The microbiome of tunnels in hidradenitis suppurativa patients

Spatial Distribution and Functional Impact of Human Scalp Hair Follicle Microbiota

Human hair follicles (HFs) constitute a unique microbiota habitat that differs substantially from the skin surface. Traditional HF sampling methods fail to eliminate skin microbiota contaminants or assess the HF microbiota incompletely, and microbiota functions in human HF physiology remain ill explored. Therefore, we used laser-capture microdissection, metagenomic shotgun sequencing, and FISH to characterize the human scalp HF microbiota in defined anatomical compartments. This revealed significant compartment-, tissue lineage-, and donor age-dependent variations in microbiota composition. Greatest abundance variations between HF compartments were observed for viruses, archaea, Staphylococcus epidermidis, Cutibacterium acnes, and Malassezia restricta, with the latter 2 being the most abundant viable HF colonizers (as tested by propidium monoazide assay) and, surprisingly, most abundant in the HF mesenchyme. Transfection of organ-cultured human scalp HFs with S. epidermidis-specific lytic bacteriophages ex vivo downregulated transcription of genes known to regulate HF growth and development, metabolism, and melanogenesis, suggesting that selected microbial products may modulate HF functions. Indeed, HF treatment with butyrate, a metabolite of S. epidermidis and other HF microbiota, delayed catagen and promoted autophagy, mitochondrial activity, and gp100 and dermcidin expression ex vivo. Thus, human HF microbiota show spatial variations in abundance and modulate the physiology of their host, which invites therapeutic targeting.

Phage therapy for hidradenitis suppurativa: a unique challenge and possible opportunity for personalized treatment of a complex, inflammatory disease

Phage therapy is an emerging antimicrobial treatment for critical multidrug-resistant pathogens. In this review, the specific potential and challenges of phage therapy for patients with hidradenitis suppurativa (HS) are discussed. This represents a unique challenge as HS is a chronic inflammatory disease, but presenting with acute exacerbations, which have an enormous negative impact on patient's quality of life. The therapeutic arsenal for HS has expanded in the past decade, for example, with adalimumab and several other biologicals that are currently under investigation. However, treatment of HS remains challenging for dermatologists because there are individuals who do not respond to any classes of the current treatment options when used for a first or second time. Furthermore, after several courses of treatment, a patient may lose their response to therapy, meaning long-term use is not always an option. Culturing studies and 16S ribosomal RNA profiling highlight the complex polymicrobial nature of HS lesions. Despite the detection of various bacterial species in lesion samples, several key pathogens, including Staphylococcus, Corynebacterium and Streptococcus, may be potential targets for phage therapy. Using phage therapy for the treatment of a chronic inflammatory disease could potentially provide new insights into the role of bacteria and the immune system in HS development. In addition, it is possible more details on the immunomodulatory effects of phages may come to light.

Bacterial Metabolites and Inflammatory Skin Diseases

The microbiome and gut-skin axis are popular areas of interest in recent years concerning inflammatory skin diseases. While many bacterial species have been associated with commensalism of both the skin and gastrointestinal tract in certain disease states, less is known about specific bacterial metabolites that regulate host pathways and contribute to inflammation. Some of these metabolites include short chain fatty acids, amine, and tryptophan derivatives, and more that when dysregulated, have deleterious effects on cutaneous disease burden. This review aims to summarize the knowledge of wealth surrounding bacterial metabolites of the skin and gut and their role in immune homeostasis in inflammatory skin diseases such as atopic dermatitis, psoriasis, and hidradenitis suppurativa.

Skin and Gut Microbiome in Hidradenitis Suppurativa: A Systematic Review

Hidradenitis suppurativa (HS) is a chronic skin disease that significantly impairs the quality of life of affected individuals. The disease is characterized by persistent purulent lesions in specific anatomical areas, and its pathophysiology involves multiple factors, including inflammation, genetics, the microbiome, and environmental components. Recent research suggests a potential role for pathogenic bacteria in HS, highlighting the importance of the communication between the human host and the microbiome in maintaining homeostasis and immune system reactivity. However, the exact mechanisms underlying the gut-skin microbial interactions in HS remain unclear. This systematic review aims to examine the existing literature on the differences in skin and gut microbiome composition between HS patients and healthy controls. The review identifies methodological inconsistencies and calls for further research to elucidate the microbiome's role in HS pathogenesis and to explore new therapeutic interventions. The review highlights the need for advancements in microbiome research methodologies, such as metataxonomics and metagenomics, to improve our understanding of the microbiota's impact on health and disease.

