Melanoma cell expression of Fas(Apo-1/CD95) ligand: implications for tumor immune escape

Constitutive Fas ligand gene transcription in Sertoli cells is regulated by Sp1

The transcriptional regulation of the Fas ligand (FasL) gene in Sertoli cells was investigated, as these cells are known to have constitutive expression of FasL and hence maintain an "immune privileged" environment within the testicle. Using the Sertoli cell line TM4, it was demonstrated that a gene segment of the 5'-untranslated region located between -318 and -237 relative to the translation start site is required for constitutive FasL transcription. Deletion and mutation analysis demonstrate that an Sp1 rather than an NFAT or NFKB-like DNA binding motif present within this region is necessary and sufficient for constitutive FasL gene transcription. Nuclear extracts of Sertoli cells contain Sp1 and Sp3 that specifically binds to the Sp1 motif present in the FasL gene, and overexpression of Sp1 but not Sp3 leads to a further increase of transcription from the FasL promoter-enhancer region. The data presented demonstrates that constitutive FasL gene transcription in Sertoli cells is regulated by Sp1. In addition, it is shown that basal FasL expression in Jurkat T cells is also controlled by Sp1 and this is in contrast to induced FasL expression, which is NFAT-dependent.

Fas-Mediated Suicide of Tumor-Reactive T Cells Following Activation by Specific Tumor: Selective Rescue by Caspase Inhibition

CD8+ T lymphocytes that specifically recognize tumor cells can be isolated and expanded ex vivo. While the lytic properties of these cells have been well described, their fate upon encounter with cognate tumor is not known. We performed reverse 51Cr release assays in which the lymphocyte effectors rather than the tumor cell targets were radioactively labeled. We found that melanoma tumor cells caused the apoptotic death of tumor-specific T cells only upon specific MHC class I-restricted recognition. This death was entirely blockable by the addition of an Ab directed against the Fas death receptor (APO-1, CD95). Contrary to the prevailing view that tumor cells cause the death of anti-tumor T cells by expressing Fas ligand (FasL), our data suggested that FasL was instead expressed by T lymphocytes upon activation. While the tumor cells did not express FasL by any measure (including RT-PCR), functional FasL (as well as FasL mRNA) was consistently found on activated anti-tumor T cells. We could successfully block the activation-induced cell death with z-VAD-fmk, a tripeptide inhibitor of IL-1β-converting enzyme homologues, or with anti-Fas mAbs. Most importantly, these interventions did not inhibit T cell recognition as measured by IFN-γ release, nor did they adversely affect the specific lysis of tumor cell targets. These results imply that Fas-mediated activation-induced cell death could be a limiting factor in the in vivo efficacy of adoptive transfer of class I-restricted CD8+ T cells and provide a means of potentially enhancing their growth in vitro as well as their function in vivo.

FasL is more frequently expressed in liver metastases of colorectal cancer than in matched primary carcinomas

Colorectal carcinoma cells have recently been shown to express Fas ligand (FasL). This ligand could allow the tumour cells to evade activated tumour-infiltrating lymphocytes (TILs) by inducing their apoptosis and would thus promote tumour survival and possibly metastasis formation. To test this hypothesis in vivo we analysed the expression of FasL mRNA and protein in paired tissue samples of normal colonic mucosa (N), primary colorectal carcinomas (T) and their metastases (M) from a total of 21 patients by four different methods. Additionally, the presence and activation status of infiltrating lymphocytes, which might contribute to the total amount of FasL in the tissue, was determined by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) in the same samples. The frequency of FasL detection was 30-40% in T and was 60-100% in M, depending on the sensitivity of the method. Simultaneously, the amount of CD25 mRNA, used as a measure of the number of activated TILs, was in 90% of patients lower in M than in T. The increased frequency of FasL detection in liver metastases was therefore not due to the presence of activated TILs. We conclude that metastasizing subpopulations of colorectal tumour cells express FasL more frequently than the primary carcinomas and may be able to eliminate activated TILs in vivo via Fas/FasL-induced apoptosis or other hitherto unknown mechanisms.

Involvement of Fas (CD95/APO-1) and Fas ligand in apoptosis induced by ganciclovir treatment of tumor cells transduced with herpes simplex virus thymidine kinase

Transduction of cancer cells with herpes simplex virus thymidine kinase gene (HSVtk) followed by prodrug ganciclovir (GCV) treatment has been shown to induce apoptosis. In this study, four murine tumors including B16F10 melanoma, NG4TL4 sarcoma, H6 hepatoma and 1MEA 7R.1 hepatoma were found to vary in sensitivity to this gene therapy strategy in vitro but, at effective doses of GCV, the HSVtk-transduced cells of all four tumors showed similar kinetics of early rise in p53 protein levels, then cell cycle S-/G2-phase arrest and finally signs of apoptosis. Immunoblot analyses revealed that Fas (CD95/APO-1), Fas ligand (FasL) and two downstream mediators, RIP and caspase-3, (CPP32, YAMA, Apopain) were increased in GCV-treated HSVtk-transduced tumor cells the cell cycle arrest and before apoptosis. Increased expression of FasL could also be observed in vivo in HSVtk-transduced tumors induced to regress by GCV treatment. Enzyme measurements using specific substrate showed that the caspase-3 activation followed kinetically the FasL expression. More than half of the HSVtk/GCV-induced cell death could be abrogated by addition to the cell culture medium of a specific antisense oligonucleotide to block FasL synthesis, a recombinant Fas/Fc chimeric protein to compete with Fas receptor for FasL binding, or cell-permeable specific tetrapeptide inhibitors of caspase-3 or caspase-8.

