Mast cell-sensory neuron crosstalk in allergic diseases

Mast cells (MCs) are tissue-resident immune cells, well-positioned at the host-environment interface for detecting external antigens and playing a critical role in mobilizing innate and adaptive immune responses. Sensory neurons are afferent neurons innervating most areas of the body but especially in the periphery, where they sense external and internal signals and relay information to the brain. The significance of MC-sensory neuron communication is now increasingly becoming recognized, especially because both cell types are in close physical proximity at the host-environment interface and around major organs of the body and produce specific mediators that can activate each other. In this review, we explore the roles of MC-sensory neuron crosstalk in allergic diseases, shedding light on how activated MCs trigger sensory neurons to initiate signaling in pruritus, shock, and potentially abdominal pain in allergy, and how activated sensory neurons regulate MCs in homeostasis and atopic dermatitis associated with contact hypersensitivity and type 2 inflammation. Throughout the review, we also discuss how these 2 sentinel cell types signal each other, potentially resulting in a positive feedback loop that can sustain inflammation. Unraveling the mysteries of MC-sensory neuron crosstalk is likely to unveil their critical roles in various disease conditions and enable the development of new therapeutic approaches to combat these maladies.

Beyond Allergies-Updates on The Role of Mas-Related G-Protein-Coupled Receptor X2 in Chronic Urticaria and Atopic Dermatitis

Mast cells (MCs) are an important part of the immune system, responding both to pathogens and toxins, but they also play an important role in allergic diseases, where recent data show that non-IgE-mediated activation is also of relevance, especially in chronic urticaria (CU) and atopic dermatitis (AD). Skin MCs express Mas-related G-protein-coupled receptor X2 (MRGPRX2), a key protein in non-IgE-dependent MC degranulation, and its overactivity is one of the triggering factors for the above-mentioned diseases, making MRGPRX2 a potential therapeutic target. Reviewing the latest literature revealed our need to focus on the discovery of MRGPRX2 activators as well as the ongoing vast research towards finding specific MRGPRX2 inhibitors for potential therapeutic approaches. Most of these studies are in their preliminary stages, with one drug currently being investigated in a clinical trial. Future studies and improved model systems are needed to verify whether any of these inhibitors may have the potential to be the next therapeutic treatment for CU, AD, and other pseudo-allergic reactions.

Adipose tissue-derived exosomes alleviate particulate matter-induced inflammatory response and skin barrier damage in atopic dermatitis-like triple-cell model

Recently, particulate matter (PM) has been shown to exacerbate atopic dermatitis (AD) by inducing an inflammatory response. Meanwhile, several studies revealed that exosomes derived from adipose tissue-derived mesenchymal stem cells promote wound healing and alleviate inflammation via their regenerative and immunomodulatory capacities. Our study aimed to investigate the effects of human adipose tissue-derived mesenchymal stem cell-derived (ASC)-exosomes in PM-induced AD. An AD-like triple-cell model was established by treating human keratinocytes, dermal fibroblasts, and mast cells with polyinosinic:polycytidylic acid (Poly I:C) and interleukin 1 alpha (IL-1α). The effects of PM and ASC-exosomes on the expression of pro-inflammatory cytokines and skin barrier proteins were examined using quantitative real-time polymerase chain reaction, western blotting, and immunofluorescence. PM increased pro-inflammatory cytokines (IL-6, IL-1β, and IL-1α) and decreased the anti-inflammatory cytokine IL-10, while the mRNA expression of skin barrier proteins (loricrin and filaggrin) decreased. However, when the cells were treated with ASC-exosomes, the PM-induced effects on pro-inflammatory cytokines and skin barrier proteins were reversed. Our results confirmed that PM-induced inflammation and skin barrier damage were alleviated by ASC-exosomes in our AD-like triple-cell model. These data suggest that ASC-exosomes can serve as a therapeutic agent for PM-exacerbated AD.

Mast cell-mediated immune regulation in health and disease

Mast cells are important components of the immune system, and they perform pro-inflammatory as well as anti-inflammatory roles in the complex process of immune regulation in health and disease. Because of their strategic perivascular localization, sensitivity and adaptability to the microenvironment, and ability to release a variety of preformed and newly synthesized effector molecules, mast cells perform unique functions in almost all organs. Additionally, Mast cells express a wide range of surface and cytoplasmic receptors which enable them to respond to a variety of cytokines, chemicals, and pathogens. The mast cell's role as a cellular interface between external and internal environments as well as between vasculature and tissues is critical for protection and repair. Mast cell interactions with different immune and nonimmune cells through secreted inflammatory mediators may also turn in favor of disease promoting agents. First and forefront, mast cells are well recognized for their multifaceted functions in allergic diseases. Reciprocal communication between mast cells and endothelial cells in the presence of bacterial toxins in chronic/sub-clinical infections induce persistent vascular inflammation. We have shown that mast cell proteases and histamine induce endothelial inflammatory responses that are synergistically amplified by bacterial toxins. Mast cells have been shown to exacerbate vascular changes in normal states as well as in chronic or subclinical infections, particularly among cigarette smokers. Furthermore, a potential role of mast cells in SARS-CoV-2-induced dysfunction of the capillary-alveolar interface adds to the growing understanding of mast cells in viral infections. The interaction between mast cells and microglial cells in the brain further highlights their significance in neuroinflammation. This review highlights the significant role of mast cells as the interface that acts as sensor and early responder through interactions with cells in systemic organs and the nervous system.

Stem Cells from Human Exfoliated Deciduous Teeth Attenuate Atopic Dermatitis Symptoms in Mice through Modulating Immune Balance and Skin Barrier Function

Atopic dermatitis (AD) is a chronic skin inflammatory disease associated with immune abnormalities and disrupted skin barrier function. Mesenchymal stem cells (MSCs) have been suggested as an alternative therapeutic option in AD. Stem cells from human exfoliated deciduous teeth (SHEDs) are a unique postnatal stem cell population with high immunomodulatory properties. The aim of this study was to explore the effects of SHEDs on AD in the BALB/c mouse model induced by 2,4-dinitrochlorobenzene (DNCB). SHEDs were administrated intravenously or subcutaneously, and clinical severity, histopathological findings, skin barrier function, and organ indexes were evaluated. Skin tissue cytokine mRNA levels and serum cytokine protein levels were further analysed. SHED administration significantly alleviated AD clinical severity, including dermatitis scores, ear thickness, scratching behaviour, and infiltration of mast cells. In addition, disrupted skin barrier function and enlarged spleens were restored by SHED administration. Further, SHED treatment reduced the levels of IgE, IgG1, and thymic stromal lymphopoietin (TSLP) in the serum and the modulated expression of Th1-, Th2-, and Th17-associated cytokines in skin lesions. In conclusion, SHEDs attenuated AD-like skin lesions in mice by modulating the immune balance and skin barrier function. SHEDs could be a potential new treatment agent for AD.

Development of In Vitro Co-Culture Model to Mimic the Cell to Cell Communication in Response to Urban PM2.5

Background

Airborne particulate matter (PM), a widespread air contaminant, is a complex mixture of solids and aerosols composed of particles suspended in the air. PM is associated with inflammatory responses and may worsen inflammatory skin diseases. However, the mechanisms through which PM affects atopic dermatitis (AD) remain unclear.

