P69 Modelling epidermal melanocyte behaviour during plaque psoriasis progression and resolution

Abstract Melanocytes in human epidermis reside in the basal layer in a ratio of 1 melanocyte to ∼10 keratinocytes. In healthy epidermis, melanocytes neither divide nor undergo apoptosis. In psoriasis, the 1:10 ratio is maintained despite significant keratinocyte hyperplasia, suggesting an increase in melanocyte numbers. We investigated whether melanocyte numbers change in active psoriasis plaques and after plaque resolution upon treatment with biologics, and via the impact of psoriasis-related cytokines on melanogenesis in primary normal melanocytes in vitro. Immunohistochemistry (IHC) with MITF (melanocyte marker) and Ki67 (proliferation marker) on lesional and non-lesional samples (n = 11) at baseline and after 12 weeks of treatment assessed melanocyte proliferation. Melanocyte number/mm of basal layer and melanocyte/keratinocyte ratio were estimated by analysis on ImageJ. The TUNEL-assay and IHC staining with pro-apoptotic marker, Cleaved Caspase 3, and anti-apoptotic marker, BCL-2 were performed on lesional and non-lesional samples (n = 5) at baseline and after 4 and 12 weeks of treatment to determine if melanocytes undergo apoptosis upon treatment with biologics. Primary normal human melanocytes were treated with proinflammatory cytokines (IL-17, TNF-α, IL-22, and IL-23) to assess their influence on melanogenesis via immunocytochemistry (ICC) and Western blot (WB) analysis. Proliferating melanocytes were detected in non-lesional and lesional samples and in resolving plaques after treatment but not in healthy control epidermis. Melanocyte number/mm of the basal layer was significantly higher in non-lesional compared with both lesional sections and healthy controls (P < 0.0001), suggesting melanocyte expansion occurs prior to psoriasis plaque establishment. Twelve weeks after treatment, melanocyte numbers in lesional and non-lesional samples were still significantly higher than controls (P < 0.05) despite the reduction in keratinocyte numbers. The TUNEL-assay and Cleaved Caspase 3 staining did not detect evidence of melanocyte apoptosis. ICC and WB analysis of treated primary melanocytes revealed that TNF-α decreases the activity of the rate-limiting melanogenesis enzyme tyrosinase and down-regulates TRP1 both when this cytokine was used alone and in combination with the other cytokines. This study shows early melanocyte dysregulation supporting a role for melanocytes in psoriasis pathogenesis perhaps via an ADAMSTL5-initiated inflammation. The higher melanocyte numbers are retained even after treatment, and we were unable to detect melanocyte deletion/apoptosis despite a reduction in keratinocyte number. We show the direct inhibitory impact of psoriasis-associated cytokines on melanogenesis. This raises intriguing questions as to the nature of melanocyte proliferation in this benign inflammatory skin disease and suggests that such ectopic melanocytic proliferation in the epidermis may not be a harbinger of malignancy risk.