Autoimmunity in mice bearing lprcg: a novel mutant gene

Role of metalloproteases in the CD95 signaling pathways

CD95L (also known as FasL or CD178) is a member of the tumor necrosis family (TNF) superfamily. Although this transmembrane ligand has been mainly considered as a potent apoptotic inducer in CD95 (Fas)-expressing cells, more recent studies pointed out its role in the implementation of non-apoptotic signals. Accordingly, this ligand has been associated with the aggravation of inflammation in different auto-immune disorders and in the metastatic occurrence in different cancers. Although it remains to decipher all key factors involved in the ambivalent role of this ligand, accumulating clues suggest that while the membrane bound CD95L triggers apoptosis, its soluble counterpart generated by metalloprotease-driven cleavage is responsible for its non-apoptotic functions. Nonetheless, the metalloproteases (MMPs and ADAMs) involved in the CD95L shedding, the cleavage sites and the different stoichiometries and functions of the soluble CD95L remain to be elucidated. To better understand how soluble CD95L triggers signaling pathways from apoptosis to inflammation or cell migration, we propose herein to summarize the different metalloproteases that have been described to be able to shed CD95L, their cleavage sites and the biological functions associated with the released ligands. Based on these new findings, the development of CD95/CD95L-targeting therapeutics is also discussed.

The genetic landscape of the FAS pathway deficiencies

Dysfunction of the FAS-FASLG pathway causes a lymphoproliferative disorder with autoimmunity called Autoimmune lymphoproliferative syndrome (ALPS) mainly caused by FAS mutations. The goal of this review is to describe the genetic bases of the autoimmune lymphoproliferative syndrome and to underline their genetic complexity with the contribution of both germline and somatic events accounting for the variable clinical penetrance of the FAS mutations. Starting from the cohort of patients studied in the French cohort (>165 cases), we also reviewed the literature cases in order to depict a full description of the mutations affecting the FAS-FASLG pathway involved in the outcome of this rare non-malignant and non-infectious pediatric lymphoproliferative disease. We also discussed the variable clinical penetrance associated with mutations affecting the extracellular domain of the protein. Such non-penetrant germline mutations are frequently associated with an additional somatic event impacting the second allele of FAS. Moreover, the uncomplete clinical penetrance associated with mutations affecting the intracellular domain of FAS, in patient lacking additional FAS somatic event, suggested a potential digenic inheritance with a FAS mutation accompanied by a genetic modifier possibly impacting another player of the lymphocytes homeostasis (affecting the survival, activation or apoptosis of the peripheral leukocytes).

Non-apoptotic Fas (CD95) Signaling on T Cells Regulates the Resolution of Th2-Mediated Inflammation

Fas (CD95/APO-1) and its ligand (FasL/CD95L) promote the resolution of type 2 lung inflammation and eosinophilia. We previously found that Fas-deficiency on T cells, but not eosinophils, delayed resolution of inflammation. However, Fas can signal both cell death and have a positive signaling function that can actually activate cells. In this study, we investigated whether Fas-induced death or Fas-activated signaling pathways promote resolution of allergic lung inflammation. By increasing T cell survival through two Fas-independent pathways, using Bim-deficient T cells or Bcl-xL overexpressing T cells, no differences in resolution of Th2-mediated inflammation was observed. Furthermore, Th2 cells were inherently resistant to Fas-mediated apoptosis and preferentially signaled through non-apoptotic pathways following FasL treatment. Utilizing Fas-mutant mice deficient in apoptotic but sufficient for non-apoptotic Fas signaling pathways, we demonstrate that non-apoptotic Fas signaling in T cells drives resolution of Th2-mediated airway inflammation. Our findings reveal a previously unknown role for non-apoptotic Fas signaling on Th2 cells in the induction of resolution of type 2 inflammation.

The role of CD95 and CD95 ligand in cancer

CD95 (Fas/APO-1) and its ligand, CD95L, have long been viewed as a death receptor/death ligand system that mediates apoptosis induction to maintain immune homeostasis. In addition, these molecules are important in the immune elimination of virus-infected cells and cancer cells. CD95L was, therefore, considered to be useful for cancer therapy. However, major side effects have precluded its systemic use. During the last 10 years, it has been recognized that CD95 and CD95L have multiple cancer-relevant nonapoptotic and tumor-promoting activities. CD95 and CD95L were discovered to be critical survival factors for cancer cells, and were found to protect and promote cancer stem cells. We now discuss five different ways in which inhibiting or eliminating CD95L, rather than augmenting, may be beneficial for cancer therapy alone or in combination with standard chemotherapy or immune therapy.

