The molecular basis for apoptotic defects in patients with CD95 (Fas/Apo-1) mutations

Genetic Testing in Patients with Autoimmune Lymphoproliferative Syndrome: Experience of 802 Patients at Cincinnati Children's Hospital Medical Center

Autoimmune lymphoproliferative syndrome (ALPS) is a rare genetic disorder featuring chronic lymphadenopathy, splenomegaly, cytopenias, and increased lymphoma risk. Differentiating ALPS from immunodeficiencies with overlapping symptoms is challenging. This study evaluated the performance and the diagnostic yield of a 15-gene NGS panel for ALPS at Cincinnati Children's Hospital Medical Center. Samples from 802 patients submitted for ALPS NGS panel were studied between May 2014 and January 2023. A total of 62 patients (7.7%) had a definite diagnosis: 52/62 cases (84%) showed 37 unique pathogenic/likely pathogenic germline FAS variants supporting ALPS diagnosis (6.5%, 52/802). The ALPS diagnostic yield increased to 30% in patients who additionally fulfilled abnormal ALPS immunology findings criteria. 17/37 (46%) diagnostic FAS variants were novel variants reported for the first time in ALPS. 10/802 cases (1.2%) showed diagnostic findings in five genes (ADA2, CTLA4, KRAS, MAGT1, NRAS) which are related to autoimmune lymphoproliferative immunodeficiency (ALPID). Family studies enabled the reclassification of variants of unknown significance (VUS) and also the identification of at-risk family members of FAS-positive patients, which helped in the follow-up diagnosis and treatment. Alongside family studies, complete clinical phenotypes and abnormal ALPS immunology and Fas-mediated apoptosis results helped clarify uncertain genetic findings. This study describes the largest cohort of genetic testing for suspected ALPS in North America and highlights the effectiveness of the ALPS NGS panel in distinguishing ALPS from non-ALPS immunodeficiencies. More comprehensive assessment from exome or genome sequencing could be considered for undefined ALPS-U patients or non-ALPS immunodeficiencies after weighing cost, completeness, and timeliness of different genetic testing options.

Bifidobacterium animalis sup F1-7 Acts as an Effective Activator to Regulate Immune Response Via Casepase-3 and Bak of FAS/CD95 Pathway

Intestinal microecology was closely related to immune regulation, but the related mechanism was still unclear. This study aimed to reveal how microorganisms improved immune response via casepase-3 and Bak of FAS/CD95 pathway. Bifidobacterium animalis F1-7 inhibited the melanoma B16-F10 cells in vitro effectively; had a potent anticancer effect of lung cancer mice; effectively improved the spleen immune index and CD3+ (75.8%) and CD8+ (19.8%) expression level; strengthened the phagocytosis of macrophages; inhibited the overexpression of inflammatory factors IL-6 (319.10 ± 2.46 pg/mL), IL-8 (383.05 ± 9.87 pg/mL), and TNF-α (2003.40 ± 11.42 pg/mL); and promoted the expression of anti-inflammatory factor IL-10 (406.00 ± 3.59 pg/mL). This process was achieved by promoting caspase-8/3 and BH3-interacting domain death agonist (Bid), Bak genes, and protein expression. This study confirmed the B. animalis F1-7 could act as an effective activator to regulate immune response by promoting the expression of caspase-8/3, Bid and Bak genes, and proteins and by activating the FAS/CD95 pathway. Our study provided a data support for the application of potentially beneficial microorganisms of B. animalis F1-7 as an effective activator to improve immunity.

Synthetic receptor platform to identify loss-of-function single nucleotide variants and designed mutants in the death receptor Fas/CD95

Synthetic biology has emerged as a useful technology for studying cytokine signal transduction. Recently, we described fully synthetic cytokine receptors to phenocopy trimeric receptors such as the death receptor Fas/CD95. Using a nanobody as an extracellular-binding domain for mCherry fused to the natural receptor's transmembrane and intracellular domain, trimeric mCherry ligands were able to induce cell death. Among the 17,889 single nucleotide variants in the SNP database for Fas, 337 represent missense mutations that functionally remained largely uncharacterized. Here, we developed a workflow for the Fas synthetic cytokine receptor system to functionally characterize missense SNPs within the transmembrane and intracellular domain of Fas. To validate our system, we selected five functionally assigned loss-of-function (LOF) polymorphisms and included 15 additional unassigned SNPs. Moreover, based on structural data, 15 gain-of-function or LOF candidate mutations were additionally selected. All 35 nucleotide variants were functionally investigated through cellular proliferation, apoptosis and caspases 3 and 7 cleavage assays. Collectively, our results showed that 30 variants resulted in partial or complete LOF, while five lead to a gain-of-function. In conclusion, we demonstrated that synthetic cytokine receptors are a suitable tool for functional SNPs/mutations characterization in a structured workflow.

Impact of human CD95 mutations on cell death and autoimmunity: a model

CD95/Fas/APO-1 can trigger apoptotic as well as nonapoptotic pathways in immune cells. CD95 signaling in humans can be inhibited by several mechanisms, including mutations in the gene encoding CD95. CD95 mutations lead to autoimmune disorders, such as autoimmune lymphoproliferative syndrome (ALPS). Gaining further insight into the reported mutations of CD95 and resulting alterations of its signaling networks may provide further understanding of their presumed role in certain autoimmune diseases. For illustrative purposes and to better understand the potential outcomes of CD95 mutations, here we assign their positions to the recently determined 3D structures of human CD95. Based on this, we make certain predictions and speculate on the putative role of CD95 mutation defects in CD95-mediated signaling for certain autoimmune diseases.

