Fas (CD95)-transduced signal preferentially stimulates lupus peripheral T lymphocytes

FAS mRNA editing in Human Systemic Lupus Erythematosus

FAS/FASL system plays a central role in maintaining peripheral immune tolerance. Human Systematic Lupus Erythematosus (SLE) is a prototypic systemic autoimmune disease characterized by expansion of autoreactive lymphocytes. It remains unclear whether a defective FAS/FASL system is involved in the pathogenesis of SLE. In this study, we have discovered a novel nucleotide insertion in FAS mRNA. We demonstrate that this novel FAS mutation occurs at mRNA levels, likely through a site-specific mRNA editing process. The mRNA editing mutation is unique for human FAS because the similar mRNA editing event is absent in other human TNF receptor (TNFR) family genes with death domains (DR5, DR6, and TNFR1) and in murine FAS. The adenine insertion mutation in the coding region message causes the alteration of human FAS mRNA reading frame. Functionally, cells expressing the edited FAS (edFAS) were refractory to FAS-mediated apoptosis. Surprisingly, cells from SLE patients produced significantly more edFAS products compared to cells from normal healthy controls. Additionally, we demonstrated that persistent engagement of T-cell receptor increases human FAS mRNA editing in human T cells. Our data suggest that the site-specific FAS mRNA editing mutation may play a critical role in human immune responses and in the pathogenesis of human chronic inflammatory diseases.

Fas ligand is responsible for CXCR3 chemokine induction in CD4+ T cell-dependent liver damage

Immune-mediated hepatic damage has been demonstrated in the pathogenesis of hepatitis C virus (HCV) and other hepatotrophic infections. Fas/Fas ligand (FasL) interaction plays a critical role in immune-mediated hepatic damage. To understand the molecular mechanism(s) of FasL-mediated liver inflammation, we examined the effect of CD4(+) T cells expressing high levels of FasL on the initiation of hepatic damage through analysis of chemokine and chemokine receptor expression in HCV core x TCR (DO11.10) double-transgenic mice. In vivo antigenic stimulation triggers a marked influx of core-expressing Ag-specific CD4(+) T cells into the liver of the immunized core(+) TCR mice but not their core(-) TCR littermates. Strikingly, the inflammatory process in the liver of core(+) TCR mice was accompanied by a dramatic increase in IFN-inducible protein 10 and monokine induced by IFN-gamma production. The intrahepatic lymphocytes were primarily CXCR3-positive and anti-CXCR3 Ab treatment abrogates migration of CXCR3(+) lymphocytes into the liver and hepatic damage. Importantly, the blockade of Fas/FasL interaction reduces the expression of IFN-inducible protein 10 and monokine induced by IFN-gamma and cellular infiltration into the liver. These findings suggest that activated CD4(+) T cells with elevated FasL expression are involved in promoting liver inflammation and hepatic damage through the induction of chemokines.

Peripheral blood lymphocyte apoptosis and circulating dendritic cells in patients with systemic lupus erythematosus: correlation with immunological status and disease-related symptoms

