Contribution of plasma cells and B cells to hidradenitis suppurativa pathogenesis

Beyond the skin: endocrine, psychological and nutritional aspects in women with hidradenitis suppurativa

Hidradenitis suppurativa (HS), also known as acne inversa, is a chronic inflammatory skin disorder that primarily affects body folds and the genital area, with a higher prevalence in women across Europe. The pathogenesis of HS involves a complex interplay of intrinsic and extrinsic factors, including genetics, immunity, hormones, and environmental influences. HS is frequently associated with a variety of comorbidities, such as metabolic, endocrine, and gastrointestinal conditions, as well as mental health disorders. Although the symptoms of HS are generally similar in both men and women, female patients may experience exacerbations of HS due to hormonal fluctuations during menstruation, pregnancy, breastfeeding, and menopause. These hormonal changes require special consideration by clinicians when managing HS in women. Due to its chronic nature and frequent flare-ups, HS significantly impacts patients' quality of life, affecting social interactions, emotional well-being, and psychological health. Women with HS may also experience sexual dysfunction, which is further exacerbated by emotional burdens such as shame, loss of femininity, and diminished intimacy. This review highlights key aspects of HS, extending beyond its skin manifestations to address endocrine, psychological, and nutritional aspects in the female population. It also underscores the importance of multidisciplinary collaboration in providing comprehensive care for women with this debilitating condition. Given the limited and largely off-label treatment options, a holistic approach is essential to ensure an appropriate management.

Senescence as a molecular target in skin aging and disease

Skin aging represents a multifactorial process influenced by both intrinsic and extrinsic factors, collectively known as the skin exposome. Cellular senescence, characterized by stable cell cycle arrest and secretion of pro-inflammatory molecules, has been implicated as a key driver of physiological and pathological skin aging. Increasing evidence points towards the role of senescence in a variety of dermatological diseases, where the accumulation of senescent cells in the epidermis and dermis exacerbates disease progression. Emerging therapeutic strategies such as senolytics and senomorphics offer promising avenues to target senescent cells and mitigate their deleterious effects, providing potential treatments for both skin aging and senescence-associated skin diseases. This review explores the molecular mechanisms of cellular senescence and its role in promoting age-related skin changes and pathologies, while compiling the observed effects of senotherapeutics in the skin and discussing the translational relevance.

B cells in non-lymphoid tissues

B cells have long been understood to be drivers of both humoral and cellular immunity. Recent advances underscore this importance but also indicate that in infection, inflammatory disease and cancer, B cells function directly at sites of inflammation and form tissue-resident memory populations. The spatial organization and cellular niches of tissue B cells have profound effects on their function and on disease outcome, as well as on patient response to therapy. Here we review the role of B cells in peripheral tissues in homeostasis and disease, and discuss the newly identified cellular and molecular signals that are involved in regulating their activity. We integrate emerging data from multi-omic human studies with experimental models to propose a framework for B cell function in tissue inflammation and homeostasis.

Unravelling the complex pathogenesis of hidradenitis suppurativa

Hidradenitis suppurativa (HS) is a complex inflammatory disease, with rapid advances being made in our understanding of the complex immunological pathogenesis of the condition. New insights into the genomic landscape of HS have identified a number of genes that contribute to the development of HS in a polygenic manner, contributing to inflammatory dysregulation and alterations in epidermal stem cell fate in the follicular unit. These genomic variations can explain unique aspects of the disease such as the development and presence of epithelialized tunnels and abnormalities in wound healing. From genetic and translational studies, it is likely that these genetic alterations predispose to an innate immune dysregulation that can be triggered through sex hormone-responsive transcription factors with hormonal changes such as puberty, pregnancy and the menstrual cycle. The role of sex hormones in HS also has direct effects upon the development and maturation of inflammatory cells such as monocytes, which has the potential to explain differential patient response to treatments such as interleukin-23 antagonism. The role of adipose tissue as an active immunological organ also plays a role in the immune dysregulation seen in the disease. Fibrotic tissue and immunologically active fibroblasts play a significant role in the perpetuation of inflammation and development of adaptive immune dysfunction in the disease. The cutaneous and gut microbiomes play significant roles in the activation of innate immunity, although conflicting data exist as to their central or peripheral role in disease pathogenesis. Overall, our understanding of disease pathogenesis in HS is moving toward a more nuanced, complex paradigm in which patient heterogeneity in presentation and immunological characteristics are moving closer to the identification of therapeutic biomarkers to guide therapeutic modalities in the management of this burdensome condition.

Hidradenitis suppurativa

Hidradenitis suppurativa is a chronic inflammatory disease characterised by painful, deep-seated nodules, abscesses, and draining tunnels in the skin of axillary, inguinal, genitoanal, or inframammary areas. In recent years, the body of knowledge in hidradenitis suppurativa has advanced greatly. This disorder typically starts in the second or third decade of life. The average worldwide prevalence is 1% but varies geographically. Hidradenitis suppurativa has a profound negative effect on patients' quality of life and on the gross value added to society. Comorbidities (eg, metabolic syndrome, inflammatory arthritis, and inflammatory bowel disease) frequently accompany skin alterations, because of systemic inflammation. Pathogenesis of hidradenitis suppurativa is complex and includes innate immune mechanisms (eg, macrophages, neutrophils, IL-1β, tumour necrosis factor [TNF], and granulocyte colony-stimulating factor), T-cell mechanisms (eg, IL-17 and IFN-γ), and B-cell mechanisms (eg, associated with dermal tertiary lymphatic structures and autoantibodies). Chronic inflammation leads to irreversible skin damage with tunnel formation and morbid scarring. Current treatment includes drug therapy (for the initial, purely inflammatory phase), combined drug and surgical therapy (for the destructive phase), or surgery alone (for the burnout phase). The first systemic therapies approved for hidradenitis suppurativa targeting TNF (adalimumab) and IL-17 (secukinumab and bimekizumab) have expanded drug therapy options for moderate-to-severe disease, which were previously mainly restricted to oral antibiotics. Moreover, there is a robust pipeline of immunomodulatory drugs in various stages of development for hidradenitis suppurativa. Aims of management should include early intervention to prevent irreversible skin damage, adequate control of symptoms including pain, and mitigation of extra-cutaneous comorbidities, all requiring early diagnosis and an interdisciplinary, holistic and personalised approach.

Hidradenitis Suppurativa

Hidradenitis suppurativa (HS) is a chronic inflammatory skin disease characterized by the formation of nodules, abscesses, and fistulae at intertriginous sites. Pain, pruritus, malodor, and suppuration have a significant impact on quality of life for HS patients. Prevalence figures vary greatly in the literature from 0.05% to 4.1%, and HS is more common in females. The current understanding of the etiology and pathogenesis of HS is incomplete; numerous hypotheses concern the interplay of lifestyle factors, skin microbiota, genetics, and a dysregulated immune system. Due to its phenotypic heterogeneity and multifactorial pathogenesis, HS is a complex disease that can prove challenging to manage. Two approved biologic therapies for the management of HS have led to clinical response in approximately 50% of treated patients. New therapies targeting the interleukin (IL)-1, IL-17, IL-36, and Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathways are in ongoing clinical trials, and preliminary data offer hope for greater clinical efficacy in HS in the future.

Hidradenitis suppurativa: TLSs take the center stage

Hidradenitis suppurativa (HS) is a severe chronic inflammatory skin disease with limited response to therapy. In this issue of Immunity, Yu et al.1 identify skin tertiary lymphoid structures (TLSs) as primary sites for lymphocyte clonal expansion and autoantibody production, driving disease progression, and provide insight into how formation and maintenance of TLS impact therapeutic outcomes.

