3-Dimensional Optical Clearing and Imaging of Pruritic Atopic Dermatitis and Psoriasis Skin Reveals Downregulation of Epidermal Innervation

Visualising cancer in 3D: 3-Dimensional Tissue Imaging for management of cutaneous basal cell carcinoma

Surgical management of basal cell carcinoma (BCC) typically involves surgical excision with post-operative margin assessment using the bread-loafing technique; or gold-standard Mohs micrographic surgery (MMS), where margins are iteratively examined for residual cancer after tumour removal, with additional excisions performed upon detecting residual tumour at margins. There is limited sampling of resection margins with bread loafing, with detection of positive margins 44% of the time using 2 mm intervals. To resolve this, we have developed three-dimensional (3D) Tissue Imaging for: (1) complete examination of cancer margins and (2) detection of tumour proximity to nerves and blood vessels. 3D Tissue optical clearing with a light sheet imaging protocol was developed for margin assessment in two datasets assessed by two independent evaluators: (1) 48 samples from 29 patients with varied BCC subtypes, sizes and pigmentation levels; (2) 32 samples with matching Mohs' surgeon reading of tumour margins using two-dimensional haematoxylin & eosin-stained sections. The 3D Tissue Imaging protocol permits a complete examination of deeper and peripheral margins. Two independent evaluators achieved negative predictive values of 92.3% and 88.24% with 3D Tissue Imaging. Images obtained from 3D Tissue Imaging recapitulates histological features of BCC, such as nuclear crowding, palisading and retraction clefting and provides a 3D context for recognising normal skin adnexal structures. Concurrent immunofluorescence labelling of nerves and blood vessels allows visualisation of structures closer to tumour-positive regions, which may have a higher risk for neural and vascular infiltration. Together, this method provides more information in a 3D spatial context, enabling better cancer management by clinicians.

Atopic Dermatitis Itch: Scratching for an Explanation

Chronic pruritus is a cardinal symptom of atopic dermatitis (AD). The mechanisms underlying atopic itch involve intricate crosstalk among skin, immune components, and neural components. In this review, we explore these mechanisms, focusing on key players and interactions that induce and exacerbate itch. We discuss the similarities and differences between pruritus and pain in patients with AD as well as the relationship between pruritus and factors such as sweat and the skin microbiome. Furthermore, we explore novel targets that could provide significant itch relief in these patients as well as exciting future research directions to better understand atopic pruritus in darker skin types.

Similarities and differences in peripheral itch and pain pathways in atopic dermatitis

Atopic dermatitis (AD) is predominantly characterized by intense itching, but concomitant skin pain is experienced by more than 40% of patients. Patients with AD display considerable somatosensory aberrations, including increased nerve sensitivity to itch stimuli (hyperknesis), perception of itch from innocuous stimuli (alloknesis), or perception of pain from innocuous stimuli (allodynia). This review summarizes the current understanding of the similarities and differences in the peripheral mechanisms underlying itch and pain in AD. These distinct yet reciprocal sensations share many similarities in the peripheral nervous system, including common mediators (such as serotonin, endothelin-1, IL-33, and thymic stromal lymphopoietin), receptors (such as members of the G protein-coupled receptor family and Toll-like receptors), and ion channels for signal transduction (such as certain members of the transient receptor potential [TRP] cation channels). Itch-responding neurons are also sensitive to pain stimuli. However, there are distinct differences between itch and pain signaling. For example, specific immune responses are associated with pain (type 1 and/or type 3 cytokines and certain chemokine C-C [CCL2, CCL5] and C-X-C [CXCL] motif ligands) and itch (type 2 cytokines, including IL-31, and periostin). The TRP melastatin channels TRPM2 and TRPM3 have a role in pain but no known role in itch. Activation of μ-opioid receptors is known to alleviate pain but exacerbate itch. Understanding the connection between itch and pain mechanisms may offer new insights into the treatment of chronic pain and itch in AD.