Fas ligand (CD95 ligand) controls angiogenesis beneath the retina

A principal cause of blindness is subretinal neovascularization associated with age-related macular degeneration. Excised neovascular membranes from patients with age-related macular degeneration demonstrated a pattern of Fas+ new vessels in the center of the vascular complex, surrounded by FasL+ retinal pigment epithelial cells. In a murine model, Fas (CD95)-deficient (Ipr) and FasL-defective (gld) mice had a significantly increased incidence of neovascularization compared with normal mice. Furthermore, in gld mice there is massive subretinal neovascularization with uncontrolled growth of vessels. We found that cultured choroidal endothelial cells were induced to undergo apoptosis by retinal pigment epithelial cells through a Fas-FasL interaction. In addition, antibody against Fas prevented vascular tube formation of choroidal endothelial cells derived from the eye in a three-dimensional in vitro assay. Thus, FasL expressed on retinal pigment epithelial cells may control the growth and development of new subretinal vessels that can damage vision.

Functional Analysis of TRAIL Receptors Using Monoclonal Antibodies

mAbs were generated against the extracellular domain of the four known TNF-related apoptosis-inducing ligand (TRAIL) receptors and tested on a panel of human melanoma cell lines. The specificity of the mAb permitted a precise evaluation of the TRAIL receptors that induce apoptosis (TRAIL-R1 and -R2) compared with the TRAIL receptors that potentially regulate TRAIL-mediated apoptosis (TRAIL-R3 and -R4). Immobilized anti-TRAIL-R1 or -R2 mAbs were cytotoxic to TRAIL-sensitive tumor cells, whereas tumor cells resistant to recombinant TRAIL were also resistant to these mAbs and only became sensitive when cultured with actinomycin D. The anti-TRAIL-R1 and -R2 mAb-induced death was characterized by the activation of intracellular caspases, which could be blocked by carbobenzyloxy-Val-Ala-Asp (OMe) fluoromethyl ketone (zVAD-fmk) and carbobenzyloxy-Ile-Glu(OMe)-Thr-Asp (OMe) fluoromethyl ketone (zIETD-fmk). When used in solution, one of the anti-TRAIL-R2 mAbs was capable of blocking leucine zipper-human TRAIL binding to TRAIL-R2-expressing cells and prevented TRAIL-induced death of these cells, whereas two of the anti-TRAIL-R1 mAbs could inhibit leucine zipper-human TRAIL binding to TRAIL-R1:Fc. Furthermore, use of the blocking anti-TRAIL-R2 mAb allowed us to demonstrate that the signals transduced through either TRAIL-R1 or TRAIL-R2 were necessary and sufficient to mediate cell death. In contrast, the expression of TRAIL-R3 or TRAIL-R4 did not appear to be a significant factor in determining the resistance or sensitivity of these tumor target cells to the effects of TRAIL.

Regulation of CD95 (Apo-1/Fas) ligand and receptor expression in squamous-cell carcinoma by interferon-gamma and cisplatin

CD95 (Apo-1/Fas) ligand (CD95L) expression has been observed in various malignancies. In human primary cell lines from a squamous cell carcinoma (SCC) of the vulva, the effect of cisplatin (CDDP) and IFNgamma on the expression of CD95L and its 2 receptor isoforms, CD95 transmembrane (CD95tm) and CD95 soluble receptor, was studied at the mRNA and protein levels. Addition of CDDP and IFNgamma increased CD95L mRNA levels in the primary cell line 6-fold and 1.7-fold, respectively. In comparison, CD95tm mRNA levels were diminished by CDDP but increased 8-fold upon IFNgamma challenge. CD95L expressed by SCC cells was functionally relevant since these cells were able to induce CD95-specific apoptosis in autologous lymphocytes from the SCC-bearing patient. Thus, CD95L expression in SCC may contribute to tumor-associated immunosuppression, which may be modulated by CDDP and IFNgamma. In tumor samples of the primary SCC, CD95L expression was enhanced in the area of the border between invasive tumor tissue and surrounding stroma cells. The locally restricted over-expression of CD95L was congruent with the arrangement of apoptotic stroma cells in the direct vicinity of invading tumor tongues, suggesting a role as invasion factor for CD95L.

Expression of apoptotic regulatory molecules in renal cell carcinoma: elevated expression of Fas ligand

