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Melanosomes are melanocyte-specific lysosome-related organelles that are transferred to keratinocytes of the epidermis and anagen hair bulb. Transferred melanin forms supra-nuclear caps that protect epidermal keratinocytes against UV irradiation. The mechanism(s) responsible for melanosome transfer into keratinocytes and their subsequent intra-keratinocyte distribution has long remained one of the most enigmatic of heterotypic cell interactions. Although there have been many attempts to study this process, significant progress has been hindered by the absence of an adequate in vitro model. During our ongoing study of melanocyte-keratinocyte interactions in skin and hair follicle, we have developed a novel in vitro assay that exploits the specificity of Silv ⁄ Pmel17 ⁄ gp100 expression for melanosome ⁄ melanin granules. Using matched cultures of keratinocytes and melanocytes isolated from normal healthy epidermis together with double immunofluorescence, we have determined that gp100 is a surprisingly useful tracker of transferred melanin. Moreover, transferred gp100 stained melanin granules emit a bright fluorescence signal, facilitating ready quantification of melanin transfer levels between melanocytes and keratinocytes. This quantitative approach was validated using known inducers and inhibitors of the melanocyte phenotype. This assay further confirmed that cytophagocytosis of melanocyte components (e.g. dendrite tips) by keratinocytes is one route for melanin incorporation into keratinocytes. Lastly, a role for the recently proposed filopodium as a direct conduit for melanin transfer was substantiated using this novel approach. In conclusion, this assay promises to significantly aid our investigations of the molecular basis of melanosome transfer and offers a new tool for the clinical evaluation of melanocyte modulators.Key words: filopodia -gp100 -keratinocyte -melanocytemelanosome transfer -phagocytosis -Pmel17Please cite this paper as: The silver locus product (Silv ⁄ gp100 ⁄ Pmel17) as a new tool for the analysis of melanosome transfer in human melanocyte-keratinocyte co-culture. Experimental Dermatology 2008; 17: 418-426.

Collectively, these data show that chronic ultraviolet irradiation induces alterations of innate immune cells which are recruited in sun-exposed skin without being activated.