Antigen-mediated T cell expansion regulated by parallel pathways of death

RIPK1 ablation in T cells results in spontaneous enteropathy and TNF-driven villus atrophy

RIPK1 is a crucial regulator of cell survival, inflammation and cell death. Human RIPK1 deficiency leads to early-onset intestinal inflammation and peripheral T cell imbalance, though its role in αβT cell-mediated intestinal homeostasis remains unclear. In this study, we demonstrate that mice with RIPK1 ablation in conventional αβT cells (Ripk1ΔCD4) developed a severe small intestinal pathology characterized by small intestinal elongation, crypt hyperplasia, and duodenum-specific villus atrophy. Using mixed bone marrow chimeras reveals a survival disadvantage of αβT cells compared to γδT cells in the small intestine. Broad-spectrum antibiotic treatment ameliorates crypt hyperplasia and prevents intestinal elongation, though villus atrophy persists. Conversely, crossing Ripk1ΔCD4 with TNF receptor 1 Tnfr1-/- knockout mice rescues villus atrophy but not intestinal elongation. Finally, combined ablation of Ripk1∆CD4 and Casp8∆CD4 fully rescues intestinal pathology, revealing that αβT cell apoptosis in Ripk1∆CD4 drives the enteropathy. These findings demonstrate that RIPK1-mediated survival of αβT cells is essential for proximal small intestinal homeostasis. In Ripk1∆CD4 mice, the imbalanced T cell compartment drives microbiome-mediated intestinal elongation and TNF-driven villus atrophy.

NF-κB regulated expression of A20 controls IKK dependent repression of RIPK1 induced cell death in activated T cells

IKK signalling is essential for survival of thymocytes by repressing RIPK1 induced cell death rather than its canonical function of activating NF-κB. The role of IKK signalling in activated T cells is unclear. To investigate this, we analysed activation of IKK2 deficient T cells. While TCR triggering was normal, proliferation and expansion was profoundly impaired. This was not due to defective cell cycle progression, rather dividing T cells became sensitised to TNF induced cell death, since inhibition of RIPK1 kinase activity rescued cell survival. Gene expression analysis of activated IKK2 deficient T cells revealed defective expression of Tnfaip3, that encodes A20, a negative regulator of NF-κB. To test whether A20 expression was required to protect IKK2 deficient T cells from cell death, we generated mice with T cells lacking both A20 and IKK2. Doing this resulted in near complete loss of peripheral T cells, in contrast to mice lacking one or other gene. Strikingly, this phenotype was completely reversed by inactivation of RIPK1 kinase activity in vivo. Together, our data show that IKK signalling in activated T cells protects against RIPK1 dependent death, both by direct phosphorylation of RIPK1 and through NF-κB mediated induction of A20, that we identify for the first time as a key modulator of RIPK1 activity in T cells.

RIPK1 protects naive and regulatory T cells from TNFR1-induced apoptosis

The T cell population size is stringently controlled before, during, and after immune responses, as improper cell death regulation can result in autoimmunity and immunodeficiency. RIPK1 is an important regulator of peripheral T cell survival and homeostasis. However, whether different peripheral T cell subsets show a differential requirement for RIPK1 and which programmed cell death pathway they engage in vivo remains unclear. In this study, we demonstrate that conditional ablation of Ripk1 in conventional T cells (Ripk1ΔCD4) causes peripheral T cell lymphopenia, as witnessed by a profound loss of naive CD4+, naive CD8+, and FoxP3+ regulatory T cells. Interestingly, peripheral naive CD8+ T cells in Ripk1ΔCD4 mice appear to undergo a selective pressure to retain RIPK1 expression following activation. Mixed bone marrow chimeras revealed a competitive survival disadvantage for naive, effector, and memory T cells lacking RIPK1. Additionally, tamoxifen-induced deletion of RIPK1 in CD4-expressing cells in adult life confirmed the importance of RIPK1 in post-thymic survival of CD4+ T cells. Ripk1K45A mice showed no change in peripheral T cell subsets, demonstrating that the T cell lymphopenia was due to the scaffold function of RIPK1 rather than to its kinase activity. Enhanced numbers of Ripk1ΔCD4 naive T cells expressed the proliferation marker Ki-67+ despite the peripheral lymphopenia and single-cell RNA sequencing revealed T cell-specific transcriptomic alterations that were reverted by additional caspase-8 deficiency. Furthermore, Ripk1ΔCD4Casp8 ΔCD4 and Ripk1ΔCD4Tnfr1-/- double-knockout mice rescued the peripheral T cell lymphopenia, revealing that RIPK1-deficient naive CD4+ and CD8+ cells and FoxP3+ regulatory T cells specifically die from TNF- and caspase-8-mediated apoptosis in vivo. Altogether, our findings emphasize the essential role of RIPK1 as a scaffold in maintaining the peripheral T cell compartment and preventing TNFR1-induced apoptosis.

