There is no direct evidence of histamine playing a role in the pruritus associated with EB. Indeed, the weak or minimal response to standard antihistamines in the management of pruritus in case studies of EB [10, 11] appears to indicate either that the pathogenesis of pruritus is not histamine dependent or that a histamine receptor other than H1 is involved.

Proteases are enzymes that are essential to epidermal permeability barrier homeostasis. Serine proteases play a role in blister formation in EB [12]. In addition, they are central to the pathophysiology of pruritus. They are agonists of a crucial itch receptor, PAR-2, which shall be described below.

Protease-activated receptor-2 (PAR-2) plays a key role in epithelial inflammation. There is a suprabasal localisation of functional PAR-2 in the human epidermis [13]. PAR-2 receptors are expressed in cells active in scar formation including keratinocytes and fibroblasts [5]. In addition, PAR-2 receptors lie on nociceptive nerve endings and are one of several itch receptors that transmit the signal of pruritus [14]. PAR-2 receptors also interact synergistically with another important itch receptor, the transient receptor potential vanilloid type 1 (TRPV-1) [15]. PAR-2 is not only expressed in the peripheral nerves but also in the central nervous system. PAR-2 signalling in the periphery stimulates the release of neuropeptides from central nerve endings at the spinal cord level, thereby activating central itch receptors to transmit itch responses to the brain.

The above findings suggest that serine protease inhibitors or PAR-2 antagonists such as GB88 might be promising therapeutic tools for the management of itch in EB. Certainly, in terms of the overall management of EB, the idea of such therapies having a potential efficacy has been raised by several authors [16, 17].

Prostaglandins and leukotrienes are two classes of biologic substances produced by arachidonic acid that are released from cell membrane phospholipids [18]. Both prostaglandins and leukotrienes are produced by nearly all cells in the body in response to injury to the cell membrane [19]. Both prostaglandin E2 (PGE2) and leukotriene B4 (LB4) have chemotactic activity and attract leucocytes to the wound area. Both are pruritogenic [20]. LB4 is an agonist of the TRPV-1 receptor [4]. PGE2 is indirectly pruritogenic by being one of many inflammatory mediators that sensitises TRPV-1.

While the role of PGE2 in the pruritus of EB has not been studied, an argument may be made by an analogy with skin in the postburns environment. In burns patients there is an increase in PGE2 in hypertrophic-derived fibroblasts compared to normal matched dermal fibroblasts [21]. The presence of PGE2 contributes to postburns pruritus. Prostaglandin E2 is upregulated in the epidermis of patients with uraemic pruritus, almost certainly contributing to the pruritogenic milieu.

Leukotriene B4 is highly pruritogenic. The presence of LB4 has not been studied in EB. Nevertheless, it should be noted that, in one study, the fluid in a blister located on the top of a large EB nevus contained increased concentrations of both PGE2 and LB4 [22].

The role of substance P (SP) in the pathogenesis of pruritus in EB has not been studied. It is likely to be involved. SP is integral to the neurotransmission of the itch signal at afferent nerve endings. In addition, SP is released by those nerve endings back into the inflammatory milieu. It is an important messenger between skin cells in pruritus. SP receptors are expressed by both epidermal keratinocytes and dermal mast cells. SP is also a neurotransmitter at the NK-1 receptor at the dorsal horn in the spinal cord, one of several receptors that transmit the itch signal centrally.

Nerve growth factor (NGF) has several regulatory functions in cutaneous nocioception, cutaneous nerve development and reconstruction after injury. All these functions are relevant to EB. Stimulation of epidermal keratinocytes induces a release of NGF that act on nocioceptors, sensitising them to noxious stimuli including pruritogens. A key itch receptor that is sensitised by NGF is TRPV-1 [23]. In addition, NGF is transported along the axon towards the dorsal root ganglion to induce an upregulation of a variety of proteins involved in neuronal growth and sensitivity. This mechanism leads to altered peripheral nocioception to pruritus. In cutaneous inflammatory diseases, NGF is over-expressed in atopic dermatitis and prurigo nodularis [24].

IL-31 is a cytokine mainly produced by Th2 lymphocytes. It is a pruritogen and a mediator of pruritus. In atopic dermatitis there is an increased concentration of IL-31 in both the serum and skin, and levels of both correlate with the severity of pruritus. The lesional skin of prurigo nodularis has high levels of IL-31. IL-31 is implicated in the pathogenesis of familial primary cutaneous amyloidosis.

Nagy and colleagues failed to demonstrate an association between EBP and IL-31 haplotype. They conceded that they “could not fully discount a role for IL-31 in EBP since their investigation focused on genomic DNA rather than IL-31 expression or signalling in the skin”. Whether IL-31 has a role in the pathogenesis of pruritus in EB requires further study.

TNF-alpha is a pro-inflammatory cytokine. It exerts its effect by binding to TNF-alpha receptors 1 and 2. It induces Th1 lymphocyte production over Th2 lymphocytes. It induces apoptosis in some cell types. It activates the TRPV-1 receptor, an itch receptor.

Does TNF-alpha play a role in the pathogenesis of EB? Research teams led by Yoneda et al. [25] and Lu et al. [26] reached opposite conclusions. Yoneda postulated that TNF-alpha has a critical role in keratinocyte apoptosis. The argument commenced with a recognition that keratin 5 (K5) and keratin 14 (K14) are expressed in keratinocytes in the basal and suprabasal layers of the epithelium. Mutations in the genes of K5 and K14 cause EB simplex: the absence of keratin filaments in the basal epithelium is followed by a complete detachment of the epidermis from the dermis. In their study, transfection of mutant K14 into keratinocytes caused K14 aggregation. That aggregation causes stress on keratinocyte endoplasmic reticulum which induces the keratinocyte to excrete TNF-alpha. TNF-alpha, in turn, in an autocrine mechanism, triggers TNF-alpha receptors on the surface of the keratinocyte precipitating an apoptotic cascade and cell death.

Lu et al. did not find any alteration in TNF-alpha levels in a cell culture model of EBS. They concluded that TNF-alpha was not involved in EB. Rather, they found that pro-inflammatory cytokines IL-6 and IL-1 beta were significantly upregulated at the mRNA and protein level in K5 mutant mouse skin.