Immune dysregulation underlies a subset of patients with chronic idiopathic pruritus

Neuroimmune interplay during type 2 inflammation: Symptoms, mechanisms, and therapeutic targets in atopic diseases

Type 2 inflammation is characterized by overexpression and heightened activity of type 2 cytokines, mediators, and cells that drive neuroimmune activation and sensitization to previously subthreshold stimuli. The consequences of altered neuroimmune activity differ by tissue type and disease; they include skin inflammation, sensitization to pruritogens, and itch amplification in atopic dermatitis and prurigo nodularis; airway inflammation and/or hyperresponsiveness, loss of expiratory volume, airflow obstruction and increased mucus production in asthma; loss of sense of smell in chronic rhinosinusitis with nasal polyps; and dysphagia in eosinophilic esophagitis. We describe the neuroimmune interactions that underlie the various sensory and autonomic pathologies in type 2 inflammatory diseases and present recent advances in targeted treatment approaches to reduce type 2 inflammation and its associated symptoms in these diseases. Further research is needed to better understand the neuroimmune mechanisms that underlie chronic, sustained inflammation and its related sensory pathologies in diseases associated with type 2 inflammation.

Immunosenescence and Skin: A State of Art of Its Etiopathogenetic Role and Crucial Watershed for Systemic Implications

Immunosenescence is a complex multifactorial phenomenon consisting of wide-ranging remodeling of the immune system during the life span, resulting in an age-related qualitative-quantitative decline of immune cells and cytokines. A growing body of evidence in the international literature is highlighting the etiopathogenetic role of skin immunosenescence in the onset of various dermatologic conditions. Skin immunosenescence also serves as an interesting watershed for the onset of system-wide conditions in the context of allergic inflammation. Moreover, in recent years, an increasingly emerging and fascinating etiopathogenetic parallelism has been observed between some mechanisms of immunosenescence, both at cutaneous and systemic sites. This would help to explain the occurrence of apparently unconnected comorbidities. Throughout our review, we aim to shed light on emerging immunosenescent mechanisms shared between dermatologic disorders and other organ-specific diseases in the context of a more extensive discussion on the etiopathogenetic role of skin immunosenescence. A promising future perspective would be to focus on better understanding the mutual influence between skin and host immunity, as well as the influence of high inter-individual variability on immunosenescence/inflammaging. This can lead to a more comprehensive "immunobiographic" definition of each individual.

Itch: Epidemiology, clinical presentation, and diagnostic workup

Itch, or pruritus, is the uncomfortable sensation underlying the desire to scratch. Itch is a very common complaint in the general population that can result from dermatologic, systemic (eg, renal, hepatobiliary, endocrine), paraneoplastic, neuropathic, and psychogenic etiologies. Chronic itch is associated with significant sleep disturbances and profoundly reduces overall quality of life. Certain populations, including elderly and African Americans, are at increased risk of experiencing heightened burden of itch. Because of the variable clinical presentation and wide-ranging etiologies, itch presents a challenge for clinicians. The initial evaluation should include a complete blood count, with differential, hepatic, renal, and thyroid function testing along with diabetes screening. Further testing should be guided by history and physical examination findings. There should be a heightened concern for underlying malignancy in individuals older than 60 years of age who have a history of liver disease and diffuse itch less than 12 months of duration. For individuals with chronic pruritus of unknown origin, increased blood eosinophils may serve as a biomarker of T helper cell type 2 polarization and response to immunomodulator therapies. In this first part of a 2-part continuing medical education series, we describe the broader epidemiology and specific conditions associated with itch and the clinical presentation and diagnostic workup for patients with itch.

