CD95 ligand--death factor and costimulatory molecule?

Decoding dengue's neurological assault: insights from single-cell CNS analysis in an immunocompromised mouse model

BACKGROUND

Dengue encephalitis, a severe neurological complication of dengue virus infection, is increasingly recognized for its rising incidence and significant public health burden. Despite its growing prevalence, the underlying mechanisms and effective therapeutic strategies remain poorly understood.

METHODS

Cellular atlas of dengue encephalitis was determined by single-nucleus RNA sequencing. Viral load of dengue virus and the level of cytokines expression was detected by RT-qPCR. The target cells of dengue virus were verified by immunofluorescence. The cytotoxic effect of CD8+ T cell was determined by flow cytometry, immunofluorescence, in vivo CD8+ T cell depletion, adoptive transfer and CCK-8-based cell viability assay. Axonal and synaptic reduction induced by dengue virus infection was demonstrated by RT-qPCR, Western blot, transmission electron microscope and immunofluorescence. Finally, motor and sensory functions of mice were detected by open field test and hot plate test, respectively.

RESULTS

In this study, we utilized single-nucleus RNA sequencing on brain tissues from a dengue-infected murine model to construct a comprehensive cellular atlas of dengue encephalitis. Our findings identify neurons, particularly inhibitory GABAergic subtypes, as the primary targets of dengue virus. Additionally, immune cell infiltration was observed, contributing to significant neurological damage. Comprehensive analyses of cell-cell communication, combined with CD8+ T cell depletion and transfer restoration experiments, have elucidated the critical role of CD8+ T cells in triggering encephalitis through their interaction with neurons. These cells infiltrate the brain from peripheral circulation, interact with neurons, and induce damage of synapse and axon, accompanied by neurological dysfunction.

CONCLUSION

We defined cellular atlas of dengue encephalitis in mouse model and identified the primary target neuron of dengue virus. In addition, we demonstrated the significant cytotoxic effect of CD8+ T cell, which leads to apoptosis of neuron and neurological dysfunction of mice. Our study provides a molecular and cellular framework for understanding dengue encephalitis through advanced sequencing technologies. The insights gained serve as a foundation for future investigations into its pathogenesis and the development of targeted therapeutic approaches.

The role of immune cell death in spermatogenesis and male fertility

The male reproductive system provides a distinctive shield to the immune system, safeguarding germ cells (GCs) from autoimmune harm. The testis in mammals creates a unique immunological setting due to its exceptional immune privilege and potent local innate immunity. which can result from a number of different circumstances, including disorders of the pituitary gland, GC aplasia, and immunological elements. Apoptosis, or programmed cell death (PCD), is essential for mammalian spermatogenesis to maintain and ensure an appropriate number of GCs that correspond with the supporting capability of the Sertoli cells. Apoptosis is substantial in controlling the number of GCs in the testis throughout spermatogenesis, and any dysregulation of this process has been linked to male infertility. There is a number of evidence about the potential of PCD in designing novel therapeutic approaches in the treatment of infertility. A detailed understanding of PCD and the processes that underlie immunological infertility can contribute to the progress in designing strategies to prevent and treat male infertility. This review will provide a summary of the role of immune cell death in male reproduction and infertility and describe the therapeutic strategies and agents for treatment based on immune cell death.

Synthesis of N-Glycosylated Soluble Fas Ligand

Controlled cell death is essential for the regulation of the immune system and plays a role in pathogen defense. It is often altered in pathogenic conditions such as cancer, viral infections and autoimmune diseases. The Fas receptor and its corresponding membrane-bound ligand (FasL) are part of the extrinsic apoptosis pathway activated in these cases. A soluble form of FasL (sFasL), produced by ectodomain shedding, displays a diverse but still elusive set of non-apoptotic functions and sometimes even serves as a pro-survival factor. To gather more knowledge about the characteristics of this protein and the impact N-glycosylations may have, access to homogeneous posttranslationally modified variants of sFasL is needed. Therefore, we developed a flexible strategy to obtain such homogeneously N-glycosylated variants of sFasL by applying chemical protein synthesis. This strategy can be flexibly combined with enzymatic methods to introduce more complex, site selective glycosylations.

Oregano (Origanum vulgare) Essential Oil and Its Constituents Prevent Rat Kidney Tissue Injury and Inflammation Induced by a High Dose of L-Arginine

This study aimed to evaluate the protective action of oregano (Origanum vulgare) essential oil and its monoterpene constituents (thymol and carvacrol) in L-arginine-induced kidney damage by studying inflammatory and tissue damage parameters. The determination of biochemical markers that reflect kidney function, i.e., serum levels of urea and creatinine, tissue levels of neutrophil-gelatinase-associated lipocalin (NGAL), and kidney injury molecule-1 (KIM-1), as well as a panel of oxidative-stress-related and inflammatory biomarkers, was performed. Furthermore, histopathological and immunohistochemical analyses of kidneys obtained from different experimental groups were conducted. Pre-treatment with the investigated compounds prevented an L-arginine-induced increase in serum and tissue kidney damage markers and, additionally, decreased the levels of inflammation-related parameters (TNF-α and nitric oxide concentrations and myeloperoxidase activity). Micromorphological kidney tissue changes correlate with the alterations observed in the biochemical parameters, as well as the expression of CD95 in tubule cells and CD68 in inflammatory infiltrate cells. The present results revealed that oregano essential oil, thymol, and carvacrol exert nephroprotective activity, which could be, to a great extent, associated with their anti-inflammatory, antiradical scavenging, and antiapoptotic action and, above all, due to their ability to lessen the disturbances arising from acute pancreatic damage. Further in-depth studies are needed in order to provide more detailed explanations of the observed activities.

CD95 (Fas) and CD95L (FasL)-mediated non-canonical signaling pathways

Although the interaction of CD95L (also known as FasL) with its so-called death receptor CD95 (Fas) induces an apoptotic signal responsible for the elimination of infected and cancer cells and maintenance of tissue homeostasis, this receptor can also implement non apoptotic signaling pathways. This latter signaling is involved in metastatic dissemination in certain cancers and the severity of auto-immune disorders. The signaling complexity of this pair is increased by the fact that CD95 expression itself seems to contribute to oncogenesis via a CD95L-independent manner and, that both ligand and receptor might interact with other partners modulating their pathophysiological functions. Finally, CD95L itself can trigger cell signaling in immune cells rendering complex the interpretation of mouse models in which CD95 or CD95L are knocked out. Herein, we discuss these non-canonical responses and their biological functions.

