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

1Itching is a very unpleasant sensation that may provoke scratching or the desire to scratch. Chronic itch has clinically been classified into 3 groups according to skin changes: (1) pruritus on primary diseased, inflamed skin; (2) pruritus on primary non-diseased, non-inflamed skin; and (3) pruritus with chronic secondary scratch lesions. Pruritus has also been categorized based on underlying diseases as following: (1) dermatological diseases; (2) systemic diseases including diseases of pregnancy and drug-induced pruritus; (3) neurological diseases; (4) psychiatric/psychosomatic diseases; (5) mixed; and (6) others. Based on its duration, itch can be divided into acute (less than 6 weeks) and chronic (more than 6 weeks).

2In the skin, the perception of itch originates from the combined action of nervous, cutaneous and inflammatory cells and substances released by them in the microenvironment. In humans, the nerves responsible for the perception of itch also react to stimuli of pain. There are two types of pruriceptors, specialized nerve fibers sensing itch: fastly conducting A-fibers and non-myelinated, slowly conducting C-fibers, which can be further subdivided based on their responsiveness to different chemical or physical stimuli [reviewed in detail in].

3The itch-specific peripheral sensory neurons may be a subgroup of nociceptive neurons. Subsets of mechano-insensitive nociceptive C-fibers can respond to histamine resulting in the release neuropeptides, including substance P and calcitonin gene -related peptide (CGRP). Instead, the mechano/heat-sensitive nociceptors are not reactive to histamine, but show a response to a cowhage plant protease, leading to stinging itch, which is possibly mediated via protease-activated receptor-2 or -4 (PAR-2 or -4). This type of stinging itch resembles that in skin inflammatory diseases. Nevertheless, cutaneous nerves contain a range of different receptors and ion channels. Therefore, different combinations of receptors may produce a certain type of itch and/or pain sensation. In the spinal dorsal horn, gastrin-releasing peptide-expressing neurons participate in the transmission of itch. The sensing system is further complexed by excitatory interneurons, spinothalamic tract neurons and the central nervous system where these sensations are recognized. Furthermore, it is noteworthy that chronic inflammation in skin causes additional changes to the neural and proinflammatory cell network.

4Free nerve endings in the epidermis are important for sensing itch. It has been proposed that there is a reciprocal synaptic-like interaction between nerves and keratinocytes. One possible mediator in this interaction is ß-endorphin, as keratinocytes expressing this mediator have been found around unmyelinated fibers that can be activated via μ-opioid receptor. In fact, the whole complex of nerve fibers and the epidermal cells have been proposed to form the “itch receptor”. The skin cells themselves express a wide array of mediators and their receptors involved in the perception of itch. In addition to nerve fibers and epidermal cells, the cells of the immune system constitute yet another part in pruritus. In regard to cellular events in itching skin, mast cells are important players that are involved in neurogenic inflammation and its associated itch. In this review, the fundamental role of mast cells and sensory nerves in itch is discussed.

5Mast cells are important effector cells in allergic reactions and immunity [reviewed in ], but they also contribute to carcinogenesis. Mast cells are abundant in body-environment interfaces in the skin and the gastrointestinal tract as well as present in the meninges of the central nervous system, and in the lung. In the skin, mast cell numbers are highest in the upper dermis and their numbers are not affected by age or sex, but increase in response to various environmental stimuli as shown after ultraviolet radiation in human skin, and after exposure to topical sensitizing agents in mice. It is thus obvious that mast cells constitute an inherent component of itch.

6Mast cells are fully loaded with preformed mediators or they produce newly-synthesized mediators, including proteases, histamine, lipid-derived mediators, cytokines, and chemokines which they release upon activation through a variety of mechanisms leading to degranulation, piecemeal degranulation, and/or mediator secretion without degranulation [reviewed in and in ]. In type I hypersensitivity, mast cell degranulation is a fundamental event and involves crosslinking of an antigen with two IgE molecules bound to FcεRI receptors. Mast cell activation has been studied especially in the context of urticaria. It is known that several factors such as infections, stress, certain foods, pseudoallergens, hormones, neuropeptides, and Th2 inflammation may prime mast cells [reviewed in ]. Also autoimmune mechanisms may lead to mast cell activation. In these cases, IgE recognizes dermis-derived autoantigens, of which more than 200 are present in chronic spontaneous urticaria. Another mechanism is the presence of IgG or IgM class antibodies against IgE or against its FcεRI receptor. These autoantibodies may also target the eosinophils, which release mast-cell activating factors.

