An Interleukin-25-Mediated Autoregulatory Circuit in Keratinocytes Plays a Pivotal Role in Psoriatic Skin Inflammation

Cytokines in psoriasis: From pathogenesis to targeted therapy

Psoriasis is a multifactorial disease that affects 0.84% of the global population and it can be associated with disabling comorbidities. As patients present with thick scaly lesions, psoriasis was long believed to be a disorder of keratinocytes. Psoriasis is now understood to be the outcome of the interaction between immunological and environmental factors in individuals with genetic predisposition. While it was initially thought to be solely mediated by cytokines of type-1 immunity, namely interferon-γ, interleukin-2, and interleukin-12 because it responds very well to cyclosporine, a reversible IL-2 inhibitor; the discovery of Th-17 cells advanced the understanding of the disease and helped the development of biological therapy. This article aims to provide a comprehensive review of the role of cytokines in psoriasis, highlighting areas of controversy and identifying the connection between cytokine imbalance and disease manifestations. It also presents the approved targeted treatments for psoriasis and those currently under investigation.

IL-6 Up-Regulates Expression of LIM-Domain Only Protein 4 in Psoriatic Keratinocytes through Activation of the MEK/ERK/NF-κB Pathway

Psoriasis is a chronic inflammatory skin disease characterized by the activation of keratinocytes and the infiltration of immune cells. Overexpression of the transcription factor LIM-domain only protein 4 (LMO4) promoted by IL-23 has critical roles in regulating the proliferation and differentiation of psoriatic keratinocytes. IL-6, an autocrine cytokine in psoriatic epidermis, is a key mediator of IL-23/T helper 17-driven cutaneous inflammation. However, little is known about how IL-6 regulates the up-regulation of LMO4 expression in psoriatic lesions. In this study, human immortalized keratinocyte cells, clinical biopsy specimens, and an animal model of psoriasis induced by imiquimod cream were used to investigate the role of IL-6 in the regulation of keratinocyte proliferation and differentiation. Psoriatic epidermis showed abnormal expression of IL-6 and LMO4. IL-6 up-regulated the expression of LMO4 and promoted keratinocyte proliferation and differentiation. Furthermore, in vitro and in vivo studies showed that IL-6 up-regulates LMO4 expression by activating the mitogen-activated extracellular signal-regulated kinase (MEK)/extracellular signal-regulated kinase (ERK)/NF-κB signaling pathway. These results suggest that IL-6 can activate the NF-κB signaling pathway, up-regulate the expression of LMO4, lead to abnormal proliferation and differentiation of keratinocytes, and promote the occurrence and development of psoriasis.

The Immunology of Psoriasis-Current Concepts in Pathogenesis

Psoriasis is one of the most common inflammatory skin diseases with a chronic, relapsing-remitting course. The last decades of intense research uncovered a pathological network of interactions between immune cells and other types of cells in the pathogenesis of psoriasis. Emerging evidence indicates that dendritic cells, TH17 cells, and keratinocytes constitute a pathogenic triad in psoriasis. Dendritic cells produce TNF-α and IL-23 to promote T cell differentiation toward TH17 cells that produce key psoriatic cytokines IL-17, IFN-γ, and IL-22. Their activity results in skin inflammation and activation and hyperproliferation of keratinocytes. In addition, other cells and signaling pathways are implicated in the pathogenesis of psoriasis, including TH9 cells, TH22 cells, CD8+ cytotoxic cells, neutrophils, γδ T cells, and cytokines and chemokines secreted by them. New insights from high-throughput analysis of lesional skin identified novel signaling pathways and cell populations involved in the pathogenesis. These studies not only expanded our knowledge about the mechanisms of immune response and the pathogenesis of psoriasis but also resulted in a revolution in the clinical management of patients with psoriasis. Thus, understanding the mechanisms of immune response in psoriatic inflammation is crucial for further studies, the development of novel therapeutic strategies, and the clinical management of psoriasis patients. The aim of the review was to comprehensively present the dysregulation of immune response in psoriasis with an emphasis on recent findings. Here, we described the role of immune cells, including T cells, B cells, dendritic cells, neutrophils, monocytes, mast cells, and innate lymphoid cells (ILCs), as well as non-immune cells, including keratinocytes, fibroblasts, endothelial cells, and platelets in the initiation, development, and progression of psoriasis.

Tuft cell IL-17RB restrains IL-25 bioavailability and reveals context-dependent ILC2 hypoproliferation

The tuft cell-ILC2 circuit orchestrates rapid type 2 responses upon detecting microbe-derived succinate and luminal helminths. Our findings delineate key mechanistic steps, involving IP3R2 engagement and Ca 2+ flux, governing IL-25 production by tuft cells triggered by succinate detection. While IL-17RB plays a pivotal intrinsic role in ILC2 activation, it exerts a regulatory function in tuft cells. Tuft cells exhibit constitutive Il25 expression, placing them in an anticipatory state that facilitates rapid production of IL-25 protein for ILC2 activation. Tuft cell IL-17RB is crucial for restraining IL-25 bioavailability, preventing excessive tonic ILC2 stimulation due to basal Il25 expression. Suboptimal ILC2 stimulation by IL-25 resulting from tuft cell Il17rb -deficiency or prolonged succinate exposure induces a state of hypoproliferation in ILC2s, also observed in chronic helminth infection. Our study offers critical insights into the regulatory dynamics of IL-25 in this circuit, highlighting the delicate tuning required for responses to diverse luminal states.

Folate supports IL-25-induced tuft cell expansion following enteroviral infections

Intestinal tuft cells, a kind of epithelial immune cells, rapidly expand in response to pathogenic infections, which is associated with infection-induced interleukin 25 (IL-25) upregulation. However, the metabolic mechanism of IL-25-induced tuft cell expansion is largely unknown. Folate metabolism provides essential purine and methyl substrates for cell proliferation and differentiation. Thus, we aim to investigate the roles of folate metabolism playing in IL-25-induced tuft cell expansion by enteroviral infection and recombinant murine IL-25 (rmIL-25) protein-stimulated mouse models. At present, enteroviruses, such as EV71, CVA16, CVB3, and CVB4, upregulated IL-25 expression and induced tuft cell expansion in the intestinal tissues of mice. However, EV71 did not induce intestinal tuft cell expansion in IL-25-/- mice. Interestingly, compared to the mock group, folate was enriched in the intestinal tissues of both the EV71-infected group and the rmIL-25 protein-stimulated group. Moreover, folate metabolism supported IL-25-induced tuft cell expansion since both folate-depletion and anti-folate MTX-treated mice had a disrupted tuft cell expansion in response to rmIL-25 protein stimulation. In summary, our data suggested that folate metabolism supported intestinal tuft cell expansion in response to enterovirus-induced IL-25 expression, which provided a new insight into the mechanisms of tuft cell expansion from the perspective of folate metabolism.

The m6 A modification of Il17a in CD4+ T cells promotes inflammation in psoriasis

Psoriasis is a chronic inflammatory skin disorder. The mechanism of psoriasis pathogenesis is not entirely clear. Here, we reported that the level of the N6-methyladenosine (m6 A) modification was increased in psoriatic CD4+ T cells compared with healthy controls. In the psoriasis mouse model, depletion of the RNA demethylase, Alkbh5, from CD4+ T cells promoted the psoriasis-like phenotype and inflammation. Intriguingly, this phenotype and inflammation were alleviated by the ablation of the m6 A methyltransferase Mettl3 in CD4+ T cells. Mechanistically, we found that the m6 A modification of IL17A mRNA increased the expression of IL-17A (an important pro-inflammatory factor in psoriasis) and promoted psoriasis. Thus, our study provided evidence that the m6 A modification of IL17A in CD4+ T cells regulates inflammation in psoriasis.