The CD95/CD95L signaling pathway: a role in carcinogenesis

Apoptosis is a fundamental process that contributes to tissue homeostasis, immune responses, and development. The receptor CD95, also called Fas, is a member of the tumor necrosis factor receptor (TNF-R) superfamily. Its cognate ligand, CD95L, is implicated in immune homeostasis and immune surveillance, and various lineages of malignant cells exhibit loss-of-function mutations in this pathway; therefore, CD95 was initially classified as a tumor suppressor gene. However, more recent data indicate that in different pathophysiological contexts, this receptor can transmit non-apoptotic signals, promote inflammation, and contribute to carcinogenesis. A comparison with the initial molecular events of the TNF-R signaling pathway leading to non-apoptotic, apoptotic, and necrotic pathways reveals that CD95 is probably using different molecular mechanisms to transmit its non-apoptotic signals (NF-κB, MAPK, and PI3K). As discussed in this review, the molecular process by which the receptor switches from an apoptotic function to an inflammatory role is unknown. More importantly, the biological functions of these signals remain elusive.

The role of apoptosis-induced proliferation for regeneration and cancer

Genes dedicated to killing cells must have evolved because of their positive effects on organismal survival. Positive functions of apoptotic genes have been well established in a large number of biological contexts, including their role in eliminating damaged and potentially cancerous cells. More recently, evidence has suggested that proapoptotic proteins-mostly caspases-can induce proliferation of neighboring surviving cells to replace dying cells. This process, that we will refer to as "apoptosis-induced proliferation," may be critical for stem cell activity and tissue regeneration. Depending on the caspases involved, at least two distinct types of apoptosis-induced proliferation can be distinguished. One of these types have been studied using a model in which cells have initiated cell death, but are prevented from executing it because of effector caspase inhibition, thereby generating "undead" cells that emit persistent mitogen signaling and overgrowth. Such conditions are likely to contribute to certain forms of cancer. In this review, we summarize the current knowledge of apoptosis-induced proliferation and discuss its relevance for tissue regeneration and cancer.

Incomplete Killing And Enhanced Activation of Islet-Reactive CD8+ T Cells by FasL-Expressing Dendritic Cells Limits Protection from Diabetes

AIMS

Autologous dendritic cells (DC) are a promising tool for induction of cytotoxic CD8+ T cell immunity against tumors and chronic viral infections. When armed with the death-inducing Fas-ligand (FasL, CD195), DC attenuate delayed-type hypersensitivity reactions and allotransplant rejection by promoting activation-induced cell death in T cells. We investigated the possibility of using FasL-expressing DC to induce deletion of islet-reactive CD8+ T cells in vivo, and to prevent destruction of pancreatic islets in a model of autoimmune diabetes.

METHODS

DC, propagated from mouse bone marrow cells, were purified and made to express FasL and islet-antigen via plasmid transfection. CD8+ T cells (OT-I cells) recognizing the antigen, ovalbumin, were adoptively transferred to transgenic mice expressing ovalbumin in islets (RIP-OVA(lo) mice), and these mice were primed with ovalbumin. To test the potential of DC to prevent diabetes in this model, the mice were later intravenously vaccinated with the transfected DC.

RESULTS

Transfected DC induced partial deletion of antigen-reactive CD8+ T cells in vivo and reduced the level of lymphocyte infiltration into pancreatic islets. Diabetes developed less frequently in vaccinated mice, but this effect was limited. Further in vitro analysis showed that FasL-expressing DC not only deleted many of the responding CD8+ T cells but also promoted the expansion of surviving cells and their IFN-gamma production.

CONCLUSIONS

FasL-expressing DC can also have stimulatory effects on CD8+ T cells warranting further investigation into the optimal design of tolerance-promoting DC-vaccination to prevent autoimmune diabetes.

Targeting Notch signaling in autoimmune and lymphoproliferative disease

Patients with autoimmune lymphoproliferative syndrome (ALPS) and systemic lupus erythematosis (SLE) have T-cell dysregulation and produce abnormal, activated T lymphocytes and an atypical peripheral T-cell population, termed double negative T cells (DNTs). T-cell functions, including DNT transition in T-cell development and T-cell activation, are critically dependent on Notch signaling. We hypothesized that inhibiting Notch signaling would be effective in ALPS and SLE by reducing the production of abnormal DNTs and by blocking aberrant T-cell activation. We tested this hypothesis using murine models of ALPS and SLE. Mice were randomized to treatment with the notch pathway inhibitor (gamma-secretase inhibitor), N-S-phenyl-glycine-t-butyl ester (DAPT), or vehicle control. Response to treatment was assessed by measurement of DNTs in blood and lymphoid tissue, by monitoring lymph node and spleen size with ultrasound, by quantifying cytokines by bead-array, by ELISA for total IgG and anti-double-stranded DNA (dsDNA) specific antibodies, and by histopathologic assessment for nephritis. We found a profound and statistically significant decrease in all disease parameters, comparing DAPT-treated mice to controls. Using a novel dosing schema, we avoided the reported toxicities of gamma-secretase inhibitors. Inhibiting the Notch signaling pathway may thus present an effective, novel, and well-tolerated treatment for autoimmune and lymphoproliferative diseases.