Extrafollicular responses in humans and SLE

Chronic autoimmune diseases, and in particular Systemic Lupus Erythematosus (SLE), are endowed with a long-standing autoreactive B-cell compartment that is presumed to reactivate periodically leading to the generation of new bursts of pathogenic antibody-secreting cells (ASC). Moreover, pathogenic autoantibodies are typically characterized by a high load of somatic hypermutation and in some cases are highly stable even in the context of prolonged B-cell depletion. Long-lived, highly mutated antibodies are typically generated through T-cell-dependent germinal center (GC) reactions. Accordingly, an important role for GC reactions in the generation of pathogenic autoreactivity has been postulated in SLE. Nevertheless, pathogenic autoantibodies and autoimmune disease can be generated through B-cell extrafollicular (EF) reactions in multiple mouse models and human SLE flares are characterized by the expansion of naive-derived activated effector B cells of extrafollicular phenotype. In this review, we will discuss the properties of the EF B-cell pathway, its relationship to other effector B-cell populations, its role in autoimmune diseases, and its contribution to human SLE. Furthermore, we discuss the relationship of EF B cells with Age-Associated B cells (ABCs), a TLR-7-driven B-cell population that mediates murine autoimmune and antiviral responses.

A variant of death-receptor 3 associated with rheumatoid arthritis interferes with apoptosis-induction of T cell

Rheumatoid arthritis (RA) is a chronic polyarthritis of unknown etiology. To unravel the molecular mechanisms in RA, we performed targeted DNA sequencing analysis of patients with RA. This analysis identified a variant of the death receptor 3 (DR3) gene, a member of the family of apoptosis-inducing Fas genes, which contains four single-nucleotide polymorphisms (SNPs) and a 14-nucleotide deletion within exon 5 and intron 5. We found that the deletion causes the binding of splicing regulatory proteins to DR3 pre-mRNA intron 5, resulting in a portion of intron 5 becoming part of the coding sequence, thereby generating a premature stop codon. We also found that this truncated DR3 protein product lacks the death domain and forms a heterotrimer complex with wildtype DR3 that dominant-negatively inhibits ligand-induced apoptosis in lymphocytes. Myelocytes from transgenic mice expressing the human DR3 variant produced soluble truncated DR3, forming a complex with TNF-like ligand 1A (TL1A), which inhibited apoptosis induction. In summary, our results reveal that a DR3 splice variant that interferes with ligand-induced T cell responses and apoptosis may contribute to RA pathogenesis.

Soluble fas and the -670 polymorphism of fas in lupus nephritis

This study was performed to clarify the role of soluble Fas (sFas) in lupus nephritis (LN) and establish a potential relationship between LN and the −670 polymorphism of Fas in 67 patients with systemic lupus erythematosus (SLE), including a subset of 24 LN patients with proteinuria. Additionally, a group of 54 healthy subjects (HS) was included. The allelic frequency of the −670 polymorphism of Fas was determined using PCR-RFLP analysis, and sFas levels were assessed by ELISA. Additionally, the WT-1 protein level in urine was measured. The Fas receptor was determined in biopsies by immunohistochemistry (IHC) and in situ hybridization (FISH) and apoptotic features by TUNEL. Results. The −670 Fas polymorphism showed that the G allele was associated with increased SLE susceptibility, with an odds ratio (OR) of 1.86. The sFas was significantly higher in LN patients with the G/G genotype, and this subgroup exhibited correlations between the sFas level and proteinuria and increased urinary WT-1 levels. LN group shows increased expression of Fas and apoptotic features. In conclusion, our results indicate that the G allele of the −670 polymorphism of Fas is associated with genetic susceptibility in SLE patients with elevated levels of sFas in LN with proteinuria.

FAS system deregulation in T-cell lymphoblastic lymphoma

The acquisition of resistance towards FAS-mediated apoptosis may be required for tumor formation. Tumors from various histological origins exhibit FAS mutations, the most frequent being hematological malignancies. However, data regarding FAS mutations or FAS signaling alterations are still lacking in precursor T-cell lymphoblastic lymphomas (T-LBLs). The available data on acute lymphoblastic leukemia, of precursor origin as well, indicate a low frequency of FAS mutations but often report a serious reduction in FAS-mediated apoptosis as well as chemoresistance, thus suggesting the occurrence of mechanisms able to deregulate the FAS signaling pathway, different from FAS mutation. Our aim at this study was to determine whether FAS-mediated apoptotic signaling is compromised in human T-LBL samples and the mechanisms involved. This study on 26 T-LBL samples confirms that the FAS system is impaired to a wide extent in these tumors, with 57.7% of the cases presenting any alteration of the pathway. A variety of mechanisms seems to be involved in such alteration, in order of frequency the downregulation of FAS, the deregulation of other members of the pathway and the occurrence of mutations at FAS. Considering these results together, it seems plausible to think of a cumulative effect of several alterations in each T-LBL, which in turn may result in FAS/FASLG system deregulation. Since defective FAS signaling may render the T-LBL tumor cells resistant to apoptotic cell death, the correct prognosis, diagnosis and thus the success of anticancer therapy may require such an in-depth knowledge of the complete scenario of FAS-signaling alterations.

Somatic loss of heterozygosity, but not haploinsufficiency alone, leads to full-blown autoimmune lymphoproliferative syndrome in 1 of 12 family members with FAS start codon mutation

We describe a family with 12 members carrying a heterozygous germline FAS c.3G>T start codon mutation leading to FAS haploinsufficiency. One patient had autoimmune lymphoproliferative syndrome (ALPS), one had recovered from ALPS, and ten mutation-positive relatives (MPRs) were healthy. FAS-mediated apoptosis and surface expression of FAS in single-positive T cells were lower for MPRs but did not discriminate between them and the ALPS patient. However, double-negative (DN) T cells of the ALPS patient had no FAS expression due to somatic loss of heterozygosity. Our results in this kindred suggest that FAS haploinsufficiency does not cause ALPS-FAS, but that modifying genetic events are crucial for its pathogenesis. FAS surface expression on DN T cells should be assessed routinely and FAS haploinsufficient patients should be followed as its potential for lymphomagenesis is not well defined and a second hit might occur later on.

