innate immune pathways that regulate immunity to helminth parasites and describe how better understanding of these pathways may lead to the development of new therapeutic strategies.

Intestinal helminths and protozoan infections in patients with colorectal cancer: prevalence and possible association with cancer pathogenesis

The purpose of the present study was to determine the prevalence of intestinal helminths and protozoa in colorectal cancer (CRC) patients and to evaluate the possible association between the prevalence and CRC pathogenesis. A total of 200 CRC patients and 200 residents of Tashkent, who had no complaints related to the gastrointestinal tract, were examined by triple coproscopy using a concentration method and estimations of protozoan infection intensity. Of the CRC patients tested, 144 were classified as T₁-₄N₀M₀ (without metastases) and 56 were classified as T_1-4N_1-2M_0-1 (with metastases). Parasitological examination was performed during CRC diagnosis before and after surgery and chemotherapy. A significantly higher prevalence of Blastocystis sp., Chilomastix mesnili, Jodamoeba butschlii, and Endolimax nana was found in CRC patients than in the control population (p < 0.0001), amounting to 80, 20, 22.5, and 11.5%, respectively. The high prevalence of Blastocystis sp., as well as the patterns of infection intensity, was stable at all stages of examination. The ratio of the number of CRC patients with and without Blastocystis sp. in the T_1-4N₀M₀ and T_1-4N_1-2M_0-1 groups amounted to 3.3 and 7.0, respectively. The ratios for C. mesnili, E. coli, J. butschlii, and E. nana in both groups were 0.2 and 0.2, 0.07 and 0.07, 0.3 and 0.16, and 0.18 and 0.01, respectively. The prevalence of helminths and Giardia lamblia in CRC patients and the control population was not significantly different. Taken together, these data indicate a possible role for Blastocystis sp. in CRC pathogenesis. Diagnosis, treatment, and further observation of patients with Blastocystis sp. are necessary at all stages of CRC, including during diagnosis and before and after surgery and chemotherapy.

Thymic Stromal Lymphopoietin Isoforms, Inflammatory Disorders, and Cancer

Thymic stromal lymphopoietin (TSLP) is a pleiotropic cytokine originally isolated from a murine thymic stromal cell line. TSLP exerts its biological effects by binding to a high-affinity heteromeric complex composed of thymic stromal lymphopoietin receptor chain and IL-7Rα. TSLP is primarily expressed by activated lung and intestinal epithelial cells, keratinocytes, and fibroblasts. However, dendritic cells (DCs), mast cells, and presumably other immune cells can also produce TSLP. Different groups of investigators have demonstrated the existence of two variants for TSLP in human tissues: the main isoform expressed in steady state is the short form (sf TSLP), which plays a homeostatic role, whereas the long form (lfTSLP) is upregulated in inflammatory conditions. In addition, there is evidence that in pathological conditions, TSLP can be cleaved by several endogenous proteases. Several cellular targets for TSLP have been identified, including immune (DCs, ILC2, T and B cells, NKT and Treg cells, eosinophils, neutrophils, basophils, monocytes, mast cells, and macrophages) and non-immune cells (platelets and sensory neurons). TSLP has been originally implicated in a variety of allergic diseases (e.g., atopic dermatitis, bronchial asthma, eosinophilic esophagitis). Emerging evidence indicates that TSLP is also involved in chronic inflammatory (i.e., chronic obstructive pulmonary disease and celiac disease) and autoimmune (e.g., psoriasis, rheumatoid arthritis) disorders and several cancers. These emerging observations greatly widen the role of TSLP in different human diseases. Most of these studies have not used tools to analyze the expression of the two TSLP isoforms. The broad pathophysiologic profile of TSLP has motivated therapeutic targeting of this cytokine. Tezepelumab is a first-in-class human monoclonal antibody (1) that binds to TSLP inhibiting its interaction with TSLP receptor complex. Tezepelumab given as an add-on-therapy to patients with severe uncontrolled asthma has shown safety and efficacy. Several clinical trials are evaluating the safety and the efficacy of tezepelumab in different inflammatory disorders. Monoclonal antibodies used to neutralize TSLP should not interact or hamper the homeostatic effects of sf TSLP.