7Mast cells release the contents of their secretory granules to their surroundings upon degranulation. Many of these granule mediators or mediators synthesized de novo participate in the development of itch.

8Histamine is the most important pruritogenic mediator of mast cells. Histamine has four receptors, namely H1–H4, of which H1 and H4 are important in pruritus. The function of these receptors in itch has been mainly studied in mouse models, and it has been shown that skin sensory neurons express H1, H3 and H4. In mouse models, H1-antagonists have been effective in decreasing itch, which has been known already for decades, although H4-antagonists have proved to be more potent. Histamine acts also on Transient receptor potential vanilloid receptor-1 (TRPV-1) in sensory neurons. In keratinocytes, also TRPV-4 is a histaminergic pruriceptor.

9Tryptase, one of the main proteinases secreted by mast cells, can induce pruritus in mice and its effects are inhibited by PAR-2 antibody or PAR-2 antagonist, showing that PAR-2 is involved in tryptase-induced pruritus. Involvement of tryptase and PAR-2 in itch has also been reported in a mouse model of atopic dermatitis. In line with these data, non-lesional and lesional skin biopsies from patients with atopic dermatitis show PAR-2 in sensory nerves with closely located mast cells.

10Interleukin-31 (IL-31) is important in the pruritus of atopic dermatitis and it also participates in the itch of cutaneous lymphoma. IL-31 has been shown to increase the growth and sprouting of cutaneous sensory nerves, which express its receptor, IL-31RA.

11Interleukin-31 has been demonstrated to induce mild itch that appears slowly starting at 143 min after skin prick test, which is associated with a long-lasting erythema. By comparison, histamine induces immediate itch that starts within 5 min after skin prick test. Human mast cells and T-cells are sources of IL-31 in skin, thus participating in the development of itch. Moreover, mast cell-derived histamine in addition to IL-31 increase the secretion of brain-derived natriuretic peptide, which in turn affects dendritic cells and keratinocytes to produce cytokines and other mediators, leading to inflammation, and increased itch signaling.

12Leukotrienes and prostaglandins are also involved in itch, but by different mechanism. When administered intradermally, leukotriene B4 induces itch while prostaglandin E2 does not. Leukotriene B4 is released from keratinocytes in response to PAR-2 receptor activation and it is involved in the itch-causing cascades of substance P and IL-31. PAR-2 activation and leukotriene B4 release participate also in dermatophyte-induced itch. In addition to producing leukotriene B4 by themselves, human, and murine mast cells also express leukotriene B4 receptors BLT1 and BLT2. On the contrary, prostaglandin D2, also produced by mast cells themselves, decreases histamine release from mast cells and inhibits scratching in a mouse model. Thus, it seems that mast cells release many mediators that also control their own function.

13There are several neuropeptides released by the sensory neurons in the skin, which then activate mast cells. Mast cells degranulate in response to nerve growth factor (NGF) and this signaling acts through TrkA tyrosine receptor. Interestingly, mast cells can secrete NGF also by themselves suggesting an autocrine or paracrine mechanism.

14The potency of substance P in degranulating mast cells and causing itchy wheals was found already decades ago. The effects of substance P on mast cells are mediated either through neurokinin-1 receptor (NK-1R) or Mas-related G protein coupled receptor-X2 (MRGPRX2). In the initial phases of topical therapy with calcineurin inhibitors pimecrolimus and tacrolimus, pruritus and burning is often present. These compounds have been shown to release substance P and CGRP from primary afferent nerve endings in murine skin, leading to mast cell degranulation and thus release of pruritogenic histamine, and tryptase. Mast cells degranulate rapidly in response to substance P resulting in wheal reaction in the skin. The ability of substance P to induce histamine release seems to appear only at high concentrations. In line with this, substance P decreases mast cell recruitment and degranulation when used as a topical treatment in a murine model of atopic dermatitis, pointing to a concentration-dependent dual role of substance P.