Evolving approaches to profiling the microbiome in skin disease

Despite its harsh and dry environment, human skin is home to diverse microbes, including bacteria, fungi, viruses, and microscopic mites. These microbes form communities that may exist at the skin surface, deeper skin layers, and within microhabitats such as the hair follicle and sweat glands, allowing complex interactions with the host immune system. Imbalances in the skin microbiome, known as dysbiosis, have been linked to various inflammatory skin disorders, including atopic dermatitis, acne, and psoriasis. The roles of abundant commensal bacteria belonging to Staphylococcus and Cutibacterium taxa and the fungi Malassezia, where particular species or strains can benefit the host or cause disease, are increasingly appreciated in skin disorders. Furthermore, recent research suggests that the interactions between microorganisms and the host's immune system on the skin can have distant and systemic effects on the body, such as on the gut and brain, known as the "skin-gut" or "skin-brain" axes. Studies on the microbiome in skin disease have typically relied on 16S rRNA gene sequencing methods, which cannot provide accurate information about species or strains of microorganisms on the skin. However, advancing technologies, including metagenomics and other functional 'omic' approaches, have great potential to provide more comprehensive and detailed information about the skin microbiome in health and disease. Additionally, inter-species and multi-kingdom interactions can cause cascading shifts towards dysbiosis and are crucial but yet-to-be-explored aspects of many skin disorders. Better understanding these complex dynamics will require meta-omic studies complemented with experiments and clinical trials to confirm function. Evolving how we profile the skin microbiome alongside technological advances is essential to exploring such relationships. This review presents the current and emerging methods and their findings for profiling skin microbes to advance our understanding of the microbiome in skin disease.

Consistency of Bacterial Triggers in the Pathogenesis of Hidradenitis Suppurativa

Hidradenitis suppurativa (HS) is an inflammatory skin disease whose pathogenesis remains poorly defined. Over the past decades, the bacterial role in HS patients has been a focus of research. According to the literature, the HS skin (and probably gut) bacterial composition is different to that of healthy controls. To date, a key question is whether compositional changes in the microbial populations are responsible for the development of HS (primum movens), or only secondarily reflect the ongoing inflammatory process. The great diversity of methodologies that have been used to study microbial role in HS have led to an accumulation of conflicting results. Thus, in view of these considerations, the aim of this article is to provide the reader with an overview about different hypotheses proposed to explain the bacterial role in HS pathogenesis.

Rifampin-moxifloxacin-metronidazole combination therapy for severe Hurley stage 1 hidradenitis suppurativa: prospective short-term trial and 1-year follow-up in 28 consecutive patients

BACKGROUND

Severe Hurley stage 1 hidradenitis suppurativa (HS1) is a difficult-to-treat form of the disease.

OBJECTIVE

To assess the efficacy and tolerance of the oral combination of rifampin (10 mg/kg once daily)/moxifloxacin (400 mg once daily)/metronidazole (250-500 mg 3 times daily) (RMoM) treatment strategy in patients with severe HS1.

METHODS

Prospective, open-label, noncomparative cohort study in 28 consecutive patients. Nineteen patients were treated for 6 weeks by RMoM, followed by 4 weeks of rifampin/moxifloxacin alone, then by cotrimoxazole after remission. Moxifloxacin was replaced by pristinamycin (1 g 3 times daily) in 9 patients because of contraindications or intolerance. The primary endpoint was a Sartorius score of 0 (clinical remission) at week 12.

RESULTS

The median Sartorius score dropped from 14 to 0 (P = 6 × 10^-6) at week 12, with 75% of patients reaching clinical remission. A low initial Sartorius score was a prognosis factor for clinical remission (P = .049). The main adverse effects were mild gastrointestinal discomfort, mucosal candidiasis, and asthenia. At 1 year of follow-up, the median number of flares dropped from 21/year to 1 (P = 1 × 10^-5).

LIMITATIONS

Small, monocentric, noncontrolled study.

CONCLUSIONS

Complete and prolonged remission can be obtained in severe HS1 by using targeted antimicrobial treatments.

Microbiome in Hidradenitis Suppurativa—What We Know and Where We Are Heading

Recently, interest in the microbiome of cutaneous diseases has increased tremendously. Of particular interest is the gut-brain-skin axis proposed by Stokes and Pillsbury in 1930. The microbiome has been suggested in the pathogenesis of hidradenitis suppurativa, however the link between the commensals and the host is yet to be established. Across all studies, the increased abundance of Porphyromonas, Peptoniphilus, and Prevotella spp., and a loss of skin commensal species, such as Cutibacterium in HS lesions, is a consistent finding. The role of gut and blood microbiome in hidradenitis suppurativa has not been fully elucidated. According to studies, the main link with the intestine is based on the increased risk of developing Crohn’s disease and ulcerative colitis, however, further research is highly needed in this area. Lifestyle, dietary approaches, and probiotics all seem to influence the microbiome, hence being a promising modality as adjuvant therapy. The aim of this review was to present the latest reports in the field of research on skin, blood, and gut microbiome in terms of hidradenitis suppurativa.

Characteristics of the Skin Microbiome in Selected Dermatological Conditions: A Narrative Review

The skin is the largest and outermost organ of the human body. The microbial diversity of the skin can be influenced by several variable factors such as physiological state, lifestyle, and geographical locations. Recent years have seen increased interest in research aiming at an improved understanding of the relationship between the human microbiota and several diseases. Albeit understudied, interesting correlations between the skin microbiota and several dermatological conditions have been observed. Studies have shown that a decrease or increase in the abundance of certain microbial communities can be implicated in several dermatological pathologies. This narrative review (i) examines the role of the skin microbiota in the maintenance of skin homeostasis and health, (ii) provides examples on how some common skin diseases (acne inversa, candidiasis, psoriasis) are associated with the dysbiosis of microbial communities, and (iii) describes how recent research approaches used in skin microbiome studies may lead to improved, more sensitive diagnostics and individual therapeutics in the foreseeable future.

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.