Skin neuropathy and immunomodulation in diseases

Skin is a vital barrier tissue of the body. Immune responses in the skin must be precisely controlled, which would otherwise cause severe disease conditions such as psoriasis, atopic dermatitis, or pathogenic infection. Research evidence has increasingly demonstrated the essential roles of neural innervations, i.e., sensory and sympathetic signals, in modulating skin immunity. Notably, neuropathic changes of such neural structures have been observed in skin disease conditions, implicating their direct involvement in various pathological processes. An in-depth understanding of the mechanism underlying skin neuropathy and its immunomodulatory effects could help reveal novel entry points for therapeutic interventions. Here, we summarize the neuroimmune interactions between neuropathic events and skin immunity, highlighting the current knowledge and future perspectives of this emerging research frontier.

Cutaneous Components Leading to Pruritus, Pain, and Neurosensitivity in Atopic Dermatitis: A Narrative Review

Atopic dermatitis (AD) is a chronic, relapsing immunoinflammatory skin condition characterized by sensations such as pruritis, pain, and neuronal hypersensitivity. The mechanisms underlying these sensations are multifactorial and involve complex crosstalk among several cutaneous components. This review explores the role these components play in the pathophysiology of atopic dermatitis. In the skin intercellular spaces, sensory nerves interact with keratinocytes and immune cells via myriad mediators and receptors. These interactions generate action potentials that transmit pruritis and pain signals from the peripheral nervous system to the brain. Keratinocytes, the most abundant cell type in the epidermis, are key effector cells, triggering crosstalk with immune cells and sensory neurons to elicit pruritis, pain, and inflammation. Filaggrin expression by keratinocytes is reduced in atopic dermatitis, causing a weakened skin barrier and elevated skin pH. Fibroblasts are the main cell type in the dermis and, in atopic dermatitis, appear to reduce keratinocyte differentiation, further weakening the skin barrier. Fibroblasts and mast cells promote inflammation while dermal dendritic cells appear to attenuate inflammation. Inflammatory cytokines and chemokines play a major role in AD pathogenesis. Type 2 immune responses typically generate pruritis, and the type 1 and type 3 responses generate pain. Type 2 responses and increased skin pH contribute to barrier dysfunction and promote dysbiosis of the skin microbiome, causing the proliferation of Staphyloccocus aureus. In conclusion, understanding the dynamic interactions between cutaneous components in AD could drive the development of therapies to improve the quality of life for patients with AD.

Scratching increases epidermal neuronal branching and alters psychophysical testing responses in atopic dermatitis and brachioradial pruritus

Background

Chronic scratching imposes a major stress on the skin and can lead to itch intensity worsening, and consequently, patients may enter an itch-scratch cycle. This repetitive mechanical stress can result in lichenification, worsening of epidermal barrier function, and enhanced cutaneous inflammation. Furthermore, a reduction of intraepidermal nerve fibers was previously described in lichenification.

Aim

The aim of this study was to investigate the influence of chronic scratching on the epidermal neuroanatomy and on sensory changes, in particular the prevalence of hyperknesis and alloknesis in patients after mechanical, chemical, and electrical stimuli.

Methods

Analyses were performed on pruritic lichenified (chronically scratched), pruritic non-lichenified (not chronically scratched), and non-pruritic non-lesional (unaffected) skin areas of patients with inflammatory pruritus, i.e., atopic dermatitis (n = 35), and neuropathic pruritus, i.e., brachioradial pruritus (n = 34) vs. healthy matched controls (n = 64). Our fine-grained spatial skin characterization enabled specifically studying the differential effects of chronic scratching in inflammatory and neuropathic itch.

Results

Analysis of intraepidermal nerve fiber density showed rarefaction of fibers in all three skin areas of patients compared with healthy controls in both diagnoses. Even more, the two pruritic areas had significantly less nerve fibers than the unaffected skin, whereas electrically induced itch was massively increased. Epidermal branching of the remaining nerve fibers in lichenified/chronically scratched skin was increased, particularly in patients with brachioradial pruritus, which may contribute to the pronounced local neuronal sensitivity. Hyperknesis and alloknesis were found to increase independently of lichenification.

