Phagocytosis by normal human melanocytes in vitro

Phagocytosis is an important first step in processing of antigens ultimately to be presented in the context of MHC class II molecules on the cell surface. Degradation of internalized antigens can then proceed by fusion of the phagosome with organelles containing lysosomal enzymes. Evidence for the phagocytic capacity of cultured normal human skin melanocytes is provided, both by electron microscopy and by confocal laser scanning microscopy. The phagocytic capacity of melanocytes is approximately 5% of that found for cultured human dermal skin fibroblasts. However, in a mixed culture of keratinocytes and melanocytes, uptake of 1 micron latex beads in 72 h was found to be comparable for both epidermal cell types, implicating in vivo significance for phagocytosis by melanocytes. Furthermore, it is shown that phagosomes containing latex beads will fuse with melanosomes, indicating that melanosomes can function as specialized lysosomal organelles within normal human skin melanocytes. The present investigation indicates that melanocytes are equipped with the phagocytic machinery important for the processing of antigens and thus may function as accessory cells within the skin immune system.