A non-cleavable mutant of Fas ligand does not prevent neutrophilic destruction of islet transplants

Therapeutic approaches targeting CD95L/CD95 signaling in cancer and autoimmune diseases

Cell death plays a pivotal role in the maintenance of tissue homeostasis. Key players in the controlled induction of cell death are the Death Receptors (DR). CD95 is a prototypic DR activated by its cognate ligand CD95L triggering programmed cell death. As a consequence, alterations in the CD95/CD95L pathway have been involved in several disease conditions ranging from autoimmune diseases to inflammation and cancer. CD95L-induced cell death has multiple roles in the immune response since it constitutes one of the mechanisms by which cytotoxic lymphocytes kill their targets, but it is also involved in the process of turning off the immune response. Furthermore, beyond the canonical pro-death signals, CD95L, which can be membrane-bound or soluble, also induces non-apoptotic signaling that contributes to its tumor-promoting and pro-inflammatory roles. The intent of this review is to describe the role of CD95/CD95L in the pathophysiology of cancers, autoimmune diseases and chronic inflammation and to discuss recently patented and emerging therapeutic strategies that exploit/block the CD95/CD95L system in these diseases.

Cell death-independent activities of the death receptors CD95, TRAILR1, and TRAILR2

Since their identification more than 20 years ago, the death receptors CD95, TRAILR1, and TRAILR2 have been intensively studied with respect to their cell death-inducing activities. These receptors, however, can also trigger a variety of cell death-independent cellular responses reaching from the activation of proinflammatory gene transcription programs over the stimulation of proliferation and differentiation to induction of cell migration. The cell death-inducing signaling mechanisms of CD95 and the TRAIL death receptors are well understood. In contrast, despite the increasing recognition of the biological and pathophysiological relevance of the cell death-independent activities of CD95, TRAILR1, and TRAILR2, the corresponding signaling mechanisms are less understood and give no fully coherent picture. This review is focused on the cell death-independent activities of CD95 and the TRAIL death receptors and addresses mainly three questions: (a) how are these receptors linked to noncell death pathways at the molecular level, (b) which factors determine the balance of cell death and cell death-independent activities of CD95 and the TRAIL death receptors at the cellular level, and (c) what are the consequences of the cell death-independent functions of these receptors for their role in cancer and inflammatory diseases.

Posttransplantation systemic immunomodulation with SA-FasL-engineered donor splenocytes has robust efficacy in preventing cardiac allograft rejection in mice

Apoptosis induced by the engagement of FasL with Fas receptor on the surface of lymphocytes is an important immune homeostatic mechanism that ensures tolerance to self-antigens under normal physiologic conditions. As such, FasL has been extensively tested as a tolerogenic molecule with the use of gene therapy in settings of autoimmunity and transplantation with conflicting outcomes. Although the mechanistic basis of these contradictory observations is largely unknown, the use of wild-type FasL and the means by which the gene was expressed may provide an explanation. To overcome these complications, we generated a chimeric FasL protein with streptavidin (SA-FasL) having potent apoptotic activity and displayed this molecule effectively and rapidly on biotinylated biologic membranes for immunomodulation. In the present study, we displayed SA-FasL on the surface of BALB/c splenocytes and injected 5 × 10(6) cells intraperitoneally into C57BL/6 recipients of BALB/c heart grafts on days 1, 3, and 5 after-transplantation. To control initial graft-reactive immune responses and facilitate FasL-mediated apoptosis, rapamycin was used as an immunosuppressant at 0.2 mg/kg daily for a total of 15 doses immediately after heart transplantation. All mice injected with SA-FasL-engineered donor splenocytes accepted their grafts during the 100-day observation period. In marked contrast, immunomodulation with control streptavidin protein-engineered BALB/c splenocytes had minimal effect on graft survival (mean survival, 21.4 ± 1.5 d). Taken together, these results establish posttransplantation systemic immunomodulation with SA-FasL-engineered donor splenocytes under transient cover of rapamycin as an effective regimen in preventing cardiac allograft rejection in rodents with important clinical implications.

