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

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.

Discrepancies in hidradenitis suppurativa lesion characterization by providers and patients

BACKGROUND

The hidradenitis suppurativa (HS) clinical response (HiSCR) has come under scrutiny as several HS clinical trials failed to meet primary endpoints with high placebo responses. This may be due to limitations of the tool and raters' ability to accurately characterize and count lesions, rather than lack of efficacy of the studied drug. Due to HS lesion complexity and potential differences in rater training, it was hypothesized that there would be discrepancies in how providers characterize and count lesions for HS clinical trials.

OBJECTIVE

To evaluate how HS providers and patients name and count HS lesions and to identify discrepancies among providers to initiate the development of consensus-driven guidance for HS rater training.

METHODS

An online survey was distributed to the members of HIdradenitis SuppuraTiva cORe outcomes set International Collaboration (HISTORIC). Respondents were asked to classify lesion images composed of multiple and different morphology types and answer questions regarding inclusion of associated dermatological conditions.

RESULTS

Forty-seven HISTORIC members responded (29 providers; 18 patients). There was variability in how respondents classified HS lesions. Of 12 questions containing images, four had ≥50% of respondents choosing the same answer. With an image of a lesion composed of different morphologies, 45% of providers counted it as a single lesion and 45% counted it as multiple distinct lesions. With an image of multiple interconnected draining tunnels, 7% of providers classified it as a single draining tunnel while 79% categorized it as multiple draining tunnels with the number estimated by visual inspection. There was also variability in deciding whether lesions occurring in associated conditions should be considered separately or included in HS lesion counts. Patient responses were also variable.

CONCLUSIONS

The result of the current study reaffirms the gap in how providers characterize and count HS lesions for clinical trials and the need to develop consensus-driven rater training related to HS outcome measures.

Inflammatory proteins and hidradenitis suppurativa: Insights from genetic correlation and Mendelian randomization

Previous research has highlighted a significant association between inflammatory proteins and the development and progression of hidradenitis suppurativa (HS). Nevertheless, the potential causative link between these factors remains to be definitively established. To investigate the genetic correlation between inflammatory proteins and HS, linkage disequilibrium score regression (LDSC) was employed. Mendelian randomization (MR) analysis, incorporating inverse variance weighted, MR-Egger, and weighted median methodologies, was utilized to evaluate the possible causal relationship between circulating inflammatory proteins (CIPs) and HS. Additionally, reverse MR analysis was carried out to explore reverse causality. The data set for 91 CIPs was derived from a genome-wide protein quantitative trait loci study, while HS-related data were acquired from the FinnGen study. Moreover, the stability of the causal relationships was assessed via sensitivity analyses, encompassing tests for pleiotropy, heterogeneity, and leave-one-out analysis. The LDSC analysis suggested the existence of genetic correlations between the levels of Fibroblast growth factor 21 (FGF-21), stem cell factor, and HS. The MR analysis identified a suggestive association of T-cell surface glycoprotein CD5 and C-X-C motif chemokine 11 with an elevated risk of HS. Conversely, C-C motif chemokine 4, Protein S100-A12, Interleukin-10 receptor subunit beta, and Programmed cell death 1 ligand 1 were associated with a diminished risk of HS. Moreover, HS was demonstrated to increase the levels of four CIPs: Interleukin-20, Leukemia inhibitory factor (LIF), LIF receptor, and Thymic stromal lymphopoietin. The findings of this investigation offer suggestive evidence for possible genetic correlations and causal links between various genetically predicted inflammatory proteins and HS. There exists a pressing requirement for additional studies to elucidate the fundamental processes driving these associations.

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.

Apocrine Gland Damage and the Release of Specific Keratins in Early Stage Indicate the Crucial Involvement of Apocrine Glands in Hidradenitis Suppurativa

The apocrine glands (AGs) are not considered to be primarily involved in hidradenitis suppurativa (HS). This study investigated the potential role of AGs in HS pathogenesis using immunohistochemistry and single-cell sequencing of nonlesional skin and early lesional skin (LS) from patients with HS (n = 12) and healthy controls (n = 8). AG cell destruction was more frequent, and AG size was significantly reduced in the nonlesional skin and LS. Barrier-related genes (eg, CLDN1 and CDH1) were downregulated in the AGs of the nonlesional skin and LS. Damaged AGs in the LS primarily recruited and activated neutrophils through the CXCL-CXCR and SAA1-FPR2 pathways. Elevated levels of specific keratins (keratin 18 and keratin 19) released from damaged AGs were observed on the skin surface of patients and were associated with disease severity. Keratin 19 was also detected in the dermis of the nonlesional skin and LS and was surrounded by neutrophils and macrophages. Moreover, serum keratin 19 levels in patients (N = 20) were significantly negatively correlated with the age at HS onset. Collectively, our findings provide previously unreported evidence that the AGs are damaged and release specific keratins in early HS lesions, indicating a crucial role of the AGs in HS pathogenesis.

