Cutaneous immunosurveillance and regulation of inflammation by group 2 innate lymphoid cells

The microbiota: a crucial mediator in gut homeostasis and colonization resistance

The gut microbiota is a complex and diverse community of microorganisms that colonizes the human gastrointestinal tract and influences various aspects of human health. These microbes are closely related to enteric infections. As a foreign entity for the host, commensal microbiota is restricted and regulated by the barrier and immune system in the gut and contributes to gut homeostasis. Commensals also effectively resist the colonization of pathogens and the overgrowth of indigenous pathobionts by utilizing a variety of mechanisms, while pathogens have developed strategies to subvert colonization resistance. Dysbiosis of the microbial community can lead to enteric infections. The microbiota acts as a pivotal mediator in establishing a harmonious mutualistic symbiosis with the host and shielding the host against pathogens. This review aims to provide a comprehensive overview of the mechanisms underlying host-microbiome and microbiome-pathogen interactions, highlighting the multi-faceted roles of the gut microbiota in preventing enteric infections. We also discuss the applications of manipulating the microbiota to treat infectious diseases in the gut.

Type 2 Innate Lymphoid Cells and Skin Fibrosis in a Murine Model of Atopic Dermatitis-Like Skin Inflammation

BACKGROUND

Atopic dermatitis (AD) is a chronic relapsing inflammatory skin disease. Although murine studies have demonstrated that type 2 innate lymphoid cells (ILCs) mediate type 2 skin inflammation, their role in skin fibrosis in AD remains unclear. This study investigated whether type 2 ILCs are involved in skin fibrosis using an AD-like murine model.

METHODS

C57BL/6 mice were treated epicutaneously with Aspergillus fumigatus (Af) for 5 consecutive days per week for 5 weeks to induce skin fibrosis. Mature lymphocyte deficient Rag1-/- mice were also used to investigate the role of type 2 ILCs in skin fibrosis.

RESULTS

The clinical score and transepidermal water loss (TEWL) were significantly higher in the AD group than in the control group. The AD group also showed significantly increased epidermal and dermal thicknesses and significantly higher numbers of eosinophils, neutrophils, mast cells, and lymphocytes in the lesional skin than the control group. The lesional skin of the AD group showed increased stain of collagen and significantly higher levels of collagen than the control group (10.4 ± 2.2 µg/mg vs. 1.6 ± 0.1 µg/mg, P < 0.05). The AD group showed significantly higher populations of type 2 ILCs in the lesional skin compared to the control group (0.08 ± 0.01% vs. 0.03 ± 0.01%, P < 0.05). These findings were also similar with the AD group of Rag1-/- mice compared to their control group. Depletion of type 2 ILCs with anti-CD90.2 monoclonal antibodies significantly improved clinical symptom score, TEWL, and infiltration of inflammatory cells, and significantly decreased levels of collagen were observed in the AD group of Rag1-/- mice (1.6 ± 0.0 μg/mg vs. 4.5 ± 0.3 μg/mg, P < 0.001).

CONCLUSION

In the Af-induced AD-like murine model, type 2 ILCs were elevated, with increased levels of collagen. Additionally, removal of type 2 ILCs resulted in decreased collagen levels and improved AD-like pathological findings. These findings suggest that type 2 ILCs play a role in the mechanism of skin fibrosis in AD.

Cellular and molecular mechanisms of skin wound healing

Wound healing is a complex process that involves the coordinated actions of many different tissues and cell lineages. It requires tight orchestration of cell migration, proliferation, matrix deposition and remodelling, alongside inflammation and angiogenesis. Whereas small skin wounds heal in days, larger injuries resulting from trauma, acute illness or major surgery can take several weeks to heal, generally leaving behind a fibrotic scar that can impact tissue function. Development of therapeutics to prevent scarring and successfully repair chronic wounds requires a fuller knowledge of the cellular and molecular mechanisms driving wound healing. In this Review, we discuss the current understanding of the different phases of wound healing, from clot formation through re-epithelialization, angiogenesis and subsequent scar deposition. We highlight the contribution of different cell types to skin repair, with emphasis on how both innate and adaptive immune cells in the wound inflammatory response influence classically studied wound cell lineages, including keratinocytes, fibroblasts and endothelial cells, but also some of the less-studied cell lineages such as adipocytes, melanocytes and cutaneous nerves. Finally, we discuss newer approaches and research directions that have the potential to further our understanding of the mechanisms underpinning tissue repair.

Crosstalk between gut microbiota and host immune system and its response to traumatic injury

Millions of microorganisms make up the complex microbial ecosystem found in the human gut. The immune system's interaction with the gut microbiota is essential for preventing inflammation and maintaining intestinal homeostasis. Numerous metabolic products that can cross-talk between immune cells and the gut epithelium are metabolized by the gut microbiota. Traumatic injury elicits a great and multifaceted immune response in the minutes after the initial offense, containing simultaneous pro- and anti-inflammatory responses. The development of innovative therapies that improve patient outcomes depends on the gut microbiota and immunological responses to trauma. The altered makeup of gut microbes, or gut dysbiosis, can also dysregulate immunological responses, resulting in inflammation. Major human diseases may become more common as a result of chronic dysbiosis and the translocation of bacteria and the products of their metabolism beyond the mucosal barrier. In this review, we briefly summarize the interactions between the gut microbiota and the immune system and human disease and their therapeutic probiotic formulations. We also discuss the immune response to traumatic injury.

Cellular metabolism and hypoxia interfacing with allergic diseases

Allergic diseases display significant heterogeneity in their pathogenesis. Understanding the influencing factors, pathogenesis, and advancing new treatments for allergic diseases is becoming more and more vital as currently, prevalence continues to rise, and mechanisms of allergic diseases are not fully understood. The upregulation of the hypoxia response is linked to an elevated infiltration of activated inflammatory cells, accompanied by elevated metabolic requirements. An enhanced hypoxia response may potentially contribute to inflammation, remodeling, and the onset of allergic diseases. It has become increasingly clear that the process underlying immune and stromal cell activation during allergic sensitization requires well-tuned and dynamic changes in cellular metabolism. The purpose of this review is to examine current perspectives regarding metabolic dysfunction in allergic diseases. In the past decade, new technological platforms such as "omic" techniques have been applied, allowing for the identification of different biomarkers in multiple models ranging from altered lipid species content, increased nutrient transporters, and altered serum amino acids in various allergic diseases. Better understanding, recognition, and integration of these alterations would increase our knowledge of pathogenesis and potentially actuate a novel repertoire of targeted treatment approaches that regulate immune metabolic pathways.

Tonic type 2 immunity is a critical tissue checkpoint controlling autoimmunity in the skin

Immunoregulatory mechanisms established in the lymphoid organs are vital for preventing autoimmunity. However, the presence of similar mechanisms in non-lymphoid tissues remains unclear. Through transcriptomic and lipidomic analyses, we find a negative association between psoriasis and fatty acid metabolism, as well as Th2 signature. Homeostatic expression of liver X receptor (LXR) and peroxisome proliferator-activated receptor gamma (PPARγ) is essential for maintaining fatty acid metabolism and for conferring resistance to psoriasis in mice. Perturbation of signal transducer and activator of transcription 6 (STAT6) diminishes the homeostatic levels of LXR and PPARγ. Furthermore, mice lacking STAT6, interleukin 4 receptor alpha (IL-4Rα), or IL-13, but not IL-4, exhibit increased susceptibility to psoriasis. Under steady state, innate lymphoid cells (ILCs) are the primary producers of IL-13. In human skin, inhibiting tonic type 2 immunity exacerbates psoriasis-like inflammation and IL-17A, while activating LXR or PPARγ inhibits them. Hence, we propose that tonic type 2 immunity, driven by IL-13-producing ILCs, represents a crucial tissue checkpoint that represses autoimmunity and maintains lipid homeostasis in the skin.