Objective

To establish an in vitro model that more accurately mimics AD using human keratinocyte (HaCaT), dermal fibroblast (HDF), and mast cell (HMC-1) and using this model to investigate the mechanism through which PMs affect AD.

Methods

An AD-like in vitro model was established by seeding HaCaT, HDF, and HMC-1 cells with recombinant human interleukin (IL)-1α and polyinosinic:polycytidylic acid. We confirmed the effect of PM on the inflammatory cytokine expression of a triple-cell culture model. SRM 1649b Urban Dust, which is mainly composed of polycyclic aromatic hydrocarbons, was used as the reference PM. The effects of PM on the expression levels of proinflammatory cytokines and skin barrier markers were assessed using quantitative real-time polymerase chain reaction and western blotting. Inflammatory cytokine levels were measured using an enzyme-linked immunosorbent assay.

Results

Interactions between various skin cell types were evaluated using a co-culture system. PM treatment increased mRNA and protein levels of the inflammatory cytokines IL-6, IL-1α, tumor necrosis factor-α, IL-4, and IL-1β and decreased the expression of the skin barrier markers filaggrin and loricrin.

Conclusion

Our results suggest that an in vitro triple-cell culture model using HaCaT, HDF, and HMC-1 cells may be reliable for obtaining more physiological, functional, and reproducible data on AD and skin barriers.

Neuron‒Mast Cell Cross-Talk in the Skin

Skin-resident mast cells (MCs) and cutaneous sensory neurons both play crucial roles in microbial‒host defense and inflammatory diseases. MCs can be directly activated by pathogens or their products, resulting in the release of numerous mediators that promote innate immune responses and also activate sensory neurons. Cutaneous sensory neurons can also directly detect the presence of pathogens, resulting in the release of neuropeptides that modulate MC function. In this review, we will focus on the reciprocal interactions between cutaneous sensory neurons and MCs and the importance of this cross-talk in skin diseases.

Intractable Itch in Atopic Dermatitis: Causes and Treatments

Itch or pruritus is the hallmark of atopic dermatitis and is defined as an unpleasant sensation that evokes the desire to scratch. It is also believed that itch is a signal of danger from various environmental factors or physiological abnormalities. Because histamine is a well-known substance inducing itch, H₁-antihistamines are the most frequently used drugs to treat pruritus. However, H₁-antihistamines are not fully effective against intractable itch in patients with atopic dermatitis. Given that intractable itch is a clinical problem that markedly decreases quality of life, its treatment in atopic dermatitis is of high importance. Histamine-independent itch may be elicited by various pruritogens, including proteases, cytokines, neuropeptides, lipids, and opioids, and their cognate receptors, such as protease-activated receptors, cytokine receptors, Mas-related G protein-coupled receptors, opioid receptors, and transient receptor potential channels. In addition, cutaneous hyperinnervation is partly involved in itch sensitization in the periphery. It is believed that dry skin is a key feature of intractable itch in atopic dermatitis. Treatment of the underlying conditions that cause itch is necessary to improve the quality of life of patients with atopic dermatitis. This review describes current insights into the pathophysiology of itch and its treatment in atopic dermatitis.

Mast Cells and Sensory Nerves Contribute to Neurogenic Inflammation and Pruritus in Chronic Skin Inflammation

The intimate interaction between mast cells and sensory nerves can be illustrated by the wheal and surrounding flare in an urticarial reaction in human skin. This reaction is typically associated with an intense itch at the reaction site. Upon activation, cutaneous mast cells release powerful mediators, such as histamine, tryptase, cytokines, and growth factors that can directly stimulate corresponding receptors on itch-mediating sensory nerves. These include, e.g., H1- and H4-receptors, protease-activated receptor-2, IL-31 receptor, and the high-affinity receptor of nerve growth factor (TrkA). On the other hand, sensory nerves can release neuropeptides, including substance P and vasoactive intestinal peptide, that are able to stimulate mast cells to release mediators leading to potentiation of the reciprocal interaction, inflammation, and itch. Even though mast cells are well recognized for their role in allergic skin whealing and urticaria, increasing evidence supports the reciprocal function between mast cells and sensory nerves in neurogenic inflammation in chronic skin diseases, such as psoriasis and atopic dermatitis, which are often characterized by distressing itch, and exacerbated by psychological stress. Increased morphological contacts between mast cells and sensory nerves in the lesional skin in psoriasis and atopic dermatitis as well as experimental models in mice and rats support the essential role for mast cell-sensory nerve communication in consequent pruritus. Therefore, we summarize here the present literature pointing to a close association between mast cells and sensory nerves in pruritic skin diseases as well as review the essential supporting findings on pruritic models in mice and rats.

Mast cell-neural interactions contribute to pain and itch

Mast cells are best recognized for their role in allergy and anaphylaxis, but increasing evidence supports their role in neurogenic inflammation leading to pain and itch. Mast cells act as a "power house" by releasing algogenic and pruritogenic mediators, which initiate a reciprocal communication with specific nociceptors on sensory nerve fibers. Consequently, nerve fibers release inflammatory and vasoactive neuropeptides, which in turn activate mast cells in a feedback mechanism, thus promoting a vicious cycle of mast cell and nociceptor activation leading to neurogenic inflammation and pain/pruritus. Mechanisms underlying mast cell differentiation, activation, and intercellular interactions with inflammatory, vascular, and neural systems are deeply influenced by their microenvironment, imparting enormous heterogeneity and complexity in understanding their contribution to pain and pruritus. Neurogenic inflammation is central to both pain and pruritus, but specific mediators released by mast cells to promote this process may vary depending upon their location, stimuli, underlying pathology, gender, and species. Therefore, in this review, we present the contribution of mast cells in pathological conditions, including distressing pruritus exacerbated by psychologic stress and experienced by the majority of patients with psoriasis and atopic dermatitis and in different pain syndromes due to mastocytosis, sickle cell disease, and cancer.

Chinese Society of Allergy Guidelines for Diagnosis and Treatment of Allergic Rhinitis

Allergic rhinitis (AR) is a global health problem that causes major illnesses and disabilities worldwide. Epidemiologic studies have demonstrated that the prevalence of AR has increased progressively over the last few decades in more developed countries and currently affects up to 40% of the population worldwide. Likewise, a rising trend of AR has also been observed over the last 2-3 decades in developing countries including China, with the prevalence of AR varying widely in these countries. A survey of self-reported AR over a 6-year period in the general Chinese adult population reported that the standardized prevalence of adult AR increased from 11.1% in 2005 to 17.6% in 2011. An increasing number of Journal Articles and imporclinical trials on the epidemiology, pathophysiologic mechanisms, diagnosis, management and comorbidities of AR in Chinese subjects have been published in international peer-reviewed journals over the past 2 decades, and substantially added to our understanding of this disease as a global problem. Although guidelines for the diagnosis and treatment of AR in Chinese subjects have also been published, they have not been translated into English and therefore not generally accessible for reference to non-Chinese speaking international medical communities. Moreover, methods for the diagnosis and treatment of AR in China have not been standardized entirely and some patients are still treated according to regional preferences. Thus, the present guidelines have been developed by the Chinese Society of Allergy to be accessible to both national and international medical communities involved in the management of AR patients. These guidelines have been prepared in line with existing international guidelines to provide evidence-based recommendations for the diagnosis and management of AR in China.

