Role of the Complement Pathway in Inflammatory Skin Diseases: A Focus on Hidradenitis Suppurativa

The complement system as a key modulator of the oral microbiome in health and disease

In this review, we address the interplay between the complement system and host microbiomes in health and disease, focussing on oral bacteria known to contribute to homeostasis or to promote dysbiosis associated with dental caries and periodontal diseases. Host proteins modulating complement activities in the oral environment and expression profiles of complement proteins in oral tissues were described. In addition, we highlight a sub-set of bacterial proteins involved in complement evasion and/or dysregulation previously characterized in pathogenic species (or strains), but further conserved among prototypical commensal species of the oral microbiome. Potential roles of these proteins in host-microbiome homeostasis and in the emergence of commensal strain lineages with increased virulence were also addressed. Finally, we provide examples of how commensal bacteria might exploit the complement system in competitive or cooperative interactions within the complex microbial communities of oral biofilms. These issues highlight the need for studies investigating the effects of the complement system on bacterial behaviour and competitiveness during their complex interactions within oral and extra-oral host sites.

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.

Exploring the Phytochemical Composition and Biological Potential of Balkan Endemic Species Stachys scardica Griseb

Stachys scardica Griseb. is a Balkan endemic species listed in The Red Data Book of Bulgaria with the conservation status "endangered". Successful micropropagation was achieved on MS medium supplemented with 1.5 mg/L benzyladenine (BA), followed by a subsequent ex vitro adaptation in an experimental field resulting in 92% regenerated plants. Using nuclear magnetic resonance (NMR), phenylethanoid glycosides (verbascoside, leucosceptoside A), phenolic acids (chlorogenic acid), iridoids (allobetonicoside and 8-OAc-harpagide), and alkaloids (trigonelline) were identified, characteristic of plants belonging to the genus Stachys. High antioxidant and radical scavenging activities were observed in both in situ and ex vitro acclimated S. scardica plants, correlating with the reported high concentrations of total phenols and flavonoids in these variants. Ex vitro adapted plants also exhibited a well-defined anti-inflammatory potential, demonstrating high inhibitory activity against the complement system. Employing a disk diffusion method, a 100% inhibition effect was achieved compared to positive antibiotic controls against Staphylococcus epidermidis and Propionibacterium acnes, with moderate activity against Bacillus cereus. The induced in vitro and ex vitro model systems can enable the conservation of S. scardica in nature and offer future opportunities for the targeted biosynthesis of valuable secondary metabolites, with potential applications in the pharmaceutical and cosmetic industries.

Complement as a vital nexus of the pathobiological connectome for acute respiratory distress syndrome: An emerging therapeutic target

The hallmark of acute respiratory distress syndrome (ARDS) pathobiology is unchecked inflammation-driven diffuse alveolar damage and alveolar-capillary barrier dysfunction. Currently, therapeutic interventions for ARDS remain largely limited to pulmonary-supportive strategies, and there is an unmet demand for pharmacologic therapies targeting the underlying pathology of ARDS in patients suffering from the illness. The complement cascade (ComC) plays an integral role in the regulation of both innate and adaptive immune responses. ComC activation can prime an overzealous cytokine storm and tissue/organ damage. The ARDS and acute lung injury (ALI) have an established relationship with early maladaptive ComC activation. In this review, we have collected evidence from the current studies linking ALI/ARDS with ComC dysregulation, focusing on elucidating the new emerging roles of the extracellular (canonical) and intracellular (non-canonical or complosome), ComC (complementome) in ALI/ARDS pathobiology, and highlighting complementome as a vital nexus of the pathobiological connectome for ALI/ARDS via its crosstalking with other systems of the immunome, DAMPome, PAMPome, coagulome, metabolome, and microbiome. We have also discussed the diagnostic/therapeutic potential and future direction of ALI/ARDS care with the ultimate goal of better defining mechanistic subtypes (endotypes and theratypes) through new methodologies in order to facilitate a more precise and effective complement-targeted therapy for treating these comorbidities. This information leads to support for a therapeutic anti-inflammatory strategy by targeting the ComC, where the arsenal of clinical-stage complement-specific drugs is available, especially for patients with ALI/ARDS due to COVID-19.