Itch: Pathogenesis and treatment

Itch pathogenesis is broadly characterized into histaminergic and nonhistaminergic pathways and transmitted via 2 main receptor families: G protein-coupled receptors and transient receptor potential channels. In the skin, itch is primarily transmitted by unmyelinated type C and thinly myelinated type Aδ nerve fibers. Crosstalk between the immune and neural systems modulates itch transmission at the skin, spinal cord, and brain. Among the many known pruritogens, Th2 cytokines, such as interleukin-4, interleukin-13, interleukin-31, and thymic stromal lymphopoietin, are particularly important mediators that signal through shared Janus kinase pathways, representing novel targets for novel itch therapeutics. Emerging evidence has also revealed that the opioidergic system is a potent modulator of itch transmission, with increased μ-opioid activity and decreased κ-opioid activity contributing to itch pathogenesis. Optimal management of itch requires that treatment approaches be tailored to specific etiologic itch subtypes. When the etiology is unknown and patients are given a diagnosis of chronic pruritus of unknown origin, treatment should be guided by the presence of Th2 polarization, often reflected by increased blood eosinophils. In the second article of this 2-part series, we outline our current understanding of itch pathogenesis and discuss available and emerging treatments for itch.

Autoimmune Progesterone Dermatitis: A Systematic Review

ABSTRACT

Autoimmune progesterone dermatitis (AIPD) is a cyclical, cutaneous reaction to endogenous progesterone that occurs throughout the menstrual cycle. The cutaneous manifestations of AIPD vary greatly from patient to patient, ranging anywhere from urticaria to erythema multiforme to anaphylaxis. As such, recognition, diagnosis, and management of this condition are difficult for clinicians. In the present article, we conducted a systematic review of 112 articles and 132 individual cases to summarize the clinical features and presentation of AIPD while also summarizing the successes and failures of different treatment plans. Despite the great variety in clinical presentations, it is clear from the data that ovulation-suppressing medical therapies and surgery have the greatest success in treating AIPD, whereas more commonly used therapies such as antihistamines and systemic corticosteroids frequently fail in providing any relief. Further research is necessary to determine the exact pathogenesis of AIPD and allow for more targeted treatment.

The pruritogenic role of the type 2 immune response in diseases associated with chronic itch

While there is a vast array of aetiologies that may lead to chronic pruritus, recent data suggests that many of these conditions share similar interactions between keratinocytes, nerves, and the immune system. Specifically, the type 2 immune response, including Th2 T Cells and their related cytokines, has been noted to play a major role in the development of pruritus in a variety of itchy conditions. To date, atopic dermatitis is the most striking example of this pathogenesis. However, the body of literature supporting its role in many other itchy conditions, including other inflammatory, bullous, as well as systemic diseases, continues to grow. In addition, new treatments targeting this type 2 immune system continue to be developed and investigated. In the current review, we present the current body of literature supporting the role of the type 2 immune response in itchy conditions beyond atopic dermatitis as well as potential therapeutic options that target this pathway for chronic itch.

Autoreactive Peripheral Blood T Helper Cell Responses in Bullous Pemphigoid and Elderly Patients With Pruritic Disorders

Bullous pemphigoid (BP) is a prototypic autoimmune disorder of the elderly, characterized by serum IgG autoantibodies, namely anti-BP180 and anti-BP230, directed against components of the basal membrane zone that lead to sub-epidermal loss of adhesion. Pruritus may be indicative of a pre-clinical stage of BP, since a subset of these patients shows serum IgG autoantibodies against BP230 and/or BP180 while chronic pruritus is increasingly common in the elderly population and is associated with a variety of dermatoses. Clinical and experimental evidence further suggests that pruritus of the elderly may be linked to autoimmunity with loss of self-tolerance against cutaneous autoantigens. Thus, the objective of this study was to determine autoreactive T cell responses against BP180 in elderly patients in comparison to patients with BP. A total of 22 elderly patients with pruritic disorders, 34 patients with bullous or non-bullous BP and 34 age-matched healthy controls were included in this study. The level of anti-BP180 and anti-BP230 IgG serum autoantibodies, Bullous Pemphigoid Disease Area Index (BPDAI), and pruritus severity were assessed for all patients and controls. For characterization of the autoreactive T cell response, peripheral blood mononuclear cells were stimulated ex vivo with recombinant BP180 proteins (NH2- and COOH-terminal domains) and the frequencies of BP180-specific T cells producing interferon-γ, interleukin (IL)-5 or IL-17 were subsequently determined by ELISpot assay. Patients with BP showed a mixed Th1/Th2 response against BP180 while autoreactive Th1 cells were identified in a minor subset of elderly patients with pruritic disorders. Furthermore, our T cell characterization revealed that therapeutic application of topical clobetasol propionate ointment in BP patients significantly reduced peripheral blood BP180-specific T cells, along with clinically improved symptoms, strongly suggesting a systemic immunosuppressive effect of this treatment.

