Homeostatic Control of Sebaceous Glands by Innate Lymphoid Cells Regulates Commensal Bacteria Equilibrium

A spatial portrait of the human sebaceous gland transcriptional program

Sebaceous glands (SG) and their oily secretion (sebum) are indispensable for maintaining skin structure and function, and their deregulation causes skin disorders including but not limited to acne. Recent studies also indicate that sebum may have important immunomodulatory activities and may influence whole-body energy metabolism. However, the progressive transcriptional changes of sebocytes that lead to sebum production have never been characterized in detail. Here, we exploited the high cellular resolution provided by sebaceous hyperplasia and integrated spatial transcriptomics, pseudotime analysis, RNA velocity, and functional enrichment to map the landscape of sebaceous differentiation by employing spatial transcriptomics. Our results were validated by comparison with published SG transcriptome data and further corroborated by assessing the protein expression pattern of a subset of the transcripts in the public repository Human Protein Atlas. Departing from four sebocyte differentiation stages generated by unsupervised clustering, we demonstrate consecutive modulation of cellular functions associable with specific gene sets, from cell proliferation and oxidative phosphorylation via lipid synthesis to cell death. Both validation methods confirmed the biological significance of our results. Our report is complemented by a freely available and browsable online tool. Our data provide the first high-resolution spatial portrait of the SG transcriptional landscape and deliver starting points for experimentally assessing novel candidate molecules for regulating SG homeostasis in health and disease.

TRPV3 promotes sebocyte inflammation via transcriptional modulating TLR2 in acne

Acne is a common chronic inflammatory disease of the pilosebaceous unit. Transient receptor potential vanilloid 3 (TRPV3) is an ion channel that is involved in inflammatory dermatosis development. However, the involvement of TRPV3 in acne-related inflammation remains unclear. Here, we used acne-like mice and human sebocytes to examine the role of TRPV3 in the development of acne. We found that TRPV3 expression increased in the skin lesions of Propionibacterium acnes (P. acnes)-injected acne-like mice and the facial sebaceous glands (SGs) of acne patients. TRPV3 promoted inflammatory cytokines and chemokines secretion in human sebocytes and led to neutrophil infiltration surrounding the SGs in acne lesions, further exacerbating sebaceous inflammation and participating in acne development. Mechanistically, TRPV3 enhanced TLR2 level by promoting transcriptional factor phosphorylated-FOS-like antigen-1 (p-FOSL1) expression and its binding to the TLR2 promoter, leading to TLR2 upregulation and downstream NF-κB signaling activation. Genetic or pharmacological inhibition of TRPV3 both alleviated acne-like skin inflammation in mice via the TLR2-NF-κB axis. Thus, our study revealed the critical role of TRPV3 in sebaceous inflammation and indicated its potential as an acne therapeutic target.

Group 3 innate lymphoid cells: A trained Gutkeeper

Group 3 innate lymphoid cells (ILC3s) are tissue-resident immune lymphocytes that critically regulate intestinal homeostasis, organogenesis, and immunity. ILC3s possess the capacity to "sense" the inflammatory environment within tissues, especially in the context of pathogen challenges that imprints durable non-antigen-specific changes in ILC3 function. As such, ILC3s become a new actor in the emerging field of trained innate immunity. Here, we summarize recent discoveries regarding ILC3 responses to bacterial challenges and the role these encounters play in triggering trained innate immunity. We further discuss how signaling events throughout ILC3 ontogeny potentially control the development and function of trained ILC3s. Finally, we highlight the open questions surrounding ILC3 "training" the answers to which may reveal new insights into innate immunity. Understanding the fundamental concepts behind trained innate immunity could potentially lead to the development of new strategies for improving immunity-based modulation therapies for inflammation, infectious diseases, and cancer.

Adding Fuel to the Fire? The Skin Microbiome in Atopic Dermatitis

Atopic dermatitis (AD) is a multifactorial, heterogeneous disease characterized by epidermal barrier dysfunction, immune system dysregulation, and skin microbiome alterations. Skin microbiome studies in AD have demonstrated that disease flares are associated with microbial shifts, particularly Staphylococcus aureus predominance. AD-associated S. aureus strains differ from those in healthy individuals across various genomic loci, including virulence factors, adhesion proteins, and proinflammatory molecules-which may contribute to complex microbiome barrier-immune system interactions in AD. Different microbially based treatments for AD have been explored, and their future therapeutic successes will depend on a deeper understanding of the potential microbial contributions to the disease.

Sebaceous gland organoid engineering

Sebaceous glands (SGs), as holocrine-secreting appendages, lubricate the skin and play a central role in the skin barrier. Large full-thickness skin defects cause overall architecture disruption and SG loss. However, an effective strategy for SG regeneration is lacking. Organoids are 3D multicellular structures that replicate key anatomical and functional characteristics of in vivo tissues and exhibit great potential in regenerative medicine. Recently, considerable progress has been made in developing reliable procedures for SG organoids and existing SG organoids recapitulate the main morphological, structural and functional features of their in vivo counterparts. Engineering approaches empower researchers to manipulate cell behaviors, the surrounding environment and cell-environment crosstalk within the culture system as needed. These techniques can be applied to the SG organoid culture system to generate functionally more competent SG organoids. This review aims to provide an overview of recent advancements in SG organoid engineering. It highlights some potential strategies for SG organoid functionalization that are promising to forge a platform for engineering vascularized, innervated, immune-interactive and lipogenic SG organoids. We anticipate that this review will not only contribute to improving our understanding of SG biology and regeneration but also facilitate the transition of the SG organoid from laboratory research to a feasible clinical application.

Tissue-specific features of innate lymphoid cells in antiviral defense

Innate lymphocytes (ILCs) rapidly respond to and protect against invading pathogens and cancer. ILCs include natural killer (NK) cells, ILC1s, ILC2s, ILC3s, and lymphoid tissue inducer (LTi) cells and include type I, type II, and type III immune cells. While NK cells have been well recognized for their role in antiviral immunity, other ILC subtypes are emerging as players in antiviral defense. Each ILC subset has specialized functions that uniquely impact the antiviral immunity and health of the host depending on the tissue microenvironment. This review focuses on the specialized functions of each ILC subtype and their roles in antiviral immune responses across tissues. Several viruses within infection-prone tissues will be highlighted to provide an overview of the extent of the ILC immunity within tissues and emphasize common versus virus-specific responses.

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.

