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(American Journal of Pathology. 2001;158:387-391.)
© 2001 American Society for Investigative Pathology

Short Communication

Identification of Fas-L-Expressing Apoptotic T Lymphocytes in Normal Human Peripheral Blood

In Vivo Suicide

Giuseppe De Panfilis^*, Arnaldo Caruso, Paolo Sansoni, Giorgio Pasolini^*, Diego Semenza^* and Claudio Torresani^§

From the Departments of Dermatology ^*
and Virology,
Brescia University Hospital, Brescia; and the Departments of Medicine
and Dermatology,^§
University of Parma, Parma, Italy

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.