Preclinical studies for pharmacokinetics and biodistribution of Ad-stTRAIL, an adenovirus delivering secretable trimeric TRAIL for gene therapy

Malignant glioma is the most frequent type in brain tumors. The prognosis of this tumor has not been significantly improved for the past decades and the average survival of patients is less than one year. Thus, an effective novel therapy is urgently needed. TNF-related apoptosis inducing ligand (TRAIL), known to have tumor cell-specific killing activity, has been investigated as a novel therapeutic for cancers. We have developed Ad-stTRAIL, an adenovirus delivering secretable trimeric TRAIL for gene therapy and demonstrated the potential to treat malignant gliomas. Currently, this Ad-stTRAIL gene therapy is under phase I clinical trial for malignant gliomas. Here, we report preclinical studies for Ad-stTRAIL carried out using rats. We delivered Ad-stTRAIL intracranially and determined its pharmacokinetics and biodistribution. Most Ad-stTRAIL remained in the delivered site and the relatively low number of viral genomes was detected in the opposite site of brain and cerebrospinal fluid. Similarly, only small portion of the viral particles injected was found in the blood plasma and major organs and tissues, probably due to the brain-blood barrier. Multiple administrations did not lead to accumulation of Ad-stTRAIL at the injection site and organs. Repeated delivery of Ad-stTRAIL did not show any serious side effects. Our data indicate that intracranially delivered Ad-stTRAIL is a safe approach, demonstrating the potential as a novel therapy for treating gliomas.

Pancreatic islets engineered with SA-FasL protein establish robust localized tolerance by inducing regulatory T cells in mice

Allogeneic islet transplantation is an important therapeutic approach for the treatment of type 1 diabetes. Clinical application of this approach, however, is severely curtailed by allograft rejection primarily initiated by pathogenic effector T cells regardless of chronic use of immunosuppression. Given the role of Fas-mediated signaling in regulating effector T cell responses, we tested if pancreatic islets can be engineered ex vivo to display on their surface an apoptotic form of Fas ligand protein chimeric with streptavidin (SA-FasL) and whether such engineered islets induce tolerance in allogeneic hosts. Islets were modified with biotin following efficient engineering with SA-FasL protein that persisted on the surface of islets for >1 wk in vitro. SA-FasL-engineered islet grafts established euglycemia in chemically diabetic syngeneic mice indefinitely, demonstrating functionality and lack of acute toxicity. Most importantly, the transplantation of SA-FasL-engineered BALB/c islet grafts in conjunction with a short course of rapamycin treatment resulted in robust localized tolerance in 100% of C57BL/6 recipients. Tolerance was initiated and maintained by CD4(+)CD25(+)Foxp3(+) regulatory T (Treg) cells, as their depletion early during tolerance induction or late after established tolerance resulted in prompt graft rejection. Furthermore, Treg cells sorted from graft-draining lymph nodes, but not spleen, of long-term graft recipients prevented the rejection of unmodified allogeneic islets in an adoptive transfer model, further confirming the Treg role in established tolerance. Engineering islets ex vivo in a rapid and efficient manner to display on their surface immunomodulatory proteins represents a novel, safe, and clinically applicable approach with important implications for the treatment of type 1 diabetes.

The role of FasL and Fas in health and disease

The FS7-associated cell surface antigen (Fas, also named CD95, APO-1 or TNFRSF6) attracted considerable interest in the field of apoptosis research since its discovery in 1989. The groups of Shin Yonehara and Peter Krammer were the first reporting extensive apoptotic cell death induction upon treating cells with Fas-specific monoclonal antibodies.1,2 Cloning of Fas3 and its ligand,4,5 FasL (also known as CD178, CD95L or TNFSF6), laid the cornerstone in establishing this receptor-ligand system as a central regulator of apoptosis in mammals. Therapeutic exploitation of FasL-Fas-mediated cytotoxicity was soon an ambitous goal and during the last decade numerous strategies have been developed for its realization. In this chapter, we will briefly introduce essential general aspects of the FasL-Fas system before reviewing its physiological and pathophysiological relevance. Finally, FasL-Fas-related therapeutic tools and concepts will be addressed.