Immunity to gastrointestinal nematode infections

Numerous species of nematodes have evolved to inhabit the gastrointestinal tract of animals and humans, with over a billion of the world's population infected with at least one species. These large multicellular pathogens present a considerable and complex challenge to the host immune system given that individuals are continually exposed to infective stages, as well as the high prevalence in endemic areas. This review summarizes our current understanding of host-parasite interactions, detailing induction of protective immunity, mechanisms of resistance, and resolution of the response. It is clear from studies of well-defined laboratory model systems that these responses are dominated by innate and adaptive type 2 cytokine responses, regulating cellular and soluble effectors that serve to disrupt the niche in which the parasites live by strengthening the physical mucosal barrier and ultimately promoting tissue repair.

Curing tumor-bearing mice by shifting a Th2 to a Th1 anti-tumor response

Over the past several years remarkable therapeutic responses have been obtained with immunomodulatory monoclonal antibodies (mAbs), both in mice \cite{10,18,20,48,54,61} and human cancer patients \cite{1,3,14,28,30,39,80}. However, complete regressions and cures are infrequent and not predictable and some tumor types respond much worse than others. As an attempt to increase curability, we have investigated in mouse models the therapeutic efficacy of several mAb combinations, focusing on anti-PD-1/CTLA-4/CD137 and anti-PD-1/CTLA-4/CD137/CD19, and we have also combined mAbs with the chemotherapeutic drug cisplatin. Our data demonstrate an important contribution of anti-CD19 mAb to therapeutic efficacy, they show that intratumoral delivery of the mAbs is therapeutically more effective than systemic delivery, and that there is synergy when the mAbs are combined with cisplatin. In an attempt to improve predictability, we developed an in vitro model that may also be employed to search for novel immunomodulatory agents and combinations. This article reviews our data and discusses what is known about the underlying mechanisms.

Combinatorial targeting of TSLP, IL-25, and IL-33 in type 2 cytokine-driven inflammation and fibrosis

Thymic stromal lymphopoietin (TSLP), interleukin-25 (IL-25), and IL-33 are important initiators of type 2-associated mucosal inflammation and immunity. However, their role in the maintenance of progressive type 2 inflammation and fibrosis is much less clear. Using chronic models of helminth infection and allergic lung inflammation, we show that collective disruption of TSLP, IL-25, and IL-33 signaling suppresses chronic and progressive type 2 cytokine-driven inflammation and fibrosis. In a schistosome lung granuloma model or during chronic Schistosoma mansoni infection in the liver, individual ablation of TSLP, IL-25, or IL-33/ST2 had no impact on the development of IL-4/IL-13-dependent inflammation or fibrosis. However, significant reductions in granuloma-associated eosinophils, hepatic fibrosis, and IL-13-producing type 2 innate lymphoid cells (ILC2s) were observed when signaling of all three mediators was simultaneously disrupted. Combined blockade through monoclonal antibody (mAb) treatment also reduced IL-5 and IL-13 expression during primary and secondary granuloma formation in the lungs. In a model of chronic house dust mite-induced allergic lung inflammation, combined mAb treatment did not decrease established inflammation or fibrosis. TSLP/IL-33 double-knockout mice treated with anti-IL-25 mAb during priming, however, displayed decreased inflammation, mucus production, and lung remodeling in the chronic phase. Together, these studies reveal partially redundant roles for TSLP, IL-25, and IL-33 in the maintenance of type 2 pathology and suggest that in some settings, early combined targeting of these mediators is necessary to ameliorate progressive type 2-driven disease.

Preventive Trichuris suis ova (TSO) treatment protects immunocompetent rabbits from DSS colitis but may be detrimental under conditions of immunosuppression