The Role of Gut-derived Short-Chain Fatty Acids in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic condition affecting the central nervous system (CNS), where the interplay of genetic and environmental factors influences its pathophysiology, triggering immune responses and instigating inflammation. Contemporary research has been notably dedicated to investigating the contributions of gut microbiota and their metabolites in modulating inflammatory reactions within the CNS. Recent recognition of the gut microbiome and dietary patterns as environmental elements impacting MS development emphasizes the potential influence of small, ubiquitous molecules from microbiota, such as short-chain fatty acids (SCFAs). These molecules may serve as vital molecular signals or metabolic substances regulating host cellular metabolism in the intricate interplay between microbiota and the host. A current emphasis lies on optimizing the health-promoting attributes of colonic bacteria to mitigate urinary tract issues through dietary management. This review aims to spotlight recent investigations on the impact of SCFAs on immune cells pivotal in MS, the involvement of gut microbiota and SCFAs in MS development, and the considerable influence of probiotics on gastrointestinal disruptions in MS. Comprehending the gut-CNS connection holds promise for the development of innovative therapeutic approaches, particularly probiotic-based supplements, for managing MS.

The role of thymic stromal lymphopoietin in cutaneous disorders

Thymic Stromal Lymphopoietin (TSLP) is an important cytokine that invokes early immune responses. TSLP, an IL-7-like cytokine encoded by the TSLP gene, activates JAK1 and JAK2 signaling pathways, stimulating dendritic cells to induce inflammatory Th2 cells. This cytokine is associated with pruritus in various cutaneous disorders, particularly atopic dermatitis. Varying levels of the cytokine TSLP have been demonstrated in studies of different cutaneous disorders. Pharmacological treatment targeting TSLP has been explored recently, particularly in the realm of atopic dermatitis.This review explores the relation of TSLP to cutaneous diseases, highlighting its potential as a biomarker for monitoring disease progression in discoid lupus erythematosus (DLE). The pharmacological therapy involving TSLP is discussed, along with the potential role of TSLP promotion in the treatment of alopecia areata. This overview examines the background, structure, and functions of TSLP, with a focus on its association with cutaneous disorders and a special focus on the impact of the atopic march.

Sexual dimorphism in skin immunity is mediated by an androgen-ILC2-dendritic cell axis

Males and females exhibit profound differences in immune responses and disease susceptibility. However, the factors responsible for sex differences in tissue immunity remain poorly understood. Here, we uncovered a dominant role for type 2 innate lymphoid cells (ILC2s) in shaping sexual immune dimorphism within the skin. Mechanistically, negative regulation of ILC2s by androgens leads to a reduction in dendritic cell accumulation and activation in males, along with reduced tissue immunity. Collectively, our results reveal a role for the androgen-ILC2-dendritic cell axis in controlling sexual immune dimorphism. Moreover, this work proposes that tissue immune set points are defined by the dual action of sex hormones and the microbiota, with sex hormones controlling the strength of local immunity and microbiota calibrating its tone.

Thymic Stromal Lymphopoietin (TSLP) Is Cleaved by Human Mast Cell Tryptase and Chymase

Thymic stromal lymphopoietin (TSLP), mainly expressed by epithelial cells, plays a central role in asthma. In humans, TSLP exists in two variants: the long form TSLP (lfTSLP) and a shorter TSLP isoform (sfTSLP). Macrophages (HLMs) and mast cells (HLMCs) are in close proximity in the human lung and play key roles in asthma. We evaluated the early proteolytic effects of tryptase and chymase released by HLMCs on TSLP by mass spectrometry. We also investigated whether TSLP and its fragments generated by these enzymes induce angiogenic factor release from HLMs. Mass spectrometry (MS) allowed the identification of TSLP cleavage sites caused by tryptase and chymase. Recombinant human TSLP treated with recombinant tryptase showed the production of 1-97 and 98-132 fragments. Recombinant chymase treatment of TSLP generated two peptides, 1-36 and 37-132. lfTSLP induced the release of VEGF-A, the most potent angiogenic factor, from HLMs. By contrast, the four TSLP fragments generated by tryptase and chymase failed to activate HLMs. Long-term TSLP incubation with furin generated two peptides devoid of activating property on HLMs. These results unveil an intricate interplay between mast cell-derived proteases and TSLP. These findings have potential relevance in understanding novel aspects of asthma pathobiology.

Dupilumab-associated head and neck dermatitis shows a pronounced type 22 immune signature mediated by oligoclonally expanded T cells

Dupilumab, an IL4R-blocking antibody, has shown clinical efficacy for atopic dermatitis (AD) treatment. In addition to conjunctivitis/blepharitis, the de novo appearance of head/neck dermatitis is now recognized as a distinct side effect, occurring in up to 10% of patients. Histopathological features distinct from AD suggest a drug effect, but exact underlying mechanisms remain unknown. We profiled punch biopsies from dupilumab-associated head and neck dermatitis (DAHND) by using single-cell RNA sequencing and compared data with untreated AD and healthy control skin. We show that dupilumab treatment was accompanied by normalization of IL-4/IL-13 downstream activity markers such as CCL13, CCL17, CCL18 and CCL26. By contrast, we found strong increases in type 22-associated markers (IL22, AHR) especially in oligoclonally expanded T cells, accompanied by enhanced keratinocyte activation and IL-22 receptor upregulation. Taken together, we demonstrate that dupilumab effectively dampens conventional type 2 inflammation in DAHND lesions, with concomitant hyperactivation of IL22-associated responses.

Skin microbe-dependent TSLP-ILC2 priming axis in early life is co-opted in allergic inflammation

Although early life colonization of commensal microbes contributes to long-lasting immune imprinting in host tissues, little is known regarding the pathophysiological consequences of postnatal microbial tuning of cutaneous immunity. Here, we show that postnatal exposure to specific skin commensal Staphylococcus lentus (S. lentus) promotes the extent of atopic dermatitis (AD)-like inflammation in adults through priming of group 2 innate lymphoid cells (ILC2s). Early postnatal skin is dynamically populated by discrete subset of primed ILC2s driven by microbiota-dependent induction of thymic stromal lymphopoietin (TSLP) in keratinocytes. Specifically, the indole-3-aldehyde-producing tryptophan metabolic pathway, shared across Staphylococcus species, is involved in TSLP-mediated ILC2 priming. Furthermore, we demonstrate a critical contribution of the early postnatal S. lentus-TSLP-ILC2 priming axis in facilitating AD-like inflammation that is not replicated by later microbial exposure. Thus, our findings highlight the fundamental role of time-dependent neonatal microbial-skin crosstalk in shaping the threshold of innate type 2 immunity co-opted in adulthood.