Decreased GATA3 levels cause changed mouse cutaneous innate lymphoid cell fate, facilitating hair follicle recycling

In mice, skin-resident type 2 innate lymphoid cells (ILC2s) exhibit some ILC3-like characteristics. However, the underlying mechanism remains elusive. Here, we observed lower expression of the ILC2 master regulator GATA3 specifically in cutaneous ILC2s (cILC2s) compared with canonical ILC2s, in line with its functionally divergent role in transcriptional control in cILC2s. Decreased levels of GATA3 enabled the expansion of RORγt fate-mapped (RORγtfm+) cILC2s after postnatal days, displaying certain similarities to ILC3s. Single-cell trajectory analysis showed a sequential promotion of the RORγtfm+ cILC2 divergency by RORγt and GATA3. Notably, during hair follicle recycling, these RORγtfm+ cILC2s accumulated around the hair follicle dermal papilla (DP) region to facilitate the process. Mechanistically, we found that GATA3-mediated integrin α3β1 upregulation on RORγtfm+ cILC2s was required for their positioning around the DP. Overall, our study demonstrates a distinct regulatory role of GATA3 in cILC2s, particularly in promoting the divergence of RORγtfm+ cILC2s to facilitate hair follicle recycling.

The Roles of Innate Immune Cells in Atopic Dermatitis

BACKGROUND

Atopic dermatitis (AD) is a chronic inflammatory skin condition characterized by recurrent eczematous lesions and severe pruritus. The economic burden and time penalty caused by the relapse of AD reduce patients' life quality.

SUMMARY

AD has complex pathogenesis, including genetic disorders, epidermal barrier dysfunction, abnormal immune responses, microbial dysbiosis of the skin, and environmental factors. Recently, the role of innate immune cells in AD has attracted considerable attention. This review highlighted recent findings on innate immune cells in the onset and progression of AD.

KEY MESSAGES

Innate immune cells play essential roles in the pathogenesis of AD and enough attention should be given for treating AD from the perspective of innate immunity in clinics.

Tissue microenvironment induces tissue specificity of ILC2

Type 2 innate lymphoid cells were found to be members of the innate immune cell family, which is involved in innate and adaptive immunity to resist the invasion of foreign antigens and induce allergic reactions caused by allergens. The advancement of ILC2 research has pointed out that ILC2s have a high degree of diversity, challenging the notion of their homogeneity as a cellular population. An increasing number of studies indicate that ILC2 is a cell population with tissue specificity which can be induced by the tissue microenvironment. In addition, crosstalk between tissues can change ILC2 functions of migration and activation. Here, we emphasize that ILC2 undergoes adaptive changes under the regulation of the tissue microenvironment and distant tissues, thereby coordinating the organization's operation. In addition, ILC2 alterations induced by the tissue microenvironment are not limited to the ILC2 cell population, and ILC2 can also transdifferentiate into another class of ILC cell population (ILC1 or ILC3). In this review, we summarized the tissue-specific effects of ILC2 by tissue microenvironment and focused on the function of ILC2 in inter-tissue crosstalk. Lastly, we discussed the transdifferentiations of ILC2 caused by the abnormal change in tissue environment.

The protective roles of integrin α4β7 and Amphiregulin-expressing innate lymphoid cells in lupus nephritis

Type 2 innate lymphoid cells (ILC2s) have emerged as key regulators of the immune response in renal inflammatory diseases such as lupus nephritis. However, the mechanisms underlying ILC2 adhesion and migration in the kidney remain poorly understood. Here, we revealed the critical role of integrin α4β7 in mediating renal ILC2 adhesion and function. We found that integrin α4β7 enables the retention of ILC2s in the kidney by binding to VCAM-1, E-cadherin, or fibronectin on structural cells. Moreover, integrin α4β7 knockdown reduced the production of the reparative cytokine amphiregulin (Areg) by ILC2s. In lupus nephritis, TLR7/9 signaling within the kidney microenvironment downregulates integrin α4β7 expression, leading to decreased Areg production and promoting the egress of ILC2s. Notably, IL-33 treatment upregulated integrin α4β7 and Areg expression in ILC2s, thereby enhancing survival and reducing inflammation in lupus nephritis. Together, these findings highlight the potential of targeting ILC2 adhesion as a therapeutic strategy for autoimmune kidney diseases.

IL-26 Potentiates Type 2 Skin Inflammation in the Presence of IL-1β

Atopic dermatitis (AD) is a debilitating inflammatory skin disorder. Biologics targeting the IL-4/IL-13 axis are effective in AD, but there is still a large proportion of patients who do not respond to IL-4R blockade. Further exploration of potentially pathogenic T-cell-derived cytokines in AD may lead to new effective treatments. This study aimed to investigate the downstream effects of IL-26 on skin in the context of type 2 skin inflammation. We found that IL-26 alone exhibited limited inflammatory activity in the skin. However, in the presence of IL-1β, IL-26 potentiated the secretion of TSLP, CXCL1, and CCL20 from human epidermis through Jak/signal transducer and activator of transcription signaling. Moreover, in an in vivo AD-like skin inflammation model, IL-26 exacerbated skin pathology and locally increased type 2 cytokines, most notably of IL13 in skin T helper cells. Neutralization of IL-1β abrogated IL-26-mediated effects, indicating that the presence of IL-1β is required for full IL-26 downstream action in vivo. These findings suggest that the presence of IL-1β enables IL-26 to be a key amplifier of inflammation in the skin. As such, IL-26 may contribute to the development and pathogenesis of inflammatory skin disorders such as AD.

The Role of the Microbiota in the Pathogenesis and Treatment of Atopic Dermatitis-A Literature Review

Atopic dermatitis (AD) is a chronic inflammatory skin condition with a high prevalence worldwide. AD pathogenesis is complex and consists of immune system dysregulation and impaired skin barrier, influenced by genetic and environmental factors. The purpose of the review is to show the complex interplay between atopic dermatitis and the microbiota. Human microbiota plays an important role in AD pathogenesis and the course of the disease. Dysbiosis is an important factor contributing to the development of atopic diseases, including atopic dermatitis. The gut microbiota can influence the composition of the skin microbiota, strengthening the skin barrier and regulating the immune response via the involvement of bacterial metabolites, particularly short-chain fatty acids, in signaling pathways of the gut-skin axis. AD can be modulated by antibiotic intake, dietary adjustments, hygiene, and living conditions. One of the promising strategies for modulating the course of AD is probiotics. This review offers a summary of how the microbiota influences the development and treatment of AD, highlighting aspects that warrant additional investigation.

RAG suppresses group 2 innate lymphoid cells

Antigen specificity is the central trait distinguishing adaptive from innate immune function. Assembly of antigen-specific T cell and B cell receptors occurs through V(D)J recombination mediated by the Recombinase Activating Gene endonucleases RAG1 and RAG2 (collectively called RAG). In the absence of RAG, mature T and B cells do not develop and thus RAG is critically associated with adaptive immune function. In addition to adaptive T helper 2 (Th2) cells, group 2 innate lymphoid cells (ILC2s) contribute to type 2 immune responses by producing cytokines like Interleukin-5 (IL-5) and IL-13. Although it has been reported that RAG expression modulates the function of innate natural killer (NK) cells, whether other innate immune cells such as ILC2s are affected by RAG remains unclear. We find that in RAG-deficient mice, ILC2 populations expand and produce increased IL-5 and IL-13 at steady state and contribute to increased inflammation in atopic dermatitis (AD)-like disease. Further, we show that RAG modulates ILC2 function in a cell-intrinsic manner independent of the absence or presence of adaptive T and B lymphocytes. Lastly, employing multiomic single cell analyses of RAG1 lineage-traced cells, we identify key transcriptional and epigenomic ILC2 functional programs that are suppressed by a history of RAG expression. Collectively, our data reveal a novel role for RAG in modulating innate type 2 immunity through suppression of ILC2s.