Skin immune-mesenchymal interplay within tertiary lymphoid structures promotes autoimmune pathogenesis in hidradenitis suppurativa

Hidradenitis suppurativa (HS) is a chronic, debilitating inflammatory skin disease characterized by keratinized epithelial tunnels that grow deeply into the dermis. Here, we examined the immune microenvironment within human HS lesions. Multi-omics profiling and multiplexed imaging identified tertiary lymphoid structures (TLSs) near HS tunnels. These TLSs were enriched with proliferative T cells, including follicular helper (Tfh), regulatory (Treg), and pathogenic T cells (IL17A+ and IFNG+), alongside extensive clonal expansion of plasma cells producing antibodies reactive to keratinocytes. HS fibroblasts express CXCL13 or CCL19 in response to immune cytokines. Using a microfluidic system to mimic TLS on a chip, we found that HS fibroblasts critically orchestrated lymphocyte aggregation via tumor necrosis factor alpha (TNF-α)-CXCL13 and TNF-α-CCL19 feedback loops with B and T cells, respectively; early TNF-α blockade suppressed aggregate initiation. Our findings provide insights into TLS formation in the skin, suggest therapeutic avenues for HS, and reveal mechanisms that may apply to other autoimmune settings, including Crohn's disease.

Selective small molecule inhibitors for hidradenitis suppurativa: Today and tomorrow

Hidradenitis suppurativa (HS) is an autoinflammatory condition characterized by abscesses, inflammatory nodules, and tunnels in intertriginous sites of the body. The pathogenesis of HS involves follicular occlusion in combination with environmental, genetic, hormonal, and metabolic factors. HS lesions are characterized by an influx of neutrophils, histiocytes, B and T cells, and upregulation of proinflammatory cytokines such as tumor necrosis factor-α, interleukin-1, interleukin-17, and interferons. Selective small molecule inhibitors (SMIs) are organic compounds that bind to active sites on target proteins involved in inflammatory signaling pathways, most commonly blocking enzymes, ion channels and receptors. SMIs are divided into conventional and selective SMIs. Selective SMIs are further subdivided into kinase and nonkinase SMIs. Currently there are five selective SMIs available in the United States with demonstrated efficacy for HS in clinical studies including apremilast, topical ruxolitinib, upadacitinib, fostamatinib, and sirolimus. These selective SMIs target four pathways hypothesized to be important to HS pathogenesis including phosphodiestase 4, Janus kinases, spleen tyrosine kinase, and mammalian target of rapamycin. Several new SMIs are currently in the clinical trial pipeline targeting Bruton's tyrosine kinase, aryl hydrocarbon receptors, heat shock protein 90 as well as interleukin-1 and -17 signaling pathways.

Intrinsic factors in the pathogenesis of hidradenitis suppurativa: Genetics, hormones, and the microbiome

The pathogenesis of hidradenitis suppurativa is complex and multifaceted. Our understanding of disease mechanisms is in constant flux with new genetic, inflammatory, and microbiological insights upending previous paradigms and enlightening us as to the complex connections between different drivers of disease. This updated review integrates novel genetic insights from genome wide association studies, along with novel basic science data from single cell transcriptomic studies and recent interventional translational work to inform our evolving understanding of the disease. The overarching premise is that hidradenitis suppurativa is a chronic autoinflammatory disorder characterized by inflammatory dysregulation in a background of stem cell fate aberrations highly responsive to hormonal and metabolic changes. Genetic drivers are linked to both hair follicle stem cell fate and cellular response to androgen signaling. The complex mixed inflammatory circuits involve both innate and adaptive immunity, with activation of inflammatory fibroblast subsets and tertiary lymphoid organs in chronic disease. Interplay between inflammation, the cutaneous and gut microbiomes are observed both pre- and posttherapy, however causality remain unclear. Metabolic syndrome, sex hormones and insulin resistance are all interlinked, with adipose tissue being a hormonally active organ able to modulate endogenous sources of sex hormones. Future enquiry regarding factors contributing to disease progression and the identification of novel therapeutic targets will aid in more effective therapeutic strategies for management of this burdensome disease.

CD2 expressing innate lymphoid and T cells are critical effectors of immunopathogenesis in hidradenitis suppurativa

Hidradenitis suppurativa (HS) is a chronic, debilitating inflammatory skin disease with a poorly understood immunopathogenesis. Here, we report that HS lesional skin is characterized by the expansion of innate lymphocytes and T cells expressing CD2, an essential activation receptor and adhesion molecule. Lymphocytes expressing elevated CD2 predominated with unique spatial distribution throughout the epidermis and hypodermis in the HS lesion. CD2+ cells were mainly innate lymphocytes expressing the NK cell marker, CD56, and CD4+ T cells. Importantly, these CD2+ cells interacted with CD58 (LFA3) expressing epidermal keratinocytes and fibroblasts in the hypodermis. Granzyme Abright NKT cells (CD2+CD3+CD56bright) clustered with α-SMA expressing fibroblasts juxtaposed to epithelialized tunnels and fibrotic regions of the hypodermis. Whereas NK cells (CD2+CD56dim) were perforin+, granzymes A+ and B+, and enriched adjacent to hyperplastic follicular epidermis and tunnels of HS showing presence of apoptotic cells. The cytokines IL-12, IL-15, and IL-18, which enhance NK cell maturation and function were significantly elevated in HS. Ex vivo HS skin explant cultures treated with CD2:CD58 interaction-blocking anti-CD2 monoclonal antibody attenuated secretion of inflammatory cytokines/chemokines and suppressed inflammatory gene signature. Additionally, CD2:CD58 blockade altered miRNAs involved in NK/NKT differentiation and/or function. In summary, we show that a cellular network of heterogenous NKT and NK cell populations drives inflammation and is critical in the pathobiology of HS, including tunnel formation and fibrosis. Finally, CD2 blockade is a viable immunotherapeutic approach for the effective management of HS.

Australasian hidradenitis suppurativa management guidelines

Hidradenitis Suppurativa is a burdensome inflammatory skin disease with significant quality of life impact. These management guidelines were developed to direct appropriate clinical management in the Australasian context. A systematic review was used for the basis of the consensus guidelines. Thirteen clinical experts were involved in a modified Delphi consensus process to develop the guidelines and treatment algorithms. Overall management strategies include appropriate severity assessment of disease and comorbidities, multimodal therapy with systemic and local treatments, and evidence-based progression along the therapeutic ladder in the event of inadequate response. Sequential monotherapy with antibiotics and/or single agent therapy is discouraged and aggressive treatment of moderate to severe disease to capture the window of opportunity is highly emphasised. Specific considerations in the setting of disease comorbidities, pregnancy and breastfeeding are also addressed. Overall, the complex nature of HS requires a complex and multimodal therapeutic response with medical, physical and surgical therapies to achieve best patient outcomes.

Disease heterogeneity and molecular classification of inflammatory palmoplantar diseases

BACKGROUND

Palmoplantar pustulosis (PPP) is an inflammatory disease characterized by relapsing eruptions of neutrophil-filled, sterile pustules on the palms and soles that can be clinically difficult to differentiate from non-pustular palmoplantar psoriasis (palmPP) and dyshidrotic palmoplantar eczema (DPE).

OBJECTIVE

We sought to identify overlapping and unique PPP, palmPP, and DPE drivers to provide molecular insight into their pathogenesis.

METHODS

We performed bulk RNA sequencing of lesional PPP (n = 33), palmPP (n = 5), and DPE (n = 28) samples, as well as 5 healthy nonacral and 10 healthy acral skin samples.

RESULTS

Acral skin showed a unique immune environment, likely contributing to a unique niche for palmoplantar inflammatory diseases. Compared to healthy acral skin, PPP, palmPP, and DPE displayed a broad overlapping transcriptomic signature characterized by shared upregulation of proinflammatory cytokines (TNF, IL-36), chemokines, and T-cell-associated genes, along with unique disease features of each disease state, including enriched neutrophil processes in PPP and to a lesser extent in palmPP, and lipid antigen processing in DPE. Strikingly, unsupervised clustering and trajectory analyses demonstrated divergent inflammatory profiles within the 3 disease states. These identified putative key upstream immunologic switches, including eicosanoids, interferon responses, and neutrophil degranulation, contributing to disease heterogeneity.

CONCLUSION

A molecular overlap exists between different inflammatory palmoplantar diseases that supersedes clinical and histologic assessment. This highlights the heterogeneity within each condition, suggesting limitations of current disease classification and the need to move toward a molecular classification of inflammatory acral diseases.

Proof-of-concept study exploring the effect of spesolimab in patients with moderate-to-severe hidradenitis suppurativa: a randomized double-blind placebo-controlled clinical trial

BACKGROUND

Hidradenitis suppurativa (HS) is a chronic inflammatory skin disease with a considerable disease burden. Existing treatment options are limited and often suboptimal; a high unmet need exists for effective targeted therapies.