Renal cell carcinoma (RCC) is the most common renal neoplasm. Despite being infiltrated by tumour infiltrating lymphocytes (TIL), these TIL are unable to control tumour growth in vivo, suggesting that the cytotoxic capacity of TIL against RCC is impaired, or that the tumour cells are resistant to killing and therefore escape detection by the immune system. It is postulated that the expression of apoptotic regulatory molecules in RCC favours tumour cell survival. The present study has therefore determined the expression of Fas (APO-1/CD95), Fas ligand (Fas L) and bcl-2 in these tumours. The expression of Fas, Fas L and bcl-2 mRNA transcripts was determined in RCC, normal kidney and peripheral blood by semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR), following RNA extraction and cDNA synthesis from tissues and cell samples. Transcript levels were measured by densitometry after Southern blot hybridization of PCR products with internal radio-labelled oligonucleotide probes; a densitometry score was assigned to each hybridizing DNA band and expressed as a ratio of the glyceraldehyde-3-phosphate dehydrogenase content. In peripheral blood, the expression of Fas L and bcl-2 transcripts was similar between patients and normal healthy individuals; however, Fas transcript expression was significantly down-regulated in the patients' versus normal peripheral blood (P = 0.026). Most interestingly, significantly up-regulated Fas L expression was observed in RCC compared to normal kidney (P = 0.041). In contrast, bcl-2 transcripts were well represented in normal kidney but markedly decreased in RCC (P = 0.021). The expression of Fas transcripts in normal kidney and RCC was variable. These data demonstrate elevated expression of Fas L transcripts in RCC, but the functional relevance of this remains to be investigated.

DNA damage signals induction of fas ligand in tumor cells

Many anticancer agents exert their cytotoxicity through DNA damage and induction of apoptosis. Fas ligand (FasL), a key component of T lymphocytes, has been shown to be induced by some of those agents. To address what is an early signal for this induction, we constructed a FasL promoter-luciferase reporter gene to investigate effects of DNA topoisomerase (Topo) II inhibitors on FasL promoter activity. Transient transfection assays in HeLa and other tumor cell lines demonstrated that induction of FasL promoter activity in response to Topo II inhibitors such as VM-26 mimicked endogenous FasL expression under the same conditions. The ability of these agents to induce FasL expression correlated with their ability to cause DNA damage. For instance, complex-stabilizing Topo II inhibitors such as etoposide, teniposide, and doxorubicin, which cause DNA damage, strongly induce FasL expression; by contrast, non-DNA-damaging catalytic Topo II inhibitors such as ICRF-187 and merbarone do not do this. In support of the notion that DNA damage triggers FasL induction, we found that DNA-damaging irradiation also induced FasL promoter activity in a dose-dependent manner. Finally, the catalytic Topo II inhibitor ICRF-187 suppressed VM-26-induced-FasL expression. This suppression correlated with the ability of this drug to inhibit VM-26-induced DNA strand breaks. Together, our results suggest that DNA damage in response to agents such as etoposide and teniposide might serve as an early signal to induce FasL expression.

Tumoricidal activity of tumor necrosis factor-related apoptosis-inducing ligand in vivo

To evaluate the utility of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) as a cancer therapeutic, we created leucine zipper (LZ) forms of human (hu) and murine (mu) TRAIL to promote and stabilize the formation of trimers. Both were biologically active, inducing apoptosis of both human and murine target cells in vitro with similar specific activities. In contrast to the fulminant hepatotoxicity of LZ-huCD95L in vivo, administration of either LZ-huTRAIL or LZ-muTRAIL did not seem toxic to normal tissues of mice. Finally, repeated treatments with LZ-huTRAIL actively suppressed growth of the TRAIL-sensitive human mammary adenocarcinoma cell line MDA-231 in CB.17 (SCID) mice, and histologic examination of tumors from SCID mice treated with LZ-huTRAIL demonstrated clear areas of apoptotic necrosis within 9-12 hours of injection.

Impediments to successful immunotherapy

Over the last decade, there has been a considerable increase in understanding of immune responses against cancers, the antigenic structures on tumor cells recognised by the immune system, and the development of more effective vaccines. There is, however, very limited understanding of why the immune system most often fails to control tumor growth and progression. In some patients, it is difficult to demonstrate immune responses to their tumors, and it may be assumed that this reflects poor recognition of tumor antigens, induction of anergy in lymphocytes, or suppression of immune responses by tumor-derived factors. In other patients, tumor progression appears to occur despite the presence of antibody or cell-mediated responses. This may indicate selection of tumor cells that have lost tumor antigens or HLA antigens by immune responses against the tumor. Tumor cells may also become resistant to mediators of apoptosis, such as Fas ligand and tumor necrosis factor-related apoptosis-inducing ligand used by lymphocytes to kill tumor cells. It is suggested that development of effective immunotherapy will need to include strategies that take into account these limitations of immune responses and classification of tumors according to the treatment approach most likely to succeed.

Is extravasation a Fas-regulated process?

The monolayer of endothelial cells that coats the luminal surface of the vessel wall has numerous physiological functions, including the prevention of coagulation, control of vascular permeability, maintenance of vascular tone and regulation of leukocyte extravasation. Recently, we detected functional Fas ligand (FasL) expression on the endothelial lining of blood vessels. FasL induces apoptotic cell death in the multitude of cell types that express its receptor, Fas. Here, we review the function of vascular endothelium in controlling leukocyte extravasation, and illustrate how the regulation of endothelial FasL expression might contribute to this process. We also describe the role of leukocyte extravasation in angiogenesis and atherosclerosis, and we suggest that FasL gene transfer might provide a means of treating diseases of the proliferative vessel wall, particularly those that result from the detrimental infiltration of inflammatory cells.