The Skin and Inflamm-Aging

With its unique anatomical location facing both the external and internal environment, the skin has crucial functions, including shielding the body from damage caused by ultraviolet radiation and chemicals, preventing water loss, acting as a primary barrier against pathogens, participating in metabolic processes like vitamin D production and temperature control and relaying information to the body through sensory and proprioceptor nerves. Like all organ systems, skin is known to undergo multiple changes with aging. A better understanding of the mechanisms that mediate aging-related skin dysfunction may allow the creation of targeted therapeutics that have beneficial effects not only on aged skin but also on other organs and tissues that experience a loss of or decline in function with aging. The skin is the largest organ of the body and can contribute to serum inflammatory mediator levels. One alteration known to occur with age is an impairment of skin barrier function; since disruption of the barrier is known to induce inflammation, skin may be a major contributor to the sustained, sub-clinical systemic inflammation associated with aging. Such "inflamm-aging" may underlie many of the deleterious changes observed in aged individuals. This review explores the role of age-related skin changes, skin inflammation and inflamm-aging.

The IL-17 family in diseases: from bench to bedside

The interleukin-17 (IL-17) family comprises six members (IL-17A-17F), and recently, all of its related receptors have been discovered. IL-17 was first discovered approximately 30 years ago. Members of this family have various biological functions, including driving an inflammatory cascade during infections and autoimmune diseases, as well as boosting protective immunity against various pathogens. IL-17 is a highly versatile proinflammatory cytokine necessary for vital processes including host immune defenses, tissue repair, inflammatory disease pathogenesis, and cancer progression. However, how IL-17 performs these functions remains controversial. The multifunctional properties of IL-17 have attracted research interest, and emerging data have gradually improved our understanding of the IL-17 signaling pathway. However, a comprehensive review is required to understand its role in both host defense functions and pathogenesis in the body. This review can aid researchers in better understanding the mechanisms underlying IL-17's roles in vivo and provide a theoretical basis for future studies aiming to regulate IL-17 expression and function. This review discusses recent progress in understanding the IL-17 signaling pathway and its physiological roles. In addition, we present the mechanism underlying IL-17's role in various pathologies, particularly, in IL-17-induced systemic lupus erythematosus and IL-17-related tumor cell transformation and metastasis. In addition, we have briefly discussed promising developments in the diagnosis and treatment of autoimmune diseases and tumors.

Cellular Mechanisms of Psoriasis Pathogenesis: A Systemic Review

Psoriasis is a common inflammatory skin disease characterized by abnormal proliferation of epidermal keratinocytes and massive infiltration of inflammatory cells. Many kinds of cells, including keratinocytes, T lymphocytes, dendritic cells, neutrophils, and macrophages, are reported to play critical roles in the pathogenesis and progression of psoriasis. However, to date, the role of each kind of cell in the pathogenesis and development of psoriasis has not been systematically reviewed. In addition, although antibodies developed targeting cytokines (e.g. IL-23, IL-17A, and TNF-α) released by these cells have shown promising results in the treatment of psoriasis patients, these targeted antibodies still do not cure psoriasis and only provide short-term relief of symptoms. Furthermore, long-term use of these antibodies has been reported to have adverse physical and psychological effects on psoriasis patients. Therefore, gaining a deeper understanding of the cellular and molecular pathogenesis of psoriasis and providing new thoughts on the development of psoriasis therapeutic drugs is of great necessity. In this review, we summarize the roles of various cells involved in psoriasis, aiming to provide new insights into the pathogenesis and development of psoriasis at the cellular level and hoping to provide new ideas for exploring new and effective psoriasis treatments.

A systematic review of natural products for skin applications: Targeting inflammation, wound healing, and photo-aging

BACKGROUND

Every day the skin is constantly exposed to several harmful factors that induce oxidative stress. When the cells are incapable to maintain the balance between antioxidant defenses and reactive oxygen species, the skin no longer can keep its integrity and homeostasis. Chronic inflammation, premature skin aging, tissue damage, and immunosuppression are possible consequences induced by sustained exposure to environmental and endogenous reactive oxygen species. Skin immune and non-immune cells together with the microbiome are essential to efficiently trigger skin immune responses to stress. For this reason, an ever-increasing demand for novel molecules capable of modulating immune functions in the skin has risen the level of their development, particularly in the field of natural product-derived molecules.

PURPOSE

In this review, we explore different classes of molecules that showed evidence in modulate skin immune responses, as well as their target receptors and signaling pathways. Moreover, we describe the role of polyphenols, polysaccharides, fatty acids, peptides, and probiotics as possible treatments for skin conditions, including wound healing, infection, inflammation, allergies, and premature skin aging.

METHODS

Literature was searched, analyzed, and collected using databases, including PubMed, Science Direct, and Google Scholar. The search terms used included "Skin", "wound healing", "natural products", "skin microbiome", "immunomodulation", "anti-inflammatory", "antioxidant", "infection", "UV radiation", "polyphenols", "polysaccharides", "fatty acids", "plant oils", "peptides", "antimicrobial peptides", "probiotics", "atopic dermatitis", "psoriasis", "auto-immunity", "dry skin", "aging", etc., and several combinations of these keywords.

RESULTS

Natural products offer different solutions as possible treatments for several skin conditions. Significant antioxidant and anti-inflammatory activities were reported, followed by the ability to modulate immune functions in the skin. Several membrane-bound immune receptors in the skin recognize diverse types of natural-derived molecules, promoting different immune responses that can improve skin conditions.

CONCLUSION

Despite the increasing progress in drug discovery, several limiting factors need future clarification. Understanding the safety, biological activities, and precise mechanisms of action is a priority as well as the characterization of the active compounds responsible for that. This review provides directions for future studies in the development of new molecules with important pharmaceutical and cosmeceutical value.

Revisiting the interleukin 17 family of cytokines in psoriasis: pathogenesis and potential targets for innovative therapies

Psoriasis is a common chronic inflammatory skin disease, associated with substantial comorbidity. TH17 lymphocytes, differentiating under the influence of dendritic cell-derived IL-23, and mediating their effects via IL-17A, are believed to be central effector cells in psoriasis. This concept is underlined by the unprecedented efficacy of therapeutics targeting this pathogenetic axis. In recent years, numerous observations made it necessary to revisit and refine this simple "linear" pathogenetic model. It became evident that IL-23 independent cells exist that produce IL-17A, that IL-17 homologues may exhibit synergistic biological effects, and that the blockade of IL-17A alone is clinically less effective compared to the inhibition of several IL-17 homologues. In this review, we will summarize the current knowledge around IL-17A and its five currently known homologues, namely IL-17B, IL-17C, IL-17D, IL-17E (also known as IL-25) and IL-17F, in relation to skin inflammation in general and psoriasis in particular. We will also re-visit the above-mentioned observations and integrate them into a more comprehensive pathogenetic model. This may help to appreciate current as well as developing anti-psoriatic therapies and to prioritize the selection of future drugs' mode(s) of action.

d-mannose blocks the interaction between keratinocytes and Th17 cells to alleviate psoriasis by inhibiting HIF-1α/CCL20 in mice

Psoriasis is an autoimmune chronic inflammatory skin disease with an unclear pathogenesis that is difficult to cure, causing serious physical and mental burdens for patients. Previous research showed that a mutually reinforcing vicious cycle caused by keratinocytes (KC) and a variety of immune cells plays an important role in psoriatic inflammation. d-Mannose, a widely distributed metabolite in the body, has been found to treat several metabolic diseases, but its impact on psoriasis remains unknown. Our study aims to investigate the effects of d-mannose on psoriasis and its specific mechanism. Here, we found that d-mannose alleviates psoriasis in mice both as oral and topical agents. Specifically, d-mannose down-regulated the expression of hypoxia-inducible factor 1A(HIF-1α) and inhibited the expression of chemokine CCL20 in keratinocytes, thereby inhibiting the local infiltration of Th17 cells and breaking the cycle of keratinocytes-Th17 cells. Overall, our study indicates that d-mannose alleviates cutaneous inflammation in psoriasis by inhibiting the HIF-1α/CCL20/Th17 cells axis, and d-mannose has the potential to be used as an oral and topical agent in the treatment of psoriasis.

