36 Neuropeptides and Atopic Eczema

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36 Neuropeptides and Atopic Eczema

F. Fantini, C. Pincelli

36.1 Neuropeptides and the Skin

A neural network consisting of dense ramifications of nerve branches permeates all skin layers, and has an intimate relationship with every cutaneous appendage, cell, and functional structure. Peripheral nerves are traditionally divided into sensory (afferent) and autonomic (efferent), according to their main neurophysio- logical activities. More than a century ago a series of investigational studies led to the concept that sensory nerves could also exert a peripheral „efferent“ activity, i.e., sensory fibers play a role in the cutaneous inflammation induced by various experimental stimuli. In fact, a chemical or anatomical impairment of sensory fibers reduces the peripheral inflammatory responses (reviewed in [1]). The „axon reflex“ model, i.e., the antidromic activation of peripheral sensory branches, can explain this apparently paradoxical activity of sensory fibers [2]. An increasing body of evidence indi- cates that neuropeptides (NP) are the chemical mediators of the neurogenic inflammatory responses, and in recent years much investigational effort has been made to clarify the role of NP in inflammatory dermatoses and other kinds of peripheral inflammation. Moreover, many other pathophysiological activities, including local immunity, cell growth, and differentiation, as well as tissue repair have now been demonstrated to be modulated by nerve-skin interactions.

NP are peptides which are synthesized in the body of both central and peripheral neurons, peripherally transported along the axons, and then released in the peripheral tissues to act as neurotransmitters or mod- ulators [3]. After their release, NP are degraded by tissue and plasmatic peptidases (neutral endopeptidases, angiotensin-converting enzyme). NP can coexist in the same nerve fibers and be coreleased both with other

„classic“ neurotransmitters (acetylcholine, norepi- nephrine) and with other NP. Several NP have been found in normal and inflamed human skin, mostly in the C-type unmyelinated fibers [4, 5]. They are particu- larly distributed around vascular and adnexal struc- tures, consistently with their presence both in sensory and in autonomic nerves. NP-reactive nerve endings have been demonstrated to take intimate contact with cells crucial to the immuno-inflammatory reactions in the skin (mast cells, Langerhans cells) [6, 7]. The distribution of NP-reactive fibers in the skin provides the anatomic support for the modulatory effects that NP exert on cutaneous immune inflammation, particular- ly on the vasodynamic and cell-mediated components.

Several NP, such as substance P (SP), neurokinin A (NKA), and vasoactive intestinal peptide (VIP), may induce vasodilatation and plasma extravasation acting either directly on the cutaneous microvasculature or indirectly, through degranulation of mast cells [8 – 10].

Calcitonin gene-related peptide (CGRP) induces a long-lasting erythema [11]. Neuropeptide Y (NPY), contained in adrenergic autonomic fibers, evokes vaso- constriction [12]. Specific membrane receptors mediate the modulation of immune and inflammatory cell activities by NP [13]. In different experimental models, NP modulate lymphocyte proliferation, leukocyte traf- ficking and phagocytosis, adhesion molecule expression, cytokine release, and immunoglobulin production. NP exert trophic activities on different cutaneous cells, such as keratinocytes, fibroblasts, smooth mus- cle, and endothelial cells. Interestingly Langerhans cells, crucial to the pathomechanisms of AE lesions, have shown an intimate association with CGRP-containing epidermal nerve endings, and CGRP was demonstrated to inhibit the Langerhans cell antigen pre- sentation activity [7]. CGRP also acts as inhibitor in other cell-mediated immune responses [14, 15].

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36.2 Role of Neuropeptides in Atopic Eczema

A participation of neural mechanisms in the pathogen- esis of atopic eczema (AE) has been hypothesized for a long time, on the basis of clinical observations. Sym- metry and exacerbation of the lesions after psychologi- cal stress can be explained with the involvement of nerve fibers. Damage of peripheral nerve fibers can be followed by a localized disappearance of AE lesions in the distribution area of the impaired fibers [16]. Simi- lar anecdotal reports, actually, are not limited to eczema [17], but have also been made for other inflammatory dermatoses, such as psoriasis [18] and seborrheic dermatitis [19]. More unique to AE, instead, are basic clinical features which could be induced by a direct activation of cutaneous nerves: the acute erythematous rash and the itch.

