Putative mechanism of the itch-scratch circle: repeated scratching decreases the cutaneous level of prostaglandin D2, a mediator that inhibits itching

Transient receptor potential channels and itch: how deep should we scratch?

Over the past 30 years, transient receptor potential (TRP) channels have evolved from a somewhat obscure observation on how fruit flies detect light to become the center of drug discovery efforts, triggering a heated debate about their potential as targets for therapeutic applications in humans. In this review, we describe our current understanding of the diverse mechanism of action of TRP channels in the itch pathway from the skin to the brain with focus on the peripheral detection of stimuli that elicit the desire to scratch and spinal itch processing and sensitization. We predict that the compelling basic research findings on TRP channels and pruritus will be translated into the development of novel, clinically useful itch medications.

The ion channel TRPA1 is required for chronic itch

Chronic itch is a debilitating condition that affects one in 10 people. Little is known about the molecules that mediate chronic itch in primary sensory neurons and skin. We demonstrate that the ion channel TRPA1 is required for chronic itch. Using a mouse model of chronic itch, we show that scratching evoked by impaired skin barrier is abolished in TRPA1-deficient animals. This model recapitulates many of the pathophysiological hallmarks of chronic itch that are observed in prevalent human diseases such as atopic dermatitis and psoriasis, including robust scratching, extensive epidermal hyperplasia, and dramatic changes in gene expression in sensory neurons and skin. Remarkably, TRPA1 is required for both transduction of chronic itch signals to the CNS and for the dramatic skin changes triggered by dry-skin-evoked itch and scratching. These data suggest that TRPA1 regulates both itch transduction and pathophysiological changes in the skin that promote chronic itch.

Increased frequencies of IL-31-producing T cells are found in chronic atopic dermatitis skin

Interleukin (IL)-31 has been associated with pruritus, a characteristic feature of atopic dermatitis (AD). Local T cell responses may be responsible for the increased level of IL-31 mRNA observed in AD. We investigated the frequency of IL-31-producing T cells in AD lesions, as well as their cytokine profile. T cells were isolated from chronic AD lesions, autologous blood and healthy donor skin. Intracellular expression of IL-31, IFN-γ, IL-13, IL-17 and IL-22 was measured using flow cytometry. T cells from AD lesions contained significantly higher percentages of IL-31-producing T cells compared to autologous blood and donor skin. Many IL-31-producing T cells co-produced IL-13 and to lesser extent IL-22, but rarely IFN-γ or IL-17. A substantial part of the IL-31-producing T cells did not co-produce any of the other cytokines and could therefore not be linked to any of the known functionally different T cell subsets. The T cell infiltrates were also relatively enriched for Th2/Tc2 and Th22/Tc22 cells, while frequencies of Th1/Tc1 and Th17 cells were decreased. This is the first report describing the detection of IL-31 at protein level in skin-infiltrating T cells. We show here that T cells in chronic AD skin produce IL-31 and that AD lesions contain increased levels of these IL-31-producing T cells. This suggests that a substantial part of previously reported increased IL-31 mRNA levels in AD skin is T cell derived and that these cells may be involved in the pathogenesis of AD.

The pleasurability of scratching an itch: a psychophysical and topographical assessment

BACKGROUND

Scratching an itch is perceived as being pleasurable. However, an analysis of topographical variations in itch intensity, the effectiveness of scratching to provide itch relief and the associated pleasurability has not been performed at different body sites.

OBJECTIVES

To examine the role of scratching pleasurability in providing itch relief by investigating whether itch intensity is perceived differently at three different sites and to assess a potential correlation between the pleasurability and itch attenuation induced by scratching.

METHODS

Itch was induced on the forearm, ankle and back using cowhage spicules in 18 healthy subjects. These sites were subsequently scratched by an investigator with a cytology brush immediately following itch induction. The intensity of itch with and without scratching at these sites and the pleasurability of scratching were recorded by taking visual analogue scale ratings at 30-s intervals.

RESULTS

Mean itch intensity and scratching pleasurability ratings at the ankle and back were significantly higher than on the forearm. For the forearm and ankle, the greater the itch while scratching, the higher was the pleasurability. A higher baseline itch was linked to a higher itch reduction secondary to scratching in all tested areas. Pleasurability paralleled the curve of itch reduction for the back and forearm; however, scratching pleasurability at the ankle remained elevated and only slightly decreased while itch was diminishing.

CONCLUSIONS

There are topographical differences in itch intensity, the effectiveness of scratching in relieving itch and the associated pleasurability. Experimental itch induced by cowhage was more intensely perceived at the ankle, while scratching attenuated itch most effectively on the back.