New insights into the function of Interleukin-25 in disease pathogenesis

Interleukin-25 (IL-25), also known as IL-17E, is a cytokine belonging to the IL-17 family. IL-25 is abundantly expressed by Th2 cells and various kinds of epithelial cells. IL-25 is an alarm signal generated upon cell injury or tissue damage to activate immune cells through the interaction with IL-17RA and IL-17RB receptors. The binding of IL-25 to IL-17RA/IL-17RB complex not only initiates and maintains type 2 immunity but also regulates other immune cells (e.g., macrophages and mast cells) via various signaling pathways. It has been well-documented that IL-25 is critically involved in the development of allergic disorders (e.g., asthma). However, the roles of IL-25 in the pathogenesis of other diseases and the underlying mechanisms are still unclear. This review presents current evidence on the roles of IL-25 in cancers, allergic disorders, and autoimmune diseases. Moreover, we discuss the unanswered key questions underlying IL-25-mediated disease pathology, which will provide new insights into the targeted therapy of this cytokine in clinical treatment.

FXYD3 enhances IL-17A signaling to promote psoriasis by competitively binding TRAF3 in keratinocytes

Psoriasis is a common chronic inflammatory skin disease characterized by inflammatory cell infiltration and epidermal hyperplasia. However, the regulatory complexity of cytokine and cellular networks still needs to be investigated. Here, we show that the expression of FXYD3, a member of the FXYD domain-containing regulators of Na+/K+ ATPases family, is significantly increased in the lesional skin of psoriasis patients and mice with imiquimod (IMQ)-induced psoriasis. IL-17A, a cytokine important for the development of psoriatic lesions, contributes to FXYD3 expression in human primary keratinocytes. FXYD3 deletion in keratinocytes attenuated the psoriasis-like phenotype and inflammation in an IMQ-induced psoriasis model. Importantly, FXYD3 promotes the formation of the IL-17R-ACT1 complex by competing with IL-17R for binding to TRAF3 and then enhances IL-17A signaling in keratinocytes. This promotes the activation of the NF-κB and MAPK signaling pathways and leads to the expression of proinflammatory factors. Our results clarify the mechanism by which FXYD3 serves as a mediator of IL-17A signaling in keratinocytes to form a positive regulatory loop to promote psoriasis exacerbation. Targeting FXYD3 may serve as a potential therapeutic approach in the treatment of psoriasis.

Management of Infections in Psoriatic Patients Treated with Systemic Therapies: A Lesson from the Immunopathogenesis of Psoriasis

Modern treatments continue to be developed based on identifying targets within the innate and adaptive immune pathways associated with psoriasis. Whilst there is a sound biologic rationale for increased risk of infection following treatment with immunomodulators, the clinical evidence is confounded by these agents being used in patients affected with several comorbidities. In an era characterized by an ever greater and growing risk of infections, it is necessary to always be updated on this risk. In this mini-review, we will discuss recent updates in psoriasis immunopathogenesis as a rationale for systemic therapy, outline the risk of infections linked to the disease itself and systemic therapy as well, and provide an overview of the prevention and management of infections.

Treatment with Interleukin-25 Suppresses Short-Term High-Fructose Diet-Induced Hepatic Gene Expression and Activities of Fatty Acid Synthesis Enzymes in Rats

This study aimed to investigate the effects of interleukin-25, which belongs to the interleukin-17 family, on short-term high-fructose diet-induced hepatic triacylglycerol accumulation. Rats were fed a high-starch (control) or high-fructose diet for 7 d, with or without intraperitoneal administration of recombinant interleukin-25 from days 3-7. Treatment with interleukin-25 significantly reduced the mRNA levels and activity of fatty acid synthesis enzymes and caused a nominal reduction in hepatic triacylglycerol levels in rats fed a high-fructose diet but not in those fed a control diet. Interleukin-25 treatment did not affect the mRNA levels of β-oxidation enzymes in either the control or fructose-fed rats. These results suggest that treatment with interleukin-25 suppresses short-term high-fructose diet-induced fatty acid synthesis and leads to the alleviation of triacylglycerol accumulation in the liver.

Pyruvate kinase M2 mediates IL-17 signaling in keratinocytes driving psoriatic skin inflammation

Psoriasis is an inflammatory skin disease characterized by keratinocyte proliferation and inflammatory cell infiltration induced by IL-17. However, the molecular mechanism through which IL-17 signaling in keratinocytes triggers skin inflammation remains not fully understood. Pyruvate kinase M2 (PKM2), a glycolytic enzyme, has been shown to have non-metabolic functions. Here, we report that PKM2 mediates IL-17A signaling in keratinocytes triggering skin psoriatic inflammation. We find high expression of PKM2 in the epidermis of psoriatic patients and mice undergoing psoriasis models. Specific depletion of PKM2 in keratinocytes attenuates the development of experimental psoriasis by reducing the production of pro-inflammatory mediators. Mechanistically, PKM2 forms a complex with Act1 and TRAF6 regulating NF-κB transcriptional signaling downstream of the IL-17 receptor. As IL-17 also induces PKM2 expression in keratinocytes, our findings reveal a sustained signaling circuit critical for the psoriasis-driving effects of IL-17A, suggesting that PKM2 is a potential therapeutic target for psoriasis.

Skin Microbiome, Metabolome and Skin Phenome, from the Perspectives of Skin as an Ecosystem

Skin is a complex ecosystem colonized by millions of microorganisms, including bacteria, fungi, and viruses. Skin microbiota is believed to exert critical functions in maintaining host skin health. Profiling the structure of skin microbial community is the first step to overview the ecosystem. However, the community composition is highly individualized and extremely complex. To explore the fundamental factors driving the complexity of the ecosystem, namely the selection pressures, we review the present studies on skin microbiome from the perspectives of ecology. This review summarizes the following: (1) the composition of substances/nutrients in the cutaneous ecological environment that are derived from the host and the environment, highlighting their proposed function on skin microbiota; (2) the features of dominant skin commensals to occupy ecological niches, through self-adaptation and microbe-microbe interactions; (3) how skin microbes, by their structures or bioactive molecules, reshape host skin phenotypes, including skin immunity, maintenance of skin physiology such as pH and hydration, ultraviolet (UV) protection, odor production, and wound healing. This review aims to re-examine the host-microbe interactions from the ecological perspectives and hopefully to give new inspiration to this field.