We investigated in vivo the relationship between the degree of peripheral blood lymphocyte apoptosis and circulating dendritic cells (DC) as well as the immunological status in 45 patients with systemic lupus erythematous (SLE). Apoptosis was detected by phosphatidylserine externalization and assays to detect caspase activation. The total DC count (tDC) and their myeloid, mDC1 (BDCA1+) and mDC2 (BDCA3+), and plasmacytoid, pDC (BDCA3+), subtypes were assessed. Moreover, several immunological parameters, such as complement proteins, interferons (IFN), tumor necrosis factor (TNF)-alpha, interleukin (IL)-4, and IL-6 levels were assessed. There were no significant differences in both the intensity of apoptosis and DC counts between active and inactive SLE as well as between untreated patients and those treated with steroids. The incidence of lymphocyte apoptosis correlated positively with T-cell count, both T-helper (p=0.004) and cytotoxic T cells (p=0.001), but not with B or natural killer (NK) cells. The intensity of apoptosis enhanced along with the increase in complement C3 (p=0.016) and decrease in IFN-gamma (p=0.040) levels. The apoptotic cell count correlated with tDC (p=0.031), mDC1 (p=0.007), and pDC (p=0.039) counts. Both tDC and mDC1 counts correlated positively with antinuclear antibody (ANA) titers (p=0.017 and 0.032, respectively). Moreover, tDC correlated with C4 (p=0.039) and pDC with both C3 (p=0.032) and C4 (p=0.007) levels. The DC counts correlated inversely with IFN-gamma (tDC, p=0.038; mDC1, p=0.009), IL-6 (mDC2, p=0.031), or serum IgG levels (tDC, p=0.006; mDC1, p<0.001; mDC2, p=0.001). We found a positive correlation between lymphocyte apoptosis and peripheral blood DC count as well as the level of complement proteins in patients with SLE. The enhanced lymphocyte apoptosis was accompanied by the decrease in concentration of some cytokines, such as IFN-gamma or IL-6, as well as serum IgG level. This finding may reflect pathogenetic events during development of the disease, which include a persistent signal derived from circulating apoptotic lymphocytes, mobilizing the complement system, and attracting peripheral blood DC.

Increased Fas ligand expression of CD4+ T cells by HCV core induces T cell-dependent hepatic inflammation

Hepatitis C virus (HCV) infection is associated with a high rate of viral persistence and the development of chronic liver disease. The expression of HCV core protein in T cells has previously been reported to alter T cell activation and has been linked to the development of liver inflammation. However, the molecular and cellular basis for the role of HCV core-expressing T cells in liver inflammation is not understood. Here, using double-transgenic mice of CD2/HCV-core transgenic mice and ovalbumin (OVA)-specific T cell receptor transgenic mice, we demonstrated that in vivo antigenic stimulation (OVA peptide administration) triggers a marked influx of core-expressing, antigen-specific, transgenic CD4+ T cells into the liver of these mice. Phenotypic analysis of the liver-infiltrating T cells revealed high expression levels of CD44 and Fas ligand (FasL). Adoptive transfer of liver-infiltrating, core-expressing CD4+ T cells into severe combined immunodeficiency mice directly demonstrated the capacity of these activated T cells to induce liver inflammation. It is important that anti-FasL antibody treatment of the mice at the time of cell transfer abrogated the liver inflammation induced by core-expressing CD4+ T cells. These findings suggest that activated T lymphocytes expressing elevated levels of FasL may be involved in the bystander killing of hepatocyte, as well as the induction of chronic liver inflammation, by promoting recruitment of proinflammatory cells to the liver.

The −844C/T polymorphism in the Fas ligand promoter associates with Taiwanese SLE

FasL expression is critical in T-cell activation-induced apoptosis, which is involved in lupus pathogenesis. This study identified two SNPs in the FasL promoter regions from -1145 to -45 by genomic DNA sequencing. The -844C/T polymorphism was previously described by its location in and affect on the CCAAT/enhancer-binding protein beta (C/EBPB beta)-binding site and the other (-1094A/C, a novel polymorphism) was located at the NF-kappaB transcription-binding site. FasL gene promoter polymorphisms were genotyped in 260 systemic lupus erythematosus (SLE) patients and 280 healthy controls using MassArray matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectrometry. The distribution of FasL promoter -844C/C genotype, predominant in Taiwanese, was skewed in Taiwanese SLE patients (odds ratio: 1.53; P-value=0.014). FasL promoter -844C/T polymorphism genotype distributions of Taiwanese, African Americans, and Caucasians differed. Moreover, no particular clinical association of -844C/T and -1094A/C polymorphisms with SLE was found in patients in Taiwan. This study confirmed that -844C/C genotype is associated with lupus susceptibility. The -1094A/C polymorphism is not significantly associated with lupus disease susceptibility, albeit the role of NF-kappaB pathway in FasL promoter activation remains unclear. Fas/FasL pathway may contribute to SLE polygenic disease entity.