OBJECTIVES

To explore the effects of spesolimab treatment in patients with HS.

METHODS

This randomized double-blind placebo-controlled proof-of-clinical-concept (PoCC) study was conducted at 25 centres across 12 countries from 3 May 2021 to 21 April 2022. Patients had moderate-to-severe HS for ≥ 1 year before enrolment. Patients were randomized (2 : 1) to receive a loading dose of 3600-mg intravenous spesolimab (1200 mg at weeks 0, 1 and 2) or matching placebo, followed by maintenance with either 1200-mg subcutaneous spesolimab every 2 weeks from weeks 4 to 10 or matching placebo. The primary endpoint was the percentage change from baseline in total abscess and inflammatory nodule (AN) count at week 12. Secondary endpoints were the absolute change from baseline in the International Hidradenitis Suppurativa Severity Score System (IHS4), percentage change from baseline in draining tunnel (dT) count, the proportion of patients achieving a dT count of 0, absolute change from baseline in the revised Hidradenitis Suppurativa Area and Severity Index (HASI-R), the proportion of patients achieving Hidradenitis Suppurativa Clinical Response (HiSCR50), the proportion of patients with ≥ 1 flare (all at week 12) and patient-reported outcomes.

RESULTS

In this completed trial, randomized patients (n = 52) received spesolimab (n = 35) or placebo (n = 17). The difference vs. placebo in least squares mean is reported. At week 12, the percentage change in total AN count was similar between treatment arms: -4.1% [95% confidence interval (CI) -31.7 to 23.4]. There was greater numerical improvement in the spesolimab arm, as measured by IHS4 (13.9, 95% CI -25.6 to -2.3); percentage change from baseline in dT count (-96.6%, 95% CI -154.5 to -38.8); and the proportion of patients achieving a dT count of 0 (18.3%, 95% CI -7.9 to 37.5). Spesolimab treatment also improved HASI-R and HiSCR50 vs. placebo. Spesolimab demonstrated a favourable safety profile, similar to that observed in trials in other diseases.

CONCLUSIONS

This exploratory PoCC study supports the development of spesolimab as a new therapeutic option in HS.

Cellular indexing of transcriptomes and epitopes (CITE-Seq) in hidradenitis suppurativa identifies dysregulated cell types in peripheral blood and facilitates diagnosis via machine learning

Hidradenitis suppurativa (HS) is a chronic inflammatory skin condition characterized by painful nodules, abscesses, and scarring, predominantly affecting intertriginous regions and it is often underdiagnosed. This study aimed to utilize single cell RNA and cell-surface protein sequencing (CITE-Seq) to delineate the immune composition of circulating cells in Hidradenitis suppurativa (HS) peripheral blood compared to healthy controls. CITE-Seq was used to analyze the gene and protein expression profiles of peripheral blood mononuclear cells (PBMCs) from 9 HS and 29 healthy controls. The study identified significant differences cell composition between HS patients and healthy controls, including increased proportions of CD14+ and CD16+ monocytes, cDC2, plasmablasts, and proliferating CD4+ T cells in HS patients. Differential expression analysis revealed upregulation of inflammatory markers such as TNF, IL1B, and NF-κB in monocytes, as well as chemokines and cell adhesion molecules involved in immune cell recruitment and tissue infiltration. Pathway enrichment analysis highlighted the involvement of IL-17, IL-26 and TNF signaling pathways in HS pathogenesis. Machine learning identified key markers for diagnostics and therapeutic development. The findings also support the potential for machine learning models to aid in the diagnosis of HS based on immune cell markers. These insights may inform future therapeutic strategies targeting specific immune pathways in HS.

Single-cell and spatial transcriptomics of vulvar lichen sclerosus reveal multi-compartmental alterations in gene expression and signaling cross-talk

Vulvar diseases are a critical yet often neglected area of women's health, profoundly affecting patients' quality of life and frequently resulting in long-term physical and psychological challenges. Lichen sclerosus (LS) is a chronic inflammatory skin disorder that predominantly affects the vulva, leading to severe itching, pain, scarring, and an increased risk of malignancy. Despite its profound impact on affected individuals, the molecular pathogenesis of vulvar LS (VLS) is not well understood, hindering the development of FDA-approved therapies. Here, we utilize single-cell and spatial transcriptomics to analyze lesional and non-lesional skin from VLS patients, as well as healthy control vulvar skin. Our findings demonstrate histologic, cellular, and molecular heterogeneities within VLS, yet highlight unifying molecular changes across keratinocytes, fibroblasts, immune cells, and melanocytes in lesional skin. They reveal cellular stress and damage in fibroblasts and keratinocytes, enhanced T cell activation and cytotoxicity, aberrant cell-cell signaling, and increased activation of the IFN, JAK/STAT, and p53 pathways in specific cell types. Using both monolayer and organotypic culture models, we also demonstrate that knockdown of select genes, which are downregulated in VLS lesional keratinocytes, partially recapitulates VLS-like stress-associated changes. Collectively, these data provide novel insights into the pathogenesis of VLS, identifying potential biomarkers and therapeutic targets for future research.

The potential of Bruton's tyrosine kinase (BTK) inhibitors in the pharmacotherapeutic management of immune and dermatological disease

INTRODUCTION

The review article explores the evolving role of Bruton's tyrosine kinase (BTK) inhibitors in immune-mediated dermatological conditions, addressing significant gaps in current treatment approaches.

AREAS COVERED

The review comprehensively discusses the mechanisms of action of BTK inhibitors, including irreversible and reversible inhibitors. Clinical applications of BTK inhibitors in dermatological diseases such as pemphigus, chronic spontaneous urticaria (CSU), hidradenitis suppurativa (HS), systemic lupus erythematosus (SLE), and atopic dermatitis are explored, highlighting recent advancements and ongoing clinical trials. Potential advantages of BTK inhibitors over existing therapies and challenges in translating preclinical findings to clinical outcomes are discussed.

EXPERT OPINION/COMMENTARY

BTK inhibitors represent a promising therapeutic avenue for immune-mediated dermatological conditions, offering oral administration, targeted pathway inhibition, and a favorable safety profile compared to biologic therapies. Ongoing research and clinical trials hold the potential to address unmet needs and reshape the therapeutic landscape in dermatology.

Repeated stress to the skin amplifies neutrophil infiltration in a keratin 17- and PKCα-dependent manner

Neutrophils are the first immune cells to reach inflamed sites and contribute to the pathogenesis of chronic inflammatory skin diseases. Yet, little is known about the pattern of neutrophil infiltration in inflamed skin in vivo and the mechanisms mediating their recruitment. Here, we provide insight into the dynamics of neutrophil infiltration in skin in response to acute or repeated inflammatory stress, highlighting a novel keratinocyte- and keratin 17 (K17)-dependent mechanism that regulates neutrophil recruitment to inflamed skin. We used the phorbol ester TPA and UVB, alone or in combination, to induce sterile inflammation in mouse skin. A single TPA treatment results in a neutrophil influx in the dermis that peaks at 12 h and resolves within 24 h. A subsequent TPA treatment or a UVB challenge, when applied 24 h but not 48 h later, accelerates, amplifies, and prolongs neutrophil infiltration. This transient amplification response (TAR) is mediated by local signals in inflamed skin, can be recapitulated in ex vivo culture, and involves the K17-dependent sustainment of protein kinase Cα (PKCα) activity and release of chemoattractants by stressed keratinocytes. K17 binds RACK1, a scaffold protein essential for PKCα activity. The N-terminal head domain of K17 is crucial for its association with RACK1 and regulation of PKCα activity. Analysis of RNAseq data reveals a signature consistent with TAR and PKCα activation in inflammatory skin diseases. These findings uncover a novel, keratin-dependent mechanism that amplifies neutrophil recruitment in skin under stress, with direct implications for inflammatory skin disorders.

