TGF-β-responsive myeloid cells suppress type 2 immunity and emphysematous pathology after hookworm infection

TGF-β signaling in health, disease, and therapeutics

Transforming growth factor (TGF)-β is a multifunctional cytokine expressed by almost every tissue and cell type. The signal transduction of TGF-β can stimulate diverse cellular responses and is particularly critical to embryonic development, wound healing, tissue homeostasis, and immune homeostasis in health. The dysfunction of TGF-β can play key roles in many diseases, and numerous targeted therapies have been developed to rectify its pathogenic activity. In the past decades, a large number of studies on TGF-β signaling have been carried out, covering a broad spectrum of topics in health, disease, and therapeutics. Thus, a comprehensive overview of TGF-β signaling is required for a general picture of the studies in this field. In this review, we retrace the research history of TGF-β and introduce the molecular mechanisms regarding its biosynthesis, activation, and signal transduction. We also provide deep insights into the functions of TGF-β signaling in physiological conditions as well as in pathological processes. TGF-β-targeting therapies which have brought fresh hope to the treatment of relevant diseases are highlighted. Through the summary of previous knowledge and recent updates, this review aims to provide a systematic understanding of TGF-β signaling and to attract more attention and interest to this research area.

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.

Tissue-specific immunity in helminth infections

A characteristic feature of host responses to helminth infections is the development of profound systemic and tissue-localised Type 2 immune responses that play critical roles in immunity, tissue repair and tolerance of the parasite at tissue sites. These same Type 2 responses are also seen in the tissue-associated immune-pathologies seen in asthma, atopic dermatitis and many forms of allergies. The recent identification of new subtypes of immune cells and cytokine pathways that influence both immune and non-immune cells and tissues creates the opportunity for reviewing helminth parasite-host responses in the context of tissue specific immunity. This review focuses on the new discoveries of the cells and cytokines involved in tissue specific immune responses to helminths and how these contribute to host immunity against helminth infection and allow the host to accommodate the presence of parasites when they cannot be eliminated.

The multi-faceted roles of TGF-β in regulation of immunity to infection

Transforming Growth Factor-β is a potent regulator of the immune system, acting at every stage from thymic differentiation, population of the periphery, control of responsiveness, tissue repair and generation of memory. It is therefore a central player in the immune response to infectious pathogens, but its contribution is often clouded by multiple roles acting on different cells in time and space. Hence, context is all-important in understanding when TGF-β is beneficial or detrimental to the outcome of infection. In this review, a full range of infectious agents from viruses to helminth parasites are explored within this framework, drawing contrasts and general conclusions about the importance of TGF-β in these diseases.

Regulatory cytokine function in the respiratory tract

The respiratory tract is an important site of immune regulation; required to allow protective immunity against pathogens, while minimizing tissue damage and avoiding aberrant inflammatory responses to inhaled allergens. Several cell types work in concert to control pulmonary immune responses and maintain tolerance in the respiratory tract, including regulatory and effector T cells, airway and interstitial macrophages, dendritic cells and the airway epithelium. The cytokines transforming growth factor β, interleukin (IL-) 10, IL-27, and IL-35 are key coordinators of immune regulation in tissues such as the lung. Here, we discuss the role of these cytokines during respiratory infection and allergic airway disease, highlighting the critical importance of cellular source and immunological context for the effects of these cytokines in vivo.

Antecedent Nippostrongylus infection alters the lung immune response to Plasmodium berghei

In endemic regions, it is not uncommon for patients to be co-infected with soil-transmitted helminths and malaria. Although both malaria and many helminth species use the lungs as a site of development, little attention has been paid to the impact that pulmonary immunity induced by one parasite has on the lung response to the other. To model the consequences of a prior hookworm exposure on the development of immunity to malaria in the lungs, mice were infected with Nippostrongylus brasiliensis and 2 weeks later challenged with Plasmodium berghei. We found that a pre-existing hookworm-induced type 2 immune environment had a measurable but modest impact on the nature of the malaria-driven type 1 cytokine response in the lungs that was associated with a transient effect on parasite development and no significant changes in morbidity and mortality after malaria infection. However, prior hookworm infection did have a lasting effect on lung macrophages, where the malaria-induced M1-like response was blunted by previous M2 polarization. These results demonstrate that, although helminth parasites confer robust changes to the immunological status of the pulmonary microenvironment, lung immunity is plastic and capable of rapidly adapting to consecutive heterologous infections.

