Cell Cycle-Dependent Regulation of FLIP Levels and Susceptibility to Fas-Mediated Apoptosis

Activation-induced cell death of peripheral T cells results from the interaction between Fas and Fas ligand. Resting peripheral T cells are resistant to Fas-induced apoptosis and become susceptible only after their activation. We have investigated the molecular mechanism mediating the sensitization of resting peripheral T cells to Fas-mediated apoptosis following TCR stimulation. TCR activation decreases the steady state protein levels of FLIP (FLICE-like inhibitory protein), an inhibitor of the Fas signaling pathway. Reconstitution of intracellular FLIP levels by the addition of a soluble HIV transactivator protein-FLIP chimera completely restores resistance to Fas-mediated apoptosis in TCR primary T cells. Inhibition of IL-2 production by cyclosporin A, or inhibition of IL-2 signaling by rapamycin or anti-IL-2 neutralizing Abs prevents the decrease in FLIP levels and confers resistance to Fas-mediated apoptosis following T cell activation. Using cell cycle-blocking agents, we demonstrate that activated T cells arrested in G1 phase contain high levels of FLIP protein, whereas activated T cells arrested in S phase have decreased FLIP protein levels. These findings link regulation of FLIP protein levels with cell cycle progression and provide an explanation for the increase in TCR-induced apoptosis observed during the S phase of the cell cycle.

Expression and Function of Fas on Cells Damaged by γ-Irradiation in B6 and B6/lpr Mice

Fas (CD95) is a cell surface protein that mediates apoptosis. lpr is a mutation of the Fas gene caused by a retroviral insertion resulting in premature termination of transcription and aberrant splicing of Fas mRNA. Mice homozygous for the lpr gene develop lymphoproliferation and produce autoantibodies closely resembling those of human systemic lupus erythematosus. While lpr mice have been reported to express low levels of normally spliced Fas mRNA, it is unknown whether they express functional Fas protein. Here we show that splenocytes from lpr mice that have been damaged by γ-irradiation expressed Fas protein. Fas was up-regulated on irradiated B6 cells and could be detected on B6/lpr cells undergoing apoptosis following in vitro culture. Detection of Fas on live lpr cells was demonstrable when apoptosis was blocked by zinc. In a short term chimera system, Fas was shown to play a role, in vivo, in the disposition of radiation-injured cells from both normal and lpr mice. The addition of anti-Fas Ab to in vitro cultures resulted in an increase in apoptosis in both B6 and B6/lpr cells. Detection of intact Fas message and low levels of Fas protein in lpr mice has led to the consideration of lpr as a leaky mutation. This study demonstrates that lpr mice can produce functional Fas protein. This system is also appropriate for identifying the in vivo role of Fas/FasL in apoptosis following other cell manipulations.

Differential Expression of Fas and Fas Ligand in Acute and Chronic Graft-Versus-Host Disease: Up-Regulation of Fas and Fas Ligand Requires CD8+ T Cell Activation and IFN-γ Production

The parent-into-F1 model of acute and chronic graft-vs-host disease (GVHD) was used as an example of in vivo cell-mediated or Ab-mediated responses, respectively, and the roles of Fas and Fas ligand (FasL) were investigated. Using both flow cytometry and PCR methodologies, we found that acute GVHD mice exhibited significant up-regulation of Fas and FasL, whereas Fas/FasL up-regulation in chronic GVHD mice was equal to or marginally greater than that in uninjected mice. Functional studies confirmed that Fas/FasL contributed to the anti-host CTL activity of splenocytes from acute GVHD mice, although a perforin-dependent pathway was also identified. Despite the presence of FasL on both donor CD4+ and CD8+ T cells in acute GVHD mice, depletion studies demonstrated that all the in vitro anti-host CTL activity resided in the CD8+ population. Furthermore, injection of CD8-depleted B6 spleen cells into F1 mice blocked Fas/FasL up-regulation and IFN-γ production, resulting in chronic GVHD. Lastly, up-regulation of Fas/FasL in acute GVHD mice could be blocked by anti-IFN-γ mAb in vivo. Thus, in this in vivo model of alloantigen immune responsiveness, Fas/FasL up-regulation is critically dependent on Ag-specific (donor) CD8+ T cell activation and IFN-γ production. Donor CD4+ T cell activation in the absence of CD8+ T cell activation results in an autoantibody-mediated response, no significant Fas/FasL up-regulation, impaired elimination of autoreactive B cells, and persistent humoral autoimmunity.

Fas/Fas Ligand Interactions Are Involved in Ultraviolet-B-Induced Human Lymphocyte Apoptosis

We wondered whether the apoptosis known to occur after UV-B irradiation might involve the Fas/Fas ligand (FasL) signaling pathway. We exposed PBLs from normal individuals, and also the Jurkat (E6-1) and U937 cell lines, to graded doses of UV-B irradiation and observed a prompt and marked increase in Fas expression at doses as low as 0.5 mJ/cm2. Increased Fas expression did not require new protein synthesis, since cycloheximide-treated cells also showed an increase in Fas after UV-B. UV-B-irradiated cells cultured in the presence of zinc showed inhibition of apoptosis coincident with a marked increase in Fas+ cells, apparently indicating the accumulation of Fas-bearing cells unable to undergo apoptosis. After UV-B irradiation, PBLs showed increased expression of Fas ligand; the E6-1 lymphocytic cell line also released soluble FasL. UV-B induced apoptosis could be partially blocked by neutralizing FasL Abs, and a FasL-resistant variant of E6-1 cell line showed reduced apoptosis after UV-B irradiation, implying that the increase in Fas expression signified a role for Fas in UV-induced apoptosis. UV-induced Fas expression may serve to target stress-injured cells for removal by FasL-bearing cells or by FasL produced by the cells themselves in response to the stimuli, and may represent a general function of the Fas/FasL pathway in facilitating the apoptosis and elimination of undesirable or harmful cells.