Chronic Itch of Unknown Origin Is Associated With an Enhanced Th2 Skin Immune Profile

ABSTRACT

Chronic pruritus of unknown origin (CPUO) is described as chronic itch lasting longer than 6 weeks in the absence of a defined skin rash and any known causative disease process. A retrospective study was performed on biopsy samples from patients with CPUO and normal controls to compare the immune profiles of these patients with healthy individuals. We used dual CD3/T-bet and CD3/GATA3 immunohistochemical staining to assess for T-cells expressing Th1 versus Th2 transcription factors, respectively. Our data showed that CD3+ cells of patients with CPUO co-express significantly more GATA3 compared with normal controls. Meanwhile, the normal control skin showed a much more balanced T-bet/GATA3 ratio of co-expression. Our data suggest an enrichment of Th2 cells in CPUO skin by T cell/GATA3 co-staining, supporting that CPUO is increasingly considered a type 2/Th2 cell-associated disease. We thus speculate that type 2 cytokine blockade-based therapies may represent effective treatments for CPUO.

Pathophysiology and Treatment of Pruritus in Elderly

Pruritus is a relatively common symptom that anyone can experience at any point in their life and is more common in the elderly. Pruritus in elderly can be defined as chronic pruritus in a person over 65 years old. The pathophysiology of pruritus in elderly is still unclear, and the quality of life is reduced. Generally, itch can be clinically classified into six types: Itch caused by systemic diseases, itch caused by skin diseases, neuropathic pruritus, psychogenic pruritus, pruritus with multiple factors, and from unknown causes. Senile pruritus can be defined as a chronic pruritus of unknown origin in elderly people. Various neuronal mediators, signaling mechanisms at neuronal terminals, central and peripheral neurotransmission pathways, and neuronal sensitizations are included in the processes causing itch. A variety of therapies are used and several novel drugs are being developed to relieve itch, including systemic and topical treatments.

Efficacy of pregabalin for the treatment of chronic pruritus of unknown origin, assessed based on electric current perception threshold

Chronic pruritus of unknown origin (CPUO) is defined as itching lasting more than 6 weeks in the absence of discernible skin lesions. Pregabalin is used to treat patients with CPUO. In this study, we aimed to investigate differences in the perception threshold of itch sensation between patients with CPUO and healthy individuals and to evaluate the efficacy of pregabalin for CPUO. At baseline, week 2, and week 4 after treatment initiation, the visual analogue scale (VAS) score was measured to assess pruritus severity, and electric current perception threshold (CPT) was measured at 250 and 5 Hz using a NEUROMETER CPT/C stimulator. Twenty healthy individuals and 41 patients with CPUO were enrolled in this study. The patients with CPUO were categorised as those who responded to antihistamines (Antihistamine group), were not improved by antihistamines (Pregabalin group), and were not improved by antihistamines and pregabalin (Refractory group). The baseline CPT values were not significantly different between patients with CPUO and healthy control. Pruritus was improved in 7 of 10 patients in the Pregabalin group after treatment with pregabalin, showing decreased CPT at 5 Hz. The sensitive C-fibres presented a high threshold to detect itch sensation, and this sensitivity decreased in response to treatment with pregabalin.

Assessment of prurigo of pregnancy in patients without atopic background

Background

Prurigo of pregnancy (PP), a specific pregnancy dermatosis, has been associated with atopic background in the patient and/or the patient's family. However, this association was not validated in some studies, and cases without atopic background have been reported.

