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Stasis dermatitis (SD) is a common disease in the elderly population, with pruritus being one of the troublesome symptoms. However, there are few therapeutic modalities available for SD-associated itch because little is known about its pathophysiological mechanism. Therefore, we sought to investigate the mediators of itch in SD using an immunofluorescence study on patient lesions focusing on IL-31. Ex vivo stimulation studies using murine peritoneal macrophages were also used to elucidate the pathological mechanisms of the generation of IL-31. In SD lesions, dermal infiltrating IL-31(þ) cells were increased in number compared with the healthy controls, and the majority of IL-31(þ) cells were CD68(þ) macrophages. The presence of itch in SD was significantly associated with the amount of CD68(þ)/IL-31(þ) macrophages and CD68(þ)/CD163(þ) M2 macrophages. The number of CD68(þ)/IL-31(þ) macrophages was correlated with the number of dermal CC chemokine receptor type 4(þ) T helper type 2 cells, IL-17(þ) cells, basophils, substance P(þ) cells, and dermal deposition of periostin and hemosiderin. Furthermore, murine peritoneal macrophages expressed an M2 marker arginase-1 and generated IL-31 when stimulated with a combination of substance P, periostin, and red blood cell lysate (representing hemosiderin). IL-31 from macrophages may play a role in itch in SD.

Prurigo nodularis (PN) is a chronic skin dermatosis with hyperkeratotic and intensely pruritic nodules. Managing PN‐associated itch is difficult because its aetiology is still unknown. This study aimed to investigate the correlation between itch intensity in PN and the expression of a pruritogenic cytokine interleukin (IL)‐31, its receptor complex components IL‐31 receptor α (IL‐31RA) and oncostatin M receptor β (OSMRβ), and oncostatin M (OSM), which is a ligand of OSMR β, through immunofluorescence staining examination. Itch intensity in PN was closely correlated with the number of dermal IL‐31(+) cells (Spearman's r = 0.551, p < 0.05), dermal IL‐31RA(+) cells (r = 0.475, p < 0.05) and dermal OSM(+) cells (r = 0.505, p < 0.05). In addition, the number of dermal OSMRβ (+) cells was increased in PN (t test, p < 0.05), despite not being correlated with itch intensity (Spearman's r = 0.375, p > 0.05). Major cellular sources of dermal IL‐31 were T cells (27.0% of total IL‐31–expressing cells) and macrophages (35.0%), while those of OSM were mainly T cells (49.8%) and mast cells (26.8%). IL‐31RA–expressing dermal cells were mostly mast cells (49.3%) and macrophages (36.6%), and OSMRβ was mainly expressed by macrophages (51.8%) in the dermis. These findings indicate that IL‐31 (mainly from macrophages and T cells) and OSM (principally from T cells and mast cells) stimulate dermal cells expressing IL‐31RA and OSMRβ (e.g. macrophages), which may further promote itch and inflammation in PN. This complex dermal milieu of cell/cytokine/receptor network can be a therapeutic target for PN‐associated itch.

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