Melanocytes of the human epidermis are located in the stratum basale in a ratio of 1 melanocyte to 10 keratinocytes. Melanocytes do not divide in healthy epidermis, even after UVR stimulation. While psoriasis is characterised by keratinocyte hyperplasia, pigmentary anomalies can occur (hypopigmented Woronoff ring, hyperpigmentation). Curiously, the normal s.basale melanocyte to keratinocyte ratio is retained in the hyperplastic epidermis. This may infer a commensurate (compensatory) expansion in melanocyte number. Yet how this occurs, is unknown. In our (immuno)histochemical study we examined the epidermal-melanin unit (EMU) in 10 patients with plaque psoriasis. Healthy SPT II/III skin and untreated psoriasis skin (lesional & non-lesional, SPT I-III) were examined for evidence of melanocyte proliferation, using antibodies to MITF and Ki67. Melanin distribution was evaluated by Warthin-Starry staining. The following were assessed in the s. basale: a) total melanocyte number/mm; b) overall ratio of melanocytes to keratinocytes; c) number of proliferating melanocytes as % of total melanocytes; and d) pattern of melanin distribution. Proliferating melanocytes were not detected in healthy epidermis but were detected in lesional and also non-lesional psoriasis. Melanocytes/mm of s.basale (normalized for rete ridge length) were significantly higher (p<0.01) in non-lesional compared to lesional psoriasis skin, suggesting melanocytes already proliferate before keratinocyte hyperplasia extends the rete ridges. Melanin was similarly restricted to the s.basale in both control and non-lesional psoriasis skin but was strikingly scattered throughout the entire epidermis in lesional skin. This study shows a remarkable and unambiguous expansion of melanocyte number in psoriasis epidermis, which in lesional skin was associated with significant alteration in melanin distribution. Thus, psoriasis can be regarded as a disorder of the entire EMU and prompts further investigation into the suggested role of melanocytes in the pathogenesis of psoriasis.

ESDR215 - Unambiguous evidence of melanocyte proliferation in plaque psoriasis

50th Annual Meeting of the European Society for Dermatological Research. VIRTUAL MEETING