Interleukin-2-induced skin inflammation

Recombinant human IL-2 has been used to treat inflammatory diseases and cancer; however, side effects like skin rashes limit the use of this therapeutic. To identify key molecules and cells inducing this side effect, we characterized IL-2-induced cutaneous immune reactions and investigated the relevance of CD25 (IL-2 receptor α) in the process. We injected IL-2 intradermally into WT mice and observed increases in immune cell subsets in the skin with preferential increases in frequencies of IL-4- and IL-13-producing group 2 innate lymphoid cells and IL-17-producing dermal γδ T cells. This overall led to a shift toward type 2/type 17 immune responses. In addition, using a novel topical genetic deletion approach, we reduced CD25 on skin, specifically on all cutaneous cells, and found that IL-2-dependent effects were reduced, hinting that CD25 - at least partly - induces this skin inflammation. Reduction of CD25 specifically on skin Tregs further augmented IL-2-induced immune cell infiltration, hinting that CD25 on skin Tregs is crucial to restrain IL-2-induced inflammation. Overall, our data support that innate lymphoid immune cells are key cells inducing side effects during IL-2 therapy and underline the significance of CD25 in this process.

Communication between Mast Cells and Group 2 Innate Lymphoid Cells in the Skin

The skin is a dynamic organ with a complex immune network critical for maintaining balance and defending against various pathogens. Different types of cells in the skin, such as mast cells (MCs) and group 2 innate lymphoid cells (ILC2s), contribute to immune regulation and play essential roles in the early immune response to various triggers, including allergens. It is beneficial to dissect cell-to-cell interactions in the skin to elucidate the mechanisms underlying skin immunity. The current manuscript concentrates explicitly on the communication pathways between MCs and ILC2s in the skin, highlighting their ability to regulate immune responses, inflammation, and tissue repair. Furthermore, it discusses how the interactions between MCs and ILC2s play a crucial role in various skin conditions, such as autoimmune diseases, dermatological disorders, and allergic reactions. Understanding the complex interactions between MCs and ILC2s in different skin conditions is crucial to developing targeted treatments for related disorders. The discovery of shared pathways could pave the way for novel therapeutic interventions to restore immunological balance in diseased skin tissues.

Two regulatory T cell populations in the visceral adipose tissue shape systemic metabolism

Visceral adipose tissue (VAT) is an energy store and endocrine organ critical for metabolic homeostasis. Regulatory T (Treg) cells restrain inflammation to preserve VAT homeostasis and glucose tolerance. Here, we show that the VAT harbors two distinct Treg cell populations: prototypical serum stimulation 2-positive (ST2+) Treg cells that are enriched in males and a previously uncharacterized population of C-X-C motif chemokine receptor 3-positive (CXCR3+) Treg cells that are enriched in females. We show that the transcription factors GATA-binding protein 3 and peroxisome proliferator-activated receptor-γ, together with the cytokine interleukin-33, promote the differentiation of ST2+ VAT Treg cells but repress CXCR3+ Treg cells. Conversely, the differentiation of CXCR3+ Treg cells is mediated by the cytokine interferon-γ and the transcription factor T-bet, which also antagonize ST2+ Treg cells. Finally, we demonstrate that ST2+ Treg cells preserve glucose homeostasis, whereas CXCR3+ Treg cells restrain inflammation in lean VAT and prevent glucose intolerance under high-fat diet conditions. Overall, this study defines two molecularly and developmentally distinct VAT Treg cell types with unique context- and sex-specific functions.

Staphylococcus aureus exacerbates dermal IL-33/ILC2 axis activation through evoking RIPK3/MLKL-mediated necroptosis of dry skin

Atopic dermatitis (AD) is a persistent skin disease typified by symptoms of dry skin and recurrent eczema. Patients with AD are at heightened risk for Staphylococcus aureus infection. Group 2 innate lymphoid cells (ILC2s) are mainly activated by epithelial cell-derived cytokines IL-33 and involved in the pathogenesis of AD. However, little is known about the effect of skin delipidization on the epithelial cell-derived cytokines and dermal ILC2s in AD. In our study, we investigated the mechanism by which S. aureus infection modulates and exacerbates the pathogenesis of dry skin, leading to type 2 inflammation in the context of innate immunity. In vivo, we found that S. aureus infection aggravated delipidization-induced dermal IL-33 release and dermal ILC2 accumulation, which exacerbated skin inflammation. We also noticed that Il33fl/fl K14cre mice and Tlr2-/- mice exhibited attenuated skin inflammation. In vitro, treatment with necroptosis inhibitors reduced IL-33 release from S. aureus-infected keratinocytes. Mechanistically, we observed an increase in the necroptosis-associated kinases, MLKL and RIPK3, in S. aureus-infected mice, indicating that IL-33 release was associated with necroptotic cell death responses. Our results reveal that S. aureus infection-elicited keratinocyte necroptosis contributes to IL-33-mediated type 2 inflammation, which exacerbates the pathogenesis of dry skin.

Intravital microscopy visualizes innate immune crosstalk and function in tissue microenvironment

Significant advances have been made in the field of intravital microscopy (IVM) on myeloid cells due to the growing number of validated fluorescent probes and reporter mice. IVM provides a visualization platform to directly observe cell behavior and deepen our understanding of cellular dynamics, heterogeneity, plasticity, and cell-cell communication in native tissue environments. This review outlines the current studies on the dynamic interaction and function of innate immune cells with a focus on those that are studied with IVM and covers the advances in data analysis with emerging artificial intelligence-based algorithms. Finally, the prospects of IVM on innate immune cells are discussed.

ILC2-mediated immune crosstalk in chronic (vascular) inflammation

Crosstalk between innate and adaptive immunity is pivotal for an efficient immune response and to maintain immune homeostasis under steady state conditions. As part of the innate immune system, type 2 innate lymphoid cells (ILC2s) have emerged as new important regulators of tissue homeostasis and repair by fine-tuning innate-adaptive immune cell crosstalk. ILC2s mediate either pro- or anti-inflammatory immune responses in a context dependent manner. Inflammation has proven to be a key driver of atherosclerosis, resembling the key underlying pathophysiology of cardiovascular disease (CVD). Notably, numerous studies point towards an atheroprotective role of ILC2s e.g., by mediating secretion of type-II cytokines (IL-5, IL-13, IL-9). Boosting these protective responses may be suitable for promising future therapy, although these protective cues are currently incompletely understood. Additionally, little is known about the mechanisms by which chemokine/chemokine receptor signaling shapes ILC2 functions in vascular inflammation and atherosclerosis. Hence, this review will focus on the latest findings regarding the protective and chemokine/chemokine receptor guided interplay between ILC2s and other immune cells like T and B cells, dendritic cells and macrophages in atherosclerosis. Further, we will elaborate on potential therapeutic implications which result or could be distilled from the dialogue of ILC2s with cells of the immune system in cardiovascular diseases.

