Epicutaneous sensitization to house dust mite allergen requires interferon regulatory factor 4-dependent dermal dendritic cells

The role of dendritic cells in respiratory viral infection

Respiratory viral infections represent one of the major causes of death worldwide. The recent coronavirus disease 2019 pandemic alone claimed the lives of over 6 million people around the globe. It is therefore crucial to understand how the immune system responds to these threats and how respiratory infection can be controlled and constrained. Dendritic cells (DCs) are one of the key players in antiviral immunity because of their ability to detect pathogens. They can orchestrate an immune response that will, in most cases, lead to viral clearance. Different subsets of DCs are present in the lung and each subset can contribute to antiviral responses through various mechanisms. In this review, we discuss the role of the different lung DC subsets in response to common respiratory viruses, with a focus on respiratory syncytial virus, influenza A virus and severe acute respiratory syndrome coronavirus 2. We also review how lung DC-mediated responses to respiratory viruses can lead to the worsening of an existing chronic pulmonary disease such as asthma. Throughout the review, we discuss results obtained from animal studies as well as results generated from infected patients.

Sublingual allergen immunotherapy prevents house dust mite inhalant type 2 immunity through dendritic cell-mediated induction of Foxp3+ regulatory T cells

Sublingual allergen immunotherapy (SLIT) is an emerging treatment option for allergic asthma and a potential disease-modifying strategy for asthma prevention. The key cellular events leading to such long-term tolerance remain to be fully elucidated. We administered prophylactic SLIT in a mouse model of house dust mite (HDM)-driven allergic asthma. HDM extract was sublingually administered over 3 weeks followed by intratracheal sensitization and intranasal challenges with HDM. Prophylactic SLIT prevented allergic airway inflammation and hyperreactivity with a low lab-to-lab variation. The HDM-specific T helper (Th)2 (cluster of differentiation 4 Th) response was shifted by SLIT toward a regulatory and Th17 response in the lung and mediastinal lymph node. By using Derp1-specific cluster of differentiation 4+ T cells (1-DER), we found that SLIT blocked 1-DER T cell recruitment to the mediastinal lymph node and dampened IL-4 secretion following intratracheal HDM sensitization. Sublingually administered Derp1 protein activated 1-DER T cells in the cervical lymph node via chemokine receptor7+ migratory dendritic cells (DC). DCs migrating from the oral submucosa to the cervical lymph node after SLIT-induced Foxp3+ regulatory T cells. When mice were sensitized with HDM, prior prophylactic SLIT increased Derp1 specific regulatory T cells (Tregs) and lowered Th2 recruitment in the lung. By using Foxp3-diphtheria toxin receptor mice, Tregs were found to contribute to the immunoregulatory prophylactic effect of SLIT on type 2 immunity. These findings in a mouse model suggest that DC-mediated functional Treg induction in oral mucosa draining lymph nodes is one of the driving mechanisms behind the disease-modifying effect of prophylactic SLIT.

Interplay of IL-33 and IL-35 Modulates Th2/Th17 Responses in Cigarette Smoke Exposure HDM-Induced Asthma

Cigarette smoke (CS) facilitates adverse effects on the airway inflammation and treatment of asthma. Here, we investigated the mechanisms by which CS exacerbates asthma. The roles of IL-33 and IL-35 in asthma development were examined by treatment with IL-33 knockout (IL-33 KO) or transfection of adenovirus encoding IL-35 (Ad-IL-35) in a murine model of cigarette smoke-exposure asthma. Furthermore, the involvement of IL-33 and IL-35 in regulating DCs and Th2/Th17 cells was examined in a coculture system of DCs with CD4+ T cells. Additionally, we observed the effect of CpG-ODNs on the balance of IL-33 and IL-35. We show that CS and house dust mite (HDM) exposure induced IL-33 and suppressed IL-35 levels in cigarette smoke-exposure asthma in vivo and in vitro. Treatment with IL-33 KO or Ad-IL-35 significantly attenuated airway hyperreactivity, goblet hyperplasia, airway remodelling, and eosinophil and neutrophil infiltration in the lung tissues from asthmatic mice. Furthermore, we demonstrated reciprocal regulation between CS and HDM-modulated IL-33 and IL-35. Mechanistically, IL-33 KO (or anti-ST2) and Ad-IL-35 attenuated Th2- and Th17-associated inflammation by downregulating TSLP-DC signalling. Finally, administration of CpG-ODNs suppressed the expression of IL-33/ST2 and elevated the levels of IL-35, which is mainly derived from CD4+Foxp+ Tregs, to alleviate Th2- and Th17-associated inflammation by inhibiting the activation of BMDCs. Taken together, the IL-33/ST2 pathway drives the DC-Th2 and Th17 responses of cigarette smoke-exposure asthma, while IL-35 has the opposite effect. CpG-ODNs represent a potential therapeutic strategy for modulating the balance of IL-33 and IL-35 to suppress allergic airway inflammation.

Homeostatic activation of aryl hydrocarbon receptor by dietary ligands dampens cutaneous allergic responses by controlling Langerhans cells migration

Dietary compounds can affect the development of inflammatory responses at distant sites. However, the mechanisms involved remain incompletely understood. Here, we addressed the influence on allergic responses of dietary agonists of aryl hydrocarbon receptor (AhR). In cutaneous papain-induced allergy, we found that lack of dietary AhR ligands exacerbates allergic responses. This phenomenon was tissue-specific as airway allergy was unaffected by the diet. In addition, lack of dietary AhR ligands worsened asthma-like allergy in a model of 'atopic march.' Mice deprived of dietary AhR ligands displayed impaired Langerhans cell migration, leading to exaggerated T cell responses. Mechanistically, dietary AhR ligands regulated the inflammatory profile of epidermal cells, without affecting barrier function. In particular, we evidenced TGF-β hyperproduction in the skin of mice deprived of dietary AhR ligands, explaining Langerhans cell retention. Our work identifies an essential role for homeostatic activation of AhR by dietary ligands in the dampening of cutaneous allergic responses and uncovers the importance of the gut-skin axis in the development of allergic diseases.