Oxidative stress-induced apoptosis and autophagy: Balancing the contrary forces in spermatogenesis

Spermatogenesis is a complex process in the testis and is a cornerstone of male infertility. The abundance of unsaturated fatty acid and high cell division rate make male germs cells prone to DNA deterioration. ROS-mediated oxidative stress triggers DNA damage, autophagy, and apoptosis in male germ cells, which are critical causative factors that lead to male infertility. The complex connection and molecular crosstalk between apoptosis and autophagy is seen at multifaceted levels that interconnect the signaling pathways of these two processes. Multilevel interaction between apoptosis and autophagy is a seamless state of survival and death in response to various stressors. Interaction between multiple genes and proteins such as the mTor signaling pathway, Atg12 proteins, and the death adapter proteins, such as Beclin 1, p53, and Bcl-2 family proteins, validates such a link between these two phenomena. Testicular cells being epigenetically different from somatic cells, undergo numerous significant epigenetic transitions, and ROS modulates the epigenetic framework of mature sperm. Epigenetic deregulation of apoptosis and autophagy under oxidative stress conditions can cause sperm cell damage. The current review recapitulates the current role of prevailing stressors that generate oxidative stress leading to the induction of apoptosis and autophagy in the male reproductive system. Considering the pathophysiological consequences of ROS-mediated apoptosis and autophagy, a combinatorial approach, including apoptosis inhibition and autophagy activation, a therapeutic strategy to treat male idiopathic infertility. Understanding the crosslink between apoptosis and autophagy under stress conditions in male germ cells may play an essential role in developing therapeutic strategies to treat infertility.

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.

COVID-19 and Diabetic Nephropathy

Diabetic nephropathy is the most common condition that requires a chronic renal replacement therapy, such as hemodialysis, peritoneal dialysis, kidney transplantation, or simultaneous kidney-pancreas transplantation. Chronic kidney disease progression, that is the loss of nephrons, which causes the continuous decline of the eGFR, underlies the pathogenesis of diabetic nephropathy. During the COVID-19 pandemic, it became clear that diabetic nephropathy is amongst the independent risk factors that predicts unfavourable outcome upon SARS-CoV2 infection. While we still lack conclusive mechanistic insights into how nephrons are rapidly lost upon SARS-CoV2 infection and why patients with diabetic nephropathy are more susceptible to severe outcomes upon SARS-CoV2 infection, here, we discuss several aspects of the interface of COVID-19 with diabetic nephropathy. We identify the shortage of reliable rodent models of diabetic nephropathy, limited treatment options for human diabetic nephropathy and the lack of knowledge about virus-induced signalling pathways of regulated necrosis, such as necroptosis, as key factors that explain our failure to understand this system. Finally, we focus on immunosuppressed patients and discuss vaccination efficacy in these and diabetic patients. We conclude that more basic science and mechanistic understanding will be required both in diabetic nephropathy as well as in host immune responses to the SARS-CoV2 virus if novel therapeutic strategies are desired.

sFasL-The Key to a Riddle: Immune Responses in Aging Lung and Disease

By dint of the aging population and further deepened with the Covid-19 pandemic, lung disease has turned out to be a major cause of worldwide morbidity and mortality. The condition is exacerbated when the immune system further attacks the healthy, rather than the diseased, tissue within the lung. Governed by unremittingly proliferating mesenchymal cells and increased collagen deposition, if inflammation persists, as frequently occurs in aging lungs, the tissue develops tumors and/or turns into scars (fibrosis), with limited regenerative capacity and organ failure. Fas ligand (FasL, a ligand of the Fas cell death receptor) is a key factor in the regulation of these processes. FasL is primarily found in two forms: full length (membrane, or mFasL) and cleaved (soluble, or sFasL). We and others found that T-cells expressing the mFasL retain autoimmune surveillance that controls mesenchymal, as well as tumor cell accumulation following an inflammatory response. However, mesenchymal cells from fibrotic lungs, tumor cells, or cells from immune-privileged sites, resist FasL⁺ T-cell-induced cell death. The mechanisms involved are a counterattack of immune cells by FasL, by releasing a soluble form of FasL that competes with the membrane version, and inhibits their cell death, promoting cell survival. This review focuses on understanding the previously unrecognized role of FasL, and in particular its soluble form, sFasL, in the serum of aged subjects, and its association with the evolution of lung disease, paving the way to new methods of diagnosis and treatment.

Physcion and Physcion 8-O-β-D-glucopyranoside: Natural Anthraquinones with Potential Anticancer Activities

Nature has provided prodigious reservoirs of pharmacologically active compounds for drug development since times. Physcion and physcion 8-O-β-D-glucopyranoside (PG) are bioactive natural anthraquinones which exert anti-inflammatory and anticancer properties with minimum or no adverse effects. Moreover, physcion also exhibits anti-microbial and hepatoprotective properties, while PG is known to have anti-sepsis as well as ameliorative activities against dementia. This review aims to highlight the natural sources and anticancer activities of physcion and PG, along with associated mechanisms of actions. On the basis of the literature, physcion and PG regulate multitudinous cell signaling pathways through the modulation of various regulators of cell cycle, protein kinases, microRNAs, transcriptional factors, and apoptosis linked proteins resulting in the effective killing of cancerous cells in vitro as well as in vivo. Both compounds effectively suppress metastasis, furthermore, physcion acts as an inhibitor of 6PGD and also plays an important role in chemosensitization. This review article suggests that physcion and PG are potent anticancer drug candidates, but further investigations on their mechanism of action and pre-clinical trials are mandatory in order to comprehend the full potential of these natural cancer killers in anticancer remedies.

Recent progress in therapeutic drug delivery systems for treatment of traumatic CNS injuries

Most therapeutics for the treatment of traumatic central nervous system injuries, such as traumatic brain injury and spinal cord injury, encounter various obstacles in reaching the target tissue and exerting pharmacological effects, including physiological barriers like the blood-brain barrier and blood-spinal cord barrier, instability rapid elimination from the injured tissue or cerebrospinal fluid and off-target toxicity. For central nervous system delivery, nano- and microdrug delivery systems are regarded as the most suitable and promising carriers. In this review, the pathophysiology and biomarkers of traumatic central nervous system injuries (traumatic brain injury and spinal cord injury) are introduced. Furthermore, various drug delivery systems, novel combinatorial therapies and advanced therapies for the treatment of traumatic brain injury and spinal cord injury are emphasized.

Differential gene expression of tumor-infiltrating CD8+ T cells in advanced versus early-stage colorectal cancer and identification of a gene signature of poor prognosis

Background

Cytotoxic CD8⁺ T cell-mediated response is the most important arm of adaptive immunity, which dictates the capacity of the host immune response in eradicating tumor cells. Due to tumor intrinsic and/or extrinsic factors, the density and function of CD8⁺ tumor-infiltrating lymphocytes (TILs) could be compromised, leading to poor prognosis and survival.

Methods

Using RNA-Seq, transcriptomes of sorted CD3⁺CD8⁺ TILs from treatment-naïve colorectal cancer (CRC) patients at advanced stages (III and IV) were compared with those from patients with early stages (I and II). A signature referred to as ‘poor prognosis CD8 gene signature (ppCD8sig)’ was identified and analyzed in The Cancer Genome Atlas CRC dataset. Scores for the ppCD8sig were calculated and classified as high, intermediate and low, and its prognostic significance was assessed using multivariate analysis and Cox proportional hazard model. Densities of CD3⁺ and CD8⁺ T cell infiltration in tumors from patients with high and low ppCD8sig scores were assessed by flow cytometry and immunostaining.