Apoptosis, Fas and Fas-ligand expression in melanocytic tumors

Impaired regulation of apoptosis is known to be associated with the development of various forms of cancer. Fas binding to its ligand, Fas ligand (Fas-L), has been shown to trigger apoptosis in various cell types. Fas-L is expressed by melanoma cells and has been suggested to play a role in melanoma escape from immune surveillance. In the present study, we assessed apoptotic activity and examined Fas and Fas-L expression in malignant melanomas, Spitz nevi and ordinary melanocytic nevi. We evaluated apoptotic activity using terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling. Apoptotic activity was found to be minimal in melanomas and moderate in Spitz nevi. In contrast, common nevi demonstrated significant levels of apoptosis in the deep parts of the tumor. Fas was found to be expressed by all Spitz nevi, most melanocytic nevi and approximately half of the malignant melanoma specimens. Fas expression was also significantly more pronounced in Spitz nevus cells as compared with the two other tumors. The anti-Fas-L antibody was found to stain all three melanocytic tumors. Staining was shown to be stronger and more frequent in melanoma cells as compared to the nevus cells. Using the Spearman test, no significant correlation between Fas-L expression in melanoma cells and apoptosis in MM-infiltrating mononuclear cells was found, suggesting that Fas-L expression in melanoma cells may not be instrumental in their ability to escape immune mechanisms of defense. In contrast, increased levels of apoptosis in the deep parts of melanocytic nevi may reflect and possibly contribute to their benign nature.

Pancreatic cancer cells can evade immune surveillance via nonfunctional Fas (APO-1/CD95) receptors and aberrant expression of functional Fas ligand

BACKGROUND

The Fas (APO-1/CD95) receptor/Fas ligand (FasR/FasL) system plays a key role in immune surveillance. We investigated the possibility of a tumor escape mechanism involving the FasR/FasL system in pancreatic cancer cells.

METHODS

Fourteen pancreatic cancer cell lines and 3 pancreatic cancer surgical specimens were studied for their expression of FasR and FasL by flow cytometry, immunoblotting, and immunohistochemistry, FasR function was tested with an anti-FasR antibody. FasL function was assessed by coculture assays using pancreatic cancer cells and FasR-sensitive Jurkat T-cells.

RESULTS

FasR was expressed in normal pancreas, in 14 of 14 pancreatic cancer cell lines, and in 3 of 3 surgical specimens. However, only 1 of 14 cancer cell lines expressed functional FasR when grown in monolayer, although 3 additional cell lines displayed functional FasR when cultured in suspension. Normal pancreas did not express FasL, whereas 14 of 14 cancer cell lines and 3 of 3 surgical specimens expressed FasL. FasL expressed by pancreatic cancer cells mediated killing of Jurkat T-cells in coculture assays (P < .005).

CONCLUSIONS

These data suggest that pancreatic cancer cells have 2 potential mechanisms of evading Fas-mediated immune surveillance. A nonfunctional FasR renders them resistant to Fas-mediated apoptosis. The aberrant expression of functional FasL allows them to "counterattack" activated Fas-sensitive T-cells. Alone or in unison, these tumor escape mechanisms may contribute to the malignant and often rapid course of pancreatic cancer disease.

Tumor regressions observed in patients with metastatic melanoma treated with an antigenic peptide encoded by gene MAGE-3 and presented by HLA-A1

Thirty-nine tumor-bearing patients with metastatic melanoma were treated with 3 subcutaneous injections of the MAGE-3.A1 peptide at monthly intervals. No significant toxicity was observed. Of the 25 patients who received the complete treatment, 7 displayed significant tumor regressions. All but one of these regressions involved cutaneous metastases. Three regressions were complete and 2 of these led to a disease-free state, which persisted for more than 2 years after the beginning of treatment. No evidence for a cytolytic T lymphocyte (CTL) response was found in the blood of the 4 patients who were analyzed, including 2 who displayed complete tumor regression. Our results suggest that injection of the MAGE-3.A1 peptide induced tumor regression in a significant number of the patients, even though no massive CTL response was produced.

Mechanisms of apoptosis avoidance in cancer

Defects in the programmed cell death mechanisms can extend cell lifespan, contributing to neoplastic cell expansion independently of cell division. Deficiencies in apoptosis also contribute to carcinogenesis by creating a permissive environment for genetic instability and accumulation of gene mutations, permitting disobeyance of cell cycle checkpoints which would normally induce apoptosis, facilitating growth factor- and hormone-independent cell survival, supporting anchorage-independent survival during metastasis, reducing dependence on oxygen and nutrients, promoting resistance to immune-based destruction, and conferring resistance to cytotoxic anticancer drugs and radiation. Though much remains to be learned, identification of gene families that control the physiological cell death pathway has provided the basis for improved understanding of tumor biology. This review describes some of the recent progress made towards delineating the biochemistry and molecular biology of apoptosis control, and briefly summarizes the progress in identifying specific road blocks to apoptosis induction in cancers. These advances are beginning to reveal novel strategies for combating cancer by restoring apoptosis sensitivity.

Withdrawal of survival factors results in activation of the JNK pathway in neuronal cells leading to Fas ligand induction and cell death

The JNK pathway modulates AP-1 activity. While in some cells it may have proliferative and protective roles, in neuronal cells it is involved in apoptosis in response to stress or withdrawal of survival signals. To understand how JNK activation leads to apoptosis, we used PC12 cells and primary neuronal cultures. In PC12 cells, deliberate JNK activation is followed by induction of Fas ligand (FasL) expression and apoptosis. JNK activation detected by c-Jun phosphorylation and FasL induction are also observed after removal of either nerve growth factor from differentiated PC12 cells or KCl from primary cerebellar granule neurons (CGCs). Sequestation of FasL by incubation with a Fas-Fc decoy inhibits apoptosis in all three cases. CGCs derived from gld mice (defective in FasL) are less sensitive to apoptosis caused by KCl removal than wild-type neurons. In PC12 cells, protection is also conferred by a c-Jun mutant lacking JNK phosphoacceptor sites and a small molecule inhibitor of p38 mitogen-activated protein kinase and JNK, which inhibits FasL induction. Hence, the JNK-to-c-Jun-to-FasL pathway is an important mediator of stress-induced neuronal apoptosis.