Immune-mediated inflammation in the pathogenesis of emphysema: insights from mouse models

The cellular mechanisms that result in the initiation and progression of emphysema are clearly complex. A growing body of human data combined with discoveries from mouse models utilizing cigarette smoke exposure or protease administration have improved our understanding of emphysema development by implicating specific cell types that may be important for the pathophysiology of chronic obstructive pulmonary disease. The most important aspects of emphysematous damage appear to be oxidative or protease stress and sustained macrophage activation and infiltration of other immune cells leading to epithelial damage and cell death. Despite the identification of these associated processes and cell types in many experimental studies, the reasons why cigarette smoke and other pollutants result in unremitting damage instead of injury resolution are still uncertain. We propose an important role for macrophages in the sequence of events that lead and maintain this chronic tissue pathologic process in emphysema. This model involves chronic activation of macrophage subtypes that precludes proper healing of the lung. Further elucidation of the cross-talk between epithelial cells that release damage-associated signals and the cellular immune effectors that respond to these cues is a critical step in the development of novel therapeutics that can restore proper lung structure and function to those afflicted with emphysema.

A Trematode Parasite Derived Growth Factor Binds and Exerts Influences on Host Immune Functions via Host Cytokine Receptor Complexes

The trematode Fasciola hepatica is responsible for chronic zoonotic infection globally. Despite causing a potent T-helper 2 response, it is believed that potent immunomodulation is responsible for rendering this host reactive non-protective host response thereby allowing the parasite to remain long-lived. We have previously identified a growth factor, FhTLM, belonging to the TGF superfamily can have developmental effects on the parasite. Herein we demonstrate that FhTLM can exert influence over host immune functions in a host receptor specific fashion. FhTLM can bind to receptor members of the Transforming Growth Factor (TGF) superfamily, with a greater affinity for TGF-β RII. Upon ligation FhTLM initiates the Smad2/3 pathway resulting in phenotypic changes in both fibroblasts and macrophages. The formation of fibroblast CFUs is reduced when cells are cultured with FhTLM, as a result of TGF-β RI kinase activity. In parallel the wound closure response of fibroblasts is also delayed in the presence of FhTLM. When stimulated with FhTLM blood monocyte derived macrophages adopt an alternative or regulatory phenotype. They express high levels interleukin (IL)-10 and arginase-1 while displaying low levels of IL-12 and nitric oxide. Moreover they also undergo significant upregulation of the inhibitory receptor PD-L1 and the mannose receptor. Use of RNAi demonstrates that this effect is dependent on TGF-β RII and mRNA knock-down leads to a loss of IL-10 and PD-L1. Finally, we demonstrate that FhTLM aids newly excysted juveniles (NEJs) in their evasion of antibody-dependent cell cytotoxicity (ADCC) by reducing the NO response of macrophages—again dependent on TGF-β RI kinase. FhTLM displays restricted expression to the F. hepatica gut resident NEJ stages. The altered fibroblast responses would suggest a role for dampened tissue repair responses in facilitating parasite migration. Furthermore, the adoption of a regulatory macrophage phenotype would allow for a reduced effector response targeting juvenile parasites which we demonstrate extends to an abrogation of the ADCC response. Thus suggesting that FhTLM is a stage specific evasion molecule that utilises host cytokine receptors. These findings are the first to clearly demonstrate the interaction of a helminth cytokine with a host receptor complex resulting in immune modifications that facilitate the non-protective chronic immune response which is characteristic of F. hepatica infection.

Parasitic worms, helminths, can cause long-lived chronic infection in many hosts that they infection. The liver fluke, Fasciola hepatica, is one such parasite causing global infection of both humans and animals. F. hepatica exerts an influence over the immune system such that it avoids effector mechanisms and prevents the development of effective immunity. Here we characterise a molecule—FhTLM—derived from juvenile parasites that is similar to the regulatory cytokine TGF-β. We show that FhTLM will bind to host TGF-β receptors with a reduced affinity when compared with mammalian TGF-β. Despite this FhTLM can induce Smad2/3 signalling in host leukocytes, which is key to initiating gene transcription. Phenotypically FhTLM causes fibroblasts to slow their growth and replication response resulting in slower wound healing. Importantly FhTLM induces a macrophage phenotype that resembles a regulatory macrophage phenotype identified in other species undergoing helminth infection. Finally we Our work highlights the potential of FhTLM to play important roles in controlling host immunity when initially infected with juvenile parasites, thereby preventing the development of effective immunity.