TSLP, IL-33, and IL-25: Not just for allergy and helminth infection

The release of cytokines from epithelial and stromal cells is critical for the initiation and maintenance of tissue immunity. Three such cytokines, thymic stromal lymphopoietin, IL-33, and IL-25, are important regulators of type 2 immune responses triggered by parasitic worms and allergens. In particular, these cytokines activate group 2 innate lymphoid cells, TH2 cells, and myeloid cells, which drive hallmarks of type 2 immunity. However, emerging data indicate that these tissue-associated cytokines are not only involved in canonical type 2 responses but are also important in the context of viral infections, cancer, and even homeostasis. Here, we provide a brief review of the roles of thymic stromal lymphopoietin, IL-33, and IL-25 in diverse immune contexts, while highlighting their relative contributions in tissue-specific responses. We also emphasize a biologically motivated framework for thinking about the integration of multiple immune signals, including the 3 featured in this review.

Neuro-epithelial-ILC2 crosstalk in barrier tissues

Group 2 innate lymphoid cells (ILC2s) contribute to the maintenance of mammalian barrier tissue homeostasis. We review how ILC2s integrate epithelial signals and neurogenic components to preserve the tissue microenvironment and modulate inflammation. The epithelium that overlies barrier tissues, including the skin, lungs, and gut, generates epithelial cytokines that elicit ILC2 activation. Sympathetic, parasympathetic, sensory, and enteric fibers release neural signals to modulate ILC2 functions. We also highlight recent findings suggesting neuro-epithelial-ILC2 crosstalk and its implications in immunity, inflammation and resolution, tissue repair, and restoring homeostasis. We further discuss the pathogenic effects of disturbed ILC2-centered neuro-epithelial-immune cell interactions and putative areas for therapeutic targeting.

IL-17D-induced inhibition of DDX5 expression in keratinocytes amplifies IL-36R-mediated skin inflammation

Aberrant RNA splicing in keratinocytes drives inflammatory skin disorders. In the present study, we found that the RNA helicase DDX5 was downregulated in keratinocytes from the inflammatory skin lesions in patients with atopic dermatitis and psoriasis, and that mice with keratinocyte-specific deletion of Ddx5 (Ddx5^∆KC) were more susceptible to cutaneous inflammation. Inhibition of DDX5 expression in keratinocytes was induced by the cytokine interleukin (IL)-17D through activation of the CD93-p38 MAPK-AKT-SMAD2/3 signaling pathway and led to pre-messenger RNA splicing events that favored the production of membrane-bound, intact IL-36 receptor (IL-36R) at the expense of soluble IL-36R (sIL-36R) and to the selective amplification of IL-36R-mediated inflammatory responses and cutaneous inflammation. Restoration of sIL-36R in Ddx5^∆KC mice with experimental atopic dermatitis or psoriasis suppressed skin inflammation and alleviated the disease phenotypes. These findings indicate that IL-17D modulation of DDX5 expression controls inflammation in keratinocytes during inflammatory skin diseases.

Emerging roles for IL-25 and IL-33 in colorectal cancer tumorigenesis

Colorectal cancer (CRC) is the second leading cause of cancer-related death worldwide, and is largely refractory to current immunotherapeutic interventions. The lack of efficacy of existing cancer immunotherapies in CRC reflects the complex nature of the unique intestinal immune environment, which serves to maintain barrier integrity against pathogens and harmful environmental stimuli while sustaining host-microbe symbiosis during homeostasis. With their expression by barrier epithelial cells, the cytokines interleukin-25 (IL-25) and IL-33 play key roles in intestinal immune responses, and have been associated with inappropriate allergic reactions, autoimmune diseases and cancer pathology. Studies in the past decade have begun to uncover the important roles of IL-25 and IL-33 in shaping the CRC tumour immune microenvironment, where they may promote or inhibit tumorigenesis depending on the specific CRC subtype. Notably, both IL-25 and IL-33 have been shown to act on group 2 innate lymphoid cells (ILC2s), but can also stimulate an array of other innate and adaptive immune cell types. Though sometimes their functions can overlap they can also produce distinct phenotypes dependent on the differential distribution of their receptor expression. Furthermore, both IL-25 and IL-33 modulate pathways previously known to contribute to CRC tumorigenesis, including angiogenesis, tumour stemness, invasion and metastasis. Here, we review our current understanding of IL-25 and IL-33 in CRC tumorigenesis, with specific focus on dissecting their individual function in the context of distinct subtypes of CRC, and the potential prospects for targeting these pathways in CRC immunotherapy.

Interleukin-17 Family Cytokines in Metabolic Disorders and Cancer

Interleukin-17 (IL-17) family cytokines are potent drivers of inflammatory responses. Although IL-17 was originally identified as a cytokine that induces protective effects against bacterial and fungal infections, IL-17 can also promote chronic inflammation in a number of autoimmune diseases. Research in the last decade has also elucidated critical roles of IL-17 during cancer development and treatment. Intriguingly, IL-17 seems to play a role in the risk of cancers that are associated with metabolic disorders. In this review, we summarize our current knowledge on the biochemical basis of IL-17 signaling, IL-17′s involvement in cancers and metabolic disorders, and postulate how IL-17 family cytokines may serve as a bridge between these two types of diseases.

ANGPTL4 Regulates Psoriasis via Modulating Hyperproliferation and Inflammation of Keratinocytes

Background: Psoriasis is characterized by keratinocyte proliferation and massive inflammatory leukocytes infiltration, affecting 0.14%–1.99% of the world’s population. Our aim was to identify novel potential therapeutic strategies for psoriasis.

Methods: Weighted gene co-expression network analysis (WGCNA) was performed to identify gene modules that were closely related to psoriasis based on the GSE30999 dataset, which contained expression data from 85 patients with moderate-to-severe psoriasis. Then, angiopoietin-like 4 (ANGPTL4), one of the most related hub genes, was selected for in vitro and in vivo functional assays. In our experiments, imiquimod (IMQ)-induced psoriasiform dermatitis in mice and human keratinocytes (HaCaT) cells were used to study the potential roles and mechanisms of ANGPTL4 in psoriasis.

Results: WGCNA analysis revealed the turquoise module was most correlated with psoriasis, and ANGPTL4 is one of the most related hub genes that significantly upregulated in psoriasis lesions compared with non-lesional skin. Consistent with the bioinformatic analysis, the expression of ANGPTL4 was significantly upregulated in IMQ-induced psoriasiform skin of mice. Exogenous recombinant ANGPLT4 protein treatment could promote the proliferation and induce the expression of inflammatory cytokines in HaCaTs, whereas silencing of ANGPTL4 effectively inhibited these effects. Then we demonstrated that recombinant ANGPTL4 protein exacerbated psoriasiform inflammation and epidermal hyperproliferation in vivo. Mechanismly, extracellular signal-regulated kinase 1/2 (ERK1/2) and signal transducer and activator of transcription 3 (STAT3) pathways were involved in ANGPTL4-mediated regulation of proliferation and inflammation.

Conclusion: We found ANGPTL4 was significantly increased in IMQ-induced psoriasiform skin of mice. ANGPTL4 could promote keratinocyte proliferation and inflammatory response via ERK1/2 and STAT3 dependent signaling pathways in psoriasis.

Role of IL-25 on Eosinophils in the Initiation of Th2 Responses in Allergic Asthma

Background

Eosinophils act as a secondary antigen-presenting cell (APC) to stimulate Th cell responses against antigens. IL-25 plays a significant role in eosinophil activation in allergic asthma. The role of IL-25 on the classic APC functions of dendritic cells has been elucidated. However, whether IL-25 facilitates eosinophils for antigen presentation is unknown.

Objective

To elucidate the role of IL-25 on eosinophils antigen presenting function during allergic asthma and its related mechanism.