Fas transduces dual apoptotic and trophic signals in hematopoietic progenitors

Stem cells and progenitors are often required to realize their differentiation potential in hostile microenvironments. The Fas/Fas ligand (FasL) interaction is a major effector pathway of apoptosis, which negatively regulates the expansion of differentiated hematopoietic cells. The involvement of this molecular interaction in the function of hematopoietic stem and progenitor cells is not well understood. In the murine syngeneic transplant setting, both Fas and FasL are acutely upregulated in bone marrow-homed donor cells; however, the Fas(+) cells are largely insensitive to FasL-induced apoptosis. In heterogeneous populations of lineage-negative (lin(-)) bone marrow cells and progenitors isolated by counterflow centrifugal elutriation, trimerization of the Fas receptor enhanced the clonogenic activity. Inhibition of caspases 3 and 8 did not affect the trophic signals mediated by Fas, yet it efficiently blocked the apoptotic pathways. Fas-mediated tropism appears to be of physiological significance, as pre-exposure of donor cells to FasL improved the radioprotective qualities of hematopoietic progenitors, resulting in superior survival of myeloablated hosts. Under these conditions, the activity of long-term reconstituting cells was not affected, as determined in sequential secondary and tertiary transplants. Dual caspase-independent tropic and caspase-dependent apoptotic signaling place the Fas receptor at an important junction of activation and death. This regulatory mechanism of hematopoietic homeostasis activates progenitors to promote the recovery from aplasia and converts into a negative regulator in distal stages of cell differentiation. Disclosure of potential conflicts of interest is found at the end of this article.

Dominant-negative Fas mutation is reversed by down-expression of c-FLIP

Fas triggering by agonistic antibodies or by its cognate ligand, FasL, induces apoptotic cell death, whereas mutation in the Fas death domain is associated with lymphoma progression. On prolonged culture in the presence of an agonistic anti-Fas antibody, we raised a Jurkat cell line resistant to agonistic antibodies but still sensitive to soluble FasL, which carried at the heterozygous state, a point mutation into the Fas death domain. Down-modulation of c-FLIP expression reversed the blockade of the Fas pathway. We show that the activation threshold for the Fas receptor is more easily overcome by multimeric FasL than by agonistic antibodies and that the increase of this threshold due to mutation in the Fas death domain can be overcome by acting on a downstream effector of the Fas signal, c-FLIP. These findings put forward a new approach to eradicate Fas-resistant tumor cells.

Genetic disorders of programmed cell death in the immune system

Human genetics offers new possibilities for understanding physiological regulatory mechanisms and disorders of the immune system. Genetic abnormalities of lymphocyte cell death programs have provided insights into mechanisms of receptor biology and principles of immune homeostasis and tolerance. Thus far, there are two major diseases of programmed cell death associated with inherited human mutations: the autoimmune lymphoproliferative syndrome and the caspase-eight deficiency state. We describe the details of their molecular pathogenesis and discuss how these diseases illustrate important concepts in immune regulation and genetics.

Fas binding to calmodulin regulates apoptosis in osteoclasts

Promotion of osteoclast apoptosis is one therapeutic approach to osteoporosis. Calmodulin, the major intracellular Ca(2+) receptor, modulates both osteoclastogenesis and bone resorption. The calmodulin antagonist, trifluoperazine, rescues bone loss in ovariectomized mice (Zhang, L., Feng, X., and McDonald, J. M. (2003) Endocrinology 144, 4536-4543). We show here that a 3-h treatment of mouse osteoclasts with either of the calmodulin antagonists, tamoxifen or trifluoperazine, induces osteoclast apoptosis dose-dependently. Tamoxifen, 10 microm, and trifluoperazine, 10 microm, induce 7.3 +/- 1.8-fold and 5.3 +/- 0.9-fold increases in osteoclast apoptosis, respectively. In Jurkat cells, calmodulin binds to Fas, the death receptor, and this binding is regulated during Fas-mediated apoptosis (Ahn, E. Y., Lim, S. T., Cook, W. J., and McDonald, J. M. (2004) J. Biol. Chem. 279, 5661-5666). In osteoclasts, calmodulin also binds Fas. When osteoclasts are treated with 10 microm trifluoperazine, the binding between Fas and calmodulin is dramatically decreased at 15 min and gradually recovers by 60 min. A point mutation of the Fas death domain in the Lpr(-cg) mouse renders Fas inactive. Using glutathione S-transferase fusion proteins, the human Fas cytoplasmic domain is shown to bind calmodulin, whereas a point mutation (V254N) comparable with the Lpr(-cg) mutation in mice has markedly reduced calmodulin binding. Osteoclasts derived from Lpr(-cg) mice have diminished calmodulin/Fas binding and are more sensitive to calmodulin antagonist-induced apoptosis than those from wild-type mice. Both tamoxifen- and trifluoperazine-induced apoptosis are increased 1.6 +/- 0.2-fold in Lpr(-cg)-derived osteoclasts compared with osteoclasts derived from wild-type mice. In summary, calmodulin antagonists induce apoptosis in osteoclasts by a mechanism involving interference with calmodulin binding to Fas. The effects of calmodulin/Fas binding on calmodulin antagonist-induced apoptosis may open a new avenue for therapy for osteoporosis.