Trichuris suis ova (TSO) have been tested for therapeutic application in inflammatory bowel diseases (IBD) yet understanding of the underlying mechanisms and safety in an immunocompromised host is limited due to lack of a suitable animal model. We used a recently established rabbit model of dextran sodium sulphate (DSS) induced colitis to study the efficacy, mechanisms and safety of TSO therapy in immunocompetent and immunosuppressed animals. TSO treatment prevented the DSS induced weight loss, delayed the onset of DSS induced symptoms by 2 days and significantly reduced the disease activity (DAI). TSO treatment protected caecal histology and prevented the colitis-associated loss in faecal microbiota diversity. Mainly the transcriptome of lamina propria mononuclear cells (LPMC) was affected by TSO treatment, showing dampened innate and adaptive inflammatory responses. The protective effect of TSO was lost in immunosuppressed rabbits, where TSO exacerbated colitis. Our data show that preventive TSO treatment ameliorates colitis severity in immunocompetent rabbits, modulates LPMC immune responses and reduces faecal dysbiosis. In contrast, the same TSO treatment exacerbates colitis in immunosuppressed animals. Our data provide further evidence for a therapeutic effect of TSO in IBD, yet caution is required with regard to TSO treatment in immunosuppressed patients.

Endogenous Calcitriol Synthesis Controls the Humoral IgE Response in Mice

The vitamin D receptor participates in the control of IgE class-switch recombination in B cells. The physiologic vitamin D receptor agonist, 1,25(OH)₂D₃ (calcitriol), is synthesized by the essential enzyme 25-hydroxyvitamin D₃-1α-hydroxylase (CYP27B1), which can be expressed by activated immune cells. The role of endogenous calcitriol synthesis for the regulation of IgE has not been proven. In this study, we investigated IgE-responses in Cyp27b1-knockout (KO) mice following sensitization to OVA or intestinal infection with Heligmosomoides polygyrus Specific Igs and plasmablasts were determined by ELISA and ELISpot, Cyp27b1 expression was measured by quantitative PCR. The data show elevated specific IgE and IgG1 concentrations in the blood of OVA-sensitized Cyp27b1-KO mice compared with wild-type littermates (+898 and +219%). Accordingly, more OVA-specific IgG1-secreting cells are present in spleen and fewer in the bone marrow of Cyp27b1-KO mice. Ag-specific mechanisms are suggested as the leucopoiesis is in general unchanged and activated murine B and T lymphocytes express Cyp27b1 Accordingly, elevated specific IgE concentrations in the blood of sensitized T cell-specific Cyp27b1-KO mice support a lymphocyte-driven mechanism. In an independent IgE-inducing model, i.e., intestinal infection with H. polygyrus, we validated the increase of total and specific IgE concentrations of Cyp27b1-KO compared with wild-type mice, but not those of IgG1 or IgA. We conclude that endogenous calcitriol has an impact on the regulation of IgE in vivo. Our data provide genetic evidence supporting previous preclinical and clinical findings and suggest that vitamin D deficiency not only promotes bone diseases but also type I sensitization.

Suppression of inflammation and tissue damage by a hookworm recombinant protein in experimental colitis

Gastrointestinal parasites, hookworms in particular, have evolved to cause minimal harm to their hosts when present in small numbers, allowing them to establish chronic infections for decades. They do so by creating an immunoregulatory environment that promotes their own survival, but paradoxically also benefits the host by protecting against the onset of many inflammatory diseases. To harness the therapeutic value of hookworms without using live parasites, we have examined the protective properties of the recombinant protein anti-inflammatory protein (AIP)-1, secreted in abundance by hookworms within the intestinal mucosa, in experimental colitis. Colitic inflammation assessed by weight loss, colon atrophy, oedema, ulceration and necrosis, as well as abdominal adhesion was significantly suppressed in mice treated with a single intraperitoneal dose of AIP-1 at 1 mg kg⁻¹. Local infiltration of inflammatory cells was also significantly reduced, with minimal goblet cell loss and preserved mucosal architecture. Treatment with AIP-1 promoted the production of colon interleukin (IL)-10, transforming growth factor (TGF)-β and thymic stromal lymphopoietin (TSLP), resulting in the suppression of tumour necrosis factor (TNF)-α, IL-13 and IL-17 A cytokines and granulocyte macrophage colony-stimulating factor (GM-CSF), CX motif chemokine (CXCL)-11 and cyclooxygenase synthase (COX)-2 mRNA transcripts. AIP-1 promoted the accumulation of regulatory T cells in the colon likely allowing rapid healing of the colon mucosa. Hookworm recombinant AIP-1 is a novel therapeutic candidate for the treatment of inflammatory bowel diseases that can be explored for the prevention of acute inflammatory relapses, an important cause of colorectal cancer.