Precise mapping of the CD95 pre-ligand assembly domain

Pre-association of CD95 at the plasma membrane is mandatory for efficient death receptor signaling. This homotrimerization occurs through self-association of an extracellular domain called the pre-ligand assembly domain (PLAD). Using novel molecular and cellular tools, we confirmed that CD95-PLAD is necessary to promote CD95 multimerization and plays a pivotal role in the transmission of apoptotic signals. However, while a human CD95 mutant deleted of the previously described PLAD domain (amino acids 1 to 66) fails to interact with its wild-type counterpart and trigger autonomous cell death, deletion of amino acids 1 to 42 does not prevent homo- or hetero (human/mouse)-oligomerization of CD95, and thus does not alter transmission of the apoptotic signal. Overall, these findings indicate that the region between amino acids 43 to 66 corresponds to the minimal motif involved in CD95 homotypic interaction and is necessary to convey an efficient apoptotic signal. Interfering with this PLAD may represent a new therapeutic strategy for altering CD95-induced apoptotic and non-apoptotic signals.

CD95-mediated cell signaling in cancer: mutations and post-translational modulations

Apoptosis has emerged as a fundamental process important in tissue homeostasis, immune response, and during development. CD95 (also known as Fas), a member of the tumor necrosis factor receptor (TNF-R) superfamily, has been initially cloned as a death receptor. Its cognate ligand, CD95L, is mainly found at the plasma membrane of activated T-lymphocytes and natural killer cells where it contributes to the elimination of transformed and infected cells. According to its implication in the immune homeostasis and immune surveillance, and since several malignant cells of various histological origins exhibit loss-of-function mutations, which cause resistance towards the CD95-mediated apoptotic signal, CD95 has been classified as a tumor suppressor gene. Nevertheless, this assumption has been recently challenged, as in certain pathophysiological contexts, CD95 engagement transmits non-apoptotic signals that promote inflammation, carcinogenesis or liver/peripheral nerve regeneration. The focus of this review is to discuss these apparent contradictions of the known function(s) of CD95.

A survey of 90 patients with autoimmune lymphoproliferative syndrome related to TNFRSF6 mutation

Autoimmune lymphoproliferative syndrome (ALPS) is a genetic disorder characterized by early-onset, chronic, nonmalignant lymphoproliferation, autoimmune manifestations, and susceptibility to lymphoma. The majority of ALPS patients carry heterozygous germline (ALPS-FAS) or somatic mutations (ALPS-sFAS) of the TNFRSF6 gene coding for FAS. Although the clinical features of ALPS have been described previously, long-term follow-up data on morbidity and mortality are scarce. We performed a retrospective analysis of clinical and genetic features of 90 ALPS-FAS and ALPS-sFAS patients monitored over a median period of 20.5 years. Heterozygous germline mutations of TNFRSF6 were identified in 83% of probands. Somatic TNFRSF6 mutations were found in 17% of index cases (all located within the intracellular domain of FAS). Sixty percent of the ALPS-FAS patients with mutations in the extracellular domain had a somatic mutation affecting the second allele of TNFRSF6; age at onset was later in these patients. No other genotype-phenotype correlations could be found. Long-term analysis confirmed a trend toward spontaneous remission of lymphoproliferation in adulthood but mixed outcomes for autoimmune manifestations. We observed significant and potentially life-threatening disease and treatment-related morbidity, including a high risk of sepsis after splenectomy that calls for careful long-term monitoring of ALPS patients. We also noted a significantly greater occurrence of disease-related symptoms in male than in female patients.

Disease causing mutations in the TNF and TNFR superfamilies: Focus on molecular mechanisms driving disease

The tumor necrosis factor (TNF) and TNF receptor (TNFR) superfamilies comprise multidomain proteins with diverse roles in cell activation, proliferation and cell death. These proteins play pivotal roles in the initiation, maintenance and termination of immune responses and have vital roles outside the immune system. The discovery and analysis of diseases associated with mutations in these families has revealed crucial mechanistic details of their normal functions. This review focuses on mutations causing four different diseases, which represent distinct pathological mechanisms that can exist within these superfamilies: autoimmune lymphoproliferative syndrome (ALPS; FAS mutations), common variable immunodeficiency (CVID; TACI mutations), tumor necrosis factor receptor associated periodic syndrome (TRAPS; TNFR1 mutations) and hypohidrotic ectodermal dysplasia (HED; EDA1/EDAR mutations). In particular, we highlight how mutations have revealed information about normal receptor-ligand function and how such studies might direct new therapeutic approaches.

Autoimmune lymphoproliferative syndrome: response to mycophenolate mofetil and pyrimethamine/sulfadoxine in a 5-year-old child

Autoimmune Lymphoproliferative Syndrome (ALPS) is a rare inherited disorder of disrupted lymphocyte homeostasis characterized by chronic splenomegaly and lymphadenopathy of early onset, hypergammaglobulinemia (Ig G and Ig A), autoimmune phenomena, and expanded populations of TCR-α/β+, CD3+, CD4-, CD8-T cells (Fisher et al. Cell 81:935-946; 1995), called double negative T-cells [(DN) T cells]. We discuss a case of ALPS which showed good response to immunosuppressant drug Mycophenolate-Mofetil in combination with Pyrimethamine/Sulfadoxine.

