Ca2+ signaling modulates cytolytic T lymphocyte effector functions

Regulation of IFN-gamma signaling is essential for the cytotoxic activity of CD8(+) T cells

Previous studies have demonstrated that, as naive murine CD4(+) cells differentiate into Th1 cells, they lose expression of the second chain of IFN-gammaR (IFN-gammaR2). Hence, the IFN-gamma-producing subset of Th cells is unresponsive to IFN-gamma. Analysis of IFN-gamma-producing CD8(+) T cells demonstrates that, like Th1 cells, these cells do not express IFN-gammaR2. To define the importance of IFN-gamma signaling for the development of functional CD8(+) T cells, mice either lacking IFN-gammaR2 or overexpressing this protein were examined. While CD8(+) T cell development and function appear normal in IFN-gammaR2(-/-) mice, CD8(+) T cell function in IFN-gammaR2 transgenic is altered. IFN-gammaR2 transgenic CD8(+) T cells are unable to lyse target cells in vitro. However, these cells produce Fas ligand, perforin, and granzyme B, the effector molecules required for killing. Interestingly, TG CD8(+) T cells proliferate normally and produce cytokines, such as IFN-gamma in response to antigenic stimulation. Therefore, although IFN-gamma signaling is not required for the generation of normal cytotoxic T cells, constitutive IFN-gamma signaling can selectively impair the cytotoxic function of CD8(+) T cells.

CD8(+) tumor-infiltrating T cells are deficient in perforin-mediated cytolytic activity due to defective microtubule-organizing center mobilization and lytic granule exocytosis

Tumor-infiltrating lymphocytes (TIL) are well known to be functionally impaired typified by the inability to lyse cognate tumor cells in vitro. We have investigated the basis for defective TIL lytic function in transplantable murine tumor models. CD8(+) TIL are nonlytic immediately on isolation even though they express surface activation markers, contain effector phase cytokine mRNAs, and contain perforin and granzyme B proteins which are packaged into lytic granules. Ag-specific lytic capability is rapidly recovered if purified TIL are briefly cultured in vitro and tumor lysis is perforin-, but not Fas ligand mediated. In response to TCR ligation of nonlytic TIL in vitro, proximal and distal signaling events are normal; calcium flux is rapid; mitogen-activated protein/extracellular signal-related kinase kinase, extracellular regulatory kinase 2, phosphoinositide-3 kinase, and protein kinase C are activated; and IL-2 and IFN-gamma is secreted. However, on conjugate formation between nonlytic TIL and cognate tumor cells in vitro, the microtubule-organizing center (MTOC) does not localize to the immunological synapse, thereby precluding exocytosis of preformed lytic granules and accounting for defective TIL lytic function. Recovery of TCR-mediated, activation-dependent MTOC mobilization and lytic activity requires proteasome function, implying the existence of an inhibitor of MTOC mobilization. Our findings show that the regulated release of TIL cytolytic granules is defective despite functional TCR-mediated signal transduction.

Signaling alterations in activation-induced nonresponsive CD8 T cells

Costimulation-dependent production and autocrine use of IL-2 by activated CD8 T cells results in initial clonal expansion, but this is transient. The cells quickly become anergic, unable to produce IL-2 in response to Ag and costimulation, irrespective of the form of costimulation. This activation-induced non-responsiveness (AINR) differs from "classical" anergy in that it results despite the cells receiving both signal 1 and signal 2. AINR cells can still proliferate in response to exogenous IL-2, but can no longer produce it. Other TCR-mediated events including cytolytic function and IFN-gamma production are not affected in the AINR state. To characterize the mechanism(s) responsible for lack of IL-2 production in CD8 T cells in the AINR state, microspheres bearing immobilized anti-TCR Abs or peptide-MHC complexes, B7-1, and ICAM-1 were used to provide well-defined stimuli to the cells. Comparison of normal and AINR cells revealed that in AINR cells extracellular signal-regulated kinase (ERK) is upregulated more transiently, Janus kinase activation is substantially reduced, and activation of p38 is eliminated. PMA and ionomycin restored proliferation and IL-2 production in AINR cells, indicating a signaling defect upstream of Ras and protein kinase C. Inhibitors of ERK (PD98059) and of p38 kinase (SB202190) blocked IL-2 mRNA expression and proliferation of both peptide-MHC/B7-1/ICAM-1-stimulated normal cells and PMA/ionomycin-stimulated AINR cells. Together these results demonstrate that activation of at least ERK and p38 is essential for IL-2 production by CD8 T cells and that up-regulation of these mitogen-activated protein kinases, along with Janus kinase, is defective in AINR cells.