Host Responses to Malassezia spp. in the Mammalian Skin

The skin of mammalian organisms is home for a myriad of microbes. Many of these commensals are thought to have beneficial effects on the host by critically contributing to immune homeostasis. Consequently, dysbiosis can have detrimental effects for the host that may manifest with inflammatory diseases at the barrier tissue. Besides bacteria, fungi make an important contribution to the microbiota and among these, the yeast Malassezia widely dominates in most areas of the skin in healthy individuals. There is accumulating evidence that Malassezia spp. are involved in a variety of skin disorders in humans ranging from non- or mildly inflammatory conditions such as dandruff and pityriasis versicolor to more severe inflammatory skin diseases like seborrheic eczema and atopic dermatitis. In addition, Malassezia is strongly linked to the development of dermatitis and otitis externa in dogs. However, the association of Malassezia spp. with such diseases remains poorly characterized. Until now, studies on the fungus–host interaction remain sparse and they are mostly limited to experiments with isolated host cells in vitro. They suggest a multifaceted crosstalk of Malassezia spp. with the skin by direct activation of the host via conserved pattern recognition receptors and indirectly via the release of fungus-derived metabolites that can modulate the function of hematopoietic and/or non-hematopoietic cells in the barrier tissue. In this review, we discuss our current understanding of the host response to Malassezia spp. in the mammalian skin.

Emerging concepts: mast cell involvement in allergic diseases

In a process known as overt degranulation, mast cells can release all at once a diverse array of products that are preformed and present within cytoplasmic granules. This occurs typically within seconds of stimulation by environmental factors and allergens. These potent, preformed mediators (ie, histamine, heparin, serotonin, and serine proteases) are responsible for the acute symptoms experienced in allergic conditions such as allergic conjunctivitis, allergic rhinitis, allergy-induced asthma, urticaria, and anaphylaxis. Yet, there is reason to believe that the actions of mast cells are important when they are not degranulating. Mast cells release preformed mediators and inflammatory cytokines for periods after degranulation and even without degranulating at all. Mast cells are consistently seen at sites of chronic inflammation, including nonallergic inflammation, where they have the ability to temper inflammatory processes and shape tissue morphology. Mast cells can trigger actions and chemotaxis in other important immune cells (eg, eosinophils and the newly discovered type 2 innate lymphocytes) that then make their own contributions to inflammation and disease. In this review, we will discuss the many known and theorized contributions of mast cells to allergic diseases, focusing on several prototypical allergic respiratory and skin conditions: asthma, chronic rhinosinusitis, aspirin-exacerbated respiratory disease, allergic conjunctivitis, atopic dermatitis, and some of the more common medication hypersensitivity reactions. We discuss traditionally accepted roles that mast cells play in the pathogenesis of each of these conditions, but we also delve into new areas of discovery and research that challenge traditionally accepted paradigms.

Induction of Mast Cell Accumulation by Tryptase via a Protease Activated Receptor-2 and ICAM-1 Dependent Mechanism

Mast cells are primary effector cells of allergy, and recruitment of mast cells in involved tissue is one of the key events in allergic inflammation. Tryptase is the most abundant secretory product of mast cells, but little is known of its influence on mast cell accumulation. Using mouse peritoneal model, cell migration assay, and flow cytometry analysis, we investigated role of tryptase in recruiting mast cells. The results showed that tryptase induced up to 6.7-fold increase in mast cell numbers in mouse peritoneum following injection. Inhibitors of tryptase, an antagonist of PAR-2 FSLLRY-NH2, and pretreatment of mice with anti-ICAM-1, anti-CD11a, and anti-CD18 antibodies dramatically diminished tryptase induced mast cell accumulation. On the other hand, PAR-2 agonist peptides SLIGRL-NH2 and tc-LIGRLO-NH2 provoked mast cell accumulation following injection. These implicate that tryptase induced mast cell accumulation is dependent on its enzymatic activity, activation of PAR-2, and interaction between ICAM-1 and LFA-1. Moreover, induction of trans-endothelium migration of mast cells in vitro indicates that tryptase acts as a chemoattractant. In conclusion, provocation of mast cell accumulation by mast cell tryptase suggests a novel self-amplification mechanism of mast cell accumulation. Mast cell stabilizers as well as PAR-2 antagonist agents may be useful for treatment of allergic reactions.

Mast cell proteases as pharmacological targets

Mast cells are rich in proteases, which are the major proteins of intracellular granules and are released with histamine and heparin by activated cells. Most of these proteases are active in the granule as well as outside of the mast cell when secreted, and can cleave targets near degranulating mast cells and in adjoining tissue compartments. Some proteases released from mast cells reach the bloodstream and may have far-reaching actions. In terms of relative amounts, the major mast cell proteases include the tryptases, chymases, cathepsin G, carboxypeptidase A3, dipeptidylpeptidase I/cathepsin C, and cathepsins L and S. Some mast cells also produce granzyme B, plasminogen activators, and matrix metalloproteinases. Tryptases and chymases are almost entirely mast cell-specific, whereas other proteases, such as cathepsins G, C, and L are expressed by a variety of inflammatory cells. Carboxypeptidase A3 expression is a property shared by basophils and mast cells. Other proteases, such as mastins, are largely basophil-specific, although human basophils are protease-deficient compared with their murine counterparts. The major classes of mast cell proteases have been targeted for development of therapeutic inhibitors. Also, a human β-tryptase has been proposed as a potential drug itself, to inactivate of snake venins. Diseases linked to mast cell proteases include allergic diseases, such as asthma, eczema, and anaphylaxis, but also include non-allergic diseases such as inflammatory bowel disease, autoimmune arthritis, atherosclerosis, aortic aneurysms, hypertension, myocardial infarction, heart failure, pulmonary hypertension and scarring diseases of lungs and other organs. In some cases, studies performed in mouse models suggest protective or homeostatic roles for specific proteases (or groups of proteases) in infections by bacteria, worms and other parasites, and even in allergic inflammation. At the same time, a clearer picture has emerged of differences in the properties and patterns of expression of proteases expressed in human mast cell subsets, and in humans versus other mammals. These considerations are influencing prioritization of specific protease targets for therapeutic inhibition, as well as options of pre-clinical models, disease indications, and choice of topical versus systemic routes of inhibitor administration.