Developing recombinant and synthetic vaccines for the treatment of melanoma

To develop new vaccines for the treatment of patients with cancer, target antigens presented on tumor cell surfaces have been cloned. Many of these antigens are non-mutated differentiation antigens and are expressed by virtually all melanomas, making them attractive components for a widely efficacious melanoma vaccine. These antigens are also expressed by melanocytes, however, and are likely to be subject to immune tolerance. A central challenge for tumor immunologists has thus been the breaking of tolerance to cancer antigens. We review recent clinical trials using experimental cancer vaccines, including recent evidence that therapeutic vaccines can induce objective responses in patients with metastatic malignant melanoma. We focus on the foundations of these approaches in new experimental animal models designed to test novel vaccines and report on what these new models predict for the future development of therapeutic vaccines for cancer.

Peripheral T and putative natural killer cell lymphomas commonly coexpress CD95 and CD95 ligand

The CD95 (Fas)/CD95 ligand (CD95L) system is an important mechanism triggering apoptosis, and CD95L expression has recently been implicated for immune evasion and aggressive behavior in malignancies. This study aimed to investigate CD95 and CD95L expression in lymphomas and the possible relationship with tumor cell apoptosis, with emphasis on the natural killer (NK) cell lymphomas, which are highly aggressive neoplasms and frequently exhibit tumor cell apoptosis/necrosis. Frozen sections of 82 cases of lymphomas obtained from Queen Elizabeth Hospital and Caritas Medical Center, Hong Kong, were immunostained with polyclonal anti-CD95 and anti-CD95L antibodies. The NK-cell lymphomas were also studied for apoptosis by in situ end labeling (ISEL) method, and zonal tumor cell death was evaluated semiquantitatively. The cases studied included 27 NK-, 22 T-, and 33 B-cell lymphomas. CD95 was expressed in 25 (93%) NK-, 11 (50%) T-, and 14 (42%) B-cell lymphomas. CD95L was expressed in 19 (70%) NK-, 15 (68%) T-, and 3 (9%) B-cell lymphomas. There was significant difference in the frequency of CD95 expression between B- and NK- (P < .001), and between T- and NK-cell lymphomas (P < .05), and in CD95L expression between B- and T- (P < .01) or NK-cell (P < .01) lymphomas. Zonal tumor cell death was present in 21 (78%) NK-cell lymphomas and 1 (4.5%) T-cell lymphoma and showed no correlation with CD95 or CD95L expression. ISEL analysis showed apoptosis predominantly in the viable areas in only 5 (24%) NK-cell lymphomas. In conclusion, CD95L is frequently expressed in NK- and T-cell lymphomas, but rarely in B-cell lymphomas. Zonal tumor cell death is not correlated with CD95 or CD95L expression and thus the CD95/CD95L system probably does not contribute significantly to this phenomenon. We postulate that the frequent expression of CD95L by NK- and T-cell lymphomas may mediate local or systemic tissue damage and immune evasion, and may contribute to the clinical aggressiveness of these tumors.

CD95 ligand (CD95L) in normal human lymphoid tissues: a subset of plasma cells are prominent producers of CD95L

CD95(Fas/APO-1)-ligand (CD95L) mediates apoptosis by trimerization of the CD95 receptor on the surface of sensitive cells. In vitro studies have shown CD95L expression mainly by activated T cells and suggested a role for CD95L in the regulation of immune responses. Little is known, however, about the cellular distribution of CD95L in situ in the normal human immune system. We investigated CD95L expression in tissue sections of the thymus, lymph node, spleen, tonsil, and gastrointestinal tract using in situ hybridization and two monoclonal antibodies. In all these organs, cells expressing CD95L message and protein were scarce and comprised scattered lymphocytes, rare nonlymphoid cells, and a subset of epithelioid endothelial cells. Surprisingly, a subset of plasma cells turned out to be the most prominent producers of CD95L, matching the reports on CD95L in myeloma cells. CD95L+ plasma cells were most numerous in the mucosa-associated lymphoid tissue. This also applied to acquired mucosa-associated lymphoid tissue in chronic gastritis in which CD95L+ plasma cells were found scattered in the lamina propria. Our data suggest that plasma cells as yet may be neglected modulators of immune responses.

The central role of CD4(+) T cells in the antitumor immune response

The induction of optimal systemic antitumor immunity involves the priming of both CD4(+) and CD8(+) T cells specific for tumor-associated antigens. The role of CD4(+) T helper cells (Th) in this response has been largely attributed to providing regulatory signals required for the priming of major histocompatibility complex class I restricted CD8(+) cytolytic T lymphocytes, which are thought to serve as the dominant effector cell mediating tumor killing. However, analysis of the effector phase of tumor rejection induced by vaccination with irradiated tumor cells transduced to secrete granulocyte/macrophage colony-stimulating factor indicates a far broader role for CD4(+) T cells in orchestrating the host response to tumor. This form of immunization leads to the simultaneous induction of Th1 and Th2 responses, both of which are required for maximal systemic antitumor immunity. Cytokines produced by these CD4(+) T cells activate eosinophils as well as macrophages that produce both superoxide and nitric oxide. Both of these cell types then collaborate within the site of tumor challenge to cause its destruction.