The role of caspase-8 in inflammatory signalling and pyroptotic cell death

The programmed cell death machinery exhibits surprising flexibility, capable of crosstalk and non-apoptotic roles. Much of this complexity arises from the diverse functions of caspase-8, a cysteine-aspartic acid protease typically associated with activating caspase-3 and - 7 to induce apoptosis. However, recent research has revealed that caspase-8 also plays a role in regulating the lytic gasdermin cell death machinery, contributing to pyroptosis and immune responses in contexts such as infection, autoinflammation, and T-cell signalling. In mice, loss of caspase-8 results in embryonic lethality from unrestrained necroptotic killing, while in humans caspase-8 deficiency can lead to an autoimmune lymphoproliferative syndrome, immunodeficiency, inflammatory bowel disease or, when it can't cleave its substrate RIPK1, early onset periodic fevers. This review focuses on non-canonical caspase-8 signalling that drives immune responses, including its regulation of inflammatory gene transcription, activation within inflammasome complexes, and roles in pyroptotic cell death. Ultimately, a deeper understanding of caspase-8 function will aid in determining whether, and when, targeting caspase-8 pathways could be therapeutically beneficial in human diseases.

Comprehensive analysis of necroptosis-related genes in renal ischemia-reperfusion injury

Background

Oxidative stress is the primary cause of ischemia-reperfusion injury (IRI) in kidney transplantation, leading to delayed graft function (DGF) and implications on patient health. Necroptosis is believed to play a role in renal IRI. This research presents a comprehensive analysis of necroptosis-related genes and their functional implications in the context of IRI in renal transplantation.

Methods

The necroptosis-related differentially expressed genes (NR-DEGs) were identified using gene expression data from pre- and post-reperfusion renal biopsies, and consensus clustering analysis was performed to distinguish necroptosis-related clusters. A predictive model for DGF was developed based on the NR-DEGs and patients were divided into high- and low-risk groups. We investigated the differences in functional enrichment and immune infiltration between different clusters and risk groups and further validated them in single-cell RNA-sequencing (scRNA-seq) data. Finally, we verified the expression changes of NR-DEGs in an IRI mouse model.

Results

Five NR-DEGs were identified and were involved in various biological processes. The renal samples were further stratified into two necroptosis-related clusters (C1 and C2) showing different occurrences of DGF. The predictive model had a reliable performance in identifying patients at higher risk of DGF with the area under the curve as 0.798. Additionally, immune infiltration analysis indicated more abundant proinflammatory cells in the high-risk group, which was also found in C2 cluster with more DGF patients. Validation of NR-DEG in scRNA-seq data further supported their involvement in immune cells. Lastly, the mouse model validated the up-regulation of NR-DEGs after IR and indicated the correlations with kidney function markers.

Conclusions

Our research provides valuable insights into the identification and functional characterization of NR-DEGs in the context of renal transplantation and sheds light on their involvement in immune responses and the progression of IRI and DGF.

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

Bioluminescent RIPoptosome Assay for FADD/RIPK1 Interaction Based on Split Luciferase Assay in a Human Neuroblastoma Cell Line SH-SY5Y

Different programed cell death (PCD) modalities involve protein-protein interactions in large complexes. Tumor necrosis factor α (TNFα) stimulated assembly of receptor-interacting protein kinase 1 (RIPK1)/Fas-associated death domain (FADD) interaction forms Ripoptosome complex that may cause either apoptosis or necroptosis. The present study addresses the interaction of RIPK1 and FADD in TNFα signaling by fusion of C-terminal (CLuc) and N-terminal (NLuc) luciferase fragments to RIPK1-CLuc (R1C) or FADD-NLuc (FN) in a caspase 8 negative neuroblastic SH-SY5Y cell line, respectively. In addition, based on our findings, an RIPK1 mutant (R1C K612R) had less interaction with FN, resulting in increasing cell viability. Moreover, presence of a caspase inhibitor (zVAD.fmk) increases luciferase activity compared to Smac mimetic BV6 (B), TNFα -induced (T) and non-induced cell. Furthermore, etoposide decreased luciferase activity, but dexamethasone was not effective in SH-SY5Y. This reporter assay might be used to evaluate basic aspects of this interaction as well as for screening of necroptosis and apoptosis targeting drugs with potential therapeutic application.