15Interestingly, stimulation of human mast cells with substance P and an IL-1 family member, IL-33, increase the secretion of proinflammatory TNF and IL-1β and these responses are inhibited by a natural flavonoid, methoxyluteolin. A pure luteolin with Ashwagandha has been proposed as a relief to patients suffering from stress and inflammation-associated diseases. This release of cytokines is potentially inhibited also by IL-37, another IL-1 family cytokine [reviewed in,, ]. The effect of substance P to release proinflammatory cytokines from mast cells points to a mechanism how mast cells participate in the neurogenic inflammation in psoriasis and atopic dermatitis as discussed below in more detail. Recently, reported increased levels of IL-33 in the stratum corneum of patients with atopic dermatitis. All these data emphasize the role of these cytokines in the context of neuroinflammation and itch, in which mast cells obviously participate.

16Vasoactive intestinal peptide (VIP) is another potent neuropeptide to degranulate mast cells, a reaction mediated through VPAC2, and/or MRGPRX2 receptors on mast cells. In addition, substance P and VIP have been found to stimulate human mast cells in vitro conditions to release cytokines and chemokines, including TNF-α, GM-CSF, IL-3, CCL2, CCL5, CXCL8, CXCL9, and CXCL10.

17In a retrospective study conducted by, dermatological disease was the probable cause of the itch in 41,8 % of patients, while almost a similar number of cases (44,8 %) showed no apparent origin of the symptoms. The role of mast cells in the development of itch has been studied mostly in psoriasis, atopic dermatitis and urticaria. It is likely that the increased contacts between nerve fibers and mast cells often seen in these dermatoses constitute the morphological basis for itch chronicization during chronic skin inflammation. The current understanding of the role of mast cells and sensory nerves in itch in these and selected other pruritic dermatoses is discussed here next.

18Psoriasis, a common chronic inflammatory and scaly skin disease, is characterized by pruritus that affects 60–90% of the patients and can appear in different forms, such as stinging, pinching, tickling, crawling, burning or pain sensations. On the other hand, psychosocial stress can exacerbate psoriasis in 40–80% of the patients. Thus, pruritus and stress may be reciprocally interconnected factors in psoriasis.

19Emotional stress is associated with the activation of a variety of neuro-immune-endocrine systems. For example, the hypothalamic-pituitary-adrenal (HPA) axis is activated and stress hormones are released, including corticotroping-releasing hormone (CRH), adrenocorticotropic hormone, and glucocorticoids. Interestingly, human skin has its own functional peripheral equivalent of the HPA axis. Furthermore, numerous other factors are activated in stress, such as α-MSH, neuropeptides, neurotrophins and the sympathetic nervous system.

20Several studies have previously shown that sensory nerve fibers and neuropeptides, including substance P, neurokinin A and VIP, are increased in the psoriatic lesion. Furthermore, the morphologic contacts between neurofilament+ nerves and tryptase+ mast cells are more frequent in psoriatic lesions than in non-lesional psoriatic skin or normal skin. Even the morphologic contacts between substance P+ and CGRP+ fibers and tryptase+ mast cells, but not the contacts between VIP+ fibers and tryptase+ mast cells, have been found to be increased in the psoriatic lesion. Therefore, there is a morphologic basis for mast cell-neural interaction as well as neurogenic inflammation in psoriasis.

21The increase in mast cell numbers, especially the MCTC-type (tryptase+ and chymase+) of mast cell, in the psoriatic lesion has been known for decades. However, in contrast to the resistant tryptase, chymase is sensitive to the action of serum protease inhibitors, which may explain the finding that the enzyme activity of chymase is decreased in the psoriatic lesion. The net outcome of the partial inactivation of chymase may be an uncontrolled and enhanced substance P-mediated neurogenic inflammation, as chymase degrades substance P and VIP, but tryptase degrades VIP and CGRP. Tryptase has the capability to cleave and activate the PAR-2 receptor. Therefore, the serine proteinase may not only activate the receptor on nerves and numerous proinflammatory cells but it may activate PAR-2 on mast cells themselves in a para- or autocrine fashion, as the percentage of tryptase+ mast cells containing PAR-2 immunoreactivity is increased in the psoriatic lesion. Furthermore, the activation of PAR-2 sensitizes TRPV-1 leading to increased substance P and CGRP release and TRPV-1 is expressed in substance P+ fibers as well as in mast cells in human skin.