Hepatocyte Growth Factor Has Unique Functions in Keratinocytes that Differ from those of IL-17A and TNF and May Contribute to Inflammatory Pathways in Hidradenitis Suppurativa

Hidradenitis suppurativa (HS) is a chronic inflammatory disease that is difficult to control, and its mechanism remains unclear. Hepatocyte GF (HGF) has been reported to be significantly upregulated in the serum and skin of patients with HS, especially in the lesions with tunnels. In this study, we examined the transcriptome of HGF-treated keratinocytes and compared it with genetic profiling of HS lesions. HGF was highly expressed in HS skin, especially in the deep dermis, compared with that in healthy controls, and its source was mainly fibroblasts. HGF upregulated more genes in keratinocytes than IL-17A or TNF-a, and these genes included multiple epithelial-mesenchymal transition-related genes. Differentially expressed genes in HGF-stimulated keratinocytes were involved in activation of epithelial-mesenchymal transition-related pathways. These HGF-induced genes were significantly upregulated in HS lesions compared with those in healthy skin and nonlesions and were more strongly associated with HS tunnels. In summary, HGF was highly expressed in HS and induced epithelial-mesenchymal transition-related genes in keratinocytes; HGF-induced genes were highly associated with gene profiling of HS with tunnels, suggesting that HGF may be involved in HS tunnel formation through epithelial-mesenchymal transition.

Single-cell mass cytometry in immunological skin diseases

Immune-related skin diseases represent a collective of dermatological disorders intricately linked to dysfunctional immune system processes. These conditions are primarily characterized by an immoderate activation of the immune system or deviant immune responses, involving diverse immune components including immune cells, antibodies, and inflammatory mediators. However, the precise molecular dysregulation underlying numerous individual cases of these diseases and unique subsets respond under disease conditions remains elusive. Comprehending the mechanisms and determinants governing the homeostasis and functionality of diseases could offer potential therapeutic opportunities for intervention. Mass cytometry enables precise and high-throughput quantitative measurement of proteins within individual cells by utilizing antibodies labeled with rare heavy metal isotopes. Imaging mass cytometry employs mass spectrometry to obtain spatial information on cell-to-cell interactions within tissue sections, simultaneously utilizing more than 40 markers. The application of single-cell mass cytometry presents a unique opportunity to conduct highly multiplexed analysis at the single-cell level, thereby revolutionizing our understanding of cell population heterogeneity and hierarchy, cellular states, multiplexed signaling pathways, proteolysis products, and mRNA transcripts specifically in the context of many autoimmune diseases. This information holds the potential to offer novel approaches for the diagnosis, prognostic assessment, and monitoring responses to treatment, thereby enriching our strategies in managing the respective conditions. This review summarizes the present-day utilization of single-cell mass cytometry in studying immune-related skin diseases, highlighting its advantages and limitations. This technique will become increasingly prevalent in conducting extensive investigations into these disorders, ultimately yielding significant contributions to their accurate diagnosis and efficacious therapeutic interventions.

Comparative transcriptome analysis of acne vulgaris, rosacea, and hidradenitis suppurativa supports high-dose dietary zinc as a therapeutic agent

Acne vulgaris, rosacea, and hidradenitis suppurativa are enduring inflammatory skin conditions that frequently manifest with akin clinical attributes, posing a considerable challenge for their distinctive diagnosis. While these conditions do exhibit certain resemblances, they also demonstrate distinct underlying pathophysiological mechanisms and treatment modalities. Delving into both the molecular parallels and disparities among these three disorders can yield invaluable insights for refined diagnostics, effective management, and targeted therapeutic interventions. In this report, we present a comparative analysis of transcriptomic data across these three diseases, elucidating differentially expressed genes and enriched pathways specific to each ailment, as well as those shared among them. Specifically, we identified multiple zinc-binding proteins (SERPINA1, S100A7, S100A8, S100A9 and KRT16) as consistently highly upregulated genes across all three diseases. Our hypothesis suggests that these proteins could bind and sequester zinc, potentially leading to localized zinc deficiency and heightened inflammation. We identified high-dose dietary zinc as a promising therapeutic approach and confirmed its effectiveness through validation in an acne mouse model.

Positionally distinct interferon stimulated dermal immune acting fibroblasts promote neutrophil recruitment in Sweet's syndrome

Sweet's syndrome is a poorly understood inflammatory skin disease characterized by neutrophil infiltration to the dermis. Single-nucleus and bulk transcriptomics of archival clinical samples of Sweet's syndrome revealed a prominent interferon signature in Sweet's syndrome skin that was reduced in tissue from other neutrophilic dermatoses. This signature was observed in different subsets of cells, including fibroblasts that expressed interferon-induced genes. Functionally, this response was supported by analysis of cultured primary human dermal fibroblasts that were observed to highly express neutrophil chemokines in response to activation by type I interferon. Furthermore, single-molecule resolution spatial transcriptomics of skin in Sweet's syndrome identified positionally distinct immune acting fibroblasts that included a CXCL1+ subset proximal to neutrophils and a CXCL12+ subset distal to the neutrophilic infiltrate. This study defines the cellular landscape of neutrophilic dermatoses and suggests dermal immune acting fibroblasts play a role in the pathogenesis of Sweet's syndrome through recognition of type I interferons.

Biologics for Hidradenitis suppurativa: evolution of the treatment paradigm

INTRODUCTION

Hidradenitis suppurativa (HS) is an autoinflammatory skin disease with a high unmet need for effective medical management. Clinically, it is characterized by inflammatory nodules that may progress into abscesses, draining tunnels and extensive scarring, mainly affecting apocrine gland-bearing areas.

AREAS COVERED

Treatment options include topical and systemic medications and a variety of surgical procedures. The anti-TNF-α antibody adalimumab and the anti-IL-17 secukinumab are the only two approved biologics for HS, showing moderate efficacy. HS research is a rapidly growing field, with a wide range of agents leveraging distinct mechanisms of action currently under development. Drugs targeting the IL-17 and Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathways are the most advanced in both ongoing and completed Phase 3 studies, promising deeper levels of response. Use of other, off-label biologics is also discussed.

EXPERT OPINION

A therapeutic algorithm is proposed based on comorbidities and existing evidence. Patient-tailored combinations between biologics and other biologics or small molecules will hopefully allow clinicians to target most events in HS pathophysiology in a complementary way while obtaining a meaningful effect on their devastating manifestations.

Proteomic Profiling of Central Centrifugal Cicatricial Alopecia Reveals Role of Humoral Immune Response Pathway and Metabolic Dysregulation

Proteomic profiling on other primary cicatricial alopecias, such as frontal fibrosing alopecia and lichen planopilaris, have suggested a T helper 1-mediated inflammatory pathway, but in central centrifugal cicatricial alopecia (CCCA), the protein expression patterns are unknown. In this study, we sought to characterize protein expression patterns in CCCA to identify biomarkers of disease activity that will identify potential therapeutic avenues for treatment. Scalp protein quantification was performed to understand protein expression patterns in affected versus unaffected scalps in CCCA. A total of 5444 proteins were identified, of which 148 proteins were found to be differentially expressed in CCCA-affected scalp, with upregulation of adaptive immune pathways (IGHG3, P = .034; IGHG4, P = .01; IGG1, P = .026) and markers of fibrosis (ITGA1, P = .016; SFRP2, P = .045; TPM2, P = .029; SLMAP, P = .016) and downregulation of metabolic proteins (ALOX15B, P = .003; FADS2, P = .006; ELOVL5, P = .007; FA2H, P = .017; FAR2, P = .011; SC5D, P < .001). Our analysis revealed, to our knowledge, previously unknown humoral immune canonical pathways, notably IgG, implicated in CCCA and additionally confirmed aberrant lipid metabolism pathways implicated in diabetes mellitus, suggesting unique mechanisms of disease in patients with CCCA.