Understanding the development of Th2 cell-driven allergic airway disease in early life

Allergic diseases, including atopic dermatitis, allergic rhinitis, asthma, and food allergy, are caused by abnormal responses to relatively harmless foreign proteins called allergens found in pollen, fungal spores, house dust mites (HDM), animal dander, or certain foods. In particular, the activation of allergen-specific helper T cells towards a type 2 (Th2) phenotype during the first encounters with the allergen, also known as the sensitization phase, is the leading cause of the subsequent development of allergic disease. Infants and children are especially prone to developing Th2 cell responses after initial contact with allergens. But in addition, the rates of allergic sensitization and the development of allergic diseases among children are increasing in the industrialized world and have been associated with living in urban settings. Particularly for respiratory allergies, greater susceptibility to developing allergic Th2 cell responses has been shown in children living in urban environments containing low levels of microbial contaminants, principally bacterial endotoxins [lipopolysaccharide (LPS)], in the causative aeroallergens. This review highlights the current understanding of the factors that balance Th2 cell immunity to environmental allergens, with a particular focus on the determinants that program conventional dendritic cells (cDCs) toward or away from a Th2 stimulatory function. In this context, it discusses transcription factor-guided functional specialization of type-2 cDCs (cDC2s) and how the integration of signals derived from the environment drives this process. In addition, it analyzes observational and mechanistic studies supporting an essential role for innate sensing of microbial-derived products contained in aeroallergens in modulating allergic Th2 cell immune responses. Finally, this review examines whether hyporesponsiveness to microbial stimulation, particularly to LPS, is a risk factor for the induction of Th2 cell responses and allergic sensitization during infancy and early childhood and the potential factors that may affect early-age response to LPS and other environmental microbial components.

Epicutaneous allergen immunotherapy induces a profound and selective modulation in skin dendritic cell subsets

BACKGROUND

Epicutaneous immunotherapy (EPIT) protocols have recently been developed to restore tolerance in patients with food allergy (FA). The mechanisms by which EPIT protocols promote desensitization rely on a profound immune deviation of pathogenic T and B cell responses.

OBJECTIVE

To date, little is known about the contribution of skin dendritic cells (skDCs) to T cell remodeling and EPIT efficacy.

METHODS

We capitalized on a preclinical model of food allergy to ovalbumin (OVA) to characterize the phenotype and functions of OVA⁺ skDCs throughout the course of EPIT.

RESULTS

Our results showed that both Langerhans cells (LCs) and dermal conventional cDC1 and cDC2 subsets retained their ability to capture OVA in the skin and to migrate toward the skin-draining lymph nodes during EPIT. However, their activation/maturation status was significantly impaired, as evidenced by the gradual and selective reduction of CD86, CD40, and OVA protein expression in respective subsets. Phenotypic changes during EPIT were also characterized by a progressive diversification of single cell gene signatures within each DC subset. Interestingly, we observed that OVA⁺ LCs progressively lost their capacity to prime CD4⁺ T_EFF, but gained T_REG stimulatory properties. In contrast, cDC1 were inefficient in priming CD4⁺ T_EFF or in reactivating T_MEMin vitro, while cDC2 retained moderate stimulatory properties, and progressively biased type-2 immunity toward type-1 and type-17 responses.

CONCLUSIONS

Our results therefore emphasize that the acquisition of distinct phenotypic and functional specializations by skDCs during EPIT is at the cornerstone of the desensitization process.

Dectin-2 promotes house dust mite-skewed Th2 response through the activation of cDC2s

The role of Dectin-2 (gene symbol, Clec4n) in house dust mite (HDM) induced Th2 immune response and the exact mechanism remains controversial. In this study, we illustrated that, Clec4n^-/- mice had decreased Th2 immune response following HDM challenge, which may ascribe to dramatically reduced type 2 conventional dendritic cells (cDC2s) in lung of Clec4n^-/- mice, as cDC2s from lung of Clec4n^-/- mice after challenging had less ability to induce Th2 response with decreased production of IL-4/IL-13. Further in vitro experiments showed the activation of Clec4n^-/--BMDCs significantly decreased after HDM stimulation accompanied with decreased activation of Syk-NF-κB and Syk-JNK signal pathway. Importantly, Dectin-2 expression in PBMCs from asthmatic patients was significantly higher than that in healthy controls. Taken together, these results demonstrated that Dectin-2 could promote cDC2s activation in lung, which polarizes Th2 immune response outlining a novel mechanism of asthma development.

Proteolytic activity accelerates the TH17/TH22 recall response to an epicutaneous protein allergen-induced TH2 response

Epicutaneous exposure to protein allergens, such as papain, house dust mite (HDM), and ovalbumin (OVA), represents an important mode of sensitization for skin diseases including protein contact dermatitis, immunologic contact urticaria, and atopic dermatitis. These diseases are inducible by re-exposure to an allergen at both original skin sensitization and distant skin sites. In this study, we examined the serum IgE/IgG₁ response, differentiation of T-helper (T_H) cells, and epicutaneous T_H recall response in mice pre-sensitized with protein allergens through the back skin and subsequently challenged on the ear skin. Repeated epicutaneous sensitization with allergenic proteins including papain, HDM, OVA, and protease inhibitor-treated papain, but not bovine serum albumin, induced serum allergen-specific antibody production, passive cutaneous anaphylaxis responses, and T_H2 differentiation in the skin draining lymph node (DLN) cells. Sensitization with papain or HDM, which have protease activity, resulted in the differentiation of T_H17 as well as T_H2. In papain- or HDM-sensitized mice, a subsequent single challenge on the ear skin induced the expression of T_H2 and T_H17/T_H22 cytokines. These results suggest that allergenic proteins induce the differentiation of T_H2 in skin DLN cells and an antibody response. These findings may be useful for identifying proteins of high and low allergenic potential. Moreover, allergenic proteins containing protease activity may also differentiate T_H17 and induce T_H2 and T_H17/T_H22 recall responses at epicutaneous challenge sites. This suggests that allergen protease activity accelerates the onset of skin diseases caused by protein allergens.

Clinical Characteristics of Atopic Dermatitis in Korean School-Aged Children and Adolescents According to Onset Age and Severity

BACKGROUND

Atopic dermatitis (AD) is a heterogeneous disease with different age of onset, disease course, clinical symptoms, severity, and risk of comorbidity. The characteristics of children with AD also vary by age or country. However, little is known about the clinical characteristics of AD in Korean school-aged children and adolescents. Furthermore, there are few studies on phenotypic differences according to onset age. This study aimed to explore the clinical characteristics and phenotypes according to onset age and severity of AD in children and adolescents in Korea.

METHODS

AD patients aged 6-18 years who presented to 18 hospitals nationwide were surveyed. The patients were examined for disease severity by pediatric allergy specialists, and data on history of other allergic diseases, familial allergy history, onset age, trigger factors, lesion sites, treatment history and quality of life were collected. The results of the patient's allergy test were also analyzed. The patients were classified into infancy-onset (< 2 years of age), preschool-onset (2-5 years of age), and childhood-onset (≥ 6 years of age) groups. Study population was analyzed for clinical features according to onset-age groups and severity groups.