Impact of neutrophil extracellular traps on fluid properties, blood flow and complement activation

Introduction

The intravascular formation of neutrophil extracellular traps (NETs) is a trigger for coagulation and blood vessel occlusion. NETs are released from neutrophils as a response to strong inflammatory signals in the course of different diseases such as COVID-19, cancer or antiphospholipid syndrome. NETs are composed of large, chromosomal DNA fibers decorated with a variety of proteins such as histones. Previous research suggested a close mechanistic crosstalk between NETs and the coagulation system involving the coagulation factor XII (FXII), von Willebrand factor (VWF) and tissue factor. However, the direct impact of NET-related DNA fibers on blood flow and blood aggregation independent of the coagulation cascade has remained elusive.

Methods

In the present study, we used different microfluidic setups in combination with fluorescence microscopy to investigate the influence of neutrophil-derived extracellular DNA fibers on blood rheology, intravascular occlusion and activation of the complement system.

Results

We found that extended DNA fiber networks decelerate blood flow and promote intravascular occlusion of blood vessels independent of the plasmatic coagulation. Associated with the DNA dependent occlusion of the flow channel was the strong activation of the complement system characterized by the production of complement component 5a (C5a). Vice versa, we detected that the local activation of the complement system at the vascular wall was a trigger for NET release.

Discussion

In conclusion, we found that DNA fibers as the principal component of NETs are sufficient to induce blood aggregation even in the absence of the coagulation system. Moreover, we discovered that complement activation at the endothelial surface promoted NET formation. Our data envisions DNA degradation and complement inhibition as potential therapeutic strategies in NET-induced coagulopathies.

Serum Immunoglobulin G Is a Marker of Hidradenitis Suppurativa Disease Severity

Hidradenitis suppurativa (HS) is a chronic inflammatory condition of the skin that is brought about by autoinflammation and hyperkeratosis at the pilosebaceous unit. The clinical severity of HS can be measured using static (Hurley Severity Scoring (HSS)) and/or dynamic (International HS Severity Scoring System (IHS4)) severity scoring instruments. However, few clinically available serological parameters have been found to correlate with disease severity. In this study, we sought to investigate the role of serum immunoglobulin (Ig) G, M and A levels as biomarkers of disease severity and to compare them with other, more conventional inflammatory indices, such as the erythrocyte sedimentation rate, C-reactive protein, the neutrophil-lymphocyte ratio, the platelet-lymphocyte ratio and the systemic immune-inflammation index. In this cross-sectional study, patients were recruited from the only dermatology referral centre in Malta, Europe, and subjected to clinical examination and the assessment of inflammatory and immunologic parameters. Serum IgG, M and A levels were assessed using the Atellica^® NEPH 630 System (SIEMENS-Healthineers AF, Erlangen, Germany) nephelometric analyser. Serum IgG, M and A levels correlate with both dynamic and static HS severity scoring systems. Serum IgG behaves as a marker of severe HS disease as categorised by HSS and the IHS4. Our findings suggest that the serum IgG level can be used in the clinical setting as a biomarker of disease severity and, therefore, as an adjunct to clinical severity scoring.