Results

Genes related to epigenetic regulation and response to hypoxia were upregulated in CD8⁺ TILs from patients with advanced stages, while genes related to T cell activation, cell proliferation and cell cycle were downregulated. Patients with high ppCD8sig score had poorer disease-specific survival (DSS) and shorter progression-free interval (PFI). The ppCD8sig was an independent prognostic indicator for DSS (HR 1.83, 95% CI 1.40 to 2.38, p<0.0001) and PFI (HR 1.42, 95% CI 1.04 to 1.93, p=0.026). Additionally, patients with high ppCD8sig score were more likely to have advanced stages (χ² p<0.0001) and residual disease after primary therapy (χ² p=0.046). Patients with high ppCD8sig score had reduced levels of CD3⁺ and CD8⁺ TILs and low Immunoscores (IS), compared to patients with low ppCD8sig score.

Conclusions

Our data provided insights into the altered regulation of biological mechanisms and signaling pathways in CD8⁺ TILs during CRC progression, and revealed a gene signature as an independent prognostic indicator. Patients with high ppCD8sig score had lower levels of TILs and low IS. These data further confirm the prognostic value of the identified ppCD8sig and potentially highlight its clinical relevance.

Tau Pathology Triggered by Spinal Cord Injury Can Play a Critical Role in the Neurotrauma Development

Traumatic spinal cord injury (SCI) can result in substantial neurological impairment along with significant emotional and psychological distress. It is clear that there is profound neurodegeneration upon SCI, gradually spread to other spinal cord regions and brain areas. Despite extensive considerations, it remains uncertain how pathogenicity diffuses in the cord. It has been reported that tau protein abnormal hyperphosphorylation plays a central role in neurodegeneration triggered by traumatic brain injury (TBI). Tau is a microtubule-associated protein, heavily implicated in neurodegenerative diseases. Importantly, tau pathology spreads in a traumatic brain in a timely manner. In particular, we have recently demonstrated that phosphorylated tau at Thr231 exists in two distinct cis and trans conformations, in which that cis P-tau is extremely neurotoxic, has a prion nature, and spreads to various brain areas and cerebrospinal fluid (CSF) upon trauma. On the other hand, tau pathology, in particular hyperphosphorylation at Thr231, has been observed upon SCI. Taken these together, we conclude that cis pT231-tau may accumulate and spread in the spinal cord as well as CSF and diffuse tau pathology in the central nervous system (CNS). Moreover, antibody against cis P-tau can target intracellular cis P-tau and protect pathology spreading. Thus, considering cis P-tau as a driver of tau pathology and neurodegeneration upon SCI would open new windows toward understanding the disease development and early biomarkers. Furthermore, it would help us develop effective therapies for SCI patients.

Blockade of BAFF Receptor BR3 on T Cells Enhances Their Activation and Cytotoxicity

The BAFF receptor BR3 plays key roles in B-cell activation, maturation, and survival whereas the function of BR3 on T lymphocytes is less well characterized. Previous reports have demonstrated that BR3 costimulates human T-cell activation in vitro in the presence of high nonphysiological levels of plate-bound BAFF. Here, relying on the soluble and membrane-bound BAFF expressed by T cells themselves, we investigated the function of BR3 on activated primary CD4 and CD8 T lymphocytes using a BR3-specific neutralization antibody and shRNA gene down-modulation. Interestingly, the anti-BR3 blocking antibody resulted in significant augmentation of CD25 and IFN-γ expression by both subsets, as did shRNA-mediated down-modulation of BR3. In addition, granzyme B expression was substantially elevated in anti-BR3-treated and BR3-silenced T cells. Anti-BR3 blockade increased the expression of CD25 on cytolytic CRTAM T cells. Importantly, anti-BR3 significantly enhanced redirected killing of P-815 cells by both CD4 and CD8 cytotoxic T cells [cytotoxic T lymphocytes (CTLs)]. Furthermore, anti-BR3-augmented CD4 T-cell-mediated killing of class II melanoma cell line A375 and cervical cancer cell line HeLa in vitro, increasing the level of granzyme B activity as measured by PARP-1 cleavage and active caspase 3. Together, our data indicate that BR3 neutralization increases the activation and cytolytic function of CD4 and CD8 cytotoxic T lymphocytes. Our findings provide a novel strategy for ex vivo T-cell activation applicable to T-cell immunotherapy platforms such as TIL or CAR-T cell therapeutics.

Biomarkers in Spinal Cord Injury: Prognostic Insights and Future Potentials

Spinal Cord Injury (SCI) is a major challenge in Neurotrauma research. Complex pathophysiological processes take place immediately after the injury and later on as the chronic injury develops. Moreover, SCI is usually accompanied by traumatic injuries because the most common modality of injury is road traffic accidents and falls. Patients develop significant permanent neurological deficits that depend on the extent and the location of the injury itself and in time they develop further neurological and body changes that may risk their mere survival. In our review, we explored the recent updates with regards to SCI biomarkers. We observed two methods that may lead to the appearance of biomarkers for SCI. First, during the first few weeks following the injury the Blood Spinal Cord Barrier (BSCB) disruption that releases several neurologic structure components from the injured tissue. These components find their way to Cerebrospinal Fluid (CSF) and the systemic circulation. Also, as the injury develops several components of the pathological process are expressed or released such as in neuroinflammation, apoptosis, reactive oxygen species, and excitotoxicity sequences. Therefore, there is a growing interest in examining any correlations between these components and the degrees or the outcomes of the injury. Additionally, some of the candidate biomarkers are theorized to track the progressive changes of SCI which offers an insight on the patients' prognoses, potential-treatments-outcomes assessment, and monitoring the progression of the complications of chronic SCI such as Pressure Ulcers and urinary dysfunction. An extensive literature review was performed covering literature, published in English, until February 2018 using the Medline/PubMed database. Experimental and human studies were included and titles, PMID, publication year, authors, biomarkers studies, the method of validation, relationship to SCI pathophysiology, and concluded correlation were reported. Potential SCI biomarkers need further validation using clinical studies. The selection of the appropriate biomarker group should be made based on the stage of the injuries, the accompanying trauma and with regards to any surgical, or medical interference that might have been done. Additionally, we suggest testing multiple biomarkers related to the several pathological changes coinciding to offer a more precise prediction of the outcome.

Tobacco smoke and nicotine suppress expression of activating signaling molecules in human dendritic cells

Cigarette smoke has significant toxic effects on the immune system, and increases the risk of developing autoimmune diseases; one immunosuppressive effect of cigarette smoke is that it inhibits the T cell-stimulating, immunogenic properties of myeloid dendritic cells (DCs). As the functions of DCs are regulated by intra-cellular signaling pathways, we investigated the effects of cigarette smoke extract (CSE) and nicotine on multiple signaling molecules and other regulatory proteins in human DCs to elucidate the molecular basis of the inhibition of DC maturation and function by CSE and nicotine. Maturation of monocyte-derived DCs was induced with the TLR3-agonist poly I:C or with the TLR4-agonist lipopolysaccharide, in the absence or presence of CSE or nicotine. Reverse-phase protein microarray was used to quantify multiple signaling molecules and other proteins in cell lysates. Particularly in poly I:C-matured DCs, cigarette smoke constituents and nicotine suppressed the expression of signaling molecules associated with DC maturation and T cell stimulation, cell survival and cell migration. In conclusion, constituents of tobacco smoke suppress the immunogenic potential of DCs at the signaling pathway level.