Conclusion

Our results indicate that chronic scratching may not affect intraepidermal nerve fiber density but leads to a stronger branching pattern of intraepidermal nerve fibers, which may contribute to local hypersensitivity. The increased sensitivity in the pruritic areas suggests mechanisms of peripheral sensitization, whereas the increased sensation of electrically and chemically induced itch in unaffected skin indicates central sensitization for itch.

Transcriptomic, Epigenomic, and Neuroanatomic Signatures Differ in Chronic Prurigo, Atopic Dermatitis, and Brachioradial Pruritus

Scratching and scratch-induced injuries, including neuroanatomical alterations, are key characteristics of chronic pruritus entities of different origins. The aim of this study was to link gene expression (array hybridization, qPCR) with DNA methylation (array hybridization) and neuroanatomy (PGP9.5 staining) in chronic nodular prurigo (CNPG), atopic dermatitis (AD), brachioradial pruritus (BRP), and matched healthy controls. Specific signatures of gene expression and DNA methylation clearly discriminated pruritic lesional skin from nonpruritic skin in CNPG and from healthy skin of volunteers, respectively. Although intraepidermal nerve fiber density was indiscriminately reduced, the level of epidermal branching, assessed by a semiquantitative pattern analysis, differentiated the entities (CNPG > BRP > AD). Correspondingly, repellent SEMA3A showed the highest expression in AD, whereas axonal growth-promoting nerve GF was most prominent in CNPG and BRP. Overexpression of genes for nerve fiber regeneration (NELL2/NFKB/ARTN) was found in AD and CNPG but not in BRP. Our findings suggest that differential branching patterns rather than mere innervation density separate chronic itch conditions and reflect disease-specific local expression profiles. In pruritic dermatoses (AD and CNPG), nerve injury and subsequent sprouting may primarily result from chronic scratching, whereas genuine neuropathy is expected to underlie BRP.

Skin optical clearing enabled by dissolving hyaluronic acid microneedle patches

Optical imaging and phototherapy are of great significance in the detection, diagnosis, and therapy of diseases. Depth of light in the skin tissues in optical imaging and phototherapy can be significantly improved with the assistance of optical clearing technology by weakening the scattering from the refractive indexes inhomogeneity among skin constituents. However, the barrier of the stratum corneum restricts the penetration of optical clearing agents into deep tissues and limits the optical clearing effects. Herein, we develop an optical clearing strategy by using dissolving microneedle (MN) patches made of hyaluronic acid (HA), which can effortlessly and painlessly penetrate the stratum corneum to reach the epidermis and dermis. By using the HA MN patches, the transmittance of skin tissues is improved by about 12.13 %. We show that the HA MN patches enhance the clarity of blood vessels to realize naked-eyes observation. Moreover, a simulated subcutaneous tumor cells experiment also verifies that the optical clearing effects of the HA MN patch efficiently boost the efficiency of the photodynamic killing of tumor cells by 26.8 %. As a courageous attempt, this study provides a promising avenue to improve the optical clearing effects for further clinical application of optical imaging and phototherapy.

Peripheral itch sensitization in atopic dermatitis

Atopic dermatitis is a skin disorder caused by skin dryness and barrier dysfunction, resulting in skin inflammation and chronic itch (or pruritus). The pathogenesis of atopic dermatitis is thought to be initiated by a lowering of the itch threshold due to dry skin. This lowering of the itch threshold is at least partially due to the increase in intraepidermal nerve fibers and sensitization of sensory nerves by interleukin (IL)-33 produced and secreted by keratinocytes. Such skin is easily prone to itch due to mechanical stimuli, such as rubbing of clothing and chemical stimuli from itch mediators. In patients with atopic dermatitis, once itch occurs, further itch is induced by scratching, and the associated scratching breaks down the skin barrier. Disruption of the skin barrier allows entry into the epidermis of external foreign substances, such as allergens derived from house dust mites, leading to an increased induction of type 2 inflammatory responses. As a result, type 2 cytokines IL-4, IL-13, and IL-31 are mainly secreted by Th2 cells, and their action on sensory nerve fibers causes further itch sensitization. These sequences of events are thought to occur simultaneously in patients with atopic dermatitis, leading to a vicious itch-scratch cycle. This vicious cycle becomes a negative spiral that leads to disease burden. Therefore, controlling itch is essential for the treatment of atopic dermatitis. In this review, we summarize and discuss advances in the mechanisms of peripheral itch sensitization in atopic dermatitis, focusing on skin barrier-neuro-immune triadic connectivity.