TGF-β in tolerance, development and regulation of immunity

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The broader superfamily of TGF-β-like proteins is reviewed, and signaling pathways summarised.

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The role of TGF-β in the immune tolerance and control of infectious disease is discussed.

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The superfamily member AMH is involved in embryonic sexual differentiation.

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Helminth parasites appear to exploit the TGF-β pathway to suppress host immunity.

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TGF-β homologues and mimics from parasites offer a new route for therapeutic tolerance induction.

The TGF-β superfamily is an ancient metazoan protein class which cuts across cell and tissue differentiation, developmental biology and immunology. Its many members are regulated at multiple levels from intricate control of gene transcription, post-translational processing and activation, and signaling through overlapping receptor structures and downstream intracellular messengers. We have been interested in TGF-β homologues firstly as key players in the induction of immunological tolerance, the topic so closely associated with Ray Owen. Secondly, our interests in how parasites may manipulate the immune system of their host has also brought us to study the TGF-β pathway in infections with longlived, essentially tolerogenic, helminth parasites. Finally, within the spectrum of mammalian TGF-β proteins is an exquisitely tightly-regulated gene, anti-Müllerian hormone (AMH), whose role in sex determination underpins the phenotype of freemartin calves that formed the focus of Ray’s seminal work on immunological tolerance.

Myeloid expression of the AP-1 transcription factor JUNB modulates outcomes of type 1 and type 2 parasitic infections

Activation of macrophages is a key step in the initiation of immune responses, but the transcriptional mechanisms governing macrophage activation during infection are not fully understood. It was recently shown that the AP-1 family transcription factor JUNB positively regulates macrophage activation in response to Toll-like receptor agonists that promote classical or M1 polarization, as well as to the cytokine interleukin-4 (IL-4), which elicits an alternatively activated or M2 phenotype. However, a role for JUNB in macrophage activation has never been demonstrated in vivo. Here, to dissect the role of JUNB in macrophage activation in a physiological setting, mice lacking JUNB specifically in myeloid cells were tested in two infection models: experimental cerebral malaria, which elicits a pathological type 1 immune response, and helminth infection, in which type 2 responses are protective. Myeloid-restricted deletion of Junb reduced type 1 immune activation, which was associated with reduced cerebral pathology and improved survival during infection with Plasmodium berghei. Myeloid JUNB deficiency also compromised type 2 activation during infection with the hookworm Nippostrongylus brasiliensis, leading to diminished cytokine production and eosinophil recruitment and increased parasite burden. These results demonstrate that JUNB in myeloid cells shapes host responses and outcomes during type 1 and type 2 infections.

The Dectin-2 family of C-type lectin-like receptors: an update

New discoveries reveal crucial roles for the Dectin-2 family in many aspects of the immune response.

Myeloid and non-myeloid cells express members of the C-type lectin-like receptor (CTLR) family, which mediate crucial cellular functions during immunity and homeostasis. Of relevance here is the dendritic cell-associated C-type lectin-2 (Dectin-2) family of CTLRs, which includes blood dendritic cell antigen 2 (BDCA-2), dendritic cell immunoactivating receptor (DCAR), dendritic cell immunoreceptor (DCIR), Dectin-2, C-type lectin superfamily 8 (CLECSF8) and macrophage-inducible C-type lectin (Mincle). These CTLRs possess a single extracellular conserved C-type lectin-like domain and are capable of mediating intracellular signalling either directly, through integral signalling domains, or indirectly, by associating with signalling adaptor molecules. These receptors recognize a diverse range of endogenous and exogenous ligands, and can function as pattern recognition receptors for several classes of pathogens including fungi, bacteria and parasites, driving both innate and adaptive immunity. In this review, we summarize our knowledge of each of these receptors, highlighting the exciting discoveries that have been made in recent years.