Neutralization of CD95 ligand protects the liver against ischemia-reperfusion injury and prevents acute liver failure

Ischemia-reperfusion injury is a common pathological process in liver surgery and transplantation, and has considerable impact on the patient outcome and survival. Death receptors are important mediators of ischemia-reperfusion injury, notably the signaling pathways of the death receptor CD95 (Apo-1/Fas) and its corresponding ligand CD95L. This study investigates, for the first time, whether the inhibition of CD95L protects the liver against ischemia-reperfusion injury. Warm ischemia was induced in the median and left liver lobes of C57BL/6 mice for 45 min. CD95Fc, a specific inhibitor of CD95L, was applied prior to ischemia. Hepatic injury was assessed via consecutive measurements of liver serum enzymes, histopathological assessment of apoptosis and necrosis and caspase assays at 3, 6, 12, 18 and 24 h after reperfusion. Serum levels of liver enzymes, as well as characteristic histopathological changes and caspase assays indicated pronounced features of apoptotic and necrotic liver damage 12 and 24 h after ischemia-reperfusion injury. Animals treated with the CD95L-blocker CD95Fc, exhibited a significant reduction in the level of serum liver enzymes and showed both decreased histopathological signs of parenchymal damage and decreased caspase activation. This study demonstrates that inhibition of CD95L with the CD95L-blocker CD95Fc, is effective in protecting mice from liver failure due to ischemia-reperfusion injury of the liver. CD95Fc could therefore emerge as a new pharmacological therapy for liver resection, transplantation surgery and acute liver failure.

Impairment of Fas-ligand-caveolin-1 interaction inhibits Fas-ligand translocation to rafts and Fas-ligand-induced cell death

Fas-ligand/CD178 belongs to the TNF family proteins and can induce apoptosis through death receptor Fas/CD95. The important requirement for Fas-ligand-dependent cell death induction is its localization to rafts, cholesterol- and sphingolipid-enriched micro-domains of membrane, involved in regulation of different signaling complexes. Here, we demonstrate that Fas-ligand physically associates with caveolin-1, the main protein component of rafts. Experiments with cells overexpressing Fas-ligand revealed a FasL N-terminal pre-prolin-rich region, which is essential for the association with caveolin-1. We found that the N-terminal domain of Fas-ligand bears two caveolin-binding sites. The first caveolin-binding site binds the N-terminal domain of caveolin-1, whereas the second one appears to interact with the C-terminal domain of caveolin-1. The deletion of both caveolin-binding sites in Fas-ligand impairs its distribution between cellular membranes, and attenuates a Fas-ligand-induced cytotoxicity. These results demonstrate that the interaction of Fas-ligand and caveolin-1 represents a molecular basis for Fas-ligand translocation to rafts, and the subsequent induction of Fas-ligand-dependent cell death. A possibility of a similar association between other TNF family members and caveolin-1 is discussed.

Crystal Structure of the Complex of Human FasL and Its Decoy Receptor DcR3

The apoptotic effect of FasL:Fas signaling is disrupted by DcR3, a unique secreted member of the tumor necrosis factor receptor superfamily, which also binds and neutralizes TL1A and LIGHT. DcR3 is highly elevated in patients with various tumors and contributes to mechanisms by which tumor cells to evade host immune surveillance. Here we report the crystal structure of FasL in complex with DcR3. Comparison of FasL:DcR3 structure with our earlier TL1A:DcR3 and LIGHT:DcR3 structures supports a paradigm involving the recognition of invariant main-chain and conserved side-chain functionalities, which is responsible for the recognition of multiple TNF ligands exhibited by DcR3. The FasL:DcR3 structure also provides insight into the FasL:Fas recognition surface. We demonstrate that the ability of recombinant FasL to induce Jurkat cell apoptosis is significantly enhanced by native glycosylation or by structure-inspired mutations, both of which result in reduced tendency to aggregate. All of these activities are efficiently inhibited by recombinant DcR3.

Role of caspases in CD95-induced biphasic activation of acid sphingomyelinase

Acid sphingomyelinase (A-SMase) plays an important role in the initiation of CD95 signaling by forming ceramide-enriched membrane domains that enable clustering and activation of the death receptors. In TNF-R1 and TRAIL-R1/R2 signaling, A-SMase also contributes to the lysosomal apoptosis pathway triggered by receptor internalization. Here, we investigated the molecular mechanism of CD95-mediated A-SMase activation, demonstrating that A-SMase is located in internalized CD95-receptosomes and is activated by the CD95/CD95L complex in a biphasic manner.Since several caspases have been described to be involved in the activation of A-SMase, we evaluated expression levels of caspase-8, caspase-7 and caspase-3 in CD95-receptosomes. The occurrence of cleaved caspase-8 correlated with the first peak of A-SMase activity and translocation of the A-SMase to the cell surface which could be blocked by the caspase-8 inhibitor IETD.Inhibition of CD95-internalization selectively reduced the second phase of A-SMase activity, suggesting a fusion between internalized CD95-receptosomes and an intracellular vesicular pool of A-SMase. Further analysis demonstrated that caspase-7 activity correlates with the second phase of the A-SMase activity, whereas active caspase-3 is present at early and late internalization time points. Blocking caspases-7/ -3 by DEVD reduced the second phase of A-SMase activation in CD95-receptosomes suggesting the potential role of caspase-7 or -3 for late A-SMase activation.In summary, we describe a biphasic A-SMase activation in CD95-receptosomes indicating (I.) a caspase-8 dependent translocation of A-SMase to plasma membrane and (II.) a caspase-7 and/or -3 dependent fusion of internalized CD95-receptosomes with intracellular A-SMase-containing vesicles.

Intramembrane proteolysis within lysosomes

Regulated intramembrane proteolysis is of pivotal importance in a diverse set of developmental and physiological processes. Altered intramembrane substrate turnover may be associated with neurodegeneration, cancer and impaired immune function. In this review we will focus on the intramembrane proteases which have been localized in the lysosomal membrane. Members of the γ-secretase complex and γ-secretase activity are found in the lysosomal membrane and are discussed to contribute to intracellular amyloid β production. Mutant or deficient γ-secretase may cause disturbed lysosomal function. The signal peptide peptidase-like (SPPL) protease 2a is a lysosomal membrane component and cleaves CD74, the invariant chain of the MHC II complex, as well as FasL, TNF, ITM2B and TMEM106, type II transmembrane proteins involved in the regulation of immunity and neurodegeneration. Therefore, it can be concluded, that not only proteolysis within the lysosomal lumen but also within lysosomal membranes regulates important cellular functions and contributes essentially to proteostasis of membrane proteins what may become increasingly compromised in the aged individual.