Methods

Eosinophils from allergic asthma subjects were cultured with IL-25 and HDM to identify the co-stimulator molecules expression. Co-cultures of patient eosinophils and autologous naïve CD4⁺ T cells in the same culture system were to explore whether eosinophils had the capacity to promote Th cell differentiation in response to IL-25 engagement. In asthma mouse model, IL-25^-/- mice were exposed to HDM to investigate the effect of IL-25 on eosinophils during the sensitization phase. The impact of IL-25 on the capacity for eosinophils taking up antigens was evaluated. Mouse bone marrow derived eosinophils (BmEOS) were co-cultured with naïve CD4⁺T cells sorted from spleens under HDM and IL-25 stimulation to identify T cell differentiation.

Results

IL-25 upregulated HLA-DR, PD-L1, and OX-40L expression on eosinophils from allergic asthma patients. IL-25 and HDM co-sensitized eosinophils promoted Th2 differentiation. In mouse model, IL-25^-/- mice experienced restrained allergic pulmonary inflammation and reduced eosinophils recruitment and antigen uptake capacity during the early sensitization phase. In vitro, IL-25 promoted antigen uptake by eosinophils. In BmEOS and naïve CD4⁺T cells co-culture, IL-25 accreted the proportion of CD4⁺Th2 cells, which was absent in CD4⁺T cells culture alone.

Conclusion

Our data identify a novel role of IL-25 in enhancing eosinophils antigen uptake and co-stimulator molecules expression to induce Th2 priming in the context of allergic inflammation.

Updated Perspectives on Keratinocytes and Psoriasis: Keratinocytes are More Than Innocent Bystanders

Psoriasis is a complex disease triggered by genetic, immunologic, and environmental stimuli. Many genes have been linked to psoriasis, like the psoriasis susceptibility genes, some of which are critical in keratinocyte biology and epidermal barrier function. Still, the exact pathogenesis of psoriasis is unknown. In the disease, the balance between the proliferative and differentiative processes of keratinocytes becomes altered. Multiple studies have highlighted the role of dysregulated immune cells in provoking the inflammatory responses seen in psoriasis. In addition to immune cells, accumulating evidence shows that keratinocytes are involved in psoriasis pathogenesis, as discussed in this review. Although certain immune cell-derived factors stimulate keratinocyte hyperproliferation, activated keratinocytes can also produce anti-microbial peptides, cytokines, and chemokines that can promote their proliferation, as well as recruit immune cells to help initiate and reinforce inflammatory feedback loops. Psoriatic keratinocytes also show intrinsic differences from normal keratinocytes even after removal from the in vivo inflammatory environment; thus, psoriatic keratinocytes have been found to exhibit abnormal calcium metabolism and possible epigenetic changes that contribute to psoriasis. The Koebner phenomenon, in which injury promotes the development of psoriatic lesions, also provides evidence for keratinocytes' contributions to disease pathogenesis. Furthermore, transgenic mouse studies have confirmed the importance of keratinocytes in the etiology of psoriasis. Finally, in addition to immune cells and keratinocytes, data in the literature support roles for other cell types, tissues, and systems in psoriasis development. These other contributors are all potential targets for therapies, suggesting the importance of a holistic approach when treating psoriasis.

Alarmin Cytokines as Central Regulators of Cutaneous Immunity

Skin acts as the primary interface between the body and the environment. The skin immune system is composed of a complex network of immune cells and factors that provide the first line of defense against microbial pathogens and environmental insults. Alarmin cytokines mediate an intricate intercellular communication between keratinocytes and immune cells to regulate cutaneous immune responses. Proper functions of the type 2 alarmin cytokines, thymic stromal lymphopoietin (TSLP), interleukin (IL)-25, and IL-33, are paramount to the maintenance of skin homeostasis, and their dysregulation is commonly associated with allergic inflammation. In this review, we discuss recent findings on the complex regulatory network of type 2 alarmin cytokines that control skin immunity and highlight the mechanisms by which these cytokines regulate skin immune responses in host defense, chronic inflammation, and cancer.

Contribution of Interleukin-17A to Retinal Degenerative Diseases

Retinal degenerative diseases are a leading cause of vision loss and blindness throughout the world, characterized by chronic and progressive loss of neurons and/or myelin. One of the common features of retinal degenerative diseases and central neurodegenerative diseases is chronic neuroinflammation. Interleukin-17A (IL-17A) is the cytokine most closely related to disease in its family. Accumulating evidence suggests that IL-17A plays a key role in human retinal degenerative diseases, including age-related macular degeneration, diabetic retinopathy and glaucoma. This review aims to provide an overview of the role of IL-17A participating in the pathogenesis of retinal degenerative diseases, which may open new avenues for potential therapeutic interventions.

IL-25 participates in keratinocyte-driven dermal matrix turnover and is reduced in Systemic Sclerosis epidermis

OBJECTIVES

Evidence shows that dysfunctional SSc keratinocytes contribute to fibrosis by altering dermal homeostasis. Whether interleukin-25 (IL-25), an IL-17 family member regulating many epidermal functions, takes part in skin fibrosis is unknown. Here we address the role of IL-25 in skin fibrosis.

METHODS

The expression of IL-25 was evaluated by immunofluorescence and in situ hybridization in 10 SSc and 7 healthy donors (HD) skin biopsies. Epidermal equivalents (EE) reconstituted by primary HD keratinocytes were used as a model to study transcriptomic changes induced by IL-25 in the epidermis. RNA expression profile in EE was characterized by RNAseq. The conditioned medium (CM) from primary SSc and HD keratinocytes primed with IL-25 was used to stimulate fibroblasts. IL-6, IL-8, MMP-1, type-I collagen (col-I), and fibronectin production by fibroblasts was assessed by ELISA.

RESULTS

SSc epidermis expressed lower levels of IL-25 compared with HD. In EE, IL-25 regulated several molecular pathways related to wound healing and ECM remodeling. Compared with control CM, the CM from IL-25-primed keratinocytes enhanced the fibroblast production of MMP-1, IL-6, IL-8, but not of Col-I nor fibronectin. However, IL-25 significantly reduced the production of Col-I when applied directly to fibroblasts. The activation of keratinocytes by IL-25 was receptor-dependent and evident after a very short incubation time (10 min), largely mediated by IL-1, suggesting enhanced and specific release of preformed mediators.

CONCLUSIONS

These results show that IL-25 participates to skin homeostasis and its decreased expression in SSc may contribute to skin fibrosis by favoring ECM deposition over degradation.

A Proximal-to-Distal Survey of Healthy Adult Human Small Intestine and Colon Epithelium by Single-Cell Transcriptomics

BACKGROUND & AIMS

Single-cell transcriptomics offer unprecedented resolution of tissue function at the cellular level, yet studies analyzing healthy adult human small intestine and colon are sparse. Here, we present single-cell transcriptomics covering the duodenum, jejunum, ileum, and ascending, transverse, and descending colon from 3 human beings.

METHODS

A total of 12,590 single epithelial cells from 3 independently processed organ donors were evaluated for organ-specific lineage biomarkers, differentially regulated genes, receptors, and drug targets. Analyses focused on intrinsic cell properties and their capacity for response to extrinsic signals along the gut axis across different human beings.

RESULTS

Cells were assigned to 25 epithelial lineage clusters. Multiple accepted intestinal stem cell markers do not specifically mark all human intestinal stem cells. Lysozyme expression is not unique to human Paneth cells, and Paneth cells lack expression of expected niche factors. Bestrophin 4 (BEST4)⁺ cells express Neuropeptide Y (NPY) and show maturational differences between the small intestine and colon. Tuft cells possess a broad ability to interact with the innate and adaptive immune systems through previously unreported receptors. Some classes of mucins, hormones, cell junctions, and nutrient absorption genes show unappreciated regional expression differences across lineages. The differential expression of receptors and drug targets across lineages show biological variation and the potential for variegated responses.