Cutaneous nerve fibers participate in the progression of psoriasis by linking epidermal keratinocytes and immunocytes

Recent studies have illustrated that psoriatic lesions are innervated by dense sensory nerve fibers. Psoriatic plaques appeared to improve after central or peripheral nerve injury. Therefore, the nervous system may play a vital role in psoriasis. We aimed to clarify the expression of nerve fibers in psoriasis and their relationship with immune cells and keratinocytes, and to explore the effect of skin nerve impairment. Our results illustrated that nerve fibers in psoriatic lesions increased and were closely innervated around immune cells and keratinocytes. RNA-seq analysis showed that peripheral sensory nerve-related genes were disrupted in psoriasis. In spinal cord hemi-section mice, sensory impairment improved psoriasiform dermatitis and inhibited the abnormal proliferation of keratinocytes. Botulinum toxin A alleviated psoriasiform dermatitis by inhibiting the secretion of calcitonin gene-related peptide. Collectively, cutaneous nerve fibers participate in the progression of psoriasis by linking epidermal keratinocytes and immunocytes. Neurological intervention may be a new treatment strategy for psoriasis.

CB2R Deficiency Exacerbates Imiquimod-Induced Psoriasiform Dermatitis and Itch Through the Neuro-Immune Pathway

Background: Cannabinoid receptor 2 (CB2R) is a potential target for anti-inflammatory and pain therapeutics given its significant immunomodulatory and analgesic effects. However, the role of CB2R in imiquimod (IMQ)-induced psoriasiform dermatitis (PsD) and itch is poorly understood.

Objective: To investigate the function and mechanism of CB2R in PsD and itch in mice.

Methods: Following daily treatment with topical IMQ cream for 5-7 consecutive days in C56BL/6 wild-type (WT) and CB2R gene knockout (KO) mice, we assessed the Psoriasis Area and Severity Index (PASI) scores and the scratch bouts every day, and hematoxylin and eosin (H&E) staining, toluidine blue staining were used to observe the histological changes. mRNA levels were analyzed by quantitative real-time polymerase chain reaction (qRT-PCR). Protein levels were detected by western blotting (WB), immunohistochemistry (IHC), immunofluorescence (IF) and cytometric bead array (CBA). Flow cytometry (FCM) was used to examine the proportion of Th17/Treg cells.

Results: We found that CB2R expression levels were increased in mice with psoriasis. Compared with WT mice, CB2R deficiency exacerbated IMQ-induced PsD and scratching bouts and upregulated the expression of proinflammatory cytokines by increasing the infiltration of CD4⁺ T cells and the Th17/Treg ratio. Obvious proliferation and prolongation of nerve fibers and high expression of nerve growth factor (NGF) were observed in PsD and CB2R KO mice. Pretreatment with the CB2R agonist, JWH-133 significantly reversed inflammation and scratching bouts. CB2R didn't participate in the induction of itch in psoriasis by regulating the expression of IL-31, thymic stromal lymphopoietin (TSLP) and mast cells in mouse skins.

Conclusion: Our results demonstrate that CB2R plays a pivotal role in the pathophysiology of psoriasis, providing a new potential target for anti-inflammatory and antipruritic drugs.

