J Tschopp

Malignant melanoma accounts for most of the increasing mortality from skin cancer. Melanoma cells were found to express Fas (also called Apo-1 or CD95) ligand (FasL). In metastatic lesions, Fas-expressing T cell infiltrates were proximal to FasL+ tumor cells. In vitro, apoptosis of Fas-sensitive target cells occurred upon incubation with melanoma tumor cells; and in vivo, injection of FasL+ mouse melanoma cells in mice led to rapid tumor formation. In contrast, tumorigenesis was delayed in Fas-deficient lpr mutant mice in which immune effector cells cannot be killed by FasL. Thus, FasL may contribute to the immune privilege of tumors.

Staphylococcal enterotoxin B (SEB) is a bacterial superantigen (SAg) that predominantly interacts with V(beta)8+ T cells. In vivo treatment of mice with SEB leads to an initial increase in the percentage of V(beta)8+ T cells, followed by a decrease in the numbers of these cells, eventually reaching lower levels than those found before treatment with the SAg. This decrease is due to apoptosis of the SEB-responding cells. In the present study, we use the distinct light scattering characteristics of apoptotic cells to characterize T cells that are being deleted in response to SEB in vivo. We show that dying, SEB-reactive T cells express high levels of Fas and Fas ligand (Fas-L), which are implicated in apoptotic cell death. In addition, the B cell marker B220 is upregulated on apoptotic cells. Moreover, we show that the generation of cells with an apoptotic phenotype is severely impaired in response to SEB in functional Fas-L-deficient mutant gld mice, confirming the role of the Fas pathway in SAg mediated peripheral deletion in vivo.

The expression and function of Fas (CD95/APO-1), a cell surface receptor directly responsible for triggering cell death by apoptosis, was investigated on human T lymphocytes resident within the intestinal lamina propria, a major site of antigen challenge and persistent lymphocyte activation. Three color immunofluorescence and FACS analysis indicated that virtually all freshly isolated human gut lamina propria T lymphocytes (T-LPL) express Fas, together with the marker of progress activation CD45R0. A discrete fraction of freshly isolated T-LPL also constitutively expressed Fas ligand (FasL), perhaps as a result of recent in vivo activation. Importantly, whereas Fas cross-linking did not result in apoptosis induction in peripheral blood T lymphocytes (T-PBL), Fas was found to be fully effective in generating the apoptotic signal in T-LPL. This was associated with the activation of an acidic sphingomyelinase and with ceramide generation, early events known to be involved in Fas-mediated apoptotic signaling. By contrast, acidic sphingomyelinase activation and ceramide production were not detectable in T-PBL after Fas cross-linking. However C2-ceramide, a cell permeant synthetic analog of ceramide, could efficiently induce apoptosis in T-LPL and T-PBL when added exogenously. These data indicate that T-LPL constitutively express both Fas and FasL and that Fas cross-linking generates signals resulting in sphingomyelin hydrolysis and apoptosis, outlining a potential mechanism involved in intestinal tolerance. Moreover, they provide the first evidence of a role for ceramide-mediated pathways in normal immunoregulation.

The interaction of Fas (Apo-1) and TNF receptor-1 with their respective ligands can lead to cell death. The so-called death domain, a sequence motif present in the cytoplasmic portion of the two receptors, has been identified as a critical structural element involved in signal transduction that leads to apoptosis. Here we describe several additional proteins which contain a death domain. Novel members of this family include proteins known to be implicated not only in apoptosis but also in neuron guidance.