Characteristics of bacterial community in eyelashes of patients with Demodex blepharitis

BACKGROUND

Demodex blepharitis (DB) is a common disease of the ocular surface. The characteristics of the bacterial community in eyelash roots after Demodex infestation are still unknown. Knowledge of the characteristics of the bacterial community of eyelash follicles in patients with DB can provide valuable insights for guiding the diagnosis and treatment of DB.

METHODS

Twenty-five patients with DB (DB group) and 21 non-DB volunteers (control group) were enrolled in the study. Eyelashes from the upper eyelid of the right eye were sampled, and 16S ribosomal DNA (rDNA) sequencing was performed to determine the V3-V4 regions of the microbial 16S rDNA gene within 1 month of infestation. The sequencing data of the two groups were analyzed and compared. The effect of the bacterium Burkholderia on the survival of Demodex mites was evaluated using Demodex obtained from 12 patients with DB other that the patients in the DB group.

RESULTS

A total of 31 phyla and 862 genera were identified in the DB and control groups. The five most abundant phyla in the two groups were Proteobacteria, Firmicutes, Actinobacteria, Bacteroidetes and Cyanobacteria. The abundance of Actinomycetes was significantly higher in the DB group than in the control group. At the genus level, the five most abundant genera in the two groups were Pseudomonas, Burkholderia-Caballeronia-Paraburkholderia, Rolstonia and Acinetobacter; Clostridium sensu stricto 1 was abundant in the control group and Corynebacterium_1 was abundant in the DB group. Compared with the control group, the abundance of Burkholderia-Caballeronia-Paraburkholderia was 2.36-fold lower in the DB group. Linear discriminant analysis Effect Size (LEfSe) analysis revealed Burkholderia-Caballeronia-Paraburkholderia, SC_I_84_unclassified, Nonmyxobacteria and Succinvibrio to be the major biomarkers in the control group and Catenibacterium and Lachnospiraceae NK4A136 group to be the major biomarkers in the DB group. To explore the performance of these optimal marker models, receiver operational characteristic curve analysis was performed, and the average area under the curve value of Burkholderia-Caballeronia-Paraburkholderia was 0.7448. Burkholderia cepacia isolated from normal human eyelashes was fermented, and the Demodex mites isolated from patient eyelashes were cultured together with its fermented supernatant. The results showed that the fermentation supernatant could significantly reduce the survival time of the Demodex mites, suggesting the potential therapeutic value of this bacterium against Demodex.

CONCLUSIONS

The composition of the bacterial community in the eyelashes of DB patients differed from that in eyelashes of healthy volunteers, revealing a decrease in bacterial diversity in infested eyelashes. This decrease may be related to the occurrence and development of DB. The supernatant of Burkholderia cepacia culture medium was found to inhibit the growth of Demodex in eyelash hair follicles, providing a new insight with potential applications for the clinical treatment of Demodex infestation.

Understanding Type 3 Innate Lymphoid Cells and Crosstalk with the Microbiota: A Skin Connection

Innate lymphoid cells (ILCs) are a diverse population of lymphocytes classified into natural killer (NK) cells, ILC1s, ILC2s, ILC3s, and ILCregs, broadly following the cytokine secretion and transcription factor profiles of classical T cell subsets. Nonetheless, the ILC lineage does not have rearranged antigen-specific receptors and possesses distinct characteristics. ILCs are found in barrier tissues such as the skin, lungs, and intestines, where they play a role between acquired immune cells and myeloid cells. Within the skin, ILCs are activated by the microbiota and, in turn, may influence the microbiome composition and modulate immune function through cytokine secretion or direct cellular interactions. In particular, ILC3s provide epithelial protection against extracellular bacteria. However, the mechanism by which these cells modulate skin health and homeostasis in response to microbiome changes is unclear. To better understand how ILC3s function against microbiota perturbations in the skin, we propose a role for these cells in response to Cutibacterium acnes, a predominant commensal bacterium linked to the inflammatory skin condition, acne vulgaris. In this article, we review current evidence describing the role of ILC3s in the skin and suggest functional roles by drawing parallels with ILC3s from other organs. We emphasize the limited understanding and knowledge gaps of ILC3s in the skin and discuss the potential impact of ILC3-microbiota crosstalk in select skin diseases. Exploring the dialogue between the microbiota and ILC3s may lead to novel strategies to ameliorate skin immunity.

Skin single-cell transcriptomics reveals a core of sebaceous gland-relevant genes shared by mice and humans

BACKGROUND

Single-cell RNA sequencing (scRNA-seq) has been widely applied to dissect cellular heterogeneity in normal and diseased skin. Sebaceous glands, essential skin components with established functions in maintaining skin integrity and emerging roles in systemic energy metabolism, have been largely neglected in scRNA-seq studies.

METHODS

Departing from mouse and human skin scRNA-seq datasets, we identified gene sets expressed especially in sebaceous glands with the open-source R-package oposSOM.

RESULTS

The identified gene sets included sebaceous gland-typical genes as Scd3, Mgst1, Cidea, Awat2 and KRT7. Surprisingly, however, there was not a single overlap among the 100 highest, exclusively in sebaceous glands expressed transcripts in mouse and human samples. Notably, both species share a common core of only 25 transcripts, including mitochondrial and peroxisomal genes involved in fatty acid, amino acid, and glucose processing, thus highlighting the intense metabolic rate of this gland.

CONCLUSIONS

This study highlights intrinsic differences in sebaceous lipid synthesis between mice and humans, and indicates an important role for peroxisomal processes in this context. Our data also provides attractive starting points for experimentally addressing novel candidates regulating sebaceous gland homeostasis.

Deciphering the molecular mechanisms of stem cell dynamics in hair follicle regeneration

Hair follicles, which are connected to sebaceous glands in the skin, undergo cyclic periods of regeneration, degeneration, and rest throughout adult life in mammals. The crucial function of hair follicle stem cells is to maintain these hair growth cycles. Another vital aspect is the activity of melanocyte stem cells, which differentiate into melanin-producing melanocytes, contributing to skin and hair pigmentation. Sebaceous gland stem cells also have a pivotal role in maintaining the skin barrier by regenerating mature sebocytes. These stem cells are maintained in a specialized microenvironment or niche and are regulated by internal and external signals, determining their dynamic behaviors in homeostasis and hair follicle regeneration. The activity of these stem cells is tightly controlled by various factors secreted by the niche components around the hair follicles, as well as immune-mediated damage signals, aging, metabolic status, and stress. In this study, we review these diverse stem cell regulatory and related molecular mechanisms of hair regeneration and disease conditions. Molecular insights would provide new perspectives on the disease mechanisms as well as hair and skin disorder treatment.