FasL and the NKG2D receptor are required for the secretion of the Tag7/PGRP-S-Hsp70 complex by the cytotoxic CD8+ lymphocytes

Tag7 (PGRP-S or PGLYRP1), while possessing an antimicrobial activity, also exhibits an antitumor effect when in complex with the major heat shock protein Hsp70. The cytotoxic Tag7-Hsp70 complex is secreted by lymphocytes after interaction with the HLA-negative tumors. Previously, we have shown that IL-2 induces formation of the CD4⁺ and CD8⁺ cytotoxic subpopulations of human lymphocytes, which kill tumor cells through the FasL-Fas interaction. Here, we show that only the CD8⁺ T cells are able to secrete the Tag7-Hsp70 complex. For its secretion the same proteins on the surface of the lymphocytes and target cells, which are involved in the contact lysis, are necessary as well. The interaction of Fas receptor with FasL leads to an activation of the Tag7-Hsp70 complex in the lymphocyte membrane fraction, and here FasL acts as a receptor that induces intracellular signaling in lymphocytes. An interaction of the MicA stress ligand with the NKG2D receptor is necessary for the release of this cytotoxic complex. It is possible, that CD8⁺ T lymphocytes interacting with a target cell can both carry out the contact killing of these cells and to secrete the cytotoxic factor. © 2016 IUBMB Life, 69(1):30-36, 2017.

Apoptosis Markers in Breast Cancer Therapy

Cancer is a disease characterized by a very little apoptosis, ie, genetically programmed cell death. Aberrations in apoptotic pathways are central to tumorigenesis, tumor progression, and overall tumor growth and regression in response to chemotherapy. It is now increasingly accepted that chemotherapeutic drug efficacy is partially related to its ability to induce apoptosis. Apoptosis, therefore, represents not only a vital target in cancer therapy but also a unique biomarker opportunity that has thus far been largely unexploited. In response to therapy, tumor cells undergo apoptosis and release their cellular components in the circulation. As such, these materials may serve as biomarkers to assess response. Apoptosis markers in breast cancer include circulating soluble FasL, granzyme B, and cytochrome c that increase following chemotherapy. Unfortunately, there is a paucity of information in the literature with respect to this approach. As such, large-scale prospective studies are clearly needed to validate this approach and more fully elucidate clinical usefulness.

Targeting the Fas/FasL system in Rheumatoid Arthritis therapy: Promising or risky?

Rheumatoid Arthritis (RA) is a chronic inflammatory disease affecting synovial joints. Tumor necrosis factor (TNF) α is a key component of RA pathogenesis and blocking this cytokine is the most common strategy to treat the disease. Though TNFα blockers are very efficient, one third of the RA patients are unresponsive or present side effects. Therefore, the development of novel therapeutic approaches is required. RA pathogenesis is characterized by the hyperplasia of the synovium, closely associated to the pseudo-tumoral expansion of fibroblast-like synoviocytes (FLS), which invade and destroy the joint structure. Hence, depletion of RA FLS has been proposed as an alternative therapeutic strategy. The TNF family member Fas ligand (FasL) was reported to trigger apoptosis in FLS of arthritic joints by binding to its receptor Fas and therefore suggested as a promising candidate for targeting the hyperplastic synovial tissue. However, this cytokine is pleiotropic and recent data from the literature indicate that Fas activation might have a disease-promoting role in RA by promoting cell proliferation. Therefore, a FasL-based therapy for RA requires careful evaluation before being applied. In this review we aim to overview what is known about the apoptotic and non-apoptotic effects of Fas/FasL system and discuss its relevance in RA.

The combination of type I IFN, TNF-α, and cell surface receptor engagement with dendritic cells enables NK cells to overcome immune evasion by dengue virus

Clinical studies have suggested the importance of the NK cell response against dengue virus (DenV), an arboviral infection that afflicts >50 million individuals each year. However, a comprehensive understanding of the NK cell response against dengue-infected cells is lacking. To characterize cell-contact mechanisms and soluble factors that contribute to the antidengue response, primary human NK cells were cocultured with autologous DenV-infected monocyte-derived dendritic cells (DC). NK cells responded by cytokine production and the lysis of target cells. Notably, in the absence of significant monokine production by DenV-infected DC, it was the combination of type I IFNs and TNF-α produced by DenV-infected DC that was important for stimulating the IFN-γ and cytotoxic responses of NK cells. Cell-bound factors enhanced NK cell IFN-γ production. In particular, reduced HLA class I expression was observed on DenV-infected DC, and IFN-γ production was enhanced in licensed/educated NK cell subsets. NK-DC cell contact was also identified as a requirement for a cytotoxic response, and there was evidence for both perforin/granzyme as well as Fas/Fas ligand-dependent pathways of killing by NK cells. In summary, our results have uncovered a previously unappreciated role for the combined effect of type I IFNs, TNF-α, and cell surface receptor-ligand interactions in triggering the antidengue response of primary human NK cells.

Serum levels of soluble Fas ligand, granzyme B and cytochrome c during adjuvant chemotherapy of breast cancer

BACKGROUND

Anticancer agents used in chemotherapy for tumors induce apoptosis in malignant cells. Soluble Fas ligand, granzyme B and cytochrome c are key elements in the process of apoptosis. The objective of this preliminary study was to evaluate the changes in the serum concentrations of these parameters in breast cancer patients undergoing adjuvant chemotherapy.

MATERIALS AND METHODS

Sixty patients with histopathologically proven breast cancer were included in the present study. The blood samples were taken after surgery before chemotherapy and after 3weeks of administration of the first cycle of chemotherapy. Thirty healthy female controls were selected for comparison. Soluble FasL, granzyme B and cytochrome c were estimated from serum by ELISA.

RESULTS

Significantly increased concentrations of soluble FasL, granzyme B and cytochrome c were found in stage II and stage III of breast cancer patients after chemotherapy compared with concentrations before chemotherapy (P<0.0001). A significant positive correlation was found between soluble FasL and cytochrome c as well as between granzyme B and cytochrome c in breast cancer patients after chemotherapy.

CONCLUSION

Serum concentrations of apoptotic markers such as soluble FasL, granzyme B and cytochrome c were increased after administration of the first cycle of chemotherapeutic drugs. The measurement of these circulating apoptotic markers may help clinicians in evaluating treatment efficacy in breast cancer.

Membrane trafficking of death receptors: implications on signalling

Death receptors were initially recognised as potent inducers of apoptotic cell death and soon ambitious attempts were made to exploit selective ignition of controlled cellular suicide as therapeutic strategy in malignant diseases. However, the complexity of death receptor signalling has increased substantially during recent years. Beyond activation of the apoptotic cascade, involvement in a variety of cellular processes including inflammation, proliferation and immune response was recognised. Mechanistically, these findings raised the question how multipurpose receptors can ensure selective activation of a particular pathway. A growing body of evidence points to an elegant spatiotemporal regulation of composition and assembly of the receptor-associated signalling complex. Upon ligand binding, receptor recruitment in specialized membrane compartments, formation of receptor-ligand clusters and internalisation processes constitute key regulatory elements. In this review, we will summarise the current concepts of death receptor trafficking and its implications on receptor-associated signalling events.