The rash, a common sign in AE patients in response to various stimuli acting either locally or systemically, can be interpreted as the „clinical translation“ of the erythematous flare in the axon reflex experimental model. In the classic experiment, a triple response was induced in the skin after activation of sensory receptors by peripheral noxious stimuli [20]. The flare was generated via the recruitment of adjacent sensory fibers, extending the erythematous response beyond the site of the initial damage. The dynamics of the acute rash in AE patients, which often is a prelude to the re- exacerbation of subacute/chronic lesions, fit well with a neurogenic mechanism responsible for the initial vasodynamic response, which is followed by a cascade of subsequent cellular events.

Itch is a cardinal symptom both in the acute and in the chronic lesions of AE. Regardless of the fine mechanisms of itch induction, still largely hypothetical, there is no doubt that the itch sensation involves the activation of a class of cutaneous sensory fibers. Recent experimental data indicate that the itch sensation is encoded by a specific subset of unmyelinated sensory C-fibers (polymod- al receptors) [21]. This is strong, direct evidence that cutaneous NP-containing nerve fibers are activated in the course of AE. Pruritogenic substances (e.g., mechani- cal, thermal, chemical, allergenic stimuli, which are all potentially pruritogenic in AE) activate specific nerve terminals to integrate the itch sensation at a central level.

At the same time, through the axon reflex pathways, the same factors that induce the itch sensation could also induce a peripheral neuropeptidergic efferent response.

Different experimental approaches have been tried in recent years to support these suggestive clinical evi- dences. For example, anatomical and/or biochemical variations in the cutaneous nerve fibers and/or NP content could be assumed as an indirect proof of a neurogenic involvement in AE. The density of nerve fibers in chronic lesional skin of AE (prurigo and lichenified lesions), as revealed by immunohistochemistry with pan-neural markers, is consistently increased [22 – 24].

Electron microscopy reveals normal cutaneous free nerve endings, but some ultrastructural features in AE lesions, such as bulging of axons, increased number of mitochondria and loss of Schwann cells have been related to functional activation [23]. Several semiquan- titative immunohistochemical evaluations of NP-specific cutaneous nerve fibers have been performed, both in lesional and in nonlesional skin of AE [25 – 29]. The results have not been consistent. The discrepancies between the different studies could be partly explained by the difficulty in standardizing the counting methods and by the extreme paucity of NP-reactive fibers. More- over, the results should be normalized for the epidermal/dermal thickness and for the anatomical site of biopsy. Radioimmunological quantitative evaluations of cutaneous NP, theoretically more accurate than fiber counting, have been performed on suction blister fluid and on tissue homogenates, both in lesional and in nonlesional skin [25, 27, 30]. More consistent results have been obtained: in general, the vasoactive intestinal peptide (VIP) is increased, while the substance P (SP) levels are either decreased or unchanged in lesional skin of AE as compared to nonlesional and normal skin. Caution is mandatory in the interpretation of these results: radioimmunological levels of NP reflect an instant description of a chronic process (generally the biopsies are taken on lichenified skin) and NP could intervene in selective, short-lived phases of the process; the metabolism of NP, once released from nerve endings, is rapid; the quantitative alterations can give only very indirect, rough evidence of the NP involvement (does an increase reflect an enhanced production, transport, storage, release, or a reduced catab- olism of NP?). The same criticism applies to the evalua- tion of plasmatic levels of NP in the course of AE: both SP and VIP levels were found to be increased in atopic sera, but a correlation with the clinical activity of AE could be demonstrated only for SP [31, 32]. Serum levels of q -endorphin is claimed to correlate well with AE activity [33, 34].