A proinflammatory role for Fas in joints of mice with collagen-induced arthritis

Collagen-induced arthritis (CIA) is a chronic inflammatory disease bearing all the hallmarks of rheumatoid arthritis, e.g. polyarthritis, synovitis, and subsequent cartilage/bone erosions. One feature of the disease contributing to joint damage is synovial hyperplasia. The factors responsible for the hyperplasia are unknown; however, an imbalance between rates of cell proliferation and cell death (apoptosis) has been suggested. To evaluate the role of a major pathway of cell death – Fas (CD95)/FasL – in the pathogenesis of CIA, DBA/1J mice with a mutation of the Fas gene (lpr) were generated. The susceptibility of the mutant DBA-lpr/lpr mice to arthritis induced by collagen type II was evaluated. Contrary to expectations, the DBA-lpr/lpr mice developed significantly milder disease than the control littermates. The incidence of disease was also significantly lower in the lpr/lpr mice than in the controls (40% versus 81%; P < 0.05). However DBA-lpr/lpr mice mounted a robust immune response to collagen, and the expression of local proinflammatory cytokines such as, e.g., tumor necrosis factor α (TNF-α) and IL-6 were increased at the onset of disease. Since the contribution of synovial fibroblasts to inflammation and joint destruction is crucial, the potential activating effect of Fas on mouse fibroblast cell line NIH3T3 was investigated. On treatment with anti-Fas in vitro, the cell death of NIH3T3 fibroblasts was reduced and the expression of proinflammatory cytokines TNF-α and IL-6 was increased. These findings suggest that impairment of immune tolerance by increased T-cell reactivity does not lead to enhanced susceptibility to CIA and point to a role of Fas in joint destruction.

Ceramide-mediated clustering is required for CD95-DISC formation

Early events required for induction of apoptosis by CD95 are preassociation of CD95, the formation of the death-inducing signaling complex (DISC) and clustering of CD95 in distinct membrane domains. Here, we identify the molecular ordering of these events and show that the acid sphingomyelinase (ASM) functions upstream of the DISC to mediate CD95 clustering in ceramide-enriched membrane platforms, an event that is required for DISC formation. Experiments in ASM-deficient cells revealed that CD95 ligation, in the absence of ceramide generation, triggers <1% of full caspase 8 activation at the receptor. This event, however, is both necessary and sufficient to trigger translocation of ASM onto the outer leaflet of the plasma membrane, ASM activation and ceramide release, but insufficient for apoptosis induction. Ceramide-mediated CD95 clustering then amplifies the primary CD95 signaling and drives the second step of CD95 signaling, that is, formation of the DISC yielding 100% caspase activity and apoptosis. These studies suggest that the most parsimonious interpretation of the molecular ordering of the earliest events in CD95 signaling, at least in some cells, is: CD95 ligation-->1% of maximum caspase 8 activation-->ASM translocation-->ceramide generation-->CD95 clustering-->DISC formation-->100% of maximum caspase 8 activation-->apoptosis.

Characterization of CD3+ CD4- CD8- (double negative) T cells in patients with systemic lupus erythematosus: activation markers

Systemic lupus erythematosus (SLE) is characterized by B cell hyperactivity and the production of autoantibodies, some of which (antibodies to dsDNA) are thought to be pathogenic. T helper cells drive the production of autoantibodies and the aim of this study is to characterize phenotypically a subpopulation of T cells (the CD3+ CD4- CD8-, double negative (DN) T cells) previously identified as helping to enhance anti-DNA antibodies in patients with SLE. Data were obtained using FACS staining of DN T cells that had been purified from PBMCs by magnetic bead separation. The percentage of TCR alphabeta+ DN T cells was found to be significantly higher in patients with SLE as compared with controls (P = 0.02), although there was no significant increase in total percentage of DN T cells, which includes TCR gammadelta+ cells. Activation markers HLA-DR and CD69, the costimulatory molecule CD28 and CTLA-4 were all expressed on the surface of a higher percentage of DN T cells in patients with SLE than in patients with rheumatoid arthritis (RA) or healthy controls (HC). More DN T cells from patients with SLE were of CD45RA phenotype than was found in controls, while CD45RO-expressing cells were reduced. In addition, DN T cells from patients with SLE expressed significantly higher levels of HLA-DR (P = 0.006), CD28 (P = 0.05), CTLA4 (P = 0.03) and CD45RA (P = 0.05) on the cell surface than those from the CD4/8 population. Correlation of expression of the markers measured with various parameters of disease activity and severity showed that high levels of HLA-DR expression correlated with high circulating serum C3 (> 0.9 IU/ml), indicating that an activated phenotype is consistent with severe disease.

