Keratinocytes constitutively express the CD95 ligand molecule on the plasma membrane:: an in situ immunoelectron microscopy study on ultracryosections of normal human skin

Background Tissue homeostasis is mainly preserved by cytolytic functions. Cytolytic cells, when expressing the CD95 ligand (Fas-L) molecule on the cell membrane, are able to kill CD95 (Fas)-expressing target cells. Although cultured epidermal keratinocytes (KC) have been shown to express Fas-L, and normal skin has been shown to bear Fas-L mRNA, efforts so far to find possible constitutive Fas-L expression on the cell membrane by resting KC in normal human epidermis (i.e. in a functionally active location) have been inconclusive. Objectives The aim of the present study was therefore to show the constitutive expression of Fas-L on the plasma membrane of KC. Methods Gold immunoelectronmicroscopy, a highly specific and sensitive immunodetection system, was performed in situ on skin sections obtained by ultracryomicrotomy, without previous embedding (i.e. in conditions strictly similar to the in vivo situation). Results Relatively few (51.55 +/- 28.61), 10-nm colloidal gold particles were observed at the cell surface of KC in the basal layer of the epidermis and an even smaller (P < 0.005) number of gold granules was detected in the KC of the spinous layer. Conclusions Although scanty, the constitutive Fas-L expressed on the surface of KC can bind Fas expressed by possible occasional inflammatory cells entering the epidermis, and kill them, so preventing inflammation. Fas-L-expressing KC could moreover induce apoptosis of epidermal cells bearing viral or neoplastic antigens. Thus, the expression of Fas-L by KC may contribute to the preservation of epidermal homeostasis in vivo.