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.

Endothelial cell apoptosis induced by oxidized LDL is associated with the down-regulation of the cellular caspase inhibitor FLIP

Fas (CD-95/APO-1) is a death receptor that initiates an apoptotic signal when activated by its ligand, FasL. Normal vascular endothelial cells are resistant to Fas-mediated apoptosis though they express both Fas and FasL. Oxidized low density lipoprotein (OxLDL) or lysophosphatidylcholine (LPC), a major component of OxLDL, induces endothelial cell suicide by sensitizing endothelial cells to Fas-mediated apoptosis. Here, we show that endothelial cell apoptosis by OxLDL and LPC-C16:0 was dose-dependent and correlated with down-regulation of FLICE-inhibitory protein (FLIP), an intracellular caspase inhibitor. FLIP down-regulation also occurred when endothelial cells were treated with toxic doses of LPC-C18:0 or minimally modified low density lipoprotein (LDL). In contrast, FLIP was not down-regulated by native LDL, acetylated LDL, LPC-C12:0, cholesterol, or 7-ketocholesterol, which are not toxic to endothelial cells. The cytotoxicity of oxidized lipids was reversed by transfecting endothelial cells with a FLIP expression plasmid. The results demonstrate, for the first time, FLIP regulation under conditions that lead to pathological tissue destruction.

Newly discovered role for Fas ligand in the cell-cycle arrest of CD4+ T cells

Fas Ligand (FasL) can induce apoptosis of Fas-bearing cells. It is expressed on the cell surface of many tumor cells, immune-privileged tissues and activated lymphocytes. We report here that FasL can itself transduce signals, leading to cell-cycle arrest and cell death in CD4+ T cells. In vitro, FasL engagement inhibited CD4+ T-cell proliferation, cell-cycle progression, and IL-2 secretion. In vivo, FasL engagement prevented superantigen-mediated CD4+, but not CD8+, T-cell expansion. These findings demonstrate that FasL engagement regulates cell-cycle progression, and show that FasL engagement in vivo has a potent anti-inflammatory effect specific for CD4+ T cells.

Adenovirus mediated gene therapy in a glioblastoma vaccine model; specific antitumor immunity and abrogation of immunosuppression

Clinical trials are being performed using tumor genetically engineered to produce cytokines as a vaccine. The design of such a vaccine may be made more effective by further study using in-vitro as well as in-vivo models. We studied an in-vitro tumor 'vaccine' model in glioblastoma. We have demonstrated high efficiency transfection of the Interleukin-2 (IL-2) gene into glioblastoma cell lines using adenoviral vectors. Glioblastoma cell lines transduced with this vector could produce high levels of IL-2 for up to 2 weeks, long enough to elicit an antitumor immune response. We studied tumor/effector cell interactions using cytotoxicity assays coupled with flow cytometric analysis. Activation of CD8+ and expansion of CD3+/CD16+ effector cell subpopulations were observed, suggesting the generation of a specific anti-tumor response and the potential for systemic immunity. We demonstrated that glioblastoma produce immunosuppressive factors which reduce the antitumor response by peripheral blood effector cells. These immunosuppressive factors could be neutralized to improve antitumor response. A better understanding of tumor/effector cell interactions may improve the design of gene therapy trials.

Fas ligand is present in tumors of the Ewing's sarcoma family and is cleaved into a soluble form by a metalloproteinase

Fas ligand (FasL) exists in transmembrane and soluble forms and induces apoptosis on cross-linking with the Fas receptor. We evaluated the biological significance of FasL and Fas in 61 tumor tissues and 9 cell lines of the Ewing's sarcoma family of tumors (ESFT). FasL was present in 62.5% and Fas in 79.4% of primary ESFT. Metastatic tumors had higher expression of FasL (95%), suggesting association with a metastatic phenotype. FasL was detected in the cytoplasm and membrane of ESFT cells by immunofluorescence. Western blotting revealed transmembrane and soluble FasL in cytosolic extracts and soluble FasL in conditioned media. Both transmembrane and soluble FasL induced apoptosis of Fas-sensitive Jurkat cells in co-culture experiments with ESFT cells or their media. Treatment with phenanthroline and the synthetic metalloproteinase inhibitor BB-3103 reduced the levels of soluble FasL in the media, suggesting that in ESFT, FasL is processed by a metalloproteinase and released in the extracellular milieu. The released soluble FasL may serve to attack cells of the immune system and/or interfere with the binding of transmembrane FasL with Fas, and results in down-regulation of transmembrane FasL. Synthetic metalloproteinase inhibitors may modify the ratio of transmembrane to soluble FasL.