FAS haploinsufficiency is a common disease mechanism in the human autoimmune lymphoproliferative syndrome

The autoimmune lymphoproliferative syndrome (ALPS) is characterized by early-onset lymphadenopathy, splenomegaly, immune cytopenias, and an increased risk for B cell lymphomas. Most ALPS patients harbor mutations in the FAS gene, which regulates lymphocyte apoptosis. These are commonly missense mutations affecting the intracellular region of the protein and have a dominant-negative effect on the signaling pathway. However, analysis of a large cohort of ALPS patients revealed that ∼30% have mutations affecting the extracellular region of FAS, and among these, 70% are nonsense, splice site, or insertions/deletions with frameshift for which no dominant-negative effect would be expected. We evaluated the latter patients to understand the mechanism(s) by which these mutations disrupted the FAS pathway and resulted in clinical disease. We demonstrated that most extracellular-region FAS mutations induce low FAS expression due to nonsense-mediated RNA decay or protein instability, resulting in defective death-inducing signaling complex formation and impaired apoptosis, although to a lesser extent as compared with intracellular mutations. The apoptosis defect could be corrected by FAS overexpression in vitro. Our findings define haploinsufficiency as a common disease mechanism in ALPS patients with extracellular FAS mutations.

Onset of autoimmune lymphoproliferative syndrome (ALPS) in humans as a consequence of genetic defect accumulation

Autoimmune diseases develop in approximately 5% of humans. They can arise when self-tolerance checkpoints of the immune system are bypassed as a consequence of inherited mutations of key genes involved in lymphocyte activation, survival, or death. For example, autoimmune lymphoproliferative syndrome (ALPS) results from defects in self-tolerance checkpoints as a consequence of mutations in the death receptor-encoding gene TNF receptor superfamily, member 6 (TNFRSF6; also known as FAS). However, some mutation carriers remain asymptomatic throughout life. We have now demonstrated in 7 ALPS patients that the disease develops as a consequence of an inherited TNFRSF6 heterozygous mutation combined with a somatic genetic event in the second TNFRSF6 allele. Analysis of the patients' CD4(-)CD8(-) (double negative) T cells--accumulation of which is a hallmark of ALPS--revealed that in these cells, 3 patients had somatic mutations in their second TNFRSF6 allele, while 4 patients had loss of heterozygosity by telomeric uniparental disomy of chromosome 10. This observation provides the molecular bases of a nonmalignant autoimmune disease development in humans and may shed light on the mechanism underlying the occurrence of other autoimmune diseases.

Solution NMR investigation of the CD95/FADD homotypic death domain complex suggests lack of engagement of the CD95 C terminus

We have addressed complex formation between the death domain (DD) of the death receptor CD95 (Fas/APO-1) with the DD of immediate adaptor protein FADD using nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and size-exclusion chromatography with in-line light scattering. We find complexation to be independent of the C-terminal 12 residues of CD95 and insensitive to mutation of residues that engage in the high-order clustering of CD95-DD molecules in a recently reported crystal structure obtained at pH 4. Differential NMR linewidths indicate that the C-terminal region of the CD95 chains remains in a disordered state and (13)C-methyl TROSY data are consistent with a lack of high degree of symmetry for the complex. The overall molecular mass of the complex is inconsistent with that in the crystal structure, and the complex dissociates at pH 4. We discuss these findings using sequence analysis of CD95 orthologs and the effect of FADD mutations on the interaction with CD95.

The Fas-FADD death domain complex structure reveals the basis of DISC assembly and disease mutations

The death inducing signaling complex (DISC) formed by the death receptor Fas, the adapter protein FADD and caspase-8 mediates the extrinsic apoptotic program. Mutations in Fas that disrupt the DISC cause autoimmune lymphoproliferative syndrome (ALPS). Here we show that the Fas–FADD death domain (DD) complex forms an asymmetric oligomeric structure composed of 5–7 Fas DD and 5 FADD DD, whose interfaces harbor ALPS-associated mutations. Structure-based mutations disrupt the Fas–FADD interaction in vitro and in living cells; the severity of a mutation correlates with the number of occurrence of a particular interaction in the structure. The highly oligomeric structure explains the requirement for hexameric or membrane-bound FasL in Fas signaling. It also predicts strong dominant negative effects of Fas mutations, which are confirmed by signaling assays. The structure optimally positions the FADD death effector domain (DED) to interact with the caspase-8 DED for caspase recruitment and higher order aggregation.

Somatic FAS mutations are common in patients with genetically undefined autoimmune lymphoproliferative syndrome

Autoimmune lymphoproliferative syndrome (ALPS) is characterized by childhood onset of lymphadenopathy, hepatosplenomegaly, autoimmune cytopenias, elevated numbers of double-negative T (DNT) cells, and increased risk of lymphoma. Most cases of ALPS are associated with germline mutations of the FAS gene (type Ia), whereas some cases have been noted to have a somatic mutation of FAS primarily in their DNT cells. We sought to determine the proportion of patients with somatic FAS mutations among a group of our ALPS patients with no detectable germline mutation and to further characterize them. We found more than one-third (12 of 31) of the patients tested had somatic FAS mutations, primarily involving the intracellular domain of FAS resulting in loss of normal FAS signaling. Similar to ALPS type Ia patients, the somatic ALPS patients had increased DNT cell numbers and elevated levels of serum vitamin B(12), interleukin-10, and sFAS-L. These data support testing for somatic FAS mutations in DNT cells from ALPS patients with no detectable germline mutation and a similar clinical and laboratory phenotype to that of ALPS type Ia. These findings also highlight the potential role for somatic mutations in the pathogenesis of nonmalignant and/or autoimmune hematologic conditions in adults and children.

Underexpression and overexpression of Fas and Fas ligand: a double-edged sword

OBJECTIVE

To compare autoimmune lymphoproliferative syndrome (ALPS) and Stevens-Johnson syndrome (SJS) with respect to the defects in Fas- and Fas ligand (FasL)-mediated apoptosis.