Stress signaling and STAT1 activation characterize the keratinocytic gene expression pattern in Hidradenitis suppurativa

BACKGROUND

The pathogenetic factors generating the innate immune signal necessary for T cell activation, initiation and chronification of Hidradenitis suppurativa (HS, also known as Acne inversa) are still poorly understood. Emerging evidence suggests that a defective keratinocyte function critically contributes to HS disease development and progression.

OBJECTIVES

To elucidate the role of keratinocytes in HS lesion formation, we compared the transcriptomes of lesional and perilesional epidermis isolated from HS patients by RNA sequencing (RNA Seq).

METHODS

Pairwise-matched lesional and perilesional HS skin samples of five different donors were obtained and epidermal keratinocytes freshly isolated and processed for RNA extraction and RNA seq. Lesionally regulated genes were analyzed by large scale promotor analysis and functional annotation clustering to identify epidermally overrepresented transcription factor binding sites and functionally related gene groups. Results were experimentally validated with independent epidermal isolates of patient-matched lesional and perilesional HS skin employing qRT-PCR, cell culture, immunoblot, and immunostaining.

RESULTS

We show that HS is characterized by a strong epidermal stress state evident by a significant overrepresentation of an AP-1-driven gene signature and a substantial activation of the stress-activated cJun N-terminal kinase (JNK) pathway in lesional epidermis. Additionally, our data reveal a strong induction of STAT1 activation in lesional HS epidermis that likely results from IFNγ production and triggered expression of key inflammatory genes coordinating innate immune activation and the adaptive T cell response in HS.

CONCLUSIONS

Our data implicate a key role of stress signaling and JAK/STAT1 activation in disease progression of HS and suggest interference with JAK/STAT1 signaling as a potentially promising therapeutic approach for HS.

Which hidradenitis suppurativa comorbidities should I take into account?

Hidradenitis suppurativa (HS) is a chronic, recurrent skin inflammatory disease associated with a variety of comorbidities, like metabolic syndrome, reduced quality of life, sexual dysfunction, working disability, inflammatory bowel disease, axial spondyloarthritis, depression and anxiety. Like psoriasis, HS patients have been found to have higher risk of cardiovascular death and suicide risk. Evidence suggests that for such a chronic, multi-comorbid disease, the use of validated outcomes to assess severity and effect of treatment, along with the use of clinically important patient reported outcomes, is essential. Clinicians should be informed about these comorbidities so that appropriate screening is implemented. The potential of available treatments to negatively and positively affect these comorbidities should also be taken into account when designing treatment strategies. This article aims to provide an outline of important HS comorbidities with emphasis on possible implications for daily clinical practice.

Prevotella species as oral residents and infectious agents with potential impact on systemic conditions

Oral Prevotella are known as anaerobic commensals on oral mucosae and in dental plaques from early life onwards, including pigmented P. melaninogenica, P. nigrescens, and P. pallens and non-pigmented Prevotella species. Many Prevotella species contribute to oral inflammatory processes, being frequent findings in dysbiotic biofilms of periodontal diseases (P. intermedia, P. nigrescens), cariotic lesions (P. denticola, Alloprevotella (formerly Prevotella) tannerae), endodontic infections (P. baroniae, P. oris, P. multisaccharivorax), and other clinically relevant oral conditions. Over the years, several novel species have been recovered from the oral cavity without knowledge of their clinical relevance. Within this wide genus, virulence properties and other characteristics like biofilm formation seemingly vary in a species- and strain-dependent manner, as shown for the P. intermedia group organisms (P. aurantiaca, P. intermedia, P. nigrescens, and P. pallens). Oral Prevotella species are identified in various non-oral infections and chronic pathological conditions. Here, we have updated the knowledge of the genus Prevotella and the role of Prevotella species as residents and infectious agents of the oral cavity, as well as their detection in non-oral infections, but also gathered information on their potential link to cancers of the head and neck, and other systemic disorders.

Microbial Isolates and Association with Disease Severity and Quality of Life in Individuals with Hidradenitis Suppurativa: An Observational Study

Introduction

Hidradenitis suppurativa (HS) is a chronic inflammatory disease characterized by recurrent nodules, abscesses, and sinus tracts, resulting in scarring. Patients suffer significantly impacted quality of life (QoL), manifested by anxiety and depression. We describe microbial isolates identified from active sites in adults with HS, and explore associations between organisms isolated, disease severity, and QoL.

Methods

Observational study over 2 years. Assessment was made of disease severity using the Hurley staging score and of QoL scores, and subjective assessment. Wound swab samples were obtained from sites of active disease. Descriptive summary statistics and tests of significance were used to analyse the data.

Results

Two hundred and twelve patients participated, resulting in 352 episodes, and 501 lesion swab samples. 54% were female and 59% were obese, median age of 37 years. A lower proportion of Gram-negative organisms and more staphylococcal and streptococcal organisms were isolated from sites of disease in individuals with stage 3 disease compared to those with stage 1-2 disease (p = 0.001). We found no association between microbial isolation and QoL.

Discussion/Conclusions

Hurley stage 3 disease is associated with more infected lesions than Hurley stage 1-2 disease, but the QoL experienced by patients with HS is determined by factors other than infection.

