Is there a role for mast cells in psoriasis?
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Mast cells have traditionally been considered as effector cells in allergy but during the last decade it has been realized that mast cells are essentially involved in the mechanisms of innate and acquired immunity. Upon activation by anaphylactic, piecemeal degranulation or degranulation-independent mechanisms mast cells can secrete rapidly or slowly a number of soluble mediators, such as serine proteinases, histamine, lipid-derived mediators, cytokines, chemokines and growth factors. Mast cells can express cell surface co-stimulatory receptors and ligands, and they can express MHC class II molecules and thereby present antigens. These soluble factors and cell surface molecules can interact with other cells, such as endothelial cells, keratinocytes, sensory nerves, neutrophils, T cell subsets and antigen presenting cells which are essential effectors in the development of skin inflammation. Besides promoting inflammation, mast cells may attempt in some circumstances to suppress the inflammation and epidermal growth but the regulation between suppressive and proinflammatory mechanisms is unclear. Psoriasis is characterized by epidermal hyperplasia and chronic inflammation where tryptase- and chymase-positive MCTC mast cells are activated early in the developing lesion and later the cells increase in number in the upper dermis with concomitant expression of cytokines and TNF superfamily ligands as well as increased contacts with neuropeptide-containing sensory nerves. Due to the intimate involvement of mast cells in immunity and chronic inflammation the role of mast cells in psoriasis is discussed in this review.
KeywordsMast cell Mediator Psoriasis Proteinase Cytokine Nerve Pathogenesis
Mast cells are long-lived cells that are strategically located at sites, such as the upper dermal skin, respiratory tract and bowel mucosa, where the host tissue is exposed to external antigens and microbes. Human mast cells can be divided into MCTC and MCT subtypes based on their proteinase composition: MCTC cells contain tryptase, chymase, carboxypeptidase and a cathepsin G-like proteinase whereas MCT cells contain only tryptase [78, 162, 202]. MCC mast cell type with only chymase and carboxypeptidase has also been identified in human tissues  though there are results from skin organ cultures suggesting that it may represent an apoptotic mast cell from which tryptase has dissolved away . MCTC mast cell is the predominant cell type in the skin and bowel submucosa, whereas MCT cells predominate in the lung and bowel mucosa [78, 202].
Traditionally, mast cells are known to be activated to release mediators by allergen cross-linking IgE molecules and their high affinity FcεRI receptors on the cell membrane. However, the activation mechanisms are far more complex and versatile. In addition to IgE-dependent mechanism, human skin mast cells can be activated by an IgG-dependent mechanism since they constitutively express the FCγRIIa receptor . Furthermore, the high-affinity IgG receptor, FcγRI/CD64, can be induced in in vitro cultured mast cells by interferon-γ . Interestingly, mast cells in the psoriatic lesion, where also interferon-γ is expressed, show expression of CD64 . Recently, mast cells have been reported to express a range of toll-like receptors which can be activated by microbial products . Mast cells can also be activated for mediator release by numerous endogenous peptides and proteins, including complement products C3a and C5a , neuropeptides, such as substance P (SP) and vasoactive intestinal peptide (VIP) , stem cell factor (SCF) , tumor necrosis factor (TNF) , tryptase , and α-melanocyte-stimulating hormone . Corticotropin-releasing hormone has been found to induce selective release of vascular endothelial growth factor (VEGF) from human mast cells , a factor which is essentially involved in angiogenesis and capillary growth. Human mast cell activation by means of reverse signalling through CD30 ligand is a novel pathway for mast cell activation and chemokine secretion . It should be realized that the activation of mast cells for mediator release is not simply an on–off mechanism, i.e., no activation or rapid and extensive anaphylactic degranulation. Mast cells can undergo also slow peacemeal degranulation , exosome secretion , and selective degranulation-independent mediator secretion [17, 42].
The physiological role of mast cells in the skin is not well known. It appears that mast cells are essential effector cells in protecting the skin from, e.g., severe bacterial and parasite infections [113, 171] and from severe venom reactions after snake bites and insect stings . However, there is growing evidence suggesting that mast cells are also essentially involved in inflammatory pathomechanisms of chronic diseases. This is understandable since mast cells can produce, store and release upon activation numerous mediators that can potentially affect the immune system and promote inflammation. These can be categorised into preformed mediators and newly-synthesized mediators. Preformed mediators stored in the secretory granules include, e.g., tryptase, chymase, carboxypeptidase, a cathepsin G-like proteinase, histamine, heparin proteoglycan, chondroitin sulphate E, acidic hydrolases, and various cytokines and growth factors. Newly synthesized mediators include prostaglandin D2, leukotriene C4 and varying amounts of other lipid-derived mediators. In addition, mast cells can be induced to de novo synthesis of a range of cytokines, chemokines, growth factors, and cell membrane molecules, such as chemokine receptors, receptor ligands, toll-like receptors, MHC class II molecules and co-stimulatory molecules, all of which are crucial in inducing an immune and inflammatory reaction [29, 46, 83, 160].
