Fas ligand-transfected myoblasts and islet cell transplantation

The FasLane to ocular pathology-metalloproteinase cleavage of membrane-bound FasL determines FasL function

Fas ligand (FasL) is best known for its ability to induce cell death in a wide range of Fas-expressing targets and to limit inflammation in immunoprivileged sites such as the eye. In addition, the ability of FasL to induce a much more extensive list of outcomes is being increasingly explored and accepted. These outcomes include the induction of proinflammatory cytokine production, T cell activation, and cell motility. However, the distinct and opposing functions of membrane-associated FasL (mFasL) and the C-terminal soluble FasL fragment (sFasL) released by metalloproteinase cleavage is less well documented and understood. Both mFasL and sFasL can form trimers that engage the trimeric Fas receptor, but only mFasL can form a multimeric complex in lipid rafts to trigger apoptosis and inflammation. By contrast, a number of reports have now documented the anti-apoptotic and anti-inflammatory activity of sFasL, pointing to a critical regulatory function of the soluble molecule. The immunomodulatory activity of FasL is particularly evident in ocular pathology where elimination of the metalloproteinase cleavage site and the ensuing increased expression of mFasL can severely exacerbate the extent of inflammation and cell death. By contrast, both homeostatic and increased expression of sFasL can limit inflammation and cell death. The mechanism(s) responsible for the protective activity of sFasL are discussed but remain controversial. Nevertheless, it will be important to consider therapeutic applications of sFasL for the treatment of ocular diseases such as glaucoma.

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.

Life in the Fas lane: differential outcomes of Fas signaling

Fas, also known as CD95 or APO-1, is a member of the tumor necrosis factor/nerve growth factor superfamily. Although best characterized in terms of its apoptotic function, recent studies have identified several other cellular responses emanating from Fas. These responses include migration, invasion, inflammation, and proliferation. In this review, we focus on the diverse cellular outcomes of Fas signaling and the molecular switches identified to date that regulate its pro- and anti-apoptotic functions. Such switches occur at different levels of signal transduction, ranging from the receptor through to cross-talk with other signaling pathways. Factors identified to date including other extracellular signals, proteins recruited to the death-inducing signaling complex, and the availability of different intracellular components of signal transduction pathways. The success of therapeutically targeting Fas will require a better understanding of these pathways, as well as the regulatory mechanisms that determine cellular outcome following receptor activation.

Characteristics of the early immune response following transplantation of mouse ES cell derived insulin-producing cell clusters

BACKGROUND

The fully differentiated progeny of ES cells (ESC) may eventually be used for cell replacement therapy (CRT). However, elements of the innate immune system may contribute to damage or destruction of these tissues when transplanted.

METHODOLOGY/PRINCIPAL FINDINGS

Herein, we assessed the hitherto ill-defined contribution of the early innate immune response in CRT after transplantation of either ESC derived insulin producing cell clusters (IPCCs) or adult pancreatic islets. Ingress of neutrophil or macrophage cells was noted immediately at the site of IPCC transplantation, but this infiltration was attenuated by day three. Gene profiling identified specific inflammatory cytokines and chemokines that were either absent or sharply reduced by three days after IPCC transplantation. Thus, IPCC transplantation provoked less of an early immune response than pancreatic islet transplantation.

CONCLUSIONS/SIGNIFICANCE

Our study offers insights into the characteristics of the immune response of an ESC derived tissue in the incipient stages following transplantation and suggests potential strategies to inhibit cell damage to ensure their long-term perpetuation and functionality in CRT.

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.

Fas Ligand as a Tool for Immunosuppression and Generation of Immune Tolerance

The role of Fas ligand (FasL) in physiologically limiting immune responses and maintaining immune-privileged sites has led to a body of research aiming to confer protection to allogeneic grafts by expressing FasL on the allogeneic tissue or by administrating FasL-transduced donor dendritic cells. In addition, several studies have used FasL to abrogate autoimmune responses. This review presents the results of these studies and discusses the problems associated with FasL usage.