DATA SOURCES

Selected reviews, case reports, and original studies were searched in PubMed and MEDLINE for the keywords ALPS, SJS, Fas, FasL, and apoptosis.

STUDY SELECTION

Case reports of ALPS and SJS were selected as examples of Fas- and FasL-mediated diseases. In addition, we selected articles that examined the pathophysiology of apoptosis in the context of Fas-FasL interaction.

RESULTS

Failure to initiate apoptosis of abnormal T lymphocytes occurs in such diseases as ALPS, leading to the accumulation of double negative T cells with an increase in autoimmunity. In contrast to apoptotic failure, SJS is associated with a pathological increase in programmed keratinocyte cell death.

CONCLUSION

The consequences of dysregulated Fas- and FasL-mediated apoptosis leads to self-reactivity, malignant transformation, and immune dysfunction. An understanding of underlying mechanisms and qualitative assessment of Fas and FasL may have clinical benefits when control of these homeostatic mechanisms is in question.

Childhood polyarteritis nodosa in autoimmune lymphoproliferative syndrome

Autoimmune lymphoproliferative syndrome (ALPS) is an uncommon disorder of Fas-mediated apoptosis that results in impaired lymphocyte death and, therefore, disturbed immune homeostasis. Besides presentation with lymphadenopathy and splenomegaly, patients with ALPS have a high incidence of autoimmune phenomena. To our knowledge, this is the first description of polyarteritis nodosa that includes numerous arterial aneurysms in a child with ALPS. Active vasculitis resolved after allogeneic hematopoietic stem cell transplantation. This report of polyarteritis nodosa associated with human ALPS supports previous findings in Fas-deficient mouse models that frequently develop vasculitic manifestations and suggests that apoptotic defects of lymphocytes may play a role in the pathophysiology of systemic vasculitis. Thus, patients with ALPS might be more susceptible to autoimmune vessel inflammation. This case furthermore emphasizes that even rare autoimmune manifestations should be considered and investigated in patients with immunodeficiencies, because that might help in planning treatment strategies for these patients.

Advances in the management and understanding of autoimmune lymphoproliferative syndrome (ALPS)

Autoimmune lymphoproliferative syndrome (ALPS) is a disorder of T cell dysregulation caused by defective Fas-mediated apoptosis. Patients with ALPS can develop a myriad of clinical manifestations including lymphadenopathy, hepatosplenomegaly, autoimmunity and increased rates of malignancy. ALPS may be more common that originally thought, and testing for ALPS should be considered in patients with unexplained lymphadenopathy, hepatosplenomegaly, and/or autoimmunity. As the pathophysiology of ALPS is better characterized, a number of targeted therapies are in preclinical development and clinical trials with promising early results. This review describes the clinical and laboratory manifestations found in ALPS patients, as well as the molecular basis for the disease and new advances in treatment.

Structure of the Fas/FADD complex: a conditional death domain complex mediating signaling by receptor clustering

Death domain complexes are key protein arrangements in the regulation of various cellular signaling events. One of the most prominent death domain complexes first described in the initiation of apoptosis is formed by the transmembrane receptor Fas, the cytosolic adaptor protein FADD, and caspase-8 and is referred to as the Fas/FADD/caspase-8 death inducing signaling complex (DISC). The recent structure of the Fas/FADD death domain complex reveals how formation of this signaling platform can be stringently regulated utilizing only Fas receptor clustering to form a death domain network. This work reveals that an opening mechanism of Fas is needed to expose binding sites for the FADD death domain and sets the stage for a conditional interaction, which is characterized by weak interactions adapted for a regulatory function. The overall crystal structure reveals a tetrameric arrangement of four primary Fas/FADD complexes. Intriguingly all contacts mediating the tetramer are solely provided through Fas/Fas interactions and are entirely dependent on the open form. These findings are instrumental in depicting a mechanism for DISC regulation where Fas receptor clustering leads to the stabilization of the open Fas death domains which are then capable of binding FADD in a weak interaction. At the same time this mechanism ensures that in the absence of a sufficient stimulus no interaction between Fas and FADD is possible. Therefore the conformation dependent, conditional Fas/FADD death domain interaction represents the regulatory element per se. This interaction contrasts the classic constitutive interactions of adaptor domains, which cannot provide regulatory function themselves. This model portrays how sole death domains are able to mediate signaling upon receptor clustering in the complete absence of enzyme activity.

[Late onset of primary immune deficiencies]

Primary immune deficiencies (PID) are characterized by a failure of the immune system that is not explained by any infectious, neoplastic, or iatrogenic cause. The diagnosis of PID should be considered in cases of severe or recurrent infections but also in cases with granulomatosis, autoimmune diseases, hemophagocytic syndrome, lymphoproliferative disorders, or even some solid tumors. The onset of PID may be late, most often in adulthood. Nonetheless, late onset may also mean in the first years rather than months of life or in adolescence rather than early childhood. In adults, the diagnosis of PID cannot be considered before acquired immunodeficiencies--far more frequent--are ruled out. Factors affecting the late onset of PID are not known.

The final destiny of acantholytic cells in pemphigus is Fas mediated

BACKGROUND

Pemphigus is an autoimmune disease characterized by the formation of intra-epidermal blisters. Patients develop auto-antibodies against desmoglein 1 and 3 proteins and induce acantholysis.

OBJECTIVE

This work addresses the issue of whether the Fas pathway mediates acantholysis. Furthermore, the possible suppliers of the Fas pathway were investigated.

METHODS

Seventeen biopsies of pemphigus patients were studied by haematoxylin and eosin staining, and apoptosis was defined by TUNEL. The expression of Fas, FasL and caspase 3 was studied by in situ hybridization and immunohistochemistry. Cell infiltrates were studied by immunofluorescence with monoclonal anti-CD3, CD4, CD8, CD19 and CD69.