Lidocaine Ameliorates Psoriasis by Obstructing Pathogenic CGRP Signaling-Mediated Sensory Neuron-Dendritic Cell Communication

Psoriasis is a chronic immune-mediated skin disorder with the nervous system contributing to its pathology. The neurogenic mediators of psoriasis are elusive and whether the intervention of cutaneous nervous system can treat psoriasis remains to be determined. Here we conducted a pilot study using epidural injection of lidocaine to treat patients with psoriasis. Lidocaine treatment markedly reduced patients' clinical scores, and improved an imiquimod (IMQ)-induced rat model of psoriasis as competent as systemic delivery of a TNF-α antibody. IMQ application elicited aberrant cutaneous nerve outgrowth and excessive generation of neuropeptide calcitonin gene-related peptide (CGRP) from dorsal root ganglion (DRG) neurons, both of which were inhibited by epidural lidocaine treatment. Single-cell RNA sequencing unveiled the overrepresentation of CGRP receptors in dermal dendritic cell (DC) populations of patients with psoriasis. Through disturbing CGRP signaling, lidocaine inhibited IL-23 production by DCs co-cultured with DRG neurons. Thus, epidural nerve block with lidocaine demonstrates an effective therapy for psoriasis, which suppresses both inordinate sensory nerve growth in the inflamed skin and CGRP-mediated IL-23 production from psoriatic DCs.

Skin-ny deeping: Uncovering immune cell behavior and function through imaging techniques

As the largest organ of the body, the skin is a key barrier tissue with specialized structures where ongoing immune surveillance is critical for protecting the body from external insults. The innate immune system acts as first-responders in a coordinated manner to react to injury or infections, and recent developments in intravital imaging techniques have made it possible to delineate dynamic immune cell responses in a spatiotemporal manner. We review here key studies involved in understanding neutrophil, dendritic cell and macrophage behavior in skin and further discuss how this knowledge collectively highlights the importance of interactions and cellular functions in a systems biology manner. Furthermore, we will review emerging imaging technologies such as high-content proteomic screening, spatial transcriptomics and three-dimensional volumetric imaging and how these techniques can be integrated to provide a systems overview of the immune system that will further our current knowledge and lead to potential exciting discoveries in the upcoming decades.

Characteristics, mechanism, and management of pain in atopic dermatitis: A literature review

BACKGROUND

Atopic dermatitis (AD) is a chronic, pruritic, immune-mediated inflammatory disease. Developments in basic science and clinical research have increased our understanding of AD. Although pain as a symptom of AD is underemphasized in previous studies, multiple researchers address pain as a frequent burden of AD. However, the exact role of pain in AD is not fully understood.

AIMS

Our review aimed to summarize the current evidence focusing on characteristics, mechanism, and management of pain in AD.

MATERIALS & METHODS

We conducted a thorough literature review in the PubMed database to figure out different aspects discussing pain in AD, including pain symptoms, burden, the relationship between pain and itch, mechanism, and pain management in AD.

RESULTS AND CONCLUSION

AD patients affected by skin pain vary from 42.7%-92.2% with remarkable intensity and heavy burden. Skin pain and itch interacted both in symptoms and mechanisms. Atopic skin with the impaired barrier, neurogenic inflammation mediators, peripheral and central sensitization of pain may possibly explain pain mechanism in AD. Future research is needed to clarify the commonality and disparity of pain and itch in AD in order to seek efficacious medications and treatment.

Developments and challenges in dermatology: an update from the Interactive Derma Academy (IDeA) 2019

The 2019 Interactive Derma Academy (IDeA) meeting was held in Lisbon, Portugal, 10-12 May, bringing together leading dermatology experts from across Europe, the Middle East and Asia. Over three days, the latest developments and challenges in relation to the pathophysiology, diagnosis, evaluation and management of dermatological conditions were presented, with a particular focus on acne, atopic dermatitis (AD) and actinic keratosis (AK). Interesting clinical case studies relating to these key topics were discussed with attendees to establish current evidence-based best practices. Presentations reviewed current treatments, potential therapeutic approaches and key considerations in the management of acne, AK and AD, and discussed the importance of the microbiome in these conditions, as well as the provision of patient education/support. It was highlighted that active treatment is not always required for AK, depending on patient preferences and clinical circumstances. In addition to presentations, two interactive workshops on the diagnosis and treatment of sexually transmitted infections/diseases (STIs/STDs) presenting to the dermatology clinic, and current and future dermocosmetics were conducted. The potential for misdiagnosis of STIs/STDs was discussed, with dermoscopy and/or reflectance confocal microscopy suggested as useful diagnostic techniques. In addition, botulinum toxin was introduced as a potential dermocosmetic, and the possibility of microbiome alteration in the treatment of dermatological conditions emphasized. Furthermore, several challenges in dermatology, including the use of lasers, the complexity of atopic dermatitis, wound care, use of biosimilars and application of non-invasive techniques in skin cancer diagnosis were reviewed. In this supplement, we provide an overview of the presentations and discussions from the fourth successful IDeA meeting, summarizing the key insights shared by dermatologists from across the globe.

