Human keratinocyte adhesion and phagocytosis promoted by fibronectin

Epidermal cells from human foreskin or cadaver skin were found to bind and phagocytose fibronectin-coated latex beads but not serum albumin-coated beads. Three lines of evidence suggested that the beads actually were internalized by the cells and not bound only at the cell surfaces. First, the fibronectin coat on internalized beads could be detected with antifibronectin antibodies only if the cells were permeabilized prior to indirect immunofluorescence staining. Second, the internalized beads were not released from the cells by trypsin treatment. Finally, electron microscopic observations showed that the internalized beads were packaged individually and in clusters inside endocytic vesicles in the perinuclear region of the cells. These vesicles had the typical appearance of lysosomes, and, based on the release of trichloroacetic acid-soluble radioactive fragments of fibronectin into the incubation medium, it was concluded that degradation of the fibronectin on the bead surfaces occurred. The epidermal cells that phagocytosed the fibronectin-coated beads were confirmed as keratinocytes according to their electron microscopic appearance and prominent immunofluorescence staining with antikeratin but not anti-plasma fibronectin antibodies. Fibronectin also was found to promote the attachment and spreading of keratinocytes on culture dishes, although the concentration of fibronectin required for half-maximal activity (approximately 5 micrograms/ml) was severalfold higher than the concentration of fibronectin required for spreading of human fibroblasts (approximately 1 microgram/ml). The results suggest the possible importance of fibronectin in mediating adhesion and phagocytosis by keratinocytes, which may be important for the migration of these cells during wound repair.