Psoriasis is characterized by chronic inflammation and epidermal hyperplasia where numerous inflammatory and immune cells function simultaneously, and the involvement of mast cells in this complex cellular and molecular network has become obvious. During the last decade many reports have shown that mast cells are not just bystander cells in inflammatory diseases but rather crucial for immunity as reported in a news article entitled “Mast cells show their might” (Science 2007; 317:614–616). Therefore, in this review the role of mast cells in the development of psoriatic lesion is discussed.
Mast cells in the psoriatic lesion
The presence of numerous mast cells in the psoriatic lesion, especially in the subepidermal connective tissue, was recognized many decades ago as reviewed by Cox . After these observations, several other investigators confirmed that the number of mast cells is increased in the upper dermis of the psoriatic lesion [59, 60, 80, 189], and it is the MCTC type of mast cells that are increased in number in the uppermost dermis of the psoriatic lesion . However, this change is not specific to psoriasis since MCTC mast cells are similarly increased in number in superficial spreading basal cell carcinoma , and mast cells in chronic leg ulcers are predominantly of this MCTC type . Even though mast cells can only rarely be seen in the epidermis of the healthy-looking skin, the presence of mast cells in close association with the epidermis of the psoriatic lesion is a typical feature and sometimes mast cells are present even inside the lesional epidermis . Intraepidermal mast cells, however, have also been associated with other chronic inflammatory skin diseases with epidermal proliferation, such as pemphigus vegetans, dermatitis vegetans and pseudoepitheliomatous diseases  as well as chronic leg ulcers .
The appearance of degranulated mast cells is a typical feature in the early developing psoriatic lesion as reported by several investigators [13, 27, 168]. Schubert and Christophers  treated psoriatic plaques with a potent steroid under occlusion for 6–10 days and then allowed a relapse to occur. During the developing relapse, the earliest changes were found to be endothelial swelling followed by the appearance of degranulated mast cells around the postcapillary venules, and hours later activated macrophages were present. Brody  examined a series of biopsies taken from the early phases of acute eruptive guttate psoriasis and demonstrated different types of mast cell degranulation in the psoriatic lesion. He reported that degranulated mast cells are an early and constant feature in the evolution of acute guttate psoriasis. Toruniowa and Jablonska  induced a Köbner reaction in the healthy-looking psoriatic skin and analyzed biopsies taken from the developing lesion. A significant increase in mast cell numbers took place on day 4, when compared to mast cell numbers in control skin, and this increase peaked on day 14 simultaneously with the appearance of the Köbner′s phenomenon. These findings suggest that mast cells residing in the dermis are among the earliest cells which show activation in the initial phases leading to mast cell degranulation and increase in cell number. The increased interstitial histamine concentration in the psoriatic plaque, as measured with the microdialysis technique, also suggests elevated mast cell activity and degranulation [97, 147].
Regulation of mast cell number and activity in the psoriatic skin
The mechanisms for the increase in mast cell numbers in chronic inflammation, such as in psoriasis, is not clear. The stem cell factor (SCF) (Kit ligand) is a well-known and essential factor for the growth, migration, activation and survival of human mast cells developed and cultured in vitro [93, 123, 133, 134] or isolated from skin specimens [45, 85]. SCF injected subcutaneously induces anaphylactic-like activation and degranulation of dermal mast cells in situ followed by inflammatory cell recruitment such as neutrophils, eosinophils and basophils . Prolonged subcutaneous injection of SCF for up to 14 days results in mast cell and melanocyte hyperplasia in the skin . Mature mast cells isolated from human skin specimens can be cultured in serum-free medium in the presence of SCF, and these cells even proliferate and maintain their proteinase phenotype in such conditions  suggesting that a mature MCTC mast cell is capable of proliferation in response to SCF during a disease process. In a mouse model of skin inflammation, Kit expression is essential for mast cell accumulation which further supports the importance of SCF–Kit interaction . Another explanation for the increased mast cell numbers may be that the survival of resting and activated mast cells is enhanced due to survival gene expression or other mechanisms [148, 210] and/or mast cells and their hematopoietic progenitors are recruited from the circulation. For example, mast cells express at least nine chemokine receptors  and SCF, TGF-β, RANTES and stromal cell-derived factor-1α have efficiently induced migration of human mast cells in vitro and therefore they are potent candidates for chemoattraction of mast cells to the tissue site [82, 132, 139]. In addition to SCF, several other factors have been reported to modulate human mast cell development or survival in vitro, such as IL-3 to IL-6 , IL-9 , thrombopoietin , nerve growth factor , and endothelial cells .