Distinct DC subsets regulate adaptive Th1 and 2 responses during Trichuris muris infection

Low- and high-dose infections with the murine large intestinal nematode Trichuris muris are associated with induction of adaptive Th1 and Th2 responses, respectively, in mesenteric lymph nodes (MLN). Classical dendritic cells (cDC) accumulate in the large intestinal mucosa and MLN upon T. muris infection, yet their role in driving adaptive responses to infection remains largely unknown. We performed low- and high-dose T. muris infections of mice deficient in defined cDC subsets to investigate their role in induction of adaptive immune responses. Mice lacking IRF4-dependent cDC failed to clear a high-dose infection and displayed impaired Th2 responses. Conversely, mice lacking IRF8-dependent cDC cleared a low-dose infection and displayed an impaired Th1 response while increased production of Th2 cytokines. Finally, mice lacking both IRF4- and IRF8-dependent cDC were able to generate a Th2 response and clear a low-dose infection. Collectively, these results suggest that IRF4- and IRF8-dependent cDC act antagonistically during T. muris infection, and demonstrate that intestinal Th2 responses can be generated towards T. muris in the absence of IRF4-dependent cDC.

Toxoplasma Co-infection Prevents Th2 Differentiation and Leads to a Helminth-Specific Th1 Response

Nematode infections, in particular gastrointestinal nematodes, are widespread and co-infections with other parasites and pathogens are frequently encountered in humans and animals. To decipher the immunological effects of a widespread protozoan infection on the anti-helminth immune response we studied a co-infection with the enteric nematode Heligmosomoides polygyrus in mice previously infected with Toxoplasma gondii. Protective immune responses against nematodes are dependent on parasite-specific Th2 responses associated with IL-4, IL-5, IL-13, IgE, and IgG1 antibodies. In contrast, Toxoplasma gondii infection elicits a strong and protective Th1 immune response characterized by IFN-γ, IL-12, and IgG2a antibodies. Co-infected animals displayed significantly higher worm fecundity although worm burden remained unchanged. In line with this, the Th2 response to H. polygyrus in co-infected animals showed a profound reduction of IL-4, IL-5, IL-13, and GATA-3 expressing T cells. Co-infection also resulted in the lack of eosinophilia and reduced expression of the Th2 effector molecule RELM-β in intestinal tissue. In contrast, the Th1 response to the protozoan parasite was not diminished and parasitemia of T. gondii was unaffected by concurrent helminth infection. Importantly, H. polygyrus specific restimulation of splenocytes revealed H. polygyrus-reactive CD4⁺ T cells that produce a significant amount of IFN-γ in co-infected animals. This was not observed in animals infected with the nematode alone. Increased levels of H. polygyrus-specific IgG2a antibodies in co-infected mice mirrored this finding. This study suggests that polarization rather than priming of naive CD4⁺ T cells is disturbed in mice previously infected with T. gondii. In conclusion, a previous T. gondii infection limits a helminth-specific Th2 immune response while promoting a shift toward a Th1-type immune response.

TSLP is a direct trigger for T cell migration in filaggrin-deficient skin equivalents

Mutations in the gene encoding for filaggrin (FLG) are major predisposing factors for atopic dermatitis (AD). Besides genetic predisposition, immunological dysregulations considerably contribute to its pathophysiology. For example, thymic stromal lymphopoietin (TSLP) is highly expressed in lesional atopic skin and significantly contributes to the pathogenesis of AD by activating dendritic cells that then initiate downstream effects on, for example, T cells. However, little is known about the direct interplay between TSLP, filaggrin-deficient skin and other immune cells such as T lymphocytes. In the present study, FLG knockdown skin equivalents, characterised by intrinsically high TSLP levels, were exposed to activated CD4⁺ T cells. T cell exposure resulted in an inflammatory phenotype of the skin equivalents. Furthermore, a distinct shift from a Th1/Th17 to a Th2/Th22 profile was observed following exposure of T cells to filaggrin-deficient skin equivalents. Interestingly, TSLP directly stimulated T cell migration exclusively in filaggrin-deficient skin equivalents even in the absence of dendritic cells, indicating a hitherto unknown role of TSLP in the pathogenesis of AD.

