Cutting edge: inhaled antigen upregulates retinaldehyde dehydrogenase in lung CD103+ but not plasmacytoid dendritic cells to induce Foxp3 de novo in CD4+ T cells and promote airway tolerance

Leveraging current insights on IL-10-producing dendritic cells for developing effective immunotherapeutic approaches

Dendritic cells (DC) are professional antigen-presenting cells involved in promoting and controlling immune responses. Different subsets of DC, named tolerogenic (tol)DC, play a critical role in the maintenance of tissue homeostasis and in fostering tolerance. These unique skills make tolDC especially attractive for strategies aimed at re-establishing/inducing tolerance in immune-mediated conditions. The generation of potent tolDC in vitro from peripheral blood monocytes has seen remarkable advancements. TolDC modulate T cell dynamics by favoring regulatory T cells (Tregs) and curbing effector/pathogenic T cells. Among the several methods developed for in vitro tolDC generation, IL-10 conditioning has been proven to be the most efficient, as IL-10-modulated tolDC were demonstrated to promote Tregs with the strongest suppressive activities. Investigating the molecular, metabolic, and functional profiles of tolDC uncovers essential pathways that facilitate their immunoregulatory functions. This Review provides an overview of current knowledge on the role of tolDC in health and disease, focusing on IL-10 production, functional characterization of in vitro generated tolDC, molecular and metabolic changes occurring in tolDC induced by tolerogenic agents, clinical applications of tolDC-based therapy, and finally new perspectives in the generation of effective tolDC.

CD11c+ dendritic cells PlexinD1 deficiency exacerbates airway hyperresponsiveness, IgE and mucus production in a mouse model of allergic asthma

Dendritic cells (DCs) are pivotal in regulating allergic asthma. Our research has shown that the absence of Sema3E worsens asthma symptoms in acute and chronic asthma models. However, the specific role of PlexinD1 in these processes, particularly in DCs, remains unclear. This study investigates the role of PlexinD1 in CD11c+ DCs using a house dust mite (HDM) model of asthma. We generated CD11c+ DC-specific PlexinD1 knockout (CD11cPLXND1 KO) mice and subjected them, alongside wild-type controls (PLXND1fl/fl), to an HDM allergen protocol. Airway hyperresponsiveness (AHR) was measured using FlexiVent, and immune cell populations were analyzed via flow cytometry. Cytokine levels and immunoglobulin concentrations were assessed using mesoscale and ELISA, while collagen deposition and mucus production were examined through Sirius-red and periodic acid Schiff (PAS) staining respectively. Our results indicate that CD11cPLXND1 KO mice exhibit significantly exacerbated AHR, characterized by increased airway resistance and tissue elastance. Enhanced mucus production and collagen gene expression were observed in these mice compared to wild-type counterparts. Flow cytometry revealed higher CD11c+ MHCIIhigh CD11b+ cell recruitment into the lungs, and elevated total and HDM-specific serum IgE levels in CD11cPLXND1 KO mice. Mechanistically, co-cultures of B cells with DCs from CD11cPLXND1 KO mice showed significantly increased IgE production compared to wild-type mice.These findings highlight the critical regulatory role of the plexinD1 signaling pathway in CD11c+ DCs in modulating asthma features.

Lung Interstitial Macrophages Can Present Soluble Antigens and Induce Foxp3+ Regulatory T Cells

Lung macrophages constitute a sophisticated surveillance and defense system that contributes to tissue homeostasis and host defense and allows the host to cope with the myriad of insults and antigens to which the lung mucosa is exposed. As opposed to alveolar macrophages, lung interstitial macrophages (IMs) express high levels of Type 2 major histocompatibility complex (MHC-II), a hallmark of antigen-presenting cells. Here, we showed that lung IMs, like dendritic cells, possess the machinery to present soluble antigens in an MHC-II-restricted way. Using ex vivo ovalbumin (OVA)-specific T cell proliferation assays, we found that OVA-pulsed IMs could trigger OVA-specific CD4+ T cell proliferation and Foxp3 expression through MHC-II-, IL-10-, and transforming growth factor β-dependent mechanisms. Moreover, we showed that IMs efficiently captured locally instilled antigens in vivo, did not migrate to the draining lymph nodes, and enhanced local interactions with CD4+ T cells in a model of OVA-induced allergic asthma. These results support that IMs can present antigens to CD4+ T cells and trigger regulatory T cells, which might attenuate lung immune responses and have functional consequences for lung immunity and T cell-mediated disorders.

Tissue-specific antigen-presenting cells contribute to distinct phenotypes of allergy

Antigen-presenting cells (APCs) are critical cells bridging innate and adaptive immune responses by taking up, processing, and presenting antigens to naïve T cells. At steady state, APCs thus control both tissue homeostasis and the induction of tolerance. In allergies however, APCs drive a Th2-biased immune response that is directed against otherwise harmless antigens from the environment. The main types of APCs involved in the induction of allergy are dendritic cells, monocytes, and macrophages. However, these cell types can be further divided into local, tissue-specific populations that differ in their phenotype, migratory capacity, T-cell activating potential, and production of effector molecules. Understanding if distinct populations of APCs contribute to either tissue-specific immune tolerance, allergen sensitization, or allergic inflammation will allow us to better understand disease pathology and develop targeted treatment options for different stages of allergic disease. Therefore, this review describes the main characteristics, phenotypes, and effector molecules of the APCs involved in the induction of allergen-specific Th2 responses in affected barrier sites, such as the skin, nose, lung, and gastrointestinal tract. Furthermore, we highlight open questions that remain to be addressed to fully understand the contribution of different APCs to allergic disease.

Regulatory ILC2-Role of IL-10 Producing ILC2 in Asthma

Over the past two decades, a growing body of evidence observations have shown group two innate lymphoid cells (ILC2) to be critical drivers of Type 2 (T2) inflammatory responses associated with allergic inflammatory conditions such as asthma. ILC2 releases copious amounts of pro-inflammatory T2 cytokines-interleukin (IL)-4, IL-5, IL-9, and IL-13. This review provides a comprehensive overview of the newly discovered regulatory subtype of ILC2 described in murine and human mucosal tissue and blood. These KLRG1+ILC2 have the capacity to produce the anti-inflammatory cytokine IL-10. Papers compiled in this review were based on queries of PubMed and Google Scholar for articles published from 2000 to 2023 using keywords "IL-10" and "ILC2". Studies with topical relevance to IL-10 production by ILC2 were included. ILC2 responds to microenvironmental cues, including retinoic acid (RA), IL-2, IL-4, IL-10, and IL-33, as well as neuropeptide mediators such as neuromedin-U (NMU), prompting a shift towards IL-10 and away from T2 cytokine production. In contrast, TGF-β attenuates IL-10 production by ILC2. Immune regulation provided by IL-10+ILC2s holds potential significance for the management of T2 inflammatory conditions. The observation of context-specific cues that alter the phenotype of ILC warrants examining characteristics of ILC subsets to determine the extent of plasticity or whether the current classification of ILCs requires refinement.