Induction of rapid T cell death and phagocytic activity by Fas-deficient lpr macrophages

Peripheral T cells are maintained by the apoptosis of activated T cells through the Fas-Fas ligand system. Although it is well known that normal T cells fail to survive in the Fas-deficient immune condition, the molecular mechanism for the phenomenon has yet to be elucidated. In this study, we demonstrate that rapid cell death and clearance of normal T cells were induced by Fas-deficient lpr macrophages. Transfer of normal T cells into lpr mice revealed that Fas expression on donor T cells was promptly enhanced through the IFN-γ/IFN-γR. In addition, Fas ligand expression and phagocytic activity of lpr macrophages were promoted through increased NF-κB activation. Controlling Fas expression on macrophages plays an essential role in maintaining T cell homeostasis in the peripheral immune system. Our data suggest a critical implication to the therapeutic strategies such as transplantation and immunotherapy for immune disorder or autoimmunity related to abnormal Fas expression.

Alginate encapsulated cells secreting Fas-ligand reduce lymphoma carcinogenicity

Fas ligand (CD95L/APO-1) is considered as a potent anti-tumor agent due to its mediated cell death properties. We have designed a polymeric microencapsulation system, which encapsulates soluble FasL secreting cells. The encapsulated cells continuously release soluble FasL (sFasL) at the tumor site, while the device protects the encapsulated cells from the host immune system. The potential and efficacy of this system are demonstrated in vitro and in vivo for tumor inhibition. Polymeric microcapsules composed of Alginate Poly-l-lysine were optimized to encapsulate L5 secreting sFasL cells. The expression and anti-tumor activities of the sFasL were confirmed in vitro and tumor inhibition was studied in vivo in SCID mice bearing subcutaneous lymphoma tumors. In vitro, sFasL secreted by the encapsulated L5-sFasL cells was biologically active, inhibited proliferation and induced apoptotic cell death in Fas sensitive tumor cells. Mice injected with encapsulated L5-sFasL cells on the day of tumor injection or 10 days after tumor injection showed significant reduction in tumor volume, of 87% and 95%, respectively. Our findings show that encapsulated cells expressing sFasL can be used as a local device and efficiently suppress malignant Fas sensitive tumors with no side effects.

Role of Bcl-2 family proteins and caspases in the regulation of apoptosis

Apoptosis, or programmed cell death, plays a pivotal role in the elimination of unwanted, damaged, or infected cells in multicellular organisms and also in diverse biological processes, including development, cell differentiation, and proliferation. Apoptosis is a highly regulated form of cell death, and dysregulation of apoptosis results in pathological conditions including cancer, autoimmune and neurodegenerative diseases. The Bcl-2 family proteins are key regulators of apoptosis, which include both anti- and pro-apoptotic proteins, and a slight change in the dynamic balance of these proteins may result either in inhibition or promotion of cell death. Execution of apoptosis by various stimuli is initiated by activating either intrinsic or extrinsic pathways which lead to a series of downstream cascade of events, releasing of various apoptotic mediators from mitochondria and activation of caspases, important for the cell fate. In view of recent research advances about underlying mechanism of apoptosis, this review highlights the basics concept of apoptosis and its regulation by Bcl-2 family of protein. Furthermore, this review discusses the interplay of various apoptotic mediators and caspases to decide the fate of the cell. We expect that this review will add to the pool of basic information necessary to understand the mechanism of apoptosis which may implicate in designing better strategy to develop biomedical therapy to control apoptosis.

Insights into the molecular regulation of FasL (CD178) biology

Fas ligand (FasL, CD95L, APO-1L, CD178, TNFSF6, APT1LG1) is the key death factor of receptor-triggered programmed cell death in immune cells. FasL/Fas-dependent apoptosis plays a pivotal role in activation-induced cell death, termination of immune responses, elimination of autoreactive cells, cytotoxic effector function of T and NK cells, and the establishment of immune privilege. Deregulation or functional impairment of FasL threatens the maintenance of immune homeostasis and defense and results in severe autoimmunity. In addition, FasL has been implicated as an accessory or costimulatory receptor in T cell activation. The molecular mechanisms underlying this reverse signaling capacity are, however, poorly understood and still controversially discussed. Many aspects of FasL biology have been ascribed to selective protein-protein interactions mediated by a unique polyproline region located in the membrane-proximal intracellular part of FasL. Over the past decade, we and others identified a large number of putative FasL-interacting molecules that bind to this polyproline stretch via Src homology 3 or WW domains. Individual interactions were analyzed in more detail and turned out to be crucial for the lysosomal storage, the transport and the surface appearance of the death factor and potentially also for reverse signaling. This review summarizes the work in the framework of the Collaborative Research Consortium 415 (CRC 415) and provides facts and hypotheses about FasL-interacting proteins and their potential role in FasL biology.

Palmitoylation of human FasL modulates its cell death-inducing function

Fas ligand (FasL) is a transmembrane protein that regulates cell death in Fas-bearing cells. FasL-mediated cell death is essential for immune system homeostasis and the elimination of viral or transformed cells. Because of its potent cytotoxic activity, FasL expression at the cell surface is tightly regulated, for example, via processing by ADAM10 and SPPL2a generating soluble FasL and the intracellular fragments APL (ADAM10-processed FasL form) and SPA (SPPL2a-processed APL). In this study, we report that FasL processing by ADAM10 counteracts Fas-mediated cell death and is strictly regulated by membrane localization, interactions and modifications of FasL. According to our observations, FasL processing occurs preferentially within cholesterol and sphingolipid-rich nanodomains (rafts) where efficient Fas–FasL contact occurs, Fas receptor and FasL interaction is also required for efficient FasL processing, and FasL palmitoylation, which occurs within its transmembrane domain, is critical for efficient FasL-mediated killing and FasL processing.

Male germ cell apoptosis: regulation and biology

Cellular apoptosis appears to be a constant feature in the adult testis and during early development. This is essential because mammalian spermatogenesis is a complex process that requires precise homeostasis of different cell types. This review discusses the latest information available on male germ cell apoptosis induced by hormones, toxins and temperature in the context of the type of apoptotic pathway either the intrinsic or the extrinsic that may be used under a variety of stimuli. The review also discusses the importance of mechanisms pertaining to cellular apoptosis during testicular development, which is independent of exogenous stimuli. Since instances of germ cell carcinoma have increased over the past few decades, the current status of research on apoptotic pathways in teratocarcinoma cells is included. One other important aspect that is covered in this review is microRNA-mediated control of germ cell apoptosis, a field of research that is going to see intense activity in near future. Since knockout models of various kinds have been used to study many aspects of germ cell development, a comprehensive summary of literature on knockout mice used in reproduction studies is also provided.