The percentage of tryptase-positive mast cells immunopositive for different proteins in the upper dermis of the psoriatic skin based on the sequential double-staining technique
Number of subjects
46 ± 26%
88 ± 12%
33 ± 19%
72 ± 15%
60 ± 13%
87 ± 7%
6 ± 4%
17 ± 7%
11 ± 6%
14 ± 10%
44 ± 17%
67 ± 18%
15 ± 15%
16 ± 25%
8 ± 11%
33 ± 21%
(data compiled from references listed above)
Several effective drugs used in the treatment of psoriasis can affect the activity and/or number of mast cells. Low-strength anthralin therapy has been found to reduce the number of mast cells in the psoriatic plaque . The treatment with 0.05% clobetasol-17-propionate cream for up to 4 weeks inhibits allergen-induced wheal reaction and reduces markedly the number of mast cells in the skin . Corticosteroids may possibly inhibit the production of SCF in dermal cells leading to reduced survival of mast cells . However, a topical clobetasol-17-propionate treatment of psoriatic plaques for up to 3–4 weeks did not result in a decrease in mast cell number, nor did the 3- to 4-week treatment with bath-PUVA, and a longer treatment period appears to be required in order to deplete mast cell hyperplasia in the chronic psoriatic plaque . This resistance of mast cells to topical corticosteroid may be explained by highly increased SCF production in psoriasis [73, 212]. Retinoids, which are widely used in the treatment of psoriasis, have been reported to inhibit the development of human mast cells from progenitors in vitro [67, 92] but without reducing the number of mature mast cells in the normal human skin . Also, retinoids have been found to inhibit histamine release from concanavalin-A stimulated human mast cells isolated from the adenoid tissue . Thus, one therapeutic action of retinoids may be targeted to inhibition of mast cell activity as well as maturation of recruited progenitor mast cells entered to the tissue site. Cyclosporin A is also a potent immunomodulatory drug used in the treatment of psoriasis. However, besides its well-known inhibitory action on T cells cyclosporin A has long been known to inhibit histamine and LTC4 or PGD2 release from isolated human lung or skin mast cells [110, 177]. Cyclosporin A can also inhibit SCF-induced histamine release  as well as chemokine gene induction in human mast cells . Moreover, subcutaneous injections of cyclosporin A into asebia mice, which show epidermis hyperplasia and dermal inflammatory changes with elevated mast cell numbers, have been found to decrease mast cell numbers in the dermis . Thus, a part of the therapeutic effect of cyclosporin A in psoriasis may be attributed to the inhibition of mast cell activity and the decrease in mast cell numbers. In addition to potent immunomodulators, also H1- and H2-antihistamines have been tested in psoriatic patients [145, 147, 219]. Based on the skin microdialysis technique and histamine assays mast cells in the psoriatic lesion are hyperreactive. A prolonged high-dose ranitidine administration for up to 6 months to psoriatic patients appears to improve psoriatic skin disease and normalize mast cell hyperreactivity towards substance P and codeine injections, but without affecting mast cell numbers . However, a double-blind and placebo-controlled multicenter study could not reveal any significant therapeutic effect of ranitidine on psoriasis . In a double-blind controlled study, 10 mg cetirizine administered three times a day for 15 days significantly reduced the number of tryptase-positive mast cells in the lesional skin, resulted in some improvement in the psoriatic area and severity index (PASI) score and erythema , and reduced the expression of adhesion molecules ICAM-1, ICAM-3 and LFA-1 . Nevertheless, an inhibitor of mast cell degranulation, Tiacrilast, administered topically as a 3% hydrogel for 4 weeks did not produce any significant improvement in the psoriatic plaques .
Mast cell serine proteinases in psoriasis
The major protein in mast cell granules, tryptase, is a trypsin-like serine proteinase with 4 subunits and catalytic sites and it shows heterogeneity in respect to glycosylated side chains and can follow sigmoidal reaction kinetics typical for allosteric enzymes [58, 64, 144]. The gene locus of human tryptase in the chromosome 16p13.3 is far more complex than originally thought: there are two major forms, α- and β-tryptases, which can be divided further into α1- and α2-forms and β1-, β2- and β3-forms. Furthermore, additional forms include γ-, ε- and δ-tryptases [19, 20, 199, 207, 214] of which γ-tryptase is a functionally and structurally distinct membrane-bound enzyme in mast cells that can efficiently be inactivated by α1-proteinase inhibitor [208, 214], ε-tryptase is expressed in airway epithelial cells but apparently not markedly in mast cells , and δ-tryptase has marked amino acid similarity with α- and β-tryptases but is truncated by 40 amino acids in the carboxyterminal end and cannot hydrolyze the canonical tryptase substrate N-p-tosyl-Gly-Pro-Lys-p-nitronilide . There is, however, a report showing that δ-tryptase gene is a pseudogene without transcription in human mast cells in vitro . Nevertheless, inactive α- and β-protryptases are constitutively secreted and active β-tryptase is the principal enzyme that is packed and stored in large quantities in the secretory granules of mast cells [64, 169]. Human β-tryptase is a ring-like tetramer with four active centers facing towards the central oval pore . This feature explains the high selectivity of the catalytic action of β-tryptase on different proteins and peptides since a protein has to be in a proper orientation and has to possess a suitable conformation to reach the buried active centers. Indeed, β-tryptase is resistant to common endogenous protease inhibitors and, in fact, there are no known physiologic inhibitors for tetrameric β-tryptase. Furthermore, heparin glycosaminoglycan is needed to stabilize β-tryptase as an active tetramer and degradation of heparin or replacement of β-tryptase from heparin by another protein or molecule can be physiological control mechanisms—in the absence of heparin tetrameric β-tryptase dissociates reversibly into inactive-destabilized tetramer and further to inactive monomers and monomers with limited activity [57, 64, 164]. In accordance with this resistance to protease inhibitors, tryptase has been found to be catalytically active detected on skin sections from inflamed skin and only occasional tryptase immunoreactive cells without enzyme activity have been detected [31, 61, 86].