Innate immunity and microbial dysbiosis in hidradenitis suppurativa – vicious cycle of chronic inflammation

Hidradenitis Suppurativa (HS) is a chronic multifactorial inflammatory skin disease with incompletely understood mechanisms of disease pathology. HS is characterized by aberrant activation of the innate immune system, resulting in activation of pathways that aim to protect against pathogenic microorganisms, and also contribute to failure to resolve inflammation. Imbalance in innate immunity is evident in deregulation of host antimicrobial peptides (AMPs) and the complement system associated with the microbiome dysbiosis. The pathology is further complicated by ability of pathogens associated with HS to overcome host immune response. Potential roles of major AMPs, cathelicidin, defensins, dermcidin, S100 proteins, RNAse 7 and complement proteins are discussed. Dysregulated expression pattern of innate immunity components in conjunction with bacterial component of the disease warrants consideration of novel treatment approaches targeting both host immunity and pathogenic microbiome in HS.

Soluble Factors and Receptors Involved in Skin Innate Immunity-What Do We Know So Far?

The pattern recognition receptors, complement system, inflammasomes, antimicrobial peptides, and cytokines are innate immunity soluble factors. They sense, either directly or indirectly, the potential threats and produce inflammation and cellular death. High interest in their modulation has emerged lately, acknowledging they are involved in many cutaneous inflammatory, infectious, and neoplastic disorders. We extensively reviewed the implication of soluble factors in skin innate immunity. Furthermore, we showed which molecules target these factors, how these molecules work, and how they have been used in dermatological practice. Cytokine inhibitors have paved the way to a new era in treating moderate to severe psoriasis and atopic dermatitis.

Primary alterations during the development of hidradenitis suppurativa

Background

Hidradenitis suppurativa (HS) is a chronic, inflammatory disease of the apocrine gland‐rich (AGR) skin region. The initial steps of disease development are not fully understood, despite intense investigations into immune alterations in lesional HS skin.

Objectives

We aimed to systematically investigate the inflammatory molecules involved in three stages of HS pathogenesis, including healthy AGR, non‐lesional HS and lesional HS skin, with the parallel application of multiple mRNA and protein‐based methods.

Methods

Immune cell counts (T cells, dendritic cells, macrophages), Th1/Th17‐related molecules (IL‐12B, TBX21, IFNG, TNFA, IL‐17, IL10, IL‐23A, TGFB1, RORC, CCL20), keratinocyte‐related sensors (TLR2,4), mediators (S100A7, S100A8, S100A9, DEFB4B, LCN2, CAMP, CCL2) and pro‐inflammatory molecules (IL1B, IL6, TNFA, IL‐23A) were investigated in the three groups by RNASeq, RT‐qPCR, immunohistochemistry and immunofluorescence.

Results

Epidermal changes were already detectable in non‐lesional HS skin; the epidermal occurrence of antimicrobial peptides (AMPs), IL‐1β, TNF‐α and IL‐23 was highly upregulated compared with healthy AGR skin. In lesional HS epidermis, TNF‐α and IL‐1β expression remained at high levels while AMPs and IL‐23 increased even more compared with non‐lesional skin. In the dermis of non‐lesional HS skin, signs of inflammation were barely detectable (vs. AGR), while in the lesional dermis, the number of inflammatory cells and Th1/Th17‐related mediators were significantly elevated.

Conclusions

Our findings that non‐lesional HS epidermal keratinocytes produce not only AMPs and IL‐1β but also high levels of TNF‐α and IL‐23 confirm the driver role of keratinocytes in HS pathogenesis and highlight the possible role of keratinocytes in the transformation of non‐inflammatory Th17 cells (of healthy AGR skin) into inflammatory cells (of HS) via the production of these mediators. The fact that epidermal TNF‐α and IL‐23 appear also in non‐lesional HS seems to prove these cytokines as excellent therapeutic targets.

Insights into the Pathogenesis of HS and Therapeutical Approaches

Hidradenitis suppurativa (HS) is a debilitating, chronic, (auto)inflammatory disease primarily affecting apocrine gland-rich areas of the body. Although pathogenic mechanisms responsible for HS have not yet been fully elucidated, it is a multifactorial process whose main target is the terminal follicle. The role of the inflammatory process (and consequently of cytokine milieu) and of several other factors (genetics, lifestyle, hormonal status, microbiome, innate and adaptive immune systems) involved in HS pathogenesis has been investigated (and often defined) over the years with a view to transferring research results from bench to bedside and describing a unique and universally accepted pathogenetic model. This review will update readers on recent advances in our understanding of HS pathogenesis and novel (potential) medical therapies for patients with moderate-to-severe HS.