Caspase Inhibition Modulates Monocyte-Derived Macrophage Polarization in Damaged Tissues

Circulating monocytes are recruited in damaged tissues to generate macrophages that modulate disease progression. Colony-stimulating factor-1 (CSF-1) promotes the generation of monocyte-derived macrophages, which involves caspase activation. Here, we demonstrate that activated caspase-3 and caspase-7 are located to the vicinity of the mitochondria in CSF1-treated human monocytes. Active caspase-7 cleaves p47^PHOX at aspartate 34, which promotes the formation of the NADPH (nicotinamide adenine dinucleotide phosphate) oxidase complex NOX2 and the production of cytosolic superoxide anions. Monocyte response to CSF-1 is altered in patients with a chronic granulomatous disease, which are constitutively defective in NOX2. Both caspase-7 down-regulation and radical oxygen species scavenging decrease the migration of CSF-1-induced macrophages. Inhibition or deletion of caspases prevents the development of lung fibrosis in mice exposed to bleomycin. Altogether, a non-conventional pathway that involves caspases and activates NOX2 is involved in CSF1-driven monocyte differentiation and could be therapeutically targeted to modulate macrophage polarization in damaged tissues.

Necroptosis in inflammatory bowel disease: A potential effective target

The morbidity of inflammatory bowel diseases (IBD) is rising rapidly but no curative therapies to prevent its recurrence. Cell death is crucial to maintaining homeostasis. Necroptosis is a newly identified programmed cell death and its roles played in IBD need to be explored. Necroptosis is mediated by receptor interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL), which resulted in cell swelling, plasma membrane rupture, intracellular content leaking, and eventually cell death as well as the promotion of inflammation. Studies have found that inhibiting necroptosis alleviated IBD in animal models and IBD patients with an increased level of necroptosis in inflammatory tissues, indicating that necroptosis is related to the pathogenesis of IBD. However, due to the complexity in regulation of necroptosis and the involvement of multiple functions of relevant signaling molecules, the specific mechanism remains elusive. Necroptosis may play a vital regulatory role in the pathogenesis of IBD, which provides a new idea and method for further exploring the therapeutic target of IBD.

Caspase-8: Friend or Foe in Bortezomib/Lenalidomide-Based Therapy for Myeloma

Antiproliferation and proapoptosis are two major molecular mechanisms of action of drugs used for the treatment of multiple myeloma. Proteasome inhibitors, such as bortezomib (PS-341), and immunomodulatory drugs (IMiDs), such as lenalidomide, are the two drug types approved for the treatment of myeloma. Bortezomib and lenalidomide activate caspase-8 and promote the apoptosis of myeloma cells. However, caspase-8 inhibition potentiated the antiproliferative effect of lenalidomide and bortezomib in myeloma cells, suggesting that caspase-8 could regulate proliferation and apoptosis in the opposite pathway. In this mini-review, I summarized recent advances in determining the molecular mechanisms of caspase-8 in bortezomib–lenalidomide-based therapy for myeloma and explored the possible functions of caspase-8 in the proliferation and apoptosis of myeloma cells. Furthermore, future directions of caspase-8-based therapy for myeloma have been discussed.

