Proteinase-activated receptor-2 mediates itch: a novel pathway for pruritus in human skin

Abstract

We examined whether neuronal proteinase-activated receptor-2 (PAR-2) may be involved in pruritus of human skin. The endogenous PAR-2 agonist tryptase was increased up to fourfold in atopic dermatitis (AD) patients. PAR-2 was markedly enhanced on primary afferent nerve fibers in skin biopsies of AD patients. Intracutaneous injection of endogenous PAR-2 agonists provoked enhanced and prolonged itch when applied intralesionally. Moreover, itch upon mast cell degranulation was abolished by local antihistamines in controls but prevailed in AD patients. Thus, we identified enhanced PAR-2 signaling as a new link between inflammatory and sensory phenomena in AD patients. PAR-2 therefore represents a promising therapeutic target for the treatment of cutaneous neurogenic inflammation and pruritus.

Figure 1.

Dose—response relationship of codeine-induced release of histamine (a) and tryptase (b) (mean ± SD) and of intensity of itch (scale from 0 to 10) (c) (median, quartiles) in controls (open triangles) and patients with AD (filled triangles). Codeine phosphate was applied via intradermal microdialysis catheters for 30 min after a baseline perfusion of 60 min, and mediator release was measured in the perfusate at 15 min intervals. In a second session, codeine applications were repeated with a histamine receptor antagonist (ceterizine; 100 μg/ml) applied during the entire protocol (d—f, open and filled diamonds). The numbers of subjects or patients are indicated in b and e. AUC, Area under the curve; max., maximum.

Figure 2.

Double immunofluorescence staining of PAR-2 (B5 antiserum) and mast cell tryptase in lesional and nonlesional human skin biopsies of patients with AD. a, In lesional skin of patients with atopic dermatitis, staining for PAR-2 (red) can be observed in keratinocytes, blood vessels, certain inflammatory cells, and nerve-fiber-like structures. Mast cells (green) associated with PAR-2-positive blood vessels (100×). b, Omission of antibodies against PAR-2 demonstrates only staining of mast cells by tryptase (100×). c, Higher magnification reveals staining of small nerve fibers (arrow) in the dermis associated with blood vessels (red) and mast cells (green) (400×). d, In lesional skin of patients with AD, increased staining for PAR-2 was observed in nerve fibers (arrows) closely associated with mast cells (green) (630×) at higher magnification. e, Staining for PAR-2 (arrows) was also observed in nerve fibers of nonlesional skin from patients with AD (630×). f, Control staining using the appropriate peptide for preabsorption (B5 antiserum) did not result in any PAR-2-like or tryptase-like immunoreactivity in either human skin tissue (630×).

Figure 3.

Dose—response curves for itch induction by intracutaneous injection of a PAR-2 agonist [tethered ligand; 50 μl; 13 controls (open circles) and 14 AD patients (filled diamonds, lesional skin; filled circles, nonlesional skin)] and the reverse peptide (VKGILS-NH₂; 6 controls and 4 AD patients) are shown. Intensity of itch sensation after the injection was assessed on a scale from 0 to 10 at 10 sec intervals for 5 min. auc, Area under the curve; mean ± SEM.