Recruitment of Hepatic NK Cells by IL-12 Is Dependent on IFN-γ and VCAM-1 and Is Rapidly Down-Regulated by a Mechanism Involving T Cells and Expression of Fas

NK cells have been shown to be important antitumor or antiviral effector cells in the liver. In the present study we have examined the factors that regulate the initial recruitment and subsequent fate of hepatic NK and T cells in mice treated with IL-12 or IL-2. Daily administration of IL-12 caused a rapid initial increase in NK cells followed by a subsequent decrease that coincided with an accumulation of T cells. The recruitment of hepatic NK cells by IL-12, but not the subsequent T cell infiltrate, was abrogated in IFN-γ−/− mice. In contrast, daily administration of IL-2 caused a sustained increase in liver-associated NK cells that was not diminished in IFN-γ−/− mice. The IL-12-induced recruitment in both hepatic NK and T cells was abrogated by in vivo treatment with anti-VCAM-1 mAbs, while treatment with anti-ICAM-1 Abs decreased only the recruitment of T cells in the IL-12-treated mice. The rapid loss of newly recruited hepatic NK cells in IL-12-treated mice did not occur in SCID mice or in B.MRL-Faslpr (Fas−) and B6Smn.C3H-Faslgld (FasL−) mutant mice, suggesting that T cells can actively eliminate hepatic NK cells through a Fas-dependent mechanism. These findings also imply that during the endogenous innate immune response to infectious agents or tumors or in the host response induced by cytokine therapies, the biologic effects of NK cells may be limited by T cell-mediated effects.

Regulation of the proinflammatory effects of Fas ligand (CD95L)

Fas ligand (CD95L) inhibits T cell function in immune-privileged organs such as the eye and testis, yet in most tissues CD95L expression induces potent inflammatory responses. With a stably transfected colon carcinoma cell line, CT26-CD95L, the molecular basis for these divergent responses was defined. When injected subcutaneously, rejection of CT26-CD95L was caused by neutrophils activated by CD95L. CT26-CD95L survived in the intraocular space because of the presence of transforming growth factor-beta (TGF-beta), which inhibited neutrophil activation. Providing TGF-beta to subcutaneous sites protected against tumor rejection. Thus, these cytokines together generate a microenvironment that promotes immunologic tolerance, which may aid in the amelioration of allograft rejection.

Ex vivo staining of metastatic lymph nodes by class I major histocompatibility complex tetramers reveals high numbers of antigen-experienced tumor-specific cytolytic T lymphocytes

Characterization of cytolytic T lymphocyte (CTL) responses to tumor antigens has been impeded by a lack of direct assays of CTL activity. We have synthesized reagents ("tetramers") that specifically stain CTLs recognizing melanoma antigens. Tetramer staining of tumor-infiltrated lymph nodes ex vivo revealed high frequencies of tumor-specific CTLs which were antigen-experienced by surface phenotype. In vitro culture of lymph node cells with cytokines resulted in very large expansions of tumor-specific CTLs that were dependent on the presence of tumor cells in the lymph nodes. Tetramer-guided sorting by flow cytometer allowed isolation of melanoma-specific CTLs and confirmation of their specificity and their ability to lyse autologous tumor cells. Our results demonstrate the value of these novel reagents for monitoring tumor-specific CTL responses and for generating CTLs for adoptive immunotherapy. These data also indicate that strong CTL responses to melanoma often occur in vivo, and that the reactive CTLs have substantial proliferative and tumoricidal potential.

Oxidized LDL activates fas-mediated endothelial cell apoptosis

Oxidized low density lipoproteins (OxLDL) promote chronic inflammatory responses in the vasculature that give rise to atherosclerotic plaques. Fas ligand (FasL) is naturally expressed on the vascular endothelium where it can induce apoptosis in Fas-expressing immune cells as they enter the vessel wall. Although vascular endothelial cells are normally resistant to Fas-mediated cell death, OxLDL were shown to induce apoptosis in cultured endothelial cells and endothelium of arterial explants by a process that could be inhibited with Fas L neutralizing antibodies. OxLDL-induced cell death was also reduced in the aortic endothelium cultured from gld (FasL-/-) and lpr (Fas-/-) mice as compared with wild-type mice. OxLDL acted by sensitizing endothelial cells to death signals from the Fas receptor. Thus, the ability of OxLDL to promote Fas-mediated endothelial cell suicide may be a feature that contributes to their atherogenicity.

Cutting Edge: Chemotactic Activity of Soluble Fas Ligand Against Phagocytes

A recombinant soluble form of human Fas ligand (sFasL) was tested for its chemotactic activity against human and mouse polymorphonuclear neutrophils (PMN) by the Boyden chamber method. sFasL exhibited a potent chemotactic activity against both human and mouse PMN and HL-60 cells when differentiated into neutrophils or monocytes. A neutralizing anti-FasL mAb abolished the chemotactic activity, while control mAb did not. Ligation of Fas by either IgM- or IgG-type anti-Fas mAb also induced PMN migration. PMN derived from lpr mice that express few Fas molecules did not respond to sFasL. In contrast, those derived from lprcg mice that express Fas molecules with a mutated death domain normally responded to sFasL chemotaxis. These results directly indicated a chemotactic activity of sFasL against PMN and suggest a novel signaling function of Fas, which appears to be independent of the death domain-mediated apoptosis.

