FasL cross-linking inhibits activation of human peripheral T cells

Revisiting autoimmune lymphoproliferative syndrome caused by Fas ligand mutations

BACKGROUND

Fas ligand (FasL) is expressed by activated T cells and induces death in target cells upon binding to Fas. Loss-of-function FAS or FASLG mutations cause autoimmune-lymphoproliferative syndrome (ALPS) characterized by expanded double-negative T cells (DNT) and elevated serum biomarkers. While most ALPS patients carry heterozygous FAS mutations, FASLG mutations are rare and usually biallelic. Only 2 heterozygous variants were reported, associated with an atypical clinical phenotype.

OBJECTIVE

We revisited the significance of heterozygous FASLG mutations as a cause of ALPS.

METHODS

Clinical features and biomarkers were analyzed in 24 individuals with homozygous or heterozygous FASLG variants predicted to be deleterious. Cytotoxicity assays were performed with patient T cells and biochemical assays with recombinant FasL.

RESULTS

Homozygous FASLG variants abrogated cytotoxicity and resulted in early-onset severe ALPS with elevated DNT, raised vitamin B₁₂, and usually no soluble FasL. In contrast, heterozygous variants affected FasL function by reducing expression, impairing trimerization, or preventing Fas binding. However, they were not associated with elevated DNT and vitamin B₁₂, and they did not affect FasL-mediated cytotoxicity. The dominant-negative effects of previously published variants could not be confirmed. Even Y166C, causing loss of Fas binding with a dominant-negative effect in biochemical assays, did not impair cellular cytotoxicity or cause vitamin B₁₂ and DNT elevation.

CONCLUSION

Heterozygous loss-of-function mutations are better tolerated for FASLG than for FAS, which may explain the low frequency of ALPS-FASLG.

Therapeutic approaches targeting CD95L/CD95 signaling in cancer and autoimmune diseases

Cell death plays a pivotal role in the maintenance of tissue homeostasis. Key players in the controlled induction of cell death are the Death Receptors (DR). CD95 is a prototypic DR activated by its cognate ligand CD95L triggering programmed cell death. As a consequence, alterations in the CD95/CD95L pathway have been involved in several disease conditions ranging from autoimmune diseases to inflammation and cancer. CD95L-induced cell death has multiple roles in the immune response since it constitutes one of the mechanisms by which cytotoxic lymphocytes kill their targets, but it is also involved in the process of turning off the immune response. Furthermore, beyond the canonical pro-death signals, CD95L, which can be membrane-bound or soluble, also induces non-apoptotic signaling that contributes to its tumor-promoting and pro-inflammatory roles. The intent of this review is to describe the role of CD95/CD95L in the pathophysiology of cancers, autoimmune diseases and chronic inflammation and to discuss recently patented and emerging therapeutic strategies that exploit/block the CD95/CD95L system in these diseases.

The Autoimmune Lymphoproliferative Syndrome with Defective FAS or FAS-Ligand Functions

The autoimmune lymphoproliferative syndrome (ALPS) is a non-malignant and non-infectious uncontrolled proliferation of lymphocytes accompanied by autoimmune cytopenia. The genetic etiology of the ALPS was described in 1995 by the discovery of the FAS gene mutations. The related apoptosis defect accounts for the accumulation of autoreactive lymphocytes as well as for specific clinical and biological features that distinguish the ALPS-FAS from other monogenic defects of this apoptosis pathway, such as FADD and CASPASE 8 deficiencies. The ALPS-FAS was the first description of a monogenic cause of autoimmunity, but its non-Mendelian expression remained elusive until the description of somatic and germline mutations in ALPS patients. The recognition of these genetic diseases brought new information on the role of this apoptotic pathway in controlling the adaptive immune response in humans.

On How Fas Apoptosis-Independent Pathways Drive T Cell Hyperproliferation and Lymphadenopathy in lpr Mice

Fas induces massive apoptosis in T cells after repeated in vitro T cell receptor (TCR) stimulation and is critical for lymphocyte homeostasis in Fas-deficient (lpr) mice. Although the in vitro Fas apoptotic mechanism has been defined, there is a large conceptual gap between this in vitro phenomenon and the pathway that leads to in vivo development of lymphadenopathy and autoimmunity. A striking abnormality in lpr mice is the excessive proliferation of CD4⁺ and CD8⁺ T cells, and more so of the double-negative TCR⁺CD4⁻CD8⁻B220⁺ T cells. The basis of lpr T cell hyperproliferation remains elusive, as it cannot be explained by Fas-deficient apoptosis. T cell-directed p21 overexpression reduces hyperactivation/hyperproliferation of all lpr T cell subtypes and lymphadenopathy in lpr mice. p21 controls expansion of repeatedly stimulated T cells without affecting apoptosis. These results confirm a direct link between hyperactivation/hyperproliferation, autoreactivity, and lymphadenopathy in lpr mice and, with earlier studies, suggest that Fas apoptosis-independent pathways control lpr T cell hyperproliferation. lpr T cell hyperproliferation could be an indirect result of the defective apoptosis of repeatedly stimulated lpr T cells. Nonetheless, in this perspective, we argue for an alternative setting, in which lack of Fas would directly cause lpr T cell hyperactivation/hyperproliferation in vivo. We propose that Fas/Fas ligand (FasL) acts as an activation inhibitor of recurrently stimulated T cells, and that its disruption causes overexpansion of T cells in lpr mice. Research to define the underlying mechanism of this Fas/FasL effect could resolve the phenotype of lpr mice and lead to therapeutics for related human syndromes.