In-Depth Proteomic Map of Innate Lymphoid Cells from Healthy Human Skin and Blood

Innate lymphoid cells (ILCs) are essential players in the skin-associated immune system, nevertheless little is known about their proteomes and proteomic diversity. In this study, we describe about 6,600 proteins constitutively expressed by ILC2s and ILC3s from healthy human skin and blood using state-of-the-art proteomics. Although the vast majority of proteins was expressed by both ILC subsets and in both compartments, the skin ILC2s and ILC3s were more distinct than their counterparts in blood. Only skin ILC3s expressed uniquely detected proteins. Our in-depth proteomic dataset allowed us to define the cluster of differentiation marker profiles of the ILC subsets, explore distribution and abundance of proteins known to have immunological functions, as well as identify subset-specific proteins that have not previously been implicated in ILC biology. Taken together, our analyses substantially expand understanding of the protein expression signatures of ILC subsets. Going forward, these proteomic datasets will serve as valuable resources for future studies of ILC biology.

Association of different cell types and inflammation in early acne vulgaris

Acne vulgaris, one of the most common skin diseases, is a chronic cutaneous inflammation of the upper pilosebaceous unit (PSU) with complex pathogenesis. Inflammation plays a central role in the pathogenesis of acne vulgaris. During the inflammatory process, the innate and adaptive immune systems are coordinately activated to induce immune responses. Understanding the infiltration and cytokine secretion of differential cells in acne lesions, especially in the early stages of inflammation, will provide an insight into the pathogenesis of acne. The purpose of this review is to synthesize the association of different cell types with inflammation in early acne vulgaris and provide a comprehensive understanding of skin inflammation and immune responses.

Regulatory T cells in skin mediate immune privilege of the hair follicle stem cell niche

Immune tolerance is maintained in lymphoid organs (LOs). Despite the presence of complex immune cell networks in non-LOs, it is unknown whether self-tolerance is maintained in these tissues. We developed a technique to restrict genetic recombination to regulatory T cells (Tregs) only in skin. Selective depletion of skin Tregs resulted in T cell-mediated inflammation of hair follicles (HFs). Suppression did not rely on CTLA-4, but instead on high-affinity interleukin-2 (IL-2) receptor expression by skin Tregs, functioning exclusively in a cell-extrinsic manner. In a novel model of HF stem cell (HFSC)-driven autoimmunity, we reveal that skin Tregs immunologically protect the HFSC niche. Finally, we used spatial transcriptomics to identify aberrant IL-2 signaling at stromal-HF interfaces in a rare form of human alopecia characterized by HFSC destruction and alopecia areata. Collectively, these results reveal the fundamental biology of Tregs in skin uncoupled from the systemic pool and elucidate a mechanism of self-tolerance.

AgNPs-Modified Polylactic Acid Microneedles: Preparation and In Vivo/In Vitro Antimicrobial Studies

OBJECTIVE

To prepare polylactic acid microneedles (PLAMNs) with sustained antibacterial effect to avoid skin infection caused by traditional MNs-based biosensors.

METHODS

Silver nanoparticles (AgNPs) were synthesized using an in-situ reduction process with polydopamine (PDA). PLAMNs were fabricated using the hot-melt method. A series of pressure tests and puncture experiments were conducted to confirm the physicochemical properties of PLAMNs. Then AgNPs were modified on the surface of PLAMNs through in-situ reduction of PDA, resulting in the formation of PLAMNs@PDA-AgNPs. The in vitro antibacterial efficacy of PLAMNs@PDA-AgNPs was evaluated using agar diffusion assays and bacterial liquid co-culture approach. Wound healing and simulated long-term application were performed to assess the in vivo antibacterial effectiveness of PLAMNs@PDA-AgNPs.

RESULTS

The MNs array comprised 169 tiny needle tips in pyramidal rows. Strength and puncture tests confirmed a 100% puncture success rate for PLAMNs on isolated rat skin and tin foil. SEM analysis revealed the integrity of PLAMNs@PDA-AgNPs with the formation of new surface substances. EDS analysis indicated the presence of silver elements on the surface of PLAMNs@PDA-AgNPs, with a content of 14.44%. Transepidermal water loss (TEWL) testing demonstrated the rapid healing of micro-pores created by PLAMNs@PDA-AgNPs, indicating their safety. Both in vitro and in vivo tests confirmed antibacterial efficacy of PLAMNs@PDA-AgNPs.

CONCLUSIONS

In conclusion, the sustained antibacterial activity exhibited by PLAMNs@PDA-AgNPs offers a promising solution for addressing skin infections associated with MN applications, especially when compared to traditional MN-based biosensors. This advancement offers significant potential for the field of MN technology.

Thymic stromal lymphopoietin induces IL-4/IL-13 from T cells to promote sebum secretion and adipose loss

BACKGROUND

The cytokine TSLP promotes type 2 immune responses and can induce adipose loss by stimulating lipid loss from the skin through sebum secretion by sebaceous glands, which enhances the skin barrier. However, the mechanism by which TSLP upregulates sebaceous gland function is unknown.

OBJECTIVES

This study investigated the mechanism by which TSLP stimulates sebum secretion and adipose loss.

METHODS

RNA-sequencing analysis was performed on sebaceous glands isolated by laser capture microdissection and single-cell RNA-sequencing analysis was performed on sorted skin T cells. Sebocyte function was analyzed by histological analysis and sebum secretion in vivo and by measuring lipogenesis and proliferation in vitro.

RESULTS

This study found that TSLP sequentially stimulated the expression of lipogenesis genes followed by cell death genes in sebaceous glands to induce holocrine secretion of sebum. TSLP did not affect sebaceous gland activity directly. Rather, single-cell RNA-sequencing revealed that TSLP recruited distinct T-cell clusters that produce IL-4 and IL-13, which were necessary for TSLP-induced adipose loss and sebum secretion. Moreover, IL-13 was sufficient to cause sebum secretion and adipose loss in vivo and to induce lipogenesis and proliferation of a human sebocyte cell line in vitro.

CONCLUSIONS

This study proposes that TSLP stimulates T cells to deliver IL-4 and IL-13 to sebaceous glands, which enhances sebaceous gland function, turnover, and subsequent adipose loss.