Fas ligand expression in primary colon adenocarcinomas: evidence that the Fas counterattack is a prevalent mechanism of immune evasion in human colon cancer

Fas ligand (FasL) kills sensitive Fas receptor (FasR)-bearing cells by inducing apoptosis. FasL expressed by non-lymphoid cells within the eye and the testis mediates immune privilege by inducing apoptosis of Fas-sensitive infiltrating pro-inflammatory immune effector cells. It has previously been demonstrated by the present authors that the colon cancer cell SW620 expresses FasL and can kill lymphoid cells by Fas-mediated apoptosis in vitro. This 'Fas counterattack' was subsequently confirmed by others as a potential mechanism of immune privilege in various malignancies. The aim of the present study was to ascertain the prevalence of FasL expression in human colon cancer and to confirm that neoplastic colonic epithelial cells express FasL in vivo. The study of FasL expression by colon cancer cell lines was extended: it was shown that seven of eight colon adenocarcinoma cell lines expressed FasL mRNA, using reverse transcription-polymerase chain reaction (RT-PCR). Prevalent expression of FasL was confirmed in vivo: all the resected colonic tumours examined (31/31) were found to express FasL. In the tumours, FasL were co-localized to neoplastic colonic epithelial cells, using immunohistochemistry and in situ hybridization, respectively. FasL expression was independent of Dukes' stage, suggesting that it may occur throughout colon cancer progression. These results suggest that FasL is a common mediation of immune privilege in colon cancer.

On the role and significance of Fas (Apo-1/CD95) ligand (FasL) expression in immune privileged tissues and cancer cells using multiple myeloma as a model

Our knowledge in immunology has been dramatically increased by several excellent investigations elucidating the role of the Fas (Apo-1/CD95) receptor/ligand (FasL) system in complex immunological processes such as the acquisition of self tolerance in T cells, progression of autoimmunity, clonal deletion of activated T cells, B-cell regulation and the establishment of "immune privileged" sites such as testis or retina. In addition to these regulatory immunological activities, Fas/FasL interaction was also shown to participate in active defense mechanisms of the host against infected or transformed cells thereby inducing apoptosis in target cells. However, the same mechanism seems also to be part of an escape strategy utilized by tumor cells in various neoplastic malignancies of both hematopoetic as also non-hematopoetic origin. We ourselves were able to demonstrate that neoplastic plasma cell lines, as well as native malignant myeloma cells constitutively express FasL mRNA and protein. The FasL molecule is functionally active and able to induce programmed cell death in Fas sensitive target T cells in vitro. These target T cells were protected from programmed cell death by preincubation of T cells with a Fas-blocking monoclonal antibody (mAb) or of myeloma cells with a FasL-neutralizing mAb. respectively. Furthermore, overexpression of the caspase inhibitor, cowpoxvirus protein CrmA, also protected target T cells from being killed by myeloma cells, identifying Fas/FasL mediated signaling as the effector pathway utilized by malignant plasma cells. Our observations strongly suggest the engagement of Fas/FasL interaction in the escape strategy of this malignancy. The molecular basis of this evasive mechanism differs in essential respects from those described in melanoma, lung cancer, hepatocellular carcinoma, or astrocytoma, since downregulation of Fas or instrinsic insensitivity towards Fas-mediated signaling were not prerequisites for the occurrence of this phenomenon in Fas-sensitive multiple myeloma cell lines. However, myeloma cell lines resisted cocultivation with FasL-expressing target T cells in vitro. The aim of this review is to discuss the role of Fas/FasL interaction in the establishment of malignant disease, in the light of our findings on myeloma cells and also by drawing upon similar observations of other investigators on different kinds of tumor cells and cell lines and further to consider its possible relevance in formulating novel approaches to cancer therapy.

Fas/Fas ligand (FasL)-deregulated apoptosis and IL-6 insensitivity in highly malignant myeloma cells

IL-6 is a growth factor which interferes in the apoptosis of malignant plasma cells. Here we explore its role in the spontaneous and Fas/FasL-regulated apoptosis of seven myeloma cell clones (MCC). MCC-2 and -7 were constitutively defective in Fas antigen in the presence of large membrane exposure of FasL, and showed a high rate of cell proliferation irrespective of the presence of IL-6. Cytofluorimetric analysis following propidium iodide (PI) staining revealed a minimal extent of spontaneous apoptosis, as in other IL-6-insensitive, though Fas-positive MCC, namely MCC-3 and -5. By contrast, a regular amplitude of apoptosis occurred in the remaining IL-6-dependent clones. Their propensity to cell death, as well as their FasL membrane expression, were promptly down-modulated by the cytokine, whereas no substantial effect was detected in IL-6-independent MCC. Furthermore, we investigated the quantitative secretion of FasL. Both [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] (MTT) cytotoxicity assay and PI staining of WC8 lymphoblasts from a Fas-transfected mouse lymphoma, incubated with supernatants from MCC, showed a variable cytocidal property, thus confirming the cellular release of FasL. However, a significant elevation of FasL secretion occurred in both Fas- MCC, whereas molecular cloning and sequencing of Fas revealed the presence of a splicing variant, namely Fas Exo4,6Del, in the cDNA from both MCC-3 and -5, which were previously demonstrated to be unresponsive to Fas stimulation. Taken together, these data provide evidence that concurrence of IL-6 insensitivity and deregulation of apoptosis in myeloma cells reflects a high malignancy grade. It is suggested that the secretion of Fas splicing variants in Fas+ plasma cells, as well as the over-production of FasL in Fas- myelomas, are differential mechanisms by which myeloma cells escape host immune surveillance.