Hidradenitis Suppurativa: Host-Microbe and Immune Pathogenesis Underlie Important Future Directions

Hidradenitis suppurativa (HS) is an inflammatory disease of the skin with a chronic, relapsing-remitting course. The pathogenesis of the disease is poorly understood and involves multiple factors, including genetics, environment, host-microbe interactions, and immune dysregulation. In particular, the composition of the cutaneous microbiome shifts as the disease progresses, although it is unclear whether this is a primary or secondary process. Trials with immunomodulatory therapy elucidate the role of specific immune pathways and cytokine signaling in disease mechanism, such as TNF-α, IL-1β, IL-12, IL-17, IL-23, and complement. Future studies should continue examining the causes of and contributing factors to microbial changes and immune dysregulation in HS pathogenesis.

Drug Survival of Biologics in Patients With Hidradenitis Suppurativa

Importance

Biologics are important in treating patients with hidradenitis suppurativa (HS). However, to our knowledge, data on their real-life performance and treatment patterns in HS are limited.

Objective

To examine the drug survival of biologic therapies for HS in a real-world setting.

Design, Setting, participants

This cohort study included all patients with HS between January 1, 2005, and December 31, 2018, who were treated with biologics at the 5 academic hospital clinics where all biologic treatment for HS is conducted in Denmark. Biologics included adalimumab, anakinra, certolizumab pegol, etanercept, golimumab, infliximab, secukinumab, and ustekinumab. Data were analyzed between June 1, 2021, and June 20, 2021.

Main Outcomes and Measures

Drug survival was depicted through Kaplan-Meier curves, and Cox regression models were used to calculate adjusted (age, sex, previous number of biologic treatment series) hazard ratios (aHRs) with 95% CIs for the risk of treatment discontinuation. Switching patterns were visualized through a Sankey diagram.

Results

The study comprised 241 patients (176 women [61.8%]; total of 386 treatment series) with a mean (SD) age of 41.8 (12.6) years at initiation of first biologic therapy. There were a total of 256 (189 [73.8%] biologic naive), 66 (32 [48.5%] biologic naive), 23 (9 [39.1%] biologic naive), and 22 (9 [40.9%] biologic naive) treatment series with adalimumab, infliximab, etanercept, and ustekinumab, respectively. The median time to discontinuation was 36.0 (IQR, 21.9-63.0), 28.7 (IQR, 15.1-62.9), 26.0 (IQR, 16.9-155.9), and 17.9 weeks (IQR, 12.9-41.0) for adalimumab, infliximab, ustekinumab and etanercept, respectively. The risk of drug discontinuation was significantly higher for etanercept compared with adalimumab (aHR, 1.81; 95% CI, 1.16-2.82), infliximab (aHR, 1.77; 95% CI, 1.03-3.05), and ustekinumab (aHR, 2.49; 95% CI, 1.12-5.52), whereas no difference was observed when comparing these 3 therapies with each other. We found no significant differences in drug survival for biologic-naive vs nonnaive treatment series. Increasing C-reactive protein levels (aHR, 1.01; 95% CI, 1.00-1.03) and concomitant antibiotic treatment (aHR, 2.82; 95% CI, 1.36-5.86) were associated with the risk of discontinuing infliximab therapy. Men (aHR, 0.69; 95% CI, 0.51-0.91) had a reduced risk of discontinuing use of adalimumab.

Conclusions and Relevance

In this cohort study, drug survival was comparable between adalimumab, infliximab, and ustekinumab but significantly lower for etanercept. There were no differences in drug survival among biologic-naive and nonnaive patients.

The Clinical Relevance of the Microbiome in Hidradenitis Suppurativa: A Systematic Review

Hidradenitis suppurativa is a chronic disease of the pilosebaceous unit. The name of the condition is a testament to the presumed relationship between the disease and the microbiome. The pathophysiology of hidradenitis suppurativa is, however, complex and believed to be the product of a multifactorial interplay between the interfollicular epithelium, pilosebaceous unit, microbiome, as well as genetic and environmental factors. In this review we assimilate the existing literature regarding the role played by the human microbiome in HS in various contexts of the disease, including the pathophysiologic, therapeutic, and potentially, diagnostic as well prognostic. In conclusion, the role played by the microbiome in HS is extensive and relevant and can have bench-to-bedside applications.

Characterisation of the nail microbiome in psoriatic and nonpsoriatic patients with onychomycosis

BACKGROUND

Onychomycosis (OM) is the most common infectious nail disease, and it occurs frequently in patients with psoriasis. Microbial community shifts have been suggested to play a role in psoriasis and fungal infection occurrence.

OBJECTIVES

To investigate and compare nail microbial community compositions in psoriatic and nonpsoriatic patients with OM.

METHODS

Toenail samples were collected from nonpsoriatic patients with OM, psoriatic patients with nail psoriasis (NP) and OM, patients with only NP and healthy controls. Bacterial and fungal community compositions were analysed by amplicon sequencing of the V3-V4 regions of the 16S rDNA gene and the ITS1 region, respectively.