In contrast to tryptase, only a single α-form of human chymase gene has been found in the protease gene locus in chromosome 14q11.1–11.2 next to cathepsin G gene. Mature chymase is a 30-kDa monomeric and chymotryptic serine proteinase that is processed from prochymase by dipeptidyl peptidase I and is stored in high quantities in mast cell secretory granules [19, 20]. Even though both tryptase and chymase bind efficiently to heparin, chymase-heparin proteoglycan complexes (M r 400–560 kDa) are larger in size than tryptase–heparin proteoglycan complexes (M r 200–250 kDa) . Furthermore, the subcellular localization of chymase differs from that of tryptase within the secretory granules—chymase and cathepsin G are preferentially localized in more electron-dense amorphous granule subregions whereas tryptase resides in less electron-dense crystalline subregions . These differences may explain the findings that tryptase protein diffuses away together with histamine whereas chymase protein tends to remain at the site of mast cells in human skin biopsies cultured and activated ex vivo [95, 96]. Thus, tryptase can diffuse through the extracellular matrix and reach more distant target sites in the skin whereas chymase functions rather locally. However, different forms of chymase have been described in respect to the binding affinity to heparin–agarose column, possibly owing to differences in glycosylation, and therefore chymase form with low binding affinity may diffuse more easily after release than the high-affinity form . Although chymase binds efficiently to heparin, the enzyme remains active also in the absence of heparin, a feature which differs from that of tryptase. However, heparin can modulate the interaction of chymase with cells, protein substrates and inhibitors [74, 159, 195]. A clearly distinct feature of chymase is that it can be inactivated by a range of endogenous protease inhibitors, including α1-proteinase inhibitor (α1-PI), α1-antichymotrypsin (α1-AC) [114, 161] α2-macroglobulin , secretory leukocyte proteinase inhibitor  and squamous cell carcinoma antigen-2 . However, chymase can also effectively degrade α1-PI and α1-AC and, in fact, this reaction proceeds faster than the inhibition of chymase by these inhibitors  suggesting a complex regulation between the proteinase and its inhibitors. Thus, the activity of chymase is controlled by protease inhibitors normally present in inflamed tissue whereas tryptase may have a prolonged action time after being released from mast cells. In support for this, inactivation of chymase has been shown to take place rapidly in the skin, e.g., in the allergic skin wheal reaction , cutaneous herpes zoster lesions  and skin organ cultures [95, 96], as detected enzyme- and immunohistochemically.
Mast cells contain large amounts of preformed β-tryptase and α-chymase and their possible function in the inflammatory process cannot be ignored . Even though the pathophysiological significance of β-tryptase in vivo is not clear, there are several experimental findings which suggest an implication for the development of psoriatic inflammation. Tryptase has been reported to induce vascular tube formation of endothelial cells in vitro suggesting a role in angiogenesis . In addition, tryptase can stimulate human endothelial cells for increased production of MCP-1 and IL-8 [24, 25, 91], chemokines which can attract neutrophils and many other cells of the immune system. Furthermore, tryptase activates peripheral blood mononuclear cells for TNF-α, IL-6 and IL-1β production , human neutrophils for IL-8 secretion , and human T cells for IL-6 release . Activation of these cells is very likely mediated via activation of the proteinase-activated receptor-2 (PAR-2) by tryptase. In experimental animals, tryptase injected into the skin of guinea pigs induced accumulation of neutrophils and eosinophils within 6 h and later also other cells of the immune system . In addition, also human skin mast cells express the PAR-2 receptor and can thus be activated by tryptase and receptor agonists . Furthermore, the percentage of mast cells showing PAR-2 immunoreactivity is increased in the psoriatic lesion (Carvalho et al., unpublished results). This suggests that tryptase released from mast cells can induce mast cell activation in an autocrine or paracrine fashion leading to potentiation of mast cell-mediated inflammation in psoriasis (Fig. 1). Besides its proinflammatory function tryptase may also control cell recruitment to some extent since it is capable of cleaving eotaxin and RANTES, but not GM-CSF, MCP-1, IL-8, TGF-β1 and VEGF . Tryptase-positive mast cells are increased in number close to the psoriatic epidermis and, therefore, the enzyme may well affect the basement membrane and basal keratinocytes. Indeed, tryptase has been found to degrade fibronectin in the basement membrane zone . In addition, tryptase can activate metalloproteinases, such as MMP-3 and MMP-9 [55, 75, 102], and pro-urokinase , or it can act as a gelatinase . Thus, tryptase can modify the extracellular matrix and basement membrane directly or indirectly in order to make space for T cells and neutrophils for migrating into the epidermis. Tryptase may also affect keratinocytes directly via activation of PAR-2 on their surface . Interestingly, in vitro activation of keratinocytes by PAR-2 agonists results in increased keratinocyte phagocytosis , ICAM-1 expression  and IL-8 release . Furthermore, tryptase has been reported to stimulate bronchial epithelial cells to release IL-8 and express ICAM-1 .