RESULTS

All of the biopsies showed intra-epidermal blisters, acantholytic cells and inflammatory infiltrates. The blisters expressed Fas, FasL and caspase 3. Cell infiltrates were composed of CD8 and a few CD4(+)CD69(+) cells. Additionally, CD19(+) cells were detected. Interestingly, the Fas expression was increased in acantholytic cells and perilesional keratinocytes. Incidentally, these cells exhibited apoptotic features. Interestingly, the CD8 cells expressed FasL.

CONCLUSION

This paper presents the morphological evidence that apoptosis and acantholysis are linked. Therefore, the Fas pathway is associated with CD8 cells in pemphigus lesions.

Fas promoter region gene polymorphism is associated with an increased risk for myocardial infarction

A growing body of evidence has shown that Fas-mediated apoptosis is involved in atherosclerosis progression. Recent studies have revealed that a single nucleotide polymorphism (SNP) in the Fas promoter region (-670G/A) influences Fas expression. Here, we investigated whether -670G/A SNP influences the incidence of myocardial infarction (MI) by examining a comparison between MI patients (n=154) and control subjects (n=462) in a Japanese population. The allele frequency in each group was A 53.6%/G 46.4% in the MI patients, and A 43.9%/G 56.1% in the non-MI subjects (chi(2)=8.6; P=0.003). The odds ratio was 2.62 (95% CI: 1.43-4.88). As subjects with the -670AA genotype had a signal transducer and activator of transcription 1 (STAT1)-binding site in the Fas promoter region, STAT-1 activation by interferon-gamma may upregulate Fas expression in human vascular smooth muscle cells (VSMCs) of -670AA genotype subjects as described earlier. The Fas upregulation induces excess apoptosis to VSMCs, which leads to unstable plaque formation in atherosclerotic lesions and then potentially to plaque rupture, which can cause MI. Further investigation of hypertensive subjects revealed that the -670AA genotype does not induce hypertension occurrence, supporting that this difference of MI occurrence between the -670AA genotype and the -670GG genotype may be because of plaque rupture followed by excess apoptosis of VSMCs in the atherosclerotic lesion. We conclude that the Fas promoter gene, SNP (-670G/A), may be a risk factor of MI occurrence.

Residual CD95-pathway function in children with autoimmune lymphoproliferative syndrome is independent from clinical state and genotype of CD95 mutation

Human autoimmune lymphoproliferative syndrome (ALPS) is caused by defective CD95-mediated apoptosis of lymphocytes. In most patients, heterozygous mutations within the CD95 gene are found. Mutated proteins interfere with CD95-signaling in a dominant-negative way. However, the penetrance of clinical disease is variable. We describe 13 patients out of nine families with the clinical presentation of ALPS. Eight different mutations were detected. Sensitivity to CD95-induced cell-death, assembly of the CD95-death-inducing signaling complex (DISC), and activity of initiator caspases-8 and -10 were compared in EBV-transformed B-lymphoblastoid cells of these patients. All CD95-mutations led to a reduced DISC formation and diminished initiator caspase activity upon CD95-stimulation, whereas a marked heterogeneity in sensitivity to CD95-induced killing was found. Residual apoptosis sensitivity to almost normal levels could be achieved upon cross-linking by addition of protein A. Thus, no correlation between residual CD95 sensitivity and clinical phenotype or genotype of ALPS was found. This observation is only partially explained by the variable effects of the CD95-mutations themselves. It also points to a pronounced influence of additional factors, such as modifier pathways or exogenous effects apart from the CD95 pathway in the pathogenesis of ALPS.

Many checkpoints on the road to cell death: regulation of Fas-FasL interactions and Fas signaling in peripheral immune responses

Interactions between the TNF-family receptor Fas (CD95) and Fas Ligand (FasL, CD178) can efficiently induce apoptosis and are critical for the maintenance of immunological self-tolerance. FasL is kept under strict control by transcriptional and posttranslational regulation. Surface FasL can be cleaved by metalloproteases, resulting in shed extracellular domains, and FasL can also traffic to secretory lysosomes. Each form of FasL has distinct biological functions. Fas is more ubiquitously expressed, but its apoptosis-inducing function is regulated by a number of mechanisms including submembrane localization, efficiency of receptor signaling complex assembly and activation, and bcl-2 family members in some circumstances. When apoptosis is not induced, Fas-FasL interactions can also trigger a number of activating and proinflammatory signals. Harnessing the apoptosis-inducing potential of Fas for therapy of cancer and autoimmune disease has been actively pursued, and despite a number of unexpected side-effects that result from manipulating Fas-FasL interactions, this remains a worthy goal.

Genetic defects of apoptosis and primary immunodeficiency

Programmed cell death is important for maintaining lymphocyte homeostasis. Several human-inherited diseases with impaired apoptosis have been identified at the genetic level: autoimmune lymphoproliferative syndrome, caspase-8 deficiency state, and X-linked lymphoproliferative syndrome. These diseases feature excess lymphocyte accumulation, autoimmunity, or immunodeficiency. Elucidating their molecular pathogenesis has also provided new insights into the signaling mechanisms regulating apoptosis and lymphocyte activation.

Death receptors and apoptosis

Interactions between death receptors from the tumor necrosis factor superfamily and their ligands play a crucial role in the development and the integrity of the epidermis. The major consequence resulting from death receptor targeting is apoptosis. Evidence for dysregulation of death receptor signaling associated with the pathogenesis of selected cutaneous diseases, including toxic epidermal necrolysis, graft versus host disease, and skin cancer, are reviewed herein.