Objective

This study aimed to evaluate the clinical features of PP not related to atopic background and search for comorbid conditions in medical and obstetric records.

Methods

In this case series, patients with typical PP presentation (i.e., pruritic, discrete papulonodules on the extensor surfaces of the extremities) diagnosed at the dermatology clinics of tertiary referral hospitals were evaluated. The exclusion criteria included missing historical data, inadequate follow-up, laboratory test results suggestive of other pruritic conditions, history of atopic disease, and family history of atopy. Clinical and laboratory data including course, response to treatment, serum total immunoglobulin E level, and comorbidities in the medical and obstetric history were collected.

Results

Twenty patients developed PP predominantly during the late second or early third trimester. Nine patients reported that itching developed first, versus 10 patients who reported that lesions started first (one patient was uncertain about onset). There was no recurrence postpartum (mean postpartum follow-up, 17 months). Serum total immunoglobulin E level was elevated in 3 of 14 patients tested (21.4%). Obstetric history (n = 12) included polymorphic eruption of pregnancy (16.6%), PP (16.6%), and gestational pruritus (8.3%). Two of 20 patients (10%) had a history of pruritic skin condition (prurigo nodularis and pruritus of unclear etiology) while not pregnant. Medical history (n = 20) included psychiatric disease (20%) (obsessive compulsive disorder and anxiety), hypothyroidism (10%), and obesity (10%).

Conclusion

PP can develop in patients without an atopic background. This finding should be considered when classifying specific pregnancy dermatoses. A thorough medical and family history with a focus on atopy should be obtained from every patient with a gestational eruption, and patients should be counseled accordingly.

Kimura's disease: effects of age on clinical presentation

BACKGROUND

Kimura's disease (KD) is known to be dominant among young Asian men, but it can also occur in middle- and advanced-aged people. The clinical characteristics of KD, especially by age, are not well known.

AIM

This study was performed to investigate the effects of age on the clinical characteristics of KD.

DESIGN

We conducted a case series study.

METHODS

All case studies of patients diagnosed with KD were collected via a PubMed search of studies published until August 2018. The data were analyzed by age group.

RESULTS

In total, 215 studies were reviewed (238 patients; mean age of 36 years). The male:female ratio was 4:1 overall, 17:1 in patients aged <20 years, 4:1 in patients aged 20-39 years and 2:1 in patients aged ≥40 years (P = 0.01). The percentage of patients with pruritus was 15.4% overall, 3.8% in patients aged <20 years, 15.5% in patients aged 20-39 years and 21.7% in patients aged ≥40 years (P = 0.02). The time to diagnosis was 5.3 years overall, 3.2 years in patients aged <20 years, 4.7 years in patients aged 20-39 years and 7.1 years in patients aged ≥40 years (P < 0.01).

CONCLUSIONS

The proportion of female patients affected the incidence of pruritus, and the time to diagnosis increased as the patients' age increased. There were no significant age-related differences in region/race, complications, multiplicity, laterality, anatomical distribution, maximum size, eosinophil count, immunoglobulin E level, initial treatment, recurrence or outcomes. This may be useful information for the diagnosis of KD.

The Return of the Mast Cell: New Roles in Neuroimmune Itch Biology

The mast cell-nerve unit classically has represented a fundamental neuroimmune axis in the development of itch because of the traditional prominence of histamine as a pruritogen. However, it is appreciated increasingly that most chronic itch disorders are likely nonhistaminergic in nature, provoking the hypothesis that other novel effector itch mechanisms derived from mast cells are important. In this review, we present an overview of classical mast cell biology and put these concepts into the context of recent advances in our understanding of the regulation and function of the mast cell-nerve unit in itch biology.