Combination of molecular mimicry and aberrant autoantigen expression is important for development of anti-Fas ligand autoantibodies in patients with systemic lupus erythematosus

We have reported previously that circulating anti-Fas ligand (FasL) autoantibodies able to inhibit Fas/FasL-mediated apoptosis were present in patients with systemic lupus erythematosus (SLE). In the present study, we describe the epitopes recognized by these anti-FasL autoantibodies. Rabbit antihuman antibody, raised against a FasL fragment consisting of amino acids (aa) 103-179 (fragment 2.0), inhibited Fas/FasL-mediated apoptosis, whereas an antibody against a FasL aa 103-146 fragment (fragment 1.0) did not. This suggested that an epitope around aa 146-179 was important for Fas/FasL interaction. Epitope mapping of anti-FasL autoantibodies using deletion mutants indicated that the epitope was located around aa 163-179. Three-dimensional molecular modelling of the Fas/FasL complex revealed that the aa 162-169 region was located on the outermost side of FasL, which suggested that the anti-FasL autoantibody would easily have access to the epitope. FasL point mutants involving aa positions 162-169 resulted in complete loss of apoptosis-inducing capability, which suggested that the aa 162-169 region was important for Fas/FasL interaction. A synthetic FasL peptide consisting of aa 161-170 blocked the binding of anti-FasL autoantibodies to FasL fragment 2.0 (aa 103-179). The FasL aa 161-170 sequence was found to be highly homologous with aa sequences from several infectious agents. Synthetic peptides derived from some of these microorganisms cross-reacted with the epitope recognized by the autoantibodies, suggesting that several foreign infectious agent-derived proteins may share an epitope with human FasL. As lymphocytes from SLE patients aberrartly expressed FasL, it is possible that infection by one of several infectious agents may trigger cross-reactive antibody responses, after which aberrantly expressed endogenous FasL might induce the shift from a cross-reactive response to an authentic autoimmune response. Therefore, a combination of molecular mimicry and aberrant autoantigen expression may be important for the development of anti-FasL autoantibodies in SLE patients.

Activation-induced cell death

The death of T lymphocytes following their activation involves several signal pathways that converge on a series of proteases, known as caspases, that degrade cellular proteins and activate a DNAse. Caspases are activated through ligation of cell surface death receptors as well as via direct activation of downstream caspases, often through metabolic stress such as cytokine withdrawal or generation of oxygen radicals, that culminates in mitochondrial dysfunction and release of the pro-apoptotic molecules, cytochrome c and Smac/DIABLO. The Bcl-2 family members serve to regulate the mitochondrial membrane integrity. Recent studies are now revealing the significant contribution to the activation-induced cell death of T cells by downstream caspases such as caspase-3 and Bcl-2-homology domain 3 (BH3)-only members of the Bcl-2 family.