Hidradenitis suppurativa: A folliculotropic disease of innate immune barrier dysfunction?

The innate immune system of human skin consists of a multi-layered barrier consisting of cells and soluble effector molecules charged with maintaining homeostasis and responding to insults and infections. It has become increasingly clear that these barrier layers become compromised in skin diseases, especially in disorders of an (auto)inflammatory nature. In the case of hidradenitis suppurativa, great strides have been made in recent years in characterizing the underlying breakdown in homeostatic innate immunity, including an increasing understanding of the central role of the hair follicle in this process. This breakdown appears to occur at multiple levels: the pilosebaceous unit, associated epithelium, the cutaneous microbiome, alteration of immune cell function and local molecular events such as complement activation. This review seeks to summarize, contextualize and analyse critically our current understanding of how these innate immune barriers become dysregulated in the early stage(s) of hidradenitis suppurativa, and to speculate on where potential hidradenitis suppurativa research could be most fruitful.

Clinical Implementation of Biologics and Small Molecules in the Treatment of Hidradenitis Suppurativa

Hidradenitis suppurativa (HS) is a chronic, recurrent, auto-inflammatory skin disease originating from the hair follicles. The typical inflammatory nodules, abscesses, and draining sinus tracts (tunnels) are characterized by a massive influx of neutrophils, macrophages, B-cells, plasma cells, T helper (Th)1, Th17 cells and upregulation of pro-inflammatory cytokines such as IL-1, IL-17, IL-12/23, and TNF-α. Over the last decades, several clinical trials evaluated the clinical efficacy of different biologics targeting these pro-inflammatory cytokines, in particular TNF-α and IL-1. However, adalimumab is still the only registered drug for HS. This review discusses biologics and small molecules with high level of evidence for their clinical application, provides guidance on when and how to use these biologics and small molecules in clinical practice, and elaborates on the combination with medical and surgical treatment options beyond the current guidelines. Furthermore this review provides an overview of potential biologics and small molecules currently under investigation for novel targets in HS such as IL-36, C5a, Janus kinase family members, CD-40, LTA4 and CXCR1/2.

Systemic lupus erythematosus patients have a distinct structural and functional skin microbiota compared with controls

OBJECTIVE

The skin is the second most affected organ after articular involvement in systemic lupus erythematosus (SLE) patients. Cutaneous involvement occurs in approximately 80% of patients during the course of SLE. Interaction between the host and skin microorganism is a complex process. There are few studies on the diversity of skin microbes in SLE patients. Therefore, this study aims to explore the relationship between skin microorganisms and SLE.

METHODS

A total of 20 SLE patients, 20 controls with rosacea and 20 healthy controls were selected as study subjects. Both the skin microbiota of rash region and non-rash region for each SLE patient were collected.16S rRNA gene sequencing was used to detected skin microbiota from 80 specimens. α-Diversity and β-diversity of skin microbiota were analyzed based on operational taxonomic units (OTUs) and minimal entropy decomposition (MED). Using Wilcoxon test and Linear Discriminate Analysis Effect Size (LEfSe), skin microbial diversity and composition were analyzed. Functional capabilities of microbiota were estimated through Kyoto Encyclopedia of Genes and Genomes database.

RESULTS

Compared to rash region of SLE, diversity and richness were increased in healthy controls, and decreased in non-rash region of SLE and rash region of controls with rosacea. Additionally, changes of skin microbial composition were found at different taxonomic levels between four groups. For example, genus Halomonas was increased and genera Pelagibacterium, Novosphingobium, and Curvibacter were decreased in rash region compared to non-rash region of SLE based on OTUs and MED. Based on OTUs, metabolic pathways were also found differences in SLE patients, such as Xenobiotics Biodegradation and Metabolism.