Inflammatory cell death induced by cytotoxic lymphocytes: a dangerous but necessary liaison

Cytotoxic lymphocytes (CLs), and more specifically Tc and NK cells, are the main executors of cell death in the immune system, playing a key role during both immunosurveillance and immunotherapy. These cells induce regulated cell death (RCD) by different mechanisms, being granular exocytosis and expression of death ligands the most prominent and best characterized ones. Apoptosis, a traditionally considered low-inflammatory type of cell death, has been accepted for years as the paradigm of RCD induced by CLs. However, several recent studies have demonstrated that NK cells and Tc cells can also induce more inflammatory forms of cell death, namely, necroptosis, pyroptosis, and ferroptosis. Activation of these highly inflammatory types of cell death appears to critically contribute to the activation of a successful antitumour immune response. Additionally, the role of specific cell death pathways in immunogenic cell death is still under intense debate, especially considering the interconnections with other inflammatory forms of cell death. These evidences, together with the advent of new cancer immunotherapies, highlight the necessity to deepen our understanding of the link between the cell death triggered by CLs and inflammation. This knowledge will be instrumental to maximize the antitumour potential of immunotherapies, minimizing deleterious effects associated with these treatments. In this review, we will briefly summarize the main features of apoptosis, necroptosis, pyroptosis and ferroptosis, to subsequently discuss the most recent evidences about the role of these RCD pathways during the elimination of cancer cells mediated by CLs and its modulation to increase the efficacy of cancer immunotherapy.

Ethanol induces necroptosis in gastric epithelial cells in vitro

The stomach frequently suffers from acute gastric diseases after excessive ingestion of high-concentration alcoholic beverages, but little is known about the pathological mechanism by which ethanol affects the gastric mucosa. The aim of this study was to explore the mechanism of gastric epithelial cell death induced by relatively high concentrations of ethanol in vitro. Ethanol was demonstrated to induce rapid cell death in a concentration-dependent manner (Spearman r = .943, p = .017) and to activate the phosphorylation of key mediators in necroptosis pathway without influencing the key mediators in apoptosis pathway. The receptor-interacting serine-threonine kinase 1 (RIP1) kinase inhibitor necrostatin-1s (nec-1s) was found to reverse necroptotic cell death (from 65.5% necrosis to 35.8% necrosis, p = .006) and to inhibit the formation of necrosome complexes. These results indicate necroptosis rather than apoptosis pathway is an essential mechanism and is a novel therapeutic target in acute alcoholic gastric diseases. PRACTICAL APPLICATIONS: Alcohol consumption is related with a variety of diseases in many organs, but its pathological mechanism might be quite different due to the exposure extent between the stomach and other organs. Although there have been plenty of studies on alcoholic liver diseases and those in other organs, the pathological mechanism of alcoholic gastric diseases has been poorly investigated. Considering the unique distribution of ethanol on gastric mucosa, it is worthwhile to explore the specific cell death pattern of gastric epithelial cells under high-concentration ethanol treatment. Further investigation of the mechanisms of alcoholic gastric diseases would provide potential therapeutic strategies for the treatment of acute alcoholic gastric diseases as well as other acute alcoholic diseases.

Caspase-Independent Regulated Necrosis Pathways as Potential Targets in Cancer Management

Regulated necrosis is an emerging type of cell death independent of caspase. Recently, with increasing findings of regulated necrosis in the field of biochemistry and genetics, the underlying molecular mechanisms and signaling pathways of regulated necrosis are gradually understood. Nowadays, there are several modes of regulated necrosis that are tightly related to cancer initiation and development, including necroptosis, ferroptosis, parthanatos, pyroptosis, and so on. What’s more, accumulating evidence shows that various compounds can exhibit the anti-cancer effect via inducing regulated necrosis in cancer cells, which indicates that caspase-independent regulated necrosis pathways are potential targets in cancer management. In this review, we expand the molecular mechanisms as well as signaling pathways of multiple modes of regulated necrosis. We also elaborate on the roles they play in tumorigenesis and discuss how each of the regulated necrosis pathways could be therapeutically targeted.

NF-κB and Extrinsic Cell Death Pathways - Entwined Do-or-Die Decisions for T cells

NF-κB signaling is required at multiple stages of T cell development and function. The NF-κB pathway integrates signals from many receptors and involves diverse adapters and kinases. Recent advances demonstrate that kinases controlling NF-κB activation, such as the IKK complex, serve dual independent functions because they also control cell death checkpoints. Survival functions previously attributed to NF-κB are in fact mediated by these upstream kinases by novel mechanisms. This new understanding has led to a refined view of how NF-κB and cell death signaling are interlinked and how they regulate cell fate. We discuss how NF-κB activation and control of cell death signaling by common upstream triggers cooperate to regulate different aspects of T cell development and function.

