Expression of human Fas ligand on mouse beta islet cells does not induce insulitis but is insufficient to confer immune privilege for islet grafts

Diabetes mellitus and apoptosis: inflammatory cells

Since the early observation that similarities between thyroiditis and insulitis existed, the important role played by inflammation in the development of diabetes has been appreciated. More recently, experiments have shown that inflammation also plays a prominent role in the development of target organ damage arising as complications, with both elements of the innate and the adaptive immune system being involved, and that cytokines contributing to local tissue damage may arise from both infiltrating and resident cells. This review will discuss the experimental evidence that shows that inflammatory cell-mediated apoptosis contributes to target organ damage, from beta cell destruction to both micro- and macro-vascular disease complications, and also how alterations in leukocyte turnover affects immune function.

Elevated serum decoy receptor 3 with enhanced T cell activation in systemic lupus erythematosus

Decoy receptor 3 (DcR3/TR6) is a decoy receptor for the Fas ligand (FasL) and can inhibit FasL-induced apoptosis. It has been reported recently that DcR3 can induce T cell activation via co-stimulation of T cells, suggesting that DcR3 may be involved in the pathophysiology of autoimmune diseases. This study aims to analyse the serum DcR3 in patients with systemic lupus erythematosus (SLE) and to investigate the role of DcR3 in the pathogenesis of SLE. Significantly elevated serum DcR3 was observed in SLE patients, and the mean serum DcR3 level was significantly higher for those with active disease [SLE disease activity index (SLEDAI) >/= 10] compared with that in patients with inactive disease (SLEDAI < 10). In addition to reducing activation-induced cell death in activated T cells via neutralization of the FasL, soluble DcR3-Fc enhanced T cell proliferation and increased interleukin-2 and interferon-gamma production via co-stimulation of T cells. Moreover, enhanced T cell reactivity to DcR3-induced co-stimulation was demonstrated in lymphocytes from patients with SLE, suggesting the elevated serum DcR3 may associate with enhanced T cell activation in vivo. These findings are the first to demonstrate that serum DcR3 concentrations are increased in SLE patients, and this may imply a possible role of DcR3 in the pathogenesis of SLE via enhanced T cell hyperreactivity and reduced apoptosis in activated T cells.

Attenuation of Th1 response in decoy receptor 3 transgenic mice

The soluble decoy receptor 3 (DcR3) is a member of the TNFR superfamily. Because DcR3 is up-regulated in tumor tissues and is detectable in the sera of cancer patients, it is regarded as an immunosuppressor to down-regulate immune responses. To understand the function of DcR3 in vivo, we generated transgenic mice overexpressing DcR3 systemically. In comparison with HNT-TCR (HNT) transgenic mice, up-regulation of IL-4 and IL-10 and down-regulation of IFN-gamma, IL-12, and TNF-alpha were observed in the influenza hemagglutinin(126-138) peptide-stimulated splenocytes of HNT-DcR3 double-transgenic mice. When infected with Listeria monocytogenes, DcR3 transgenic mice show attenuated expression of IFN-gamma as well as increased susceptibility to infection. The Th2 cell-biased phenotype in DcR3 transgenic mice is attributed to decreased IL-2 secretion by T cells, resulting in the suppression of IL-2 dependent CD4(+) T cell proliferation. This suggests that DcR3 might help tumor growth by attenuating the Th1 response and suppressing cell-mediated immunity.

The role of Fas ligand and transforming growth factor beta in tumor progression: molecular mechanisms of immune privilege via Fas-mediated apoptosis and potential targets for cancer therapy

Despite the fact that expression of Fas ligand (FasL) in cytotoxic T lymphocytes (CTLs) and in natural killer (NK) cells plays an important role in Fas-mediated tumor killing, During tumor progression FasL-expressing tumor cells are involved in counterattacking to kill tumor-infiltrating lymphocytes (TILs). Soluble FasL levels also increase with tumor progression in solid tumors, and this increase inhibits Fas-mediated tumor killing by CTLs and NK cells. The increased expression of FasL in tumor cells is associated with decreased expression of Fas; and the promoter region of the FASL gene is regulated by transcription factors, such as neuronal factor kappaB (NF-kappaB) and AP-1, in the tumor microenvironment. Although the ratio of FasL expression to Fas expression in tumor cells is not strongly related to the induction of apoptosis in TILs, increased expression of FasL is associated with decreased Fas levels in tumor cells that can escape immune surveillance and facilitate tumor progression and metastasis. Transforming growth factor beta (TGF-beta) is a potent growth inhibitor and has tumor-suppressing activity in the early phases of carcinogenesis. During subsequent tumor progression, the increased secretion of TGF-beta by both tumor cells and, in a paracrine fashion, stromal cells, is involved in the enhancement of tumor invasion and metastasis accompanied by immunosuppression. Herein, the authors review the clinical significance of FasL and TGF-beta expression patterns as features of immune privilege accompanying tumor progression in the tumor microenvironment. Potential strategies for identifying which molecules can serve as targets for effective antitumor therapy also are discussed.

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.

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.