CONCLUSION

Compositions and diversity of skin microbiota in SLE patients are changed. This pilot study provides some suggestive evidence for further exploration of skin microbiota in SLE patients with cutaneous involvement.

Target molecules for future hidradenitis suppurativa treatment

The registration of the tumour necrosis factor-α inhibitor adalimumab in 2015 was a major step forward in the treatment of hidradenitis suppurativa/acne inversa (HS). However, it soon became evident that the effectiveness of adalimumab in daily practice was highly variable. A significant unmet medical need of HS patients remained, and the search for novel therapeutic targets was intensified. During the 10th European Hidradenitis Suppurativa Foundation (EHSF) e.V. Conference, reknown international HS investigators virtually presented and discussed the published data on these potential target molecules for future HS treatment. This article addresses the most promising molecules currently under investigation from a pathophysiological and clinical point of view. With phase III trials ongoing, the anti- interleukin (IL)-17 biologics bimekizumab and secukinumab are in the most advanced stage of clinical development showing promising results. In addition, targeting IL-1α with bermekimab has shown encouraging results in two clinical trials. Directing treatment at neutrophil recruitment and activation by targeting IL-36 with spesolimab fits well in the pathogenic concept of HS and clinical phase II trial results are pending. In contrast to in situ evidence, Complement 5a (C5a) and C5a receptor blockade have only shown greater clinical benefit in patients with severe HS. Inhibition of Janus kinase (JAK) 1 signalling in HS showed clinical efficacy only in the highest dosage, highlighting that careful surveillance of the balance between safety and efficacy of JAK inhibition is warranted. Overall, clinical efficacies of all novel treatments reported so far are modest. To guide drug development, more and better-defined translational data on the pathogenesis of this severe and enigmatic inflammatory skin disease are required.

High inflammation in hidradenitis suppurativa extends to perilesional skin and can be subdivided by lipocalin-2 expression

BACKGROUND

Hidradenitis suppurativa (HS) is a chronic inflammatory skin disease presenting with diverse manifestations ranging from nodules and abscesses to draining tunnels. Whether the underlying inflammation from lesions extends to relatively healthy-appearing adjacent perilesional and distant nonlesional skin has not been systematically evaluated.

OBJECTIVE

We sought to characterize lesional, perilesional, and nonlesional skin in patients with HS.

METHODS

Skin biopsy samples were collected under ultrasound guidance from patients with active, untreated moderate-to-severe HS. Site-matched control biopsy samples from healthy volunteers were used for comparison.

RESULTS

RNA sequencing demonstrated that HS skin clustered separately from healthy control skin, with perilesional and lesion skin clustering together and away from nonlesional skin. Immunohistochemistry analysis identified psoriasiform hyperplasia with keratin 16 positivity in both perilesional and lesional skin, with comparable levels of CD3⁺, CD11c⁺, and neutrophil elastase-positive cellular infiltration. There was a marked upregulation of IL-17 signaling in perilesional and lesional skin. HS samples clustered on the basis of expression of lipocalin-2 (LCN2), with samples characterized by high LCN2 expression in the skin exhibiting a differing transcriptomic profile with significantly higher overall inflammation than that of skin characterized by low LCN2 levels.

CONCLUSIONS

Perilesional HS skin has a transcriptomic and molecular profile comparable to that of lesional skin. HS can be grouped into 2 distinct subtypes based on molecular levels of LCN2 in the skin, with the LCN2-high subtype exhibiting an overall higher inflammatory burden and an upregulation of targetable cytokines. To our knowledge, this is the first study to characterize a unique HS subtype (and a potential endotype) that may guide future therapeutic targets.