Protease-activated receptors and itch

Protease-activated receptors (PARs) have been implicated in a variety of physiological functions, as well as somatosensation and particularly itch and pain. Considerable attention has focused on PARs following the finding they are upregulated in the skin of atopic dermatitis patients. The present review focuses on recent studies showing that PARs are critically involved in itch and sensitization of itch. PARs are expressed by diverse cell types including primary sensory neurons, keratinocytes, and immune cells and are activated by proteases that expose a tethered ligand. Endogenous proteases are also released from diverse cell types including keratinocytes and immune cells. Exogenous proteases released from certain plants and insects contacting the skin can also induce itch. Increased levels of proteases in the skin contribute to inflammation that is often accompanied by chronic itch which is not predominantly mediated by histamine. The neural pathway signaling itch induced by activation of PARs is distinct from that mediating histamine-induced itch. In addition, there is evidence that PARs play an important role in sensitization of itch signaling under conditions of chronic itch. These recent findings suggest that PARs and other molecules involved in the itch-signaling pathway are good targets to develop novel treatments for most types of chronic itch that are poorly treated with antihistamines.

Phase 2a, randomized, double-blind, placebo-controlled, multicenter, parallel-group study of a H4 R-antagonist (JNJ-39758979) in Japanese adults with moderate atopic dermatitis

This trial was conducted to evaluate the safety and efficacy of the H4 R-antagonist JNJ-39758979 in adult Japanese patients with moderate atopic dermatitis (AD). Eligible patients were randomly assigned to JNJ-39758979 300 mg, 100 mg or placebo once daily for 6 weeks in this phase 2a, double-blind, multicenter, placebo-controlled study. Primary efficacy was assessed via week-6 Eczema Area and Severity Index (EASI) scores. Secondary efficacy assessments included Investigator's Global Assessment (IGA) and patient-reported outcome (PRO) pruritus assessments (Pruritus Categorical Response Scale [PCRS], Pruritus Numeric Rating Scales [PNRS], Pruritus Interference Numeric Rating Scale [PINRS] and Subject's Global Impressions of Change in Pruritus [SGICP]). Eighty-eight of 105 planned patients were randomized before the study was stopped and unblinded for safety reasons. The study did not meet the primary end-point. However, numerical improvements (i.e. decreases) in median EASI were observed with JNJ-39758979 100 mg (-3.7) and 300 mg (-3.0) versus placebo (-1.3) at week 6. Nominally significant improvements across PRO PCRS, PNRS and SGICP assessments were consistently observed, particularly with JNJ-39758979 300 mg. Safety, including adverse events (AE), was comparable between JNJ-39758979 and placebo with the exception of two patients (both receiving JNJ-39758979 300 mg) with serious AE of neutropenia, leading to premature study discontinuation. No deaths were reported. Except for neutropenia, no clinically relevant changes in laboratory values were observed. Although not conclusive, findings suggest H4 R-antagonism may be beneficial for AD, particularly in controlling pruritus. JNJ-39758979 appears to be associated with drug-induced agranulocytosis, likely an off-target effect.

Tryptase is a candidate autoantigen in rheumatoid arthritis

Autoimmune processes have been implicated in the development of rheumatoid arthritis (RA); however, specific autoantigens that play a role in the aetiology of RA have been lacking. In this study, we found that sera from RA patients were particularly immunoreactive against the protein tryptase. Compared with osteoarthritis (OA) patients and healthy controls, RA patients had relatively higher levels of tryptase and concomitant anti-tryptase antibodies in their synovial tissues and sera. Similarly, synovial fluid from RA patients, but not from OA patients, contained antibodies that recognized tryptase in vitro. In addition, serum tryptase levels in both early and late RA patients significantly correlated with clinical indices usually used to diagnose RA, such as rheumatoid factor, Disease Activity Score using 28 joint counts and autoantibodies against cyclic citrullinated peptide. Our results identify tryptase as a candidate autoantigen involved in the pathogenesis of RA and monitoring its levels may have diagnostic and prognostic value.

Understanding the pathophysiology of itch

Itch is the most common symptom described by our patients. Treating this symptom can be challenging. A revolution is ongoing in understanding the pathophysiology of itch and will allow this challenge to be met. The present authors review and update the current understanding of the pathophysiology of itch.

Increased expression of cell adhesion molecule 1 by mast cells as a cause of enhanced nerve-mast cell interaction in a hapten-induced mouse model of atopic dermatitis

BACKGROUND

Neuroimmunological disorders are involved in the pathogenesis of atopic dermatitis (AD), partly through enhanced sensory nerve-skin mast cell interaction. Cell adhesion molecule 1 (CADM1) is a mast-cell adhesion molecule that mediates the adhesion to, and communication with, sympathetic nerves.

OBJECTIVES

To investigate the role of mast cell CADM1 in the pathogenesis of AD, CADM1 expression levels by comparing between lesional and nonlesional skin mast cells of an AD mouse model, which was developed by repeated application of trinitrochlorobenzene, and to examine, in cocultures, how the alterations in CADM1 detected in lesional mast cells might affect the sensory nerve-mast cell interaction.

METHODS

AD-like lesional and nonlesional skin mast cells were collected separately by laser capture microdissection. CADM1 expression was examined by reverse transcription-polymerase chain reaction and CADM1 immunohistochemistry. In cocultures, adhesion between dorsal root ganglion (DRG) neurites and IC2 mast cells was analysed by loading a femtosecond laser-induced impulsive force on neurite-attendant IC2 cells, while cellular communication was monitored as the IC2 cellular response ([Ca(2+)]i increase) after nerve-specific stimulant-induced DRG activation.

RESULTS

AD-like lesional mast cells expressed three-fold more CADM1 transcripts than nonlesional cells. This was supported at the protein level, shown by immunohistochemistry. In coculture, CADM1 overexpression in IC2 cells strengthened DRG neurite-IC2 cell adhesion and doubled the population of IC2 cells responding to DRG activation. A function-blocking anti-CADM1 antibody abolished these effects in a dose-dependent manner.

CONCLUSIONS

Increased expression of CADM1 in mast cells appeared to be a cause of enhanced sensory nerve-mast cell interaction in a hapten-induced mouse model of AD.

Eosinophils increase neuron branching in human and murine skin and in vitro

Cutaneous nerves are increased in atopic dermatitis, and itch is a prominent symptom. We studied the functional interactions between eosinophils and nerves in human and mouse skin and in culture. We demonstrated that human atopic dermatitis skin has eosinophil granule proteins present in the same region as increased nerves. Transgenic mice in which interleukin-5 (IL-5) expression is driven by a keratin-14 (K14) promoter had many eosinophils in the epidermis, and the number of nerves was also significantly increased in the epidermis. In co-cultures, eosinophils dramatically increased branching of sensory neurons isolated from the dorsal root ganglia (DRG) of mice. This effect did not occur in DRG neurons co-cultured with mast cells or with dead eosinophils. Physical contact of the eosinophils with the neurons was not required, and the effect was not blocked by an antibody to nerve growth factor. DRG neurons express eotaxin-1, ICAM-1 and VCAM-1, which may be important in the recruitment, binding, and activation of eosinophils in the region of cutaneous nerves. These data indicate a pathophysiological role for eosinophils in cutaneous nerve growth in atopic dermatitis, and suggest they may present a therapeutic target in atopic dermatitis and other eosinophilic skin conditions with neuronal symptoms such as itch.