Cutting Edge: CCR8 Signaling Regulates IL-25- and IL-33-Responsive Skin Group 2 Innate Lymphoid Cell Migration and Function

Group 2 innate lymphoid cells (ILC2s) are sentinels of barrier immunity, and their activation by the epithelial alarmins IL-25 and IL-33 is a defining trait. In this study, we identified a role for the chemokine receptor CCR8 in modulating skin ILC2 abundance and activation. CCR8 signaling facilitated IL-25-induced increases in skin and lung ILC2s, ILC2 activation and systemic IL-13 production, and ligand-directed ILC2 entry into skin and lung. CCR8 controlled ILC2 tissue entry in IL-25-treated naive mice, but only transferred bone marrow ILC2 progenitors were equipped to enter the skin, whereas multiple tissue-sourced ILC2s entered the lung. CCR8 selectively regulated IL-33-induced increases in skin ILC2s, their proliferation, and production of IL-13/IL-5, as well as IL-33-responsive transferred ILC2 trafficking only to the skin. Collectively, we illuminate (to our knowledge) novel aspects of CCR8 signaling-regulated ILC2 motility and function, especially in the skin, in response to two hallmark ILC2-activating alarmins.

Longitudinal study of the interplay between the skin barrier and facial microbiome over 1 year

Skin is a diverse ecosystem that provides a habitat for microorganisms. The skin condition and the skin microbiome interact each other under diverse environmental conditions. This study was conducted on 10 study participants for a one-year, from September 2020 to August 2021, to investigate the variability of skin microbiome and skin biophysical parameters [TEWL, hydration, and elasticity (R5)] according to season, and to understand the interplay between skin microbiome and skin characteristics. We identified that Cutibacterium, Corynebacterium, Staphyloccocus, unclassified genus within Neisseriaceae, and Streptococcus were major skin microbial taxa at the genus level, and fluctuated with the seasons. Cutibacterium was more abundant in winter, while Corynebacterium, Staphylococcus, and Streptococcus were more abundant in summer. Notably, Cutibacterium and skin barrier parameter, TEWL, exhibited a co-decreasing pattern from winter to summer and showed a significant association between Cutibacterium and TEWL. Furthermore, functional profiling using KEGG provided clues on the impact of Cutibacterium on the host skin barrier. This study enhances our understanding of the skin microbiome and its interplay with skin characteristics and highlights the importance of seasonal dynamics in shaping skin microbial composition.

Galectin-12 Regulates Immune Responses in the Skin through Sebaceous Glands

Sebaceous glands (SGs) are holocrine glands that produce sebum, which primarily contains lipids that help to maintain the barrier function of the skin. Dysregulated lipid production contributes to the progression of some diseases characterized by dry skin, including atopic dermatitis. Although the lipid production of SGs has been well-studied, few studies have assessed their role in skin immune responses. We found that SGs and sebocytes expressed IL-4 receptor and produced high levels of T helper 2-associated inflammatory mediators after IL-4 treatment, suggesting immunomodulatory effects. Galectin-12 is a lipogenic factor expressed in sebocytes that affects their differentiation and proliferation. Using galectin-12-knockdown sebocytes, we showed that galectin-12 regulated the immune response in cells exposed to IL-4 and promoted CCL26 expression by upregulating peroxisome proliferator-activated receptor-γ. Moreover, galectin-12 suppressed the expression of endoplasmic reticulum stress-response molecules, and CCL26 upregulation by IL-4 was reversed after sebocyte treatment with inducers of endoplasmic reticulum stress, suggesting that galectin-12 controls IL-4 signaling by suppressing endoplasmic reticulum stress. Using galectin-12-knockout mice, we showed that galectin-12 positively regulated the IL-4-induced enlargement of SGs and the development of an atopic dermatitis-like phenotype. Thus, galectin-12 regulates the skin immune response by promoting peroxisome proliferator-activated receptor-γ expression and suppressing endoplasmic reticulum stress in SGs.

Both Horatio and Polonius: Innate Lymphoid Cells in Tissue Homeostasis and Repair

Innate lymphoid cells (ILCs) have emerged as critical tissue-resident lymphocytes that coordinate responses to environmental stress and injury. Traditionally, their function was thought to mirror adaptive lymphocytes that respond to specific pathogens. However, recent work has uncovered a more central role for ILCs in maintaining homeostasis even in the absence of infection. ILCs are now better conceptualized as an environmental rheostat that helps maintain the local tissue setpoint during environmental challenge by integrating sensory stimuli to direct homeostatic barrier and repair programs. In this article, we trace the developmental origins of ILCs, relate how ILCs sense danger signals, and describe their subsequent engagement of appropriate repair responses using a general paradigm of ILCs functioning as central controllers in tissue circuits. We propose that these interactions form the basis for how ILC subsets maintain organ function and organismal homeostasis, with important implications for human health.

Group 1 and 3 innate lymphoid cells are increased in oral lichen planus and oral lichenoid lesions

OBJECTIVES

Innate lymphoid cells (ILCs) are vital innate immune cells cooperating with T cells. While their phenotypes and functions in oral mucosa kept unclear yet. In the present study, the relative proportions and distribution of different ILC subsets in oral mucosa of oral lichen planus (OLP), oral lichenoid lesions (OLL), and controls were compared.

SUBJECTS AND METHODS

Oral mucosal samples were collected from control (n = 29), OLP (n = 20), and OLL (n = 22) donors. ILCs subsets were characterized in single-cell suspensions by flow cytometry. Immunohistochemistry was performed to locate the CD127+ cells in situ.

RESULTS

ILCs were present in healthy and increased infiltration in OLP/OLL (p = 0.0092, p = 0.0216). Infiltration of ILC1 increased in OLP/OLL mucosa (p = 0.0225, p = 0.0399), as did the infiltration of ILC3 increase in OLL mucosa (p = 0.0128). The ILC2/ILCs ratio was significantly reduced in OLP and OLL (p = 0.0124, p = 0.0346). CD127+ cells were mainly located closely at the basement membrane.

CONCLUSIONS

The results of increased ILC1, decreased ILC2, and increased ILC3 suggested that changes of ILC distributions in oral mucosa may be relevant to persistent inflammation in local tissues, by promoting immune factors and weakening repair capacity.

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.