RESULTS

A total of 258 patients with a mean age of 10.62 ± 3.18 years were included in the study. Infancy-onset group accounted for about 60% of all patients and presented significantly more other allergic diseases, such as allergic rhinitis and asthma (P = 0.002 and P = 0.001, respectively). Food allergy symptoms and diagnoses were highly relevant to both earlier onset and more severe group. Inhalant allergen sensitization was significantly associated with both infancy-onset group and severe group (P = 0.012 and P = 0.024, respectively). A family history of food allergies was significantly associated with infancy-onset group (P = 0.036). Severe group was significantly associated with a family history of AD, especially a paternal history of AD (P = 0.048 and P = 0.004, respectively). Facial (periorbital, ear, and cheek) lesions, periauricular fissures, hand/foot eczema, and xerosis were associated with infancy-onset group. The earlier the onset of AD, the poorer the quality of life (P = 0.038). Systemic immunosuppressants were used in only 9.6% of the patients in the severe group.

CONCLUSION

This study analyzed the clinical features of AD in Korean children and adolescents through a multicenter nationwide study and demonstrated the phenotypic differences according to onset age and severity. Considering the findings that the early-onset group is more severe and accompanied by more systemic allergic diseases, early management should be emphasized in young children and infants.

Atopic dermatitis trajectories to age 8 years in the GUSTO cohort

BACKGROUND

The heterogeneity of childhood atopic dermatitis (AD) underscores the need to understand latent phenotypes that may inform risk stratification and disease prognostication.

OBJECTIVE

To identify AD trajectories across the first 8 years of life and investigate risk factors associated with each trajectory and their relationships with other comorbidities.

METHODS

Data were collected prospectively from 1152 mother-offspring dyads in the Growing Up in Singapore Towards healthy Outcomes (GUSTO) cohort from ages 3 months to 8 years. AD was defined based on parent-reported doctor's diagnosis. An unsupervised machine learning technique was used to determine AD trajectories.

RESULTS

Three AD trajectories were identified as follows: early-onset transient (6.3%), late-onset persistent (6.3%) and early-onset persistent (2.1%), alongside a no AD/reference group (85.2%). Early-onset transient AD was positively associated with male gender, family history of atopy, house dust mite sensitization and some measures of wheezing. Early-onset persistent AD was associated with antenatal/intrapartum antibiotic use, food sensitization and some measures of wheezing. Late-onset persistent AD was associated with a family history of atopy, some measures of house dust mite sensitization and some measures of allergic rhinitis and wheezing.

CONCLUSION AND CLINICAL RELEVANCE

Three AD trajectories were identified in this birth cohort, with different risk factors and prognostic implications. Further work is needed to understand the molecular and immunological origins of these phenotypes.

Dual function of Langerhans cells in skin TSLP-promoted TFH differentiation in mouse atopic dermatitis

BACKGROUND

Atopic dermatitis (AD) is among the most common chronic inflammatory skin diseases, usually occurring early in life, and often preceding other atopic diseases such as asthma. TH2 has been believed to play a crucial role in cellular and humoral response in AD, but accumulating evidence has shown that follicular helper T cell (TFH), a critical player in humoral immunity, is associated with disease severity and plays an important role in AD pathogenesis.

OBJECTIVES

This study aimed at investigating how TFHs are generated during the pathogenesis of AD, particularly what is the role of keratinocyte-derived cytokine TSLP and Langerhans cells (LCs).

METHODS

Two experimental AD mouse models were employed: (1) triggered by the overproduction of TSLP through topical application of MC903, and (2) induced by epicutaneous allergen ovalbumin (OVA) sensitization.

RESULTS

This study demonstrated that the development of TFHs and germinal center (GC) response were crucially dependent on TSLP in both the MC903 model and the OVA sensitization model. Moreover, we found that LCs promoted TFH differentiation and GC response in the MC903 model, and the depletion of Langerin+ dendritic cells (DCs) or selective depletion of LCs diminished the TFH/GC response. By contrast, in the model with OVA sensitization, LCs inhibited TFH/GC response and suppressed TH2 skin inflammation and the subsequent asthma. Transcriptomic analysis of Langerin+ and Langerin- migratory DCs revealed that Langerin+ DCs became activated in the MC903 model, whereas these cells remained inactivated in OVA sensitization model.

CONCLUSIONS

Together, these studies revealed a dual functionality of LCs in TSLP-promoted TFH and TH2 differentiation in AD pathogenesis.

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.

The basic immunology of asthma

In many asthmatics, chronic airway inflammation is driven by IL-4-, IL-5-, and IL-13-producing Th2 cells or ILC2s. Type 2 cytokines promote hallmark features of the disease such as eosinophilia, mucus hypersecretion, bronchial hyperresponsiveness (BHR), IgE production, and susceptibility to exacerbations. However, only half the asthmatics have this "type 2-high" signature, and "type 2-low" asthma is more associated with obesity, presence of neutrophils, and unresponsiveness to corticosteroids, the mainstay asthma therapy. Here, we review the underlying immunological basis of various asthma endotypes by discussing results obtained from animal studies as well as results generated in clinical studies targeting specific immune pathways.

Association of frequent moisturizer use in early infancy with the development of food allergy

Background

Objectives

Methods

Results

Conclusions

Dermal IRF4+ dendritic cells and monocytes license CD4+ T helper cells to distinct cytokine profiles

Antigen (Ag)-presenting cells (APC) instruct CD4+ helper T (Th) cell responses, but it is unclear whether different APC subsets contribute uniquely in determining Th differentiation in pathogen-specific settings. Here, we use skin-relevant, fluorescently-labeled bacterial, helminth or fungal pathogens to track and characterize the APC populations that drive Th responses in vivo. All pathogens are taken up by a population of IRF4+ dermal migratory dendritic cells (migDC2) that similarly upregulate surface co-stimulatory molecules but express pathogen-specific cytokine and chemokine transcripts. Depletion of migDC2 reduces the amount of Ag in lymph node and the development of IFNγ, IL-4 and IL-17A responses without gain of other cytokine responses. Ag+ monocytes are an essential source of IL-12 for both innate and adaptive IFNγ production, and inhibit follicular Th cell development. Our results thus suggest that Th cell differentiation does not require specialized APC subsets, but is driven by inducible and pathogen-specific transcriptional programs in Ag+ migDC2 and monocytes.

Substance P Release by Sensory Neurons Triggers Dendritic Cell Migration and Initiates the Type-2 Immune Response to Allergens

Dendritic cells (DCs) of the cDC2 lineage initiate allergic immunity and in the dermis are marked by their expression of CD301b. CD301b⁺ dermal DCs respond to allergens encountered in vivo, but not in vitro. This suggests that another cell in the dermis may sense allergens and relay that information to activate and induce the migration of CD301b⁺ DCs to the draining lymph node (dLN). Using a model of cutaneous allergen exposure, we show that allergens directly activated TRPV1⁺ sensory neurons leading to itch and pain behaviors. Allergen-activated sensory neurons released the neuropeptide Substance P, which stimulated proximally located CD301b⁺ DCs through the Mas-related G-protein coupled receptor member A1 (MRGPRA1). Substance P induced CD301b⁺ DC migration to the dLN where they initiated T helper-2 cell differentiation. Thus, sensory neurons act as primary sensors of allergens, linking exposure to activation of allergic-skewing DCs and the initiation of an allergic immune response.