The functional adaptation of effector Foxp3+ regulatory T cells to pulmonary inflammation

During infections, the timings of effector differentiation of pulmonary immune responses are of paramount importance, as pathogen persistence and unsuppressed inflammation can rapidly lead to a loss of function, increased frailty, and death. Thus, both an efficient clearance of the danger and a rapid resolution of inflammation are critical to host survival. We now know that tissue-localized FoxP3+ regulatory T cells, a subset of CD4+ T cells, are highly attuned to the type of immune response, acquiring unique phenotypic characteristics that allow them to adapt their suppressive functions with the nature of inflammatory cells. To achieve this, activated effector TREG cells acquire specialized TH 1, TH 2, and TH 17-like characteristics that allow them to migrate, survive, and time their function(s) through refined mechanisms. Herein, we describe how this process requires a unique developmental path that includes the acquisition of master transcription factors and the expression of receptors adapted to sense local danger signals that are found during pulmonary inflammation. In turn, we offer an overview of how these characteristics promote the capacity of local effector TREG cells to proliferate, survive, and display suppressive strategies to resolve lung injury.

Catching Our Breath: Updates on the Role of Dendritic Cell Subsets in Asthma

Dendritic cells (DCs), as potent antigen presenting cells, are known to play a central role in the pathophysiology of asthma. The understanding of DC biology has evolved over the years to include multiple subsets of DCs with distinct functions in the initiation and maintenance of asthma. Furthermore, asthma is increasingly recognized as a heterogeneous disease with potentially diverse underlying mechanisms. The goal of this review is to summarize the role of DCs and the various subsets therein in the pathophysiology of asthma and highlight some of the crucial animal models shaping the field today. Potential future avenues of investigation to address existing gaps in knowledge are discussed.

Donor Batf3 inhibits murine lung allograft rejection and airway fibrosis

Chronic lung allograft dysfunction (CLAD) limits survival after lung transplantation. Noxious stimuli entering the airways foster CLAD development. Classical dendritic cells (cDCs) link innate and adaptive immunity and exhibit regional and functional specialization in the lung. The transcription factor basic leucine zipper ATF-like 3 (BATF3) is absolutely required for the development of type 1 cDCs (cDC1s), which reside in the airway epithelium and have variable responses depending on the context. We studied the role of BATF3 in a mouse minor alloantigen-mismatched orthotopic lung transplant model of CLAD with and without airway inflammation triggered by repeated administration of intratracheal lipopolysaccharide (LPS). We found that cDC1s accumulated in allografts compared with isografts and that donor cDC1s were gradually replaced by recipient cDC1s. LPS administration increased the number of cDC1s and enhanced their state of activation. We found that Batf3^-/- recipient mice experienced reduced acute rejection in response to LPS; in contrast, Batf3^-/- donor grafts underwent enhanced lung and skin allograft rejection and drove augmented recipient cluster of differentiation 8⁺ T-cell expansion in the absence of LPS. Our findings suggest that donor and recipient cDC1s have differing and context-dependent roles and may represent a therapeutic target in lung transplantation.

Targeting the Semaphorin3E-plexinD1 complex in allergic asthma

Asthma is a heterogenous airway disease characterized by airway inflammation and remodeling. It affects more than 300 million people worldwide and poses a significant burden on society. Semaphorins, discovered initially as neural guidance molecules, are ubiquitously expressed in various organs and regulate multiple signaling pathways. Interestingly, Semaphorin3E is a critical molecule in lung pathophysiology through its role in both lung development and homeostasis. Semaphorin3E binds to plexinD1, mediating regulatory effects on cell migration, proliferation, and angiogenesis. Recent in vitro and in vivo studies have demonstrated that the Semaphorin3E-plexinD1 axis is implicated in asthma, impacting inflammatory and structural cells associated with airway inflammation, tissue remodeling, and airway hyperresponsiveness. This review details the Semaphorin3E-plexinD1 axis in various aspects of asthma and highlights future directions in research including its potential role as a therapeutic target in airway allergic diseases.

The innate immune brakes of the lung

Respiratory mucosal surfaces are continuously exposed to not only innocuous non-self antigens but also pathogen-associated molecular patterns (PAMPs) originating from environmental or symbiotic microbes. According to either "self/non-self" or "danger" models, this should systematically result in homeostasis breakdown and the development of immune responses directed to inhaled harmless antigens, such as T helper type (Th)2-mediated asthmatic reactions, which is fortunately not the case in most people. This discrepancy implies the existence, in the lung, of regulatory mechanisms that tightly control immune homeostasis. Although such mechanisms have been poorly investigated in comparison to the ones that trigger immune responses, a better understanding of them could be useful in the development of new therapeutic strategies against lung diseases (e.g., asthma). Here, we review current knowledge on innate immune cells that prevent the development of aberrant immune responses in the lung, thereby contributing to mucosal homeostasis.

IRF4 expression by lung dendritic cells drives acute but not Trm cell-dependent memory Th2 responses

Expression of the transcription factor interferon regulatory factor 4 (IRF4) is required for the development of lung conventional DCs type 2 (cDC2s) that elicit Th2 responses, yet how IRF4 functions in lung cDC2s throughout the acute and memory allergic response is not clear. Here, we used a mouse model that loses IRF4 expression after lung cDC2 development to demonstrate that mice with IRF4-deficient DCs display impaired memory responses to allergen. This defect in the memory response was a direct result of ineffective Th2 induction and impaired recruitment of activated effector T cells to the lung after sensitization. IRF4-deficient DCs demonstrated defects in their migration to the draining lymph node and in T cell priming. Finally, T cells primed by IRF4-competent DCs mediated potent memory responses independently of IRF4-expressing DCs, demonstrating that IRF4-expressing DCs are not necessary during the memory response. Thus, IRF4 controlled a program in mature DCs governing Th2 priming and effector responses, but IRF4-expressing DCs were dispensable during tissue-resident memory T cell-dependent memory responses.

Mouse phospholipid phosphatase 6 regulates dendritic cell cholesterol, macropinocytosis, and allergen sensitization

Lipid phosphate phosphatases are a family of enzymes with diverse cellular metabolic functions. Phospholipid phosphatase 6 (PLPP6) is a regulator of cellular polyisoprenyl phosphates; however, its in vivo functions remain to be determined. Here, mouse PLPP6 was characterized to possess similar catalytic properties as the human enzyme. Plpp6 knockout mice (Plpp6 ^-/- ) were generated and displayed decreased airway allergen sensitization, pointing to a role for PLPP6 in the early events of lung allergic responses. Dendritic cell (DC) responses were investigated and endocytosis of allergen via macropinocytosis was decreased in Plpp6 ^-/- DCs that had lower cholesterol content. When reversed by cholesterol loading, the DC macropinocytosis defect is corrected. Adoptive transfer of Plpp6 ^-/- DCs to wild-type mice during sensitization was sufficient to decrease allergen-induced responses. Together, our findings have identified PLPP6 as a pivotal regulator of DC cholesterol content and macropinocytosis, cellular mechanisms that are important for pathologic responses in allergen-induced lung inflammation.

