Adult variant of self-healing papular mucinosis in a patient with rheumatoid arthritis: Predominant proliferation of dermal dendritic cells expressing CD34 or factor XIIIa in association with dermal deposition of mucin

According to a recent classification, self-healing papular mucinosis (SHPM) is a subtype of papular mucinosis (also known as lichen myxedematosus), which is in turn a type of idiopathic localized cutaneous mucinosis. SHPM tends to occur in children, but there have been a few reports of an adult type. We report a 70-year-old Japanese woman who presented with reddish, rice-kernel-sized papules of a few days' duration on her right arm. She had a 25-year history of rheumatoid arthritis, which had been well treated with a low dose of corticosteroid as well as some other medications. No paraproteinemia or thyroid dysfunction were observed. The eruptions spontaneously resolved within 2.5 months of onset. Histological findings showed a well-circumscribed mucinous stroma surrounded by dermal mesenchymal cells, such as fibroblast-like cells in the middle of the dermis. Immunohistochemically, these cells were positive only for vimentin on the mucinous lesion. On the circumference of the mucinous lesion, these cells expressed either CD34 or factor XIIIa (FXIIIa). Because vimentin was common to dermal mesenchymal cells, we defined the cells expressing CD34 or FXIIIa, except for vimentin+ cells lacking CD34 or FXIIIa, as dermal dendritic cells (DDC). The findings of the present case suggest that CD34+ or FXIIIa+ DDC and tryptase-positive mast cells on the perilesional area in combination with vimentin+ cells on the mucinous lesion might have given rise to the dermal deposition of mucin in our case. These cells, which are possibly activated in an autoimmune manner associated with rheumatoid arthritis, might play important roles in the development of dermal deposition of mucin in SHPM.