Defined Intestinal Regions Are Drained by Specific Lymph Nodes That Mount Distinct Th1 and Th2 Responses Against Schistosoma mansoni Eggs

The balance of type 1 and type 2 immune responses plays a crucial role in anti-helminth immunity and can either support chronic infection or drive type 2 mediated expulsion of the parasite. Helminth antigens and secreted molecules directly influence this balance and induce a favorable immunological environment for the parasite’s survival. However, less is known if the site of infection also influences the balance of type 1 and type 2 immunity. Here, we report that tissue-specific immune responses are mounted against helminth antigens, which elicited strong IL-4 responses when injected into the skin, while the same antigen, delivered into the intestinal subserosa, induced increased IFN-γ and reduced Th2 responses. Immune responses in individual mesenteric lymph nodes that drain defined regions of the intestine furthermore displayed a site-specific pattern of type 1 and type 2 immunity after Schistosoma mansoni or Heligmosomoides polygyrus infection. S. mansoni egg-specific Th2 responses were detectable in all mesenteric lymph nodes but Th1 responses were only present in those draining the colon, while H. polygyrus infection elicited mixed Th1 and Th2 responses in the lymph nodes associated with the site of infection. Similar site-specific type 1 and type 2 immune responses were observed in the draining lymph nodes after the controlled delivery of S. mansoni eggs into different segments of the small and large intestine using microsurgical techniques. Different subsets of intestinal dendritic cells were hereby responsible for the uptake and priming of Th1 and Th2 responses against helminth antigens. Migratory CD11b⁺CD103⁻ and especially CD11b⁺CD103⁺ DC2s transported S. mansoni egg antigens to the draining lymph nodes to induce Th1 and Th2 responses, while CD103⁺ DC1s induced only IFN-γ responses. In contrast, H. polygyrus antigens were predominantly transported by CD11b⁺CD103⁻ DC2s and CD103⁺ DC1s and all DC subsets induced similar Th1 but weaker Th2 responses, compared to S. mansoni egg antigens. The development of adaptive anti-helminth immune responses is therefore influenced by the antigen itself, the uptake and priming characteristics of antigen-positive dendritic cell subsets and the site of infection, which shape the level of Th1 and Th2 responses in order to create a favorable immunological environment for the parasite.

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 in Th2 immune responses and allergic sensitization

Allergic responses are characterized by the activation of a specific subset of effector CD4⁺ T cells, the T-helper type 2 (Th2) cells, that respond to harmless environmental antigens causing inflammation and pathology. Th2 cells are also found in the context of parasite infections, where they can mediate parasite clearance and expulsion, and support tissue repair. The process that leads to the activation of Th2 cells in vivo is incompletely understood: while it has become clear that "conventional" dendritic cells are essential antigen-presenting cells for the initiation of Th2 immune responses, the molecules that are expressed by dendritic cells exposed to allergens, and the mediators that are produced as a consequence and signal to naïve CD4⁺ T cells to promote their development into effector Th2, remain to be defined. Here we summarize recent developments in the identification of the dendritic cell subsets involved in Th2 responses, review potential mechanisms proposed to explain the generation of these immune responses, and discuss the direct and indirect signals that condition dendritic cells to drive the development of Th2 responses during allergen or parasite exposure.

Antigen presentation by dendritic cells and their instruction of CD4+ T helper cell responses

Dendritic cells are powerful antigen-presenting cells that are essential for the priming of T cell responses. In addition to providing T-cell-receptor ligands and co-stimulatory molecules for naive T cell activation and expansion, dendritic cells are thought to also provide signals for the differentiation of CD4+ T cells into effector T cell populations. The mechanisms by which dendritic cells are able to adapt and respond to the great variety of infectious stimuli they are confronted with, and prime an appropriate CD4+ T cell response, are only partly understood. It is known that in the steady-state dendritic cells are highly heterogenous both in phenotype and transcriptional profile, and that this variability is dependent on developmental lineage, maturation stage, and the tissue environment in which dendritic cells are located. Exposure to infectious agents interfaces with this pre-existing heterogeneity by providing ligands for pattern-recognition and toll-like receptors that are variably expressed on different dendritic cell subsets, and elicit production of cytokines and chemokines to support innate cell activation and drive T cell differentiation. Here we review current information on dendritic cell biology, their heterogeneity, and the properties of different dendritic cell subsets. We then consider the signals required for the development of different types of Th immune responses, and the cellular and molecular evidence implicating different subsets of dendritic cells in providing such signals. We outline how dendritic cell subsets tailor their response according to the infectious agent, and how such transcriptional plasticity enables them to drive different types of immune responses.

The Gastrointestinal Helminth Heligmosomoides bakeri Suppresses Inflammation in a Model of Contact Hypersensitivity

Helminths regulate host immune responses to ensure their own long-term survival. Numerous studies have demonstrated that these helminth-induced regulatory mechanisms can also limit host inflammatory responses in several disease models. We used the Heligmosomoides bakeri (Hb) infection model (also known as H. polygyrus or H. polygyrus bakeri in the literature) to test whether such immune regulation affects skin inflammatory responses induced by the model contact sensitiser dibutyl phthalate fluorescein isothiocynate (DBP-FITC). Skin lysates from DBP-FITC-sensitized, Hb-infected mice produced less neutrophil specific chemokines and had significantly reduced levels of skin thickening and cellular inflammatory responses in tissue and draining lymph nodes (LNs) compared to uninfected mice. Hb-induced suppression did not appear to be mediated by regulatory T cells, nor was it due to impaired dendritic cell (DC) activity. Mice cleared of infection remained unresponsive to DBP-FITC sensitization indicating that suppression was not via the secretion of Hb-derived short-lived regulatory molecules, although long-term effects on cells cannot be ruled out. Importantly, similar helminth-induced suppression of inflammation was also seen in the draining LN after intradermal injection of the ubiquitous allergen house dust mite (HDM). These findings demonstrate that Hb infection attenuates skin inflammatory responses by suppressing chemokine production and recruitment of innate cells. These findings further contribute to the growing body of evidence that helminth infection can modulate inflammatory and allergic responses via a number of mechanisms with potential to be exploited in therapeutic and preventative strategies in the future.