Membrane-tethered proteins for basic research, imaging, and therapy

Secreted and intracellular proteins including antibodies, cytokines, major histocompatibility complex molecules, antigens, and enzymes can be redirected to and anchored on the surface of mammalian cells to reveal novel functions and properties such as reducing systemic toxicity, altering the in vivo distribution of drugs and extending the range of useful drugs, creating novel, specific signaling receptors and reshaping protein immunogenicity. The present review highlights progress in designing vectors to target and retain chimeric proteins on the surface of mammalian cells. Comparison of chimeric proteins indicates that selection of the proper cytoplasmic domain and introduction of oligiosaccharides near the cell surface can dramatically enhance surface expression, especially for single-chain antibodies. We also describe progress and limitations of employing surface-tethered proteins for preferential activation of prodrugs at cancer cells, imaging gene expression in living animals, performing high-throughput screening, selectively activating immune cells in tumors, producing new adhesion molecules, creating local immune privileged sites, limiting the distribution of soluble factors such as cytokines, and enhancing polypeptide immunogenicity. Surface-anchored chimeric proteins represent a rich source for developing new techniques and creating novel therapeutics.

Loss of functional Fas ligand enhances intestinal tumorigenesis in the Min mouse model

Fas ligand (FasL/CD95L), a member of the tumor necrosis factor family, interacts with a specific receptor Fas, ultimately leading to cell death. Tumor expression of FasL has been proposed to aid in immune evasion through a "Fas counterattack" mechanism but has also been described as a proinflammatory factor. Here, we tested the role of FasL in a mouse model of spontaneous tumor development. We used the Min mouse in which multiple benign polyps develop in the intestine due to a mutation in the Apc tumor suppressor gene. Mutant mice deficient in functional FasL, termed gld/gld, were crossed to Min mice to generate tumor-prone animals lacking functional FasL. Comparison of FasL-deficient versus proficient Min mice revealed a significant increase in polyp number in the gld/gld mice. We next assessed immune cell infiltration into adenomas. There was no difference in the number of either lymphocytes or macrophages; however, the number of tumor-infiltrating neutrophils was 3-fold lower in the gld/gld specimens compared with controls. Neutrophil migration in vitro was stimulated by wild-type but not mutant FasL. In a nontumor-bearing colitis model in vivo, neutrophil recruitment to the intestine was also reduced in gld/gld mice. Although the Fas counterattack hypothesis suggests that the absence of FasL would result in increased immune-mediated tumor elimination, the opposite is true in the Min model with lack of functional FasL associated with reduced neutrophil influx and increased tumor development. Thus, the proinflammatory rather than counterattack role of tumor FasL is more relevant.

Maternal acceptance of the fetus: true human tolerance

Induction and maintenance of immunologic tolerance in humans remains a desirable but elusive goal. Therefore, understanding the physiologic mechanisms of regulation of immune responses is highly clinically relevant for immune-mediated diseases (e.g., autoimmunity and asthma/allergy) and for cell and organ transplantation. Acceptance of the fetus, which expresses paternally inherited alloantigens, by the mother during pregnancy is a unique example of how the immune system reshapes a destructive alloimmune response to a state of tolerance. Understanding the complex mechanisms of fetomaternal tolerance has important implications for developing novel strategies to induce immunologic tolerance in humans in general and for prevention of spontaneous abortion in at-risk populations in particular.

Fas ligand exerts its pro-inflammatory effects via neutrophil recruitment but not activation

Fas ligand (FasL) expression induces apoptosis of activated T cells and has been suggested as a strategy to inhibit graft rejection. Unfortunately, the use of FasL to confer 'immune privilege' in this setting has been hampered by the finding that it may also provoke a destructive granulocytic response. While the Fas/FasL-mediated apoptotic pathways are well defined, the pro-inflammatory effects of FasL are poorly understood. Our aim in this study was to define in vitro the biological effects of FasL on neutrophil recruitment and activation. DAP-3 cells expressing human FasL on the cell membrane (mFasL) potently induced apoptosis in human neutrophils and in activated T lymphocytes. Recombinant human soluble FasL (sFasL), by contrast, was a very weak inducer of apoptosis, even at high concentrations. This latter observation suggests that cleavage of mFasL by naturally occurring matrix metalloproteinases may serve to down-regulate FasL activity in vivo. However, in the presence of a cross-linking antibody, the efficiency of apoptosis-induction by sFasL was greatly increased, suggesting that the lesser pro-apoptotic potency of sFasL reflects an inability to induce trimerization of the Fas receptor. With regard to pro-inflammatory effects, we found that sFasL is a potent neutrophil chemoattractant and, given that it induces little apoptosis, the dominance of sFasL over mFasL may mean that graft-infiltrating neutrophils will survive to mediate inflammation. Neither sFasL nor mFasL produced neutrophil activation as assessed by chemiluminescence assay. This suggests that neutrophils recruited to an inflammatory site by FasL will be activated by mechanisms other than Fas-FasL signalling.