Signaling by IL-31 and functional consequences

Cytokines are key to control cellular communication. Interleukin-31 (IL-31) was recently discovered as a new member of the IL-6 family of cytokines. IL-31 signals through a heterodimeric receptor composed of OSMR and IL-31RA, a complex that stimulates the JAK-STAT, the RAS/ERK and the PI3K/AKT signal transduction pathways. The available data suggests that IL-31 is important for both innate and adaptive immunity in tissues that are in close contact with the environment, i.e. the skin, the airways and the lung, and the lining of the intestine. Enhanced expression of IL-31 is associated with a number of diseases, including pruritic diseases such as atopic dermatitis, but also in allergy and inflammatory bowel disease. In these tissues IL-31 coordinates the interaction of different immune cells, including T-cells, mast cells, and eosinophils, with epithelial cells. In this review we have summarized the available data on IL-31 and its receptor, their expression pattern and how they are regulated. We describe the current state of knowledge of the involvement of IL-31 in diseases, both in humans and in mouse models. From these studies it is becoming clear that IL-31 plays an important role in the proper functioning of the skin and of airway and intestinal epithelia. The findings available suggest that IL-31 might be an interesting target for directed drug therapy.

Dual functions of prostaglandin D2 in murine contact hypersensitivity via DP and CRTH2

Prostaglandin D2 (PGD2) exerts its effects through two distinct receptors: the chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2) and the D prostanoid (DP) receptor. Our previous study demonstrated that CRTH2 mediates contact hypersensitivity (CHS) in mice. However, the function of DP receptor remains to be fully established. In this study, we examine the pathophysiological roles of PGD2 using DP-deficient (DP(-/-)) and CRTH2/DP-deficient (CRTH2(-/-)/DP(-/-)) mice to elucidate receptor-mediated PGD2 action in CHS. We observed profound exacerbation of CHS in DP(-/-) mice. CRTH2(-/-)/DP(-/-) mice showed similar exacerbation, but to a lesser extent. These symptoms were accompanied by increased production of interferon-γ and IL-17. The increase in IL-17 producing γδ T cells was marked and presumably contributed to the enhanced CHS. DP deficiency promoted the in vivo migration of dendritic cells to regional lymph nodes. A DP agonist added to DCs in vitro was able to inhibit production of IL-12 and IL-1β. Interestingly, production of IL-10 in dendritic cells was elevated via the DP pathway, but it was lowered by the CRTH2 pathway. Collectively, PGD2 signals through CRTH2 to mediate CHS inflammation, and conversely, DP signals to exert inhibitory effects on CHS. Thus, we report opposing functions for PGD2 that depend on receptor usage in allergic reactions.

The prostaglandin D₂ receptor CRTH2 is important for allergic skin inflammation after epicutaneous antigen challenge

BACKGROUND

Cutaneous prostaglandin (PG) D₂ levels increase after scratching. Chemoattractant receptor-homologous molecule expressed on receptor on T(H)2 cells (CRTH2) mediates chemotaxis to PGD₂ and is expressed on T(H)2 cells and eosinophils, which infiltrate skin lesions in patients with atopic dermatitis.

OBJECTIVE

We sought to examine the role of CRTH2 in a murine model of atopic dermatitis.

METHODS

CRTH2(-/-) mice and wild-type control animals were epicutaneously sensitized by means of repeated application of ovalbumin (OVA) to tape-stripped skin for 7 weeks and then challenged by means of OVA application to tape-stripped previously unsensitized skin for 1 week. Skin histology was assessed by means of hematoxylin and eosin staining and immunohistochemistry. Cytokine mRNA expression was examined by means of quantitative RT-PCR. Levels of PGD₂, antibody, and cytokines were measured by means of ELISA.

RESULTS

PGD₂ levels significantly increased in skin 24 hours after tape stripping, although not in skin subjected to repeated sensitization with OVA. Allergic skin inflammation developed normally at sites of chronic epicutaneous sensitization with OVA in CRTH2(-/-) mice but was severely impaired in previously unsensitized skin challenged with OVA, as evidenced by significantly decreased skin infiltration with eosinophils and CD4(+) cells and impaired T(H)2 cytokine mRNA expression. Impaired skin inflammation at sites of acute OVA challenge in CRTH2(-/-) mice was not due to an impaired systemic response to epicutaneous sensitization because OVA-specific IgG1 and IgE antibody levels and OVA-driven splenocyte secretion of cytokines in these mice were comparable with those seen in wild-type control animals.

CONCLUSIONS

CRTH2 promotes allergic skin inflammation in response to cutaneous exposure to antigen in previously sensitized mice.

Therapy of pruritus

IMPORTANCE OF THE FIELD

Pruritus is the predominant symptom of skin disease. Owing to the poorly understood pathophysiology, the development of effective treatment modalities for pruritus has proven to be particularly difficult. At present, there is no universally accepted therapy for itch. The purpose of this review is to provide an update on the treatment of pruritus.