Dermis resident macrophages orchestrate localized ILC2 eosinophil circuitries to promote non-healing cutaneous leishmaniasis

Tissue-resident macrophages are critical for tissue homeostasis and repair. We previously showed that dermis-resident macrophages produce CCL24 which mediates their interaction with IL-4+ eosinophils, required to maintain their M2-like properties in the TH1 environment of the Leishmania major infected skin. Here, we show that thymic stromal lymphopoietin (TSLP) and IL-5+ type 2 innate lymphoid cells are also required to maintain dermis-resident macrophages and promote infection. Single cell RNA sequencing reveals the dermis-resident macrophages as the sole source of TSLP and CCL24. Generation of Ccl24-cre mice permits specific labeling of dermis-resident macrophages and interstitial macrophages from other organs. Selective ablation of TSLP in dermis-resident macrophages reduces the numbers of IL-5+ type 2 innate lymphoid cells, eosinophils and dermis-resident macrophages, and ameliorates infection. Our findings demonstrate that dermis-resident macrophages are self-maintained as a replicative niche for L. major by orchestrating localized type 2 circuitries with type 2 innate lymphoid cells and eosinophils.

Immunological characteristics of CD103+CD8+ Tc cells in the liver of C57BL/6 mouse infected with plasmodium NSM

CD103 is an important marker of tissue-resident memory T cells (TRM) which play important roles in fighting against infection. However, the immunological characteristics of CD103+ T cells are not thoroughly elucidated in the liver of mouse infected with Plasmodium. Six- to eight-week-old C57BL/6 mice were infected with Plasmodium yoelii nigeriensis NSM. Mice were sacrificed on 12-16 days after infection and the livers were picked out. Sections of the livers were stained, and serum aspartate aminotransferase (AST) and alanine transaminase (ALT) levels were measured. Moreover, lymphocytes in the liver were isolated, and the expression of CD103 was determined by using qPCR. The percentage of CD103 on different immune cell populations was dynamically observed by using flow cytometry (FCM). In addition, the phenotype and cytokine production characteristics of CD103+CD8+ Tc cell were analyzed by using flow cytometry, respectively. Erythrocyte stage plasmodium infection could result in severe hepatic damage, a widespread inflammatory response and the decrease of CD103 expression on hepatic immune cells. Only CD8+ Tc and γδT cells expressed higher levels of CD103 in the uninfected state.CD103 expression in CD8+ Tc cells significantly decreased after infection. Compared to that of CD103- CD8+ Tc cells, CD103+ CD8+ Tc cells from the infected mice expressed lower level of CD69, higher level of CD62L, and secreted more IL-4, IL-10, IL-17, and secreted less IFN-γ. CD103+CD8+ Tc cells might mediate the hepatic immune response by secreting IL-4, IL-10, and IL-17 except IFN-γ in the mice infected with the erythrocytic phase plasmodium, which could be involved in the pathogenesis of severe liver damage resulted from the erythrocytic phase plasmodium yoelii nigeriensis NSM infection.

Understanding food allergy through neuroimmune interactions in the gastrointestinal tract

Food allergies are adverse immune reactions to food proteins in the absence of oral tolerance, and the incidence of allergies to food, including peanut, cow's milk, and shellfish, has been increasing globally. Although advancements have been made toward understanding the contributions of the type 2 immune response to allergic sensitization, crosstalk between these immune cells and neurons of the enteric nervous system is an area of emerging interest in the pathophysiology of food allergy, given the close proximity of neuronal cells of the enteric nervous system and type 2 effector cells, including eosinophils and mast cells. At mucosal sites, such as the gastrointestinal tract, neuroimmune interactions contribute to the sensing and response to danger signals from the epithelial barrier. This communication is bidirectional, as immune cells express receptors for neuropeptides and transmitters, and neurons express cytokine receptors, allowing for the detection of and response to inflammatory insults. In addition, it seems that neuromodulation of immune cells including mast cells, eosinophils, and innate lymphoid cells is critical for amplification of the type 2 allergic immune response. As such, neuroimmune interactions may be critical targets for future food allergy therapies. This review evaluates the contributions of local enteric neuroimmune interactions to the underlying immune response in food allergy and discusses considerations for future investigations into targeting neuroimmune pathways for treatment of food allergies.

Innate lymphoid cells: a new key player in atopic dermatitis

Atopic dermatitis (AD) is a common allergic inflammatory skin condition mainly caused by gene variants, immune disorders, and environmental risk factors. The T helper (Th) 2 immune response mediated by interleukin (IL)-4/13 is generally believed to be central in the pathogenesis of AD. It has been shown that innate lymphoid cells (ILCs) play a major effector cell role in the immune response in tissue homeostasis and inflammation and fascinating details about the interaction between innate and adaptive immunity. Changes in ILCs may contribute to the onset and progression of AD, and ILC2s especially have gained much attention. However, the role of ILCs in AD still needs to be further elucidated. This review summarizes the role of ILCs in skin homeostasis and highlights the signaling pathways in which ILCs may be involved in AD, thus providing valuable insights into the behavior of ILCs in skin homeostasis and inflammation, as well as new approaches to treating AD.

Heterogeneous Tfh cell populations that develop during enteric helminth infection predict the quality of type 2 protective response

T follicular helper (Tfh) cells are an important component of germinal center (GC)-mediated humoral immunity. Yet, how a chronic type 1 versus protective type 2 helminth infection modulates Tfh-GC responses remains poorly understood. Here, we employ the helminth Trichuris muris model and demonstrate that Tfh cell phenotypes and GC are differentially regulated in acute versus chronic infection. The latter failed to induce Tfh-GC B cell responses, with Tfh cells expressing Τ-bet and interferon-γ. In contrast, interleukin-4-producing Tfh cells dominate responses to an acute, resolving infection. Heightened expression and increased chromatin accessibility of T helper (Th)1- and Th2 cell-associated genes are observed in chronic and acute induced Tfh cells, respectively. Blockade of the Th1 cell response by T-cell-intrinsic T-bet deletion promoted Tfh cell expansion during chronic infection, pointing to a correlation between a robust Tfh cell response and protective immunity to parasites. Finally, blockade of Tfh-GC interactions impaired type 2 immunity, revealing the critical protective role of GC-dependent Th2-like Tfh cell responses during acute infection. Collectively, these results provide new insights into the protective roles of Tfh-GC responses and identify distinct transcriptional and epigenetic features of Tfh cells that emerge during resolving or chronic T. muris infection.

Bcl-2 supports survival and metabolic fitness of quiescent tissue-resident ILC3

Group 3 innate lymphoid cells (ILC3) are potent effector cells with critical roles in enforcing immunity, barrier integrity and tissue homeostasis along the gastrointestinal tract. ILC3 are considered primarily tissue-resident cells, seeding the gastrointestinal tract during embryonic stages and early life. However, the mechanisms through which mature ILC3 are maintained within adult tissues are poorly understood. Here, we report that lymphoid tissue-inducer-like (LTi-like) ILC3 exhibit minimal turnover in the healthy adult intestinal tract, persist for extended periods of time, and display a quiescent phenotype. Strikingly, during enteric bacterial infection LTi-like ILC3 also exhibit negligible hematopoietic replenishment and remain non-proliferative, despite robustly producing cytokines. Survival of LTi-like ILC3 was found to be dependent upon the balance between the metabolic activity required to drive effector function and anti-apoptotic programs. Notably, the pro-survival protein B-cell lymphoma-2 (Bcl-2) was required for the survival of LTi-like ILC3 ex vivo but was rendered partially dispensable if mitochondrial respiration was inhibited. Together we demonstrate LTi-like ILC3 are a tissue-resident, quiescent population that persist independently of hematopoietic replenishment to survive within the intestinal microenvironment.