Inhibition of human NK cell-mediated cytotoxicity by exposure to ammonium chloride

Ammonium-chloride-containing solutions (AC) are routinely used to lyse red blood cells during preparation of PBMC. Although exposure to AC has been described to affect the ultrastructural appearance of large granular lymphocytes and to temporarily inhibit cytolytic activity of PBMC preparations, the cellular basis of this phenomenon has not been studied. Here, the inhibitory effect of AC on human CTL and NK-mediated cytotoxicity has been analyzed in 4-h 51Cr-release assays. The results show that NK killing of K562 leukemia cells and xenogeneic endothelial cells is inhibited by AC exposure. The effect is dose-dependent and reversible, because recovery of cytotoxicity is observed within 15 h of re-culturing. AC does not reduce the viability of NK cells and the inhibitory effect is not mediated by the exhaustive release of granzymes upon AC treatment. In contrast, antigen-specific CTL killing of EBV-transformed B-lymphoblastoid cell lines and xenogeneic PHA lymphoblasts was less sensitive to AC and data are presented suggesting that FasL-induced apoptosis is not inhibited by AC. In conclusion, perforin-mediated NK killing is AC-sensitive whereas CTL killing and FasL-mediated killing appear to be AC-resistant. Therefore, AC represents a powerful tool to study different mechanisms of cell-mediated cytotoxicity and may be helpful in assessing antigen-specific CTL cytotoxicity without the influence of NK cell-mediated background killing.

Group B Streptococcus induces apoptosis in macrophages

Group B Streptococcus (GBS) is a pathogen that has developed some strategies to resist host immune defenses. Because phagocytic killing is an important pathogenetic mechanism for bacteria, we investigated whether GBS induces apoptosis in murine macrophages. GBS type III strain COH31 r/s (GBS-III) first causes a defect in cell membrane permeability, then at 24 h, apoptosis. Apoptosis was confirmed by several techniques based on morphological changes and DNA fragmentation. Cytochalasin D does not affect apoptosis, suggesting that GBS-III needs not be within the macrophage cytoplasm to promote apoptosis. Inhibition of host protein synthesis prevents apoptosis, whereas inhibition of caspase-1 or -3, does not. Therefore, GBS can trigger an apoptotic pathway independent of caspase-1 and -3, but dependent on protein synthesis. Inhibition of apoptosis by EGTA and PMA, and enhancement of apoptosis by calphostin C and GF109203X suggests that an increase in the cytosolic calcium level and protein kinase C activity status are important in GBS-induced apoptosis. Neither alteration of plasma membrane permeability nor apoptosis were induced by GBS grown in conditions impeding hemolysin expression or when we used dipalmitoylphosphatidylcholine, which inhibited GBS beta-hemolytic activity, suggesting that GBS beta-hemolysin could be involved in apoptosis. beta-Hemolysin, by causing membrane permeability defects, could allow calcium influx, which initiates macrophage apoptosis. GBS also induces apoptosis in human monocytes but not in tumor lines demonstrating the specificity of its activity. This study suggests that induction of macrophage apoptosis by GBS is a novel strategy to overcome host immune defenses.

Signaling and transcription in T helper development

The recognition of polarized T cell subsets defined by cytokine production was followed by a search to define the factors controlling this phenomenon. Suitable in vitro systems allowed the development of cytokine "recipes" that induced rapid polarization of naïve T cells into Th1 or Th2 populations. The next phase of work over the past several years has begun to define the intracellular processes set into motion during Th1/Th2 development, particularly by the strongly polarizing cytokines IL-12 and IL-4. Although somewhat incomplete, what has emerged is a richly detailed tapestry of signaling and transcription, controlling an important T cell developmental switch. In addition several new mediators of control have emerged, including IL-18, the intriguing Th2-selective T1/ST2 product, and heterogeneity in dendritic cells capable of directing cytokine-independent Th development.