Effect of processed foods on serum levels of eosinophil cationic protein among children with atopic dermatitis

The prevalence of atopic dermatitis (AD) in school-age children has increased in industrialized countries. As diet is one of the main factors provoking AD, some studies have suggested that food additives in processed foods could function as pseudoallergens, which comprise the non-immunoglobulin E-mediated reaction. Eosinophil cationic protein (ECP) is an eosinophil granule protein released during allergic reactions to food allergens in patients with AD. Thus, serum ECP levels may be a useful indicator of ongoing inflammatory processes in patients with AD. The purpose of this study was to investigate the effect of consuming MSG in processed foods on serum ECP levels among children with AD. This study was performed with 13 patients with AD (age, 7-11 years) who had a normal range of total IgE levels (< 300 IU/ml). All participants ate normal diets during the first week. Then, six patients were allocated to a processed food-restricted group (PRDG) and seven patients were in a general diet group (GDG). During the second week, children in the PRDG and their parents were asked to avoid eating all processed foods. On the third week, children in the PRDG were allowed all foods, as were the children in the GDG throughout the 3-week period. The subjects were asked to complete a dietary record during the trial period. Children with AD who received the dietary restriction showed decreased consumption of MSG and decreased serum ECP levels and an improved SCORing score on the atopic dermatitis index (P < 0.05). No differences in serum ECP levels or MSG consumption were observed in the GDG. Serum total IgE levels were not changed in either group. In conclusion, a reduction in MSG intake by restricting processed food consumption may lead to a decrease in serum ECP levels in children with AD and improve AD symptoms.

Pathophysiology and therapy of pruritus in allergic and atopic diseases

Pruritus (itch) is a major characteristic and one of the most debilitating symptoms in allergic and atopic diseases and the diagnostic hallmark of atopic dermatitis. Pruritus is regularly defined as an unpleasant sensation provoking the desire to scratch. Although we achieved rather good knowledge about certain inducers of itch such as neuropeptides, amines, mu-opioids, cytokines and proteases, for example, less is known about the pathophysiological specifities among the different diseases, and the therapeutic consequences which may derive thereoff. This review dissects the role of mediators, receptors and itch inhibitors on peripheral nerve endings, dorsal root ganglia, the spinal cord and the CNS leading to the amplification or - vice versa - suppression of pruritus. As the treatment of pruritus in allergic and atopic skin disease is still not satisfactory, knowing these pathways and mechanisms may lead to novel therapeutic approaches against this frequently encountered skin symptom.

The histamine H4 receptor mediates inflammation and pruritus in Th2-dependent dermal inflammation

The role of histamine H(4) receptor (H(4)R) was investigated in a T-helper type 2 (Th2)-cell-mediated mouse skin inflammation model that mimics several of the features of atopic dermatitis. Treatment with two specific H(4)R antagonists before challenge with FITC led to a significant reduction in ear edema, inflammation, mast cell, and eosinophil infiltration. This was accompanied by a reduction in the levels of several cytokines and chemokines in the ear tissue. Upon ex vivo antigen stimulation of lymph nodes, H(4)R antagonism reduced lymphocyte proliferation and IL-4, IL-5, and IL-17 levels. One explanation for this finding is that lymph nodes from animals dosed with the H(4)R antagonist, JNJ 7777120, contained a lower number of FITC-positive dendritic cells. The effect of H(4)R antagonism on dendritic cell migration in vivo may be an indirect result of the reduction in tissue cytokines and chemokines or a direct effect on chemotaxis. In addition to anti-inflammatory effects, JNJ 7777120 also significantly inhibited the pruritus shown in the model. Therefore, the dual effects of H(4)R antagonists on pruritus and Th2-cell-mediated inflammation point to their therapeutic potential for the treatment of Th2-mediated skin disorders, including atopic dermatitis.

Global expression profiling in atopic eczema reveals reciprocal expression of inflammatory and lipid genes

BACKGROUND

Atopic eczema (AE) is a common chronic inflammatory skin disorder. In order to dissect the genetic background several linkage and genetic association studies have been performed. Yet very little is known about specific genes involved in this complex skin disease, and the underlying molecular mechanisms are not fully understood.

METHODOLOGY/FINDINGS

We used human DNA microarrays to identify a molecular picture of the programmed responses of the human genome to AE. The transcriptional program was analyzed in skin biopsy samples from lesional and patch-tested skin from AE patients sensitized to Malassezia sympodialis (M. sympodialis), and corresponding biopsies from healthy individuals. The most notable feature of the global gene-expression pattern observed in AE skin was a reciprocal expression of induced inflammatory genes and repressed lipid metabolism genes. The overall transcriptional response in M. sympodialis patch-tested AE skin was similar to the gene-expression signature identified in lesional AE skin. In the constellation of genes differentially expressed in AE skin compared to healthy control skin, we have identified several potential susceptibility genes that may play a critical role in the pathological condition of AE. Many of these genes, including genes with a role in immune responses, lipid homeostasis, and epidermal differentiation, are localized on chromosomal regions previously linked to AE.

CONCLUSIONS/SIGNIFICANCE

Through genome-wide expression profiling, we were able to discover a distinct reciprocal expression pattern of induced inflammatory genes and repressed lipid metabolism genes in skin from AE patients. We found a significant enrichment of differentially expressed genes in AE with cytobands associated to the disease, and furthermore new chromosomal regions were found that could potentially guide future region-specific linkage mapping in AE. The full data set is available at http://microarray-pubs.stanford.edu/eczema.

Serum mast cell tryptase is not a useful marker for disease severity in psoriasis or atopic dermatitis

BACKGROUND

Severity in psoriasis and atopic dermatitis (AD) is commonly assessed with the Psoriasis Area and Severity Index (PASI) and the SCORing Atopic Dermatitis (SCORAD), respectively. Until today no serum marker is available to reflect the clinical scoring in both diseases. As mast cells play an important role in the pathogenesis of early psoriasis and AD, tryptase, a major compound of mast cell granules that is released upon activation, could in principle serve as such a marker.

OBJECTIVES

To assess the correlation between serum tryptase and severity of psoriasis and AD as well as the correlation between total IgE levels and severity of AD.

METHODS

Serum samples from patients hospitalized for psoriasis and AD were collected at time of admission and time of discharge from hospital. PASI and SCORAD assessments were performed at the same time points. Outpatients presenting with naevi and other benign noninflammatory skin lesions served as control group. Serum tryptase values and total IgE levels of patients with AD were measured using a fluoroenzyme immunoassay technique.

RESULTS

No correlation of serum tryptase level with either the severity of psoriasis or the severity of AD was seen. Total IgE levels in patients with AD at time of admission and discharge from hospital remained the same.

CONCLUSIONS

Serum total tryptase did not prove to be a useful tool in assessing severity of psoriasis or AD. Total IgE levels did not correlate with severity of AD.