RESULTS

Psoriatic OM patients had higher bacterial and fungal alpha diversities. Taxonomic analysis revealed a significantly lower relative abundance of Trichophyton rubrum (32.88% vs 82.18%, p < .001) and an increased trend of the abundance of Candida in psoriatic patients with OM than in nonpsoriatic patients. Nonpsoriatic patients with OM had a higher abundance of Staphylococcus than healthy controls (59.66% vs 45.76%, p < .05). Trichophyton, Alternaria and Malassezia could accurately differentiate psoriatic and nonpsoriatic patients with OM, with an area under the curve (AUC) of 0.86. The severity of OM was positively correlated with the relative abundance of Trichophyton rubrum. Further, Trichophyton was positively correlated with Staphylococcus and negatively correlated with Corynebacterium, Anaerococcus, Malassezia and Alternaria.

CONCLUSIONS

The nail microbiome in psoriatic patients with OM has distinct bacterial and fungal signatures, suggesting that different dysbiosis is associated with the pathogenesis of OM in psoriatic and nonpsoriatic patients.

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.

Current Surgical Management of Hidradenitis Suppurativa: A Systematic Review and Meta-Analysis

BACKGROUND

Hidradenitis suppurativa (HS) is a chronic dermatologic condition that often necessitates surgical treatment. Surgical approaches vary substantially with little data on efficacy and safety.

OBJECTIVE

Summarize the literature on HS surgery with regards to patient characteristics, surgical approaches, and study quality. Compare postsurgical recurrence rates with a meta-analysis.

METHODS

PubMed, Embase, and Scopus were searched for studies on surgical HS management published after 2004. A random effects meta-analysis of recurrence rates was performed on eligible studies.

RESULTS

Of 715 identified studies, 59 were included in the review and 33 in the meta-analysis. Twenty-two studies of wide excision had the lowest pooled recurrence rate at 8% (95% confidence interval [CI] 2%-16%); local excision had the highest at 34% (95% CI 24%-44%). For studies of wide/radical excision, flap repair had the lowest pooled recurrence rate at 0% (95% CI 0%-4%); delayed primary closure had the highest at 38% (95% CI 20%-59%).

CONCLUSIONS

Wide excision and flap-based reconstruction are associated with a lower postsurgical HS recurrence, although this must be balanced against potentially higher morbidity of extensive procedures. Heterogeneity and methodological limitations of the evidence limit the ability to make a strong conclusion about the relative recurrence rates associated with surgical techniques.

REGISTRATION

PROSPERO ID: CRD42020159948.

[Pathogenesis of hidradenitis suppurativa/acne inversa]

Hidradenitis suppurativa/acne inversa (HS) has a multifactorial pathogenesis. In addition to a sporadic form, a familial form is reported in around 40% of patients, for whom an autosomal dominant (AD) inheritance with reduced gene penetrance is assumed. The phenotype of the disease with inflammatory nodules, abscesses and secreting sinus tracts suggests an infectious origin, but the exact role of the bacteria detected in HS pathogenesis remains unclear. Smoking and metabolic syndrome are regarded as important trigger factors in HS, with obesity and hormonal changes playing a pathogenic role in the latter. Ultimately, Toll-like receptors, antimicrobial peptides, immune cells and key cytokines are involved in the excessive inflammatory reaction of HS and are also the targets of future therapies.

Yin and Yang: A disrupted skin microbiome and an aberrant host immune response in hidradenitis suppurativa

The skin microbiome plays an important role in maintaining skin homeostasis by controlling inflammation, providing immune education and maintaining host defense. However, in many inflammatory skin disorders the skin microbiome is disrupted. This dysbiotic community may contribute to disease initiation or exacerbation through the induction of aberrant immune responses in the absence of infection. Hidradenitis suppurativa (HS) is a complex, multifaceted disease involving the skin, innate and adaptive immunity, microbiota and environmental stimuli. Herein, we discuss the current state of HS skin microbiome research and how microbiome components may activate pattern recognition receptor (PRR) pathways, metabolite sensing pathways and antigenic receptors to drive antimicrobial peptide, cytokine, miRNA and adaptive immune cell responses in HS. We highlight the major open questions that remain to be addressed and how antibiotic therapies for HS likely influence both microbial burden and inflammation. Ultimately, we hypothesize that the two-way communication between the skin microbiome and host immune response in HS skin generates a chronic positive feed-forward loop that perpetuates chronic inflammation, tissue destruction and disease exacerbation.

Exploring the human hair follicle microbiome

Human hair follicles (HFs) carry complex microbial communities that differ from the skin surface microbiota. This likely reflects that the HF epithelium differs from the epidermal barrier in that it provides a moist, less acidic, and relatively ultraviolet light-protected environment, part of which is immune-privileged, thus facilitating microbial survival. Here we review the current understanding of the human HF microbiome and its potential physiological and pathological functions, including in folliculitis, acne vulgaris, hidradenitis suppurativa, alopecia areata and cicatricial alopecias. While reviewing the main human HF bacteria (such as Propionibacteria, Corynebacteria, Staphylococci and Streptococci), viruses, fungi and parasites as human HF microbiome constituents, we advocate a broad view of the HF as an integral part of the human holobiont. Specifically, we explore how the human HF may manage its microbiome via the regulated production of antimicrobial peptides (such as cathelicidin, psoriasin, RNAse7 and dermcidin) by HF keratinocytes, how the microbiome may impact on cytokine and chemokine release from the HF, and examine hair growth-modulatory effects of antibiotics, and ask whether the microbiome affects hair growth in turn. We highlight major open questions and potential novel approaches to the management of hair diseases by targeting the HF microbiome.

