Dermal dendritic cells in psoriasis, nummular dermatitis, and normal-appearing skin

Background: The reason psoriasis (PSO) favors extensor skin is unknown. We hypothesized that PSO may involve extensor skin preferentially because of differences in the number or type of dermal dendritic cells (dDCs) between flexural and extensor skin. Objective: We sought to compare dDC type and distribution in normal-appearing flexural and extensor skin, PSO, and nummular dermatitis (ND). Methods: Using immunohistochemical markers, the number, distribution, and type of Langerhans cells, myeloid dendritic cells (DCs), and plasmacytoid DCs was compared in normal-appearing skin, PSO, and ND. Results: Significant differences in dDC density were not identified between flexural and extensor skin, although extensor skin contained fewer CD11a ⁺ and CD11c ⁺ cells. Compared with normal-appearing skin, cells expressing CD11a, CD11c, CD123, CD303, and CD207 were increased in PSO. ND lesions showed similar increases. No significant difference between PSO and ND was evident with the exception of decreased S100A6 ⁺ cells in PSO. Limitations: We did not study seasonal variation in DC density or assess nonlesional skin from patients with PSO. Conclusions: The data did not support the hypothesis that PSO favors extensor skin because of differences in DC localization. However, dDCs were significantly increased in PSO by comparison with normal-appearing skin, supporting existing evidence that they are involved in the overall pathogenesis of PSO.