Adoptive transfer of splenocytes to study cell-mediated immune responses in hepatitis C infection using HCV transgenic mice

Background

Hepatitis C virus (HCV) is a major cause of chronic hepatitis and a health problem affecting over 170 million people around the world. We previously studied transgenic mice that express HCV Core, Envelope 1 and Envelope 2 proteins predominantly in the liver, resulting in steatosis, liver and lymphoid tumors, and hepatocellular carcinoma. Herein, the immune-mediated cell response to hepatitis C antigens was evaluated by adoptive transfers of carboxyfluorescein succinimidyl ester (CFSE) labelled splenocytes from HCV immunized mice into HCV transgenic mice.

Results

In comparison to non-transgenic mice, there was a significant decrease in the percentage of CFSE-labeled CD4⁺ and CD8⁺ T cells in transgenic mouse peripheral blood receiving adoptive transfers from immunized donors. Moreover, the percentage of CFSE-labeled CD4⁺ and CD8⁺ T cells were significantly higher in the spleen of transgenic and non-transgenic mice when they received splenocytes from non-immunized than from immunized mice. On the other hand, the percentages of CD4⁺ and CD8⁺ T cells in the non-transgenic recipient mouse lymph nodes were significantly higher than the transgenic mice when they received the adoptive transfer from immunized donors. Interestingly, livers of transgenic mice that received transfers from immunized mice had a significantly higher percentage of CFSE labeled T cells than livers of non-transgenic mice receiving non-immunized transfers.

Conclusions

These results suggest that the T cells from HCV immunized mice recognize the HCV proteins in the liver of the transgenic mouse model and homed to the HCV antigen expression sites. We propose using this model system to study active T cell responses in HCV infection.

The role of FasL and Fas in health and disease

The FS7-associated cell surface antigen (Fas, also named CD95, APO-1 or TNFRSF6) attracted considerable interest in the field of apoptosis research since its discovery in 1989. The groups of Shin Yonehara and Peter Krammer were the first reporting extensive apoptotic cell death induction upon treating cells with Fas-specific monoclonal antibodies.1,2 Cloning of Fas3 and its ligand,4,5 FasL (also known as CD178, CD95L or TNFSF6), laid the cornerstone in establishing this receptor-ligand system as a central regulator of apoptosis in mammals. Therapeutic exploitation of FasL-Fas-mediated cytotoxicity was soon an ambitous goal and during the last decade numerous strategies have been developed for its realization. In this chapter, we will briefly introduce essential general aspects of the FasL-Fas system before reviewing its physiological and pathophysiological relevance. Finally, FasL-Fas-related therapeutic tools and concepts will be addressed.

Identification of SH3 domain interaction partners of human FasL (CD178) by phage display screening

BACKGROUND

Fas ligand is a cytotoxic effector molecule of T and NK cells which is characterized by an intracellular N-terminal polyproline region that serves as a docking site for SH3 and WW domain proteins. Several previously described Fas ligand-interacting SH3 domain proteins turned out to be crucial for the regulation of storage, expression and function of the death factor. Recent observations, however, indicate that Fas ligand is also subject to posttranslational modifications including shedding and intramembrane proteolysis. This results in the generation of short intracellular fragments that might either be degraded or translocate to the nucleus to influence transcription. So far, protein-protein interactions that specifically regulate the fate of the intracellular fragments have not been identified.

RESULTS

In order to further define the SH3 domain interactome of the intracellular region of Fas ligand, we now screened a human SH3 domain phage display library. In addition to known SH3 domains mediating binding to the Fas ligand proline-rich domain, we were able to identify a number of additional SH3 domains that might also associate with FasL. Potential functional implications of the new binding proteins for the death factor's biology are discussed. For Tec kinases and sorting nexins, the observed interactions were verified in cellular systems by pulldown experiments.

CONCLUSION

We provide an extended list of putative Fas ligand interaction partners, confirming previously identified interactions, but also introducing several novel SH3 domain proteins that might be important regulators of Fas ligand function.

Enrichment and analysis of secretory lysosomes from lymphocyte populations

BACKGROUND

In specialized cells, such as mast cells, macrophages, T lymphocytes and Natural Killer cells in the immune system and for instance melanocytes in the skin, secretory lysosomes (SL) have evolved as bifunctional organelles that combine degradative and secretory properties. Mutations in lysosomal storage, transport or sorting molecules are associated with severe immunodeficiencies, autoimmunity and (partial) albinism. In order to analyze the function and content of secretory lysosomes in different cell populations, an efficient enrichment of these organelles is mandatory.

RESULTS

Based on a combination of differential and density gradient centrifugation steps, we provide a protocol to enrich intact SL from expanded hematopoietic cells, here T lymphocytes and Natural Killer cells. Individual fractions were initially characterized by Western blotting using antibodies against an array of marker proteins for intracellular compartments. As indicated by the presence of LAMP-3 (CD63) and FasL (CD178), we obtained a selective enrichment of SL in one of the resulting organelle fractions. The robustness and reproducibility of the applied separation protocol was examined by a high-resolution proteome analysis of individual SL preparations of different donors by 2D difference gel electrophoresis (2D-DIGE).

CONCLUSION

The provided protocol is readily applicable to enrich and isolate intact secretory vesicles from individual cell populations. It can be used to compare SL of normal and transformed cell lines or primary cell populations from healthy donors and patients with lysosomal storage or transport diseases, or from corresponding mutant mice. A subsequent proteome analysis allows the characterization of molecules involved in lysosomal maturation and cytotoxic effector function at high-resolution.

FasL expression and reverse signalling

FasL plays a central role in the induction of apoptosis within the immune system. It mediates activation-induced cell death (AICD) of T lymphocytes and contributes to the cytotoxic effector function of T and NK cells. Moreover, FasL is discussed as direct effector molecule for the establishment of immune privilege and tumour survival. Besides its death-promoting activity, FasL has been implicated in reverse signalling and might thus also play a role in T cell development and selection and the modulation of T cell activation. Considering these diverse functions, the overall FasL expression has to be tightly controlled to avoid unwanted damage. Based on an activation-associated transcriptional control, several post-transcriptional processes ensure a safe storage, a rapid mobilisation, a target-directed activity and a subsequent inactivation. Over the past years, the identification and characterisation of FasL-interacting proteins provided novel insight into the mechanisms of FasL transport, processing and reverse signalling, which might be exemplary also for the other members of the TNF family.

Distinct requirements for activation-induced cell surface expression of preformed Fas/CD95 ligand and cytolytic granule markers in T cells

Fas (CD95/Apo-1) ligand is a potent inducer of apoptosis and one of the major killing effector mechanisms of cytotoxic T cells. Thus, Fas ligand activity has to be tightly regulated, involving various transcriptional and post-transcriptional processes. For example, preformed Fas ligand is stored in secretory lysosomes of activated T cells, and rapidly released by degranulation upon reactivation. In this study, we analyzed the minimal requirements for activation-induced degranulation of Fas ligand. T cell receptor activation can be mimicked by calcium ionophore and phorbol ester. Unexpectedly, we found that stimulation with phorbol ester alone is sufficient to trigger Fas ligand release, whereas calcium ionophore is neither sufficient nor necessary. The relevance of this process was confirmed in primary CD4(+) and CD8(+) T cells and NK cells. Although the activation of protein kinase(s) was absolutely required for Fas ligand degranulation, protein kinase C or A were not involved. Previous reports have shown that preformed Fas ligand co-localizes with other markers of cytolytic granules. We found, however, that the activation-induced degranulation of Fas ligand has distinct requirements and involves different mechanisms than those of the granule markers CD63 and CD107a/Lamp-1. We conclude that activation-induced degranulation of Fas ligand in cytotoxic lymphocytes is differently regulated than other classical cytotoxic granule proteins.

