Background: Psoriasis is a common, chronic inflammatory skin disorder, which can significantly impact quality of life. Despite major breakthroughs in our understanding of the pathogenesis of psoriasis, the chronological order of the underlying mechanisms leading to the development of psoriatic plaques remains to be completely understood. Summary: Although psoriasis is classically perceived as a T-cell disease, it is now well recognized that T lymphocytes do not function in exclusivity. This theory is supported by evidence from transgenic murine models that develop marked psoriasiform disease. In addition, immune cells and cytokines regulate both early and late events involved in the pathogenesis of psoriasis. Key Messages: Psoriasis is a complex disease – a dynamic interplay between immune cells, keratinocytes, and various other skin-resident cells, such as endothelial and immune cells. The contribution of each cell type is crucial in the initiation and maintenance phases of psoriatic alterations.

Background: Recent advances have highlighted the crucial role of microbiota in the pathophysiology of chronic inflammatory diseases as well as its impact on the efficacy of therapeutic agents. Psoriasis is a chronic, multifactorial inflammatory skin disorder, which has a microbiota distinct from healthy, unaffected skin. Aim: Through an extensive review of the literature, we aim to discuss the skin and gut microbiota and redefine their role in the pathogenesis of psoriasis. Conclusions: Unfortunately, the direct link between the skin microbiota and the pathogenesis of psoriasis remains to be clearly established. Apart from improving the course of psoriasis, selective modulation of the microbiota may increase the efficacy of medical treatments as well as attenuate their side effects.

Summary Background Hidradenitis suppurativa (HS), also referred to as acne inversa, is a debilitating skin disease characterized by inflammatory nodules, chronic abscesses and tunnels (fistulae and sinuses). The association with pilonidal sinus disease (PSD) is frequently reported but not well documented. Objectives To determine the prevalence and characteristics of inflammatory skin lesions located in the intergluteal fold (IGF) of patients with HS. Methods This was an international multicentre retrospective cross‐sectional study based on data collection from a large cohort of patients with HS with and without histopathology. Results From a total of 2465 patients with HS included in the study, 661 (27%) reported lesions in the IGF. These patients were significantly more often smokers and had more severe HS. Of the 238 patients with an available clinical diagnosis, intergluteal‐HS (IG‐HS) was diagnosed in 52 patients (22%) and PSD was diagnosed in 186 patients (78%). IG‐HS was associated with the localization of HS in the proximity of the IGF, including the buttocks, genitals and the anus. There was a possibility of misclassification bias in this study as a clinical/image‐based diagnosis or histopathology of the IGF lesions was not always available. Conclusions The high prevalence of PSD suggests a strong link between both entities. Therefore, it may be useful to identify common pathophysiological mechanisms and develop common therapeutic strategies. What's already known about this topic? The occurrence of pilonidal sinus disease has not been clearly reported among patients with hidradenitis suppurativa/acne inversa. What does this study add? This is the first study that investigated the prevalence of pilonidal sinus disease among a large cohort of patients and identified the patient characteristics. Risk factors that might help to improve the management of patients were identified.

IMPORTANCEThe identification and validation of biomarkers in hidradenitis suppurativa (HS) has potential to improve the understanding and management of this chronic, burdensome disease.OBJECTIVE To systematically identify all known HS biomarkers, categorize them by biomarker type, and critically evaluate their validity according to established criteria.EVIDENCE REVIEW Eligibility criteria for this review (PROSPERO Registration 230830) included randomized clinical trials, uncontrolled clinical trials, cohort studies, case-control studies, and other observational studies with no restrictions of patient age, sex, race or ethnicity, or language of publication up until December 31, 2020. All articles were categorized into biomarker type, defined using the US Food and Drug Administration Biomarkers, Endpoints, and other Tools (BEST) glossary. Assessment of each identified biomarker was undertaken in line with the US Food and Drug Administration and European Medicines Agency guidelines for the validation of proposed biomarkers. Assessment of the strength of overall data regarding individual biomarkers was undertaken using the Grading of Recommendations, Assessment, Development, and Evaluations (GRADE) approach.FINDINGS A total of 3953 nonduplicate articles were screened, of which 1429 articles were retrieved based on the include/exclusion criteria applied. After full-text screen and data extraction, 106 articles were included in this review. The evidence of strength of 6 categories of biomarkers (susceptibility/risk, diagnostic, monitoring, predictive, prognostic, and pharmacodynamic/response biomarkers) was assessed using GRADE criteria. A total of 48 biomarkers were identified with a minimum GRADE rating of moderate. Only 1 diagnostic (serum IL-2R), 1 monitoring (dermal Doppler vascularity), and 2 predictive biomarkers (epithelialized tunnels and positive family history of HS) achieved a GRADE rating of high. None of the identified biomarkers had sufficient clinical validity to be recommended for routine use in the clinical setting. CONCLUSIONS AND RELEVANCEMajor barriers to the identification, validation, and introduction of routine biomarkers in the management of HS include lack of independent biomarker validation studies (especially assumption-free "omics"-based techniques); insufficient assessment of collinearity between identified or proposed biomarkers; and a lack of routine integration of biomarkers into the structure of clinical trials. International consensus among researchers, clinicians, and pharmaceutical stakeholders is required to standardize goals and methods and encourage biomarker integration into future HS clinical trials. This systematic review presents a number of priorities for near-term future research to overcome such barriers and limitations of biomarkers in HS.