Displacement of linker for activation of T cells from the plasma membrane due to redox balance alterations results in hyporesponsiveness of synovial fluid T lymphocytes in rheumatoid arthritis

The T lymphocytes that reside in the synovium of the inflamed joints in patients with rheumatoid arthritis display severe hyporesponsiveness upon antigenic stimulation, which is probably due to their constant subjection to high levels of oxidative stress. Here we report that the synovial fluid T lymphocytes exert severely impaired phosphorylation of the adaptor protein linker for activation of T cells (LAT), a crucial component of the TCR-mediated signaling pathways. In healthy T lymphocytes, LAT is a membrane-bound protein and becomes phosphorylated by zeta-associated protein of 70 kDa (ZAP-70) upon TCR engagement. The molecular basis underlying the deficient phosphorylation of LAT and consequently the hyporesponsiveness of the synovial fluid T lymphocytes lies in the membrane displacement of LAT. We demonstrate that the subcellular localization of LAT is sensitive to changes in the intracellular levels of the antioxidant glutathione. The membrane anchorage of LAT, and consequently the phosphorylation of LAT and the cellular activation of the synovial fluid T lymphocytes upon TCR engagement, is restored in synovial fluid T lymphocytes after supplementation of the intracellular glutathione levels with N-acetyl-l -cysteine. These data suggest a role for the membrane displacement of LAT in the hyporesponsiveness of the synovial fluid T lymphocytes as a consequence of oxidative stress.

T Cell Activation Up-Regulates the Expression of the Focal Adhesion Kinase Pyk2: Opposing Roles for the Activation of Protein Kinase C and the Increase in Intracellular Ca2+

T cell activation initiates signals that control gene expression of molecules important for T cell function. The focal adhesion kinase Pyk2 has been implicated in T cell signaling. To further analyze the involvement of Pyk2 in T cell processes, we examined the effect of T cell stimulation on the expression of Pyk2. We found that TCR ligation or PMA increased Pyk2 expression in Jurkat T cells and in normal T cells. In contrast, TCR ligation and PMA failed to induce any detectable increase in the expression of the other member of the focal adhesion kinase family, Fak, in Jurkat T cells and induced only a weak increase in Fak expression in normal T cells. The serine/threonine kinases, protein kinase C and mitogen-activated protein/extracellular signal-related kinase kinase (MEK), regulated Pyk2 expression, as inhibitors of these kinases blocked stimulus-induced Pyk2 expression. Cyclosporin A, FK506, and KN-62 did not block Pyk2 expression; thus, calcineurin and Ca2+/calmodulin-activated kinases are not critical for augmenting Pyk2 expression. TCR ligation increased Pyk2 mRNA, and the transcriptional inhibitor actinomycin D blocked Pyk2 expression. Strikingly, Ca2+ ionophores, at concentrations that in combination with other stimuli induced IL-2 expression, blocked TCR- and PMA-induced up-regulation of Pyk2 expression. Thus, the increase in Ca2+ has opposing effects on IL-2 and Pyk2 expression. Cyclosporin A and FK506, but not KN-62, blocked Ca2+ ionophore-mediated inhibition of Pyk2 expression, implicating calcineurin in down-regulating Pyk2 expression. These results show that TCR-triggered intracellular signals increase Pyk2 expression and shed light on the molecular mechanisms that regulate Pyk2 expression in T cells.

T cell receptor-induced activation and apoptosis in cycling human T cells occur throughout the cell cycle

Previous studies have found conflicting associations between susceptibility to activation-induced cell death and the cell cycle in T cells. However, most of the studies used potentially toxic pharmacological agents for cell cycle synchronization. A panel of human melanoma tumor-reactive T cell lines, a CD8+ HER-2/neu-reactive T cell clone, and the leukemic T cell line Jurkat were separated by centrifugal elutriation. Fractions enriched for the G0-G1, S, and G2-M phases of the cell cycle were assayed for T cell receptor-mediated activation as measured by intracellular Ca(2+) flux, cytolytic recognition of tumor targets, and induction of Fas ligand mRNA. Susceptibility to apoptosis induced by recombinant Fas ligand and activation-induced cell death were also studied. None of the parameters studied was specific to a certain phase of the cell cycle, leading us to conclude that in nontransformed human T cells, both activation and apoptosis through T cell receptor activation can occur in all phases of the cell cycle.