22In addition to neuropeptides, the emotional stress in psoriasis may transmit its signals to the skin through CRH and CRH-R1 receptor on mast cells as suggested previously. This hypothesis is supported by a report that the immunostaining of CRH is increased in the epidermis, sweat glands, and hair follicles in the psoriatic lesion. In addition, we have analyzed the expression of CRH-R1 immunoreactivity in mast cells in psoriasis and found that the percentage of tryptase+ mast cells expressing CRH-R1 is higher in the lesional than non-lesional skin of 8 psoriatic patients (Haimakainen S et al., unpublished results).

23There are several studies that have investigated molecular differences in skin between pruritic and non-pruritic-type of psoriasis. For example, the pruritic-type of psoriasis is characterized by increased levels of substance P and nerve fibers; decreased levels substance P-degrading neutral endopeptidase; increased levels of NGF and/or its receptor TrkA, decreased expression of semaphorin-3A (an axon-guidance molecule), and increased numbers of total and degranulated mast cells.

24Atopic dermatitis (AD) is a well-known chronic eczematous skin disease characterized by distressing pruritus that can be exacerbated by inflammatory mediators, sensory nerves, skin dryness, heat, sweat, and emotional stress.

25Like in the case of psoriasis, mast cells and sensory nerves have been suggested to play a role in neurogenic inflammation and itch in AD. Regarding mast cells, there are similarities between these diseases. In the lesional atopic dermatitis skin, the number of tryptase+ mast cells is increased slightly, but the activity of chymase is decreased. Like in psoriasis, the possible explanation for the partial inactivation of chymase in AD is the presence of chymase inhibitors in mast cells. The reduced chymase activity means that the enzyme cannot degrade and control efficiently a variety of proinflammatory peptides and proteins, including IL-6, IL-13, TNF-α, IL-4, IL-5, substance P, and VIP.

26This is relevant for the inflammation, as the percentage of mast cells expressing TNF-α, IL-4, IL-6, and CD30 ligand is increased in the lesional AD skin. In addition, the severity of itching and tryptase+ and IL-6+ mast cells correlate inversely with the (pro)filaggrin immunostaining in the epidermis of AD skin.

27Like in the case of psoriasis, there is an increased sensory nerve density in the epidermis and dermis in AD lesions, which suggests an enhanced neurogenic inflammation. Previously, it has been demonstrated that there is an increased expression of NGF and amphiregulin in the epidermis, but decreased expression of nerve repulsion factors, semaphorin-3A and anosmin-1, in AD lesions, which can explain the increased nerve density in AD. In addition, another explanation for the increased nerve density is the finding that the expression of NGF is elevated in mast cells in AD lesions, and mast cells can express the receptor of NGF, i.e., TrkA. There are also other regulatory mechanisms between NGF and tryptase, as NGF treatment of mast cells in vitro can increase the expression of TrkA as well as the contents of tryptase and histamine, and tryptase can generate mature NGF from pro-NGF. These molecular interactions can explain, at least in part, the finding that the morphologic contacts between tryptase+ mast cells and neurofilament+ fibers are increased in AD lesions.

28Acute psychologic stress can result in marked changes in nerves, neuropeptides and mast cells. For example, the exposure of AD patients to acute Trier social stress test (TSST) associated with decreased NGF+ and PGP9.5+ fibers and decreased contacts between PGP9.5+ fibers and tryptase+ mast cells in AD lesions. However, in the non-lesional AD skin, these parameters increased, rather than decreased. Moreover, a positive correlation was observed between itch and mast cell-nerve fiber contacts in the non-lesional AD skin after TSST or lesional AD skin before TSST.