Alterations to the Hidradenitis Suppurativa Serum Proteome with Spleen Tyrosine Kinase Antagonism: Proteomic Results from a Phase 2 Clinical Trial

Hidradenitis suppurativa is a disease in great need of novel therapies. Given the heterogeneous nature of the disease and the variable response to therapies, biomarkers are essential to predict response to therapies and increase our understanding of disease pathogenesis. Our recent phase 2 clinical trial of spleen tyrosine kinase antagonism using fostamatinib in hidradenitis suppurativa demonstrated a 75% clinical response, with the greatest benefit in individuals with elevated serum inflammation and IgG. In this study, we present results of an in-depth serum proteomic analysis in this patient cohort identifying downregulation of IL-12B as well as B-cell-associated proteins CCL19 and CCL20 and IFN-γ-mediated proteins CXCL10 and CX3CL1. Clinical responders demonstrated greater reduction in serum IL-17A, IL-6, IL-8, and CX3CL1 compared with clinical nonresponders. Baseline levels of CCL28 were associated with clinical response to fostamatinib therapy at week 12. Overall, this suggests that fostamatinib, by targeting B-cell receptor and Fc receptor activity in B cells, monocytes, and macrophages, has a significant molecular impact on the inflammatory serum proteome of hidradenitis suppurativa. In addition, potential therapeutic biomarkers may aid in patient selection for targeted therapy.

The NLRP3 Inflammasome Gene Is Overexpressed in Hidradenitis Suppurativa Lesions: A Preliminary Study on the Role of Pyroptosis in Disease Pathogenesis

Hidradenitis suppurativa (HS) is a debilitating inflammatory skin disorder, and its pathogenesis remains incompletely understood. This study aimed to investigate the role of the P2X7 receptor (P2X7R) and NLRP3 inflammasome in HS pathogenesis. RNA sequencing and real-time PCR were performed to assess the gene expression levels of P2X7R and NLRP3 in the skin biopsies of HS patients and healthy controls (HC). The results of our study revealed a significantly increased expression of the NLRP3 gene in both the lesional and perilesional skin of HS patients compared to healthy controls. Moreover, the mRNA levels of NLRP3 were significantly higher in lesional skin compared to non-lesional skin in HS patients, indicating the spread of inflammation to adjacent tissues. In contrast, no significant differences in P2X7R gene expression were observed between the three groups. These findings suggest the involvement of NLRP3 inflammasomes in HS pathogenesis, while P2X7R may not play a significant role in the disease. This research sheds light on the complex inflammatory pathways in HS, highlighting the potential of NLRP3 as a therapeutic target. Understanding the molecular mechanisms underlying HS is crucial for the development of targeted treatment modalities for this debilitating condition.

Significance of stress keratin expression in normal and diseased epithelia

A group of keratin intermediate filament genes, the type II KRT6A-C and type I KRT16 and KRT17, are deemed stress responsive as they are induced in keratinocytes of surface epithelia in response to environmental stressors, in skin disorders (e.g., psoriasis) and in carcinomas. Monitoring stress keratins is widely used to identify keratinocytes in an activated state. Here, we analyze single-cell transcriptomic data from healthy and diseased human skin to explore the properties of stress keratins. Relative to keratins occurring in healthy skin, stress-induced keratins are expressed at lower levels and show lesser type I-type II pairwise regulation. Stress keratins do not "replace" the keratins expressed during normal differentiation nor reflect cellular proliferation. Instead, stress keratins are consistently co-regulated with genes with roles in differentiation, inflammation, and/or activation of innate immunity at the single-cell level. These findings provide a roadmap toward explaining the broad diversity and contextual regulation of keratins.

Genetic mutations in pyoderma gangrenosum, hidradenitis suppurativa, and associated autoinflammatory syndromes: Insights into pathogenic mechanisms and shared pathways

Pyoderma gangrenosum (PG), hidradenitis suppurativa (HS), and the associated autoinflammatory syndromes, including pyogenic arthritis, pyoderma gangrenosum, and acne (PAPA) syndrome, PSTPIP1-associated myeloid-related proteinemia inflammatory (PAMI) syndrome, pyoderma gangrenosum, acne, and hidradenitis suppurativa (PASH) syndrome, and pyogenic arthritis, pyoderma gangrenosum, acne, and suppurative hidradenitis (PAPASH) syndrome are dermatological conditions characterized by chronic inflammation and tissue damage. Recent advances in genetic research have identified specific mutations associated with these disorders, shedding light on their underlying pathogenic mechanisms. This review aims to summarize the current knowledge of identified mutations and presumed pathophysiology in PG, HS, and the associated autoinflammatory syndromes.

Single-cell sequencing reveals Hippo signaling as a driver of fibrosis in hidradenitis suppurativa

Hidradenitis suppurativa (HS) is a chronic inflammatory disease characterized by abscesses, nodules, dissecting/draining tunnels, and extensive fibrosis. Here, we integrate single-cell RNA sequencing, spatial transcriptomics, and immunostaining to provide an unprecedented view of the pathogenesis of chronic HS, characterizing the main cellular players and defining their interactions. We found a striking layering of the chronic HS infiltrate and identified the contribution of 2 fibroblast subtypes (SFRP4+ and CXCL13+) in orchestrating this compartmentalized immune response. We further demonstrated the central role of the Hippo pathway in promoting extensive fibrosis in HS and provided preclinical evidence that the profibrotic fibroblast response in HS can be modulated through inhibition of this pathway. These data provide insights into key aspects of HS pathogenesis with broad therapeutic implications.

An Atlas of the Hidradenitis Suppurativa Transcriptome

INTRODUCTIONS

Hidradenitis suppurativa (HS) is a chronic inflammatory condition of the skin. Both genetic and environmental factors contribute to the risk of developing HS, but the pathogenesis of this disease is currently not fully understood. The aim of this study was to further current understanding of the molecular background of HS with the use of global transcriptome analyses.

METHODS

Transcriptome profiling of perilesional and lesional skin of five patients with HS and six healthy control patients was performed by next-generation sequencing. Groups of differentially expressed genes characteristic of the skin of patients with HS were shortlisted by bioinformatic analysis.

RESULTS

RNA sequencing followed by bioinformatic profiling revealed profound enrichment of inflammatory-related processes in both lesional and perilesional skin of patients with HS. There were, however, distinct differences in the gene expression profiles between the lesional and perilesional skin, with 1488 genes differentially expressed. Genes encoding typical proinflammatory cytokines were profoundly enriched within HS lesions. In contrast, those encoding mediators of extracellular matrix organization were highly expressed mostly in the perilesional area.

CONCLUSIONS

Our study provides novel insights into the mechanisms underlying the pathogenesis of HS, and the results also have potential clinical implications in both diagnosis and therapeutics.

Emerging Role of the IL-36/IL-36R Axis in Multiple Inflammatory Skin Diseases

IL-36 is a most recent member of the IL-1 cytokine family, primarily expressed at barrier sites of the body such as the skin, lungs, and intestine. It plays a vital role in inflammation and is implicated in the development of various cutaneous; intestinal; and pulmonary disorders, including psoriasis, inflammatory bowel disease, and chronic obstructive pulmonary disease. IL-36 comprises 4 isoforms: the proinflammatory IL-36α, IL-36β, and IL-36γ and the anti-inflammatory IL-36R antagonist. An imbalance between proinflammatory and anti-inflammatory IL-36 isoforms can contribute to the inflammatory fate of cells and tissues. IL-36 cytokines signal through an IL-36R heterodimer mediating their function through canonical signaling cacade, including the NF-B pathway. Prominent for its role in psoriasis, IL-36 has recently been associated with disease mechanisms in atopic dermatitis, hidradenitis suppurativa, neutrophilic dermatoses, autoimmune blistering disease, and Netherton syndrome. The major cutaneous source of IL-36 cytokines is keratinocytes, pointing to its role in the communication between the epidermis, innate (neutrophils, dendritic cells) immune system, and adaptive (T helper [Th]1 cells, Th17) immune system. Thus, cutaneous IL-36 signaling is crucial for the immunopathological outcome of various skin diseases. Consequently, the IL-36/IL-36R axis has recently been recognized as a promising drug target for the treatment of inflammatory disorders beyond psoriasis. This review summarizes the current update on IL-36 cytokines in inflammatory skin diseases.

