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Fas-L molecules expressed by in vitro stimulated T cells may be critically involved in suicidal activation-induced cell death (AICD) of such cells through engagement of their Fas receptors. A similar suicide of T cells was postulated to occur even in vivo, to eliminate dangerous activated lymphocytes; however, the demonstration of suicidal AICD of T cells in healthy humans in vivo is still lacking. We therefore investigated the possible occurrence of Fas-L-linked suicidal apoptosis of T cells in normal human peripheral blood. For this purpose, we took advantage of immunoelectron microscopy, which allows simultaneous visualization of the morphological apoptotic cellular changes together with surface expression of Fas-L molecules. Very few T lymphocytes were observed showing the ultrastructural features of apoptotic lymphocytes; these occasional apoptotic T cells, together with the majority of the normal T cell population, expressed the Fas molecule on the plasma membrane, as expected. Interestingly, the apoptotic cells were also Fas-L-positive, whereas normal T cells were Fas-L-negative. Such Fas-L-associated T cell suicide operating in vivo in healthy individuals is presumably able to suppress immune responses and prevent autoreactivity, thus maintaining the homeostasis of human blood.

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.

Although dendritic cells (DC) are well known for their immunogenic capacities, they may even induce peripheral T-cell tolerance, and such a tolerogenic potential can be exerted in mouse through the expression on the DC plasma membrane of the CD95-ligand (CD95-L) molecule, which is able to trigger apoptosis of CD95-expressing antigen-specific T cells. We therefore asked whether epidermal DC, namely Langerhans' cells (LC), either resting (i.e. within the epidermis, 'in situ') or activated (i.e. suspended from the epidermis) or both, could express the CD95-L molecule on the plasma membrane. For such a purpose, two colloidal gold-immunoelectron microscopy (IEM) double-step procedures were carried out: an 'in situ' method, able to investigate resting LC, was performed on ultrathin frozen sections obtained by ultracryomicrotomy (UCMT) of normal skin biopsies; a pre-embedding (P-E) method, able to investigate suspended LC, was performed on epidermal cells (EC) suspended from normal skin specimens. In UCMT/IEM sections, resting LC showed gold particles within the cytoplasm but very rarely within organelles and never along the plasma membrane: resting LC are therefore capable of synthesizing CD95-L but not of expressing it in a functional location, thus autoreactive phenomena against CD95-expressing EC being avoided in normal epidermis. On the other hand, in P-E/IEM preparations, suspended LC showed several gold particles along the plasma membrane: activated LC are therefore capable of expressing CD95-L in a functional location, thus bearing the potential to exert tolerogenic capabilities against CD95-expressing T cells, e.g. to prevent inflammatory/autoimmune cutaneous disorders and/or favor the resolution thereof.