Treatment of hidradenitis suppurativa with tetracycline, doxycycline, or lymecycline: a prospective study

AIM

To evaluate the clinical efficacy of tetracycline, doxycycline, and lymecycline in patients with hidradenitis suppurativa (HS).

METHODS

A prospective study of three different treatment regimens in patients with HS; oral tetracycline 500 mg twice daily, oral doxycycline 100 mg twice daily, and oral lymecycline 300 mg twice daily were administered in patients with HS. Outcomes were change in Hidradenitis Suppurativa Score (HSS), Dermatology Life Quality Life index (DLQI), overall disease-related distress, boil-related pain, number of boils in the preceding month, fraction of patients with no boils in the preceding month, and Physician's Global Assessment (PGA) score at follow-up.

RESULTS

In total, 108 patients, 73 (67.6%) women and 35 (32.4%) men, were included. Mean duration of treatment was 4.3 months. The mean HSS at baseline was 26.10 (SD 20.18) points, improving to 17.97 (SD 17.88) at follow-up, difference is 8.13 (95% CI 5.21-10.93), P < 0.0001. Highest improvement in HSS was observed in the tetracycline group. After multivariate adjustment, higher reduction in HSS was significantly associated with lower BMI, Hurley stage III, higher HSS at baseline, and higher number of boils in the preceding month at baseline.

CONCLUSION

Oral treatment with tetracycline, doxycycline, and lymecycline appears effective and safe in HS patients. Tetracycline provided the greatest clinical improvement measured by HSS.

The Microbiome in Hidradenitis Suppurativa: A Review

INTRODUCTION

Hidradenitis suppurativa (HS) is a chronic autoinflammatory skin disease. It is characterised by the development of abscesses and nodules in intertriginous anatomical sites. Whilst it is now recognised as an autoinflammatory condition rather than an infective disease, bacteria are implicated in disease pathogenesis.

METHODS

We performed a search of the literature from inception to 12 August 2020 using the search terms "hidradenitis suppurativa", "Verneuil's disease", "acne inversa", "microbiome", "bacteriology" and "microbiology". Studies were included if they assessed the cutaneous, gut or oral bacteria, bacteriology or microbiome in hidradenitis suppurativa.

RESULTS

Twenty-one studies examining the cutaneous microbiome and two studies examining the gastrointestinal microbiome in HS were identified. No studies examining the oral microbiome in HS were identified. A total of 972 patients and 46 healthy controls were included across studies examining the cutaneous microbiome. A total of 100 patients and 36 controls were included across both gut microbiome studies. Coagulase-negative Staphylococcus, anaerobes such as Porphyromonas and Prevotella, and Staphylococcus aureus species were commonly encountered organisms across the included cutaneous microbiome studies. The studies examining the gut microbiome were limited, with one small study demonstrating an alteration in the gut microbiome composition compared to controls. The other study found no alteration to the gut microbiome in patients with HS compared to those with inflammatory bowel disease (IBD) and HS, and IBD and/or psoriasis.

CONCLUSION

Research should be undertaken into the oral microbiome in HS. Further research should be undertaken examining the cutaneous and gut microbiome in HS, and its relationship with documented co-morbidities. Additionally, metagenomics-focused studies may help identify the relationship between microorganisms and host, and this may shed light on new pathways of disease pathogenesis. This may help identify potential future therapeutic targets.

A Systematic Review of Promising Therapeutic Targets in Hidradenitis Suppurativa: A Critical Evaluation of Mechanistic and Clinical Relevance

This systematic review identifies and critically evaluates the mechanistic and clinical evidence of new promising therapeutic targets in hidradenitis suppurativa (HS). Evidence for these targets is largely based on observational data with limited ex vivo and translational data from clinical trials. A number of placebo-controlled studies have been completed or are underway utilizing IL-1, IL-23, IL-17, complement, and Jak inhibition, although there is concern regarding elevated placebo response rates and the questionable validity of clinical scores in some participant subsets. Knowledge gaps are identified suggesting a direction for future mechanistic studies in HS, including more comprehensive inflammatory endotype profiling of disease.

Histologic progression of acne inversa/hidradenitis suppurativa: Implications for future investigations and therapeutic intervention

Since first recognized in 1839, the pathogenesis of acne inversa (AI) has undergone repeated revisions. Although there is agreement that AI involves occlusion of hair follicles with subsequent inflammation and the formation of tracts, the histologic progression of this disease still requires refinement. The objective of this study was to examine the histologic progression of AI based on the examination of a large cohort of punch biopsies and excisional samples that were examined first by hematoxylin and eosin staining. The most informative of these samples were step‐sectioned and stained by immunohistochemistry for epithelial and inflammatory markers. Based on this examination, the following observations were made: 1) AI arises from the epithelium of the infundibulum of terminal and vellus hairs; 2) These form cysts and epithelial tendrils that extend into soft tissue; 3) Immunohistochemical staining demonstrates the epithelium of AI is disordered with infundibular and isthmic differentiation and de novo expression of stem cell markers; 4) The inflammatory response in AI is heterogeneous and largely due to cyst rupture. The conclusions of this investigation were that AI is an epithelial‐driven disease caused by infiltrative, cyst forming tendrils and most of the inflammation is due to cyst rupture and release of cornified debris and bacteria. Cyst rupture often occurs below the depths of punch biopsy samples indicating their use for analysis may give an incomplete picture of the disease. Finally, our data suggest that unless therapies inhibit tendril development, it is unlikely they will cause prolonged treatment‐induced remission in AI.