Foxp3 and Bcl6 deficiency synergistically induces spontaneous development of atopic dermatitis-like skin disease

Atopic dermatitis (AD) is a common chronic skin disease caused by immune dysfunction, specifically the hyperactivation of Th2 immunity. AD is a complex disease with multiple factors contributing to its development; however, the interaction between these factors is not fully understood. In this study, we demonstrated that the conditional deletion of both the forkhead box p3 (Foxp3) and B-cell lymphoma 6 (Bcl6) genes induced the spontaneous development of AD-like skin inflammation with hyperactivation of type 2 immunity, skin barrier dysfunction, and pruritus, which were not induced by the single deletion of each gene. Furthermore, the development of AD-like skin inflammation was largely dependent on IL-4/13 signaling but not on immunoglobulin E (IgE). Interestingly, we found that the loss of Bcl6 alone increased the expression of thymic stromal lymphopoietin (TSLP) and interleukin (IL)-33 in the skin, suggesting that Bcl6 controls Th2 responses by suppressing TSLP and IL-33 expression in epithelial cells. Our results suggest that Foxp3 and Bcl6 cooperatively suppress the pathogenesis of AD. Furthermore, these results revealed an unexpected role of Bcl6 in suppressing Th2 responses in the skin.

Diet-mediated constitutive induction of novel IL-4+ ILC2 cells maintains intestinal homeostasis in mice

Group 2 innate lymphoid cells (ILC2s) expressing IL-5 and IL-13 are localized at various mucosal tissues and play critical roles in the induction of type 2 inflammation, response to helminth infection, and tissue repair. Here, we reveal a unique ILC2 subset in the mouse intestine that constitutively expresses IL-4 together with GATA3, ST2, KLRG1, IL-17RB, and IL-5. In this subset, IL-4 expression is regulated by mechanisms similar to but distinct from those observed in T cells and is partly affected by IL-25 signaling. Although the absence of the microbiota had marginal effects, feeding mice with a vitamin B1-deficient diet compromised the number of intestinal IL-4+ ILC2s. The decrease in the number of IL-4+ ILC2s caused by the vitamin B1 deficiency was accompanied by a reduction in IL-25-producing tuft cells. Our findings reveal that dietary vitamin B1 plays a critical role in maintaining interaction between tuft cells and IL-4+ ILC2s, a previously uncharacterized immune cell population that may contribute to maintaining intestinal homeostasis.

Targeting Potential of Innate Lymphoid Cells in Melanoma and Other Cancers

Reinvigorating the killing function of tumor-infiltrating immune cells through the targeting of regulatory molecules expressed on lymphocytes has markedly improved the prognosis of cancer patients, particularly in melanoma. While initially thought to solely strengthen adaptive T lymphocyte anti-tumor activity, recent investigations suggest that other immune cell subsets, particularly tissue-resident innate lymphoid cells (ILCs), may benefit from immunotherapy treatment. Here, we describe the recent findings showing immune checkpoint expression on tissue-resident and tumor-infiltrating ILCs and how their effector function is modulated by checkpoint blockade-based therapies in cancer. We discuss the therapeutic potential of ILCs beyond the classical PD-1 and CTLA-4 regulatory molecules, exploring other possibilities to manipulate ILC effector function to further impede tumor growth and quench disease progression.

Slow integrin-dependent migration organizes networks of tissue-resident mast cells

Immune cell locomotion is associated with amoeboid migration, a flexible mode of movement, which depends on rapid cycles of actin polymerization and actomyosin contraction1. Many immune cells do not necessarily require integrins, the major family of adhesion receptors in mammals, to move productively through three-dimensional tissue spaces2,3. Instead, they can use alternative strategies to transmit their actin-driven forces to the substrate, explaining their migratory adaptation to changing external environments4-6. However, whether these generalized concepts apply to all immune cells is unclear. Here, we show that the movement of mast cells (immune cells with important roles during allergy and anaphylaxis) differs fundamentally from the widely applied paradigm of interstitial immune cell migration. We identify a crucial role for integrin-dependent adhesion in controlling mast cell movement and localization to anatomical niches rich in KIT ligand, the major mast cell growth and survival factor. Our findings show that substrate-dependent haptokinesis is an important mechanism for the tissue organization of resident immune cells.

Crosstalk between the gut microbiota and innate lymphoid cells in intestinal mucosal immunity

The human gastrointestinal mucosa is colonized by thousands of microorganisms, which participate in a variety of physiological functions. Intestinal dysbiosis is closely associated with the pathogenesis of several human diseases. Innate lymphoid cells (ILCs), which include NK cells, ILC1s, ILC2s, ILC3s and LTi cells, are a type of innate immune cells. They are enriched in the mucosal tissues of the body, and have recently received extensive attention. The gut microbiota and its metabolites play important roles in various intestinal mucosal diseases, such as inflammatory bowel disease (IBD), allergic disease, and cancer. Therefore, studies on ILCs and their interaction with the gut microbiota have great clinical significance owing to their potential for identifying pharmacotherapy targets for multiple related diseases. This review expounds on the progress in research on ILCs differentiation and development, the biological functions of the intestinal microbiota, and its interaction with ILCs in disease conditions in order to provide novel ideas for disease treatment in the future.

Immune cells and associated molecular markers in dermal fibrosis with focus on raised cutaneous scars

Raised dermal scars including hypertrophic, and keloid scars as well as scalp-associated fibrosing Folliculitis Keloidalis Nuchae (FKN) are a group of fibrotic raised dermal lesions that mostly occur following cutaneous injury. They are characterized by increased extracellular matrix (ECM) deposition, primarily excessive collagen type 1 production by hyperproliferative fibroblasts. The extent of ECM deposition is thought to be proportional to the severity of local skin inflammation leading to excessive fibrosis of the dermis. Due to a lack of suitable study models, therapy for raised dermal scars remains ill-defined. Immune cells and their associated markers have been strongly associated with dermal fibrosis. Therefore, modulation of the immune system and use of anti-inflammatory cytokines are of potential interest in the management of dermal fibrosis. In this review, we will discuss the importance of immune factors in the pathogenesis of raised dermal scarring. The aim here is to provide an up-to-date comprehensive review of the literature, from PubMed, Scopus, and other relevant search engines in order to describe the known immunological factors associated with raised dermal scarring. The importance of immune cells including mast cells, macrophages, lymphocytes, and relevant molecules such as cytokines, chemokines, and growth factors, antibodies, transcription factors, and other immune-associated molecules as well as tissue lymphoid aggregates identified within raised dermal scars will be presented. A growing body of evidence points to a shift from proinflammatory Th1 response to regulatory/anti-inflammatory Th2 response being associated with the development of fibrogenesis in raised dermal scarring. In summary, a better understanding of immune cells and associated molecular markers in dermal fibrosis will likely enable future development of potential immune-modulated therapeutic, diagnostic, and theranostic targets in raised dermal scarring.