Like tryptase, chymase also can have immunoregulatory activity which may be related to the psoriatic immunopathogenesis, though the effect is dependent on the availability of enzymatically active chymase in an environment rich in protease inhibitors. Human chymase injected into the skin of guinea pigs stimulates the accumulation of neutrophils and eosinophils at the injection sites . In support for this, chymase also stimulates chemotaxis of monocytes, and neutrophils and chemokinesis of lymphocytes in vitro . Interestingly, chymase has been reported to stimulate an eosinophilic cell line EoL-1 for IL-8 synthesis and secretion in vitro . Chymase can also activate the precursors of IL-1 family, pro-IL-1β and pro-IL-18, to their corresponding active forms IL-1β and IL-18 [120, 140]. In addition, it can generate 31-amino acid endothelin-1 that acts as a potent chemoattractant for neutrophils and monocytes , and therefore chymase may promote inflammation indirectly. Another indirect mechanism could be the cleavage of membrane-bound SCF on stromal cells by chymase, which results in the release of soluble and bioactive SCF. Thus, the released SCF may have an effect on mast cell activity and growth during inflammation . Furthermore, chymase has been shown to release matrix-bound latent TGF-β1 complexes in vitro and therefore it may participate in the regulation of the immune system and matrix accumulation . Like tryptase, also chymase may have controlling effects on inflammation since chymase, as well as cathepsin G, can efficiently degrade IL-6 and IL-13, and also IL-5 and TNF-α to some extent, but not GM-CSF . In addition, chymase can degrade eotaxin, but not RANTES, MCP-1 or IL-8 . Chymase is a potent enzyme and has to be controlled by proteinase inhibitors since it can affect the epidermis and cleave epidermal keratinocytes from the substratum leading to keratinocyte apoptosis  (Diaconu et al., unpublished results). It can also degrade fibronectin  and activate procollagenase .
Interactions between mast cells and sensory nerves in psoriasis
The involvement of stress in cutaneous neurogenic inflammation and sensory nerve–mast cell interaction has been suggested in several studies [6, 143]. Neuropeptides SP and VIP can efficiently activate human skin mast cells for rapid histamine release in situ  and in vitro . Furthermore, in vitro stimulation of human mast cells with SP and VIP results in the production of proinflammatory mediators, such as MCP-1, IL-8, RANTES, TNF-α, IL-3 and GM-CSF [48, 100]. However, it appears that the amount of neuropeptides in normal skin is not sufficient for inducing marked mast cell activation, a conclusion based on the findings that capsaicin can induce only weak, if any, histamine release in skin ex vivo and in situ [71, 184]. Nevertheless, the case is different in chronic skin inflammation, since increased SP-positive nerve fibers  and increased tissue levels of SP and VIP  have been measured in the psoriatic lesion, though there are reports showing only increased VIP levels in the lesional skin [5, 149]. Accordingly, topical capsaicin cream has been reported to be effective in the treatment of psoriatic plaques  and the mechanism for this beneficial effect may be that prolonged capsaicin treatment depletes neuropeptides from nerves . Further support for the role of nerves is provided by clinical cases where nerve damage has been reported to clear chronic psoriasis and the lesions reappeared with recovery of cutaneous sensation .
In histochemical studies, morphological contacts between neurofilament-positive sensory nerves and tryptase-positive mast cells are more frequent in the developing and mature psoriatic lesions than in lesion-free skin, normal skin and lichen planus skin [129, 130]. This indicates that the morphological basis exists for nerve–mast cell interaction in psoriasis as illustrated in Fig 1. Furthermore, the contacts between SP- and CGRP-positive fibers and tryptase-positive mast cells are increased in the uppermost dermis of the psoriatic lesion as well, whereas the contacts between VIP-positive fibers and tryptase-positive mast cells were less frequent . This suggests that especially SP- and CGRP-positive nerves interact with mast cells in psoriasis. However, nerve–mast cell interaction and neuropeptides SP and CGRP have also been linked to atopic and nummular eczema lesions .