Selective expansion of allogeneic regulatory T cells by hepatic stellate cells: role of endotoxin and implications for allograft tolerance

Hepatic stellate cells (HSCs) may play an important role in hepatic immune regulation by producing numerous cytokines/chemokines and expressing Ag-presenting and T cell coregulatory molecules. Due to disruption of the endothelial barrier during cold-ischemic storage and reperfusion of liver grafts, HSCs can interact directly with cells of the immune system. Endotoxin (LPS), levels of which increase in liver diseases and transplantation, stimulates the synthesis of many mediators by HSCs. We hypothesized that LPS-stimulated HSCs might promote hepatic tolerogenicity by influencing naturally occurring immunosuppressive CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs). Following their portal venous infusion, allogeneic CD4(+) T cells, including Tregs, were found closely associated with HSCs, and this association increased in LPS-treated livers. In vitro, both unstimulated and LPS-stimulated HSCs upregulated Fas (CD95) expression on conventional CD4(+) T cells and induced their apoptosis in a Fas/Fas ligand-dependent manner. By contrast, HSCs induced Treg proliferation, which required cell-cell contact and was MHC class II-dependent. This effect was augmented when HSCs were pretreated with LPS. LPS increased the expression of MHC class II, CD80, and CD86 and stimulated the production of IL-1α, IL-1β, IL-6, IL-10 and TNF-α by HSCs. Interestingly, production of IL-1α, IL-1β, IL-6, and TNF-α was strongly inhibited, but that of IL-10 enhanced in LPS-pretreated HSC/Treg cocultures. Adoptively transferred allogeneic HSCs migrated to the secondary lymphoid tissues and induced Treg expansion in lymph nodes. These data implicate endotoxin-stimulated HSCs as important immune regulators in liver transplantation by inducing selective expansion of tolerance-promoting Tregs and reducing inflammation and alloimmunity.

Pro- and anti-apoptotic CD95 signaling in T cells

The TNF receptor superfamily member CD95 (Fas, APO-1, TNFRSF6) is known as the prototypic death receptor in and outside the immune system. In fact, many mechanisms involved in apoptotic signaling cascades were solved by addressing consequences and pathways initiated by CD95 ligation in activated T cells or other "CD95-sensitive" cell populations. As an example, the binding of the inducible CD95 ligand (CD95L) to CD95 on activated T lymphocytes results in apoptotic cell death. This activation-induced cell death was implicated in the control of immune cell homeostasis and immune response termination. Over the past years, however, it became evident that CD95 acts as a dual function receptor that also exerts anti-apoptotic effects depending on the cellular context. Early observations of a potential non-apoptotic role of CD95 in the growth control of resting T cells were recently reconsidered and revealed quite unexpected findings regarding the costimulatory capacity of CD95 for primary T cell activation. It turned out that CD95 engagement modulates TCR/CD3-driven signal initiation in a dose-dependent manner. High doses of immobilized CD95 agonists or cellular CD95L almost completely silence T cells by blocking early TCR-induced signaling events. In contrast, under otherwise unchanged conditions, lower amounts of the same agonists dramatically augment TCR/CD3-driven activation and proliferation. In the present overview, we summarize these recent findings with a focus on the costimulatory capacity of CD95 in primary T cells and discuss potential implications for the T cell compartment and the interplay between T cells and CD95L-expressing cells including antigen-presenting cells.

Immune modulation by Fas ligand reverse signaling: lymphocyte proliferation is attenuated by the intracellular Fas ligand domain

Fas ligand (FasL) not only induces apoptosis in Fas receptor-bearing target cells, it is also able to transmit signals into the FasL-expressing cell via its intracellular domain (ICD). Recently, we described a Notch-like proteolytic processing of FasL that leads to the release of the FasL ICD into the cytoplasm and subsequent translocation into the nucleus where it may influence gene transcription. To study the molecular mechanism underlying such reverse FasL signaling in detail and to analyze its physiological importance in vivo, we established a knockout/knockin mouse model, in which wild-type FasL was replaced with a deletion mutant lacking the ICD. Our results demonstrate that FasL ICD signaling impairs activation-induced proliferation in B and T cells by diminishing phosphorylation of phospholipase C γ, protein kinase C, and extracellular signal-regulated kinase 1/2. We also demonstrate that the FasL ICD interacts with the transcription factor lymphoid-enhancer binding factor-1 and inhibits lymphoid-enhancer binding factor-1–dependent transcription. In vivo, plasma cell numbers, generation of germinal center B cells, and, consequently, production of antigen-specific immunoglobulin M antibodies in response to immunization with T cell–dependent or T cell–independent antigen are negatively affected in presence of the FasL ICD, suggesting that FasL reverse signaling participates in negative fine-tuning of certain immune responses.

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.

Modulation of CD4+ T-cell activation by CD95 co-stimulation

CD95 is a dual-function receptor that exerts pro- or antiapoptotic effects depending on the cellular context, the state of activation, the signal threshold and the mode of ligation. In this study, we report that CD95 engagement modulates TCR/CD3-driven signaling pathways in resting T lymphocytes in a dose-dependent manner. While high doses of immobilized CD95 agonists silence T cells, lower concentrations augment activation and proliferation. We analyzed the co-stimulatory capacity of CD95 in detail in resting human CD4(+) T cells, and demonstrate that low-dose ligand-induced co-internalization of CD95 and TCR/CD3 complexes enables non-apoptotic caspase activation, the prolonged activation of MAP kinases, the upregulation of antiapoptotic proteins associated with apoptosis resistance, and the activation of transcription factors and cell-cycle regulators for the induction of proliferation and cytokine production. We propose that the levels of CD95L on antigen-presenting cells (APCs), neighboring T cells or epithelial cells regulate inhibitory or co-stimulatory CD95 signaling, which in turn is crucial for fine-tuning of primary T-cell activation.

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.