IL-3 produced by T cells is crucial for basophil extravasation in hapten-induced allergic contact dermatitis

Basophils have been recognized as a characterized cellular player for Th2 immune responses implicated in allergic diseases, but the mechanisms responsible for basophil recruitment to allergic skin remain not well understood. Using a hapten fluorescein isothiocyanate (FITC)-induced allergic contact dermatitis (ACD) mouse model, we show that basophils in FITC-treated IL-3-knockout mice are defective in crossing the vascular endothelium to enter the inflamed skin. By generating mice in which IL-3 is selectively ablated in T cells, we further demonstrate that IL-3 produced by T cells mediates basophil extravasation. Moreover, basophils sorted from FITC-treated IL-3-knockout mice exhibit a decreased expression of integrins Itgam, Itgb2, Itga2b and Itgb7, which are potentially implicated in extravasation process. Interestingly, we observed that these basophils had a reduced expression of retinaldehyde dehydrogenase 1 family member A2 (Aldh1a2), an enzyme responsible for the production of retinoic acid (RA), and administration of all-trans RA restored partially the extravasation of basophils in IL-3-knockout mice. Finally, we validate that IL-3 induces the expression of ALDH1A2 in primary human basophils, and provide further evidence that IL-3 stimulation induces the expression of integrins particularly ITGB7 in an RA-dependent manner. Together, our data propose a model that IL-3 produced by T cells activates ALDH1A2 expression by basophils, leading to the production of RA, which subsequently induces the expression of integrins crucially implicated in basophil extravasation to inflamed ACD skin.

Potential Role of Innate Lymphoid Cells in the Pathogenesis and Treatment of Skin Diseases

Group 2 innate lymphoid cells (ILC2s) are lymphoid cells that are resident in mucosal tissues, especially the skin, which, once stimulated by epithelial cell-derived cytokines, release IL-5, IL-13, and IL-4, as the effectors of type 2 immune responses. This research aims to evaluate the role of ILC2s in the pathogenesis of skin diseases, with a particular focus on inflammatory cutaneous disorders, in order to also elucidate potential therapeutic perspectives. The research has been conducted in articles, excluding reviews and meta-analyses, on both animals and humans. The results showed that ILC2s play a crucial role in the pathogenesis of systemic skin manifestations, prognosis, and severity, while a potential antimelanoma role is emerging from the new research. Future perspectives could include the development of new antibodies targeting or stimulating ILC2 release. This evidence could add a new therapeutic approach to inflammatory cutaneous conditions, including allergic ones.

Adriforant is a functional antagonist of histamine receptor 4 and attenuates itch and skin inflammation in mice

BACKGROUND

Histamine has been postulated to play a role in atopic dermatitis via histamine receptor 4, mediating pruritic and inflammatory effects. The H4R antagonist adriforant (PF-3893787 or ZPL389) indicated clinical efficacy in a Ph2a study in atopic dermatitis. Preclinical investigations of adriforant had been scarce as experiments in transfectants with H4R from several species suggested partial agonism, not seen in human cells.

OBJECTIVE

During the Ph2b trial in AD, we performed experiments to understand the pharmacology of adriforant in primary murine cells and in vivo models. We assessed its effects on ERK phosphorylation and transcriptional changes in bone marrow-derived mast cells, histamine-dependent Ca²⁺ flux in neurons and histamine-induced itch response. In addition, its impact on MC903-induced skin inflammation was evaluated.

RESULTS

We show that, contrary to transfectants, adriforant is a competitive antagonist of the murine histamine receptor 4, antagonizes histamine-induced ERK phosphorylation, normalizes histamine-induced transcriptional changes in mast cells and reduces histamine-dependent Ca²⁺ flux in neurons. Administration to mice reduces acute histamine-induced itch response. In addition, adriforant ameliorates inflammation in the mouse MC903 model.

CONCLUSIONS

Our results suggest that functional inhibition of histamine receptor 4 by adriforant reduces itch and inflammation in vivo. The effects observed in mice, however, did not translate to clinical efficacy in patients as the Ph2b clinical trial with adriforant did not meet pre-specified efficacy endpoints. Given the complex pathogenesis of AD, antagonism of histamine receptor 4 alone appears insufficient to reduce disease severity in AD patients, despite the effects seen in mouse models.

Dermis resident macrophages orchestrate localized ILC2-eosinophil circuitries to maintain their M2-like properties and promote non-healing cutaneous leishmaniasis

Tissue-resident macrophages (TRMs) are critical for tissue homeostasis/repair. We previously showed that dermal TRMs produce CCL24 (eotaxin2) which mediates their interaction with IL-4 producing eosinophils, required to maintain their number and M2-like properties in the TH1 environment of the Leishmania major infected skin. Here, we unveil another layer of TRM self-maintenance involving their production of TSLP, an alarmin typically characterized as epithelial cell-derived. Both TSLP signaling and IL-5+ innate lymphoid cell 2 (ILC2s) were shown to maintain the number of dermal TRMs and promote infection. Single cell RNA sequencing identified the dermal TRMs as the sole source of TSLP and CCL24. Development of Ccl24-cre mice permitted specific labeling of dermal TRMs, as well as interstitial TRMs from other organs. Genetic ablation of TSLP from dermal TRMs reduced the number of dermal TRMs, and disease was ameliorated. Thus, by orchestrating localized type 2 circuitries with ILC2s and eosinophils, dermal TRMs are self-maintained as a replicative niche for L. major.

Helper Innate Lymphoid Cells-Unappreciated Players in Melanoma Therapy

Immune checkpoint inhibitors (ICIs) and targeted therapy have dramatically changed the outcome of metastatic melanoma patients. Although immune checkpoints were developed based on the biology of adaptive T cells, they have subsequently been shown to be expressed by other subsets of immune cells. Similarly, the immunomodulatory properties of targeted therapy have been studied primarily with respect to T lymphocytes, but other subsets of immune cells could be affected. Innate lymphoid cells (ILCs) are considered the innate counterpart of T lymphocytes and include cytotoxic natural killer cells, as well as three helper subsets, ILC1, ILC2 and ILC3. Thanks to their tissue distribution and their ability to respond rapidly to environmental stimuli, ILCs play a central role in shaping immunity. While the role of NK cells in melanoma physiopathology and therapy is well established, little is known about the other helper ILC subsets. In this review, we summarize recent findings on the ability of the melanoma TME to influence the phenotype and functional plasticity of helper ILCs and highlight how this subset may in turn shape the TME. We also discuss changes in the melanoma TME induced by targeted therapy that could affect helper ILC functions, the expression of immune checkpoints on this subset and how their inhibition by ICIs may modulate helper ILC function and contribute to therapeutic efficacy.

Insights into atopic dermatitis pathogenesis lead to newly approved systemic therapies

Atopic dermatitis (AD) is a common inflammatory skin disease characterized by scaly, oozing skin and itch. In moderate-to-severe AD, treatment options have been historically very limited and off-label use has been a common method for disease management. For decades, ciclosporin A was the only systemic immunosuppressive drug approved in most European countries to address this major unmet medical need. However, increased understanding of the pathophysiology of AD has led to a revolution in the treatment of this potentially debilitating disease. Following the approval of the first biological therapy for AD in 2017, there has been a rapid expansion of compounds under development and four additional systemic therapies have been approved in Europe and the USA within the past 3 years alone. In this review, we underscore how key breakthroughs have transformed the therapeutic landscape of AD, leading to a major expansion of type 2 immunity-targeted biological therapies, exploration of neuroimmune modulatory agents, and interest in Janus kinase inhibition.

Mast Cell Interaction with Foxp3+ Regulatory T Cells Occur in the Dermis after Initiation of IgE-Mediated Cutaneous Anaphylaxis

Mast cells (MCs) are well-known for their role in IgE-mediated cutaneous anaphylactic responses, but their regulatory functions in the skin are still under intense scrutiny. Using a Red MC and Basophil reporter (RMB) mouse allowing red fluorescent detection and diphtheria toxin mediated depletion of MCs, we investigated the interaction of MCs, Foxp3⁺ regulatory T lymphocytes (Tregs) and Langerhans cells (LCs) during passive cutaneous anaphylaxis (PCA) responses. Using intravital imaging we show that MCs are sessile at homeostasis and during PCA. Breeding RMB mice with Langerin-eGFP mice revealed that dermal MCs do not interact with epidermal-localized LCs, the latter showing constant sprouting of their dendrites at homeostasis and during PCA. When bred with Foxp3-eGFP mice, we found that, although a few Foxp3⁺ Tregs are present at homeostasis, many Tregs transiently infiltrated the skin during PCA. While their velocity during PCA was not altered, Tregs increased the duration of their contact time with MCs compared to PCA-control mice. Antibody-mediated depletion of Tregs had no effect on the intensity of PCA. Hence, the observed increase in Treg numbers and contact time with MCs, regardless of an effect on the intensity of PCA responses, suggests an anti-inflammatory role dedicated to prevent further MC activation.