Differential Coupling of Second Signals for Cytotoxicity and Proliferation in CD8+ T Cell Effectors: Amplification of the Lytic Potential by B7

The role of second signals delivered through B7/CD28 interactions in T cell activation is well documented. However, once CTLs are elicited, TCR-mediated cytotoxicity appears to be uncoupled from the requirement for costimulatory signals. In this study, we show an uncoupling across a broad range of concentrations of peptide, thus demonstrating that cytolysis is a TCR-mediated response that is fully independent of costimulatory signals. However, the same T cell effectors remain fully responsive to B7 engagement, which is able to amplify Ag-mediated proliferation and cytolytic capacity. B7 expression by targets results in an IL-2-mediated proliferative expansion of the effectors concurrent with the elimination of the targets. Thus, costimulation of effectors results in a vast expansion in lytic units over time, which does not occur in the absence of IL-2 or B7. Both TCR-derived and second signals appear to be necessary to achieve this result. These results suggest that B7-expressing APC or a cohort of IL-2-producing helper cells would functionally extend the duration and effectiveness of the cytotoxic response occurring in localized immune responses.

Na(+)/Ca(2+) exchange inhibitors modulate thapsigargin-induced Ca(2+) and Na(+) influx in human lymphocytes

Thapsigargin has been shown the elevate intracellular Na(+) concentration in human lymphocytes, but mechanisms underlying thapsigargin-induced Na(+) entry are little understood. In the present study we investigated thapsigargin-induced changes in cytosolic free Na(+) and Ca(2+) concentration in human lymphocytes after inhibition of the Na(+)/Ca(2+) exchange with two structurally unrelated compounds, dimethylthiourea ad bepridil. The intracellular Na(+) increase induced by 5 microM thapsigargin was significantly enhanced in the presence of 5 mM dimethylthiourea or 40 microM bepridil. In contrast, both compounds significantly decreased the thapsigargin-induced intracellular Ca(2+) elevation. No effect of dimethylthiourea or bepridil on thapsigargin-induced Ca(2+) influx was observed in the absence of extracellular Na(+). These observations are consistent with the hypothesis that thapsigargin stimulates Na(+)/Ca(2+ )exchange in human lymphocytes. However, Na(+)/Ca(2+) exchange does not mediate Na(+) influx in human lymphocytes.

Polyamine-fas interactions: inhibition of polyamine biosynthesis in MRL-lpr/lpr mice is associated with the up-regulation of fas mRNA in thymocytes

MRL-lpr/lpr is a strain of mice that develops spontaneous signs of the autoimmune disease, systemic lupus erythematosus (SLE or lupus). The lpr (lymphoproliferation) defect has been identified as an insertion of an early transposon (ETn) derived sequence into the fas apoptosis gene. We studied the in vivo effects of difluoromethylornithine (DFMO), an irreversible inhibitor of the polyamine biosynthetic enzyme, ornithine decarboxylase (ODC), on the expression of fas in MRL-lpr/lpr mice as well as in congenic MRL- + / + and autoimmune NZB/W strains. Using Northern blot hybridization and reverse transcription polymerase chain reaction (RT-PCR), we found that DFMO treatment resulted in an increase in the expression of fas mRNA in the thymus of MRL-lpr/lpr mice. Using RT-PCR, we further found that the increased expression of fas was associated with the suppression of chimeric ETn/fas mRNA. With fractionated CD4 + and CD8 + T cells, we found a cell-specific effect of DFMO on chimeric ETn/fas expression in CD8 + cells. ETn/fas expression was detected in CD8+ T cells from untreated mice, but it was eliminated after DFMO treatment. HPLC analysis of polyamines showed depletion of putrescine and partial reduction of spermidine (35%) in DFMO-treated mice compared to controls. These results indicate that DFMO-mediated polyamine depletion is linked to the regulation of fas and chimeric ETn/fas in MRL-lpr/lpr mice. Elevated levels of polyamines in this strain, as found in earlier studies, may be associated with the progression of the autoimmune disease by altering the expression of fas gene or by facilitating the expression of chimeric ETn/fas. Our data also provide new mechanistic insights into the beneficial effects of DFMO on these mice.