CONCLUSIONS

Our study identifies novel lineage marker genes, covers regional differences, shows important differences between mouse and human gut epithelium, and reveals insight into how the epithelium responds to the environment and drugs. This comprehensive cell atlas of the healthy adult human intestinal epithelium resolves likely functional differences across anatomic regions along the gastrointestinal tract and advances our understanding of human intestinal physiology.

Association between Psoriasis Disease and IFN-λ Gene Polymorphisms

Psoriasis is one of the most common chronic immune-mediated skin diseases, having a strong genetic predisposition. Psoriasis is a T-cell-mediated disease with a mixed Th1/Th17 cytokines environment. IL-23/IL-17 axis hyperactivation is the primary pathogenesis. Psoriasis lesions have been known to exhibit high IFN-λ1 and IFN-stimulated genes (ISGs) expression, which appears to be driven by Th17 cells. However, the role and mechanism of IFN-λs in psoriasis disease remains unknown. The study aimed to investigate the relationship between IL-28B and IL-29 gene polymorphisms with psoriasis disease and clinical severity. We performed single-nucleotide polymorphisms (SNPs) of IL-28B rs12979860 (IL-28 C/T), rs8099917 (IL-28 T/G), and IL-29 rs30461 (IL-29 T/C) in 140 patients with psoriasis disease and 159 healthy controls using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. The genotype and allele frequency distributions of the rs12979860 (IL-28 C/T) and rs30461 (IL-29 T/C) polymorphisms were similar in the patient and control groups and were not statistically significant. The TG genotype of rs8099917 was statistically significantly different in patients from both groups. The TG genotype increased the risk of disease1.9-fold. The G allele may be associated with the pathogenesis of psoriasis.

Advances in the pathogenesis of psoriasis: from keratinocyte perspective

Psoriasis is a complex long-lasting inflammatory skin disease with high prevalence and associated comorbidity. It is characterized by epidermal hyperplasia and dermal infiltration of immune cells. Here, we review the role of keratinocytes in the pathogenesis of psoriasis, focusing on factors relevant to genetics, cytokines and receptors, metabolism, cell signaling, transcription factors, non-coding RNAs, antimicrobial peptides, and proteins with other different functions. The critical role of keratinocytes in initiating and maintaining the inflammatory state suggests the great significance of targeting keratinocytes for the treatment of psoriasis.

Interleukin-25-Mediated-IL-17RB Upregulation Promotes Cutaneous Wound Healing in Diabetic Mice by Improving Endothelial Cell Functions

Diabetic foot ulcer (DFU) frequently leads to non-traumatic amputation and finally even death. However, the mechanism of DFU is not fully understood. Interleukin 25 (IL-25), an alarmin cytokine that responds to tissue injury, has been reported to participate in tissue regeneration and maintaining glucose homeostasis. However, the role of IL-25 in diabetic wound healing remains unknown. Here, we showed that interleukin 17 receptor B (IL-17RB), the functional receptor of IL-25, was significantly inhibited in the wound skin of both diabetic patients with DFU and streptozotocin (STZ)-induced diabetic mice. Topical administration of recombinant IL-25 protein improved angiogenesis and collagen deposition in the wound bed and thus ameliorated delayed diabetic wound healing. IL-25 increased endothelial-specific CD31 expression in diabetic wounds and exogenous IL-25 protected endothelial cells from high glucose-impaired cell migration and tube formation in vitro. We further revealed that IL-25-mediated-IL-17RB signaling rescued the downregulation of Wnt/β-catenin pathway both in vivo in diabetic mice and in vitro in HUVECs and induced the phosphorylation of AKT and ERK 1/2 in HUVECs under high glucose conditions. This study defines a positive regulatory role of IL-25-mediated-IL-17RB signaling in diabetic wound healing and suggests that induction of IL-25-mediated-IL-17RB signaling may be a novel therapeutic strategy for treating poor healing diabetic wounds.

Identification of Keratinocyte Mitogens: Implications for Hyperproliferation in Psoriasis and Atopic Dermatitis

Psoriasis and atopic dermatitis are chronic inflammatory skin diseases characterized by keratinocyte (KC) hyperproliferation and epidermal acanthosis (hyperplasia). The milieu of disease-associated cytokines and soluble factors is considered a mitogenic factor; however, pinpointing the exact mitogens in this complex microenvironment is challenging. We employed organotypic human epidermal equivalents, faithfully mimicking native epidermal proliferation and stratification, to evaluate the proliferative effects of a broad panel of (literature-based) potential mitogens. The KC GF molecule, the T-helper 2 cytokines IL-4 and IL-13, and the psoriasis-associated cytokine IL-17A caused acanthosis by hyperplasia through a doubling in the number of proliferating KCs. In contrast, IFN-γ lowered proliferation, whereas IL-6, IL-20, IL-22, and oncostatin M induced acanthosis not by hyperproliferation but by hypertrophy. The T-helper 2‒cytokine‒mediated hyperproliferation was Jak/signal transducer and activator of transcription 3 dependent, whereas IL-17A and KC GF induced MAPK/extracellular signal‒regulated kinase kinase/extracellular signal‒regulated kinase‒dependent proliferation. This discovery that key regulators in atopic dermatitis and psoriasis are direct KC mitogens not only adds evidence to their crucial role in the pathophysiological processes but also highlights an additional therapeutic pillar for the mode of action of targeting biologicals (e.g., dupilumab) or small-molecule drugs (e.g., tofacitinib) by the normalization of KC turnover within the epidermal compartment.

The Role of Interleukins in the Pathogenesis of Dermatological Immune-Mediated Diseases

Autoimmune inflammatory diseases are primarily characterized by deregulated expression of cytokines, which drive pathogenesis of these diseases. A number of approved and experimental therapies utilize monoclonal antibodies against cytokine proteins. Cytokines can be classified into different families including the interleukins, which are secreted and act on leukocytes, the tumor necrosis factor (TNF) family, as well as chemokine proteins. In this review article, we focus on the interleukin family of cytokines, of which 39 members have been identified to this date. We outline the role of each of these interleukins in the immune system, and various dermatological inflammatory diseases with a focused discussion on the pathogenesis of psoriasis and atopic dermatitis. In addition, we describe the roles of various interleukins in psychiatric, cardiovascular, and gastrointestinal comorbidities. Finally, we review clinical efficacy and safety data from emerging late-phase anti-interleukin therapies under development for psoriasis and atopic dermatitis. Collectively, additional fundamental and clinical research remains necessary to fully elucidate the roles of various interleukin proteins in the pathogenesis of inflammatory dermatologic diseases, and treatment outcomes in patients.

Allosteric inhibition of SHP2 uncovers aberrant TLR7 trafficking in aggravating psoriasis

Psoriasis is a complex chronic inflammatory skin disease with unclear molecular mechanisms. We found that the Src homology‐2 domain‐containing protein tyrosine phosphatase‐2 (SHP2) was highly expressed in both psoriatic patients and imiquimod (IMQ)‐induced psoriasis‐like mice. Also, the SHP2 allosteric inhibitor SHP099 reduced pro‐inflammatory cytokine expression in PBMCs taken from psoriatic patients. Consistently, SHP099 significantly ameliorated IMQ‐triggered skin inflammation in mice. Single‐cell RNA sequencing of murine skin demonstrated that SHP2 inhibition impaired skin inflammation in myeloid cells, especially macrophages. Furthermore, IMQ‐induced psoriasis‐like skin inflammation was significantly alleviated in myeloid cells (monocytes, mature macrophages, and granulocytes)—but not dendritic cells conditional SHP2 knockout mice. Mechanistically, SHP2 promoted the trafficking of toll‐like receptor 7 (TLR7) from the Golgi to the endosome in macrophages by dephosphorylating TLR7 at Tyr1024, boosting the ubiquitination of TLR7 and NF‐κB‐mediated skin inflammation. Importantly, Tlr7 point‐mutant knock‐in mice showed an attenuated psoriasis‐like phenotype compared to wild‐type littermates following IMQ treatment. Collectively, our findings identify SHP2 as a novel regulator of psoriasis and suggest that SHP2 inhibition may be a promising therapeutic approach for psoriatic patients.