Inflammatory Type 2 cDCs Acquire Features of cDC1s and Macrophages to Orchestrate Immunity to Respiratory Virus Infection

The phenotypic and functional dichotomy between IRF8⁺ type 1 and IRF4⁺ type 2 conventional dendritic cells (cDC1s and cDC2s, respectively) is well accepted; it is unknown how robust this dichotomy is under inflammatory conditions, when additionally monocyte-derived cells (MCs) become competent antigen-presenting cells (APCs). Using single-cell technologies in models of respiratory viral infection, we found that lung cDC2s acquired expression of the Fc receptor CD64 shared with MCs and of IRF8 shared with cDC1s. These inflammatory cDC2s (inf-cDC2s) were superior in inducing CD4⁺ T helper (Th) cell polarization while simultaneously presenting antigen to CD8⁺ T cells. When carefully separated from inf-cDC2s, MCs lacked APC function. Inf-cDC2s matured in response to cell-intrinsic Toll-like receptor and type 1 interferon receptor signaling, upregulated an IRF8-dependent maturation module, and acquired antigens via convalescent serum and Fc receptors. Because hybrid inf-cDC2s are easily confused with monocyte-derived cells, their existence could explain why APC functions have been attributed to MCs.

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Type I interferon drives differentiation of inf-cDC2s that closely resemble MCs

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Inf-cDC2s prime CD4⁺ and CD8⁺ T cells, whereas MCs lack APC function

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Inf-cDC2s internalize antibody-complexed antigen via Fc receptors

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IRF8 controls maturation gene module in inf-cDC2s

Dendritic cells and the skin environment

The skin is inhabited by several immune cell populations that serve as a first line of defence against pathogen invasion. Amongst these populations are dendritic cells, which play an essential sentinel function by taking up antigen or infectious agents and transporting them to the lymph node for T cell recognition and the priming of immune responses. In this review, we briefly summarise recent advances showing how skin dendritic cells are connected to a network of epithelial and stromal cells, which provide structural support, growth factors, spatial cues, contact with the external environment and the skin microbiome, and favour interactions with other immune cells. We propose that this network creates a unique skin environment that may condition dendritic cell phenotype and function.

Laser-facilitated epicutaneous immunotherapy with depigmented house dust mite extract alleviates allergic responses in a mouse model of allergic lung inflammation

BACKGROUND

Skin-based immunotherapy of type 1 allergies has recently been re-investigated as an alternative for subcutaneous injections. In the current study, we employed a mouse model of house dust mite (HDM)-induced lung inflammation to explore the potential of laser-facilitated epicutaneous allergen-specific treatment.

METHODS

Mice were sensitized against native Dermatophagoides pteronyssinus extract and repeatedly treated by application of depigmented D pteronyssinus extract via laser-generated skin micropores or by subcutaneous injection with or without alum. Following aerosol challenges, lung function was determined by whole-body plethysmography and bronchoalveolar lavage fluid was analyzed for cellular composition and cytokine levels. HDM-specific IgG subclass antibodies were determined by ELISA. Serum as well as cell-bound IgE was measured by ELISA, rat basophil leukemia cell assay, and ex vivo using a basophil activation test, respectively. Cultured lymphocytes were analyzed for cytokine secretion profiles and cellular polarization by flow cytometry.

RESULTS

Immunization of mice by subcutaneous injection or epicutaneous laser microporation induced comparable IgG antibody levels, but the latter preferentially induced regulatory T cells and in general downregulated T cell cytokine production. This effect was found to be a result of the laser treatment itself, independent from extract application. Epicutaneous treatment of sensitized animals led to induction of blocking IgG, and improvement of lung function, superior compared to the effects of subcutaneous therapy. During the whole therapy schedule, no local or systemic side effects occurred.

CONCLUSION

Allergen-specific immunotherapy with depigmented HDM extract via laser-generated skin micropores offers a safe and effective treatment option for HDM-induced allergy and lung inflammation.

Transcriptional regulation of DC fate specification

Dendritic cells function in the immune system to instruct adaptive immune cells to respond accordingly to different threats. While conventional dendritic cells can be subdivided into two main subtypes, termed cDC1s and cDC2s, it is clear that further heterogeneity exists within these subtypes, particularly for cDC2s. Understanding the signals involved in specifying each of these lineages and subtypes thereof is crucial to (i) enable us to determine their specific functions and (ii) put us in a position to be able to target these cells to promote or prevent a specific function in any given disease setting. Although we still have much to learn regarding the specification of these cells, here we review the most recent advances in our understanding of this and highlight some of the next questions for the future.

Skin Exposure Contributes to Chemical-Induced Asthma: What is the Evidence? A Systematic Review of Animal Models

It is generally assumed that allergic asthma originates primarily through sensitization via the respiratory mucosa, but emerging clinical observations and experimental studies indicate that skin exposure to low molecular weight (LMW) agents, i.e. “chemicals,” may lead to systemic sensitization and subsequently develop asthma when the chemical is inhaled. This review aims to evaluate the accumulating experimental evidence that adverse respiratory responses can be elicited upon inhalation of an LMW chemical sensitizer after previous sensitization by dermal exposure. We systematically searched the PubMed and Embase databases up to April 15, 2017, and conducted forward and backward reference tracking. Animal studies involving both skin and airway exposure to LMW agents were included. We extracted 6 indicators of “selective airway hyper-responsiveness” (SAHR)—i.e. respiratory responses that only occurred in previously sensitized animals—and synthesized the evidence level for each indicator into strong, moderate or limited strength. The summarized evidence weight for each chemical agent was graded into high, middle, low or “not possible to assess.” We identified 144 relevant animal studies. These studies involved 29 LMW agents, with 107 (74%) studies investigating the occurrence of SAHR. Indicators of SAHR included physiological, cytological/histological and immunological responses in bronchoalveolar lavage, lung tissue and airway-draining lymph nodes. Evidence for skin exposure-induced SAHR was present for 22 agents; for 7 agents the evidence for SAHR was inconclusive, but could not be excluded. The ability of a chemical to cause sensitization via skin exposure should be regarded as constituting a risk of adverse respiratory reactions.

Topical Adjuvant Application during Subcutaneous Vaccination Promotes Resident Memory T Cell Generation

Skin tissue resident memory T cells (T_RM) provide superior protection to a second infection. In this study, we evaluated the use of topical CpG oligodeoxynucleotide (ODN) as adjuvant to generate skin T_RM in mice. Topical or s.c. CpG ODN adjuvant administration at the time of a s.c. Ag injection led to an accumulation of CD103^- CD8 T cells in the epidermis. However, only mice with CpG ODN administered topically had significant numbers of CD103⁺ Ag-specific CD8 T cells persisting in the local epidermal skin, enhanced circulating memory cells in the blood, and showed protection from intradermal challenge with melanoma cells. Generation of Ag-specific CD8 T cells was dependent on TLR9 expression on hematopoietic cells and partially dependent on receptor expression on stromal cells. Topical challenge of immunized mice at a distal site led to significant expansion of Ag-specific T cells in the blood and accumulation in the challenged skin. We demonstrate that local and systemic T cell memory can be generated with topical CpG ODN at the time of s.c. immunization, suggesting a new method of enhancing current vaccine formulations to generate tissue T_RM.