Pulmonary Conventional Type 1 Langerin-Expressing Dendritic Cells Play a Role in Impairing Early Protective Immune Response against Cryptococcus neoformans Infection in Mice

Lung dendritic cells (DC) are powerful antigen-presenting cells constituted by various subpopulations that differ in terms of their function and origin and differentially regulate cell-mediated antifungal immunity. The lung is the primary target organ of Cryptococcus neoformans and C. gattii infections, which makes it essential in the establishment of the first line of anti-cryptococcal defense. However, the lung-specific dynamics and function of DC subsets are poorly understood in cryptococcosis. In this study, we provide evidence for the in vivo function of a conventional langerin-expressing DC1 dendritic cell (LangDC1) population during the first week of intratracheal C. neoformans infection in mice. By using conditional depletion of LangDC1 after diphtheria toxin treatment of LangDTREGFP mice, we demonstrate that these animals better control the fungal infection and produce type 1 and 17 cytokines in the context of a type 2 immune response, favoring a predominance of iNOS over arginase-1 expression by pulmonary cells. Our results suggest that LangDC1 cells play a role in impairing immune response for the clearance of C. neoformans in the early stage of pulmonary infection.

Antigen Presentation in the Lung

The lungs are constantly exposed to environmental and infectious agents such as dust, viruses, fungi, and bacteria that invade the lungs upon breathing. The lungs are equipped with an immune defense mechanism that involves a wide variety of immunological cells to eliminate these agents. Various types of dendritic cells (DCs) and macrophages (MACs) function as professional antigen-presenting cells (APCs) that engulf pathogens through endocytosis or phagocytosis and degrade proteins derived from them into peptide fragments. During this process, DCs and MACs present the peptides on their major histocompatibility complex class I (MHC-I) or MHC-II protein complex to naïve CD8⁺ or CD4⁺ T cells, respectively. In addition to these cells, recent evidence supports that antigen-specific effector and memory T cells are activated by other lung cells such as endothelial cells, epithelial cells, and monocytes through antigen presentation. In this review, we summarize the molecular mechanisms of antigen presentation by APCs in the lungs and their contribution to immune response.

Nanoparticles for Inducing Antigen-Specific T Cell Tolerance in Autoimmune Diseases

Autoimmune diseases affect many people worldwide. Current treatment modalities focus on the reduction of disease symptoms using anti-inflammatory drugs which can lead to side effects due to systemic immune suppression. Restoration of immune tolerance by down-regulating auto-reactive cells in an antigen-specific manner is currently the “holy grail” for the treatment of autoimmune diseases. A promising strategy is the use of nanoparticles that can deliver antigens to antigen-presenting cells which in turn can enhance antigen-specific regulatory T cells. In this review, we highlight some promising cell targets (e.g. liver sinusoidal endothelial cells and splenic marginal zone macrophages) for exploiting natural immune tolerance processes, and several strategies by which antigen-carrying nanoparticles can target these cells. We also discuss how nanoparticles carrying immunomodulators may be able to activate tolerance in other antigen-presenting cell types. Finally, we discuss some important aspects that must be taken into account when translating data from animal studies to patients.

Airway administration of OM-85, a bacterial lysate, blocks experimental asthma by targeting dendritic cells and the epithelium/IL-33/ILC2 axis

BACKGROUND

Microbial interventions against allergic asthma have robust epidemiologic underpinnings and the potential to recalibrate disease-inducing immune responses. Oral administration of OM-85, a standardized lysate of human airways bacteria, is widely used empirically to prevent respiratory infections and a clinical trial is testing its ability to prevent asthma in high-risk children. We previously showed that intranasal administration of microbial products from farm environments abrogates experimental allergic asthma.

OBJECTIVES

We sought to investigate whether direct administration of OM-85 to the airway compartment protects against experimental allergic asthma; and to identify protective cellular and molecular mechanisms activated through this natural route.

METHODS

Different strains of mice sensitized and challenged with ovalbumin or Alternaria received OM-85 intranasally, and cardinal cellular and molecular asthma phenotypes were measured. Airway transfer experiments assessed whether OM-85-treated dendritic cells protect allergen-sensitized, OM-85-naive mice against asthma.

RESULTS

Airway OM-85 administration suppressed allergic asthma in all models acting on multiple innate and adaptive immune targets: the airway epithelium/IL-33/ILC2 axis, lung allergen-induced type 2 responses, and dendritic cells whose Myd88/Trif-dependent tolerogenic reprogramming was sufficient to transfer OM-85-induced asthma protection.

CONCLUSIONS

We provide the first demonstration that administering a standardized bacterial lysate to the airway compartment protects from experimental allergic asthma by engaging multiple immune pathways. Because protection required a cumulative dose 27- to 46-fold lower than the one reportedly active through the oral route, the efficacy of intranasal OM-85 administration may reflect its direct access to the airway mucosal networks controlling the initiation and development of allergic asthma.

Meteorin-β/Meteorin like/IL-41 attenuates airway inflammation in house dust mite-induced allergic asthma

We sought to examine the regulatory effect of Meteorin-β (Metrnβ)/Meteorin like (Metrnl)/IL-41 on lung inflammation in allergic asthma. We found that Metrnβ was elevated significantly in asthmatic patients and in mice with allergic asthma induced by house dust mite (HDM) extract. Upon exposure to HDM, Metrnβ was secreted predominantly by airway epithelial cells and inflammatory cells, including macrophages and eosinophils. The increased Metrnβ effectively blocked the development of airway hyperreactivity (AHR) and decreased inflammatory cell airway infiltration and type 2 cytokine production, which was associated with downregulated DC-mediated adaptive immune responses. Moreover, Metrnβ impaired the maturation and function of bone marrow-derived dendritic cells in vitro. Asthmatic mice adoptively transferred with dendritic cells isolated from Metrnβ-treated allergic mice displayed decreased AHR, airway inflammation, and lung injury. Metrnβ also displayed anti-inflammatory properties in immunodeficient SCID mice with allergic asthma and in in vitro 3D ALI airway models. Moreover, blockade of Metrnβ by anti-Metrnβ antibody treatment promoted the development of allergic asthma. These results revealed the unappreciated protective roles of Metrnβ in alleviating DC-mediated Th2 inflammation in allergic asthma, providing the novel treatment strategy of therapeutic targeting of Metrnβ in allergic asthma.

Tissue-resident immunity in the lung: a first-line defense at the environmental interface

The lung is a vital organ that incessantly faces external environmental challenges. Its homeostasis and unimpeded vital function are ensured by the respiratory epithelium working hand in hand with an intricate fine-tuned tissue-resident immune cell network. Lung tissue-resident immune cells span across the innate and adaptive immunity and protect from infectious agents but can also prove to be pathogenic if dysregulated. Here, we review the innate and adaptive immune cell subtypes comprising lung-resident immunity and discuss their ontogeny and role in distinct respiratory diseases. An improved understanding of the role of lung-resident immunity and how its function is dysregulated under pathological conditions can shed light on the pathogenesis of respiratory diseases.

Cellular and molecular mechanisms of allergic asthma

Asthma is a chronic disease of the airways, which affects more than 350 million people worldwide. It is the most common chronic disease in children, affecting at least 30 million children and young adults in Europe. Asthma is a complex, partially heritable disease with a marked heterogeneity. Its development is influenced both by genetic and environmental factors. The most common, as well as the most well characterized subtype of asthma is allergic eosinophilic asthma, which is characterized by a type 2 airway inflammation. The prevalence of asthma has substantially increased in industrialized countries during the last 60 years. The mechanisms underpinning this phenomenon are incompletely understood, however increased exposure to various environmental pollutants probably plays a role. Disease inception is thought to be enabled by a disadvantageous shift in the balance between protective and harmful lifestyle and environmental factors, including exposure to protective commensal microbes versus infection with pathogens, collectively leading to airway epithelial cell damage and disrupted barrier integrity. Epithelial cell-derived cytokines are one of the main drivers of the type 2 immune response against innocuous allergens, ultimately leading to infiltration of lung tissue with type 2 T helper (T_H2) cells, type 2 innate lymphoid cells (ILC2s), M2 macrophages and eosinophils. This review outlines the mechanisms responsible for the orchestration of type 2 inflammation and summarizes the novel findings, including but not limited to dysregulated epithelial barrier integrity, alarmin release and innate lymphoid cell stimulation.