Distinct mechanisms for sebaceous gland self-renewal and regeneration provide durability in response to injury

Sebaceous glands (SGs) release oils that protect our skin, but how these glands respond to injury has not been previously examined. Here, we report that SGs are largely self-renewed by dedicated stem cell pools during homeostasis. Using targeted single-cell RNA sequencing, we uncovered both direct and indirect paths by which resident SG progenitors ordinarily differentiate into sebocytes, including transit through a Krt5+PPARγ+ transitional basal cell state. Upon skin injury, however, SG progenitors depart their niche, reepithelialize the wound, and are replaced by hair-follicle-derived stem cells. Furthermore, following targeted genetic ablation of >99% of SGs from dorsal skin, these glands unexpectedly regenerate within weeks. This regenerative process is mediated by alternative stem cells originating from the hair follicle bulge, is dependent upon FGFR2 signaling, and can be accelerated by inducing hair growth. Altogether, our studies demonstrate that stem cell plasticity promotes SG durability following injury.

Matricellular Proteins in the Homeostasis, Regeneration, and Aging of Skin

Matricellular proteins are secreted extracellular proteins that bear no primary structural functions but play crucial roles in tissue remodeling during development, homeostasis, and aging. Despite their low expression after birth, matricellular proteins within skin compartments support the structural function of many extracellular matrix proteins, such as collagens. In this review, we summarize the function of matricellular proteins in skin stem cell niches that influence stem cells' fate and self-renewal ability. In the epidermal stem cell niche, fibulin 7 promotes epidermal stem cells' heterogeneity and fitness into old age, and the transforming growth factor-β-induced protein ig-h3 (TGFBI)-enhances epidermal stem cell growth and wound healing. In the hair follicle stem cell niche, matricellular proteins such as periostin, tenascin C, SPARC, fibulin 1, CCN2, and R-Spondin 2 and 3 modulate stem cell activity during the hair cycle and may stabilize arrector pili muscle attachment to the hair follicle during piloerections (goosebumps). In skin wound healing, matricellular proteins are upregulated, and their functions have been examined in various gain-and-loss-of-function studies. However, much remains unknown concerning whether these proteins modulate skin stem cell behavior, plasticity, or cell-cell communications during wound healing and aging, leaving a new avenue for future studies.

MacroH2A restricts inflammatory gene expression in melanoma cancer-associated fibroblasts by coordinating chromatin looping

MacroH2A has established tumour suppressive functions in melanoma and other cancers, but an unappreciated role in the tumour microenvironment. Using an autochthonous, immunocompetent mouse model of melanoma, we demonstrate that mice devoid of macroH2A variants exhibit increased tumour burden compared with wild-type counterparts. MacroH2A-deficient tumours accumulate immunosuppressive monocytes and are depleted of functional cytotoxic T cells, characteristics consistent with a compromised anti-tumour response. Single cell and spatial transcriptomics identify increased dedifferentiation along the neural crest lineage of the tumour compartment and increased frequency and activation of cancer-associated fibroblasts following macroH2A loss. Mechanistically, macroH2A-deficient cancer-associated fibroblasts display increased myeloid chemoattractant activity as a consequence of hyperinducible expression of inflammatory genes, which is enforced by increased chromatin looping of their promoters to enhancers that gain H3K27ac. In summary, we reveal a tumour suppressive role for macroH2A variants through the regulation of chromatin architecture in the tumour stroma with potential implications for human melanoma.

Role of Innate Immunity in Allergic Contact Dermatitis: An Update

Our understanding of allergic contact dermatitis mechanisms has progressed over the past decade. Innate immune cells that are involved in the pathogenesis of allergic contact dermatitis include Langerhans cells, dermal dendritic cells, macrophages, mast cells, innate lymphoid cells (ILCs), neutrophils, eosinophils, and basophils. ILCs can be subcategorized as group 1 (natural killer cells; ILC1) in association with Th1, group 2 (ILC2) in association with Th2, and group 3 (lymphoid tissue-inducer cells; ILC3) in association with Th17. Pattern recognition receptors (PRRs) including toll-like receptors (TLRs) and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) in innate immune cells recognize damage-associated molecular patterns (DAMPs) and cascade the signal to produce several cytokines and chemokines including tumor necrosis factor (TNF)-α, interferon (IFN)-α, IFN-γ, interleukin (IL)-1β, IL-4, IL-6, IL-12, IL-13, IL-17, IL-18, and IL-23. Here we discuss the recent findings showing the roles of the innate immune system in allergic contact dermatitis during the sensitization and elicitation phases.

Exploring the Pathogenesis and Mechanism-Targeted Treatments of Rosacea: Previous Understanding and Updates

Rosacea is a chronic inflammatory skin disease characterized by recurrent erythema, flushing, telangiectasia, papules, pustules, and phymatous changes in the central area of the face. Patients with this condition often experience a significant negative impact on their quality of life, self-esteem, and overall well-being. Despite its prevalence, the pathogenesis of rosacea is not yet fully understood. Recent research advances are reshaping our understanding of the underlying mechanisms of rosacea, and treatment options based on the pathophysiological perspective hold promise to improve patient outcomes and reduce incidence. In this comprehensive review, we investigate the pathogenesis of rosacea in depth, with a focus on emerging and novel mechanisms, and provide an up-to-date overview of therapeutic strategies that target the diverse pathogenic mechanisms of rosacea. Lastly, we discuss potential future research directions aimed at enhancing our understanding of the condition and developing effective treatments.

Bioprospecting the Skin Microbiome: Advances in Therapeutics and Personal Care Products

Bioprospecting is the discovery and exploration of biological diversity found within organisms, genetic elements or produced compounds with prospective commercial or therapeutic applications. The human skin is an ecological niche which harbours a rich and compositional diversity microbiome stemming from the multifactorial interactions between the host and microbiota facilitated by exploitable effector compounds. Advances in the understanding of microbial colonisation mechanisms alongside species and strain interactions have revealed a novel chemical and biological understanding which displays applicative potential. Studies elucidating the organismal interfaces and concomitant understanding of the central processes of skin biology have begun to unravel a potential wealth of molecules which can exploited for their proposed functions. A variety of skin-microbiome-derived compounds display prospective therapeutic applications, ranging from antioncogenic agents relevant in skin cancer therapy to treatment strategies for antimicrobial-resistant bacterial and fungal infections. Considerable opportunities have emerged for the translation to personal care products, such as topical agents to mitigate various skin conditions such as acne and eczema. Adjacent compound developments have focused on cosmetic applications such as reducing skin ageing and its associated changes to skin properties and the microbiome. The skin microbiome contains a wealth of prospective compounds with therapeutic and commercial applications; however, considerable work is required for the translation of in vitro findings to relevant in vivo models to ensure translatability.