Generation of IL-2-Dependent Cytolytic T Lymphocytes (CTLs) with Altered TCR Responses Derived from Antigen-Dependent CTL Clones

Ag-specific CD8+ CTL clones require TCR stimulation to respond to IL-2 for growth. Because IL-2 may be produced in the vicinity of CD8+ CTLs when Ag is limiting at the end of an immune response, we have examined the effect of culturing viral-specific CTL clones in IL-2 in the absence of antigenic stimulation. Limiting dilution analysis revealed a high precursor frequency for CTL clones derived from IL-2 propagation (termed CTL-factor dependent (FD)) that are dependent upon exogenous IL-2 for growth and survival and no longer require TCR stimulation to proliferate. Culturing CTL-FDs with infected splenocytes presenting Ag and IL-2 did not revert the clones but did lead to a TCR-induced inhibition of proliferation. The derived CTL-FDs have lost the ability to kill via the perforin/granule exocytosis mechanism of killing, although they express similar levels of TCR, CD3ε, CD8αβ, CD45, and LFA-1 compared with the parental clones. The CTL-FDs retain Fas ligand/Fas-mediated cytotoxicity, and IFN-γ production and regulate the expression of CD69 and IL-2Rα when triggered through the TCR. A parental CTL protected BALB/c mice from a lethal challenge of influenza virus, whereas a CTL-FD did not. These findings represent a novel regulatory function of IL-2 in vitro that, if functional in vivo, may serve to down-regulate cellular immune responses.

Phosphatidylinositol 3-Kinase-Dependent and -Independent Cytolytic Effector Functions

Two distinct forms of short-term cytolysis have been described for CD8+ CTLs, the perforin/granzyme- and Fas ligand/Fas (CD95 ligand (CD95L)/CD95)-mediated pathways. However, the difference in signal transduction events leading to these cytolytic mechanisms remains unclear. We used wortmannin, an irreversible antagonist of phosphatidylinositol 3-kinase (PI3-K) activity, to investigate the role of PI3-K in influenza-specific CD8+ CTL cytolytic effector function. We found that the addition of wortmannin at concentrations as low as 1 nM significantly inhibited both the Ag/MHC-induced cytolysis of CD95− target cells and serine esterase release. In strong contrast, W did not inhibit the Ag/MHC-induced CD95L expression or the CD95L/CD95-mediated cytolysis of CD95+ targets. A combination of wortmannin and blocking mAb against CD95L inhibited the cytolysis of CD95+ targets, indicating that the wortmannin-independent cytolysis was due to CD95L/CD95 mediated cytolysis. These findings suggest a differential role for PI3-K in mediating cytolysis and, thus far, the earliest difference between perforin/granzyme- and CD95L/CD95-dependent cytolysis. Our data reinforce the idea of a TCR with modular signal transduction pathways that can be triggered or inhibited selectively, resulting in differential effector function.

Stimulation through CD50 preferentially induces apoptosis of TCR1+ human peripheral blood lymphocytes

Apoptosis has an important role in several key immunological phenomena such as regulation of the immune response, and deletion of auto-reactive cells. This phenomenon is induced following the interaction of several cell membrane receptors with their respective ligands or after cell activation. We have studied the possible effect of signaling through CD50/ICAM-3 and CD69/AIM on apoptosis of peripheral blood lymphocytes. Apoptosis was assessed by both flow cytometry analysis (content of cell DNA and binding to annexin V), and detection of DNA fragmentation by agarose gel electrophoresis. We found that a stimulatory anti-CD50 mAb was able to induce a small but significant degree of apoptosis in resting peripheral blood mononuclear cells from most donors; this effect was dose-dependent and was evident as early as at 12 h, with a maximal induction at 48 h. Studies with T and non-T cells showed that only the former cell population was sensitive to the induction of apoptosis through CD50. Further experiments revealed that the anti-ICAM-3 mAb preferentially induced apoptosis of TCR gamma delta-bearing cells. In addition, we found a significant increase in Cai2+ in PBMC stimulated with an anti-CD50 mAb, suggesting the involvement of this signaling pathway in the induction of apoptosis through this adhesion receptor. In contrast, under our experimental conditions, stimulation through CD69 did not have any effect on the induction of apoptosis on either cultured T lymphoblasts or PMA-stimulated PBMC. Our findings suggest that the interaction of CD50 with its natural ligand LFA-1 results in the induction of apoptosis in a significant fraction of resting PBMC. This phenomenon may be involved in immune regulation, lymphocyte turnover and peripheral deletion of auto-reactive cells.