Immune privilege induced by regulatory T cells in transplantation tolerance

Immune privilege was originally believed to be associated with particular organs, such as the testes, brain, the anterior chamber of the eye, and the placenta, which need to be protected from any excessive inflammatory activity. It is now becoming clear, however, that immune privilege can be acquired locally in many different tissues in response to inflammation, but particularly due to the action of regulatory T cells (Tregs) induced by the deliberate therapeutic manipulation of the immune system toward tolerance. In this review, we consider the interplay between Tregs, dendritic cells, and the graft itself and the resulting local protective mechanisms that are coordinated to maintain the tolerant state. We discuss how both anti-inflammatory cytokines and negative costimulatory interactions can elicit a number of interrelated mechanisms to regulate both T-cell and antigen-presenting cell activity, for example, by catabolism of the amino acids tryptophan and arginine and the induction of hemoxygenase and carbon monoxide. The induction of local immune privilege has implications for the design of therapeutic regimens and the monitoring of the tolerant status of patients being weaned off immunosuppression.

Membrane-bound CD95 ligand expressed on human antigen-presenting cells prevents alloantigen-specific T cell response without impairment of viral and third-party T cell immunity

Genetically modified antigen-presenting cells (APC) represent an attractive strategy for in vitro immunomodulation. In the human system, APC expressing HLA-A1 and a membrane-bound form of CD95L (m-CD95L) were used for selective depletion of HLA-A1-specific T cells. In short-term assays, m-CD95L-expressing APC-induced apoptosis in activated T cells and the constitutive presence of m-CD95L and HLA-A1 expressing APC in long-term T cell cultures prevented the expansion of CD4(+) and CD8(+) HLA-A1-specific T cells and the development of HLA-A1-specific cytotoxicity. However, immunity towards third party, viral and bacterial antigens was maintained and T cells spared from depletion could be induced to develop cytotoxicity towards unrelated antigens. Interestingly, inhibition of HLA-A1-specific T cell response absolutely requires the coexpression of m-CD95L and HLA-A1 antigen on the same APC. Thus, m-CD95L expressing APC might be used in clinical settings to obtain tolerance induction in allogeneic transplantation systems or autoimmune diseases.

The dual role of Fas-ligand as an injury effector and defense strategy in diabetes and islet transplantation

The exact process that leads to the eruption of autoimmune reactions against beta cells and the evolution of diabetes is not fully understood. Macrophages and T cells may launch an initial immune reaction against the pancreatic islets of Langerhans, provoking inflammation and destructive insulitis. The information on the molecular mechanisms of the emergence of beta cell injury is controversial and points to possibly important roles for the perforin-granzyme, Fas-Fas-ligand (FasL) and tumor-necrosis-factor-mediated apoptotic pathways. FasL has several unique features that make it a potentially ideal immunomodulatory tool. Most important, FasL is selectively toxic to cytotoxic T cells and less harmful to regulatory T cells. This review discusses the intrinsic sensitivity of beta cells to FasL-mediated apoptosis, the conditions that underlie this beta cell sensitivity, and the feasibility of using FasL to arrest autoimmunity and prevent islet allograft rejection. In both the autoimmune and transplant settings, it is imperative to progress from the administration of nonspecific immunosuppressive therapy to the concept of beta-cell-specific immunomodulation. FasL evolves as a prime candidate for antigen-specific immunomodulation.

CD95L/FasL and TRAIL in tumour surveillance and cancer therapy

The membrane-bound death ligands CD95L/FasL and TRAIL, which activate the corresponding death receptors CD95/Fas, TRAILR1 and TRAILR2, induce apoptosis in many tumour cells, but can also elicit an inflammatory response. This chapter focuses on the relevance of CD95L/FasL and TRAIL for the tumour surveillance function of natural killer cells and cytotoxic T-cells and discuss current concepts of utilizing these ligands in tumour therapy.

Tumor counterattack: fact or fiction?