Posttranslational regulation of Fas ligand function

The TNF superfamily member Fas ligand acts as a prototypic death factor. Due to its ability to induce apoptosis in Fas (APO-1, CD95) expressing cells, Fas ligand participates in essential effector functions of the immune system. It is involved in natural killer cell- and T cell-mediated cytotoxicity, the establishment of immune privilege, and in termination of immune responses by induction of activation-induced cell death. In addition, Fas ligand-positive tumours may evade immune surveillance by killing Fas-positive tumour-infiltrating cells. Given these strong cytotoxic capabilities of Fas ligand, it is obvious that its function has to be strictly regulated to avoid uncontrolled damage. In hematopoietic cells, the death factor is stored in secretory lysosomes and is mobilised to the immunological synapse only upon activation. The selective sorting to and the release from this specific lysosomal compartment requires interactions of the Fas ligand cytosolic moiety, which mediates binding to various adapter proteins involved in trafficking and cytoskeletal reorganisation. In addition, Fas ligand surface expression is further regulated by posttranslational ectodomain shedding and subsequent regulated intramembrane proteolysis, releasing a soluble ectodomain cytokine into the extracellular space and an N-terminal fragment with a potential role in intracellular signalling processes. Moreover, other posttranslational modifications of the cytosolic domain, including phosphorylation and ubiquitylation, have been described to affect various aspects of Fas ligand biology. Since FasL is regarded as a potential target for immunotherapy, the further characterisation of its biological regulation and function will be of great importance for the development and evaluation of future therapeutic strategies.

Cell cycle regulation by FasL and Apo2L/TRAIL in human T-cell blasts. Implications for autoimmune lymphoproliferative syndromes

The Fas-FasL pathway plays an important role in the homeostasis of mature lymphocytes, with defects causing autoimmune lymphoproliferative syndromes (ALPS). Human T-cell blasts are not sensitive to FasL or Apo2L/TRAIL-induced apoptosis unless they get reactivated, but either of those ligands inhibits their growth in the absence of cell death induction due to a cell cycle arrest in S-G2/M. In the present work, we have studied the mechanism(s) by which FasL or Apo2L/TRAIL regulate T-cell blast cell cycle in healthy donors and in two types of ALPS patients. Our data indicate that in human CD8+ T-cell blasts, Fas ligation, and especially Apo2L/TRAIL induce the p53-dependent decrease in cyclin-B1 levels. However, the induction of the negative cell cycle regulator p21WAF1 by FasL or Apo2L/TRAIL in either CD4+ or CD8+ T-cell blasts seems to be the main regulatory mechanism. This mechanism is dependent on caspase activation and on H2O2 generation. The increase in p21 levels by FasL or Apo2L/TRAIL is concomitant with p53 increases only in CD8+ T-cell blasts, with p21 levels maintained high for longer times than p53 levels. In CD4+ T-cell blasts p21 levels are controlled through a transient and p53-independent mechanism. The present results suggest that the etiology of ALP syndromes could be related not only to defects in apoptosis induction, but also in cell cycle regulation.

The Fas ligand intracellular domain is released by ADAM10 and SPPL2a cleavage in T-cells

Fas ligand (FasL) is a type II transmembrane protein belonging to the tumor necrosis factor family. Its binding to the cognate Fas receptor triggers the apoptosis that plays a pivotal role in the maintenance of immune system homeostasis. The cell death-inducing property of FasL has been associated with its extracellular domain, which can be cleaved off by metalloprotease activity to produce soluble FasL. The fate of the remaining membrane-anchored N-terminal part of the FasL molecule has not been determined. Here we show that post-translational processing of overexpressed and endogenous FasL in T-cells by the disintegrin and metalloprotease ADAM10 generates a 17-kDa N-terminal fragment, which lacks the receptor-binding extracellular domain. This FasL remnant is membrane anchored and further processed by SPPL2a, a member of the signal peptide peptidase-like family of intramembrane-cleaving proteases. SPPL2a cleavage liberates a smaller and highly unstable fragment mainly containing the intracellular FasL domain (FasL ICD). We show that this fragment translocates to the nucleus and is capable of inhibiting gene transcription. With ADAM10 and SPPL2a we have identified two proteases implicated in FasL processing and release of the FasL ICD, which has been shown to be important for retrograde FasL signaling.

ADAM10 regulates FasL cell surface expression and modulates FasL-induced cytotoxicity and activation-induced cell death

The apoptosis-inducing Fas ligand (FasL) is a type II transmembrane protein that is involved in the downregulation of immune reactions by activation-induced cell death (AICD) as well as in T cell-mediated cytotoxicity. Proteolytic cleavage leads to the generation of membrane-bound N-terminal fragments and a soluble FasL (sFasL) ectodomain. sFasL can be detected in the serum of patients with dysregulated inflammatory diseases and is discussed to affect Fas-FasL-mediated apoptosis. Using pharmacological approaches in 293T cells, in vitro cleavage assays as well as loss and gain of function studies in murine embryonic fibroblasts (MEFs), we demonstrate that the disintegrin and metalloprotease ADAM10 is critically involved in the shedding of FasL. In primary human T cells, FasL shedding is significantly reduced after inhibition of ADAM10. The resulting elevated FasL surface expression is associated with increased killing capacity and an increase of T cells undergoing AICD. Overall, our findings suggest that ADAM10 represents an important molecular modulator of FasL-mediated cell death.

Regulation of FasL expression: A SH3 domain containing protein family involved in the lysosomal association of FasL

As a death factor of T cells and Natural Killer (NK) cells, Fas Ligand (FasL) is stored in association with secretory lysosomes. Upon stimulation, these cytotoxic granules are transported to the cell membrane where FasL is exposed on the cell surface, shed or secreted. It has been noted before that the proline-rich domain within the cytosolic part of FasL is required for its vesicular association. However, the molecular interactions involved in targeting FasL to secretory lysosomes or to the plasma membrane have not been elucidated. We now identified a family of structurally related proteins that upon co-expression with FasL reallocate the death factor from a membrane to an intracellular localization. Members of this protein family are characterized by a similar domain structure and include FBP17, PACSIN1-3, CD2BP1, CIP4, Rho-GAP C1 and several hypothetical proteins. We show that all tested members of this "FCH/SH3-family" co-precipitate FasL from transfectants. The interactions strictly depend on functional SH3 domains within the FCH/SH3 proteins. Since co-expression of FasL with individual FCH/SH3 proteins dramatically alters the intracellular localization of FasL especially in non-hematopoietic cells, our data suggest that FCH/SH3 proteins might play an important role for the subcellular distribution and lysosomal association of FasL.