The regulatory system between nerves and mast cells in psoriasis is evidently more complex than thought. For example, higher doses of topical capsaicin are needed to induce inflammation on the forearm skin of psoriatic patients than on that of healthy controls or patients with systemic scleroderma . Even though the basal histamine concentration in the psoriatic lesion is elevated, as measured with the microdialysis technique, the topical anaesthesia with lignocaine-prilocaine cream could not reduce histamine level but rather increased it together with a decrease in blood flow . One possible explanation for this may be given by the state of exhaustion in the neurogenic system in chronic psoriasis. In addition, several proteases can degrade neuropeptides and control their effects. Dipeptidyl peptidase IV (CD26) is present in the psoriatic lesion and can cleave dipeptides from the aminoterminal end of SP [136, 200]. Neutral endopeptidase can terminate SP-induced skin inflammation , and stromelysin can cleave SP . The precursor for stromelysin, prostromelysin, can be activated by mast cell tryptase . Furthermore, tryptase can cleave and inactivate VIP and CGRP, but not SP and neurokinins [43, 182, 193]. Chymase is able to degrade both SP and VIP [18, 43] (Fig. 1), and mast cell carboxypeptidase releases carboxyterminal leucine from Leu5-enkephalin, kinetensin and neurotensin . Besides controlling the neuropeptide-mediated inflammation, tryptase may also promote neurogenic inflammation by activating the PAR-2 receptor on nerves resulting in SP and CGRP release . However, it is not known whether the PAR-2 receptor is upregulated in nerves in psoriasis like it is in atopic dermatitis lesions . Interestingly, PAR-2 appears to be closely related to the capsaicin receptor, transient receptor potential vanilloid receptor-1 (TRPV-1). PAR-2 activation sensitizes TRPV-1 leading to enhancement in capsaicin-induced SP and CGRP release . Furthermore, TRPV-1 has been localized to cutaneous SP-positive nerve fibers and mast cells . Histamine is also a mediator that has traditionally been known to stimulate axon reflex, neurogenic flare and itch in the skin. In addition, CGRP has been shown to be released in the flare area elicited by histamine iontophoresis . The effect of histamine on cutaneous nerves and flare can be controlled by topical anaesthetics  and by nedocromil sodium and loop diuretics, frusemide and bumetadine, drugs which appear to inhibit the Na+/K+/2Cl− cotransporter in the sensory nerve membranes . Hence, both tryptase and histamine released from mast cells can act together and stimulate surrounding nerves and neurogenic inflammation (Fig. 1), but the regulatory mechanisms are far from complete understanding in chronic skin inflammation.
In addition to neuropeptides, also neurotrophins, such as nerve growth factor (NGF), and its high-affinity receptor (TrkA) have been suggested to play a role in psoriasis, especially in pruritic psoriasis [21, 127], and NGF may be involved in the growth of nerves in the psoriatic lesion. Nevertheless, human mast cells have previously been shown to express NGF and TrkA, and release NGF upon mast cell activation [135, 209]. Therefore, NGF released from mast cells is one possible mediator acting as an autocrine growth factor for mast cells and also interacting with nerves. Whether NGF is actually expressed in mast cells in the psoriatic lesion needs to be clarified. The neuropeptide, α-melanocyte-stimulating hormone, evidently acts as an anti-inflammatory mediator and a potent modulator of the immune system . Thus, α-melanocyte-stimulating hormone may have potential in the regulation of psoriatic inflammation but this needs to be studied further. Interestingly, α-melanocyte-stimulating hormone can sensitively, at picomolar concentrations, activate skin mast cells for histamine release .
Mast cells in relation to cells of the immune system in psoriasis
The modern aspects of the immunopathogenesis of psoriasis have been presented in recent review articles [104, 157]. Evidently, different subsets of T cells, antigen presenting cells, neutrophils and keratinocytes are all essentially involved in the immunoregulation in psoriasis. It is, however, noteworthy that mast cells can express a range of chemokines, cytokines and cell surface proteins which link them intimately to the innate and acquired immune system [29, 46]. It has long been known that the psoriatic lesion is characterized by a Th1 type of cell infiltrates . However, after recent discovery of the novel Th17 cell type producing IL-17 and IL-22 it was realized that both T cell types, Th1 and Th17, are present in the psoriatic lesion, and Th17 cells have paramount importance in the lesion development [105, 108, 218]. Similarly to Th1 cells, also Th17 cells are induced to differentiate from naïve T cells. However, the induction of Th17 cells is regulated by IL-6 and TGF-β, and IL-23 is required for the development and maintenance of the Th17 lineage. Nevertheless, numerous other factors have an impact on Th17 cell development, too [22, 218]. The defective function of regulatory T cells can also have an impact on the development of psoriatic inflammation since these T cells in psoriatic patients cannot efficiently inhibit effector T cell activity .