Macrophage-mediated extracellular matrix remodeling controls host Staphylococcus aureus susceptibility in the skin

Hypodermis is the predominant site of Staphylococcus aureus infections that cause cellulitis. Given the importance of macrophages in tissue remodeling, we examined the hypodermal macrophages (HDMs) and their impact on host susceptibility to infection. Bulk and single-cell transcriptomics uncovered HDM subsets with CCR2-dichotomy. HDM homeostasis required the fibroblast-derived growth factor CSF1, ablation of which abrogated HDMs from the hypodermal adventitia. Loss of CCR2- HDMs resulted in accumulation of the extracellular matrix component, hyaluronic acid (HA). HDM-mediated HA clearance required sensing by the HA receptor, LYVE-1. Cell-autonomous IGF1 was required for accessibility of AP-1 transcription factor motifs that controlled LYVE-1 expression. Remarkably, loss of HDMs or IGF1 limited Staphylococcus aureus expansion via HA and conferred protection against cellulitis. Our findings reveal a function for macrophages in the regulation of HA with an impact on infection outcomes, which may be harnessed to limit the establishment of infection in the hypodermal niche.

Protective and pathogenic functions of innate lymphoid cells in transplantation

Innate lymphoid cells (ILCs) are a family of lymphocytes with essential roles in tissue homeostasis and immunity. Along with other tissue-resident immune populations, distinct subsets of ILCs have important roles in either promoting or inhibiting immune tolerance in a variety of contexts, including cancer and autoimmunity. In solid organ and hematopoietic stem cell transplantation, both donor and recipient-derived ILCs could contribute to immune tolerance or rejection, yet understanding of protective or pathogenic functions are only beginning to emerge. In addition to roles in directing or regulating immune responses, ILCs interface with parenchymal cells to support tissue homeostasis and even regeneration. Whether specific ILCs are tissue-protective or enhance ischemia reperfusion injury or fibrosis is of particular interest to the field of transplantation, beyond any roles in limiting or promoting allograft rejection or graft-versus host disease. Within this review, we discuss the current understanding of ILCs functions in promoting immune tolerance and tissue repair at homeostasis and in the context of transplantation and highlight where targeting or harnessing ILCs could have applications in novel transplant therapies.

Distinct mechanisms for sebaceous gland self-renewal and regeneration provide durability in response to injury

Sebaceous glands (SGs) release oils that protect our skin, but how these glands respond to injury has not been previously examined. Here, we report that SGs are largely self-renewed by dedicated stem cell pools during homeostasis. Using targeted single cell RNA-sequencing, we uncovered both direct and indirect paths by which these resident SG progenitors ordinarily differentiate into sebocytes, including transit through a PPARγ+Krt5+ transitional cell state. Upon skin injury, however, SG progenitors depart their niche, reepithelialize the wound, and are replaced by hair follicle-derived stem cells. Furthermore, following targeted genetic ablation of >99% of SGs from dorsal skin, these glands unexpectedly regenerate within weeks. This regenerative process is mediated by alternative stem cells originating from the hair follicle bulge, is dependent upon FGFR signaling, and can be accelerated by inducing hair growth. Altogether, our studies demonstrate that stem cell plasticity promotes SG durability following injury.

The ins and outs of innate and adaptive type 2 immunity

Type 2 immunity is orchestrated by a canonical group of cytokines primarily produced by innate lymphoid cells, group 2, and their adaptive counterparts, CD4+ helper type 2 cells, and elaborated by myeloid cells and antibodies that accumulate in response. Here, we review the cytokine and cellular circuits that mediate type 2 immunity. Building from insights in cytokine evolution, we propose that innate type 2 immunity evolved to monitor the status of microbe-rich epithelial barriers (outside) and sterile parenchymal borders (inside) to meet the functional demands of local tissue, and, when necessary, to relay information to the adaptive immune system to reinforce demarcating borders to sustain these efforts. Allergic pathology likely results from deviations in local sustaining units caused by alterations imposed by environmental effects during postnatal developmental windows and exacerbated by mutations that increase vulnerabilities. This framework positions T2 immunity as central to sustaining tissue repair and regeneration and provides a context toward understanding allergic disease.

Group 2 innate lymphoid cells promote inhibitory synapse development and social behavior

The innate immune system plays essential roles in brain synaptic development, and immune dysregulation is implicated in neurodevelopmental diseases. Here we show that a subset of innate lymphocytes (group 2 innate lymphoid cells, ILC2s) is required for cortical inhibitory synapse maturation and adult social behavior. ILC2s expanded in the developing meninges and produced a surge of their canonical cytokine Interleukin-13 (IL-13) between postnatal days 5-15. Loss of ILC2s decreased cortical inhibitory synapse numbers in the postnatal period where as ILC2 transplant was sufficient to increase inhibitory synapse numbers. Deletion of the IL-4/IL-13 receptor (Il4ra) from inhibitory neurons phenocopied the reduction inhibitory synapses. Both ILC2 deficient and neuronal Il4ra deficient animals had similar and selective impairments in adult social behavior. These data define a type 2 immune circuit in early life that shapes adult brain function.

Skin γδ T cell inflammatory responses are hardwired in the thymus by oxysterol sensing via GPR183 and calibrated by dietary cholesterol

Dietary components and metabolites have a profound impact on immunity and inflammation. Here, we investigated how sensing of cholesterol metabolite oxysterols by γδ T cells impacts their tissue residency and function. We show that dermal IL-17-producing γδ T (Tγδ17) cells essential for skin-barrier homeostasis require oxysterols sensing through G protein receptor 183 (GPR183) for their development and inflammatory responses. Single-cell transcriptomics and murine reporter strains revealed that GPR183 on developing γδ thymocytes is needed for their maturation by sensing medullary thymic epithelial-cell-derived oxysterols. In the skin, basal keratinocytes expressing the oxysterol enzyme cholesterol 25-hydroxylase (CH25H) maintain dermal Tγδ17 cells. Diet-driven increases in oxysterols exacerbate Tγδ17-cell-mediated psoriatic inflammation, dependent on GPR183 on γδ T cells. Hence, cholesterol-derived oxysterols control spatially distinct but biologically linked processes of thymic education and peripheral function of dermal T cells, implicating diet as a focal parameter of dermal Tγδ17 cells.