Ex Vivo Models and Interpretation of Mechanistic Studies in Hidradenitis Suppurativa

Current ex vivo and animal models of hidradenitis suppurativa (HS) display issues with fidelity and validity to human disease. Vossen et al.'s Transwell culture system holds potential to reliably assess mechanistic pathways in HS. Consideration of the sites of control tissue and comparison of the Transwell model against skin explant models would increase the validity of this method of inquiry.

An outline of SARS-CoV-2 pathogenesis and the complement cascade of immune system

BACKGROUND

Coronavirus disease 19 is a viral infection caused by a novel coronavirus, SARS-CoV-2. It was first notified in Wuhan, China, is now spread into numerous part of the world. Thus, the world needs urgent support and encouragement to develop a vaccine or antiviral treatments to combat the atrocious outbreak.

MAIN BODY OF THE ABSTRACT

The origin of this virus is yet unknown; however, rapid transmission from human-to-human "Anthroponosis" has widely confirmed. The world is witnessing a continuous hike in SARS-CoV-2 infection. In light of the outbreak of coronavirus disease 19, we have aimed to highlight the basic and vital information about the novel coronavirus. We provide an overview of SARS-CoV-2 transmission, timeline and its pathophysiological properties which would be an aid for the development of therapeutic molecules and antiviral drugs. Immune system plays a crucial role in virus infection in order to control but may have dark side when becomes uncontrollable. The host and SARS-CoV-2 interaction describe how the virus exploits host machinery and how overactive host immune response can cause disease severity also addressed in this review.

SHORT CONCLUSION

Safe and effective vaccines may be the game-changing tools, but in the near future wearing mask, washing hands at regular intervals, avoiding crowed, maintaining physical distancing and hygienic surrounding, must be good practices to reduce and break the transmission chain. Still, research is ongoing not only on how vaccines protect against disease, but also against infection and transmission.

Pyoderma gangrenosum: proposed pathogenesis and current use of biologics with an emphasis on complement C5a inhibitor IFX-1

INTRODUCTION

Pyoderma gangrenosum (PG) is a rare neutrophilic dermatosis with no FDA-approved treatment. The complement pathway has received renewed attention because it is elevated in inflammatory cutaneous conditions such as hidradenitis suppurativa (HS) and psoriasis. IFX-1 is a complement C5a inhibitor which inhibits neutrophil activation, chemotaxis, and reduces inflammatory signaling and complement driven tissue damage in various diseases.

AREAS COVERED

The article discusses a proposed pathogenesis of PG, early clinical investigations of IFX-1 for the treatment of HS and PG, its potential as a treatment for PG, and those other biologics currently under investigation.

EXPERT OPINION

Further studies should explore how patients with PG and other neutrophilic conditions may respond to complement inhibitors such as IFX-1. C5a blockade led to a reduction in inflammatory tunnels in HS, and alteration in neutrophil migration and activation supports the role of this pathway in the development of PG. The main challenges to the approval of IFX-1 are the identification of the optimal dose, duration, and stage-dependent factors in cutaneous inflammatory disorders. Further studies are required; however, complement inhibitors such as IFX-1 could find a place in clinical practice in years to come for severe, resistant PG that does not respond to conventional therapies.

Association of Human Leukocyte Antigens Class II Variants with Susceptibility to Hidradenitis Suppurativa in a Caucasian Spanish Population