Blood natural killer cell deficiency reveals an immunotherapy strategy for atopic dermatitis

Atopic dermatitis (AD) is a widespread, chronic skin disease associated with aberrant allergic inflammation. Current treatments involve either broad or targeted immunosuppression strategies. However, enhancing the immune system to control disease remains untested. We demonstrate that patients with AD harbor a blood natural killer (NK) cell deficiency that both has diagnostic value and improves with therapy. Multidimensional protein and RNA profiling revealed subset-level changes associated with enhanced NK cell death. Murine NK cell deficiency was associated with enhanced type 2 inflammation in the skin, suggesting that NK cells play a critical immunoregulatory role in this context. On the basis of these findings, we used an NK cell-boosting interleukin-15 (IL-15) superagonist and observed marked improvement in AD-like disease in mice. These findings reveal a previously unrecognized application of IL-15 superagonism, currently in development for cancer immunotherapy, as an immunotherapeutic strategy for AD.

Nalbuphine, a kappa opioid receptor agonist and mu opioid receptor antagonist attenuates pruritus, decreases IL-31, and increases IL-10 in mice with contact dermatitis

Chronic itch is one of the disturbing symptoms of inflammatory skin diseases. Kappa opioid receptor agonists are effective in suppressing scratching in mice against different pruritogens. Nalbuphine, a nonscheduled kappa opioid receptor agonist and mu opioid receptor antagonist, has been in clinical use for post-operative pain management since the 1980s and recently has been in clinical trials for chronic itch of prurigo nodularis (https://www.trevitherapeutics.com/nalbuphine). We studied whether nalbuphine is effective against chronic scratching induced by rostral neck application of 1-fluoro-2,4-dinitrobenzene (DNFB), an accepted mouse model of contact dermatitis to study pruritoceptive itch. Mice were treated once a week with either saline or nalbuphine 20 min before the third, fifth, seventh, and ninth sensitizations with DNFB and the number of scratching bouts was counted for 30 min. Skin samples from the neck of mice at week 4 were used to measure protein levels and mRNA expressions of chemokines and cytokines. Different sets of mice were used to study sedation and anhedonic-like behavior of nalbuphine. We found that: nalbuphine (a) antagonized scratching in a dose- and time-dependent manner without affecting locomotion, b) decreased IL-31, and increased anti-inflammatory IL-10, and c) induced more elevations in the levels of CCL2, CCL3, CCL12, CXCL1, CXCL2, CXCL9, CXCL10, IL-1β, IL-16, TIMP-1, M-CSF, TREM-1 and M1-type macrophages compared to saline. Increases in chemokines and cytokines and M1 macrophages by nalbuphine suggest an inflammatory phase of healing in damaged skin due to scratching. Our data indicate that nalbuphine is an effective antipruritic in murine model of pruritoceptive itch.

Pruritus in allergy and immunology

Although evolutionarily conserved to expel ectoparasites and aid in the clearance of toxins and noxious environmental stimuli from the host, the type 2 immune response can become pathologic in the setting of a variety of allergic disorders. Itch can be a behavioral extension of type 2 immunity by evoking scratching and, in the setting of disease, can become chronic and thus highly pathologic as well. Classically, our understanding of itch mechanisms has centered around the canonical IgE-mast cell-histamine axis. However, therapies aimed at blocking the histaminergic itch pathway have been largely ineffective, suggesting the existence of nonhistaminergic itch pathways. Indeed, recent advances in itch biology have provided critical new insight into a variety of novel therapeutic avenues for chronic itch in the setting of a number of allergic disorders. Here we highlight how these new developments will likely inform the problem of pruritus in a variety of well-established and emerging conditions in the field of allergy.

Chronic Pruritus Responding to Dupilumab-A Case Series

Background: Chronic pruritus is defined as itch lasting for greater than six weeks. Pruritus is a burdensome manifestation of several internal and external disease states with a significant impact on quality of life. Dupilumab has shown promise in treating a number of conditions including atopic dermatitis (AD) and asthma. Its success in reducing pruritus in AD has generated interest regarding its potential application in other pruritic conditions, such as chronic pruritus of unknown origin, uremic pruritus, and pruigo nodularis. Methods: In this retrospective analysis, we present a series of 20 recalcitrant pruritus patients seen at a tertiary center treated with off-label dupilumab at standard AD dosing. Results: Dupilumab was successful at reducing itch in all treated patients, leading to complete resolution in 12/20 patients and an overall mean NRSi reduction of 7.55. Dupilumab was well tolerated with no significant adverse effects. Conclusions: Our case series suggests dupilumab may be a safe and efficacious therapeutic option in several pruritic conditions and demonstrates the need for further studies to better ascertain its place in the pruritus treatment armamentarium.