Dissecting lipid raft facilitated cell signaling pathways in cancer

Cancer is one of the most devastating disorders in our lives. Higher rate of proliferation than death of cells is one of the essential factors for development of cancer. The dynamicity of cell membrane plays some vital roles in cell survival and cell death, including protection, endocytosis, signaling, and increases in mechanical stability during cell division, as well as decrease of shear forces during separation of two cells after division, and cell separation from tissues for cancer metastasis. Within the membrane, there are specialized domains, known as lipid rafts. A raft can coordinate various signaling pathways. Recent data on the proteomics of lipid rafts/caveolae have highlighted the enigmatic role of various signaling proteins in cancer development. Analysis of these data of raft proteome from various tumors, cancer tissues, and cell lines cultured without and with therapeutic agents, as well as from model rafts revealed that there may be two subsets of raft assemblage in cell membrane. One subset of raft is enriched with cholesterol-sphingomyeline-ganglioside-cav-1/Src/EGFR (hereafter, "chol-raft") that is involved in normal cell signaling, and when dysregulated promotes cell transformation and tumor progression; another subset of raft is enriched with ceramide-sphingomyeline-ganglioside-FAS/Ezrin (hereafter, "cer-raft") that generally promotes apoptosis. In view of this, and to focus insight into the cancer cell physiology caused by the lipid rafts mediated signals and their receptors, and the downstream transmitters, either proliferative (for example, EGF and EGFR) or death-inducing (for example, FASL and FAS), and the precise roles of some therapeutic drugs and endogenous acid sphingomylenase in this scenario in in situ transformation of "chol-raft" into "cer-raft" are summarized and discussed in this contribution.

Camptothecin induces the transit of FasL trimers to the cell surface in apoptotic HEp-2 cells

Fas ligand (L) is a membrane protein from the tumor necrosis factor (TNF) family. It induces apoptosis upon contact with its Fas/CD95/APO1 receptor. Trimerization of FasL on the surface of effector cells is essential in the binding of the Fas trimer of the target cells. The receptor then recruits an adaptor and caspase-like proteins which lead apoptosis. This paper reports on the fate of FasL in HEp-2 cells committed to apoptosis by induction with campthotecin. Our main results demonstrated that in non-apoptotic cells, FasL aggregates in the cytoplasm forming trimers of 120 kDa. Apoptosis increases the trimeric FasL species, but also induces its dissociation into monomers of 35 kDa. In conclusion, camptothecin appears to perturb the Fas and FasL segregation in the cytoplasm by promoting the transit of FasL to the cell surface, thus fostering a process of autocrine or paracrine apoptosis. FasL is trimerized prior to Fas/FasL complex formation, and after apoptosis, FasL undergoes an intense turnover.

Dominant inhibition of Fas ligand-mediated apoptosis due to a heterozygous mutation associated with autoimmune lymphoproliferative syndrome (ALPS) Type Ib

Background:

Autoimmune lymphoproliferative syndrome (ALPS) is a disorder of lymphocyte homeostasis and immunological tolerance due primarily to genetic defects in Fas (CD95/APO-1; TNFRSF6), a cell surface receptor that regulates apoptosis and its signaling apparatus.

Methods:

Fas ligand gene mutations from ALPS patients were identified through cDNA and genomic DNA sequencing. Molecular and biochemical assessment of these mutant Fas ligand proteins were carried out by expressing the mutant FasL cDNA in mammalian cells and analysis its effects on Fas-mediated programmed cell death.

Results:

We found an ALPS patient that harbored a heterozygous A530G mutation in the FasL gene that replaced Arg with Gly at position 156 in the protein's extracellular Fas-binding region. This produced a dominant-interfering FasL protein that bound to the wild-type FasL protein and prevented it from effectively inducing apoptosis.

Conclusion:

Our data explain how a naturally occurring heterozygous human FasL mutation can dominantly interfere with normal FasL apoptotic function and lead to an ALPS phenotype, designated Type Ib.

[Autoimmune lymphoproliferative syndrome: an inherited or a somatic defect of apoptosis]

Control of lymphocyte homeostasis is essential to ensure efficient immune responses and to prevent autoimmunity. Expansion followed by contraction of the lymphocyte pool are the basis of adaptive immune responses, and apoptosis is a crucial cellular modus operandi of the contraction phase. The death receptor Fas is a key player in lymphocyte apoptosis induction and patients lacking a functional Fas receptor develop a chronic lymphoproliferation termed autoimmune lymphoproliferative syndrome (ALPS). In rare instances, defects of the Fas signaling pathway have been associated with the ALPS condition. Although these defects with familial history are usually caused by inherited mutations of the corresponding genes, somatic mosaicism of these Fas mutations were also found in sporadic cases of ALPS. These findings might have important implications in deciphering the pathophysiological bases of other autoimmune diseases.

Autoimmune lymphoproliferative syndrome: molecular basis of disease and clinical phenotype

Autoimmune lymphoproliferative syndrome (ALPS) is a variable clinical condition manifest by lymphoproliferative disease, autoimmune cytopenias and susceptibility to malignancy. Central to the cellular pathogenesis is defective FAS-induced apoptosis, which in turn leads to dysregulation of lymphocyte homeostasis. The majority of patients have heterozygous mutations in the FAS (TNFRSF6) gene, but the condition is genetically heterogeneous and mutations in FAS ligand and caspase-8 and caspase-10, all of which are involved in Fas mediated signalling, have also been identified. This review provides a detailed insight into the pathophysiology of lymphocyte apoptosis and how this relates to the variable and complex clinical manifestations of ALPS.

Fluorodeoxyglucose positron emission tomography (FDG-PET) for monitoring lymphadenopathy in the autoimmune lymphoproliferative syndrome (ALPS)

Autoimmune lymphoproliferative syndrome (ALPS) is associated with mutations that impair the activity of lymphocyte apoptosis proteins, leading to chronic lymphadenopathy, hepatosplenomegaly, autoimmunity, and an increased risk of lymphoma. We investigated the utility of fluorodeoxyglucose positron emission tomography (FDG-PET) in discriminating benign from malignant lymphadenopathy in ALPS. We report that FDG avidity of benign lymph nodes in ALPS can be high and, hence, by itself does not imply presence of lymphoma; but FDG-PET can help guide the decision for selecting which of many enlarged nodes in ALPS patients to biopsy when lymphoma is suspected.