Itch intensity in prurigo nodularis is closely related to dermal interleukin-31, oncostatin M, IL-31 receptor alpha and oncostatin M receptor beta

Prurigo nodularis (PN) is a chronic skin dermatosis with hyperkeratotic and intensely pruritic nodules. Managing PN-associated itch is difficult because its aetiology is still unknown. This study aimed to investigate the correlation between itch intensity in PN and the expression of a pruritogenic cytokine interleukin (IL)-31, its receptor complex components IL-31 receptor α (IL-31RA) and oncostatin M receptor β (OSMRβ), and oncostatin M (OSM), which is a ligand of OSMR β, through immunofluorescence staining examination. Itch intensity in PN was closely correlated with the number of dermal IL-31(+) cells (Spearman's r = 0.551, p < 0.05), dermal IL-31RA(+) cells (r = 0.475, p < 0.05) and dermal OSM(+) cells (r = 0.505, p < 0.05). In addition, the number of dermal OSMRβ (+) cells was increased in PN (t test, p < 0.05), despite not being correlated with itch intensity (Spearman's r = 0.375, p > 0.05). Major cellular sources of dermal IL-31 were T cells (27.0% of total IL-31-expressing cells) and macrophages (35.0%), while those of OSM were mainly T cells (49.8%) and mast cells (26.8%). IL-31RA-expressing dermal cells were mostly mast cells (49.3%) and macrophages (36.6%), and OSMRβ was mainly expressed by macrophages (51.8%) in the dermis. These findings indicate that IL-31 (mainly from macrophages and T cells) and OSM (principally from T cells and mast cells) stimulate dermal cells expressing IL-31RA and OSMRβ (e.g. macrophages), which may further promote itch and inflammation in PN. This complex dermal milieu of cell/cytokine/receptor network can be a therapeutic target for PN-associated itch.

Electrically Evoked Itch in Human Subjects

Administration of chemicals (pruritogens) into the skin evokes itch based on signal transduction mechanisms that generate action potentials mainly in mechanically sensitive and insensitive primary afferent C-fibers (pruriceptors). These signals from peripheral neurons are processed in spinal and supra-spinal centers of the central nervous system and finally generate the sensation of itch. Compared to chemical stimulation, electrical activation of pruriceptors would allow for better temporal control and thereby a more direct functional assessment of their activation. Here, we review the electrical stimulation paradigms which were used to evoke itch in humans in the past. We further evaluate recent attempts to explore electrically induced itch in atopic dermatitis patients. Possible mechanisms underlying successful pruritus generation in chronic itch patients by transdermal slowly depolarizing electrical stimulation are discussed.