Uncommon neoplasms mistakenly diagnosed as hidradenitis suppurativa: Report of three consecutive cases

Hidradenitis suppurativa (HS) poses diagnostic challenges due to its clinical overlap with various skin conditions and neoplasms, potentially leading to misdiagnoses. The absence of a definitive diagnostic test and infrequent use of histopathology contribute to diagnostic complexities, exacerbated by the recent increased focus on HS. Three cases initially diagnosed and treated as HS underwent clinical work-up and skin biopsies to resolve diagnostic complexities. Initially labeled as HS, the cases revealed a breast carcinoma on axillary ectopic tissue, a cutaneous gamma-delta T-cell lymphoma, and an infiltrating squamous cell carcinoma. Delayed recognition led to misguided therapies and adverse outcomes. This report stresses the need to explore alternative diagnoses for chronic skin nodules with or without ulcerations on the flexures. Timely skin biopsies are crucial for accurate diagnoses. Ongoing clinician education is essential to avoid misdiagnosis in challenging cases, in which histopathology aids in reaching a correct diagnosis.

Expanding the frontiers of therapeutic options in hidradenitis suppurativa: The valid contribution of bimekizumab

Hidradenitis suppurativa (HS) is a chronic inflammatory cutaneous disorder with a significant negative impact on quality of life. Th17 axis has a central role in the pathogenesis of HS. Kimball et al. demonstrated the efficacy and safety of bimekizumab in two double-blind, placebo-controlled phase 3 studies (BE HEARD I-II), adding a new targeting option to the therapeutic armamentarium of HS.1.

Pathogenesis of Inflammation in Skin Disease: From Molecular Mechanisms to Pathology

Many skin diseases begin with inflammatory changes on a molecular level. To develop a more thorough understanding of skin pathology and to identify new targets for therapeutic advancements, molecular mechanisms of inflammation in the context of skin disease should be studied. Current research efforts to better understand skin disease have focused on examining the role of molecular processes at several stages of the inflammatory response such as the dysregulation of innate immunity sensors, disruption of both transcriptional and post-transcriptional regulation, and crosstalk between immune and neuronal processes (neuro-immune crosstalk). This review seeks to summarize recent developments in our understanding of inflammatory processes in skin disease and to highlight opportunities for therapeutic advancements. With a focus on publications within the past 5 years (2019-2024), the databases PubMed and EBSCOhost were used to search for peer-reviewed papers regarding inflammatory molecular mechanisms and skin disease. Several themes of research interest regarding inflammatory processes in skin disease were determined through extensive review and were included based on their relative representation in current research and their focus on therapeutic potential. Several skin diseases such as psoriasis, atopic dermatitis, hidradenitis suppurativa, and scleroderma were described in the paper to demonstrate the widespread influence of inflammation in skin disease.

Essentials of hidradenitis suppurativa: a comprehensive review of diagnostic and treatment perspectives

Hidradenitis suppurativa, or acne inversa, is a chronic inflammatory skin condition with recurrent inflammatory nodules, abscesses, subcutaneous tracts, and scars. This condition may cause severe psychological distress and reduce the quality of life for affected individuals. It is considered to have one of the most damaging effects on quality of life of any skin disorder as a result of the discomfort and foul-smelling discharge from these lesions. Although the pathophysiology of HS is still unclear, multiple factors, including lifestyle, genetic, and hormonal factors, have been associated with it. The pathogenesis of HS is very complex and has wide clinical manifestations; thus, it is quite challenging to manage and often requires the use of combination treatments that must be tailored according to disease severity and other patient-specific factors. Although lifestyle changes, weight loss, quitting smoking, topical treatments, and oral antibiotics are adequate for mild cases, the challenge for healthcare professionals is dealing with moderate-to-severe HS, which often does not respond well to traditional approaches. This literature review, consisting of an overview of the various assessment tools and therapy strategies available for the diagnosis and treatment of HS from published literature, aims to be a guide for practicing clinicians in dealing with the complexities associated with this disease.