Anti-allergic function of the cell wall (DeinoWall) from Deinococcus radiodurans

Deinococcus radiodurans is an extremophile, well known to be extremely resistant to external stresses due to its unique physiological system and structure of cellular components. Although the proportion of D. radiodurans has been reported to be negatively correlated with atopic dermatitis, the exact function of D. radiodurans in allergic diseases and its precise mechanisms have not been studied. In the present study, we hypothesize that D. radiodurans or its cellular constituents play a critical role in the skin to prevent allergic inflammatory responses by modulating immunity. Heat-killed D. radiodurans inhibited the production of Th2 cytokines, such as IL-4 and IL-5, induced by ovalbumin (OVA) stimulation in splenocytes from OVA-sensitized mice. Among the cellular constituents of D. radiodurans, such as cell wall (DeinoWall), cell membrane (DeinoMem), and exopolysaccharide (DeinoPol), only DeinoWall inhibited the production of Th2 cytokines and 2,4-dinitrochlorobenzene (DNCB)-induced atopic dermatitis (AD), a Th2-predominant allergic disease in mice. Moreover, serum IgE levels and infiltration of mast cells into skin lesions, the markers of Th2 response induced by DNCB application, were significantly inhibited by treatment with DeinoWall. Remarkably, DeinoWall induced the maturation of bone marrow-derived dendritic cells (BMDCs) that promote Th1-biased immunity, which might balance Th1/Th2 and regulate allergic inflammatory responses. Collectively, these results suggest that DeinoWall acts as a major cellular constituent in the negative regulation of allergic inflammatory responses by D. radiodurans and might be a viable candidate for the treatment of allergic diseases.

Group 2 innate lymphoid cells protect mouse heart from myocardial infarction injury via interleukin 5, eosinophils, and dendritic cells

AIMS

Group 2 innate lymphoid cells (ILC2) regulate adaptive and innate immunities. In mouse heart, production of myocardial infarction (MI) increased ILC2 accumulation, suggesting a role for ILC2 in cardiac dysfunction post-MI.

METHODS AND RESULTS

We produced MI in ILC2-deficeint Rorafl/flIl7rCre/+ mice and in Icosfl-DTR-fl/+Cd4Cre/+ mice that allowed diphtheria toxin-induced ILC2 depletion. Genetic or induced deficiency of ILC2 in mice exacerbated cardiac dysfunction post-MI injury along with increased myocardial accumulation of neutrophils, CD11b+Ly6Chi monocytes, and CD4+ T cells but deficiency of eosinophils (EOS) and dendritic cells (DC). Post-MI hearts from genetic and induced ILC2-deficient mice contained many more apoptotic cells than those of control mice, and Rorafl/flIl7rCre/+ mice showed thinner and larger infarcts and more collagen-I depositions than the Il7rCre/+ mice only at early time points post-MI. Mechanistic studies revealed elevated blood IL5 in Il7rCre/+ mice at 1, 7, and 28 days post-MI. Such increase was blunted in Rorafl/flIl7rCre/+ mice. Administration of recombinant IL5 reversed EOS losses in Rorafl/flIl7rCre/+ mice, but IL5 did not correct the DC loss in these mice. Adoptive transfer of ILC2, EOS, or DC from wild-type mice, but not ILC2 from Il5-/- mice improved post-MI cardiac functions in Rorafl/flIl7rCre/+ recipient mice. EOS are known to protect cardiomyocytes from apoptosis. Here we showed that DC acted like EOS in blocking cardiomyocyte apoptosis. Yet, ILC2 or IL5 alone did not directly affect cardiomyocyte apoptosis or TGF-β-induced cardiac fibroblast Smad signaling.

CONCLUSION

This study revealed an indirect cardiac reparative role of ILC2 in post-MI hearts via the IL5, EOS, and DC mechanism.

Context-dependent function of TSLP and IL-1β in skin allergic sensitization and atopic march

Atopic diseases, including atopic dermatitis (AD) and asthma, affect a large proportion of the population, with increasing prevalence worldwide. AD often precedes the development of asthma, known as the atopic march. Allergen sensitization developed through the barrier-defective skin of AD has been recognized to be a critical step leading to asthma, in which thymic stromal lymphopoietin (TSLP) was previously shown to be critical. In this study, using a laser-assistant microporation system to disrupt targeted skin layers for generating micropores at a precise anatomic depth of mouse skin, we model allergen exposure superficially or deeply in the skin, leading to epicutaneous sensitization or dermacutaneous sensitization that is associated with a different cytokine microenvironment. Our work shows a differential requirement for TSLP in these two contexts, and identifies an important function for IL-1β, which is independent of TSLP, in promoting allergen sensitization and subsequent allergic asthma.

Allergic sensitisation in the skin can lead to allergic dermatitis and further to airway asthma in a process of atopic march. Here the authors examine the difference between superficial or deep skin sensitisation, characterise the immune cells generated and show differential TSLP and IL-1β involvement.

Mast cell tissue heterogeneity and specificity of immune cell recruitment

Mast cells occupy a unique niche within tissues as long lived perpetrators of IgE mediated hypersensitivity and anaphylaxis, as well as other immune responses. However, mast cells are not identical in different tissues and the impact of this tissue heterogeneity on the interaction with other immune cells and on defined immune responses is still unclear. In this review, we synthesize the characteristics of mast cell heterogeneity in the gut and the skin. Furthermore, we attempt to connect mast cell heterogeneity with functional diversity by exploring differences in mast cell-induced immune cell recruitment in these two model organs. The differential expression of certain receptors on mast cells of different tissues, notably tissue-specific expression patterns of integrins, complement receptors and MRGPRX2, could indicate that tissue environment-dependent factors skew mast cell-immune cell interactions, for example by regulating the expression of these receptors.

Dengue virus co-opts innate type 2 pathways to escape early control of viral replication

Mast cell products and high levels of type 2 cytokines are associated with severe dengue disease. Group 2 innate lymphoid cells (ILC2) are type-2 cytokine-producing cells that are activated by epithelial cytokines and mast cell-derived lipid mediators. Through ex vivo RNAseq analysis, we observed that ILC2 are activated during acute dengue viral infection, and show an impaired type I-IFN signature in severe disease. We observed that circulating ILC2 are permissive for dengue virus infection in vivo and in vitro, particularly when activated through prostaglandin D2 (PGD2). ILC2 underwent productive dengue virus infection, which was inhibited through CRTH2 antagonism. Furthermore, exogenous IFN-β induced expression of type I-IFN responsive anti-viral genes by ILC2. PGD2 downregulated type I-IFN responsive gene and protein expression; and urinary prostaglandin D2 metabolite levels were elevated in severe dengue. Moreover, supernatants from activated ILC2 enhanced monocyte infection in a GM-CSF and mannan-dependent manner. Our results indicate that dengue virus co-opts an innate type 2 environment to escape early type I-IFN control and facilitate viral dissemination. PGD2 downregulates type I-IFN induced anti-viral responses in ILC2. CRTH2 antagonism may be a therapeutic strategy for dengue-associated disease.