Chronic Pruritus in the Geriatric Population. Dermatol Clin. 2018;36:199-211. [PMID: 29929593 DOI: 10.1016/j.det.2018.02.004]

Chronic pruritus (>6 week's duration) in the geriatric population (≥65 years old), is an increasing health care problem. The pathophysiologic predisposing factors are abnormalities of the epidermal barrier, immune system, and nervous system. Causes can be dichotomized into histaminergic and nonhistaminergic pruritus. Topical treatments are generally safe. Systemic treatments are chosen depending on the condition, comorbid diseases, and drug interactions. Treatment options are limited. Progress has been made in identifying itch-selective mediators over the last decade. Numerous new medications are currently undergoing clinical trials and they are anticipated to enter the clinics in the near future.

Sensory TRP channels contribute differentially to skin inflammation and persistent itch

Although both persistent itch and inflammation are commonly associated with allergic contact dermatitis (ACD), it is not known if they are mediated by shared or distinct signaling pathways. Here we show that both TRPA1 and TRPV1 channels are required for generating spontaneous scratching in a mouse model of ACD induced by squaric acid dibutylester (SADBE), a small molecule hapten, through directly promoting the excitability of pruriceptors. TRPV1 but not TRPA1 channels protect the skin inflammation, as genetic ablation of TRPV1 function or pharmacological ablation of TRPV1-positive sensory nerves promotes cutaneous inflammation in the SADBE-induced ACD. Our results demonstrate that persistent itch and inflammation are mediated by distinct cellular and molecular mechanisms in a mouse model of ACD. Identification of distinct roles of TRPA1 and TRPV1 in regulating itch and inflammation may provide new insights into the pathophysiology and treatment of chronic itch and inflammation in ACD patients.

Allergic contact dermatitis is associated both with persistent itch and inflammation, but it is not known if these are mediated by shared signaling pathways. The authors show that persistent itch requires both TRPA1 and TRPV1, while TRPV1 has a protective role against skin inflammation in mice.

Retrospective study of advanced melanoma patients treated with ipilimumab after nivolumab: Analysis of 60 Japanese patients

BACKGROUND

Due to resistance and immune-related adverse events (irAE) some melanoma patients require ipilimumab after nivolumab therapy. However, little is known about the result of this switching.

OBJECTIVE

Investigate the outcome of ipilimumab switching in Japanese patients.

METHODS

We retrospectively collected 60 patients who were treated with ipilimumab after nivolumab from 9 institutes in Japan. Information of the primary tumor, treatment, response, irAE), and survival was collected.

RESULTS

In our cohort, acral lentiginous and mucosal melanoma accounted for 53% of the cases. The most common reason for initiating ipilimumab was disease progression (93%). Median interval from the last nivolumab administration to first ipilimumab administration was 29days. Only 38% of patients completed 4 injections of ipilimumab. The best overall response was 3.6%. IrAE occurred in 78% of patients and 70% of those were of grade 3/4 (G3/4) and 31% of patients experienced 2 or more irAEs. An within interval of 28days or less between the last nivolumab administration and ipilimumab administration was correlated with the development of G3/4 pyrexia and 3 or more irAEs, but irAE occurrence did not affect survival. Multivariate analysis showed that endocrine irAE (relative risk=0.22, P=0.015) and skin irAE (relative risk=2.78, P=0.048) were significant factors associated with survival.

CONCLUSION

In our study, the response ratio to ipilimumab after nivolumab was unsatisfactory and associated with a high frequency of severe irAEs. As there are few second-line treatment options for patients with BRAF wild-type advanced melanoma after nivolumab failure, patients should be closely monitored if ipilimumab is initiated.

