Apoptosis in L929 cells expressing a CD40/Fas chimeric receptor: dissociation of stimulatory from inhibitory death signalling functions

Qualitative and quantitative differences in the intensity of Fas-mediated intracellular signals determine life and death in T cells

Fas stimulation has been reported to promote the activation and proliferation of T lymphocytes, but the intracellular signalling pathways that mediate non-apoptotic responses to Fas are poorly defined. To distinguish between the activation signalling and the death-inducing pathway downstream of Fas, we generated a novel T cell line expressing a chimeric hCD8-FasC protein and found that stimulation with the anti-CD8 antibodies induced tyrosine phosphorylation of TCR-proximal proteins, activation of Raf-1/ERK, p38 and JNK, and increased expression of CD69, Fas, and Fas ligand. Stimulation of hCD8-FasC-induced activation of an atypical NF-kappaB pathway, partial cleavage of caspases, and increased expression of TRAF1, FLIP(L) and FLIP(S), thereby protecting T cells from FasL-mediated apoptosis. The proliferative response transmitted through hCD8-FasC chimeric receptors was converted into death signals when cells were stimulated, resulting in increased expression of IL-2 and Nur77 and increased caspase cleavage. Surprisingly, both the enhanced expression of FLIP(L) and FLIP(S) and the complete inhibition of FLIP(S) expression were functionally associated with cell death induction. These findings imply that Fas is able to trigger intracellular signalling events driving both apoptosis and activation of T cells but that cell fate is determined by quantitative and qualitative differences in intracellular signalling following Fas stimulation.

CD40 ligation impedes lymphoblastoid B cell proliferation and S-phase entry

The CD40 cell surface antigen and member of the tumor necrosis factor (TNF) receptor superfamily is expressed in many cell types, including normal and neoplastic B cells. Signaling through CD40 induces B cell proliferation, differentiation and, in some circumstances, protects the B cell from apoptosis. Lymphoblastoid cells (LCLs) resemble the malignant B cells that comprise the Epstein-Barr virus (EBV)-associated posttransplant lymphoproliferative disorders, in that the cells bear a highly activated phenotype and, unlike most other EBV positive tumor cells, express the majority of latent EBV genes. In this study, we use assays of cell viability, proliferation, cell cycle and apoptosis to demonstrate that ligation of the CD40 receptor in EBV-transformed LCLs inhibits their growth. The process does not involve apoptosis, but is characterized by reduced S-phase entry from G0/G1. A better understanding of the negative effects of CD40 ligation in these cells may offer clues for the development of novel therapies in EBV-related B cell disorders.

Apoptosis through CD95 (Fas/APO-1), but not a CD40/CD95 chimeric receptor, is inhibited by phorbol-12-myristate-13-acetate

CD95 (Fas/APO-1)-mediated apoptosis appears to be regulated by positive and negative signaling molecules. A human CD40/CD95 chimeric receptor was stably transfected into CD95-expressing human Jurkat T cells, and signaling through native and chimeric CD95 was compared in the same cell type to assess contributions of the CD95 extracellular and intracellular domains. Apoptosis was induced in these transfectants by soluble CD40 ligand and also by the anti-CD40 monoclonal antibodies (mAb) M2 and M3. The M2 mAb was more effective than M3 in these transfectants. In contrast to apoptosis mediated through native CD95, CD40/CD95-mediated apoptosis was not inhibited by phorbol-12-myristate-13-acetate (PMA). The apoptotic response to the anti-CD40 mAb M3, but not M2, was enhanced by PMA and dibutyryl cyclic adenosine monophosphate (db-cAMP), which also increased mRNA levels and surface expression of CD40/CD95. The enhancing effects of PMA, but not those of db-cAMP, were sensitive to cycloheximide. The M2 and M3 mAbs appeared to have virtually identical binding affinities but, when added to cells together, M3 inhibited M2-induced apoptosis. These mAbs may bind neighboring epitopes, but M2 induces a more effective signaling-competent conformation upon the chimeric receptor. These data suggest that dimerization, however only in a signaling-competent conformation, was sufficient to induce apoptosis. When expressed as a chimera with the CD40 extracellular domain, the CD95 intracellular domain was not inhibited by protein kinase C (PKC)-dependent pathways, suggesting that the CD95 extracellular domain is required for association with a molecule that inhibits the apoptotic signal.