Necroptotic-susceptible dendritic cells exhibit enhanced antitumor activities in mice

INTRODUCTION

Priming of tumor-specific T cells is a key to antitumor immune response and inflammation, in turn, is crucial for proper T-cell activation. As antigen-presenting cells can activate T cells, dendritic cells (DCs) loaded with tumor antigens have been used as immunotherapeutics against certain cancer in humans but their efficacy is modest. Necroptosis is a form of programmed cell death that results in the release of inflammatory contents. We previously generated mice with DC deficiency in a negative regulator of necroptosis, Fas-associated death domain (FADD), and found that these mice suffer from systemic inflammation due to necroptotic DCs. We hypothesize that FADD-deficient DCs could serve as a better vaccine than wild-type (WT) DCs against tumors.

MATERIALS AND METHODS

FADD-deficient and WT mouse DCs loaded with the relevant tumor peptide were injected onto mice before or after the syngeneic tumor challenge. DC vaccinations were repeated two more times and anti-PD-1 antibodies were coinjected in some experiments. Tumor sizes were measured by caliper, and the percentages of tumor-free mice or mice survived were examined over time. The cytometric analysis was carried out to analyze various immune populations.

RESULTS

In two separate tumor models, we find that mice receiving FADD-deficient DCs as vaccine rejected tumors significantly better than those receiving a WT DC vaccine. Tumor growth was severely hampered, and survival extended in these mice. More activated CD8 T cells together with elevated cytokines were observed in mice receiving the FADD-deficient DC vaccine. Furthermore, we observed these effects were potent enough to protect against tumor challenge postinjection and can work in conjunction with anti-PD-1 antibodies to reduce the tumor growth.

CONCLUSIONS

Necroptotic-susceptible DCs are better antitumor vaccines than WT DCs in mice. Our findings suggest that necroptosis-driven inflammation by DCs may be a novel avenue to generating a strong adaptive antitumor response in the clinical setting.

Necroptosis, ADAM proteases and intestinal (dys)function

Recently, an unexpected connection between necroptosis and members of the a disintegrin and metalloproteinase (ADAM) protease family has been reported. Necroptosis represents an important cell death routine which helps to protect from viral, bacterial, fungal and parasitic infections, maintains adult T cell homeostasis and contributes to the elimination of potentially defective organisms before parturition. Equally important for organismal homeostasis, ADAM proteases control cellular processes such as development and differentiation, immune responses or tissue regeneration. Notably, necroptosis as well as ADAM proteases have been implicated in the control of inflammatory responses in the intestine. In this review, we therefore provide an overview of the physiology and pathophysiology of necroptosis, ADAM proteases and intestinal (dys)function, discuss the contribution of necroptosis and ADAMs to intestinal (dys)function, and review the current knowledge on the role of ADAMs in necroptotic signaling.

Multitasking Kinase RIPK1 Regulates Cell Death and Inflammation

Receptor-interacting serine threonine kinase 1 (RIPK1) is a widely expressed kinase that is essential for limiting inflammation in both mice and humans. Mice lacking RIPK1 die at birth from multiorgan inflammation and aberrant cell death, whereas humans lacking RIPK1 are immunodeficient and develop very early-onset inflammatory bowel disease. In contrast to complete loss of RIPK1, inhibiting the kinase activity of RIPK1 genetically or pharmacologically prevents cell death and inflammation in several mouse disease models. Indeed, small molecule inhibitors of RIPK1 are in phase I clinical trials for amyotrophic lateral sclerosis, and phase II clinical trials for psoriasis, rheumatoid arthritis, and ulcerative colitis. This review focuses on which signaling pathways use RIPK1, how activation of RIPK1 is regulated, and when activation of RIPK1 appears to be an important driver of inflammation.