Mouse models of atopic dermatitis: a critical reappraisal

Mouse models for atopic dermatitis (AD) are an indispensable preclinical research tool for testing new candidate AD therapeutics and for interrogating AD pathobiology in vivo. In this Viewpoint, we delineate why, unfortunately, none of the currently available so-called "AD" mouse models satisfactorily reflect the clinical complexity of human AD, but imitate more "allergic" or "irriant" contact dermatitis conditions. This limits the predictive value of AD models for clinical outcomes of new tested candidate AD therapeutics and the instructiveness of mouse models for human AD pathophysiology research. Here, we propose to initiate a rational debate on the minimal criteria that a mouse model should meet in order to be considered relevant for human AD. We suggest that valid AD models should at least meet the following criteria: (a) an AD-like epidermal barrier defect with reduced filaggrin expression along with hyperproliferation, hyperplasia; (b) increased epidermal expression of thymic stromal lymphopoietin (TSLP), periostin and/or chemokines such as TARC (CCL17); (c) a characteristic dermal immune cell infiltrate with overexpression of some key cytokines such as IL-4, IL-13, IL-31 and IL-33; (d) distinctive "neurodermatitis" features (sensory skin hyperinnervation, defective beta-adrenergic signalling, neurogenic skin inflammation and triggering or aggravation of AD-like skin lesions by perceived stress); and (e) response of experimentally induced skin lesions to standard AD therapy. Finally, we delineate why humanized AD mouse models (human skin xenotransplants on SCID mice) offer a particularly promising preclinical research alternative to the currently available "AD" mouse models.

Prevalence, pathophysiology and management of itch in epidermolysis bullosa

Epidermolysis bullosa (EB) is a highly diverse group of inherited skin disorders, resulting from mutations in genes encoding proteins of the dermoepidermal junction. Itch (pruritus) is one of the most common symptoms across all EB subtypes. It occurs in blistered or wounded sites, or manifests as a generalized phenomenon, thereby affecting both intact skin and healing wounds. The mechanism of pruritus in EB is unclear. It is likely that skin inflammation secondary to barrier disruption, wound healing cascades and dysregulated activation of epidermal sensory nerve endings are all involved in its pathophysiology on the molecular and cellular level. Understanding these mechanisms in depth is crucial in developing optimized treatments for people with EB and improving quality of life. This review summarizes current evidence on the prevalence, mechanisms and management of itch in EB.

Visualizing the Itch-Sensing Skin Arbors

Diverse sensory neurons exhibit distinct neuronal morphologies with a variety of axon terminal arborizations subserving their functions. Because of its clinical significance, the molecular and cellular mechanisms of itch are being intensely studied. However, a complete analysis of itch-sensing terminal arborization is missing. Using an MrgprC11^CreERT2 transgenic mouse line, we labeled a small subset of itch-sensing neurons that express multiple itch-related molecules including MrgprA3, MrgprC11, histamine receptor H1, IL-31 receptor, 5-hydroxytryptamine receptor 1F, natriuretic precursor peptide B, and neuromedin B. By combining sparse genetic labeling and whole-mount placental alkaline phosphatase histochemistry, we found that itch-sensing skin arbors exhibit free endings with extensive axonal branching in the superficial epidermis and large receptive fields. These results revealed the unique morphological characteristics of itch-sensing neurons and provide intriguing insights into the basic mechanisms of itch transmission.

MRGPRX2 signals its importance in cutaneous mast cell biology: Does MRGPRX2 connect mast cells and atopic dermatitis?

The discovery of MRGPRX2 marks an important change in MC biology, explaining non-IgE-mediated clinical phenomena relying on MCs. As receptor for multiple drugs, MRGPRX2 is crucial to drug-induced hypersensitivity. However, not only drugs, but also endogenous mediators like neuropeptides and host defense peptides activate MRGPRX2, suggesting its broad impact in cutaneous pathophysiology. Here, we give a brief overview of MRGPRX2 and its regulation by microenvironmental stimuli, which support MCs and can be altered in skin disorders, and briefly touch on the functional programs elicited by MRGPRX2 ligation. Studies in Mrgprb2-deficient mice (the murine ortholog) help illuminate MRGPRX2's function in health and disease. Recent advances in this model support the long-suspected operational unit between MCs and nerves, with MRGPRX2 being a vital component. Based on the limited evidence for a major contribution of FcεRI/IgE-activated MCs to atopic dermatitis (AD), we develop the hypothesis that MRGPRX2 constitutes the missing link connecting MCs and AD, at least in selected endotypes. Support comes from the multifold changes in the MC-neuronal system of AD skin (eg greater density of MCs and closer connections between MCs and nerves, increased PAR-2/Substance P). We theorize that these deregulations suffice to initiate AD, but external triggers, many of which activating MRGPRX2 themselves (eg Staphylococcus aureus) further feed into the loop. Itch, the most burdensome hallmark of AD, is mostly non-histaminergic but tryptase-dependent, and tryptase is preferentially released upon MRGPRX2 activation. Because MRGPRX2 is a very active research field, some of the existing gaps are likely to be closed soon.