Chronic mouse model of TMA-induced contact hypersensitivity

Due to the steadily increasing incidence of atopic dermatitis (AD), especially in children, there is a high medical need for new therapies and improved animal models. In mice, trimellitic anhydride (TMA) is routinely used to trigger T-cell-dependent contact hypersensitivity (CHS) reactions. In this study, we compared the standard acute TMA-induced CHS in Balb/c mice with subacute and chronic models of TMA-induced ear inflammation. Compared to the acute model, the chronic CHS model more closely reflects characteristics of AD, such as typical morphological changes of the inflamed skin, strong infiltration with T cells, major histocompatibility complex II-positive cells, eosinophils, and mast cells, a T-helper cell-type (Th) 2 cytokine profile and a strong increase of serum IgE levels. Moreover, a strong lymph node involvement with T-helper cell dominance and a mixed Th1/Th2 T-cell differentiation and activation pattern was demonstrated. Importantly, as demonstrated by successful therapy with prednisolone, the chronic TMA-induced CHS model, in contrast to acute and subacute models, made prolonged therapeutic treatment of a pre-established skin inflammation possible. Altogether, we present an improved model of mouse T-cell-dependent skin inflammation for AD. We hope this model will enhance the predictive value of animal models for therapeutic treatment of atopic eczema.

The role of histamine H1 and H4 receptors in allergic inflammation: the search for new antihistamines

Histamine has a key role in allergic inflammatory conditions. The inflammatory responses resulting from the liberation of histamine have long been thought to be mediated by the histamine H1 receptor, and H1-receptor antagonists--commonly known as antihistamines--have been used to treat allergies for many years. However, the importance of histamine in the pathology of conditions such as asthma and chronic pruritus may have been underestimated. Here, we review accumulating evidence suggesting that histamine indeed has roles in inflammation and immune function modulation in such diseases. In particular, the discovery of a fourth histamine receptor (H4) and its expression on numerous immune and inflammatory cells has prompted a re-evaluation of the actions of histamine, suggesting a new potential for H4-receptor antagonists and a possible synergy between H1 and H4-receptor antagonists in targeting various inflammatory conditions.

Mast Cell Proteases

Mast cells (MCs) are traditionally thought of as a nuisance for its host, for example, by causing many of the symptoms associated with allergic reactions. In addition, recent research has put focus on MCs for displaying harmful effects during various autoimmune disorders. On the other hand, MCs can also be beneficial for its host, for example, by contributing to the defense against insults such as bacteria, parasites, and snake venom toxins. When the MC is challenged by an external stimulus, it may respond by degranulation. In this process, a number of powerful preformed inflammatory "mediators" are released, including cytokines, histamine, serglycin proteoglycans, and several MC-specific proteases: chymases, tryptases, and carboxypeptidase A. Although the exact effector mechanism(s) by which MCs carry out their either beneficial or harmful effects in vivo are in large parts unknown, it is reasonable to assume that these mediators may contribute in profound ways. Among the various MC mediators, the exact biological function of the MC proteases has for a long time been relatively obscure. However, recent progress involving successful genetic targeting of several MC protease genes has generated powerful tools, which will enable us to unravel the role of the MC proteases both in normal physiology as well as in pathological settings. This chapter summarizes the current knowledge of the biology of the MC proteases.

Histamine H4 receptor antagonists are superior to traditional antihistamines in the attenuation of experimental pruritus

BACKGROUND

Histamine is a potent mediator of itch in humans, yet histamine H(1) receptor antagonists have been shown to be of limited use in the treatment of certain chronic pruritic diseases. The histamine H(4) receptor is a recently described histamine receptor, expressed on hematopoietic cells, linked to the pathology of allergy and asthma.

OBJECTIVE

The contribution of the novel histamine H(4) receptor to histaminergic and allergic pruritus was investigated.

RESULTS

Histamine and a selective histamine H(4) receptor agonist caused scratching responses in mice, which were almost completely attenuated in histamine H(4) receptor knockout mice or by pretreatment with the selective histamine H(4) receptor antagonist, JNJ 7777120. Pruritus induced by allergic mechanisms was also potently inhibited with histamine H(4) receptor antagonist treatment or in histamine H(4) receptor knockout mice. In all cases, the inhibitory effect of histamine H(4) receptor antagonist was greater than those observed with histamine H(1) receptor antagonists. The histamine H(4) receptor-mediated pruritus was shown to be independent of mast cells or other hematopoietic cells and may result from actions on peripheral neurons.

CONCLUSION

These results demonstrate that the histamine H(4) receptor is involved in pruritic responses in mice to a greater extent than the histamine H(1) receptor.

CLINICAL IMPLICATIONS

Histamine H(4) receptor antagonists may have therapeutic utility for treating chronic pruritic diseases in humans where histamine H(1) receptor antagonists are not effective.

Neuronal conditions of spinal cord in dermatitis are improved by olopatadine

Intense pruritus and cutaneous reactivity represent cardinal features of eczema. The resulting scratching behaviors alter neuronal conditions of the spinal dorsal horn where the primary sensory afferent fibers transmit cutaneous stimulation and deteriorate eczematous skin lesions. We investigated the effects of olopatadine hydrochloride (olopatadine) on alteration of neuronal conditions of the spinal dorsal horn and eczematous skin lesions induced by contact dermatitis. Eczematous lesions were induced by repeated application of diphenylcyclopropenone (DCP) in BALB/c mice. Olopatadine suppressed scratching behavior caused by repeated application of DCP in mice. Increased expressions of c-Fos and substance P in the spinal dorsal horn following DCP application were improved by olopatadine. Furthermore, olopatadine diminished the number of infiltrating cells and levels of cytokines in eczematous skin lesions resulting from DCP application. Olopatadine improves neurological conditions in the spinal cord and eczematous skin lesions in a murine contact dermatitis model.

Biology of mast cell tryptase. An inflammatory mediator

In 1960, a trypsin-like activity was found in mast cells [Glenner GG & Cohen LA (1960) Nature 185, 846-847] and this activity is now commonly referred to as 'tryptase'. Over the years, much knowledge about mast cell tryptase has been gathered, and a recent (18 January 2006) PubMed search for the keywords 'tryptase + mast cell*' retrieved 1661 articles. However, still very little is known about its true biological function. For example, the true physiological substrate(s) for mast cell tryptase has not been identified, and the potential role of tryptase in mast cell-related disease is not understood. Mast cell tryptase has several unique features, with perhaps the most remarkable being its organization into a tetrameric state with all of the active sites oriented towards a narrow central pore and its consequent complete resistance towards endogenous macromolecular protease inhibitors. Much effort has been invested to elucidate these properties of tryptase. In this review we summarize the current knowledge of mast cell tryptase, including novel insights into its possible biological functions and mechanisms of regulation.

The receptor for erythropoietin is present on cutaneous mast cells

Skin samples from patients with extra-mammary Paget disease, Bowen's disease, atopic dermatitis, psoriasis and non-lesional skin of nevus pigmentosus were immunohistochemically examined with an anti-soluble erythropoietin receptor antibody (anti-sEPOR antibody), and only the dermal mast cells positively stained in all skin samples were examined. These positively stained dermal cells were proved to be mast cells by double staining with anti-sEPOR antibody and either with anti-bikunin antibody or anti-tryptase antibody. Immunoelectron microscopically these EPOR were found in the secretory granules of the dermal mast cells. Further, EPOR in the mast cells may be consisting of only the extracellular domain of erythropoietin receptor molecule as the mast cells were immunohistochemically not reacted with an antibody to the C-terminal peptide of EPOR. Human mast cell line, HMC-1 cells has immunohistochemically the erythropoietin receptor, which was consisting of a 43 kDa major protein and a 20 kDa minor protein in the immunoelectrophoresis. These data may indicate that EPOR in the mast cells may not be the whole molecule, but probably the soluble one of EPOR.