Hidradenitis suppurativa: new insights into disease mechanisms and an evolving treatment landscape

Hidradenitis suppurativa (HS), also known as acne inversa, is a chronic disabling and debilitating inflammatory disease with a high unmet medical need. The prevalence of HS reported in most studies is 1-2%, although it is likely to be under-reported and estimates vary globally owing to variance in data collection methods, ethnicity, geographical location and under-diagnosis. HS is characterized by persistent, painful cutaneous nodules, abscesses and draining tunnels commonly affecting the axillary, anogenital, inguinal and perianal/gluteal areas. Over time, chronic uncontrolled inflammation results in irreversible tissue destruction and scarring. Although the pathophysiology of HS has not been fully elucidated, the tumour necrosis factor (TNF)-α and interleukin (IL)-17 pathways have an important role, involving multiple cytokines. Currently, treatment options include topical medications; systemic therapies, including repeated and/or rotational courses of systemic antibiotics, retinoids and hormonal therapies; and various surgical procedures. The anti-TNF-α antibody adalimumab is currently the only biologic approved by both the US Food and Drug Administration and the European Medicines Agency for HS; however, its efficacy varies, with a clinical response reported in approximately 50% of patients in phase III trials. HS is a rapidly evolving field of discovery, with a diverse range of agents with distinct mechanisms of action currently being explored in clinical trials. Several other promising therapeutic targets have recently emerged, and agents targeting the IL-17 and Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathways are the most advanced in ongoing or completed phase III clinical trials. Alongside limited therapeutic options, significant challenges remain in terms of diagnosis and disease management, with a need for better treatment outcomes. Other unmet needs include significant diagnostic delays, thus missing the therapeutic 'window of opportunity'; the lack of standardized outcome measures in clinical trials; and the lack of established, well-defined disease phenotypes and biomarkers.

Single-cell profiling of prurigo nodularis demonstrates immune-stromal crosstalk driving profibrotic responses and reversal with nemolizumab

BACKGROUND

Prurigo nodularis (PN) is a chronic neuroimmune skin disease characterized by bilaterally distributed pruritic hyperkeratotic nodules on extremities and trunk. Neuroimmune dysregulation and chronic scratching are believed to both induce and maintain the characteristic lesions.

OBJECTIVES

This study sought to provide a comprehensive view of the molecular pathogenesis of PN at the single-cell level to identify and outline key pathologic processes and the cell types involved. Features that distinguish PN skin from the skin of patients with atopic dermatitis were of particular interest. We further aimed to determine the impact of the IL31RA antagonist, nemolizumab, and its specificity at the single-cell level.

METHODS

Single-cell RNA-sequencing of skin from 15 healthy donors and nonlesional and lesional skin from 6 patients each with PN and atopic dermatitis, combined with spatial-sequencing using the 10x Visium platform. Integration with bulk RNA-sequencing data from patients treated with nemolizumab.

RESULTS

This study demonstrates that PN is an inflammatory skin disease characterized by both keratinocyte proliferation and activation of profibrotic responses. This study also demonstrates that the COL11A1+ fibroblast subset is a major contributor to fibrosis and is predominantly found in the papillary dermis of PN skin. Activation of fibrotic responses is the main distinguishing feature between PN and atopic dermatitis skin. This study further shows the broad effect of nemolizumab on PN cell types, with a prominent effect driving COL11A1+ fibroblast and keratinocyte responses toward normal.

CONCLUSIONS

This study provides a high-resolution characterization of the cell types and cellular processes activated in PN skin, establishing PN as a chronic fibrotic inflammatory skin disease. It further demonstrates the broad effect of nemolizumab on pathological processes in PN skin.

Large-scale functional inference for skin-expressing lncRNAs using expression and sequence information

Long noncoding RNAs (lncRNAs) regulate the expression of protein-coding genes and have been shown to play important roles in inflammatory skin diseases. However, we still have limited understanding of the functional impact of lncRNAs in skin, partly due to their tissue specificity and lower expression levels compared with protein-coding genes. We compiled a comprehensive list of 18,517 lncRNAs from different sources and studied their expression profiles in 834 RNA-Seq samples from multiple inflammatory skin conditions and cytokine-stimulated keratinocytes. Applying a balanced random forest to predict involvement in biological functions, we achieved a median AUROC of 0.79 in 10-fold cross-validation, identifying significant DNA binding domains (DBDs) for 39 lncRNAs. G18244, a skin-expressing lncRNA predicted for IL-4/IL-13 signaling in keratinocytes, was highly correlated in expression with F13A1, a protein-coding gene involved in macrophage regulation, and we further identified a significant DBD in F13A1 for G18244. Reflecting clinical implications, AC090198.1 (predicted for IL-17 pathway) and AC005332.6 (predicted for IFN-γ pathway) had significant negative correlation with the SCORAD metric for atopic dermatitis. We also utilized single-cell RNA and spatial sequencing data to validate cell type specificity. Our research demonstrates lncRNAs have important immunological roles and can help prioritize their impact on inflammatory skin diseases.

Comparative Transcriptome Analysis of Acne vulgaris, Rosacea, and Hidradenitis Suppurativa Supports High Dose Dietary Zinc as a Therapeutic Agent

Acne vulgaris, rosacea, and hidradenitis suppurativa are enduring inflammatory skin conditions that frequently manifest with akin clinical attributes, posing a considerable challenge for their distinctive diagnosis. While these conditions do exhibit certain resemblances, they also demonstrate distinct underlying pathophysiological mechanisms and treatment modalities. Delving into both the molecular parallels and disparities among these three disorders can yield invaluable insights for refined diagnostics, effective management, and targeted therapeutic interventions. In this report, we present a comparative analysis of transcriptomic data across these three diseases, elucidating differentially expressed genes and enriched pathways specific to each ailment, as well as those shared among them. We also identified high dose dietary zinc as a potential therapeutic agent and validated its efficacy in an acne mouse model.

A history of asthma is associated with susceptibility to hidradenitis suppurativa: a population-based longitudinal study

The association of hidradenitis suppurativa (HS) and asthma remains to be investigated. To assess the bidirectional association between HS and asthma. A population-based study was conducted to compare HS patients (n = 6779) with age-, sex-, and ethnicity-matched control subjects (n = 33,259) with regard to the incidence of new onset and the prevalence of preexisting asthma. Adjusted hazard ratios (HRs) and adjusted odds ratios (ORs) were calculated. The prevalence of preexisting asthma was higher in patients with HS relative to controls (9.6% vs. 6.9%, respectively; P < 0.001). The odds of HS were 1.4-fold greater in patients with a history of asthma (fully adjusted OR 1.41; 95% CI 1.27-1.55; P < 0.001). The incidence rate of new-onset asthma was estimated at 9.0 (95% CI 6.3-12.7) and 6.2 (95% CI 5.1-7.5) cases per 10,000 person-years among patients with HS and controls, respectively. The risk of asthma was not statistically different in patients with HS and controls (fully adjusted HR 1.53; 95% CI 0.98-2.38; P = 0.062). Relative to other patients with HS, those with HS and comorbid asthma were younger at the onset of HS (30.7 [14.7] vs. 33.3 [15.1], respectively; P < 0.001) and had a comparable risk of all-cause mortality (adjusted HR 0.86; 95% CI 0.44-1.68; P = 0.660). A history of asthma confers susceptibility to subsequent development of HS. This observation is of importance for clinicians managing both patients with HS and asthma. Further research is warranted to elucidate the pathomechanism underlying this finding.

Biology of Interleukin-17 and Novel Therapies for Hidradenitis Suppurativa

Skin disorders affect ∼40% of the human population. One of the most debilitating cutaneous disorders is Hidradenitis suppurativa (HS), a noncommunicable chronic inflammatory disease with an estimated global prevalence of 0.4% to 2.5%. In January 2011, high levels of IL-17 were discovered in skin lesions of HS patients. In the following years, translational and clinical research led to a better understanding of the pathogenesis of HS. In June 2023, more than 12 years after the initial note, secukinumab, an anti-IL-17A monoclonal antibody, was approved for the treatment of moderate to severe HS. This is the next milestone in improving the treatment of these patients after the approval of the anti-TNF-α monoclonal antibody adalimumab in 2015. In this review article, we present the IL-17 pathway in HS and discuss the use of secukinumab as a therapeutic option for this disease. Our review starts with a description of the epidemiology, clinical features, etiology, and pathogenesis of HS. An overview of the IL-17/IL-17 receptor system in general and a detailed description of the known facts about the expression and action of IL-17 in HS follow. Afterward, we consider the results of clinical trials evaluating the safety and efficacy of IL-17 inhibitors in HS. Finally, a comparison is made between secukinumab and adalimumab and the characteristics of the patients that may be particularly suitable for each of these biologics are described.