Cancer development relies on a variety of mechanisms that facilitate tumor growth despite the presence of a functioning immune system. Understanding these mechanisms may foster novel therapeutic approaches for oncology and organ transplantation. By expression of the apoptosis-inducing protein CD95L (FasL, APO-1L, CD178), tumors may eliminate tumor-infiltrating lymphocytes and suppress anti-tumor immune responses, a phenomenon called "tumor counterattack". On the one hand, preliminary evidence of tumor counterattack in human tumors exists, and CD95L expression can prevent T-cell responses in vitro. On the other hand, CD95L-expressing tumors are rapidly rejected and induce inflammation in mice. Here, we summarize and discuss the consequences of CD95L expression of tumor cells and its contribution to immune escape.

Haploinsufficiency, rather than the effect of an excessive production of soluble CD95 (CD95{Delta}TM), is the basis for ALPS Ia in a family with duplicated 3' splice site AG in CD95 intron 5 on one allele

Autoimmune lymphoproliferative syndrome type Ia (ALPS Ia) is caused by mutations in the CD95/APO1/FAS (TN-FRSF6) gene, which lead to a defective CD95 ligand (CD95L)-induced apoptosis. Soluble CD95 (sCD95) has been suggested to play an important role in the pathogenesis of diverse autoimmune and malignant diseases by antagonizing CD95L. Here we evaluate a family with 4 of its 5 members harboring an ex-6-3C-->G mutation that affects the splice cis regulatory region (cctacag/ex-6-->cctagag/ex-6) of the CD95 gene. The mutation causes skipping of exon-6, which encodes the transmembrane region of CD95, and thereby leads to an excessive production of sCD95 in all 4 affected individuals. The mutation is associated with a low penetrance of disease phenotype and caused mild and transient ALPS in one male patient whereas all other family members are completely healthy. In all family members with the mutation we found that the cell surface expression of CD95 was low and the activated T cells were resistant to CD95-induced apoptosis. Unexpectedly, excessive production or addition of sCD95 had no effect on the CD95-induced apoptosis in diverse cells. In contrast, increasing the surface expression of CD95 was able to correct the defect in apoptosis. Thus we conclude that the ALPS in the one male patient was caused by haploinsufficiency of membrane CD95 expression. Our data challenge the hypothesis that sCD95 causes autoimmunity.

Induction of tolerance using Fas ligand: a double-edged immunomodulator

Apoptosis mediated by Fas ligand (FasL) interaction with Fas receptor plays a pivotal regulatory role in immune homeostasis, immune privilege, and self-tolerance. FasL, therefore, has been extensively exploited as an immunomodulatory agent to induce tolerance to both autoimmune and foreign antigens with conflicting results. Difficulties associated with the use of FasL as a tolerogenic factor may arise from (1) its complex posttranslational regulation, (2) the opposing functions of different forms of FasL, (3) different modes of expression, systemic versus localized and transient versus continuous, (4) the level and duration of expression, (5) the sensitivity of target tissues to Fas/FasL-mediated apoptosis and the efficiency of antigen presentation in these tissues, and (6) the types and levels of cytokines, chemokines, and metalloproteinases in the extracellular milieu of the target tissues. Thus, the effective use of FasL as an immunomodulator to achieve durable antigen-specific immune tolerance requires careful consideration of all of these parameters and the design of treatment regimens that maximize tolerogenic efficacy, while minimizing the non-tolerogenic and toxic functions of this molecule. This review summarizes the current status of FasL as a tolerogenic agent, problems associated with its use as an immunomodulator, and new strategies to improve its therapeutic potential.

CD1d-unrestricted human NKT cells release chemokines upon Fas engagement

Attempts at inducing allograft immune privilege by enforced Fas ligand expression have shown accelerated rejection mediated by neutrophils. While it has been proposed that Fas ligand was directly chemotactic toward neutrophils, several lines of evidence argue for an indirect recruitment mechanism. This question was addressed by using in vitro migration assays that used highly purified human leukocyte subsets. Granulocytes did not migrate in response to Fas engagement and required the presence of T cells expressing several natural killer (NK) cell markers. These rare CD8 memory T cells expressed T and NK cell markers and were not restricted to CD1d, showing that they are distinct from conventional natural killer T (NKT) cells. These cells were able to kill both NK-sensitive and -insensitive targets and secreted several CC and CXC chemokines active toward granulocytes, monocytes, and NK cells upon Fas engagement. Chemotactic factor release depended on caspase activity, in the absence of NKT cell apoptosis. The ability of CD1d-unrestricted NKT cells to recruit innate immune system cells might play a role in cancer cell eradication and contribute to inflammatory diseases.