1,2,5-Thiadiazolidin-3-one 1,1-dioxide-based heterocyclic sulfides are potent inhibitors of human tryptase

We describe herein the design, synthesis, and in vitro biochemical evaluation of a series of potent, time-dependent inhibitors of the mast cell-derived serine protease tryptase. The inhibitors were readily obtained by attaching various heterocyclic thiols, as well as a basic primary specificity residue P(1), to the 1,2,5-thiadiazolidin-3-one 1,1-dioxide scaffold. The inhibitors were found to be devoid of any inhibitory activity toward a neutral (elastase) or cysteine (papain) protease, however they were also fairly efficient inhibitors of bovine trypsin. The differential inhibition observed with trypsin suggests that enzyme selectivity can be optimized by exploiting differences in the S' subsites of the two enzymes. The results described herein demonstrate the versatility of the heterocyclic scaffold in fashioning mechanism-based inhibitors of neutral, basic, and acidic (chymo)trypsin-like serine proteases.

Mast cell chymase is increased in chronic atopic dermatitis but not in psoriasis

Mast cell chymase is a chymotrypsin-like serine proteinase primarily stored in secretory mast cell granules. Mast cell chymase has various effects on angiotensin, metalloproteases, lipoproteins, procollagen, neuropeptides and cytokines. Recent studies have demonstrated that chymase inhibitors inhibit skin inflammation. In this study we sought to determine the role of mast cell chymase in atopic dermatitis (AD) in comparison with its role in psoriasis and normal skin. Skin biopsy specimens were obtained from non-lesional and lesional skin of patients with chronic AD and psoriasis and from normal skin of non-atopic and non-psoriatic controls. The number of mast cells containing chymase was determined by immunohistochemistry using a chymase-specific monoclonal antibody. A significantly (P < 0.05) enhanced number of chymase-positive cells was found in lesional AD skin as compared to normal skin as well as to lesional and non-lesional skin of patients with psoriasis. A significant (P < 0.05) increase in the number of chymase-positive cells was also found in non-lesional AD skin in comparison to psoriasis. An enhanced, albeit not statistically significant difference was noted in non-lesional AD skin as compared to normal skin. In conclusion, these results suggest that mast cell chymase may play an integral part in eliciting and maintaining cutaneous inflammation in AD but not in psoriasis. The increased proteinase activity of mast cell chymase may also be involved in promoting a skin barrier defect in AD, which subsequently enhances the skin's permeability to allergens and microbes and thereby aggravates the eczema.

Mast Cells and Eosinophils in Feline Allergic Dermatitis: A Qualitative and Quantitative Analysis

Mast cells (MCs) and eosinophils are prominent in the perivascular infiltrate of cats with allergic dermatitis. In the skin of allergic cats MCs were mainly observed diffusely in the superficial dermis, while eosinophils were found mainly in the deep dermis in a perivascular pattern. MC counts were significantly higher in cats with allergic dermatitis (P < 0.05) than in healthy control cats, but the number varied widely. Moreover, the numbers of eosinophils in the skin of allergic and control cats differed significantly (P < 0.05) none being found in the latter. There was no significant correlation between numbers of mast cells and eosinophils in the same biopsy sample. In the allergic cats, a significantly lower number of MCs was detected by staining for tryptase than by staining for chymase or by Astra blue staining. Additionally, the chymase: tryptase ratio in healthy cats was reversed in cats with allergic dermatitis. These changes were observed in lesional and nonlesional skin of cats with allergic dermatitis. The findings indicate a generalized effect on MCs in allergic dermatitis. In addition, eosinophils are an important indicator of allergic dermatitis.

Itch associated with skin disease: advances in pathophysiology and emerging therapies

Itch, also known as pruritus, is the major symptom in skin diseases with a variety of etiologies and pathophysiologies. Significant progress has been achieved in understanding the pathophysiology of itch in the last 5 years. Neurophysiological experiments in humans and animals have revealed that itch is carried by specific C nerve fibers. Recent studies have demonstrated that peripheral mediators other than histamine are involved in induction of itch. Mast cell tryptase seems to be an important mediator in itch by its activation of proteinase activated receptor 2 in the sensory nerves. Opioids have central and peripheral itch producing activity. Neuropeptides, such as substance P, induce itch by their effect on mast cells. Based upon our improved understanding of the neurophysiology of itch a clinical classification of itch has been proposed. The classification highlights differences between peripheral pruritoceptive itch, neuropathic itch (itch related to damage to afferent nerve fibers) and neurogenic itch (itch originating in the central nervous system without any evidence of nerve damage). Emerging therapies based on these findings include topical vanilloid receptor antagonists, topical antihistamines, and topical arachidonic acid inhibitors, as well as inhibitors of non-histamine inflammatory mediators, immunomodulators and strontium salts. Systemic therapies include thalidomide, opioid antagonists, phototherapy with narrow band UVB and experimental treatments with cutaneous field stimulation and vagal nerve stimulation. With the new information it seems we will be able to better help our dermatologic patients who have itch, however we are not closer to identifying a single agent specifically targetable to this symptom.

Mast cells, nerves and neuropeptides in atopic dermatitis and nummular eczema

The association between mast cells and sensory nerves and the distribution of the neuropeptides substance P (SP), vasoactive intestinal polypeptide (VIP) and calcitonin gene-related peptide (CGRP) were studied immunohistochemically in lesional and nonlesional skin of 26 atopic dermatitis (AD) and 23 nonatopic nummular eczema (NE) patients. Mast cell-nerve contacts were counted morphometrically and confirmed by confocal laser scanning microscopy. Neuropeptide positivity was assessed semiquantitatively. Dermal contacts between mast cells and nerves were increased in number in both lesional and nonlesional samples of AD and NE when compared to those in normal controls, although only the values in lesional AD reached statistical significance ( P<0.05). Nerve-mast cell contacts in the basement membrane zone were seen practically only in lesional NE. SP and CGRP fibres were prominently increased in lesional samples when compared to their nonlesional controls both in AD and NE in the epidermis and in the papillary dermis. In both AD and NE, only small differences were found regarding VIP positivity in lesional and nonlesional biopsies. The epidermis was devoid of VIP positivity. In conclusion, SP and CGRP but not VIP fibres were more frequent in lesional than in nonlesional papillary dermis of both AD and NE. Since mast cells are also increased in number in lesions of AD and NE, they are able to maintain neurogenic inflammation through activation by SP and CGRP. The increased SP/CGRP nerves in the epidermis of AD and NE lesions may stimulate keratinocytes to release cytokines which affect various cell types enhancing inflammation.