Tissue-Specific Diversity of Group 2 Innate Lymphoid Cells in the Skin

Since the discovery of group 2 innate lymphoid cells (ILC2s), their developmental pathways, mechanisms of activation and regulation, and immunological roles in the steady state and in disease have been reported in various organs. ILC2s, which produce large amounts of IL-5 and IL-13 in response to tissue-derived factors and are essential in inducing and promoting allergic inflammation, have also been found to play multifaceted roles in maintaining tissue homeostasis. While T cells respond to foreign antigens, the activation of ILC2s is regulated by various tissue-derived factors, including cytokines, lipids, hormones, and neurotransmitters, and ILC2s show different phenotypes depending on the tissue in which they are present. In this review, we discuss tissue-specific characteristics of ILC2s in the skin. ILC2s, as defined in the lungs, intestinal tract, and adipose tissue, cannot be directly applied to cutaneous ILC biology, because skin ILC2s exhibit different aspects in the expression patterns of cell surface markers, the response to tissue-derived cytokines and the functions in both steady-state and inflammation. The skin contains ILCs with features of both ILC2s and ILC3s, and the plasticity between ILCs complicates their characters. Furthermore, the epidermis, dermis, and subcutaneous tissues contain ILCs with different characteristics; their localization has expanded our understanding of ILC function. Single-cell RNA-seq technology has further elucidated the role of ILCs in human skin and disease pathogenesis. Overall, this review discusses the phenotypical and functional heterogeneity of skin ILCs reported in recent years and highlights future directions within the field of ILC biology.

Activation of CD81+ skin ILC2s by cold-sensing TRPM8+ neuron-derived signals maintains cutaneous thermal homeostasis

As the outermost barrier tissue of the body, the skin harbors a large number of innate lymphoid cells (ILCs) that help maintain local homeostasis in the face of changing environments. How skin-resident ILCs are regulated and function in local homeostatic maintenance is poorly understood. We here report the discovery of a cold-sensing neuron-initiated pathway that activates skin group 2 ILCs (ILC2s) to help maintain thermal homeostasis. In stearoyl-CoA desaturase 1 (SCD1) knockout mice whose skin is defective in heat maintenance, chronic cold stress induced excessive activation of CCR10^-CD81⁺ST2⁺ skin ILC2s and associated inflammation. Mechanistically, stimulation of the cold-sensing receptor TRPM8 expressed in sensory neurons of the skin led to increased production of IL-18, which, in turn, activated skin ILC2s to promote thermogenesis. Our findings reveal a neuroimmune link that regulates activation of skin ILC2s to support thermal homeostasis and promotes skin inflammation after hyperactivation.

IL-13 in dermal type-2 dendritic cell specialization: from function to therapeutic targeting

Skin functions as a barrier protecting the host against physical, thermal, chemical changes and microbial insults. The skin is populated by several immune cell types which are crucial to host defence and to maintain self-tolerance as well as equilibrium with beneficial microbiota. Conventional dendritic cells (cDCs) are antigen-presenting cells that patrol the skin and all other non-lymphoid tissues for self or foreign antigens, then migrate to draining lymph nodes to initiate T cell responses. This review article describes recent developments on skin cDC specialization, focusing on the role of IL-13, a cytokine essential to allergic immune responses that is also secreted at steady state by type-2 innate lymphoid cells in healthy skin and is required for dermal cDC differentiation. Furthermore, we contextualize how different therapeutics that block IL-13 signaling and were recently approved for the treatment of atopic dermatitis might affect cDCs in human skin. This article is protected by copyright. All rights reserved.

The Role of Crosstalk of Immune Cells in Pathogenesis of Chronic Spontaneous Urticaria

Chronic spontaneous urticaria (CSU) is defined as recurrent episodes of spontaneous wheal development and/or angioedema for more than six weeks and at least twice a week. The core link in the pathogenesis of CSU is the activation of mast cells, T cells, eosinophils, and other immune cells infiltrating around the small venules of the lesion. Increased vascular permeability, vasodilatation, and recruitment of inflammatory cells directly depend on mast cell mediators’ release. Complex regulatory systems tightly influence the critical roles of mast cells in the local microenvironment. The bias toward Th2 inflammation and autoantibodies derived from B cells, histamine expressed by basophils, and initiation of the extrinsic coagulation pathway by eosinophils or monocytes exerts powerful modulatory influences on mast cells. Cell-to-cell interactions between mast cells and eosinophils/T cells also are regulators of their function and may involve CSU’s pathomechanism. This review summarizes up-to-date knowledge regarding the crosstalk between mast cells and other immune cells, providing the impetus to develop new research concepts and treatment strategies for CSU.

From Skin Barrier Dysfunction to Systemic Impact of Atopic Dermatitis: Implications for a Precision Approach in Dermocosmetics and Medicine

: Atopic dermatitis (AD) affects up to 20% of children and is considered the starting point of the atopic march with the development of food allergy, asthma, and allergic rhinitis. The heterogeneous phenotype reflects distinct and/or overlapping pathogenetic mechanisms with varying degrees of epidermal barrier disruption, activation of different T cell subsets and dysbiosis of the skin microbiome. Here, we review current evidence suggesting a systemic impact of the cutaneous inflammation in AD together with a higher risk of asthma and other comorbidities, especially in severe and persistent AD. Thus, early therapy of AD to restore the impaired skin barrier, modified microbiome, and target type 2 inflammation, depending on the (endo)phenotype, in a tailored approach is crucial. We discuss what we can learn from the comorbidities and the implications for preventive and therapeutic interventions from precision dermocosmetics to precision medicine. The stratification of AD patients into biomarker-based endotypes for a precision medicine approach offers opportunities for better long-term control of AD with the potential to reduce the systemic impact of a chronic skin inflammation and even prevent or modify the course, not only of AD, but possibly also the comorbidities, depending on the patient’s age and disease stage.

Tissue Resident and Migratory Group 2 Innate Lymphoid Cells

Group 2 innate lymphoid cells (ILC2s) are present in both mouse and human mucosal and non-mucosal tissues and implicated in initiating type 2 inflammation. ILC2s are considered to be tissue resident cells that develop in the perinatal period and persist throughout life with minimal turning over in adulthood. However, recent studies in animal models have shown their ability to circulate between different organs during inflammation and their potential functions in the destined organs, suggesting their roles in mediating multiple type 2 diseases. Here, we review recent findings on ILC2 migration, including migration within, into and out of tissues during inflammation.

Finding a Niche: Tissue Immunity and Innate Lymphoid Cells

The immune system plays essential roles in maintaining homeostasis in mammalian tissues that extend beyond pathogen clearance and host defense. Recently, several homeostatic circuits comprised of paired hematopoietic and non-hematopoietic cells have been described to influence tissue composition and turnover in development and after perturbation. Crucial circuit components include innate lymphoid cells (ILCs), which seed developing organs and shape their resident tissues by influencing progenitor fate decisions, microbial interactions, and neuronal activity. As they develop in tissues, ILCs undergo transcriptional imprinting that encodes receptivity to corresponding signals derived from their resident tissues but ILCs can also shift their transcriptional profiles to adapt to specific types of tissue perturbation. Thus, ILC functions are embedded within their resident tissues, where they constitute key regulators of homeostatic responses that can lead to both beneficial and pathogenic outcomes. Here, we examine the interactions between ILCs and various non-hematopoietic tissue cells, and discuss how specific ILC-tissue cell circuits form essential elements of tissue immunity.