Fas has a role in cerebral malaria, but not in proliferation or exclusion of the murine parasite in mice

We examined the susceptibility of murine Fas-deficient mutants to malaria infection in order to investigate the role of Fas in an experimental murine model of cerebral malaria (CM). We infected mice of B6 and CBA wild-type and mutant backgrounds with Plasmodium berghei ANKA. The incidence of CM in the mutant mice (B6-lpr, CBA-lprcg) was decreased by about 50% compared with wild-type control strains at 2 weeks after infection. We did not observe significant differences of parasitemia during a murine malaria infection with nonlethal Plasmodium yoelii 17XNL between wild-type and lymphoproliferative (lpr) mutant mice of C3H and MRL genetic backgrounds, although B6-lpr mice exhibited significantly higher parasitemia than did B6 mice 12 to 18 days after infection. These results suggest Fas has a possible role in CM but may not play a major role in the proliferation or exclusion of a murine malaria parasite in a nonlethal infection.

Induction of apoptosis and activation of NF-kappaB by CD95 require different signalling thresholds

Mutations in the death domain of the death receptor CD95 (APO-1/Fas) cause lymphoproliferation and autoimmune disease in both lpr(cg) mice and in patients with autoimmune lymphoproliferative syndrome (ALPS) type Ia. By testing lymphocytes from ALPS type Ia patients, comparing heterozygous with homozygous lpr(cg) mice and coexpressing wild-type and mutant CD95 receptors, we demonstrate that induction of apoptosis requires two wild-type alleles of CD95. By contrast, nuclear factor-kappaB (NF-kappaB) can be fully activated in cells expressing both a mutant and a wild-type CD95 allele, suggesting different thresholds to activate the two signalling pathways. This was confirmed by testing lymphocytes from heterozygous lpr mice, which showed reduced sensitivity to CD95-mediated apoptosis but normal activation of NF-kappaB when compared with wild-type mice. Mutations in CD95 may eliminate the tumour-suppressive function of CD95, at the same time allowing induction of survival or proliferative pathways, which could contribute to the increased risk for lymphoma seen in ALPS type Ia patients.

Fas ligand is enriched in the caveolae membrane domains of thymic epithelial cells

Both Fas and Fas ligand (FasL) are expressed in the thymus. Although reports suggest that they are important throughout the thymocyte maturation process their precise role remains elusive. The present paper characterizes the expression of FasL in the thymus and in the TEA3A1 and BT1B functional thymic epithelial cell (TEC) lines. FasL expression by thymus fractions, TEA3A1, and BT1B cells was detected by Northern blot analysis. In TEA3A1 cells, we discovered that FasL protein expression was localized to caveolae membrane domains. This restricted subcellular localization of FasL, together with reports describing the localization of the major histocompatibility complex proteins, the T cell receptor and Fas to caveolae membrane domains, may provide a mechanism for the deletion of thymocytes during negative selection. Finally, using semi-quantitative RT-PCR we found that FasL expression by TECs is regulated by glucocorticoids.

Fas engagement induces neurite growth through ERK activation and p35 upregulation

Fas (also known as CD95), a member of the tumour-necrosis receptor factor family of 'death receptors', can induce apoptosis or, conversely, can deliver growth stimulatory signals. Here we report that crosslinking Fas on primary sensory neurons induces neurite growth through sustained activation of the extracellular-signal regulated kinase (ERK) pathway and the consequent upregulation of p35, a mediator of neurite outgrowth. In addition, functional recovery after sciatic nerve injury is delayed in Fas-deficient lpr mice and accelerated by local administration of antibodies against Fas, which indicates that Fas engagement may contribute to nerve regeneration in vivo. Our findings define a role for Fas as an inducer of both neurite growth in vitro and accelerated recovery after nerve injury in vivo.