29The classic histaminergic itch is mediated via H1-receptors on cutaneous sensory nerves, though H4-receptors on nerves can also be involved in pruritus. However, antihistamines usually are not effective in relieving distressing pruritus in chronic skin inflammation. The reason is the non-histaminergic itch that can be mediated through other distinct pathways. The PAR-2 activation on sensory fibers is one of them, as PAR-2+ fibers are increased in AD lesions. Therefore, tryptase released from mast cells can activate PAR-2 on nerve fibers with subsequent release of substance P and CGRP and potentiation of neurogenic inflammation. Interestingly, lichenified lesions of atopic dermatitis showed no staining for substance P or VIP nor degranulated mast cells, suggesting that other factors may contribute to pruritus in chronic, lichenified lesions.

30Interleukin-31 can represent another pathway for non-histaminergic itch in AD. This cytokine is derived from Th2 cells and mast cells. In the psoriatic and AD lesions, mast cells show increased levels of IL-31 immunoreactivity compared to normal skin. The expressions of the mRNA of IL-31 and its receptor IL-31RA are increased in AD, but not psoriatic, lesions. Furthermore, the receptor is located in small-diameter neurons of human dorsal root ganglia.

31Interleukin-31 may not only induce slowly developing itch, but it may also increase the elongation, branching and density of nerves in AD lesions.

32Urticaria is a fundamentally mast cell-driven disease, in which pruritus is a key symptom in addition to wheals and/or angioedema. In urticaria, it is the degranulation of mast cells and release of their mediators that cause the symptoms by activating sensory nerves and causing vasodilatation and plasma extravasation. Although the details of mast cell activation in urticaria are not known yet, mast cells may be activated in urticaria by allergens/autoallergens or IgG antibodies against IgE or its receptor. In patients with chronic urticaria, more than 200 autoantigens reacting with IgE have been detected and of these, IL-24 is a common and functional autoantigen recognized by IgE antibodies.

33Interestingly, also a plethora of other agents are known to prime mast cells for further activation [reviewed in ]. Recently, a mouse model showed that Mrgprb2, the ortholog of the human MRGPRX2, and might be a mast cell-specific key receptor in pseudoallergic reactions. This receptor is upregulated in the skin of patients with severe chronic urticaria.

34Prurigo nodularis (PN) is a typical example of an itchy dermatosis. The close contact of mast cells and nerve fibers in PN lesions was reported already 20 years ago by. They also noticed that there are increased mast cell numbers in the lesions. There are several other pruritic skin diseases, in which the changes in mast cell numbers have been investigated and they definitely participate in the pathogenesis, although the exact details of their role in pruritus in these dermatoses is not clear yet. Mast cell numbers are increased already in the perilesional skin in patients with hidradenitis suppurativa (HS) and the numbers are further elevated in early and chronic lesions. In line with this finding, about 60 % of patients with HS report moderate or severe pruritus. Similarly, more than one third of patients with basal cell carcinoma or squamous cell carcinoma report to have pruritus and elevated mast cell numbers in both tumors have been reported. The connection between mast cell numbers and itch is not so clear in melanoma, in which less patients report itch and both increased, and decreased mast cell numbers have been reported. However, since the exact functions of mast cells in skin cancers are not known at the moment, also their role in itch remains unclear.

35The growing research performed in experimental animals has revealed that the molecular mechanisms of pruritus as well as the ligands and receptors involved in itch induction are more complex than thought, and it is beyond the scope if this review to describe them all.

36Animal models to study pruritus have been mainly developed for AD. Subcutaneous capsaicin injections to neonatal rats causes long-lasting, pruritic skin changes with histopathological findings resembling AD, including increased mast cell number. The Kyoto Fancy Rat Stock 4 is another rat model of AD showing spontaneous itchy dermatitis with increased transepidermal water loss and mast cell numbers. The bile duct ligation rat model of liver disease has provided a good insight to neuroinflammation and itch. In these rats, PAR-2 receptors are activated and this potentiates the TRPV-1 channels. As discussed above, mast cell tryptase may activate PAR-2, pointing to a role for mast cells also in hepatic pruritus. Elevated tryptase and PAR-2 levels are also found in a mouse model with ovalbumin-induced AD-like dermatitis. Mast cells and sensory nerves can be found in apparent morphologic contact in the skin. For example, in a mouse model of hapten-induced AD, mast cells in the lesional skin express high levels of cell adhesion molecule-1 (CADM-1), a molecule that was found to enhance adhesion and communication between sensory nerves and mast cells in vitro. Human mast cells have also been found to express CADM-1 that probably interacts with nectin-3 on nerves.