Human Fas-ligand expression on porcine endothelial cells does not protect against xenogeneic natural killer cytotoxicity*

Several human leukocyte subsets including natural killer (NK) cells, cytotoxic T lymphocytes (CTL), and polymorphonuclear neutrophils (PMN) participate in cellular immune responses directed against vascularized pig-to-human xenografts. As these leukocytes express the death receptor Fas either constitutively (PMN) or upon activation (NK, CTL), we explored in vitro whether the transgenic expression of Fas ligand (FasL) on porcine endothelial cells (EC) is a valuable strategy to protect porcine xenografts. The porcine EC line 2A2 was stably transfected with human FasL (2A2-FasL) and interactions of 2A2-FasL with human leukocytes were analyzed using functional assays for apoptosis, cytotoxicity, chemotaxis, adhesion under shear stress, and transmigration. FasL expressed on porcine EC induced apoptosis in human NK and T cells, but did not protect porcine EC against killing mediated by human NK cells. 2A2-FasL released soluble FasL, which induced strong chemotaxis in human PMN. Adhesion under shear stress of PMN on 2A2-FasL cells was increased whereas transendothelial migration was decreased. In contrast, FasL had no effect on the adhesion of NK cells but increased their transmigration through porcine EC. Although FasL expression on porcine EC is able to induce apoptosis in human effector cells, it did not provide protection against xenogeneic cytotoxicity. The observed impact of FasL on adhesion and transendothelial migration provides evidence for novel biological functions of FasL.

Attenuation of GVHD for allo-bone marrow transplantation recipient by FasL-Fas pathway in an H-2 haplotype disparate mouse combination

In order to explore a new special and effective way to prevent graft versus host disease (GVHD) after allogenic bone marrow transplantation (allo-BMT), the stem cell antigen-1 (Sca-1) + early hematopoietic cells (EHC) from BALB/c mouse (H-2d) were introduced with exogenous mouse Fas ligand (mFasL) cDNA gene by the retrovirus-mediated gene transfer and expanded for one week, and then they were co-cultured with the spleen mononuclear cells (SMNC) from BAC mouse (H-2dxb) as one way mixed lymphocyte reaction (OWMLR). The cytotoxicity of treated BAC mouse SMNC against Na2 51CrO4 labeling SMNC from BALB/c mouse was observed. The bone marrow mononuclear cells (BMMNC) from BAC mouse treated by the above methods were transplanted into lethally-irradiated congenic BALB/c mice to observe the occurrence of GVHD. The results showed that the SMNC from BAC mouse after OWMLR with exogenous mFasL cDNA gene-transduced hematopoietic cells (HC) from BALB/c mouse in a ratio of 1 to 5 exhibited an obvious inhibition of the cytotoxicity against the BALB/c mouse spleen cells at different effector/target ratios as compared to the control group (P<0.01). The grade I GVHD or no GVHD and the 80% survival rate at day 60 post-BMT were observed in the BALB/c mouse receiving BAC mouse BMMNC treated with similar way, while the grade II - III GVHD and the 20% survival rate were noted in the control group (P<0.01). It is suggested that the attenuation of GVHD in allo-BMT recipient could be successfully achieved through FasL-Fas pathway in an H-2 haplotype disparate mouse combination.

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.

Neutrophils mediate parenchymal tissue necrosis and accelerate the rejection of complete major histocompatibility complex-disparate cardiac allografts in the absence of interferon-gamma

A major feature of acute rejection of cardiac allografts is an intense mononuclear cell infiltration accompanied by interferon (IFN)-gamma production. In the current study we tested the role of IFN-gamma in acute rejection of allografts by comparing the histopathology of rejection in wild-type versus IFN-gamma-/- recipients of major histocompatibility complex-mismatched cardiac grafts. Wild-type recipients rejected the allografts at days 8 to 9 after transplant but rejection was accelerated 2 to 3 days in IFN-gamma-deficient recipients. During rejection in wild-type recipients, the allografts were heavily infiltrated with CD8+ T cells and other mononuclear cells. In contrast, allografts in IFN-gamma-deficient recipients had few T cells but an intense neutrophil infiltration accompanied by extensive graft parenchymal necrosis. No difference in expression levels of neutrophil chemoattractants including Groalpha/KC, MIP-2, GCP-2, and MIP-1alpha, was observed in allografts retrieved from wild-type and IFN-gamma-/- recipients. Depletion of neutrophils from IFN-gamma-deficient recipients delayed rejection until days 8 to 10 after transplant and restored the histopathology of acute allograft rejection to that observed in allografts rejected by wild-type recipients. These results indicate the potent regulatory properties of IFN-gamma during acute rejection directed at neutrophil infiltration into allografts and mediating graft tissue necrosis.