Guards at the gate: physiological and pathological roles of tissue-resident innate lymphoid cells in the lung

The lung is an important open organ and the primary site of respiration. Many life-threatening diseases develop in the lung, e.g., pneumonia, asthma, chronic obstructive pulmonary diseases (COPDs), pulmonary fibrosis, and lung cancer. In the lung, innate immunity serves as the frontline in both anti-irritant response and anti-tumor defense and is also critical for mucosal homeostasis; thus, it plays an important role in containing these pulmonary diseases. Innate lymphoid cells (ILCs), characterized by their strict tissue residence and distinct function in the mucosa, are attracting increased attention in innate immunity. Upon sensing the danger signals from damaged epithelium, ILCs activate, proliferate, and release numerous cytokines with specific local functions; they also participate in mucosal immune-surveillance, immune-regulation, and homeostasis. However, when their functions become uncontrolled, ILCs can enhance pathological states and induce diseases. In this review, we discuss the physiological and pathological functions of ILC subsets 1 to 3 in the lung, and how the pathogenic environment affects the function and plasticity of ILCs.

Mucosal Expression of T Cell Gene Variants Is Associated with Differential Resistance to Teladorsagia circumcincta

Resistance of sheep to the gastrointestinal nematode Teladorsagia circumcincta is a heritable characteristic. Control of parasite colonization and egg production is strongly linked to IgA antibody levels regulated by Th2 T cell activation within lymphoid tissue; and persistently-infected susceptible animals develop an inflammatory Th1/Th17 response within the abomasum that fails to control infection. Differential T cell polarization therefore is associated with parasite resistance and/or susceptibility and is controlled by a specific set of transcription factors and cytokine receptors. Transcript variants of these genes have been characterized in sheep, while in humans and mice different variants of the genes are associated with inflammatory diseases. RT-qPCR was used to quantify mucosal expression of the transcript variants of the sheep genes in trickle-infected animals with defined phenotypic traits. Genes that encode full-length GATA3 and IL17RB were shown to be significantly increased in resistant sheep that had controlled parasite infection. Expression levels of both were significantly negatively correlated with abomasal worm count (a parameter of susceptibility) and positively correlated with body weight (a parameter of resistance). These data show that polarized Th2 T cells within the abomasal mucosa play an important role in the maintenance of resistance.

Th2 responses are primed by skin dendritic cells with distinct transcriptional profiles

Connor et al. show that transcriptomic profiling of DCs exposed to two different Th2 stimuli in vivo reveals large numbers of differentially expressed genes but few similarities between conditions.

The dendritic cell signals required for the in vivo priming of IL-4–producing T cells are unknown. We used RNA sequencing to characterize DCs from skin LN of mice exposed to two different Th2 stimuli: the helminth parasite Nippostrongylus brasiliensis (Nb) and the contact sensitizer dibutyl phthalate (DBP)-FITC. Both Nb and DBP-FITC induced extensive transcriptional changes that involved multiple DC subsets. Surprisingly, these transcriptional changes were highly distinct in the two models, with only a small number of genes being similarly regulated in both conditions. Pathway analysis of expressed genes identified no shared pathways between Nb and DBP-FITC, but revealed a type-I IFN (IFN-I) signature unique to DCs from Nb-primed mice. Blocking the IFN-I receptor at the time of Nb treatment had little effect on DC migration and antigen transport to the LN, but inhibited the up-regulation of IFN-I–induced markers on DCs and effectively blunted Th2 development. In contrast, the response to DBP-FITC was not affected by IFN-I receptor blockade, a finding consistent with the known dependence of this response on the innate cytokine TSLP. Thus, the priming of Th2 responses is associated with distinct transcriptional signatures in DCs in vivo, reflecting the diverse environments in which Th2 immune responses are initiated.

Early local immune defences in the respiratory tract

The respiratory immune response consists of multiple tiers of cellular responses that are engaged in a sequential manner in order to control infections. The stepwise engagement of effector functions with progressively increasing host fitness costs limits tissue damage. In addition, specific mechanisms are in place to promote disease tolerance in response to respiratory infections. Environmental factors, obesity and the ageing process can alter the efficiency and regulation of this tiered response, increasing pathology and mortality as a result. In this Review, we describe the cell types that coordinate pathogen clearance and tissue repair through the serial secretion of cytokines, and discuss how the environment and comorbidity influence this response.