Sphingolipid perturbations as mechanisms for fumonisin carcinogenesis

There is a great deal of evidence that altered sphingolipid metabolism is associated with fumonisin-induced animal diseases including increased apoptotic and oncotic necrosis, and carcinogenesis in rodent liver and kidney. The biochemical consequences of fumonisin disruption of sphingolipid metabolism most likely to alter cell regulation are increased free sphingoid bases and their 1-phosphates, alterations in complex sphingolipids, and decreased ceramide (CER) biosynthesis. Because free sphingoid bases and CER can induce cell death, the fumonisin inhibition of CER synthase can inhibit cell death induced by CER but promote free sphingoid base-induced cell death. Theoretically, at any time the balance between the intracellular concentration of effectors that protect cells from apoptosis (decreased CER, increased sphingosine 1-phosphate) and those that induce apoptosis (increased CER, free sphingoid bases, altered fatty acids) will determine the cellular response. Because the balance between the rates of apoptosis and proliferation is important in tumorigenesis, cells sensitive to the proliferative effect of decreased CER and increased sphingosine 1-phosphate may be selected to survive and proliferate when free sphingoid base concentration is not growth inhibitory. Conversely, when the increase in free sphingoid bases exceeds a cell's ability to convert sphinganine/sphingosine to dihydroceramide/CER or their sphingoid base 1-phosphate, then free sphingoid bases will accumulate. In this case cells that are sensitive to sphingoid base-induced growth arrest will die and insensitive cells will survive. If the cells selected to die are normal phenotypes and the cells selected to survive are abnormal, then cancer risk will increase.

Cytokine regulation of apoptosis and Bcl-2 expression in lymphocytes of patients with systemic lupus erythematosus

Both faulty regulation of apoptosis and the inappropriate expression of several interleukins have been considered important defects of lymphocytes in the human autoimmune disease systemic lupus erythematosus (SLE). We therefore tested the in vitro effect of recombinant interleukin (IL-)-2, 4, 7, and 15 on peripheral blood mononuclear cells from patients with SLE and from healthy volunteers. Intracellular Bcl-2 and Bax expression was measured by fluorocytometry and the rate of apoptosis was determined by the TUNEL technique and propidium iodide staining. IL-2, IL-4, IL-7 and IL-15 led to a significant increase in Bcl-2 and a reduction in cell death rates, which was even more pronounced in SLE. Bax levels remained unchanged. Interestingly, the high ex vivo Bcl-2 content of lymphocytes from some SLE patients was maintained after growth factor withdrawal. Anti-apoptotic cytokine signaling may significantly influence the deregulation of cell death in SLE lymphocytes.

In vitro induction of neuronal apoptosis by anti-Fas antibody-containing sera from amyotrophic lateral sclerosis patients

Sera from 26% of patients with sporadic amyotrophic lateral sclerosis (ALS) induced in vitro apoptosis of a human neuroblastoma cell line, as detected by two methods, and most contained anti-Fas autoantibodies. In contrast, Alzheimer sera (studied as controls) very rarely induced apoptosis and did not contain detectable anti-Fas antibodies. Soluble Fas-ligand levels in ALS sera were not different from those in normal sera, except for slightly higher levels in a single case. In mixed cultures of rat embryonic brain and spinal cord cells, ALS sera (and agonistic anti-Fas monoclonal antibodies and soluble Fas-ligand) induced the apoptosis of a subpopulation of neurons. These neurons were motoneurons on the basis of staining with the monoclonal antibody SMI 32 and Fas expression was restricted to these SMI 32-positive neurons. These data are compatible with the hypothesis of the participation of an autoimmune mechanism possibly related to anti-Fas autoantibodies in certain ALS patients.

Autoantigen Ro52 directly interacts with human IgG heavy chain in vivo in mammalian cells

Previously, when we used in vivo yeast two-hybrid and in vitro protein-protein interaction analyses, we demonstrated a direct interaction between autoantigen Ro52 and the human IgG heavy chain. This interaction occurred in the absence of antibody-antigen specific interaction. Here, by employing a novel strategy, we further demonstrated that Ro52 co-localized with IgG in transfected mammalian cells. The co-localization was specific to IgG1 but not IgG3. Co-immunoprecipitating IgG with Ro52 from transfected cell lysates suggested that protein complex containing Ro52 and IgG contributed to the in vivo co-localization. In addition, IgG from normal human serum was shown to bind to the surface of apoptotic keratinocytes and the binding could be competitively blocked by 50-fold excesses of IgG1, not IgG3. With a direct binding study, we also demonstrated that IgG1 could bind to the surface of apoptotic cells while IgG3 bound barely. This binding was not competed by Fcgamma fragments indicating a non-Fcgamma receptor mediated interaction. Finally, in a competition analysis the addition of GST-RFP could reduce the IgG binding to the cell surface. Thus, we suggested that the binding of IgG to the apoptotic keratinocytes might be mediated through the interactions with the surface exposed Ro52. The potential role of forming this protein complex on the apoptotic cells will be discussed.