Cytokine Pathways and Investigational Target Therapies in Hidradenitis Suppurativa

Background: Hidradenitis suppurativa (HS) is a chronic inflammatory skin disease affecting areas with a high density of apocrine glands and characterized by subcutaneous nodules that may evolve into fistulas with pus secretion. Methods: The aim of this review is to investigate all current knowledge on cytokine regulation in the pathogenesis of HS. A systematic literature research using the words “cytokine”, “interleukin”, “pathway”, and “hidradenitis suppurativa” was performed in PubMed/Medline and Scopus/Embase databases. A search of the clinicaltrials.gov website for interventional recruiting and completed trials including the term “hidradenitis suppurativa” was also performed up to August 2020. We will discuss the pathogenetic role of various cytokines in HS and potential therapeutic targets for this debilitating disease. Results: The pathophysiology underlying this complex condition has not been clearly defined. An upregulation of various cytokines, such as tumor necrosis factor alpha (TNF-α), interleukin (IL)-1, IL-17, IL-23, and other molecules seems to be related to this inflammatory condition. Various cells, such as lymphocytes T Helper 1 and 17 and keratinocytes seem to be involved in the genesis of this condition. Conclusions: Several future studies and clinical trials are necessary in order to have new knowledge about HS and to properly treat this complex condition.

A systematic review and critical appraisal of metagenomic and culture studies in hidradenitis suppurativa

Hidradenitis suppurativa (HS), also known as acne inversa, is a chronic inflammatory skin disease with still largely unknown pathogenesis. While infectious organisms have been identified in lesions of the disease since the 1980s, questions remain over the role that bacteria and microbiome play. Recent studies using 16S ribosomal RNA gene sequencing and larger culture-based studies have begun to paint a clearer picture of the microbial world of HS. With this systematic review, we summarize all the work that has been done to date in HS bacteriology, analyse potential pitfalls and limitations of the current studies, and address future directions of investigation. This systematic review attempted to collate and analyse all bacteriology studies done to date. This review was prospectively registered with PROSPERO (1670769) performed in line with the PRISMA checklist. Twenty two studies were identified comprising 862 individual HS patients for culture studies and 206 HS patients for 16S rRNA gene sequencing studies. Methodology tended to be varied, with different sampling, culturing and sequencing methods as well as amount of analysis and stratification of patients. Bacteria identified as elevated in HS lesions in sequencing studies as well as grown from HS lesions in culture studies are identified and discussed. These primarily included the anerobic Gram-negative bacilli Prevotella, Porphyromonas and Fusibacterium, the Gram-positive bacilli Corynebacterium, and the Gram-positive cocci Staphylococcus, Streptococcus and Parvimonas. Potential interactions, as well as work in other disease models with related bacteria are also discussed. Areas of further investigation include in vitro studies of interactions between bacteria and keratinocytes, gut and oral microbiome studies and deep sequencing studies for virulence and phage factors.

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.

The Role of the Cutaneous Microbiome in Hidradenitis Suppurativa-Light at the End of the Microbiological Tunnel

The development of next generation sequencing, coupled with advances in bio-informatics, has provided new insights into the role of the cutaneous microbiome in the pathophysiology of a range of inflammatory skin diseases. In fact, it has even been suggested that the identification of specific skin microbial signatures may not only be useful in terms of diagnosis of skin diseases but they may also ultimately help inform personalised treatment strategies. To date, research investigating the role of microbiota in the development of inflammatory skin diseases has largely focused on atopic eczema and psoriasis vulgaris. The role of the microbiome in Hidradenits suppurativa (HS)-also known as acne inversa-a chronic auto-inflammatory skin disease associated with significant morbidity, has received comparatively little attention. This is despite the fact that antimicrobial therapy plays a central role in the treatment of HS. After briefly outlining the clinical features of HS and current treatment strategies, we move on to review the evidence of microbial dysbiosis in HS pathophysiology. We conclude by outlining the potential for metagenomic studies to deepen our understanding of HS biology but more importantly to identify novel and much needed treatment strategies.

Hidradenitis suppurativa: infection, autoimmunity, or both?

Hidradenitis suppurativa (HS) is a chronic inflammatory skin disease mainly affecting areas rich in apocrine glands. Clinically, is characterized by painful subcutaneous nodules and if left untreated to pus secretion, abscess and fistula formation. Its frequency is estimated to be 0.5-4% of the general population, affecting women more often. Pathogenesis of HS is still not clearly defined. It seems to be a combination of genetic factors with alterations in the skin microbiome. Furthermore, at tissue (i.e. skin) as well as at serum level, several inflammatory cytokines are upregulated. The most important of the latter are tumor necrosis factor (TNF), interleukin (IL)-1, IL-17, and IL-23. Adding another level of complexity, it has been suggested that keratinocytes might be intrinsically activated, contributing also to the observed inflammation. Interestingly, it has been noted that frequency of HS is increased in some autoimmune rheumatic diseases, such as spondyloarthropathies (SpA). Of note, both HS and SpA have relatively strong association with metabolic diseases and obesity implying that there are indeed some common underlying pathophysiological pathways. Although no specific microbe has been identified, alterations in the microbiome of the skin of these patients have been reported. Of note, microbes with a capability for biofilm formation are abundant. Treatment of HS among others, include antibiotics as well as biologic drugs targeting TNF and other cytokines and used for autoimmune rheumatic diseases. Herein, we review the current evidence on links between HS and autoimmune diseases and infectious diseases with a focus on epidemiology and pathophysiology.