Systemic impact of intestinal helminth infections

In this review, we examine the evidence that intestinal helminths can control harmful inflammatory responses and promote homeostasis by triggering systemic immune responses. Induction of separable components of immunity by helminths, which includes type 2 and immune regulatory responses, can both contribute toward the reduction in harmful type 1 immune responses that drive certain inflammatory diseases. Despite inducing type 2 responses, intestinal helminths may also downregulate harmful type 2 immune responses including allergic responses. We consider the possibility that intestinal helminth infection may indirectly affect inflammation by influencing the composition of the intestinal microbiome. Taken together, the studies reviewed herein suggest that intestinal helminth-induced responses have potent systemic effects on the immune system, raising the possibility that whole parasites or specific molecules produced by these metazoans may be an important resource for the development of future immunotherapies to control inflammatory diseases.

IL-9-mediated survival of type 2 innate lymphoid cells promotes damage control in helminth-induced lung inflammation

IL-9 acts as an autocrine amplifier of type 2 innate lymphoid cell function to promote tissue repair in the recovery phase of helminth-induced lung infection.

IL-9 fate reporter mice established type 2 innate lymphoid cells (ILC2s) as major producers of this cytokine in vivo. Here we focus on the role of IL-9 and ILC2s during the lung stage of infection with Nippostrongylus brasiliensis, which results in substantial tissue damage. IL-9 receptor (IL-9R)–deficient mice displayed reduced numbers of ILC2s in the lung after infection, resulting in impaired IL-5, IL-13, and amphiregulin levels, despite undiminished numbers of Th2 cells. As a consequence, the restoration of tissue integrity and lung function was strongly impaired in the absence of IL-9 signaling. ILC2s, in contrast to Th2 cells, expressed high levels of the IL-9R, and IL-9 signaling was crucial for the survival of activated ILC2s in vitro. Furthermore, ILC2s in the lungs of infected mice required the IL-9R to up-regulate the antiapoptotic protein BCL-3 in vivo. This highlights a unique role for IL-9 as an autocrine amplifier of ILC2 function, promoting tissue repair in the recovery phase after helminth-induced lung inflammation.

Loss of the TGFβ-activating integrin αvβ8 on dendritic cells protects mice from chronic intestinal parasitic infection via control of type 2 immunity

Chronic intestinal parasite infection is a major global health problem, but mechanisms that promote chronicity are poorly understood. Here we describe a novel cellular and molecular pathway involved in the development of chronic intestinal parasite infection. We show that, early during development of chronic infection with the murine intestinal parasite Trichuris muris, TGFβ signalling in CD4+ T-cells is induced and that antibody-mediated inhibition of TGFβ function results in protection from infection. Mechanistically, we find that enhanced TGFβ signalling in CD4+ T-cells during infection involves expression of the TGFβ-activating integrin αvβ8 by dendritic cells (DCs), which we have previously shown is highly expressed by a subset of DCs in the intestine. Importantly, mice lacking integrin αvβ8 on DCs were completely resistant to chronic infection with T. muris, indicating an important functional role for integrin αvβ8-mediated TGFβ activation in promoting chronic infection. Protection from infection was dependent on CD4+ T-cells, but appeared independent of Foxp3+ Tregs. Instead, mice lacking integrin αvβ8 expression on DCs displayed an early increase in production of the protective type 2 cytokine IL-13 by CD4+ T-cells, and inhibition of this increase by crossing mice to IL-4 knockout mice restored parasite infection. Our results therefore provide novel insights into how type 2 immunity is controlled in the intestine, and may help contribute to development of new therapies aimed at promoting expulsion of gut helminths.

Infection with intestinal parasitic worms is a major global health problem, with billions of people infected world-wide. Often these worms (known as helminths) develop a long-lasting chronic infection, due to failure of the host to mount the correct type of immune response that would normally expel the parasite. However, how the immune system is controlled leading to chronic helminth infection is not well understood. Here we identify a novel pathway of importance in the development of chronic helminth infection. Using a model parasite which infects mice, we find that a protein called transforming growth factor beta (TGFβ signals to T-cells early during the development of chronic infection and that blocking this signal protects mice from infection. We have also uncovered a key pathway and cell type that controls TGFβ function during development of chronic infection. When a protein called integrin αvβ8 is absent from dendritic cells of the immune system, TGFβ is no longer activated to signal to T-cells and mice are able to mount a protective (type 2) immune response resulting in worm expulsion. Our findings therefore provide new insights into how chronic infections develop and identify potential molecular targets for the prevention of chronic helminth infection.