Research Techniques Made Simple: Optical Clearing and Three-Dimensional Volumetric Imaging of Skin Biopsies

Skin histology is traditionally carried out using two-dimensional tissue sections, which allows for rapid staining, but these sections cannot accurately represent three-dimensional structures in skin such as nerves, vasculature, hair follicles, and sebaceous glands. Although it may be ideal to image skin in a three-dimensional manner, it is technically challenging to image deep into tissue because of light scattering from collagen fibrils in the dermis and refractive index mismatch owing to the presence of differing biological materials such as cytoplasm, and lipids in the skin. Different optical clearing methods have been developed recently, making it possible to render tissues transparent using different approaches. Here, we discuss the steps involved in tissue preparation for three-dimensional volumetric imaging and provide a brief overview of the different optical clearing methods as well as different imaging modalities for three-dimensional imaging.

Pathophysiologic mechanisms of itch in bullous pemphigoid

BACKGROUND

One of the hallmarks of bullous pemphigoid (BP) is moderate to severe chronic itch. Managing this is difficult because little is known about the mechanisms of itch in BP.

OBJECTIVE

We sought to elucidate the pathophysiologic mechanisms of itch in BP.

METHODS

The expression of itch mediators in lesions of 24 patients with BP and 6 healthy individuals were examined through immunofluorescence staining. Furthermore, the expression of itch mediators and itch severity was correlated.

RESULTS

Itch severity was correlated with eosinophils, substance P, neurokinin 1R, interleukin (IL) 31 receptor A, oncostatin M receptor-β, IL-13, periostin, and basophils. There was also a trend between itch severity and IL-31 expression. Most of the cells expressing IL-31 or neurokinin 1R were identified as eosinophils. Intraepidermal nerve fiber density was decreased. Other itch mediators, including mast cells, IL-4, thymic stromal lymphopoietin, transient receptor potential vanilloid 1 and ankyrin 1, and protease activated receptor 2 were not significantly correlated with itch severity.

LIMITATIONS

The relatively small sample size, the examination of protein expression exclusively through immunofluorescent analysis, and lack of functional assays in patients are the limitations.

CONCLUSIONS

Multiple factors are involved in BP-associated itch, including eosinophils, substance P, neurokinin 1R, IL-31, IL-31 receptor A, oncostatin M receptor-β, IL-13, periostin, and basophils. They could be useful therapeutic targets.

Pruritus in Autoimmune and Inflammatory Dermatoses

Pruritus in autoimmune and inflammatory dermatoses is a common symptom that can be severe and affect the quality of life of patients. In some diseases, pruritus is related to disorders activity and severity or may occur independent of the disease. Despite the high prevalence, the symptom is still underrated and there are only a few trials investigating the efficacy of drugs for disease-specific pruritus. In this review, the characteristics and possible pathomechanisms of pruritus in various dermatoses like autoimmune bullous diseases, connective tissue diseases as well as autoimmune-associated dermatoses (atopic dermatitis, psoriasis vulgaris) is illustrated. Additionally, studies analyzing the antipruritic treatment are discussed. Summarizing, the prevalence of pruritus in these diseases demonstrates the importance for symptom recognition and the need for an efficient antipruritic therapy.

3-Dimensional Imaging of Cutaneous Nerve Endings

Cutaneous nerve endings are assumed to change due to dermatological pathologies, such as psoriasis. Tan et al. (2019) present a method to assess changes in nerve architecture more precisely by using 3-dimensional histology. This commentary discusses the advantages of 3-dimensional histology, as well as some limitations of the presented method. Future work should focus on intravital applications.