AREAS COVERED IN THIS REVIEW

An overview of current, emerging and possible future therapies for pruritus is provided.

WHAT THE READER WILL GAIN

Insights into possible treatment regimes for pruritus in different clinical scenarios.

TAKE HOME MESSAGE

The therapy of pruritus is challenging and at present takes on an individualistic approach. Recent advancements in the mechanisms that underlie this distressing symptom have identified new targets for future therapy.

Dendritic cells express hematopoietic prostaglandin D synthase and function as a source of prostaglandin D2 in the skin

Prostaglandin D2 (PGD2), an arachidonic acid metabolite, has been implicated in allergic responses. A major source of PGD2 in the skin is mast cells that express hematopoietic PGD synthase (H-PGDS). In this study, we show the expression of H-PGDS in human dendritic cells (DCs) and the regulatory mechanisms by which DCs produce PGD2. We detected H-PGDS in epidermal Langerhans cells, dermal DCs, plasmacytoid DCs, and myeloid DCs. Monocyte-derived DCs rapidly secreted PGD2 when stimulated with the calcium ionophore A23187. More importantly, pretreatment of monocyte-derived DCs with PMA (phorbol 12-myrisate 13-acetate) synergistically enhanced the rapid PGD2 secretion induced by A23187, whereas PMA alone did not induce PGD2 secretion. Lipopolysaccharide (LPS) reduced H-PGDS expression, but interferon-gamma followed by LPS induced significant PGD2 production in a delayed time course at 6 hours. This effect was associated with inhibition of LPS-induced H-PGDS reduction. Interestingly, an irritant compound, SDS, also induced a rapid PGD2 release. PGD2 synergistically enhanced CCL22/macrophage-derived chemokine synthesis in interferon-gamma-treated human keratinocytes. In addition, bone marrow-derived DCs from wild-type mice stimulated lymph node cells to produce higher amounts of interleukin-17 than did DCs from mice lacking the H-PGDS gene. Thus, DCs could be an important source of skin PGD2 and may mediate or regulate skin inflammation by releasing PGD2 in response to various stimuli, contributing to the innate and/or acquired immune responses.

Pivotal role of mast cells in pruritogenesis in patients with myeloproliferative disorders

Pruritus is a common symptom in patients with Philadelphia chromosome-negative myeloproliferative disorders (MPDs). The pathophysiology of MPD-associated pruritus is unclear. We have demonstrated that MPD mast cells (MCs) are involved by the malignant process. In the present study, we explored the hypothesis that MCs play an important role in the development of pruritogenesis in MPDs. We found that MPD MCs released significantly greater amounts of pruritogenic factors, including histamine, leukotrienes, and interleukin-31 (IL-31) than normal MCs. Elevated levels of IL-31 were also observed in MPD CD3(+) cell-conditioned media. MPD MCs exhibited increased migratory behavior in response to stem cell factor or interleukin-8, which was associated with increased filamentous-actin content. Furthermore, the presence of pruritus in MPDs was statistically correlated with a greater number of MCs being generated by CD34(+) cells, a greater number of MC colonies being formed by CD34(+) cells, decreased apoptosis and prostaglandin D(2) release by cultured MCs, and higher plasma levels of IL-31. These data demonstrate that functional abnormalities of MPD MCs probably lead to pruritogenesis in patients with MPDs. These studies provide cellular and molecular targets for the development of antipruritus drugs for patients with MPDs.

What causes itch in atopic dermatitis?

Itch, the hallmark of atopic dermatitis, has a significant impact on quality of life for patients with this disease. Various central and peripheral mediators have been suggested to play a role in the pathophysiology of atopic eczema itch. Significant cross-talk occurs among stratum corneum, keratinocytes, immune cells, and nerve fibers, which are in close proximity to one another and induce itch. The impaired barrier function associated with the itch-scratch cycle further augments this vicious cycle. Recent advances in our understanding of itch pathophysiology shed light on peripheral and central neural sensitization of nerve fibers that contribute significantly to itch in atopic dermatitis. Recently, several new mediators have been described as associated with itch in atopic dermatitis, including serine proteases, interleukin 31, and nerve growth factor. This review covers the peripheral and central mechanisms and mediators involved in pathogenesis of itch in atopic dermatitis.

The pruritus receptor unit: a target for novel therapies

Although the role of prostanoids in itch was actively studied a decade ago, interest in this area has waned in recent years. Andoh et al. (2007, this issue) demonstrate that the prostanoid thromboxane A2 elicits scratching through its TP receptor. Identification of new itch mediators and their respective receptors within the skin will undoubtedly be the focus of future drug development for this distressing symptom.