Significant role of Fas ligand-binding but defective Fas receptor (CD95) in lymph node hyperplasia composed of abnormal double-negative T cells

The functional differences between two mutations of the Fas (CD95) locus, Faslpr (lpr) and Faslprcg (lprcg), were investigated using bone marrow (BM) transplantation on the C3H mouse background. Both lpr/lpr and lprcg/lprcg BM transferred caused lymph node (LN) hyperplasia in lpr/+ and lprcg/+ recipients, although it was clearly smaller than that in lpr/lpr and lprcg/lprcg recipients of lpr/lpr and lprcg/lprcg BM. In addition, both BM induced significantly larger LN hyperplasia in lprcg/+ than lpr/+ recipients. Appearance of CD4- CD8-[double negative (DN)] T cells in the periphery is the most consistent phenotype of Fas mutations. Importantly, the proportion of DN T cells was higher in larger LN hyperplasia in the order of lpr/+, lprcg/+ and lpr/lpr or lprcg/lprcg recipients. On the other hand, both lpr/lpr and lprcg/lprcg BM transferred into wild-type (+/+) mice caused marked LN atrophy. The former, but not the latter, induced wasting syndrome. Faslg1d (gld)-homozygous lpr/lpr BM transferred into +/+ mice elicited LN hyperplasia of the same extent as that in lpr/lpr mice transferred with lpr/lpr BM, but not wasting syndrome. Taken together with the fact that DN T cells massively express Fas ligand (FasL), this study implied that FasL overexpressed on DN cells may be involved in the accumulation of DN T cells in LN, LN atrophy and wasting syndrome, and that lprcg Fas, which can bind to Fas ligand but not transduce apoptosis signal into cells, may modulate these pathological conditions by interfering with the binding of FasL to Fas.

Clear suppression of Th1 responses but marginal amelioration of autoimmune manifestations by IL-12p40 transgene in MRL-FAS(lprcg)/FAS(lprcg) mice

To investigate the effects of overproduction of IL-12p40, a potent antagonist against IL-12, on lupus-like autoimmune disease in vivo, we generated p40 transgenic MRL-Fas(lprcg)/Fas(lprcg) mice. Serum p40 and IL-12 levels were 600- to 8000-fold and 3- to 20-fold higher in transgenic (p40-lpr(cg)) than nontransgenic (lpr(cg)) mice, respectively. Serum IFN-gamma levels increased after 3 months of age in lpr(cg) and this age-related increase was completely abrogated in p40-lpr(cg). Serum IL-4 levels were the same in both mice. Production of IgM and IgG anti-double-stranded DNA (dsDNA) antibodies was significantly lower in p40-lpr(cg). Anti-dsDNA antibodies decreased in Th1-dependent IgG2a but increased in the Th2-dependent IgG1 subclass significantly in p40-lpr(cg). Proteinuria, glomerulonephritis, and survival were only marginally ameliorated in p40-lpr(cg). The results suggest that excess p40 production in vivo may suppress Th1 responses in autoantibody and IFN-gamma production but lead to minimal improvement of clinical manifestations of autoimmune disease in this mouse model.

Alleviation of renal disease and lymphadenopathy in MRL-Fasp(lrcg)/Fas(lprcg) (MR-lpr(cg)) mice neonatally infected with mouse mammary tumor virus encoding superantigen strongly reactive with TCR Vbeta8.2 element

Mouse mammary tumor virus transmitted by FM mice (FM-MMTV) encodes a superantigen (SAg) characterized by strong reactivity with TCR Vbeta8.2 element and broad spectrum of Vbeta reactivity. To investigate what effects the expression in vivo of FM-MMTV SAg exhibits on the course of the disease in a lupus-prone model, MRL/MpJ-Fas(lprcg)/Fas(lprcg) (MRL-lpr9cg) mice, neonatally FM-MMTV-infected MRL-lprcg(MMTV) and uninfected MRL-lpr(cg) mice were compared for various disease parameters. In MRL-lprcg(MMTV), survival was significantly prolonged, glomerulonephritis, proteinuria, and lymphadenopathy were clearly ameliorated, and the production of serum immunoglobulin G (IgG), complement-activating IgG2a, and cryogenic IgG3 autoantibodies, which are thought to be pathogenic to kidneys, and circulating immune complexes (IC), and glomerular IC deposition were significantly suppressed. FM-MMTV infection deleted Vbeta8.2+ cells by about 90% and Vbeta14+ cells less efficiently in all of the CD4+, CD8+, and B220+ CD4- CD8- or double-negative (DN) T-cell populations, and Vbeta8.1+ cells in the CD4+ population but not in the others. Similar deletion profiles of CD8+ and DN T cells support that DN T cells are derived from the CD8 lineage. The results imply that the specific regulation of the immune system with viral SAg has a potential for development of an attractive immunomodulatory therapy of autoimmune diseases.