Semaphorin3E/plexinD1 Axis in Asthma: What We Know So Far!

Semaphorin3E belongs to the large family of semaphorin proteins. Semaphorin3E was initially identified as axon guidance cues in the neural system. It is universally expressed beyond the nervous system and contributes to regulating essential cell functions such as cell migration, proliferation, and adhesion. Binding of semaphorin3E to its receptor, plexinD1, triggers diverse signaling pathways involved in the pathogenesis of various diseases from cancer to autoimmune and allergic disorders. Here, we highlight the novel findings on the role of semaphorin3E in airway biology. In particular, we highlight our recent findings on the function and potential mechanisms by which semaphorin3E and its receptor, plexinD1, impact airway inflammation, airway hyperresponsiveness, and remodeling in the context of asthma.

Dendritic Cell Regulation of T Helper Cells

As the professional antigen-presenting cells of the immune system, dendritic cells (DCs) sense the microenvironment and shape the ensuing adaptive immune response. DCs can induce both immune activation and immune tolerance according to the peripheral cues. Recent work has established that DCs comprise several phenotypically and functionally heterogeneous subsets that differentially regulate T lymphocyte differentiation. This review summarizes both mouse and human DC subset phenotypes, development, diversification, and function. We focus on advances in our understanding of how different DC subsets regulate distinct CD4⁺ T helper (Th) cell differentiation outcomes, including Th1, Th2, Th17, T follicular helper, and T regulatory cells. We review DC subset intrinsic properties, local tissue microenvironments, and other immune cells that together determine Th cell differentiation during homeostasis and inflammation.

Modern View of Neutrophilic Asthma Molecular Mechanisms and Therapy

For a long time asthma was commonly considered as a homogeneous disease. However, recent studies provide increasing evidence of its heterogeneity and existence of different phenotypes of the disease. Currently, classification of asthma into several phenotypes is based on clinical and physiological features, anamnesis, and response to therapy. This review describes five most frequently identified asthma phenotypes. Neutrophilic asthma (NA) deserves special attention, since neutrophilic inflammation of the lungs is closely associated with severity of the disease and with the resistance to conventional corticosteroid therapy. This review focuses on molecular mechanisms of neutrophilic asthma pathogenesis and on the role of Th1- and Th17-cells in the development of this type of asthma. In addition, this review presents current knowledge of neutrophil biology. It has been established that human neutrophils are represented by at least three subpopulations with different biological functions. Therefore, total elimination of neutrophils from the lungs can result in negative consequences. Based on the new knowledge of NA pathogenesis and biology of neutrophils, the review summarizes current approaches for treatment of neutrophilic asthma and suggests new promising ways to treat this type of asthma that could be developed in future.

Fms-Like Tyrosine Kinase 3-Independent Dendritic Cells Are Major Mediators of Th2 Immune Responses in Allergen-Induced Asthmatic Mice

Dendritic cells (DCs) are the main mediators of Th2 immune responses in allergic asthma, and Fms-like tyrosine kinase 3 ligand (Flt3L) is an important growth factor for the development and homeostasis of DCs. This study identified the DC populations that primarily cause the initiation and development of allergic lung inflammation using Fms-like tyrosine kinase 3 (Flt3) knockout (KO) mice with allergen-induced allergic asthma. We observed type 2 allergic lung inflammation with goblet cell hyperplasia in Flt3 KO mice, despite a significant reduction in total DCs, particularly CD103⁺ DCs, which was barely detected. In addition, bone marrow-derived dendritic cells (BMDCs) from Flt3 KO mice directed Th2 immune responses in vitro, and the adoptive transfer of these BMDCs exacerbated allergic asthma with more marked Th2 responses than that of BMDCs from wild-type (WT) mice. Furthermore, we found that Flt3L regulated the in vitro expression of OX40 ligand (OX40L) in DCs, which is correlated with DC phenotype in in vivo models. In conclusion, we revealed that Flt3-independent CD11b⁺ DCs direct Th2 responses with the elevated OX40L and are the primary cause of allergic asthma. Our findings suggest that Flt3 is required to control type 2 allergic inflammation.

Dendritic Cells: Critical Regulators of Allergic Asthma

Allergic asthma is a chronic inflammatory disease of the airways characterized by airway hyperresponsiveness (AHR), chronic airway inflammation, and excessive T helper (Th) type 2 immune responses against harmless airborne allergens. Dendritic cells (DCs) represent the most potent antigen-presenting cells of the immune system that act as a bridge between innate and adaptive immunity. Pertinent to allergic asthma, distinct DC subsets are known to play a central role in initiating and maintaining allergen driven Th2 immune responses in the airways. Nevertheless, seminal studies have demonstrated that DCs can also restrain excessive asthmatic responses and thus contribute to the resolution of allergic airway inflammation and the maintenance of pulmonary tolerance. Notably, the transfer of tolerogenic DCs in vivo suppresses Th2 allergic responses and protects or even reverses established allergic airway inflammation. Thus, the identification of novel DC subsets that possess immunoregulatory properties and can efficiently control aberrant asthmatic responses is critical for the re-establishment of tolerance and the amelioration of the asthmatic disease phenotype.

Tnfaip3 expression in pulmonary conventional type 1 Langerin-expressing dendritic cells regulates T helper 2-mediated airway inflammation in mice

Background

Conventional type 1 dendritic cells (cDC1s) control anti‐viral and anti‐tumor immunity by inducing antigen‐specific cytotoxic CD8⁺ T‐cell responses. Controversy exists whether cDC1s also control CD4⁺ T helper 2 (Th2) cell responses, since suppressive and activating roles have been reported. DC activation status, controlled by the transcription factor NF‐κB, might determine the precise outcome of Th‐cell differentiation upon encounter with cDC1s. To investigate the role of activated cDC1s in Th2‐driven immune responses, pulmonary cDC1s were activated by targeted deletion of A20/Tnfaip3, a negative regulator of NF‐κB signaling.

Methods

To target pulmonary cDC1s, Cd207 (Langerin)‐mediated excision of A20/Tnfaip3 was used, generating Tnfaip3^fl/flxCd207^+/cre (Tnfaip3^Lg‐KO) mice. Mice were exposed to house dust mite (HDM) to provoke Th2‐mediated immune responses.

Results

Mice harboring Tnfaip3‐deficient cDC1s did not develop Th2‐driven eosinophilic airway inflammation upon HDM exposure, but rather showed elevated numbers of IFNγ‐expressing CD8⁺ T cells. In addition, Tnfaip3^Lg‐KO mice harbored increased numbers of IL‐12–expressing cDC1s and elevated PD‐L1 expression in all pulmonary DC subsets. Blocking either IL‐12 or IFNγ in Tnfaip3^Lg‐KO mice restored Th2 responses, whereas administration of recombinant IFNγ during HDM sensitization in C57Bl/6 mice blocked Th2 development.