Membrane Fas ligand activates innate immunity and terminates ocular immune privilege

It has been proposed that the constitutive expression of Fas ligand (FasL) in the eye maintains immune privilege, in part through inducing apoptosis of infiltrating Fas(+) T cells. However, the role of FasL in immune privilege remains controversial due to studies that indicate FasL is both pro- and anti-inflammatory. To elucidate the mechanism(s) by which FasL regulates immune privilege, we used an ocular tumor model and examined the individual roles of the membrane-bound and soluble form of FasL in regulating ocular inflammation. Following injection into the privileged eye, tumors expressing only soluble FasL failed to trigger inflammation and grew progressively. By contrast, tumors expressing only membrane FasL 1) initiated vigorous neutrophil-mediated inflammation, 2) terminated immune privilege, and 3) were completely rejected. Moreover, the rejection coincided with activation of both innate and adaptive immunity. Interestingly, a higher threshold level of membrane FasL on tumors is required to initiate inflammation within the immune privileged eye, as compared with nonprivileged sites. The higher threshold is due to the suppressive microenvironment found within aqueous humor that blocks membrane FasL activation of neutrophils. However, aqueous humor is unable to completely block the proinflammatory effects of tumor cells that express high levels of membrane FasL. In conclusion, our data indicate that the function of FasL on intraocular tumors is determined by the microenvironment in conjunction with the form and level of FasL expressed.

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.

Activation-induced cell death: the controversial role of Fas and Fas ligand in immune privilege and tumour counterattack

Activation-induced cell death (AICD) is the process by which cells undergo apoptosis in a controlled manner through the interaction of a death factor and its receptor. Programmed cell death can be induced by a number of physiological and pathological factors including Fas (CD95)-Fas ligand (FasL/CD95L) interaction, tumour necrosis factor (TNF), ceramide, and reactive oxygen species (ROS). Fas is a 48-kDa type I transmembrane protein that belongs to the TNF/nerve growth factor receptor superfamily. FasL is a 40-kDa type II transmembrane protein that belongs to the TNF superfamily. The interaction of Fas with FasL results in a series of signal transductions which initiate apoptosis. The induction of apoptosis in this manner is termed AICD. Activation-induced cell death and Fas-FasL interactions have been shown to play significant roles in immune system homeostasis. In this review the involvement of Fas and Fas ligand in cell death, with particular reference to the T cell, and the mechanism(s) by which they induce cell death is described. The role of AICD in immune system homeostasis and the controversy surrounding the role of FasL in immune privilege, inflammation, and so-called tumour counterattack is also discussed.

Cell death and immune privilege

The host response to pathogens involves complex inflammatory responses and immune reactions. While these are central to host defense and vital to clearing infections, they are often accompanied by injury to surrounding tissue. Most organ systems can tolerate these responses without permanent consequences. However, there are sites that limit the spread of inflammation because it can threaten organ function. The most prominent examples of these are the eye, brain, and reproductive organs (testis, ovary), where even minor bouts of inflammation can have long-term consequences for the survival of the organism. In these organs immune responses either do not proceed, or proceed in a manner different from other areas; thus, they are called "immunologically privileged." Here a functioning immune response can be the culprit that leads to disease.

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.

Human islet cell transplantation--future prospects

BACKGROUND

Islet transplantation has the potential to cure diabetes mellitus. Nevertheless despite successful reversal of diabetes in many small animal models, the clinical situation has been far more challenging. The aim of this review is to discuss why insulin-independence after islet allotransplantation has been so difficult to achieve.

METHODS

A literature review was undertaken using Medline from 1975 to July 2000. Results reported to the International Islet Transplant Registry (ITR) up to December 1998 were also analysed.

RESULTS

Up to December 1998, 405 islet allotransplants have been reported the ITR. Of those accurately documented between 1990 and 1998 (n = 267) only 12% have achieved insulin-independence (greater than 7 days). However with refined peri-transplant protocols insulin independence at 1 year can reach 20%.

CONCLUSIONS

There are many factors which can explain the failure of achieving insulin-independence after islet allotransplantation. These include the use of diabetogenic immunosuppressive agents to abrogate both islet allo-immunity and auto-immunity, the critical islet mass to achieve insulin-independence and the detrimental effects of transplanting islets in an ectopic site. However recent evidence most notably from the Edmonton group demonstrates that islet allotransplantation still has great potential to become an established treatment option for diabetic patients.