FIST/HIPK3: a Fas/FADD-interacting serine/threonine kinase that induces FADD phosphorylation and inhibits fas-mediated Jun NH(2)-terminal kinase activation

Fas is a cell surface death receptor that signals apoptosis. Several proteins have been identified that bind to the cytoplasmic death domain of Fas. Fas-associated death domain (FADD), which couples Fas to procaspase-8, and Daxx, which couples Fas to the Jun NH(2)-terminal kinase pathway, bind independently to the Fas death domain. We have identified a 130-kD kinase designated Fas-interacting serine/threonine kinase/homeodomain-interacting protein kinase (FIST/HIPK3) as a novel Fas-interacting protein. Binding to Fas is mediated by a conserved sequence in the COOH terminus of the protein. FIST/HIPK3 is widely expressed in mammalian tissues and is localized both in the nucleus and in the cytoplasm. In transfected cell lines, FIST/HIPK3 causes FADD phosphorylation, thereby promoting FIST/HIPK3-FADD-Fas interaction. Although Fas ligand-induced activation of Jun NH(2)-terminal kinase is impaired by overexpressed active FIST/HIPK3, cell death is not affected. These results suggest that Fas-associated FIST/HIPK3 modulates one of the two major signaling pathways of Fas.

A pivotal role of cell-bound but not soluble CD4 molecules in full development of lupus-like manifestations in MRL-Fas(lprcg)/Fas(lprcg) mice

The role of CD4 molecules in the autoimmune and lymphoproliferative syndrome caused by murine Fas mutations was studied using the novel systemic lupus erythematosus (SLE) model, MRL-Fas(lpr(cg))/Fas(lprcg) (MRL-lpr(cg)) mice, in combination with the novel mutant CD4 gene producing soluble CD4 (sCD4) instead of membrane-bound CD4 (mCD4). For this purpose, various autoimmune manifestations were compared among MRL-lpr(cg) mice homozygous (CD4slprcg), heterozygous (CD4s/mlpr(cg)), and wild-type (CD4mlpr(cg)) for the CD4 mutation. The mortality, glomerulonephritis, proteinuria, and lymphadenopathy were significantly ameliorated in CD4slprcg compared with CD4mlpr(cg) and CD4s/mlpr(cg) mice, both being comparable in these clinical characteristics. In parallel with the clinical improvement, the serum levels of immunoglobulin, anti-DNA antibodies, anti-nuclear antibodies and immune complexes, and the extent of glomerular immune deposition, were significantly lower in the former. The results indicate that mCD4 is important and can not be replaced by sCD4 in full development of SLE-like manifestations, and suggest that CD4+ T cells may aggravate the autoimmune disease by stimulating autoreactive B cells to produce autoantibodies through their helper activity in Fas mutant models. The sCD4 levels in the serum and spleen elevated with the increased accumulation of B220+CD4-CD8- (double-negative (DN)) T cells in CD4slpr(cg) mice. This, together with the significantly milder lymphadenopathy associated with lower DN T cell contents in CD4slpr(cg) than CD4mlpr(cg) mice, implies that some of abnormal DN T cells may be derived from cells of the CD4 lineage.

A novel protein (Fbf-1) that binds to CD95/APO-1/FAS and shows sequence similarity to trichohyalin and plectin

The Fas/Apo-1/CD95 cell surface receptor belongs to the TNF receptor family of cell death inducing molecules. A number of cytosolic adapter proteins that mediate signal transduction of CD95 have been characterized, but some features of the molecular mechanisms of CD95-induced cell death remain elusive. We describe here a novel protein that can interact with the cytosolic domain of the murine CD95 receptor in a yeast two-hybrid assay. This novel protein was termed Fbf-1 for Fas binding factor and bears no sequence similarity to the known CD95 adapter proteins. Fbf-1 is 1173 aa long and has a theoretical molecular weight of around 130 kDa. The protein is expressed in a wide variety of tissues and is localized in the cytoplasm. Fbf-1 is a very hydrophilic protein, highly conserved between mouse and human and bears a carboxyterminal leucine heptad repeat reminiscent of leucine zipper protein interaction domains. In addition, it shows sequence similarity to trichohyalin and plectin pointing to a function as a structural protein.

Fas engagement accelerates liver regeneration after partial hepatectomy

Fas (CD95) is a receptor involved in induction of apoptotic cell death of Fas-bearing cells, including hepatocytes and T cells. Injection of Fas-specific antibodies into mice leads to fulminant hepatic failure and death. Fas also transduces growth-promoting signals in proliferating T cells, fibroblasts and some tumor cells. Here we show that partial hepatectomy, which triggers the immediate onset of liver regeneration, protected mice against the lethal effects of Fas-specific antibodies and prevented hepatocyte apoptosis in response to Fas engagement in vivo. Furthermore, Fas engagement accelerated liver regeneration after partial hepatectomy. Liver regeneration kinetics were delayed in mutant mice with decreased cell surface Fas expression (lpr mice). In contrast, regeneration was not delayed in lpr-cg mutant mice, which have a Fas mutation that prevents Fas-induced death but not Fas-dependent proliferative stimulation. Our results indicate that Fas engagement on cells in regenerating or healing tissues may promote cell growth.