The close interaction of mast cells with T cells has been shown in several studies and mast cells can express soluble factors, cell surface molecules and co-stimulatory molecules which can activate different subsets of T cells as reviewed by Mekori and Metcalfe , and Sayed and Brown . In co-cultures of mouse mast cells and T cells, both cell-cell proximity and soluble factors, especially TNF, can contribute to T cell proliferation, and mast cells can activate multiple T cell subsets, such as CD4+, CD8+, Th1, Th2, Tc1, Tc2, γδTCR+ and CD4+CD62L− T cells . The same group demonstrated later that this mast cell-induced enhancement of T cell activation is significantly dependent on the secretion of soluble TNF and direct cell–cell interactions between mast cell OX40 ligand and T cell OX40 receptor . Similarly, human mast cells have been shown to activate T cells in vitro by direct cross-talk between OX40 ligand on mast cells and OX40 receptor on T cells . Mast cells have previously been shown to express another member of the TNF superfamily on the cell membrane, CD30 ligand, that is functional and can activate lymphoma cell lines in vitro . Mast cells can act as antigen-presenting cells to T cells in an MHC class I- and MHC class II-dependent manner in vitro [46, 116, 160], and human mast cells have been shown to express MHC class II and to present staphylococcal superantigens to CD4+ T cell hybridomas leading to T cell activation . Furthermore, human cord blood-derived mast cells exposed to different gram-negative and gram-positive bacteria in vitro have been shown to bind and phagocytose several bacteria strains, such as Staphylococcus aureus, leading to death of bacteria and TNF-α secretion from mast cells . In addition to antigen presentation, mouse and human mast cells cultured in vitro have been shown to express CD80 and CD86 that are needed as co-stimulatory signals for efficient T cell activation [44, 152]. In fact, human mast cells express a range of different CD antigens and ICAM-1, VLA-4, Mac-1 and to some extent also LFA-1 [47, 77]. The adhesion molecules on mast cells, especially ICAM-1, can interact with T cells leading to T cell activation, but on the other hand mast cells can also be activated upon cell–cell contact with activated T cells resulting in enhanced mast cell degranulation, migration and adhesion to extracellular matrix and endothelial cell ligands . In recent experiments with mice, mast cells and their TNF have been reported to enhance antigen- and Th17 cell-dependent development of a neutrophil-rich inflammatory response in airways  suggesting that mast cells can promote Th17 cell-dependent inflammation in vivo. In addition to local actions in the skin previous findings in mice show that as a result of dinitrofluorobenzene-induced contact hypersensitivity mast cells are activated and they migrate to draining lymph nodes wherein they can mediate T cell recruitment . Further support for mast cell-induced lymph node activation and hypertrophy is provided by the study showing that activation of mast cells in mouse footpad by injection of Escherichia coli bacteria or compound 48/80 results in rapid draining of mast cell-derived preformed TNF to lymph nodes wherein it induces hypertrophy and recruitment of circulating T cells . Further, Jawdat et al.  have demonstrated that the mast cell-dependent lymph node activation in mice can be mast cell TNF-dependent in an allergic response or TNF-independent in a response to bacterial peptidoglycan injection.
Different types of professional antigen presenting cells, such as Langerhans cells, macrophages, plasmacytoid dendritic cells and myeloid dendritic cells, have been presented to be essentially involved in the initiation of the psoriatic lesion and regulation of T cell responses [104, 157]. The juxtaposition of dendritic cells and mast cells in the upper dermis, however, suggests that these cells have to be in close relationship between each other. Indeed, experimental studies have shown that mast cells are capable of stimulating Langerhans cells and dendritic cells. For example, mast cell activation in mouse ear pinna induced by an IgE-dependent mechanism or by a bacterial peptidoglycan injection is crucial in the stimulation of Langerhans cell migration to draining lymph nodes . Similarly, mast cells and their TNF are needed for optimal migration of dendritic cells to local lymph nodes after induction of contact hypersensitivity in mouse ear pinna with FITC . One potent mechanism for dendritic cell activation may be via mast cell-derived exosomes. These vesicles from mouse bone marrow-derived mast cells, located in the endocytic pathway and capable of harboring exogenous antigens, have been shown to stimulate and maturate mouse dendritic cells leading to strongly enhanced antigen presenting activity to T cells . In co-cultures of human in vitro-developed mast cells and monocyte-derived dendritic cells, mast cells activated by FcεRI cross-linking induced maturation of dendritic cells which in turn polarized naïve T cells towards the Th2 lineage, an effect largely dependent on secreted histamine and contacts between mast cells and dendritic cells . In fact, histamine has previously been proved effective in dendritic cell maturation and activation. For example, histamine receptor H1 and H3 stimulation on dendritic cells results in proinflammatory cytokine production, Th1 priming and increased antigen presenting activity, whereas H2 stimulation leads to IL-10 induction and Th2 or tolerance priming . Nevertheless, it is currently not known how these interactions between mast cells and antigen presenting cells are actually working in the psoriatic skin or in mouse models of psoriasis. Histamine has also been shown to stimulate human keratinocytes through H1 receptor for increased expression of IL-6, IL-8, GM-CSF, TNF-α, ICAM-1 and different matrix metalloproteinases, and histamine even augmented the TNF-α-induced GM-CSF, IL-8 and IL-6 expression . In addition, histamine can augment through H1 receptor the interferon-γ-induced expression of MHC class I, ICAM-1, GM-CSF, CCL-5/RANTES, CCL-2/MCP-1, CCL-20/MIP-3α and CXCL-10/IP-10 in human keratinocytes .
