Identification of fas-L-expressing apoptotic T lymphocytes in normal humanperipheral blood - In vivo suicide

Fas-L molecules expressed by in vitro stimulated T cells may be critically involved in suicidal activation-induced cell death (AICD) of such cells thr ough engagement of their Fas receptors, A similar suicide of T cells was po stulated to occur even in vivo, to eliminate dangerous activated lymphocyte s; however, the demonstration of suicidal AICD of T cells in healthy humans in vivo is still lacking. We therefore investigated the possible occurrenc e of Fas-L-linked suicidal apoptosis of T cells in normal human peripheral blood. For this purpose, we took advantage of immunoelectron microscopy, wh ich allows simultaneous visualization of the morphological apoptotic cellul ar 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 majorit y 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.