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

Although the role of immune dysregulation in hidradenitis suppurativa (HS) has yet to be elucidated, recent studies identified several complement abnormalities in patients with HS. The complement system serves a critical role in the modulation of immune response and regulation of cutaneous commensal bacteria. Complement is implicated in several inflammatory skin diseases including systemic lupus erythematosus, angioedema, pemphigus, bullous pemphigoid, and HS. A model of HS pathogenesis is proposed, integrating the role of commensal bacteria, cutaneous immune responses, and complement dysregulation. The role of complement in disease pathogenesis has led to the development of novel anticomplement agents and clinical trials investigating the efficacy of such treatments in HS.

Integrating complement into the molecular pathogenesis of Hidradenitis Suppurativa

Complement inhibition has been identified as a potential therapeutic target for multiple inflammatory disorders including Hidradenitis Suppurativa (HS). It is currently unclear how complement integrates into our current model of molecular pathogenesis in HS and whether it represents a central component of pathogenesis, or a neutrophil-associated bystander. Levels of C5a in serum and tissue correlate with disease activity and degree of neutrophilic infiltrates in HS. C5a has been associated with Th17 immune axis activation in psoriasis, rheumatoid arthritis and Crohn's disease with strong similarities to TH17 activation in HS. Porphyromonas species (which are identified in the HS microbiome) are able to cleave inactive C5 into C5a implicating the cutaneous microbiome as an activator of complement. C3a and C5a are associated with activation of the NLRP3 inflammasome, implicated in the inflammatory drive in HS. Complement receptors are present upon dendritic cells, monocytes, fibroblasts and adipocytes, which may broaden the potential contribution of complement to multiple aspects of HS pathogenesis. Dysregulation of complement receptor pathways has been documented in obesity, insulin resistance and polycystic ovarian syndrome leading to the possibility that complement may explain the epidemiological associations between these conditions and HS. The therapeutic potential of complement inhibitors in HS may be related to the therapeutic target (complement receptor or complement subunit) and the presence of alternate receptors (such as C5aR2) or ligands (including C3a, PAMPs and DAMPs). Integrating complement into the known pathogenesis of HS may aid in explaining the contradictory results between Phase 2 studies of C5a antagonists. It also allows for the identification of existing knowledge gaps to target further clinical investigation and research.

Clinical, microbiological, immunological and imaging characteristics of tunnels and fistulas in hidradenitis suppurativa and Crohn's disease

Hidradenitis suppurativa (HS) tunnels and Crohn's disease (CD) fistulas are a challenge to treat. Although pathogenic similarities have been described between HS and CD, recent studies indicate that clinical, microbiological, immunological and imaging characteristics differ between these diseases. This review highlights the differences between HS tunnels and CD fistulas. Next-generation sequencing studies demonstrate a microbiome in HS tunnels dominated by Porphyromonas spp., Prevotella spp. whereas no specific bacteria have been associated with cutaneous CD. Immunologically, TNF has been found upregulated in HS tunnels along with various interleukins (IL-8, IL-16, IL-1α and IL-1β). In CD fistulas, Th1, Th17, IL-17, IFN-ɤ, TNF and IL-23 are increased. US imaging is an important tool in HS. US of HS tunnels depict hypoechoic band-like structure across skin layers in the dermis and/or hypodermis connected to the base of a widened hair follicle. In CD, MR imaging of simple perianal fistulas illustrates a linear, non-branching inflammatory tract relating to an internal opening in the anus or low rectum and an external opening to the skin surface. An increased awareness of the immediate potential differences between HS tunnels and CD fistulas may optimize treatment regimens of these intractable skin manifestations.

Contribution of fibroblasts to tunnel formation and inflammation in hidradenitis suppurativa/ acne inversa

The precise pathogenic mechanisms in the development, persistence and worsening of hidradenitis suppurativa (HS) remain ill-defined. This chronic inflammatory dermatosis displays a strong Th1 and Th17 inflammatory signature with elevated levels of TNF-α, IL-1β, IL-17 and IFNγ in lesional and perilesional tissue. HS significantly differs to other chronic inflammatory dermatoses due to the development of hypertrophic scarring and dermal tunnels. The development of scarring and tunnels suggests that fibroblastic stromal cells (including myofibroblasts, fibroblasts, pericytes etc) may be involved in the development and progression of disease. Heterogeneous populations of fibroblasts have been identified in other inflammatory disorders and malignancy which contribute to inflammation and present novel therapeutic targets for fibrotic disorders. Findings in HS are consistent with these fibroblast subpopulations and may contribute to tunnel formation, aggressive squamous cell carcinoma and the phenotypic presentation of familial HS variants. We describe the existing knowledge regarding these mechanistic pathways and methods to confirm their involvement in the pathogenesis of HS.