The Woronoff Ring in Psoriasis and the Mechanisms of Postinflammatory Hypopigmentation.

The Woronoff ring is a ring-like hypopigmentation zone around regressing psoriasis lesions. Although it was first described more than 100 years ago, its aetiology has remained a mystery. Recent insights into the pathogenesis of psoriasis can now explain the origin of the Woronoff ring. Psoriasis involves an HLA-class I-restricted autoimmune response of CD8+ T cells against melanocytes in the epidermis. The pathogenic CD8+ T cells are not cytotoxic, but are characterized by the production of interleukin-17, interleukin-22 and tumour necrosis factor-α. Interleukin-17 and tumour necrosis factor-α act synergistically on melanocytes by increasing proliferation while inhibiting melanogenesis. This reduces the cellular melanin content despite an increased number of melanocytes in psoriatic lesions. As a consequence, during healing the prior influence of interleukin-17 and tumour necrosis factor-α, despite the increased density of melanocytes, leaves a hypopigmented zone at the edge of regressing psoriasis lesions, which becomes visible as the Woronoff ring. This mechanism can explain a long-discussed puzzling phenomenon in dermatology.