Serum levels of soluble Fas correlate with indices of organ damage in systemic lupus erythematosus

OBJECTIVE

To evaluate whether the levels of soluble form of the Fas apoptosis antigen (sCD95/sFas) varied from those of healthy control subjects in a group of patients with systemic lupus erythematosus (SLE). This was done to determine whether sFas has a role in either the disease activity or the organ damage in SLE.

METHODS

Serum levels of sFas were measured over a period of 4 y (277 determinations) in 39 Arab patients with SLE and 22 age-, gender-, and race-matched healthy controls using double antibody ELISA. SLEDAI scores for disease activity and SLICC/ACR scores for cumulative organ damage were determined. Serum levels of acute phase reactants, complement, inflammatory cell counts, levels of autoantibodies, and kidney and liver function test results were obtained retrospectively from clinical records.

RESULTS

sFas levels were significantly higher in patients with SLE (n = 39, 277 determinations) (0.60 ng/ml +/- 0.38) than in healthy controls (n = 22) (0.26 ng/ml +/- 0.11) (P < 0.00001). The levels of sFas correlated with SLICC/ACR (r = 0.36; P < 0.02), but not with SLEDAI. sFas correlated with renal and liver function tests measured by s-creatinine (r = 0.38; P < 0.0001), creatinine clearance (r = -0.30, P < 0.001), s-albumin (r = -0.28, P < 0.0001), and ALT (r = 0.35; P < 0.00001), but did not correlate with the levels of acute phase reactants.

CONCLUSION

sFas is elevated in sera of SLE patient. Since sFas correlates with indices of organ damage but not with disease activity, it may be a marker of organ damage in SLE and may act to protect certain organs from further damage by inhibiting Fas-mediated apoptosis.

A dominant negative Fas-associated death domain protein mutant inhibits proliferation and leads to impaired calcium mobilization in both T-cells and fibroblasts

Death domain-containing members of the tumor necrosis factor (TNF) receptor family ("death receptors") can induce apoptosis upon stimulation by their natural ligands or by agonistic antibodies. Activated death receptors recruit death domain adapter proteins like Fas-associated death domain protein (FADD), and this ultimately leads to proteolytic activation of the caspase cascade and cell death. Recently, FADD has also been implicated in the regulation of proliferation; functional inhibition of FADD results in p53-dependent impairment of proliferation in activated T-cells. In this study we have further analyzed T-cells derived from transgenic mice expressing a dominant negative FADD mutant (FADD DN) under control of the lck promoter in vitro so as to identify the signaling pathways that become engaged upon T-cell receptor stimulation and that are regulated by death receptors. FADD DN expression inhibits T-cell proliferation, both at the G(0) --> S transition and in the G(1) phase of continuously proliferating cells. We observe a decrease in the release of calcium from intracellular stores after T-cell receptor stimulation, whereas influx of extracellular calcium seems to be unaffected. FADD DN-expressing fibroblasts show a similarly inhibited cell growth and impaired calcium mobilization indicating that the modulation of proliferation and calcium response by death receptors is not cell type-specific.

The yins and yangs of ceramide

Since their discovery over 100 years ago, sphingolipids have caught the eyes and the imagination of scientists. Modern science has made many new insights on the cell biology and day-to-day functions of many integral sphingolipids, especially those of ceramide. Ceramide is recognized as a vital second messenger in the signal transduction process mediated by receptors of many cytokines and growth factors. A great part of our current understanding of ceramide has been achieved from apoptosis-related studies, however recent data in the fields of immunology, endocrinology and neurobiology, also suggest a fundamental involvement of ceramide in the onset of diseases. Therefore, understanding the biology of ceramide could be a key to unraveling many biological mechanisms and provide information for the treatment of some common diseases.