Increased Activity and Number of Epidermal Melanocytes in Lesional Psoriatic Skin.

BACKGROUND: Psoriatic lesions may resolve with hypo- or hyperpigmentation. The involvement of melanocytes in this dichotomous clinical outcome is not fully investigated.
OBJECTIVES: Qualitative and quantitative assessment of melanocytes in untreated lesional and non-lesional psoriatic skin (n = 15) and healthy controls (n = 10).
METHODS: Skin biopsies were labelled immunohistochemically (APAAP technique) with the antimelanocyte monoclonal antibodies (MoAbs) HMB45, Melan A, tyrosinase and microphthalmia-associated transcription factor (MITF). The labelled melanocytes were evaluated by an independent investigator with a digital image analyser.
RESULTS: Lesional melanocytes, in contrast to those in non-lesional and healthy skin, exhibited features of activation in the form of dilatation, prominent and long dendrites and intense labelling. The number of melanocytes was significantly increased in psoriatic lesions in comparison with non-lesional psoriatic and healthy skin as shown by counts of cells labelled with the MoAbs HMB45 (3-fold; p < 0.001), Melan A (1.6-fold; p < 0.01) and tyrosinase (1.5-fold; p < 0.01). In contrast, labelling with MITF revealed no significant difference (1.2-fold increase; p > 0.05). Likewise, no significant difference between non-lesional psoriatic and healthy skin control was found (p > 0.05). Furthermore, no positively labelled dermal cells were detected, apart from few only detected with Melan A.
CONCLUSIONS: Epidermal melanocyte activity and numbers are increased in the epidermal compartment of psoriatic lesions providing an explanation for postinflammatory hyperpigmentation.