Mature T lymphocyte apoptosis--immune regulation in a dynamic and unpredictable antigenic environment

Apoptosis of mature T lymphocytes preserves peripheral homeostasis and tolerance by countering the profound changes in the number and types of T cells stimulated by diverse antigens. T cell apoptosis occurs in at least two major forms: antigen-driven and lymphokine withdrawal. These forms of death are controlled in response to local levels of IL-2 and antigen in a feedback mechanism termed propriocidal regulation. Active antigen-driven death is mediated by the expression of death cytokines such as FasL and TNF. These death cytokines engage specific receptors that assemble caspase-activating protein complexes. These signaling complexes tightly regulate cell death but are vulnerable to inherited defects. Passive lymphokine withdrawal death may result from the cytoplasmic activation of caspases that is regulated by mitochondria and the Bcl-2 protein. The human disease, Autoimmune Lymphoproliferative Syndrome (ALPS) is due to dominant-interfering mutations in the Fas/APO-1/CD95 receptor and other components of the death pathway. The study of ALPS patients reveals the necessity of apoptosis for preventing autoimmunity and allows the genetic investigation of apoptosis in humans. Immunological, cellular, and molecular evidence indicates that throughout the life of a T cell, apoptosis may be evoked in excessive, harmful, or useless clonotypes to preserve a healthy and balanced immune system.

Autoimmune lymphoproliferative syndrome with defective Fas: genotype influences penetrance

Autoimmune lymphoproliferative syndrome (ALPS) is a disorder of lymphocyte homeostasis and immunological tolerance. Most patients have a heterozygous mutation in the APT1 gene, which encodes Fas (CD95, APO-1), mediator of an apoptotic pathway crucial to lymphocyte homeostasis. Of 17 unique APT1 mutations in unrelated ALPS probands, 12 (71%) occurred in exons 7-9, which encode the intracellular portion of Fas. In vitro, activated lymphocytes from all 17 patients showed apoptotic defects when exposed to an anti-Fas agonist monoclonal antibody. Similar defects were found in a Fas-negative cell line transfected with cDNAs bearing each of the mutations. In cotransfection experiments, Fas constructs with either intra- or extracellular mutations caused dominant inhibition of apoptosis mediated by wild-type Fas. Two missense Fas variants, not restricted to patients with ALPS, were identified. Variant A(-1)T at the Fas signal-sequence cleavage site, which mediates apoptosis less well than wild-type Fas and is partially inhibitory, was present in 13% of African American alleles. Among the ALPS-associated Fas mutants, dominant inhibition of apoptosis was much more pronounced in mutants affecting the intracellular, versus extracellular, portion of the Fas receptor. Mutations causing disruption of the intracellular Fas death domain also showed a higher penetrance of ALPS phenotype features in mutation-bearing relatives. Significant ALPS-related morbidity occurred in 44% of relatives with intracellular mutations, versus 0% of relatives with extracellular mutations. Thus, the location of mutations within APT1 strongly influences the development and the severity of ALPS.

Sjögren's syndrome in mice carrying the Ipr(cg) gene and the therapeutic efficacy of an immunosuppressive agent FK506

The influence of the Ipr(cg) gene on the development of Sjögren's syndrome was followed up to 5 months of age in male and female mice of MRL, CBA and C3H strains. In MRL-Ipr(cg) mice, focal mononuclear cell infiltration started at 2 months and became conspicuous after 3 months of age in the lacrimal and submandibular glands but was minimal in the parotid and sublingual glands, even at 5 months of age, without any apparent sex effects found. In CBA and C3H mice carrying the Ipr(cg) gene, this manifestation of Sjögren's syndrome was much less prominent, indicating that the participation of some genes of the MRL strain may be indispensable for the development of Sjögren's syndrome in mice carrying this gene. In MRL-Ipr(cg) mice, an immunosuppressive agent, FK506, improved the serological abnormalities (decreased levels of anti-double-stranded DNA antibody of IgG2a and IgG3 subclasses) and proteinuria. It also reduced the manifestations of Sjögren's syndrome when it was intraperitoneally administered three times weekly at a dose of 2 mg/kg from 6 weeks (before disease onset) until 5 months of age (the termination of the experiment). Although VP8.2+ T cells have been demonstrated to be responsible for causing several autoimmune diseases, the selective deletion of Vp8.2+ T cells with the superantigen encoded by mouse mammary tumor virus did not affect the disease severity at all, suggesting that this T cell repertoire may not play a crucial role in induction of Sjögren's syndrome.