HLA B44 is associated with decreased severity of autoimmune lymphoproliferative syndrome in patients with CD95 defects (ALPS type Ia)

Autoimmune lymphoproliferative syndrome (ALPS) is a disorder of lymphocyte apoptosis characterized by non-malignant lymphadenopathy and splenomegaly, expansion of T cells without either CD4 or CD8 surface markers, and increased incidence of autoimmune diseases and lymphoma. Most patients with ALPS have dominant, heterozygous mutations in tumor necrosis factor receptor superfamily member 6 (TNFRSF6), which encodes CD95, also known as Fas, a mediator of apoptosis. Penetrance and range of disease manifestations in ALPS are highly variable, even among family members who share the same dominant TNFRSF6 mutation. To evaluate HLA as a candidate modifier locus, we typed HLA A, B (including subtypes), and DQB alleles in 356 individuals from 63 unrelated families with defined TNFRSF6 mutations associated with ALPS. We also developed a quantitative severity score and performed statistical analysis. Among the healthier, mutation-bearing individuals, transmission of HLA B44 was significantly overrepresented (nominal P<0.0074) as compared to transmission in patients with severe clinical features of ALPS. The B44 allele may exert a protective role in ALPS.

Detection, epitope-mapping and function of anti-Fas autoantibody in patients with silicosis

Dysregulation of apoptosis through the Fas-Fas ligand pathway is associated with the onset of autoimmune disease. Since autoantibodies directed against unknown antigens are present in the sera of these patients, sera samples were examined for the presence of autoantibodies directed against the Fas molecule. Using Western blotting and a ProteinChip analysis, autoantibodies against Fas were detected in patients with silicosis, systemic lupus erythematosus (SLE) and systemic sclerosis (SSc), and weakly detected in healthy individuals. Using epitope mapping employing 12-amino-acid polypeptides with the SPOTs system, a minimum of four epitopes and a maximum of 10 epitopes were found. Several amino acid residues involved in binding FasL, such as C66, R87, L90, E93 and H126, were presented within the epitopes. Serum containing a large amount of anti-Fas autoantibody from silicosis patients inhibited the growth of a Fas-expressing human cell line, but did not inhibit the growth of a low Fas-expresser nor a Fas-expresser in which the Fas gene had been silenced by small interference RNA. All epitopes in the intracellular region of Fas were located in the death domain. The possible roles of anti-Fas autoantibody detected in healthy volunteers and patients with silicosis or autoimmune diseases are discussed here.

Molecular defects in T- and B-cell primary immunodeficiency diseases

More than 120 inherited primary immunodeficiency diseases have been discovered in the past five decades, and the precise genetic defect in many of these diseases has now been identified. Increasing understanding of these molecular defects has considerably influenced both basic and translational research, and this has extended to many branches of medicine. Recent advances in both diagnosis and therapeutic modalities have allowed these defects to be identified earlier and to be more precisely defined, and they have also resulted in more promising long-term outcomes. The prospect of gene therapy continues to be included in the armamentarium of treatment considerations, because these conditions could be among the first to benefit from gene-therapy trials in humans.

IGM is required for efficient complement mediated phagocytosis of apoptotic cells in vivo

A variety of complement components have been detected on apoptotic cells and proposed to facilitate recognition and/or ingestion by phagocytes. The triggers for complement activation remain uncertain. To determine the role of IgM in classical pathway activation and clearance of apoptotic cells in vitro and in vivo, we quantified these parameters in mice deficient in serum IgM (sIgM). Phagocytosis by bone marrow-derived macrophages of apoptotic cells incubated with serum deficient in sIgM was markedly reduced, similar to apoptotic cells incubated with C1q deficient serum in vitro. Similarly, intraperitoneal clearance of apoptotic cells and cellular C3 deposition were significantly reduced in mice deficient in sIgM compared to wild-type mice. Clearance and C3 deposition were reconstituted by addback of IgM. In mice deficient in both sIgM and Clq, addback of both serum factors was required for restoration of clearance. These findings indicate that, on a quantitative basis, sIgM is a potent factor required for intraperitoneal phagocytosis of apoptotic cells, and further demonstrate that IgM and C1q work in concert to activate complement, resulting in C3 deposition on the apoptotic cell surface and ultimately, efficient clearance of the apoptotic cell by macrophages.

Death receptor-mediated suicide: a novel target of autoimmune disease treatment

In the thymus, based on the reactivity of their T-cell receptor with self-MHC and antigenic peptides, developing immature T-cells undergo positive and negative selection. Cells recognising self-peptides and MHC with high affinity are considered autoreactive, and thus potentially harmful, and are eliminated by induction of apoptotic cell death. Thymic negative selection is, however, only incomplete and autoreactive T-cells escape into the periphery. It is not the presence of autoreactive mature T- and B-lymphocytes as the underlying cause of tissue destruction and development of autoimmune diseases, but their uncontrolled and excessive clonal expansion upon activation by self-antigen. Thus, potent regulatory mechanisms must keep these autoreactive cells under control to avoid their inappropriate activation. Recent evidence indicates that death receptors of the tumour necrosis factor receptor family play a central role in mediating antigen receptor-induced suicide of autoreactive T-lymphocytes. Defects in these apoptosis-inducing regulatory mechanisms may result in the development of autoimmune diseases. Therefore, enhancing the cell's own suicide program, offers a most attractive therapeutic target for the treatment of autoimmune diseases.

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

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