Nanoparticle surface decoration mediated efficient protein and peptide co-encapsulation with precise ratiometric control for self-regulated drug release

Accuratly controlling drug release from a smart "self-regulated" drug delivery system is still an ongoing challenge. Herein, we developed a surface decoration strategy to achieve an efficient drug encapsulation with precise ratiometric control. Thanks to the surface decoration with cationic carrier materials by electrostatic attraction, the surface properties of different protein and peptide nanoparticles were uniformed to those adsorbed carrier materials. These carrier materials endowed protein and peptide nanoparticles with good dispersity in the oil phase and significantly inhibited the drug transfer from oil to water. With uniform surface properties, we realized the co-encapsulation of multiple types of proteins and peptides with precise ratiometric control. The encapsulation efficiency was higher than 87.8% for insulin. After solidification, the adsorbed materials on the surface of nanoparticles formed a solid protection layer, which prolonged the mean residence time of insulin from 3.3 ± 0.1 h (for insulin solution) to 47.5 ± 1.3 h. In type 1 diabetes, the spermine-modified acetalated dextran microparticle co-loaded with insulin, glucose oxidase and catalase maintained the blood glucose level within the normal range for 7 days.

Skin barrier immunology from early life to adulthood

Our skin has a unique barrier function, which is imperative for the body's protection against external pathogens and environmental insults. Although interacting closely and sharing many similarities with key mucosal barrier sites, such as the gut and the lung, the skin also provides protection for internal tissues and organs and has a distinct lipid and chemical composition. Skin immunity develops over time and is influenced by a multiplicity of different factors, including lifestyle, genetics, and environmental exposures. Alterations in early life skin immune and structural development may have long-term consequences for skin health. In this review, we summarize the current knowledge on cutaneous barrier and immune development from early life to adulthood, with an overview of skin physiology and immune responses. We specifically highlight the influence of the skin microenvironment and other host intrinsic, host extrinsic (e.g. skin microbiome), and environmental factors on early life cutaneous immunity.

Mouse Models for Atopic Dermatitis

Atopic dermatitis (AD) is a multifactorial disease with underlying barrier disruption and altered microbial flora, resulting in dry skin and eczematous inflammation with persistent pruritis. Mouse models have been heavily used to investigate AD pathophysiology. Among various AD mouse models, AD-like inflammation induced by topical calcipotriol, a vitamin D3 analog referred to as MC903 in experimental settings, is a versatile model that can be applied to any strain of mice, which can be used for immunologic and morphologic studies. Herein, we provide basic protocols for the topical application of MC903 and approaches to assess phenotypes. After inducing AD-like inflammation, the skin is harvested for flow cytometry analysis, as well as for histologic and immunofluorescence microscopy analyses. The combination of these approaches enables accurate characterization of the degree of inflammation, type of inflammatory infiltrate, and localization of immune infiltrates. Published 2023. This article is a U.S. Government work and is in the public domain in the USA. Basic Protocol 1: Application of MC903 and gross phenotype assessment Basic Protocol 2: Processing skin for flow cytometry analysis Support Protocol: Skin immune cell surface staining and flow cytometry analysis Basic Protocol 3: Harvesting skin for histologic analysis Basic Protocol 4: Immunofluorescence staining to identify immune cell infiltrates.

ILC2s in skin disorders

The activation of group 2 innate lymphoid cells (ILC2s) is controlled by various tissue-derived factors, including cytokines, whereas T cells respond to foreign antigens. This review discusses the tissue-specific properties of ILC2s in skin and their involvement in human skin diseases. In a steady state, cutaneous ILC2s contribute to tissue homeostasis. In the keratinocytes of patients with atopic dermatitis (AD), the inflammatory cytokine interleukin-33 (IL-33) is overexpressed. ILC2s are stimulated by IL-33-stimulated basophils through IL-4 to produce type 2 cytokines, such as IL-5 and IL-13. According to several studies, ILC2 expression is upregulated in human AD skin lesions, and it is involved in AD pathogenesis. Dupilumab, an antibody against IL-4 receptor α, lowered the number and percentage of ILC2s in the peripheral blood of patients with AD. Cutaneous ILC2s are divided into two subgroups: circulating and skin-resident ILC2s. However, ILC2s are homogeneous cell populations that are highly diverse and plastic, and there is no consensus on the classification that should be used. The variations in the definition for cutaneous ILC2s in different studies make comparisons among studies difficult, and in particular, the weak expression of the IL-33 receptor ST2 in cutaneous ILC2s and its lack of markers have posed a great challenge to researchers. Therefore, further comprehensive analytical studies are warranted.

Innate lymphoid cell (ILC) subsets are enriched in the skin of patients with hidradenitis suppurativa

Hidradenitis suppurativa (HS) is a chronic relapsing inflammatory skin disease manifested as painful inflamed lesions including deep-seated nodules, abscesses and sinus tracts. The exact aetiology of HS is unclear. Recent evidence suggests that immune dysregulation plays a crucial role in pathogenesis and disease progression. Innate lymphoid cells (ILC) are a recently identified immune cell subset involved in mediating immunity, however their role in HS has not yet been investigated. Three distinct subsets of ILC- ILC1, ILC2 and ILC3 have been described, and these are involved in skin tissue homeostasis and pathologic inflammation associated with autoimmunity and allergic diseases. In this study, we analysed by multiparameter flow cytometry the frequencies of ILC subsets in skin and peripheral blood mononuclear cells (PBMC) of HS patients and compared these to healthy control subjects and psoriasis patients. The absolute numbers of total ILC and subsets thereof were significantly reduced in the blood of HS patients relative to healthy controls. However, when patients were stratified according to treatment, this reduction was no longer observed in patients undergoing anti-TNF treatment. In HS lesional skin the absolute numbers of ILC were significantly increased relative to control skin. Furthermore, the frequencies of total ILC as well as ILC2 and ILC3 were significantly higher in non-lesional than lesional HS skin. This study analysed for the first time the presence of ILC subsets in the blood and skin of HS patients. Our findings suggest that ILC may participate in HS pathogenesis.

Locally sourced: site-specific immune barriers to metastasis

Tumour cells migrate very early from primary sites to distant sites, and yet metastases often take years to manifest themselves clinically or never even surface within a patient's lifetime. This pause in cancer progression emphasizes the existence of barriers that constrain the growth of disseminated tumour cells (DTCs) at distant sites. Although the nature of these barriers to metastasis might include DTC-intrinsic traits, recent studies have established that the local microenvironment also controls the formation of metastases. In this Perspective, I discuss how site-specific differences of the immune system might be a major selective growth restraint on DTCs, and argue that harnessing tissue immunity will be essential for the next stage in immunotherapy development that reliably prevents the establishment of metastases.