Compartmentalization of dendritic cell and T-cell interactions in the lymph node: Anatomy of T-cell fate decisions

Upon receiving cognate and co-stimulatory priming signals from antigen (Ag)-presenting dendritic cells (DCs) in secondary lymphoid tissues, naïve CD4+ T cells differentiate into distinct effector and memory populations. These alternate cell fate decisions, which ultimately control the T-cell functional attributes, are dictated by programming signals provided by Ag-bearing DCs and by other cells that are present in the microenvironment in which T-cell priming occurs. We know that DCs can be subdivided into multiple populations and that the various DC subsets exhibit differential capacities to initiate development of the different CD4+ T-helper populations. What is less well understood is why different subanatomic regions of secondary lymphoid tissues are colonized by distinct populations of Ag-presenting DCs and how the location of these DCs influences the type of T-cell response that will be generated. Here we review how chemokine receptors and their ligands, which position allergen and nematode-activated DCs within different microdomains of secondary lymphoid tissues, contribute to the establishment of IL-4 committed follicular helper T and type 2 helper cell responses.

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.

IgG1+ B-cell immunity predates IgE responses in epicutaneous sensitization to foods

BACKGROUND

The generation of IgE-mediated food allergy in humans is silent and only diagnosed upon manifestation of clinical symptoms. While experimental models have been used to investigate some mechanisms of allergic sensitization, the generation of humoral immunity and memory remains to be elucidated. Here, we defined the evolution of allergen-specific B-cell responses during epicutaneous sensitization to foods.

METHODS

Wild-type and genetic knockout animals, and drug or antibody strategies for cell depletion and immunoglobulin signaling blockade were used to investigate epicutaneous sensitization and disease progression; we analyzed allergen-specific germinal centers and IgG1⁺ memory B cells by flow cytometry, evaluated humoral responses, and determined clinical reactivity (anaphylaxis).

RESULTS

Epicutaneous sensitization caused microscopic skin damage, inflammation, and recruitment of activated dendritic cells to the draining lymph nodes. This process generated allergen-specific IgG1⁺ germinal center B cells, serum IgG1, and anaphylaxis that was mediated by the alternative pathway. Whether we used peanut and/or ovalbumin from the egg white for sensitization, the allergen-specific IgG1⁺ memory compartment predominantly exhibited an immature, pro-germinal center phenotype (PDL-2^- CD80^- CD35⁺ CD73⁺ ). Subsequent subclinical exposures to the allergen induced IgE⁺ germinal center B cells, serum IgE, and likely activated the classical pathway of anaphylaxis.

CONCLUSIONS

Our data demonstrate that IgG1⁺ B-cell immunity against food allergens in epicutaneous sensitization precedes the generation of IgE responses. Therefore, the assessment of allergen-specific cellular and humoral IgG1⁺ immunity may help to identify individuals at risk of developing IgE-mediated food allergy and hence provide a window for therapeutic interventions.

Mechanisms by Which Atopic Dermatitis Predisposes to Food Allergy and the Atopic March

The Atopic march denotes the progression from atopic dermatitis (AD) to the development of other allergic disorders such as immunoglobulin (Ig) E-mediated food allergy, allergic rhinitis and asthma in later childhood. There is increasing evidence from prospective birth cohort studies that early-onset AD is a risk factor for other allergic diseases or is found in strong association with them. Animal studies now provide mechanistic insights into the pathways that may be responsible for triggering the progression from the skin barrier dysfunction seen in AD to epicutaneous sensitization, food allergy and allergic airway disorders. Recent large randomized controlled trials have demonstrated the efficacy of early interventions targeted at AD and food allergy prevention. These show great promise for research into future strategies aimed at prevention of the atopic march.

Nucleic Acid Sensing in Allergic Disorders

Recent advances indicate that there is crosstalk between allergic disorders and nucleic acid sensing. Triggers that activate inflammatory mechanisms via nucleic acid sensors affect both allergic phenotypes and anti-viral responses, depending on the timing and the order of exposure. Viral respiratory infections, such as those caused by the rhinovirus, influenza, and respiratory syncytial virus, are the most frequent cause of significant asthma exacerbations through effects mediated predominantly by TLR3. However, agonists of other nucleic acid sensors, such as TLR7/8 and TLR9 agonists, may inhibit allergic inflammation and reduce clinical manifestations of disease. The allergic state can predispose the immune system to both exaggerated responses to viral infections or protection from anti-viral inflammatory responses. T_H2 cytokines appear to alter the epithelium, leading to defective viral clearance or exaggerated responses to viral infections. However, a T_H2 skewed allergic response may be protective against a T_H1-dependent inflammatory anti-viral response. This review briefly introduces the receptors involved in nucleic acid sensing, addresses mechanisms by which nucleic acid sensing and allergic responses can counteract one another, and discusses the strategies in experimental settings, both in animal and human studies, to harness the nucleic acid sensing machinery for the intervention of allergic disorders.

PDL2+ CD11b+ dermal dendritic cells capture topical antigen through hair follicles to prime LAP+ Tregs

The skin immune system must discriminate between innocuous antigens and pathogens. Antigen applied topically using a Viaskin® patch elicits immune tolerance that can suppress colitis and food allergy. Here we show how topical antigen is acquired and presented by dendritic cells in the skin. Topical antigen is acquired by Langerhans cells (LC) and CD11b⁺ cDC2s but not cDC1s, and both  LCs and CD11b⁺ cDC2s reaching the lymph node can prime T cells and expand LAP⁺ Tregs. However, LCs are neither required nor sufficient for T cell priming, and have no role in tolerance induction. Conversely, IRF-4-dependent cDC2s are required for T cell priming. Acquisition of antigen in the dermis, delivery to the draining lymph node, and generation of tolerance are all absent in hairless mice. These results indicate an important function for hair follicle niche and CD11b⁺ cDC2s in antigen acquisition, and in generation of primary immune tolerance to topical antigens.

Antigen present and presented in the structures of the skin can result in immune responses that elicit tolerance, protective immunity or allergy, depending on the immunological context. Here the authors describe a key role for the hair follicle and CD11b⁺ dendritic cells in the priming of local antigenic tolerance.

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.