Setting the Stage for Standardized Reporting of Clinical and Demographic Information in Laboratory-Based Studies of Hidradenitis Suppurativa

Hidradenitis suppurativa (HS) is a complex inflammatory skin condition affecting 0.1-4% of the population that leads to permanent scarring in the axilla, inframammary region, groin, and buttocks. Its complex pathogenesis involves genetics, innate and adaptive immunity, microbiota, and environmental stimuli. Specific populations have a higher incidence of HS, including females and Black individuals and those with associated comorbidities. HS registries and biobanks have set standards for the documentation of clinical data in the context of clinical trials and outcomes research, but collection, documentation, and reporting of these important clinical and demographic variables are uncommon in HS laboratory research studies. Standardization in the laboratory setting is needed because it helps to elucidate the factors that contribute mechanistically to HS symptoms and pathophysiology. The purpose of this article is to begin to set the stage for standardized reporting in the laboratory setting. We discuss how clinical guidelines can inform laboratory research studies, and we highlight what additional information is necessary for the use of samples in the wet laboratory and interpretation of associated mechanistic data. Through standardized data collection and reporting, data harmonization between research studies will transform our understanding of HS and lead to novel discoveries that will positively impact patient care.

Eosinophils in hidradenitis suppurativa patients exhibit pro-inflammatory traits, implicating a potential pathogenic role in the disease

Hidradenitis suppurativa (HS) is an inflammatory skin disease characterized by painful nodules, abscesses and purulent secretions in intertriginous regions. Intense pruritus frequently accompanies HS lesions, adding further discomfort for patients. While Th17 pathway activation is implicated in HS pathogenesis, disease mechanisms are still not fully understood, and therapeutics are lacking. Previous reports raise a potential role for eosinophils in HS, showing a strong association of eosinophil levels with disease severity. To investigate eosinophils in HS, we recruited patients and matched healthy controls and then performed flow-cytometry studies, eosinophil stimulation assays, and lesional skin staining for eosinophils. We found that HS patients reported similar levels of pain and itch. Compared to matched controls, HS blood exhibited decreased mature eosinophils and increased numbers of immature eosinophils, coupled with a significant increase in dermal eosinophilic infiltrates. Additionally, IL-17RA+ eosinophils were highly and significantly correlated with multiple HS-related clinical scores. In both stimulated and unstimulated conditions, HS eosinophils showed an inflammatory phenotype versus controls, including an increase in costimulatory T- and B-cell markers (e.g. CD5 and CD40) following all stimulations (TNFα/IL-17A/IL-17F). These findings highlight the significance of pruritus in HS and suggest a higher turnover of eosinophils in HS blood, potentially due to the consumption of eosinophils in skin lesions. Our data delineate the features and functions of eosinophils in HS and suggest that eosinophils participate in disease pathogenesis, advancing Th17-related inflammation. Further studies are needed to investigate eosinophils' response to current HS treatments and their potential as a therapeutic target in the disease.

Harnessing artificial intelligence for advancing early diagnosis in hidradenitis suppurativa

This perspective delves into the integration of artificial intelligence (AI) to enhance early diagnosis in hidradenitis suppurativa (HS). Despite significantly impacting Quality of Life, HS presents diagnostic challenges leading to treatment delays. We present a viewpoint on AI-powered clinical decision support system designed for HS, emphasizing the transformative potential of AI in dermatology. HS diagnosis, primarily reliant on clinical evaluation and visual inspection, often results in late-stage identification with substantial tissue damage. The incorporation of AI, utilizing machine learning and deep learning algorithms, addresses this challenge by excelling in image analysis. AI adeptly recognizes subtle patterns in skin lesions, providing objective and standardized analyses to mitigate subjectivity in traditional diagnostic approaches. The AI integration encompasses diverse datasets, including clinical records, images, biochemical and immunological data and OMICs data. AI algorithms enable nuanced comprehension, allowing for precise and customized diagnoses. We underscore AI's potential for continuous learning and adaptation, refining recommendations based on evolving data. Challenges in AI integration, such as data privacy, algorithm bias, and interpretability, are addressed, emphasizing the ethical considerations of responsible AI deployment, including transparency, human oversight, and striking a balance between automation and human intervention. From the dermatologists' standpoint, we illustrate how AI enhances diagnostic accuracy, treatment planning, and long-term follow-up in HS management. Dermatologists leverage AI to analyze clinical records, dermatological images, and various data types, facilitating a proactive and personalized approach. AI's dynamic nature supports continuous learning, refining diagnostic and treatment strategies, ultimately reshaping standards of care in dermatology.

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.