Transcriptomic Analysis of Hidradenitis Suppurativa: A Unique Molecular Signature with Broad Immune Activation

Hidradenitis suppurativa is a chronic inflammatory skin disease with limited treatment options. The poorly understood pathogenesis hinders the development of effective treatments; therefore, a pressing need exists to further elucidate the molecular mechanisms in hidradenitis suppurativa. This study investigated the underlying inflammatory pathways and cell types in hidradenitis suppurativa using transcriptomic approaches with RNA sequencing of lesional and non-lesional skin biopsies from hidradenitis suppurativa, which was jointly analyzed with previously published transcriptomic data from atopic dermatitis and psoriasis patients. The differential expression and pathway enrichment analyses demonstrated the activation of multiple inflammatory processes, including the innate and adaptive immune systems, implicated in the hidradenitis suppurativa pathogenesis. In agreement, hidradenitis suppurativa exhibited a unique and heterogeneous cell type signature involving lymphoid and myeloid cells such as B cells and macrophages. Furthermore, hidradenitis suppurativa displayed increased expression of TH1/2/17 signatures with no predominant TH signatures unlike psoriasis (TH1/17) and atopic dermatitis (TH2). In summary, our study provides molecular insights into the pathomechanisms in hidradenitis suppurativa, revealing a strong and widespread immune activation, which may benefit from treatment strategies offering a broad immunomodulation of various key inflammatory pathways. Our data not only corroborate previously reported findings but also enhance our understanding of the immune dysregulation in hidradenitis suppurativa, uncovering novel and potential therapeutic targets.

The role of the interleukin-36 axis in generalized pustular psoriasis: a review of the mechanism of action of spesolimab

Generalized pustular psoriasis (GPP) is a rare, chronic, inflammatory skin disorder characterized by recurrent flares associated with skin erythema, desquamation, and widespread superficial sterile pustules, which may be severe ("lakes of pus"). Systemic symptoms are often present, including malaise, fever, and skin pain. In GPP, innate immune responses are driven by abnormal activation of the interleukin (IL)-36-chemokine-neutrophil axis and excessive neutrophil infiltration. This review highlights the IL-36 pathway in the context of the IL-1 superfamily and describes how unopposed IL-36 signaling can lead to the development of GPP. Targeted inhibition of the IL-36 receptor (IL-36R) is an attractive therapeutic strategy in the treatment of GPP, including flare prevention and sustained disease control. Spesolimab is a first-in-class, humanized, monoclonal antibody that binds specifically to the IL-36R and antagonizes IL-36 signaling. Spesolimab was approved by the US Food and Drug Administration in September 2022 to treat GPP flares in adults and was subsequently approved for GPP flare treatment in other countries across the world. Anti-IL-36R therapy, such as spesolimab, can mitigate flares and address flare prevention in GPP, presumably through rebalancing IL-36 signaling and modulating the pro-inflammatory response of the downstream effectors.

Integrative systems biology framework discovers common gene regulatory signatures in multiple mechanistically distinct inflammatory skin diseases

More than 20% of the population across the world is affected by non-communicable inflammatory skin diseases including psoriasis, atopic dermatitis, hidradenitis suppurativa, rosacea, etc. Many of these chronic diseases are painful and debilitating with limited effective therapeutic interventions. However, recent advances in psoriasis treatment have improved the effectiveness and provide better management of the disease. This study aims to identify common regulatory pathways and master regulators that regulate molecular pathogenesis. We designed an integrative systems biology framework to identify the significant regulators across several inflammatory skin diseases. With conventional transcriptome analysis, we identified 55 shared genes, which are enriched in several immune-associated pathways in eight inflammatory skin diseases. Next, we exploited the gene co-expression-, and protein-protein interaction-based networks to identify shared genes and protein components in different diseases with relevant functional implications. Additionally, the network analytics unravels 55 high-value proteins as significant regulators in molecular pathogenesis. We believe that these significant regulators should be explored with critical experimental approaches to identify the putative drug targets for more effective treatments. As an example, we identified IKZF1 as a shared significant master regulator in three inflammatory skin diseases, which can serve as a putative drug target with known disease-derived molecules for developing efficacious combinatorial treatments for hidradenitis suppurativa, atopic dermatitis, and rosacea. The proposed framework is very modular, which can indicate a significant path of molecular mechanism-based drug development from complex transcriptomics data and other multi-omics data.

Human dermal fibroblast subpopulations and epithelial mesenchymal transition signals in hidradenitis suppurativa tunnels are normalized by spleen tyrosine kinase antagonism in vivo

Hidradenitis Suppurativa is a chronic inflammatory disease of which the pathogenesis is incompletely understood. Dermal fibroblasts have been previously identified as a major source of inflammatory cytokines, however information pertaining to the characteristics of subpopulations of fibroblasts in HS remains unexplored. Using in silico-deconvolution of whole-tissue RNAseq, Nanostring gene expression panels and confirmatory immunohistochemistry we identified fibroblast subpopulations in HS tissue and their relationship to disease severity and lesion morphology. Gene signatures of SFRP2+ fibroblast subsets were increased in lesional tissue, with gene signatures of SFRP1+ fibroblast subsets decreased. SFRP2+ and CXCL12+ fibroblast numbers, measured by IHC, were increased in HS tissue, with greater numbers associated with epithelialized tunnels and Hurley Stage 3 disease. Pro-inflammatory CXCL12+ fibroblasts were also increased, with reductions in SFRP1+ fibroblasts compared to healthy controls. Evidence of Epithelial Mesenchymal Transition was seen via altered gene expression of SNAI2 and altered protein expression of ZEB1, TWIST1, Snail/Slug, E-Cadherin and N-Cadherin in HS lesional tissue. The greatest dysregulation of EMT associated proteins was seen in biopsies containing epithelialized tunnels. The use of the oral Spleen tyrosine Kinase inhibitor Fostamatinib significantly reduced expression of genes associated with chronic inflammation, fibroblast proliferation and migration suggesting a potential role for targeting fibroblast activity in HS.

NK and NKT cells in the pathogenesis of Hidradenitis suppurativa: Novel therapeutic strategy through targeting of CD2

Hidradenitis suppurativa (HS) is a chronic debilitating inflammatory skin disease with poorly understood pathogenesis. Single-cell RNAseq analysis of HS lesional and healthy individual skins revealed that NKT and NK cell populations were greatly expanded in HS, and they expressed elevated CD2, an activation receptor. Immunohistochemistry analyses confirmed significantly expanded numbers of CD2+ cells distributed throughout HS lesional tissue, and many co-expressed the NK marker, CD56. While CD4+ T cells were expanded in HS, CD8 T cells were rare. CD20+ B cells in HS were localized within tertiary follicle like structures. Immunofluorescence microscopy showed that NK cells (CD2 + CD56 dim ) expressing perforin, granzymes A and B were enriched within the hyperplastic follicular epidermis and tunnels of HS and juxtaposed with apoptotic cells. In contrast, NKT cells (CD2 + CD3 + CD56 bright ) primarily expressed granzyme A and were associated with α-SMA expressing fibroblasts within the fibrotic regions of the hypodermis. Keratinocytes and fibroblasts expressed high levels of CD58 (CD2 ligand) and they interacted with CD2 expressing NKT and NK cells. The NKT/NK maturation and activating cytokines, IL-12, IL-15 and IL-18, were significantly elevated in HS. Inhibition of cognate CD2-CD58 interaction with blocking anti-CD2 mAb in HS skin organotypic cultures resulted in a profound reduction of the inflammatory gene signature and secretion of inflammatory cytokines and chemokines in the culture supernate. In summary, we show that a cellular network of heterogenous NKT and NK cell populations drives inflammation, tunnel formation and fibrosis in the pathogenesis of HS. Furthermore, CD2 blockade is a viable immunotherapeutic approach for the management of HS.