Conclusions

These findings indicate that the activation status of cDC1s, shown by their specific expression of co‐inhibitory molecules and cytokines, critically contributes to the development of Th2 cell–mediated disorders, most likely by influencing IFNγ production in CD8⁺ T cells.

Dendritic Cells and Microglia Have Non-redundant Functions in the Inflamed Brain with Protective Effects of Type 1 cDCs

Brain CD11c⁺ cells share features with microglia and dendritic cells (DCs). Sterile inflammation increases brain CD11c⁺ cells, but their phenotype, origin, and functions remain largely unknown. We report that, after cerebral ischemia, microglia attract DCs to the inflamed brain, and astroglia produce Flt3 ligand, supporting development and expansion of CD11c⁺ cells. CD11c⁺ cells in the inflamed brain are a complex population derived from proliferating microglia and infiltrating DCs, including a major subset of OX40L⁺ conventional cDC2, and also cDC1, plasmacytoid, and monocyte-derived DCs. Despite sharing certain morphological features and markers, CD11c⁺ microglia and DCs display differential expression of pattern recognition receptors and chemokine receptors. DCs excel CD11c^- and CD11c⁺ microglia in the capacity to present antigen through MHCI and MHCII. Of note, cDC1s protect from brain injury after ischemia. We thus reveal aspects of the dynamics and functions of brain DCs in the regulation of inflammation and immunity.

New perspectives in bronchial asthma: pathological, immunological alterations, biological targets, and pharmacotherapy

Asthma is the most common, long-lasting inflammatory airway disease that affects more than 10% of the world population. It is characterized by bronchial narrowing, airway hyperresponsiveness, vasodilatation, airway edema, and stimulation of sensory nerve endings that lead to recurring events of breathlessness, wheezing, chest tightness, and coughing. It is the main reason for global morbidity and occurs as a result of the weakening of the immune system in response to exposure to allergens or environmental exposure. In asthma condition, it results in the activation of numerous inflammatory cells like the mast and dendritic cells along with the accumulation of activated eosinophils and lymphocytes at the inflammation site. The structural cells such as airway epithelial cells and smooth muscle cells release inflammatory mediators that promote the bronchial inflammation. Long-lasting bronchial inflammation can cause pathological alterations, viz. the improved thickness of the bronchial epithelium and friability of airway epithelial cells, epithelium fibrosis, hyperplasia, and hypertrophy of airway smooth muscle, angiogenesis, and mucus gland hyperplasia. The stimulation of bronchial epithelial cell would result in the release of inflammatory cytokines and chemokines that attract inflammatory cells into bronchial airways and plays an important role in asthma. Asthma patients who do not respond to marketed antiasthmatic drugs needed novel biological medications to regulate the asthmatic situation. The present review enumerates various types of asthma, etiological factors, and in vivo animal models for the induction of asthma. The underlying pathological, immunological mechanism of action, the role of inflammatory mediators, the effect of inflammation on the bronchial airways, newer treatment approaches, and novel biological targets of asthma have been discussed in this review.

Streptococcus pneumoniae aminopeptidase N regulates dendritic cells that attenuates type-2 airway inflammation in murine allergic asthma

BACKGROUND AND PURPOSE

Epidemiological and experimental studies suggest that microbial exposure in early childhood is linked with reduced risk to suffer asthma. Thus microbial components with immunoregulatory capabilities might serve as a preventive strategy for allergic asthma. Recently, it was identified that Streptococcus pneumoniae aminopeptidase N (PepN) could suppress T cell effector function. We sought to investigate the effect of PepN on murine allergic asthma and elucidate the underlying mechanism.

EXPERIMENTAL APPROACH

The effects of intranasal administration of PepN during or before sensitization were examined in ovalbumin (OVA)-induced murine allergic asthma. The roles of CD11b⁺ dendritic cells in PepN treated OVA-induced allergic asthma were evaluated by flow cytometry, cytokines detection and adoptive transfer. Moreover, the numbers of lung type 2 innate lymphoid cells (ILC2s) were also detected.

KEY RESULTS

Administration of PepN during or before sensitization attenuated type-2 airway inflammation (eosinophilia, mucus hypersecretion, Th2 cytokines production and IgE production) in allergic asthma mice. PepN reduced lung accumulation of CD11b⁺ dendritic cells, which was accompanied by diminished dendritic cell-attracting chemokine CCL20 production as well as CCL17 and CCL22, which are Th2-cell chemokines predominantly produced by CD11b⁺ dendritic cells. Adoptive transfer of BM-derived CD11b⁺ dendritic cells abolished the inhibitory effect of PepN on OVA-induced type-2 airway inflammation. The numbers of lung ILC2s were decreased in asthmatic mice receiving PepN.

CONCLUSION AND IMPLICATIONS

PepN alleviated type-2 inflammation in OVA-induced allergic asthma mice, which was mediated by regulation of lung CD11b⁺ dendritic cells. Our study provides a novel strategy for the prevention of allergic asthma.

A profile of TNFR2+ regulatory T cells and CD103+ dendritic cells in the peripheral blood of patients with asthma

The interaction of tolerogenic CD103⁺ dendritic cells (DCs) with regulatory T (Tregs) cells modulates immune responses by inducing immune tolerance. Hence, we determined the proportion of these cells in the peripheral blood mononuclear cells (PBMC) of asthmatic patients. We observed lower trends of CD11b^-CD103⁺ DCs and CD86 within CD11b^-CD103⁺ DCs, while increased levels of Foxp3 expressing CD25^+/-TNFR2⁺ cells in asthmatics. There was a positive correlation in the expression of Foxp3 within CD3⁺CD4⁺CD25⁺TNFR2⁺ Tregs and CD11b^-CD103⁺ as well as the expression of CD86 within HLA-DR⁺CD11c⁺CD11b^-CD103⁺ DCs. In conclusion, we suggest that the increased levels of Tregs in blood could continuously suppress the T helper 2 (Th2) cells activation in the circulation which is also supported by the increase of anti-inflammatory cytokines IL-10 and TNF. Overall, functional immunoregulation of the regulatory cells, particularly Tregs, exhibit immune suppression and induce immune tolerance linked with the immune activation by the antigen presenting cells (APC).

Regulation of lung immunity by dendritic cells: Implications for asthma, chronic obstructive pulmonary disease and infectious disease

Since the first description of dendritic cells by Steinman and Cohn in 1973, this important cell type has gained increasing attention. Over 4000 papers have been published on this topic annually during the last few years. At the beginning, dendritic cells were recognized for their immune stimulatory properties and their importance in initiating an adaptive immune response. Later, it was found that dendritic cells do not only initiate but also regulate immune responses. This attribute makes the so-called regulatory dendritic cells highly important for the prevention of exaggerated immune responses. Immune cells make contact with different Ags every day and must be tightly controlled to prevent excessive inflammation and subsequent organ destruction, particularly in organs such as the gut and lungs. Here, we give a brief overview of our current knowledge on how immune responses are controlled by dendritic cells, highlighting how they are involved in the induction of peripheral tolerance. We focus on what is known about these processes in the lung, with a closer look at their role in the induction and control of diseases such as bronchial asthma, chronic obstructive pulmonary disease and lung infections. Finally, we summarize some current approaches to modulate the behavior of dendritic cells that may hopefully lead to future therapeutics to control exaggerated immune responses.