Causes, pathophysiology, and treatment of pruritus in the mature patient

Chronic itch is a common and debilitating health condition in the elderly. There are several common causes of itch in the mature population, such as skin xerosis, immunosenescence, and neuropathic changes. In addition, skin diseases, such as seborrheic dermatitis and stasis dermatitis, systemic conditions (end-stage renal disease and diabetes), or psychogenic derailments, such as depression, anxiety, and dementia, can all serve as triggers of pruritus. Polypharmacy, a common occurrence among the elderly population, may also serve as a cause of itch that may or may not be accompanied by dermatitis. Such medications as μ opioids and calcium channel blockers have been found to have a connection with pruritus in the advanced aging population. Determining the exact trigger for pruritus in the elderly may be especially challenging, because itch can be idiopathic in many cases. The role of treatments should not only take into account elimination of various underlying cutaneous, systemic, or psychogenic conditions associated with itch but also focus on the skin changes that are characteristic of the aging process. Development of such treatment options can be guided by elucidation of the mechanisms underlying the pathophysiology of itch in the geriatric population.

Sensory Neurons Co-opt Classical Immune Signaling Pathways to Mediate Chronic Itch

Mammals have evolved neurophysiologic reflexes, such as coughing and scratching, to expel invading pathogens and noxious environmental stimuli. It is well established that these responses are also associated with chronic inflammatory diseases, including asthma and atopic dermatitis. However, the mechanisms by which inflammatory pathways promote sensations such as itch remain poorly understood. Here, we show that type 2 cytokines directly activate sensory neurons in both mice and humans. Further, we demonstrate that chronic itch is dependent on neuronal IL-4Rα and JAK1 signaling. We also observe that patients with recalcitrant chronic itch that failed other immunosuppressive therapies markedly improve when treated with JAK inhibitors. Thus, signaling mechanisms previously ascribed to the immune system may represent novel therapeutic targets within the nervous system. Collectively, this study reveals an evolutionarily conserved paradigm in which the sensory nervous system employs classical immune signaling pathways to influence mammalian behavior.

Transient receptor potential vanilloid 4-expressing macrophages and keratinocytes contribute differentially to allergic and nonallergic chronic itch

BACKGROUND

Chronic itch is a highly debilitating symptom that underlies many medical disorders with no universally effective treatments. Although unique neuronal signaling cascades in the sensory ganglia and spinal cord have been shown to critically promote the pathogenesis of chronic itch, the role of skin-associated cells remains poorly understood.

OBJECTIVE

We sought to examine the cutaneous mechanisms underlying transient receptor potential vanilloid 4 (TRPV4)-mediated allergic and nonallergic chronic itch.

METHODS

Expression of TRPV4 in chronic itch and healthy control skin preparations was examined by using real-time RT-PCR. Trpv4^eGFP mice were used to study the expression and function of TRPV4 in the skin by means of immunofluorescence staining, flow cytometry, calcium imaging, and patch-clamp recordings. Genetic and pharmacologic approaches were used to examine the role and underlying mechanisms of TRPV4 in mouse models of dry skin-associated chronic itch and spontaneous scratching associated with squaric acid dibutylester-induced allergic contact dermatitis.

RESULTS

TRPV4 is selectively expressed by dermal macrophages and epidermal keratinocytes in mice. Lineage-specific deletion of TRPV4 in macrophages and keratinocytes reduces allergic and nonallergic chronic itch in mice, respectively. Importantly, TRPV4 expression is significantly increased in skin biopsy specimens from patients with chronic idiopathic pruritus in comparison with skin from healthy control subjects. Moreover, TRPV4-dependent chronic itch requires 5-hydroxytryptamine (5-HT) signaling secondary to activation of distinct 5-HT receptors in mice with allergic and those with nonallergic chronic itch conditions.

CONCLUSION

Our study reveals previously unrecognized mechanisms by which TRPV4-expressing epithelial and immune cells in the skin critically and dynamically mediate chronic itch and unravels novel targets for therapeutics in the setting of chronic itch.