In addition to the proinflammatory role of mast cells, these cells can also be involved in the induction of tolerance or immunosuppression. For example, mast cells are essentially involved in the induction of regulatory T cell-dependent peripheral tolerance in a mouse model of skin allografts, a reaction which appears to be related to the production of IL-9, a mast cell growth and activation factor, from activated regulatory T cells . In a mouse model of contact hypersensitivity using prolonged monitoring for up to 15 days after challenge, mast cells have been shown, unexpectedly, to be able to limit the inflammatory skin reaction by producing the immunosuppressive cytokine IL-10. In the same study, mast cells were also able to attenuate the mouse skin reactions induced by multiple challenges with ultraviolet irradiation for up to 30 days . Furthermore, immunosuppression of mouse skin contact hypersensitivity reaction by ultraviolet irradiation has been reported to be dependent on CXCR4-positive mast cells which migrate from the skin to the B cell area of draining lymph node as a result of chemoattraction by CXCL-12 (stromal cell-derived factor-1α) . In another mouse skin model, mast cells and concomitant IL-10 expression in lymph nodes have been reported to be critical intermediaries in the mosquito bite-induced suppression of delayed-type hypersensitivity reaction . Hence, there are open questions like in which immunological environment mast cells can promote inflammation and in which conditions they can suppress it, and how these experimental results can be linked to human skin diseases. Human mast cells have been shown to express IL-10  and therefore this cytokine is one good candidate for mediating immunosuppressive effects of mast cells in some disease states. Interestingly, IL-10 released from human mast cells can inhibit also mast cell function in an autocrine or paracrine fashion .
Once dismissed as “allergy cells” the current knowledge now indicates that mast cells have an essential role in the immunopathological mechanisms of chronic inflammatory diseases. Mast cells can be activated by a range of different mechanisms and they can synthesize and release rapidly or secrete slowly potent preformed and newly synthesized mediators as well as express cell surface ligands and receptors, molecules which can be proinflammatory or induce immunosuppression. Mast cells can also recruit the cells of the immune system, such as T cells, neutrophils and eosinophils, to the site of developing skin lesion, and they are able to affect keratinocytes, endothelial cells and sensory nerves. Mast cells can migrate to the draining lymph node and thereby affect distant immune cells, or they can stimulate the maturation of Langerhans cells and dendritic cells and their migration to lymph nodes. These cellular interactions are crucial for the development of skin inflammation, such as in the psoriatic lesion, where mast cells are activated early in the lesion development followed by the increase in MCTC mast cell number and in morphological contacts between sensory nerves and mast cells. In addition, mast cells show critical plasticity in the expression of cytokines and other TNF family ligands, a feature which strongly suggests that mast cells are involved in the immunoregulation in psoriasis. In the chronic psoriatic lesion, mast cells accumulated in the upper dermis are in the state of activation and they constantly produce proinflammatory mediators which promote perpetuation of the lesion. Several potent drugs used for the treatment of psoriasis can affect also mast cell activity and/or cell number, such as cyclosporin A, corticosteroids, retinoids and even new biological TNF-α or LFA-1 (CD11a/CD18) blocking drugs. Further studies are needed to clarify in more detail which mediators and cell surface molecules are, in fact, expressed in mast cells in psoriasis compared to other skin diseases and their relation to clinical severity. An important question to be answered is when and how mast cell functions as a proinflammatory cell and when and how as an immunosuppressive cell. It is also possible that mast cells attempt to maintain skin homeostasis by preventing excessive inflammation and epidermal growth but fail in this task. These aspects need to be analyzed also in animal models of psoriasis.
The authors wish to thank the Marie Curie Mobility Actions for Early Stage Researcher by the sixth framework of the EU commission (project no. 504926), Kuopio University Hospital, Abbott, Finland, and MacNet (Mast Cell Network) funded by Swedish Research Council. Ms Marjukka Aronen is thanked for illustrative artwork of figures.
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