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

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

Loci predisposing to autoimmunity in MRL-Fas lpr and C57BL/6-Faslpr mice

Background genes determine the incidence and severity of lymphoaccumulation and histopathologic manifestations of systemic autoimmunity in mice homozygous for the apoptosis-defective Faslpr mutation. By interval mapping of 274 F2 mice intercrossed between MRL-Faslpr (severe disease) and C57BL/6-Faslpr (minimal disease), four loci were identified with significant linkage to lymphadenopathy and/ or splenomegaly on chromosomes 4, 5, 7, and 10, which were named lupus in (MRL-Faslpr x B6-Faslpr)F2 cross1-4 (Lmb1-4), respectively. Lmb1, -2, and -3 were also linked to the production of anti-dsDNA antibodies, but not glomerulonephritis, whereas Lmb4 was associated with glomerulonephritis. Lmb2, -3, and -4 were inherited from the MRL background, but interestingly, Lmb1 was derived from the C57BL16-Faslpr. Nevertheless, each locus, regardless of the strain of origin, appeared to act in an additive manner, although certain combinations were more effective. Only a single suggestive locus on chromosome 1 could be correlated with arthritis. The identification of loci with highly significant linkage to disease manifestations in Faslpr strains will make it possible to map and clone new genetic defects contributing to autoimmunity.

Interaction of Fas(Apo-1/CD95) with proteins implicated in the ubiquitination pathway

Fas(Apo-1/CD95), a receptor belonging to the tumor necrosis factor receptor family, induces apoptosis when triggered by Fas ligand. Upon its activation, the cytoplasmic domain of Fas binds several proteins which transmit the death signal. We used the yeast two-hybrid screen to isolate Fas-associated proteins. Here we report that the ubiquitin-conjugating enzyme UBC9 binds to Fas at the interface between the death domain and the membrane-proximal region of Fas. This interaction is also seen in vivo. UBC9 transiently expressed in HeLa cells bound to the co-expressed cytoplasmic segment of Fas. FAF1, a Fas-associated protein that potentiates apoptosis (Chu et al. (1996) Proc. Natl. Acad. Sci. USA 92, 11894-11898), was found to contain sequences similar to ubiquitin. These results suggest that proteins related to the ubiquitination pathway may modulate the Fas signaling pathway.

Fas-independent apoptosis of photoreceptor cells in C3H mice

The morphogenesis of the photoreceptor cells in the retinas of C3H mice carrying the rd gene and C57BL mice carrying the normal gene was compared, and retinas of the C3H mutants (C3H-lpr/lpr, -lprcg/lprcg, and -lpr/lpr-gld/gld) defective in apoptosis through the Fas system were examined. In the C57BL retina, the inner and outer nuclear layers were separated at 8 days of age, and the photoreceptor inner and outer segments began to grow between 8-11 days after birth with their most rapid growth occurring between 14-17 days of age. In the C3H retina, the development was comparable to that of the C57BL retina at 8 days of age but the reduction in thickness of the outer nuclear and photoreceptor layers was noted at 11 days of age, and the outer nuclear layer became reduced to only a few nuclei in thickness at 14 days, being completely missing or reduced to a single row of cells at 20 days. The degeneration was by an apoptotic mechanism as confirmed morphologically and by the TUNEL method. In all the C3H mutant retinas examined over 24 days of age, the complete depletion of the outer nuclear layer or reduction to a single row comparable to 20-day-old C3H mice was seen. The rd gene action is therefore independent of Fas/Fas ligand-medicated apoptosis.

Dominant interfering Fas gene mutations impair apoptosis in a human autoimmune lymphoproliferative syndrome

Five unrelated children are described with a rare autoimmune lymphoproliferative syndrome (ALPS) characterized by massive nonmalignant lymphadenopathy, autoimmune phenomena, and expanded populations of TCR-CD3+CD4-CD8- lymphocytes. These findings, suggesting a genetic defect in the ability of T lymphocytes to respond to normal immunoregulatory mechanisms, prompted an evaluation of lymphocyte apoptosis. Each child had defective Fas-mediated T lymphocyte apoptosis associated with a unique, deleterious Fas gene mutation. One mutation appeared to cause a simple loss of function; however, four others had a dominant negative phenotype when coexpressed with normal Fas. Family studies demonstrated the inheritance of the mutant Fas alleles. The occurrence of Fas mutations together with abnormal T cell apoptosis in ALPS patients suggests an involvement of Fas in this recently recognized disorder of lymphocyte homeostasis and peripheral self-tolerance.