Spatial transcriptomic profiling reveals the pathophysiology of early-stage hidradenitis suppurativa

We investigated the role of the epithelium of nodules in Hurley stage I hidradenitis suppurativa (HS) by using spatial transcriptomics to analyse the profiles of epithelial cells and dermal-infiltrating immune cells. Compared with epidermal cysts, genes related to bacterial response, inflammatory mediators and neutrophil degranulation pathways were upregulated in the epithelial cells of early-stage HS nodules. Our analysis of dermal-infiltrating immune cells surrounding the epithelium of nodules revealed significantly elevated levels of B-cell-related genes. Similarly to the sinus tract formation observed in moderate-to-severe HS, we propose that the production of inflammatory mediators in early-stage HS may involve the activation of keratinocytes and their interaction with dermal-infiltrating immune cells.

Keratin 17- and PKCα-dependent transient amplification of neutrophil influx after repeated stress to the skin

Neutrophils contribute to the pathogenesis of chronic inflammatory skin diseases. Little is known about the source and identity of the signals mediating their recruitment in inflamed skin. We used the phorbol ester TPA and UVB, alone or in combination, to induce sterile inflammation in mouse skin and assess whether keratinocyte-derived signals impact neutrophil recruitment. A single TPA treatment results in a neutrophil influx in the dermis that peaks at 12h and resolves within 24h. A second TPA treatment or a UVB challenge, when applied at 24h but not 48h later, accelerates, amplifies, and prolongs neutrophil infiltration. This transient amplification response (TAR) is mediated by local signals in inflamed skin, can be recapitulated in ex vivo culture, and involves the K17-dependent sustainment of protein kinase Cα (PKCα) activity and release of neutrophil chemoattractants by stressed keratinocytes. We show that K17 binds RACK1, a scaffold essential for PKCα activity. Finally, analyses of RNAseq data reveal the presence of a transcriptomic signature consistent with TAR and PKCα activation in chronic inflammatory skin diseases. These findings uncover a novel, transient, and keratin-dependent mechanism that amplifies neutrophil recruitment to the skin under stress, with direct implications for inflammatory skin disorders.

A proof-of-concept open-label clinical trial of spleen tyrosine kinase antagonism using fostamatinib in moderate-to-severe hidradenitis suppurativa

BACKGROUND

Hidradenitis suppurativa (HS) is an autoinflammatory disorder of keratinization with a prominence of B cells and plasma cells. Fostamatinib is a spleen tyrosine kinase inhibitor targeting B cells and plasma cells.

OBJECTIVES

To assess the safety, tolerability, and clinical response at week 4 and week 12 of fostamatinib in moderate-to-severe HS.

METHODS

Twenty participants were administered fostamatinib 100 mg twice a day for 4 weeks, escalating to 150 mg twice a day thereafter until week 12. Participants were assessed for adverse events and clinical response assessed by HiSCR (Hidradenitis Suppurativa Clinical Response Score) and IHS4 (International Hidradenitis Suppurativa Severity Score) as well as other outcomes including DLQI (Dermatology Life Quality Index), visual analog scale, and physician global assessment.

RESULTS

All 20 participants completed the week 4 and week 12 endpoints. Fostamatinib was well tolerated in this cohort with no grade 2/3 adverse events reported. A total of 85% achieved HiSCR at week 4 and 85% at week 12. The greatest reduction in disease activity was seen at weeks 4/5 with worsening in a proportion of patients thereafter. Significant improvements were seen in pain, itch, and quality of life.

CONCLUSIONS

Fostamatinib was well tolerated in this HS cohort with no serious adverse events and improvement in clinical outcomes. Targeting B cells/plasma cells may be a viable therapeutic strategy in HS and requires further exploration.

Targeting the NLRP3 inflammasome reduces inflammation in hidradenitis suppurativa skin

BACKGROUND

Treatment for the debilitating disease hidradenitis suppurativa (HS) is inadequate in many patients. Despite an incidence of approximately 1%, HS is often under-recognized and underdiagnosed, and is associated with a high morbidity and poor quality of life.

OBJECTIVES

To gain a better understanding of the pathogenesis of HS, in order to design new therapeutic strategies.

METHODS

We employed single-cell RNA sequencing to analyse gene expression in immune cells isolated from involved HS skin vs. healthy skin. Flow cytometry was used to quantify the absolute numbers of the main immune populations. The secretion of inflammatory mediators from skin explant cultures was measured using multiplex and enzyme-linked immunosorbent assays.

RESULTS

Single-cell RNA sequencing analysis identified a significant enrichment in the frequency of plasma cells, T helper (Th) 17 cells and dendritic cell subsets in HS skin, and the immune transcriptome was distinct and more heterogeneous than healthy skin. Flow cytometry revealed significantly increased numbers of T cells, B cells, neutrophils, dermal macrophages and dendritic cells in HS skin. Genes and pathways associated with Th17 cells, interleukin (IL)-17, IL-1β and the NLRP3 inflammasome were enhanced in HS skin, particularly in samples with a high inflammatory load. Inflammasome constituent genes principally mapped to Langerhans cells and a subpopulation of dendritic cells. The secretome of HS skin explants contained significantly increased concentrations of inflammatory mediators, including IL-1β and IL-17A, and culture with an NLRP3 inflammasome inhibitor significantly reduced the secretion of these, as well as other, key mediators of inflammation.

CONCLUSIONS

These data provide a rationale for targeting the NLRP3 inflammasome in HS using small-molecule inhibitors that are currently being tested for other indications.

Single-cell transcriptomics suggest distinct upstream drivers of IL-17A/F in hidradenitis versus psoriasis

BACKGROUND

On the basis of the mounting evidence that type 17 T (T17) cells and increased IL-17 play a key role in driving hidradenitis suppurativa (HS) lesion development, biologic agents used previously in psoriasis that block signaling of IL-17A and/or IL-17F isoforms have been repurposed to treat HS.

OBJECTIVE

Our research aimed to characterize the transcriptome of HS T17 cells compared to the transcriptome of psoriasis T17 cells, along with their ligand-receptor interactions with neighborhood immune cell subsets.

METHODS

Single-cell data of 12,300 cutaneous immune cells from 8 deroofing surgical HS skin samples including dermal tunnels were compared to single-cell data of psoriasis skin (19,525 cells from 11 samples) and control skin (11,920 cells from 10 samples). All single-cell data were generated by the same protocol.

RESULTS

HS T17 cells expressed lower levels of IL23R and higher levels of IL1R1 and IL17F compared to psoriasis T17 cells (P < .05). HS Treg cells expressed higher levels of IL1R1 and IL17F compared to psoriasis Treg cells (P < .05). Semimature dendritic cells were the major immune cell subsets expressing IL1B in HS, and IL-1β ligand-receptor interactions between semimature dendritic cells and T17 cells were increased in HS compared to psoriasis (P < .05). HS dermal tunnel keratinocytes expressed inflammatory cytokines (IL17C, IL1A, IL1B, and IL6) that differed from the HS epidermis keratinocytes (IL36G) (P < .05). IL6, which synergizes with IL1B to maintain cytokine expression in T17 cells, was mainly expressed by fibroblasts in HS, which also expressed IL11+ inflammatory fibroblast genes (IL11, IL24, IL6, and POSTN) involved in the paracrine IL-1/IL-6 loop.

CONCLUSION

The IL-1β-T17 cell cytokine axis is likely a dominant pathway in HS with HS T17 cells activated by IL-1β signaling, unlike psoriasis T17 cells, which are activated by IL-23 signaling.

The genetic aspects of hidradenitis suppurativa

Genetic aspects have a substantial role in hidradenitis suppurativa (HS) pathogenesis. A positive family history of HS occurs in about one-third of HS cases and is significantly higher in patients with early onset of the disease. Recent twin studies have shown a high heritability in HS, fortifying the importance of genetic factors in disease pathogenesis. Based on existing knowledge on the genomics of HS, the disease can be categorized as familial HS, sporadic, syndromic HS, and "HS plus" associated with other syndromes. In familial HS, autosomal dominant transmission is proposed, and monogenic inheritance is rare. This monogenic trait is related to mutations of γ-secretase component genes and Notch signaling or defects in inflammasome function. With newly discovered gene mutations, such as those related to innate and adaptive immunity, skin microbiome, inflammasome, epidermal homeostasis, and keratinization pathway, we can define HS as a polygenic, multifactorial, autoinflammatory disease. To fully elucidate the genetic aspects of HS, we need extensive, long-term global collaborations.