Role of Epithelium-Derived Cytokines in Atopic Dermatitis and Psoriasis: Evidence and Therapeutic Perspectives

Atopic dermatitis and psoriasis are two of the most common chronic skin conditions. Current target therapies represent viable and safe solutions for the most severe cases of these two dermatoses but, presently, several limitations exist in terms of efficacy and side effects. A new class of products, epithelium-derived cytokines (TSLP, IL-25, IL-33), show an increasing potential for use in target therapy for these patients, and demonstrate a direct link between a generalized inflammatory and oxidative stress status and the human skin. A review was conducted to better understand their role in the aforementioned conditions. Of these three molecules, TSLP led has been most often considered in studies regarding target therapies, and most of the results in the literature are related to this cytokine. These three cytokines share common stimuli and are linked to each other in both acute and chronic phases of these diseases, and have been challenged as target therapies or biomarkers of disease activity. The results lead to the conclusion that epithelium-derived cytokines could represent a therapeutic opportunity for these patients, especially in itch control. Furthermore, they might work better when paired together with currently available therapies or in combination with in-development treatments. Further studies are needed in order to verify the efficacy and safety of the biologic treatments currently under development.

The Role of IL-17 Cytokines in Psoriasis

Psoriasis is a chronic inflammatory skin condition associated with immune dysregulation. The immunologic cascade mediated by the interleukin (IL)-17 pathway plays a critically important role in the pathogenesis of psoriasis. The IL-17 effectors (IL-17A, IL-17C, IL-17E, and IL17F) act on keratinocytes, endothelial cells, and immune cells to stimulate epidermal hyperplasia and the pro-inflammatory feed-forward cycle seen within plaque psoriasis. The IL-17 pathway is also hypothesized to modulate the inflammatory responses linking comorbid systemic diseases with psoriasis. Furthermore, the robust clinical response seen with current and emerging therapies targeting IL-17 emphasizes the importance of the IL-17 cytokines in the pathogenesis of psoriasis.

Gut and Cutaneous Microbiome Featuring Abundance of Lactobacillus reuteri Protected Against Psoriasis-Like Inflammation in Mice

Background

Psoriasis is a chronic autoinflammatory skin disease, and its aetiology remains incompletely understood. Recently, gut microbial dysbiosis is found to be tightly associated with psoriasis.

Objective

We sought to reveal the causal role of gut microbiota dysbiosis in psoriasis pathogenesis and investigate the protective effect of healthy commensal bacteria against imiquimod -induced psoriasis-like skin response.

Methods

By using fecal microbial transplantation (FMT), 16S rRNA gene-based taxonomic profiling and Lactobacillus supplement, we have assessed the effect of FMT from healthy individuals on psoriasis-like skin inflammation and associated immune disorders in imiquimod-induced psoriasis mice.

Results

Here, by using psoriasis mice humanized with the stools from healthy donors and psoriasis patients, the imiquimod-induced psoriasis in mice with psoriasis patient stool was found to be significantly aggravated as compared to the mice with healthy donor stools. Further analysis showed fecal microbiota of healthy individuals protected against Treg/Th17 imbalance in psoriasis. Moreover, we found the gut and skin microbiome in mice receipted with gut microbiota of healthy individuals (HD) differed from those of mice receipted with gut microbiota of psoriasis patients (PSD). 16S rRNA sequencing revealed that Lactobacillus reuteri was greatly enriched in fecal and cutaneous microbiome of HD mice as compared to PSD mice. Intriguingly, supplement with Lactobacillus reuteri was sufficient to increase the expression of anti-inflammatory gene IL-10, reduce Th17 cells counts and confer resistance to imiquimod-induced inflammation on the mice with gut microbiota dysbiosis.

Conclusion

Our results suggested that the gut microbiota dysbiosis is the potential causal factor for psoriasis and the gut microbiota may serve as promising therapy target for psoriasis patients.

Involvement of miR-214-3p/FOXM1 Axis During the Progression of Psoriasis

Psoriasis is a common, chronic, and relapsing skin disease characterized by hyperproliferation of keratinocytes and apoptosis delay. However, the molecular mechanisms underlying the progression of psoriasis remain elusive. MicroRNAs (miRNAs) are single-stranded, small non-coding RNAs that play a crucial role in the development of psoriasis by promoting targeted mRNA degradation or translational inhibition. Here, we report that miR-214-3p, one of the downregulated miRNAs identified in the skin of psoriatic patients and imiquimod (IMQ)-induced mouse models, can negatively regulate the expression of forkhead box M1 (FOXM1). miR-214-3p inhibition leads to hyperproliferation and increased apoptosis of keratinocytes in vitro. Moreover, we show that miR-214-3p inhibition causes an arrest of the cell cycle at the S stage by elevating the expression of NEK2, KIF20A, CENP-A, CENP-F, and Cyclin B1 and by reducing the expression of Cyclin D1 in HaCaT cells. In vivo, the administration of miR-214-3p attenuates the psoriasis-like phenotype in IMQ-induced mice. Collectively, our results suggest that miR-214-3p/FOXM1 axis in keratinocytes could be a novel target in the treatment of psoriasis.

Novel pathogenesis of atopic dermatitis from the view of cytokines in mice and humans

Type 2 immunity and inflammation underlie allergic skin disorders, such as atopic dermatitis (AD). In type 2 inflammation, IL-4, IL-13, and IL-5, which are signature type 2 cytokines, are mainly produced by type 2 helper T (Th2) cells and form the characteristic features of AD. Epithelial cell-derived cytokines such as IL-25, IL-33, and TSLP initiate type 2 inflammation by modulating various cells, including group 2 innate lymphoid cells. Moreover, IL-31, a newly identified type 2 cytokine produced mainly by Th2 cells, induces pruritus by acting on sensory neurons in the skin. Based on both basic and clinical findings, several biologics targeting Th2 cytokines have been developed and exhibited significant efficacy as therapeutic reagents for AD. We have summarized the roles of each cytokine (IL-4, 5, 13, 25, 31, and 33, and TSLP) in the development of type 2 inflammation, especially AD, from the view of basic studies in mice and clinical trials/observation in humans.

From Messengers to Receptors in Psoriasis: The Role of IL-17RA in Disease and Treatment

The paradigm of psoriasis as a Th17-driven disease has evolved in the last years towards a much deeper knowledge of the complex pathways, mechanisms, cells, and messengers involved, highlighting the crucial role played by the IL-17 family of cytokines. All IL-17 isoforms signal through IL-17R. Five subunits of IL-17R have been described to date, which couple to form a homo- or hetero-receptor complex. Characteristically, IL-17RA is a common subunit in all hetero-receptors. IL-17RA has unique structural—containing a SEFIR/TILL domain—and functional—requiring ACT-1 for signaling—properties, enabling Th17 cells to act as a bridge between innate and adaptive immune cells. In psoriasis, IL-17RA plays a key role in pathogenesis based on: (a) IL-17A, IL-17F, and other IL-17 isoforms are involved in disease development; and (b) IL-17RA is essential for signaling of all IL-17 cytokines but IL-17D, whose receptor has not been identified to date. This article reviews current evidence on the biology and role of the IL-17 family of cytokines and receptors, with focus on IL-17RA, in psoriasis and some related comorbidities, and puts them in context with current and upcoming treatments.