37The interaction between nerve fibers and mast cells can be affected by molecules leading to decreased itch and inflammation. In the NC/Nga mouse model of AD, dorsal skin lesions develop spontaneously. The treatment of these lesions with intracutaneous injections of semaphorin-3A, a nerve repulsion factor, for 5 days followed by biopsing on day 11 revealed a significant reduction in the clinical skin score and scratching behavior, decrease in PGP9.5+ nerve fibers in the epidermis, decrease in mast cell numbers, infiltrating CD4+ T cells, IL-4 production, and epidermal thickness. In addition, oral administration of chymase inhibitor has been shown to ameliorate dermatitis as well as scratching behavior in these NC/Nga mice.

38In pruritus, the neuroendocrine signals may travel from the central nervous system to periphery and vice versa. showed in 80’s that rats exposed to both audiovisual cue and antigen injection were conditioned to mast cell protease II release after reexposure to the audiovisual cue only. A sonic stress for 24 h in mice can induce degranulation of mast cells, changes in endothelium, increase in substance P+ fibers and their contacts with mast cells, and increase in the expression of NGF in skin mast cells. Furthermore, skin mast cells in rats can be activated by stress induced by immobilization. In summary, there is compelling in vivo-evidence that show activation of mast cells induced by stress.

39In an AD model induced by IL-13 in mice, the scratching behavior evoked by itch associated with increase in PGP9.5+, CGRP+, and TRPA-1+ (transient receptor potential ankyrin (1) nerves, mast cells, and particularly in TRPA-1+ mast cells. However, there was no increase in TRPV-1 in inflamed skin. In line with this, TRPA-1+ mast cells and nerves and the contacts between them were increased in the lesional skin of patients with AD. Therefore, this study emphasizes the role of TRPA-1 in itch associated with skin inflammation. In contrast to this, itch evoked by intradermal injections of β2-microglobulin in mouse skin is related, at least in part, to TRPV-1+ primary sensory nerves. Furthermore, itch evoked by IL-31 injections in mice is related to both TRPV-1 and TRPA-1 using knockout mouse models. In addition, itch induced in mice by lysophosphatidic acid-injections is related to both cation channels.

40Despite thorough and extensive research on mast cells during the past decades, these cells still remain to be an intriguing, complex cell type present in our body-environment interfaces. Although mast cells are physiologically meant to stay alert and to react to potentially noxious agents and conditions, their reactions may turn harmful. In several disorders, their functions seem to increase itch, which is not always beneficial to the host.

41The complex functional and morphologic interaction between mast cells and sensory nerves is summarized and illustrated in Figure 1. Psychic stress can aggravate itch in several skin diseases. Although the mechanism is not clear in detail, the activation of the systemic HPA axis and/or its equivalent in the skin as well as the activation of neural response can play a role in activating the neurogenic inflammation in the skin. As a consequence, several neuroendocrine mediators, including CRH, substance P and NGF, are released to the circulation and/or are produced locally in the skin. These mediators lead to activation of mast cells and release of their proinflammatory mediators that modify the inflammation of the skin disorder, often increasing itch. The mechanisms are complex owing to the intimate reciprocal communication between mast cells and sensory nerves resulting in increase in mast cell numbers and nerve fibers, development of a vicious circle, and exacerbation of neurogenic inflammation and pruritus.

42Future research should aim at further elucidating the details of mast cell biology in different physiological and pathological skin conditions in humans. Here, it will be important to focus on the association of psychic stress with mast cell-neural functions in patients with psoriasis or atopic dermatitis under standardized stress conditions. We should also aim at more in-depth understanding of mast cell heterogeneity and recruitment to the skin and how these may affect mast cell-neural responses in chronic pruritic disorders. In addition, the inhibitors of mast cell mediators or drugs preventing mast cell activation may provide new therapeutic options to treat distressing itch and thereby improve the quality of life of patients.

43Both authors wrote and edited the text.

44The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.