Hidradenitis suppurativa (HS) is a chronic inflammatory cutaneous disease of the hair follicle typically presenting recurrent, painful, and inflamed lesions on the inverse areas of the body. Although its pathogenesis remains unknown, the immune system appears to play a potential role. To date, two previous studies have not found any association between the Human Leukocyte Antigen system (HLA) and HS. In this study we analyzed the HLA-A, -B, -C; and DRB1, -DQA1, and –DQB1 allele distribution in 106 HS patients and 262 healthy controls from a Caucasian population in Cantabria (northern Spain). HLA-A*29 and B*50 were significantly more common in HS patients and A*30 and B*37 in controls, but these associations disappeared after statistical correction. DRB1*07, DQA1*02, and DQB1*02 were significantly more common in controls (p 0.026, p 0.0012, and p 0.0005, respectively) and the HLA allele DQB1*03:01 was significantly more common in HS patients (p 0.00007) after the Bonferroni correction. The DRB1*07~DQA1*02~DQB1*02 haplotype was significantly more common in controls (p < 0.0005). This is the first study showing an association between HLA-class II and HS. Our results suggest that HLA-II alleles (DRB1*07, DQA1*02, DQB1*02, and DQB1*03:01) and the DRB1*07~DQA1*02~DQB1*02 haplotype could influence resistance or susceptibility to HS.

Hidradenitis suppurativa is an autoinflammatory keratinization disease: A review of the clinical, histologic, and molecular evidence

The pathogenic model of hidradenitis suppurativa is in the midst of a paradigm shift away from a disorder of primary follicular occlusion to an autoinflammatory keratinization disease. Observational, experimental, and therapeutic evidence supports the concept of hidradenitis suppurativa as a primarily inflammatory disorder, a disorder of autoimmunity, or both, in contrast to the current prevailing paradigm of primary follicular occlusion. The lack of reliable and high-fidelity disease models has limited the available experimental and mechanistic evidence to support or refute one pathogenic model over another. This scholarly review synthesizes the existing clinical, histologic, and molecular data to evaluate the extant evidence supporting the autoinflammatory paradigm and further informing the molecular mechanisms of hidradenitis suppurativa pathogenesis. Follicular hyperkeratosis/occlusion and perifollicular inflammation coexist in histologic specimens, with interleukin 1α demonstrated to stimulate comedogenesis in the infundibulum. pH elevation in occluded body sites alters the microbiome and amplifies existing T-helper cell type 17 immunoresponses. Known metabolic comorbidities and smoking are known to upregulate interleukin 1α in follicular keratinocytes. Identified genetic variants may alter epidermal growth factor receptor signaling, leading to upregulated keratinocyte inflammatory responses. The process of follicular rupture and dermal tunnel formation can be explained as secondary responses to inflammatory activation of fibroblasts and epithelial-mesenchymal transition, with antibody production associated with inflammatory amplification in advanced disease. This review aims to reevaluate and integrate the current clinical, histologic, and molecular data into a pathogenic model of hidradenitis suppurativa. This is essential to advance our understanding of the disease and identify novel therapeutic targets and approaches.

Proceeding report of the Fourth Symposium on Hidradenitis Suppurativa Advances 2019

The Fourth Annual Symposium on Hidradenitis Suppurativa (SHSA) took place on November 1-3, 2019, at the Westin Book Cadillac Hotel in Detroit, Michigan. This symposium was a joint meeting of the US Hidradenitis Suppurativa Foundation and the Canadian Hidradenitis Suppurativa Foundation. This cross-disciplinary meeting with experts from around the world was an opportunity to discuss the most recent advances in the study of hidradenitis suppurativa (HS) pathogenesis, clinical trials, classification, scoring systems, complementary/alternative medical treatments, diet, pain management, surgical and laser treatment, and ultrasonographic assessment. A special preconference workshop was held on the use of neodymium-doped yttrium-aluminum-garnet laser hair reduction, sinus tract deroofing, and carbon dioxide laser excision with ultrasonographic mapping and tumescent anesthesia for the treatment of HS. The focused workshops on establishing an HS clinic, setting up an HS support group, the Hidradenitis Suppurativa Prospective Observational Registry and Biospecimen Repository, and wound care were held during the meeting. A special program called HS Ambassadors was established for patients who may have questions about the conference presentations, and in addition, a meet and greet for patients and HS Ambassadors was arranged. To facilitate networking between those early in their careers and clinical and research experts, a mentoring reception was held.