Myocarditis Elicits Dendritic Cell and Monocyte Infiltration in the Heart and Self-Antigen Presentation by Conventional Type 2 Dendritic Cells

Autoimmune myocarditis often leads to dilated cardiomyopathy (DCM). Although T cell reactivity to cardiac self-antigen is common in the disease, it is unknown which antigen presenting cell (APC) triggers autoimmunity. Experimental autoimmune myocarditis (EAM) was induced by immunizing mice with α-myosin loaded bone marrow APCs cultured in GM-CSF. APCs found in such cultures include conventional type 2 CD11b⁺ cDCs (GM-cDC2s) and monocyte-derived cells (GM-MCs). However, only α-myosin loaded GM-cDC2s could induce EAM. We also studied antigen presenting capacity of endogenous type 1 CD24⁺ cDCs (cDC1s), cDC2s, and MCs for α-myosin-specific TCR-transgenic TCR-M CD4⁺ T cells. After EAM induction, all cardiac APCs significantly increased and cDCs migrated to the heart-draining mediastinal lymph node (LN). Primarily cDC2s presented α-myosin to TCR-M cells and induced Th1/Th17 differentiation. Loss of IRF4 in Irf4^fl/fl.Cd11cCre mice reduced MHCII expression on GM-cDC2s in vitro and cDC2 migration in vivo. However, partly defective cDC2 functions in Irf4^fl/fl.Cd11cCre mice did not suppress EAM. MCs were the largest APC subset in the inflamed heart and produced pro-inflammatory cytokines. Targeting APC populations could be exploited in the design of new therapies for cardiac autoimmunity.

Advances in mechanisms of allergic disease in 2017

This review highlights advances in mechanisms of allergic disease, particularly type 2 innate lymphoid cells, T_H2 lymphocytes, B cells, dendritic cells, microbiome and barrier function, eosinophils, and mast cells. During the last year, considerable progress has been made in the further characterization of type 2 inflammation controlled by both adaptive (T_H2) and type 2 innate lymphoid effector cells. New pathways of lymphocyte activation, trafficking, and recruitment and effector cell mechanisms have been discovered. The plasticity of lymphocyte effector cell responses is another area in which major progress has been achieved. Accumulating evidence will influence both our understanding of allergic disease and our efforts for allergy prevention and treatment.

Antigen Uptake by Langerhans Cells Is Required for the Induction of Regulatory T Cells and the Acquisition of Tolerance During Epicutaneous Immunotherapy in OVA-Sensitized Mice

The skin is a major immunologic organ that may induce protection, sensitization or tolerance. Epicutaneous immunotherapy (EPIT) has been proposed as an attractive strategy to actively treat food allergy and has been shown to induce tolerance in sensitized mice through the induction of Foxp3⁺ regulatory T cells (Tregs), especially CD62L⁺ Tregs. Among immune cells in the skin, dendritic cells are key players in antigen-specific immune activation or regulation. The role of different populations of skin DCs in tolerance induction remains to be elucidated. Using OVA-sensitized BALB/c mice, we demonstrated that the application of a patch containing OVA-A647 to the skin resulted in allergen uptake by Langerhans cells (LCs) and CD11b⁺ dermal cDC2 and subsequent migration into skin draining lymph nodes. These 2 populations induced Foxp3 expression in CD4⁺ cells in vitro. Only LCs induced LAP⁺ cells and CD62L⁺ Tregs. Using Langerin-eGFP-DTR mice, we analyzed the role of LCs in the mechanisms of tolerance induction by EPIT in vivo. Following complete depletion of LCs, a dramatic decrease in the number of OVA⁺ DCs and OVA⁺ CD11b⁺ dermal cDC2 was observed in skin draining lymph nodes 48 h after epicutaneous application. Likewise, 2 weeks of EPIT in non-depleted mice induced Foxp3⁺ Tregs, especially CD62L⁺, and LAP⁺ Tregs in skin draining lymph nodes and spleen, whereas no induction of Tregs was observed in LC-depleted mice. Following 8 weeks of treatment, EPIT-treated mice showed significant protection against anaphylaxis accompanied by a significant increase of Foxp3⁺ Tregs, especially CD62L⁺ Tregs, which was not seen in the absence of LCs. In summary, although both LCs and CD11b⁺ dermal cDC2s could induce regulatory T cells, the absence of LCs during EPIT impaired treatment efficacy, indicating their crucial role in skin-induced tolerance.

The hygiene hypothesis: immunological mechanisms of airway tolerance

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Microbial and environmental signals set tonic activation status of barrier tissues.

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Signaling from barrier tissues licenses dendritic cells to induce T helper 2 cells.

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Pulmonary immune system in early-life prone to asthma development.

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Mechanistic understanding needed to translate epidemiological findings in therapies.

The hygiene hypothesis was initially proposed as an explanation for the alarming rise in allergy prevalence in the last century. The immunological idea behind this hypothesis was a lack of infections associated with a Western lifestyle and a consequential reduction in type 1 immune responses. It is now understood that the development of tolerance to allergens depends on microbial colonization and immunostimulatory environmental signals during early-life or passed on by the mother. These environmental cues are sensed and integrated by barrier epithelial cells of the lungs and possibly skin, which in turn instruct dendritic cells to regulate or impede adaptive T cell responses. Recent reports also implicate immunoregulatory macrophages as powerful suppressors of allergy by the microbiome. We propose that loss of adequate microbial stimulation due to a Western lifestyle may result in hypersensitive barrier tissues and the observed rise in type 2 allergic disease.

A bispecific antibody strategy to target multiple type 2 cytokines in asthma

BACKGROUND

Asthma is a chronic inflammatory airway disease in which innate and adaptive immune cells act together to cause eosinophilic inflammation, goblet cell metaplasia (GCM), and bronchial hyperreactivity (BHR). In clinical trials using biologicals against IL-4 receptor (IL-4R) α or IL-5, only a subset of patients with moderate-to-severe asthma responded favorably, suggesting that distinct pathophysiologic mechanisms are at play in subgroups of patients called endotypes. However, the effect of multiple cytokine blockade using bispecific antibodies has not been tested.

OBJECTIVE

We sought to target simultaneously the IL-4, IL-13, and IL-5 signaling pathways with a novel IL-4Rα/IL-5-bispecific antibody in a murine house dust mite (HDM) model of asthma.

METHODS

Two mAbs neutralizing IL-4Rα and IL-5 were generated by using a llama-based antibody platform. Their heavy and light chains were then cotransfected in mammalian cells, resulting in a heterogeneous antibody mixture from which the bispecific antibody was isolated by using a dual anti-idiotypic purification process. C57BL/6J mice were finally sensitized and challenged to HDM extracts and treated during challenge with the antibodies.