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Another in vivo approach evaluates the atopic skin response to the injection of NP or NP pharmacologic antagonists, assuming that altered responses could be interpreted as indirect evidence of a pathogenetic role [35 – 38]. The results, again, have not been consistent but, in general, a lowered reactivity of atopic skin after intradermal injection of NP (in particular SP and VIP) is apparent, and this has been explained as tachyphyla- xis [35].

In vitro approaches have investigated the modulatory activities of NP on selected functions of immune cells in atopic individuals. A unifying interpretation of these results is impossible, because of the differences in the experimental models; in particular, evidence of a specific modulation by NP on atopic immunologic pathways is still lacking. For example, SP enhances the production of both * -IFN and IL-4 – two crucial but antagonistic cytokines in AE – by atopic peripheral blood mononuclear leukocytes [39, 40]. VIP has no effect on cytokine release by AE leukocytes, or shows a nonspecific inhibitory action. It can be concluded that NP effects are not cytokine specific [39]. In another study SP promoted the proliferation of hapten-specific mononuclear cells, with a more selective alteration of cytokine profiles (upregulation of IL-10, downregula- tion of IL-5) [41]. A possible key for a specific activity could be found in the alteration of mononuclear cell NP-binding capabilities [42].

36.3 Neurotrophins and Atopic Eczema

In the context of the neuro-cutaneous interactions in AE, neurotrophins represent an emerging class of putative mediators. Neurotrophins, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3, 4, and 5 (NT-3, NT-4, NT-5) are a group of growth factors involved in the develop- ment, function, and survival of sympathetic and sensory neurons.

The traditional concept of neurotrophins, and in particular NGF, as target-derived and neuron-commit- ted molecules has been substantially expanded in recent years. In fact, much evidence points to a major modulatory role of neurotrophins in peripheral immune-inflammatory reactions [43, 44]. NGF is pro- duced and released by a variety of cutaneous and immune-inflammatory cells, such as keratinocytes

[45], mast cells [46], Th2 lymphocytes [47], and eosinophils [48], where it is co-stored with the major basic protein [49]. NT-4 is synthesized by human keratinocytes and NT-3 by dermal fibroblasts [50]. NGF release is stimulated by proinflammatory cytokines, such as IL-1 and TNF-[ , and its concentration is increased in peripheral inflammation [51]. Peripheral-released NGF is able to stimulate the synthesis and peripheral transport of NP by sensory neurons, and this could represent an intriguing explanation for the observed alterations of NP levels in peripheral inflammatory reactions [52 – 54]. Moreover, NGF is a powerful che- moattractant for leukocytes [55].

Few recent investigations focused on the role of neurotrophins in AE. Although data are sparse, this field seems promising for new physiopathologic insights and possible therapeutic approaches. The plasma levels of NGF are increased in AE and correlate signifi- cantly with the disease activity [31]. Different cell types, such as eosinophils, mast cells, Th2 lymphocytes, and cytokine-driven keratinocytes could be responsible for this overproduction of NGF.

More than one neurotrophin could be responsible for the increased innervation of AE lesions. NGF strongly influences the innervation density of the skin:

transgenic mice overexpressing NGF in the epidermis show an increased and abnormal innervation pattern in the skin [56]. Moreover, it is interesting to note that cutaneous NGF is increased in inflammatory dermatoses that show increased innervation, similarly to AE:

NGF levels are increased in lesional psoriatic skin [57];

an increase in NGF correlates to the neural hyperplasia in prurigo nodularis [58] and NGF mediates the nerve- fiber sprouting in the elicitation phase of human contact dermatitis [59]. Prurigo lesions of AE show an increased epidermal expression of NT-4 [50]. Interest- ingly, IFN-* , a crucial cytokine in AE, is a potent induc- er of biologically active NT-4 by human keratinocytes, both in vitro and in vivo, while it is ineffective on NT-3 [50].

Finally, NGF stimulates the proliferation of human normal keratinocytes. Since basal keratinocytes are a major source of NGF in human skin, an autocrine pro- liferative loop has been hypothesized [45, 60]. Against this background, it is tempting to attribute to this mechanism some features of chronic lesions of AE (prurigo and lichenified lesions) characterized by epidermal hyperproliferation.

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