Retinoic acid inhibits in vitro development of mast cells but has no marked effect on mature human skin tryptase- and chymase-positive mast cells

Stem cell factor plays a key role in the development of human mast cells via interaction with Kit receptor. We and other groups have previously shown that a number of cytokines can regulate the stem-cell-factor-dependent development of mast cells in vitro. In this study we investigated the effect of retinoic acid on human mast cells in vitro and in vivo. Retinoids are known to have strong modulatory effects on hematopoietic differentiation. We found that all-trans-retinoic acid, at concentrations as low as 1 nM, inhibits the stem-cell-factor-dependent differentiation of mast cells in vitro. This effect of retinoic acid was found to be on progenitor cells, whereas more mature mast cells were less affected. The use of specific agonists binding either to the RAR or the RXR nuclear receptors indicated involvement of both the RAR/RXR and RXR/RXR pathways in inhibiting mast cell differentiation. In contrast to the effects on mast cell progenitors, retinoic acid had no effect on the number of mature mast cells in skin organ cultures. Furthermore, topical treatment of normal skin with a retinoic-acid-containing cream caused an increase in the number of tryptase-positive mast cells, whereas the numbers of the major cutaneous mast cell type, tryptase- and chymase-positive mast cells, remained unaffected. Our results suggest that retinoic acid suppresses commitment of progenitor cells into the mast cell lineage and/or acts on early mast cell progenitors, whereas mature cutaneous mast cells are less susceptible to retinoic acid.

Pathophysiology of pruritus in atopic dermatitis: an overview

Pruritus is an essential feature of atopic dermatitis (AD) and the diagnosis of active AD cannot be made without the history of itching. Because of the high impact on life quality, most of the patients measure the severity of eczema by the intensity of pruritus rather than appearance of skin lesions. However, although pruritus is a cardinal symptom of AD, its mechanism and association with the cutaneous nervous system is not completely understood. Recently, a considerable progress has been achieved in clarifying the complex pathophysiology of pruritus in AD. As a cutaneous sensory perception, itch requires excitation of neuropeptide-containing free nerve endings of unmyelinated nociceptor fibers. It is well known that histamine and acetylcholine provoke itch by direct binding to 'itch receptors' and several mediators such as neuropeptides, proteases or cytokines indirectly via histamine release. Interestingly, some variations of these complex mechanisms could be demonstrated in patients with AD. This review highlights the recent knowledge of different mechanisms which may be involved in regulating pruritus in patients with AD potentially leading to new therapeutic applications for the treatment of itch in AD.

Mast cell mediators other than histamine induce pruritus in atopic dermatitis patients: a dermal microdialysis study

While histamine is the crucial mediator of pruritus in type 1 allergic reactions, its role in atopic dermatitis (AD) is unclear. In this study, the role of mast cell mediators in protein extravasation and pruritus was evaluated using intradermal microdialysis. The microdialysis capillaries were used to apply the mast cell degranulating substance compound 48/80 (C48/80; 0.05%) or histamine (0.01%) and also to deliver H1-blockers (cetirizine, 200 microg mL-1) in nine AD patients and nine controls. Large pore size membranes (3000 kDa) enabled simultaneous analysis of protein extravasation. Itch sensation was measured psychophysically and weal and flare reaction were evaluated planimetrically. Protein extravasation induced by histamine and C48/80 was significantly reduced in AD patients. Blockade of H1-receptors by cetirizine significantly reduced C48/80-induced protein extravasation in AD patients and controls to an identical level. C48/80-induced pruritus was abolished by cetirizine in controls, whereas pruritus in AD patients was unchanged after H1 blockade. We conclude that mast cell mediators others than histamine are involved in C48/80-induced pruritus in AD patients. Whether the reduced capacity of AD patients to induce protein extravasation is of pathophysiological relevance for pruritus remains to be established.

A theoretical model based upon mast cells and histamine to explain the recently proclaimed sensitivity to electric and/or magnetic fields in humans

The relationship between exposure to electromagnetic fields (EMFs) and human health is more and more in focus. This is mainly because of the rapid increasing use of such EMFs within our modern society. Exposure to EMFs has been linked to different cancer forms, e.g. leukemia, brain tumors, neurological diseases, such as Alzheimer's disease, asthma and allergy, and recently to the phenomena of 'electrosupersensitivity' and 'screen dermatitis'. There is an increasing number of reports about cutaneous problems as well as symptoms from internal organs, such as the heart, in people exposed to video display terminals (VDTs). These people suffer from subjective and objective skin and mucosa-related symptoms, such as itch, heat sensation, pain, erythema, papules and pustules. In severe cases, people can not, for instance, use VDTs or artificial light at all, or be close to mobile telephones. Mast cells (MCs), when activated, release a spectrum of mediators, among them histamine, which is involved in a variety of biological effects with clinical relevance, e.g. allergic hypersensitivity, itch, edema, local erythema and many types of dermatoses. From the results of recent studies, it is clear that EMFs affect the MC, and also the dendritic cell, population and may degranulate these cells. The release of inflammatory substances, such as histamine, from MCs in the skin results in a local erythema, edema and sensation of itch and pain, and the release of somatostatin from the dendritic cells may give rise to subjective sensations of on-going inflammation and sensitivity to ordinary light. These are, as mentioned, the common symptoms reported from patients suffering from 'electrosupersensitivity'/'screen dermatitis'. MCs are also present in the heart tissue and their localization is of particular relevance to their function. Data from studies made on interactions of EMFs with the cardiac function have demonstrated that highly interesting changes are present in the heart after exposure to EMFs. One could speculate that the cardiac MCs are responsible for these changes due to degranulation after exposure to EMFs. However, it is still not known how, and through which mechanisms, all these different cells are affected by EMFs. In this article, we present a theoretical model, based upon observations on EMFs and their cellular effects, to explain the proclaimed sensitivity to electric and/or magnetic fields in humans.

Proteinase-activated receptor-2 in human skin: tissue distribution and activation of keratinocytes by mast cell tryptase

Proteinase-activated receptor-2 (PAR-2) is a G-protein coupled receptor. Tryptic proteases cleave PAR-2 exposing a tethered ligand (SLIGKV), which binds and activates the receptor. Although PAR-2 is highly expressed by cultured keratinocytes and is an inflammatory mediator, its precise localization in the normal and inflamed human skin is unknown, and the proteases that activate PAR-2 in the skin have not been identified. We localized PAR-2 in human skin by immunohistochemistry, examined PAR-2 expression by RT-PCR and RNA blotting, and investigated PAR-2 activation by mast cell tryptase. PAR-2 was localized to keratinocytes, especially in the granular layer, to endothelial cells, hair follicles, myoepithelial cells of sweat glands, and dermal dendritic-like cells. PAR-2 was also highly expressed in keratinocytes and endothelial cells of inflamed skin. PAR-2 mRNA was detected in normal human skin by RT-PCR, and in cultured human keratinocytes and dermal microvascular endothelial cells by Northern hybridization. Trypsin, tryptase and a peptide corresponding to the tethered ligand (SLIGKVNH2) increased [Ca2+]i in keratinocytes, measured using Fura-2/AM. Although tryptase-containing mast cells were sparsely scattered in the normal dermis, they were numerous in the dermis in atopic dermatitis, and in the dermis, dermal-epidermal border, and occasionally within the lower epidermis in psoriasis. Tryptase may activate PAR-2 on keratinocytes and endothelial cells during inflammation.