Roles of CXC chemokines and macrophages in the recruitment of inflammatory cells and tumor rejection induced by Fas/Apo-1 (CD95) ligand-expressing tumor

The role of CD95 ligand (FasL/Apo-1L)-expressing tumors in immunosuppression or immunopotentiation is controversial. CD95L-transfected tumors induce immunopotentiation after vigorous neutrophil infiltration. Thus, the induction of neutrophil infiltration by CD95L seems to play an important role in tumor rejection. The mechanism by which CD95L-expressing tumors cause neutrophil infiltration and antitumor immunity has not been well understood. CXC chemokine receptor 2 (CXCR2) knockout (KO) mice are a powerful tool for studying CXC chemokine-mediated neutrophil infiltration. We investigated the roles of CD95L and chemokines in CD95L-induced antitumor activity by using CXCR2 KO mice and CD95LcDNA-transfected MethA (MethA + CD95L) fibrosarcoma. MethA + CD95L cells were completely rejected in wild-type (WT) and even in KO mice. MethA + CD95L cells injected intraperitoneally (i.p.) induced the recruitment of both neutrophils and macrophages in WT but only macrophages in KO mice, although CXC and CC chemokines were released in both mice. Macrophages incubated with MethA + CD95L cells released CXC and CC chemokines. Macrophages derived from WT and KO but not neutrophils from WT mice induced the recruitment of neutrophils when adoptively i.p. transferred with MethA + CD95L cells into CD95L/CD95-deficient mice. The different recruitment of inflammatory cells between WT and KO mice was attributed to bone marrow (BM) cells by BM transfer experiment. Our results demonstrated that CXC chemokines are essential for neutrophil recruitment and that macrophages but not neutrophils play a critical role in the CD95L-induced infiltration of inflammatory cells and the eradication of CD95L-expressing tumor cells.

Cytoplasmic microvesicular form of Fas ligand in human early placenta: switching the tissue immune privilege hypothesis from cellular to vesicular level

The local immune privilege of the fetus is created by the placenta. Fas ligand (FasL) expression in trophoblast has been implied as one of the mechanisms of fetal tolerance. However, the expression of membranal FasL by trophoblast has failed to explain this role of FasL. Two objections can be raised: (1) there have been contradictions considering which trophoblast cells, syncytiotrophoblast (ST) or cytotrophoblast, express FasL; (2) in vivo and in vitro studies have shown that the membranal form of FasL evokes inflammatory response and thus may promote fetal rejection. Using different assays and the FasL-specific antibody G247-4 we demonstrate beyond doubt that in vivo, (1) FasL is produced by and stored in the first trimester human ST only and (2) the human ST lacks surface membranal FasL. Instead, FasL, loaded in microvesicles, is stored in cytoplasmic granules. These results complement the recent in vitro studies of the microvesicular form of FasL secretion by cultured trophoblast cells, and suggest that placental FasL is synthesized by villous ST, stored in microvesicular form and secreted as exosomes. Secretion of the exosome-associated form of FasL may be one mechanism by which the placenta promotes a state of immune privilege. Additionally, FasL expression in Hofbauer cells is also demonstrated.

The Fas/Fas ligand system and cancer: immune privilege and apoptosis

The Fas/FasL system has been suggested to play an important role in the establishment of immune privilege status for tumors by inducing Fas-mediated apoptosis in tumor-specific lymphocytes. However, the role of cell-surface expressed FasL in tumor cell protection has recently become controversial. Our laboratory has focused on the study of the role of the Fas/FasL system in the normal tissue remodeling of the female reproductive tract and in immune-privileged organs. Our studies have demonstrated a connection between sex hormones and the regulation of the Fas/FasL pathway in immune and reproductive cells. More recently, we have investigated the resistance of tumor cells to Fas-mediated apoptosis. We have also characterized a new form of FasL, different from the classical membranal form, which is secreted by ovarian cancer cells. In this review we describe the main techniques used in these studies.