Interferon-gamma-dependent stimulation of Fas antigen in SV40-transformed human keratinocytes: modulation of the apoptotic process by protein kinase C

Fas antigen is a cell membrane protein that has been suggested to mediate apoptosis. Using SV40-transformed human keratinocytes, we investigated the Fas-antigen-dependent apoptotic process. The expression of Fas antigen mRNA was markedly induced by interferon-gamma (IFN-gamma) treatment (500 U/ml). After IFN-gamma treatment in the presence of anti-Fas monoclonal antibody, apoptosis was induced, as detected by the formation of nucleosome-sized fragments of DNA and morphologically by apoptotic cells with round homogeneous nuclear beads detected by acridine orange staining. The apoptotic SV40-transformed keratinocytes were analyzed quantitatively by enzyme-linked immunosorbent assay using antihistone and peroxidase-conjugated anti-DNA antibodies to detect cell death. The IFN-gamma- and anti-Fas antibody-dependent apoptotis was observed by 3 h, and the maximal response was observed by 12 h. The induction of apoptosis was significantly augmented by treatment with 10 ng/ml 12-o-tetradecanoyl-phorbol-13-acetate (TPA). TPA alone had no effect on either Fas antigen expression or on the apoptotic process. Other protein kinase C activators (1-oleoyl-2-acetylglycerol and mezerein) also stimulated IFN-gamma-dependent apoptosis, whereas 4-o-methyl phorbol myristate acetate, a very weak protein kinase C activator, had only a slight effect. The TPA-induced augmentation of apoptosis was inhibited by the protein kinase C inhibitor, 1-(5-isoquinoline-sulfonyl)-2-methyl piperazine dihydrochloride (H-7). However, H-7 inhibited only the TPA-induced augmentation of apoptosis; the basal IFN-gamma- and anti-Fas-dependent apoptosis remained in the presence of H-7. Northern blot analysis revealed that c-jun mRNA was induced by IFN-gamma plus anti-Fas antibody treatment as well as by TPA treatment; the addition of IFN-gamma alone to the incubation medium had no effect on the expression of c-jun mRNA. These results indicate that IFN-gamma induces a Fas-antigen-dependent apoptotic process in SV40-transformed keratinocytes and that TPA augments the process through the activation of protein kinase C.

Identification of a silencer, enhancer, and basal promoter region in the human CD95 (Fas/APO-1) gene

Genomic clones for the human CD95 (Fas/APO-1) and CD40 genes have been isolated and 2.3 kb of the CD95 and 0.8 kb of the CD40 gene 5'-flanking regions sequenced. Comparisons of the human CD95 gene with the human CD40 and the murine CD40 and TNFR-II genes showed a low degree of sequence similarity. However, dot matrix analyses revealed conservation of two stretches between human CD95 (-387 to -362 and -288 to 261 in CD95) and murine TNFR-II genes. Additionally, TCCTCC motifs are present within 400 bp up-stream of the ATG of all genes examined. Repeated interferon-beta (IFN-beta) silencer B motifs and a lysozyme silencer 1 motif have been found in the CD95 gene at approximately -1,600 and -1,100, respectively. Sequence comparison of the 5'-flanking regions of the murine and human CD40 genes revealed the presence of a conserved AP-4 site and two SP-1 sites. CD95, CD40, and TNFR-II genes all lack classical TATA and CAAT boxes. However, a strongly increased frequency of CpG dinucleotides was found. Primer extension analysis revealed multiple transcriptional start sites in the CD95 gene, where the usage of individual start sites appeared to be cell type-specific. Functional analysis, using reporter constructs and transient transfections, identified a silencer activity residing between nucleotide positions -1,781 and -1007 and a strong enhancer region between -1,007 and -425 in the human CD95 gene. The region between -425 and -1 retained a basal promoter activity.(ABSTRACT TRUNCATED AT 250 WORDS)