RESULTS

We successfully generated and characterized the monospecific and bispecific antibodies targeting IL-4Rα and IL-5. The monospecific antibodies could suppress eosinophilia, IgE synthesis, or both, whereas only the IL-4Rα/IL-5-bispecific antibody and the combination of monospecific antibodies additionally inhibited GCM and BHR.

CONCLUSION

Type 2 cytokines act synergistically to cause GCM and BHR in HDM-exposed mice. These preclinical results show the feasibility of generating bispecific antibodies that target multiple cytokine signaling pathways as superior inhibitors of asthma features, including the difficult-to-treat GCM.

Langerhans Cells: Sensing the Environment in Health and Disease

In the last few decades, our understanding of Langerhans cells (LCs) has drastically changed based on novel findings regarding the developmental origin and biological functions of these epidermis-specific resident immune cells. It has become clear that LCs not only exert pivotal roles in immune surveillance and homeostasis but also impact on pathology by either inducing tolerance or mediating inflammation. Their unique capabilities to self-renew within the epidermis, while also being able to migrate to lymph nodes in order to present antigen, place LCs in a key position to sample the local environment and decide on the appropriate cutaneous immune response. Exciting new data distinguishing LCs from Langerin⁺ dermal dendritic cells (DCs) on a functional and ontogenic level reveal crucial roles for LCs in trauma and various skin pathologies, which will be thoroughly discussed here. However, despite rapid progress in the field, the exact role of LCs during immune responses has not been completely elucidated. This review focuses on what mouse models that have been developed in order to enable the study of murine LCs and other Langerin-expressing DCs have taught us about LC development and function.

Co-Activation of Glucocorticoid Receptor and Peroxisome Proliferator-Activated Receptor-γ in Murine Skin Prevents Worsening of Atopic March

Children with atopic dermatitis show an increased risk to develop asthma later in life, a phenomenon referred to as "atopic march," which emphasizes the need for secondary prevention therapies. This study aimed to investigate whether relief of skin inflammation by glucocorticoids and peroxisome proliferator-activated receptor agonists might influence the subsequent development of asthma in a murine model for the atopic march in which mice were repeatedly exposed to house dust mite via the skin, followed by exposure to house dust mite in lungs. To abrogate atopic dermatitis, mice received topical treatment with glucocorticoid receptor/peroxisome proliferator-activated receptor-γ agonists. Nuclear receptor ligand effects were assessed on primary keratinocytes and dendritic cells, as central players in skin inflammation. Prior house dust mite-induced skin inflammation aggravates allergic airway inflammation and induces a mixed T helper type 2/T helper type 17 response in the lungs. Cutaneous combined activation of glucocorticoid receptor/peroxisome proliferator-activated receptor-γ reduced skin inflammation to a higher extent compared to single activation. Additive anti-inflammatory effects were more prominent in dendritic cells, as compared to keratinocytes. Alleviation of allergic skin inflammation by activation of glucocorticoid receptor/peroxisome proliferator-activated receptor-γ appeared insufficient to avoid the allergic immune response in the lungs, but efficiently reduced asthma severity by counteracting the Th17 response. Glucocorticoid receptor/peroxisome proliferator-activated receptor-γ co-activation represents a potent remedy against allergic skin inflammation and worsening of atopic march.

Ontogeny and function of murine epidermal Langerhans cells

Langerhans cells (LCs) are epidermis-resident antigen-presenting cells that share a common ontogeny with macrophages but function as dendritic cells (DCs). Their development, recruitment and retention in the epidermis is orchestrated by interactions with keratinocytes through multiple mechanisms. LC and dermal DC subsets often show functional redundancy, but LCs are required for specific types of adaptive immune responses when antigen is concentrated in the epidermis. This Review will focus on those developmental and functional properties that are unique to LCs.

T Cells in Allergic Asthma: Key Players Beyond the Th2 Pathway

Asthma is a common chronic lung disease that affects 300 million people worldwide. It causes the airways of the lungs to swell and narrow due to inflammation (swelling and excess mucus build-up in the airways) and airway constriction (tightening of the muscles surrounding the airways). Atopic asthma is the most common form of asthma, and is triggered by inhaled allergens that ultimately promote the activation of the Th2-like T cells and the development of Th2-mediated chronic inflammation. Different subsets of T cells, including T follicular helper cells, tissue-resident T, cells and Th2 effector cells, play different functions during allergic immune response. Dendritic cells (DCs) are known to play a central role in initiating allergic Th2-type immune responses and in the development of the T cell phenotype. However, this function depends on the complex interaction with other cells of the immune system and determines whether the response to environmental allergens will be one of tolerance or allergic inflammation. This review discusses cell interactions leading to the initiation and maintenance of allergic Th2-type immune responses, particularly those associated with allergic asthma.

The immunology of the allergy epidemic and the hygiene hypothesis

The immunology of the hygiene hypothesis of allergy is complex and involves the loss of cellular and humoral immunoregulatory pathways as a result of the adoption of a Western lifestyle and the disappearance of chronic infectious diseases. The influence of diet and reduced microbiome diversity now forms the foundation of scientific thinking on how the allergy epidemic occurred, although clear mechanistic insights into the process in humans are still lacking. Here we propose that barrier epithelial cells are heavily influenced by environmental factors and by microbiome-derived danger signals and metabolites, and thus act as important rheostats for immunoregulation, particularly during early postnatal development. Preventive strategies based on this new knowledge could exploit the diversity of the microbial world and the way humans react to it, and possibly restore old symbiotic relationships that have been lost in recent times, without causing disease or requiring a return to an unhygienic life style.

Dendritic Cell Subsets in Asthma: Impaired Tolerance or Exaggerated Inflammation?

Asthma is a prevalent chronic heterogeneous inflammatory disease of the airways, leading to reversible airway obstruction, in which various inflammatory responses can be observed. Mild to moderate asthma patients often present with a Th2-mediated eosinophilic inflammation whereas in severe asthma patients, a Th17-associated neutrophilic or combined Th2 and Th17-mediated eosinophilic/neutrophilic inflammation is observed. The differentiation of these effector Th2 and Th17-cells is induced by allergen-exposed dendritic cells (DCs) that migrate toward the lung draining lymph node. The DC lineage comprises conventional DCs (cDCs) and plasmacytoid DCs (pDCs), of which the cDC lineage consists of type 1 cDCs (cDC1s) and cDC2s. During inflammation, also monocytes can differentiate into so-called monocyte-derived DCs (moDCs). These DC subsets differ both in ontogeny, localization, and in their functional properties. New identification tools and the availability of transgenic mice targeting specific DC subsets enable the investigation of how these different DC subsets contribute to or suppress asthma pathogenesis. In this review, we will discuss mechanisms used by different DC subsets to elicit or hamper the pathogenesis of both Th2-mediated eosinophilic asthma and more severe Th17-mediated neutrophilic inflammation.