Signalling of multiple interleukin (IL)-17 family cytokines via IL-17 receptor A drives psoriasis-related inflammatory pathways

Background

The interleukin (IL)‐23/IL‐17 immune axis is of central importance in psoriasis. However, the impact of IL‐17 family cytokines other than IL‐17A in psoriasis has not been fully established.

Objectives

To elucidate the contribution of IL‐17 family cytokines in psoriasis.

Methods

To address the expression and localization of IL‐17 family cytokines, lesional and nonlesional skin samples from patients with psoriasis were analysed by several complementary methods, including quantitative polymerase chain reaction, immunoassays, in situ hybridization and immunohistochemistry. Mechanistic studies assessing the functional activity of IL‐17 family cytokines were performed using ex vivo cultured human skin biopsies and primary human keratinocytes.

Results

We demonstrated that IL‐17A, IL‐17F, IL‐17A/F and IL‐17C are expressed at increased levels in psoriasis lesional skin and induce overlapping gene expression responses in ex vivo cultured human skin that correlate with the transcriptomic signature of psoriasis skin. Furthermore, we showed that brodalumab, in contrast to ixekizumab, normalizes gene expression responses induced by the combination of IL‐17A, IL‐17F, IL‐17A/F and IL‐17C in human keratinocytes.

Conclusions

Several IL‐17 ligands signalling through IL‐17RA are overexpressed in psoriasis skin and induce similar psoriasis‐related inflammatory pathways demonstrating their relevance in relation to therapeutic intervention in psoriasis.

What is already known about this topic?

The key role of interleukin (IL)‐17A in psoriasis is well established.

Previous studies have shown that IL‐17A, IL‐17F and IL‐17C are overexpressed in psoriasis skin, whereas contradictory results have been published for IL‐17E.

IL‐17 family cytokines induce secretion of inflammatory mediators such as antimicrobial peptides, chemokines and cytokines involved in the pathophysiology of psoriasis.

What does this study add?

Levels of IL‐17A/F are increased in lesional psoriasis skin but markedly lower than IL‐17A and IL‐17F.

In ex vivo cultured human skin, a physiologically relevant model, IL‐17A, IL‐17F, IL‐17A/F and IL‐17C show functional redundancy in shaping the psoriasis transcriptome.

IL‐17RA antagonism normalizes expression of psoriasis‐related genes in keratinocytes induced by the combination of IL‐17 family cytokines.

What is the translational message?

Overexpression and functional redundancy of IL‐17 family cytokines in psoriasis may explain why some patients with psoriasis with primary or secondary failure of response to secukinumab or ixekizumab achieve a clinical response after switching to brodalumab.

Linked Comment: M. Sugaya. Br J Dermatol 2021; 185:483.

Roles of IL-25 in Type 2 Inflammation and Autoimmune Pathogenesis

Interleukin-17E (IL-25) is a member of the IL-17 cytokine family that includes IL-17A to IL-17F. IL-17 family cytokines play a key role in host defense responses and inflammatory diseases. Compared with other IL-17 cytokine family members, IL-25 has relatively low sequence similarity to IL-17A and exhibits a distinct function from other IL-17 cytokines. IL-25 binds to its receptor composed of IL-17 receptor A (IL-17RA) and IL-17 receptor B (IL-17RB) for signal transduction. IL-25 has been implicated as a type 2 cytokine and can induce the production of IL-4, IL-5 and IL-13, which in turn inhibits the differentiation of T helper (Th) 17. In addition to its anti-inflammatory properties, IL-25 also exhibits a pro-inflammatory effect in the pathogenesis of Th17-dominated diseases. Here, we review recent advances in the roles of IL-25 in the pathogenesis of inflammation and autoimmune diseases.

A potential inflammatory role of IL-31 in psoriatic arthritis: A correlation with Th17 cytokine profile

The goals of our study were to determine the possible association of interleukin (IL)-31 with Th17 cytokine profile in serum and to quantify retinoic acid-related orphan receptor C (RORC) mRNA expression in psoriatic arthritis (PsA) patients. This cross-sectional study was conducted in 50 patients with PsA and 30 control subjects (CS) matched by age and gender. The cytokine serum levels were quantified by magnetic bead–based assay using the Bio-Plex MAGPIX system, and RORC mRNA expression was determined by quantitative polymerase chain reaction (qPCR). As a result, significant differences in IL-31 were observed between study groups (77.23 pg/mL in PsA vs 64.4 pg/mL in CS, P < 0.001) and Th17 cytokine profile serum levels (IL-17A: 6.36 pg/mL in PsA vs 2.97 pg/mL in CS, P = 0.02; IL-17F: 44.15 pg/mL in PsA vs 23.36 pg/mL in PsA, P = 0.01; IL-17E: 3.03 pg/mL in PsA vs 0.82 pg/mL in CS, P < 0.001; IL-21: 36.45 pg/mL in PsA vs 12.44 pg/mL in CS, P = 0.02); however, significant differences were not observed for IL-23 (31.2 pg/mL in PsA vs 53.26 pg/mL in CS, P = 0.58). Furthermore, positive correlations between IL-31 and Th17 cytokine profile serum levels were found (IL-17A: r_s = 0.64, P < 0.001; IL-17F: r_s = 0.73, P < 0.001; IL-17E: r_s = 0.70, P < 0.001; IL-21: r_s = 0.54, P = 0.002; IL-23: r_s = 0.5, P < 0.01). Regarding RORC gene expression, the PsA group showed an increase of 6.85-fold compared to the CS group. We did not find any association between the serum levels of cytokines and RORC gene expression. In conclusion, in PsA, there are increased serum levels of IL-31, IL-17A, IL-17F, IL-17E, and IL-21, but not IL-23. Moreover, there was a positive correlation of IL-31 with the Th17 cytokine profile and a high RORC gene expression. Altogether, these findings suggest a proinflammatory contribution of IL-31 in close association with the Th17 cytokine profile in PsA.

IL-25 (IL-17E) in epithelial immunology and pathophysiology

IL-25, also known as IL-17E, is a unique cytokine of the IL-17 family. Indeed, IL-25 exclusively was shown to strongly induce expression of the cytokines associated with type 2 immunity. Although produced by several types of immune cells, such as T cells, dendritic cells, or group 2 innate lymphoid cells, a vast amount of IL-25 derives from epithelial cells. The functions of IL-25 have been actively studied in the context of physiology and pathology of various organs including skin, airways and lungs, gastrointestinal tract, and thymus. Accumulating evidence suggests that IL-25 is a "barrier surface" cytokine whose expression depends on extrinsic environmental factors and when upregulated may lead to inflammatory disorders such as atopic dermatitis, psoriasis, or asthma. This review summarizes the progress of the recent years regarding the effects of IL-25 on the regulation of immune response and the balance between its homeostatic and pathogenic role in various epithelia. We revisit IL-25's general and tissue-specific mechanisms of action, mediated signaling pathways, and transcription factors activated in immune and resident cells. Finally, we discuss perspectives of the IL-25-based therapies for inflammatory disorders and compare them with the mainstream ones that target IL-17A.