Modulating local airway immune responses to treat allergic asthma: lessons from experimental models and human studies

With asthma affecting over 300 million individuals world-wide and estimated to affect 400 million by 2025, developing effective, long-lasting therapeutics is essential. Allergic asthma, where Th2-type immunity plays a central role, represents 90% of child and 50% of adult asthma cases. Research based largely on animal models of allergic disease have led to the generation of a novel class of drugs, so-called biologicals, that target essential components of Th2-type inflammation. Although highly efficient in subclasses of patients, these biologicals and other existing medication only target the symptomatic stage of asthma and when therapy is ceased, a flare-up of the disease is often observed. Therefore, it is suggested to target earlier stages in the inflammatory cascade underlying allergic airway inflammation and to focus on changing and redirecting the initiation of type 2 inflammatory responses against allergens and certain viral agents. This focus on upstream aspects of innate immunity that drive development of Th2-type immunity is expected to have longer-lasting and disease-modifying effects, and may potentially lead to a cure for asthma. This review highlights the current understanding of the contribution of local innate immune elements in the development and maintenance of inflammatory airway responses and discusses available leads for successful targeting of those pathways for future therapeutics.

Mechanisms of persistence, innate immune activation and immunomodulation by the gastric pathogen Helicobacter pylori

The gastric bacterium Helicobacter pylori efficiently evades innate immune detection and persistently colonizes its human host. Understanding the genetic determinants that H. pylori uses to establish and maintain persistence, along with their cellular targets, is key to our understanding of the pathogenesis of this extraordinarily successful bacterial colonizer of the human stomach. This review highlights recent advances in elucidating innate immune recognition of H. pylori, its interactions with myeloid cells and the consequences that this very local infection has for immune responses at extragastric sites in models of allergy, autoimmunity and parasitic infection. The human-specific, gram-negative gastric colonizer and carcinogen H. pylori represents the prototype of a persistent bacterial pathogen. It is transmitted during early childhood, typically from mother to infant, and is believed to persist in its human host from the cradle to the grave. The tremendous success of H. pylori in infecting and colonizing half of the world's population, and in continuously accompanying humans since they migrated out of Africa over 60000 years ago, can largely be attributed to its ability to manipulate the host immune system to its own advantage, and to thereby ensure its own persistence and chronicity. In his final years as an active PI, Stanley Falkow increasingly recognized the need to understand bacterial persistence strategies as a prerequisite of understanding the pathogenesis of chronic bacterial infections, and, inspired in large part by Denise Monack's work on Salmonella persistence, many of our discussions at the time revolved around this topic. Multiple labs have since made important contributions to our understanding of innate immune detection of H. pylori, the types and polarization of adaptive immune responses that ensue, the ability of H. pylori to skew such immune responses to its advantage, and its ability to manipulate the host immune system with far-reaching, even systemic consequences. This review attempts to cover some of these topics, with a particular focus on the most recent contributions by researchers in the field.

Lung IFNAR1hi TNFR2+ cDC2 promotes lung regulatory T cells induction and maintains lung mucosal tolerance at steady state

The lung is a naturally tolerogenic organ. Lung regulatory T cells (T-regs) control lung mucosal tolerance. Here, we identified a lung IFNAR1^hiTNFR2⁺ conventional DC2 (iR2D2) population that induces T-regs in the lung at steady state. Using conditional knockout mice, adoptive cell transfer, receptor blocking antibodies, and TNFR2 agonist, we showed that iR2D2 is a lung microenvironment-adapted dendritic cell population whose residence depends on the constitutive TNFR2 signaling. IFNβ-IFNAR1 signaling in iR2D2 is necessary and sufficient for T-regs induction in the lung. The Epcam⁺CD45^- epithelial cells are the sole lung IFNβ producer at the steady state. Surprisingly, iR2D2 is plastic. In a house dust mite model of asthma, iR2D2 generates lung T_H2 responses. Last, healthy human lungs have a phenotypically similar tolerogenic iR2D2 population, which became pathogenic in lung disease patients. Our findings elucidate lung epithelial cells IFNβ-iR2D2-T-regs axis in controlling lung mucosal tolerance and provide new strategies for therapeutic interventions.

Tissue-resident innate immunity in the lung

The lung is a unique organ that must protect against inhaled pathogens and toxins, without mounting a disproportionate response against harmless particulate matter and without compromising its vital function. Tissue-resident immune cells within the lung provide local immunity and protection from infection but are also responsible for causing disease when dysregulated. There is a growing appreciation of the importance of tissue-resident memory T cells to lung immunity, but non-recirculating, tissue-resident, innate immune cells also exist. These cells provide the first line of defence against pulmonary infection and are essential for co-ordinating the subsequent adaptive response. In this review, we discuss the main lung-resident innate immune subsets and their functions in common pulmonary diseases, such as influenza, bacterial pneumonia, asthma and inflammatory disorders.

Immunologic mechanisms in asthma

Asthma is a chronic airway disease, which affects more than 300 million people. The pathogenesis of asthma exhibits marked heterogeneity with many phenotypes defining visible characteristics and endotypes defining molecular mechanisms. With the evolution of novel biological therapies, patients, who do not-respond to conventional asthma therapy require novel biologic medications, such as anti-IgE, anti-IL-5 and anti-IL4/IL13 to control asthma symptoms. It is increasingly important for physicians to understand immunopathology of asthma and to characterize asthma phenotypes. Asthma is associated with immune system activation, airway hyperresponsiveness (AHR), epithelial cell activation, mucus overproduction and airway remodeling. Both innate and adaptive immunity play roles in immunologic mechanisms of asthma. Type 2 asthma with eosinophilia is a common phenotype in asthma. It occurs with and without visible allergy. The type 2 endotype comprises; T helper type 2 (Th2) cells, type 2 innate lymphoid cells (ILC2), IgE-secreting B cells and eosinophils. Eosinophilic nonallergic asthma is ILC2 predominated, which produces IL-5 to recruit eosinophil into the mucosal airway. The second major subgroup of asthma is non-type 2 asthma, which contains heterogeneous group of endoypes and phenotypes, such as exercise-induced asthma, obesity induced asthma, etc. Neutrophilic asthma is not induced by allergens but can be induced by infections, cigarette smoke and pollution. IL-17 which is produced by Th17 cells and type 3 ILCs, can stimulate neutrophilic airway inflammation. Macrophages, dendritic cells and NKT cells are all capable of producing cytokines that are known to contribute in allergic and nonallergic asthma. Bronchial epithelial cell activation and release of cytokines, such as IL-33, IL-25 and TSLP play a major role in asthma. Especially, allergens or environmental exposure to toxic agents, such as pollutants, diesel exhaust, detergents may affect the epithelial barrier leading to asthma development. In this review, we focus on the immunologic mechanism of heterogenous asthma phenotypes.