Virally induced inflammation triggers fratricide of Fas-ligand-expressing beta-cells

Tissue-specific expression of Fas-ligand (Fas-L) can provide immune privilege by inducing apoptosis of "invading" lymphocytes expressing Fas. However, accelerated diabetes has been reported in transgenic mice expressing Fas-L in islets (RIP-Fas-L) as a result of Fas-dependent fratricide of beta-cells after transfer of diabetogenic clones. Here we studied whether Fas-L could protect islets from autoaggressive CD8 lymphocytes in a transgenic model of virally induced diabetes (RIP-LCMV-NP transgenic mice), in which the autoaggressive response is directed to a viral nucleoprotein (NP) expressed as a transgene in beta-cells. Indeed, disease incidence after viral (lymphocytic choriomeningitis virus [LCMV]) infection was reduced by approximately 30%, which was associated with a decrease of autoaggressive CD8 NP-specific lymphocytes in islets and pancreatic draining lymph nodes. However, surprisingly, a high degree (50%) of diabetes was seen in mice that expressed only Fas-L but not the viral transgene (NP) in beta-cells after infection with LCMV. This was due to induction of Fas on beta-cells after LCMV infection of the pancreas, resulting in Fas/Fas-L-mediated fratricide. Thus, although Fas-L can lend some immune privilege to islet cells, local virus-induced inflammation will induce Fas on beta-cells, leading to their mutual destruction if Fas-L is present. Expression of Fas-L therefore might not be protective in situations in which viral inflammation can be expected, resulting in Fas induction on the targeted cell itself.

Expression of mutant non-cleavable Fas ligand on retrovirus packaging cells causes apoptosis of immunocompetent cells and improves prodrug activation gene therapy in a malignant glioma model

Recombinant retroviruses (RV) have been widely used as vectors for clinical gene therapy of malignant brain tumors. Because of the very limited stability of these vectors in vivo, RV producing cells (VPC) are routinely used for intratumoral RV release. The host immune system, however, recognizes the intratumorally grafted allogeneic or xenogeneic VPC, and mounts an immune response against them. Humoral and cellular immune responses eventually result in reduction of VPC numbers and in limited success of RV mediated gene therapy approaches. This study presents a non-pharmacological and spatially limited approach for protection of VPC grafted in the CNS against destruction by host immune responses. Murine fibroblast-derived VPC expressing herpes-simplex-virus type I thymidine kinase (HSV-tk) were genetically modified to co-express a human Fas ligand (CD95L) deletion mutant (DeltaFasL) resistant to enzymatic cleavage and shedding. Direct interactions between Fas (CD95) on lymphocytes and DeltaFasL on VPC upon cell-cell contact rapidly caused apoptosis in lymphocytes. In addition, cultured malignant brain tumor cells (U87, LN18, LN229) transduced with DeltaFasL-RV were rendered apoptotic by Fas/DeltaFasL interaction. DeltaFasL-expressing VPC grafted in a 9L rat brain tumor model survived in significantly higher numbers compared with control VPC, and did not cause an increase in neutrophil infiltration of tumors. Gene therapy of tumor bearing animals grafted with the modified DeltaFasL-VPC and given the prodrug Ganciclovir resulted in significantly increased survival rates compared to treatment with control VPC and Ganciclovir. In conclusion, prolonged intratumoral presence of DeltaFasL-VPC seems to be a direct consequence of the expression of the membrane-bound mutant FasL, and may result in increased total RV output and improved tumor transduction with RV.

Display of Fas ligand protein on cardiac vasculature as a novel means of regulating allograft rejection

BACKGROUND

Fas ligand (FasL) is a potent death-inducing molecule with important functions in immune homeostasis and tolerance to self-antigens. The complex biological activities of FasL and its inefficient expression using conventional gene transfer approaches limit its use for immunomodulation to prevent allograft rejection. We have recently generated a chimeric FasL with core streptavidin (SA-FasL) with potent apoptotic activity and designed a novel approach to display it on the surface of several cell types via biotinylation. We herein tested whether SA-FasL can also be displayed on vascular endothelial cells in the heart and examined its effect on graft survival after transplantation into syngeneic and allogeneic hosts.

METHODS AND RESULTS

SA-FasL was efficiently displayed on the vasculature of BALB/c hearts with a half-life of 9 days in vivo. Transplantation of hearts displaying SA-FasL into syngeneic hosts resulted in indefinite graft survival without detectable toxicity to the grafts and hosts. In contrast, transplantation of allogeneic C57BL/10 hearts displaying SA-FasL into BALB/c recipients resulted in graft rejection, but in a delayed fashion as compared with control hearts (mean survival time=17.4+/-5 versus 9.6+/-1 days). Allograft survival was further extended to 21+/-2.6 and 24+/-3 days (P<0.05) by intravenous treatment of graft recipients with 1 dose of SA-FasL-decorated donor splenocytes on days 2 and 6 after transplantation, respectively.