Peptide Modification or Blocking of CD8, Resulting in Weak TCR Signaling, Can Activate CTL for Fas- But Not Perforin-Dependent Cytotoxicity or Cytokine Production

This study describes a form of partial agonism for a CD8+CTL clone, S15, in which perforin-dependent killing and IFN-γ production were lost but Fas (APO1 or CD95)-dependent cytotoxicity preserved. Cloned S15 CTL are H-2Kd restricted and specific for a photoreactive derivative of the Plasmodium berghei circumsporozoite peptide PbCS 252–260 (SYIPSAEKI). The presence of a photoactivatable group in the epitope permitted assessment of TCR-ligand binding by TCR photoaffinity labeling. Selective activation of Fas-dependent killing was observed for a peptide-derivative variant containing a modified photoreactive group. A similar functional response was obtained after binding of the wild-type peptide derivative upon blocking of CD8 participation in TCR-ligand binding. The epitope modification or blocking of CD8 resulted in an ≥8-fold decrease in TCR-ligand binding. In both cases, phosphorylation of ζ-chain and ZAP-70, as well as calcium mobilization were reduced close to background levels, indicating that activation of Fas-dependent cytotoxicity required weaker TCR signaling than activation of perforin-dependent killing or IFN-γ production. Consistent with this, we observed that depletion of the protein tyrosine kinase p56lck by preincubation of S15 CTL with herbimycin A severely impaired perforin- but not Fas-dependent cytotoxicity. Together with the observation that S15 CTL constitutively express Fas ligand, these results indicate that TCR signaling too weak to elicit perforin-dependent cytotoxicity or cytokine production can induce Fas-dependent cytotoxicity, possibly by translocation of preformed Fas ligand to the cell surface.

Selective Induction of CD8+ Cytotoxic T Lymphocyte Effector Function by Staphylococcus Enterotoxin B

Upon encounter with its antigenic stimulus, CTL characteristically proliferate, produce cytokines, and lyse the Ag-presenting cell in an attempt to impede further infection. Superantigens are extremely efficient immunostimulatory proteins that promote high levels of proliferation and massive cytokine production in reactive T cells. We compared the activation of murine influenza-specific CD8+ CTL clones stimulated with either influenza peptide or the superantigen staphylococcus enterotoxin B (SEB). We found that influenza peptide/MHC and SEB appeared equally capable of eliciting proliferation and IFN-γ production. However, while influenza peptide/MHC elicited both perforin- and Fas ligand (FasL)/Fas (CD95L/CD95)-mediated cytolytic mechanisms, SEB was unable to trigger perforin-mediated cytolysis or serine esterase release. Examination of intracellular Ca2+ mobilization events revealed that the ability to trigger intracellular Ca2+ flux was not comparable between influenza peptide and SEB. SEB stimulated only a small rise in levels of intracellular Ca2+, at times indistinguishable from background. These findings indicate that the short-term cytolytic potential of superantigen-activated CD8+ CTL clones appears to be restricted to FasL/Fas (CD95L/CD95) mediated cytolysis.

The Regulation of CD95 Ligand Expression and Function in CTL

Previous studies with CTL lines and CTL hybridomas have suggested that functional CD95 (APO-1/Fas)-ligand (CD95L) expression on effector CTLs is a consequence of specific CTL-target recognition and TCR triggering of newly transcribed CD95L. Such a control on the expression of CD95L could provide a double safeguard for killing only cognate target cells. Here the regulation of CD95L expression and function was tested in in vivo primed, alloreactive peritoneal exudate CTL (PEL) from perforin-deficient (P0) mice. CD95L-based, PEL-mediated cytotoxicity was blocked by brefeldin A, an inhibitor of intracellular protein transport, but not by the protein synthesis inhibitor emetine, the immunosuppressive drug cyclosporin A, or the DNA transcription inhibitor actinomycin D. CD95L mRNA transcripts in freshly isolated PEL were shown by RT-PCR; CD95L surface expression was evident by staining with Fas-Fc as well as CD95L Abs. Undiminished CD95L expression and cytocidal activity were found in PEL incubated for 48 h in culture, without adding Ag, mitogen, or cytokines. PEL expressed functional CD95L, yet exhibited target cell-specific killing, except when encountering high CD95-expressing cells. The results indicate that PEL use CD95L probably expressed in the Golgi and/or on the cell surface and do not require newly transcribed CD95L upon target cell conjugation. Hence the TCR-triggered recruitment of preformed CD95L, rather than its biosynthesis, controls CD95L-based specific lysis induced by CTL.