FLT3/FLT3L-mediated CD103+ dendritic cells alleviates hepatic ischemia-reperfusion injury in mice via activation of treg cells

BACKGROUND

This study was conducted to investigate the protective effect of Fms-like tyrosine kinase 3 (FLT3)/FLT3 ligand (FLT3L)-dependent CD103⁺ dendritic cells (DCs) on hepatic ischemia-reperfusion injury (IRI).

METHODS

A mouse model of hepatic IRI and cellular model following hypoxia-reperfusion (H/R) treatment were established. Peripheral blood and liver tissues were obtained and analyzed by flow cytometer in terms of percentage of CD103⁺DCs and regulatory T (Treg) cells. Serum alanine aminotransferase (ALT), aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) were determined to assess liver function. Moreover, pro-inflammatory cytokines levels including tumor necrosis factor (TNF)-α, interleukin (IL)-1β and IL-6 were measured using enzyme-linked immunosorbent assay (ELISA). The histological morphology of liver tissues was examined with hematoxylin and eosin (HE) staining. The apoptosis was detected by terminal deoxynucleotidyl transferase (TdT) dUTP Nick End Labeling (TUNEL) assay. Treg-associated cytokines transforming growth factor (TGF)-β and IL-10 expressions were measured using quantitative real time polymerase chain reaction (qRT-PCR).

RESULTS

CD103⁺ DCs were significantly decreased in peripheral blood and liver tissues of mouse model of hepatic IRI. In vivo experiments indicated that CD103⁺ DCs infusion ameliorated IRI-induced liver damage and Treg inhibition. Further investigations demonstrated that FLT3/FLT3L-dependent CD103⁺ DCs suppressed hepatocyte apoptosis via activation of Treg cells in vitro.

CONCLUSION

FLT3/FLT3L-induced CD103⁺ DCs alleviated hepatic IRI through activating Treg cells.

BATF3-dependent dendritic cells drive both effector and regulatory T-cell responses in bacterially infected tissues

The gastric lamina propria of mice that have been experimentally infected with the pathobiont Helicobacter pylori hosts a dense network of myeloid cells that includes BATF3-dependent CD103⁺ dendritic cells (DCs). We show here that CD103⁺ DCs are strictly required for gastric Th1 responses to H. pylori and for H. pylori infection control. A similar dependence of type 1 immunity on CD103⁺ DCs is observed in a Mycobacterium bovis BCG infection model, and in a syngeneic colon cancer model. Strikingly, we find that not only the expansion and/or recruitment of Th1 cells, but also of peripherally induced, neuropilin-negative regulatory T-cells to sites of infection requires BATF3-dependent DCs. A shared feature of the examined models is the strongly reduced production of the chemokines and CXCR3 ligands CXCL9, 10 and 11 in BATF3-deficient mice. The results implicate BATF3-dependent DCs in the recruitment of CXCR3⁺ effector and regulatory T-cells to target tissues and in their local expansion.

In this work, Arnold & Zhang et al report that CD103⁺ DCs are required for protective Th1 responses, infection control of mucosal and systemic bacterial pathogens, and anti-tumor immunity driven by CD4⁺ Th1 cells and CD8⁺ T cells. CD103⁺ DCs further specifically promote the recruitment of Tbet⁺ peripherally induced Tregs to sites of infection. The results implicate CD103⁺ DCs in the trafficking of CXCR3⁺ Tbet⁺ T-cells to sites of infection and tumorigenesis.

Nanoencapsulated retinoic acid as a safe tolerogenic adjuvant for intranasal vaccination against cutaneous leishmaniasis

Mucosal, but not peripheral, vaccination with whole Leishmania amazonensis antigen (LaAg) effectively protects mice against leishmaniasis, likely through a tolerogenic mechanism. Given the crucial role of retinoic acid (RA) in CD4⁺ Foxp3⁺ regulatory T cell (T_reg) differentiation and mucosal tolerance, here we evaluated the capacity of RA to improve intranasal (i.n.) vaccination with LaAg. To prevent degradation and possible mucosa irritation, RA was encapsulated in solid lipid nanoparticles (RA-SLN). Thus, BALB/c mice were given two i.n. doses of LaAg alone or in association with RA-SLN (LaAg/RA-SLN) prior to challenge with L. amazonensis. No histological sign of irritation or inflammation was produced in the nasal mucosa after RA-SLN administration. LaAg/RA-SLN vaccine was more effective in delaying lesion growth and reducing parasite burdens than LaAg alone (96% and 61% reduction, respectively). At two months after challenge, both vaccinated groups displayed similar T helper (Th) 1-skewed in situ cytokine responses, different from early infection where both Th1 and Th2 responses were suppressed, except for transforming growth factor (TGF)-β mRNA, that was higher in mice given RA-SLN. At the mucosa, RA-SLN promoted enhanced expression of interleukin (IL)-10 and CD4⁺ Foxp3⁺ T_reg population. In sum, these data show that RA-SLN is an effective and safe tolerogenic adjuvant for i.n. vaccination against leishmaniasis.

Prevention of allergy by virus-like nanoparticles (VNP) delivering shielded versions of major allergens in a humanized murine allergy model

Background

In high‐risk populations, allergen‐specific prophylaxis could protect from sensitization and subsequent development of allergic disease. However, such treatment might itself induce sensitization and allergies, thus requiring hypoallergenic vaccine formulations. We here characterized the preventive potential of virus‐like nanoparticles (VNP) expressing surface‐exposed or shielded allergens.

Methods

Full‐length major mugwort pollen allergen Art v 1 was selectively targeted either to the surface or to the inner side of the lipid bilayer envelope of VNP. Upon biochemical and immunological analysis, their preventive potential was determined in a humanized mouse model of mugwort pollen allergy.

Results

Virus‐like nanoparticles expressing shielded version of Art v 1, in contrast to those expressing surface‐exposed Art v 1, were hypoallergenic as they hardly induced degranulation of rat basophil leukemia cells sensitized with Art v 1‐specific mouse or human IgE. Both VNP versions induced proliferation and cytokine production of allergen‐specific T cells in vitro. Upon intranasal application in mice, VNP expressing surface‐exposed but not shielded allergen induced allergen‐specific antibodies, including IgE. Notably, preventive treatment with VNP expressing shielded allergen‐protected mice from subsequent sensitization with mugwort pollen extract. Protection was associated with a Th1/Treg‐dominated cytokine response, increased Foxp3⁺ Treg numbers in lungs, and reduced lung resistance when compared to mice treated with empty particles.

Conclusion

Virus‐like nanoparticles represent a novel and versatile platform for the in vivo delivery of allergens to selectively target T cells and prevent allergies without inducing allergic reactions or allergic sensitization.

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.

Aldehyde dehydrogenase in regulatory T-cell development, immunity and cancer

The role of aldehyde dehydrogenase (ALDH) in carcinogenesis and resistance to cancer therapies is well known. Mounting evidence also suggests a potentially important role for ALDH in the induction and function of regulatory T (Treg) cells. Treg cells are important cells of the immune system involved in promoting immune tolerance and preventing aberrant immune responses to beneficial or non-harmful antigens. However, Treg cells also impair tumor immunity, leading to the progression of various carcinomas. ALDH expression and the subsequent production of retinoic acid by numerous cells, including dendritic cells, macrophages, eosinophils and epithelial cells, seems important in Treg induction and function in multiple organ systems. This is particularly evident in the gastrointestinal tract, pulmonary tract and skin, which are exposed to a myriad of environmental antigens and represent interfaces between the human body and the outside world. Expression of ALDH in Treg cells themselves may also be involved in the proliferation of these cells and resistance to certain cytotoxic therapies. Hence, inhibition of ALDH expression may be useful to treat cancer. Besides the direct effect of ALDH inhibition on carcinogenesis and resistance to cancer therapies, inhibition of ALDH could potentially augment the immune response to tumor antigens by inhibiting Treg induction, function and ability to promote immune tolerance to tumor cells in multiple cancer types.