CONCLUSIONS

This study shows for the first time that exogenous proteins can be displayed on the endothelium of solid organs for therapeutic purposes. This approach provides a convenient and rapid means of displaying exogenous proteins on the surface of cells, tissues, and solid organs, with broad research and therapeutic implications.

Dendritic cells overexpressing CD95 (Fas) ligand elicit vigorous allospecific T-cell responses in vivo

Dendritic cells (DCs) genetically engineered to overexpress CD95 (Fas) ligand (CD95L-DC) were proposed as tools to induce peripheral tolerance to alloantigens. Herein, we observed that CD95L-DC obtained after retroviral gene transfer in bone marrow (BM) precursors derived from CD95-deficient (lpr/lpr) mice elicit much stronger allospecific type 1 helper T-cell and cytotoxic T-cell activities than control DCs upon injection in vivo, although they induce lower T-cell responses in vitro. Indeed, a single injection of CD95L-DC prepared from C57BL/6 mice was sufficient to prime bm13 recipients for acute rejection of C57BL/6 skin allografts that were otherwise tolerated in the context of this single weak major histocompatibility complex (MHC) class I incompatibility. Massive neutrophil infiltrates depending on interleukin (IL)-1 signaling were observed at sites of CD95L-DC injection. Experiments in IL-1 receptor-deficient mice or in animals injected with depleting anti-Gr1 monoclonal antibody (mAb) established that neutrophil recruitment is required for the development of vigorous T-cell responses after injection of CD95L-DC in vivo.

Fas-ligand--iron fist or Achilles' heel?

Experimental and physiological expression of the pro-apoptotic molecule Fas-ligand can induce inflammation under certain conditions. Discussed here are the experimental situations, possible mechanisms, and pathways that mediate this response.

Role of Fas ligand in ocular tissue

The expression of Fas ligand (FasL) in the eye is an important factor in the maintenance of immune privilege. Although FasL expression in donor corneas contributes to prolonged survival of orthotopic corneal allografts in solid organ transplantation, FasL gene-transfected tissues reportedly lead to graft destruction through neutrophil recruitment. Differences in the effects of FasL have been attributed to different roles of soluble FasL (sFasL) and membrane FasL (mFasL). This is based on the presumption that the signals through sFasL and mFasL differ, with one causing apoptosis and the other activating inflammation. It was recently reported that inflammation caused by FasL was inhibited at an immune-privileged site, and therefore the effects of FasL may depend on differences in the anatomic sites where FasL-expressing cells are located. In this article, we discuss the role of sFasL and mFasL in ocular immune privilege.

The role of Fas ligand in immune privilege

Immune privilege is a property of some sites in the body, whereby immune responses are limited or prevented. One explanation that has been proposed for this phenomenon is engagement of the pro-apoptotic molecule Fas by its ligand (FasL), which leads to apoptosis, and consequently limits an inflammatory response. This idea has recently been challenged, and here we review the evidence for and against a role for FasL in immune privilege.

Fas/Apo-1 (CD95)-mediated apoptosis of neutrophils with Fas ligand (CD95L)-expressing tumors is crucial for induction of inflammation by neutrophilic polymorphonuclear leukocytes associated with antitumor immunity

To address how FasL-expressing tumors induce neutrophil emigration and abrogate tumorigenicity, we investigated the behavior of FasLcDNA-transfected hepatoma MH134 (G2) cells injected into wild-type (+) mice, lpr(cg)/lpr(cg) (lpr(cg)) mice with death domain (DD)-mutated Fas, and gld/gld lpr/lpr (gld/lpr) mice with defects in FasL/Fas. G2 cells were eradicated after extensive infiltration of neutrophils around them in + mice but formed tumors without such infiltration in lpr(cg) and gld/lpr mice. Abundant cell debris suggestive of apoptosis of infiltrating neutrophils was found among G2 tumor cells in + mice but a few neutrophils infiltrating among G2 cells were intact in lpr(cg) and gld/lpr mice. Collectively, these results indicate the crucial role of Fas DD in Fas-mediated apoptosis of neutrophils and suggest that apoptosis of neutrophils with FasL-expressing tumors may trigger the extensive infiltration of neutrophils, resulting in violent inflammation and ultimately in the eradication of tumor cells.