Eicosanoid Control Over Antigen Presenting Cells in Asthma

Asthma is a common lung disease affecting 300 million people worldwide. Allergic asthma is recognized as a prototypical Th2 disorder, orchestrated by an aberrant adaptive CD4+ T helper (Th2/Th17) cell immune response against airborne allergens, that leads to eosinophilic inflammation, reversible bronchoconstriction, and mucus overproduction. Other forms of asthma are controlled by an eosinophil-rich innate ILC2 response driven by epithelial damage, whereas in some patients with more neutrophilia, the disease is driven by Th17 cells. Dendritic cells (DCs) and macrophages are crucial regulators of type 2 immunity in asthma. Numerous lipid mediators including the eicosanoids prostaglandins and leukotrienes influence key functions of these cells, leading to either pro- or anti-inflammatory effects on disease outcome. In this review, we will discuss how eicosanoids affect the functions of DCs and macrophages in the asthmatic lung and how this leads to aberrant T cell differentiation that causes disease.

The Canonical but Not the Noncanonical Wnt Pathway Inhibits the Development of Allergic Airway Disease

Asthma is a syndrome with multifactorial causes, resulting in a variety of different phenotypes. Current treatment options are not curative and are sometimes ineffective in certain disease phenotypes. Therefore, novel therapeutic approaches are required. Recent findings have shown that activation of the canonical Wnt signaling pathway suppresses the development of allergic airway disease. In contrast, the effect of the noncanonical Wnt signaling pathway activation on allergic airway disease is not well described. The aim of this study was to validate the therapeutic effectiveness of Wnt-1-driven canonical Wnt signaling compared with Wnt-5a-driven noncanonical signaling in murine models. In vitro, both ligands were capable of attenuating allergen-specific T cell activation in a dendritic cell-dependent manner. In addition, the therapeutic effects of Wnt ligands were assessed in two different models of allergic airway disease. Application of Wnt-1 resulted in suppression of airway inflammation as well as airway hyperresponsiveness and mucus production. In contrast, administration of Wnt-5a was less effective in reducing airway inflammation or goblet cell metaplasia. These results suggest an immune modulating function for canonical as well as noncanonical Wnt signaling, but canonical Wnt pathway activation appears to be more effective in suppressing allergic airway disease than noncanonical Wnt activation.

Immunological Processes Driving IgE Sensitisation and Disease Development in Males and Females

IgE sensitisation has increased significantly over the last decades and is a crucial factor in the development of allergic diseases. IgE antibodies are produced by B cells through the process of antigen presentation by dendritic cells, subsequent differentiation of CD4⁺ Th2 cells, and class switching in B cells. However, many of the factors regulating these processes remain unclear. These processes affect males and females differently, resulting in a significantly higher prevalence of IgE sensitisation in males compared to females from an early age. Before the onset of puberty, this increased prevalence of IgE sensitisation is also associated with a higher prevalence of clinical symptoms in males; however, after puberty, females experience a surge in the incidence of allergic symptoms. This is particularly apparent in allergic asthma, but also in other allergic diseases such as food and contact allergies. This has been partly attributed to the pro- versus anti-allergic effects of female versus male sex hormones; however, it remains unclear how the expression of sex hormones translates IgE sensitisation into clinical symptoms. In this review, we describe the recent epidemiological findings on IgE sensitisation in male and females and discuss recent mechanistic studies casting further light on how the expression of sex hormones may influence the innate and adaptive immune system at mucosal surfaces and how sex hormones may be involved in translating IgE sensitisation into clinical manifestations.

Tumor Necrosis Factor Alpha Antagonism Reveals a Gut/Lung Axis That Amplifies Regulatory T Cells in a Pulmonary Fungal Infection

Tumor necrosis factor (TNF) antagonists are popular therapies for inflammatory diseases. These agents enhance the numbers and function of regulatory T cells (Tregs), which are important in controlling inflammatory diseases. However, elevated Treg levels increase susceptibility to infections, including histoplasmosis. We determined the mechanism by which Tregs expand in TNF-neutralized mice infected with Histoplasma capsulatum Lung CD11c⁺ CD11b⁺ dendritic cells (DCs), but not alveolar macrophages, from H. capsulatum-infected mice treated with anti-TNF induced a higher percentage of Tregs than control DCs in vitro CD11b⁺ CD103⁺ DCs, understood to be unique to the intestines, were augmented in lungs with anti-TNF treatment. In the absence of this subset, DCs from anti-TNF-treated mice failed to amplify Tregs in vitro CD11b⁺ CD103⁺ DCs from TNF-neutralized mice displayed higher retinaldehyde dehydrogenase 2 (RALDH2) gene expression, and CD11b⁺ CD103⁺ RALDH⁺ DCs exhibited greater enzyme activity. To determine if CD11b⁺ CD103⁺ DCs migrated from gut to lung, fluorescent beads were delivered to the gut via oral gavage, and the lungs were assessed for bead-containing DCs. Anti-TNF induced migration of CD11b⁺ CD103⁺ DCs from the gut to the lung that enhanced the generation of Tregs in H. capsulatum-infected mice. Therefore, TNF neutralization promotes susceptibility to pulmonary H. capsulatum infection by promoting a gut/lung migration of DCs that enhances Tregs.

Distribution and Interaction of Murine Pulmonary Phagocytes in the Naive and Allergic Lung

The division of labor between pulmonary phagocytic subsets [macrophage/monocyte and dendritic cell (DC) subpopulations] has been described at the functional level. However, whether these lung phagocytes also display unique spatial distribution remains unclear. Here, to analyze cellular distribution in lung compartments and contacts between phagocyte subpopulations, we established an immunohistochemistry (IHC)-based method to clearly identify murine lung phagocyte subsets in situ based on differential expression of CD11c, CD11b, MHC-II, Langerin and mPDCA-1. Furthermore, we investigated subset-specific functional differences in antigen uptake and spatial changes upon allergic sensitization. Our staining allowed the distinction between alveolar macrophages (AMs), interstitial macrophage (IM) subpopulations, CD11b⁺ DC subpopulations, CD103⁺ DCs, and plasmacytoid DCs (pDCs). We identified interstitial regions between airways and around airways as regions of IM/CD11b⁺ DC/CD103⁺ DC clusters, where a subset of IMs (IM2) and CD103⁺ DCs formed intense contacts that decreased upon allergic sensitization. These data indicate functional interactions between both cell types either in steady state or after antigen encounter affecting the development of allergies or tolerance. Furthermore, we observed major antigen uptake in AMs and IMs rather than DC subpopulations that was not restricted to airways and adjacent areas. This will enable to focus future studies to immunologically relevant cellular interactions and to unravel which cells are tipping the balance between pro-inflammatory immune responses or tolerance.