A Novel Microglia-Specific Transcriptional Signature Correlates With Behavioral Deficits in Neuropsychiatric Lupus

Neuropsychiatric symptoms of systemic lupus erythematosus (NP-SLE) affect over one-half of SLE patients, yet underlying mechanisms remain largely unknown. We demonstrate that SLE-prone mice (CReCOM) develop NP-SLE, including behavioral deficits prior to systemic autoimmunity, reduced brain volumes, decreased vascular integrity, and brain-infiltrating leukocytes. NP-SLE microglia exhibit numerical expansion, increased synaptic uptake, and a more metabolically active phenotype. Microglia from multiple SLE-prone models express a "NP-SLE signature" unrelated to type I interferon. Rather, the signature is associated with lipid metabolism, scavenger receptor activity and downregulation of inflammatory and chemotaxis processes, suggesting a more regulatory, anti-inflammatory profile. NP-SLE microglia also express genes associated with disease-associated microglia (DAM), a subset of microglia thought to be instrumental in neurodegenerative diseases. Further, expression of "NP-SLE" and "DAM" signatures correlate with the severity of behavioral deficits in young SLE-prone mice prior to overt systemic disease. Our data are the first to demonstrate the predictive value of our newly identified microglia-specific "NP-SLE" and "DAM" signatures as a surrogate for NP-SLE clinical outcomes and suggests that microglia-intrinsic defects precede contributions from systemic SLE for neuropsychiatric manifestations.

Caspase-8 restricts antiviral CD8 T cell hyperaccumulation

The magnitude of CD8 T cell responses against viruses is checked by the balance of proliferation and death. Caspase-8 (CASP8) has the potential to influence response characteristics through initiation of apoptosis, suppression of necroptosis, and modulation of cell death-independent signal transduction. Mice deficient in CASP8 and RIPK3 (Casp8 ^-/- Ripk3 ^-/- ) mount enhanced peak CD8 T cell levels against the natural mouse pathogen murine cytomegalovirus (MCMV) or the human pathogen herpes simplex virus-1 compared with littermate control RIPK3-deficient or WT C57BL/6 mice, suggesting an impact of CASP8 on the magnitude of antiviral CD8 T cell expansion and not on contraction. The higher peak response to MCMV in Casp8 ^-/- Ripk3 ^-/- mice resulted from accumulation of greater numbers of terminally differentiated KLRG1^hi effector CD8 T cell subsets. Antiviral Casp8 ^-/- Ripk3 ^-/- T cells exhibited enhanced proliferation when splenocytes were transferred into WT recipient mice. Thus, cell-autonomous CASP8 normally restricts CD8 T cell proliferation following T cell receptor activation in response to foreign antigen. Memory inflation is a hallmark quality of the T cell response to cytomegalovirus infection. Surprisingly, MCMV-specific memory inflation was not sustained long-term in Casp8 ^-/- Ripk3 ^-/- mice even though these mice retained immunity to secondary challenge. In addition, the accumulation of abnormal B220⁺CD3⁺ T cells in these viable CASP8-deficient mice was reduced by chronic MCMV infection. Combined, these data brings to light the cell death-independent role of CASP8 during CD8 T cell expansion in mice lacking the confounding impact of RIPK3-mediated necroptosis.

Caspase-8 restricts natural killer cell accumulation during MCMV Infection

Natural killer (NK) cells provide important host defense against herpesvirus infections and influence subsequent T cell control of replication and maintenance of latency. NK cells exhibit phases of expansion, contraction and memory formation in response to the natural mouse pathogen murine cytomegalovirus (MCMV). Innate and adaptive immune responses are tightly regulated in mammals to avoid excess tissue damage while preventing acute and chronic viral disease and assuring resistance to reinfection. Caspase (CASP)8 is an autoactivating aspartate-specific cysteine protease that initiates extrinsic apoptosis and prevents receptor interacting protein (RIP) kinase (RIPK)1-RIPK3-driven necroptosis. CASP8 also promotes death-independent signal transduction. All of these activities make contributions to inflammation. Here, we demonstrate that CASP8 restricts NK cell expansion during MCMV infection but does not influence NK memory. Casp8^-/-Ripk3^-/- mice mount higher NK response levels than Casp8^+/-Ripk3^-/- littermate controls or WT C57BL/6 J mice, indicating that RIPK3 deficiency alone does not contribute to NK response patterns. MCMV m157-responsive Ly49H⁺ NK cells support increased expansion of both Ly49H^- NK cells and CD8 T cells in Casp8^-/-Ripk3^-/- mice. Surprisingly, hyperaccumulation of NK cells depends on the pronecrotic kinase RIPK1. Ripk1^-/-Casp8^-/-Ripk3^-/- mice fail to show the enhanced expansion of lymphocytes observed in Casp8^-/-Ripk3^-/- mice even though development and homeostasis are preserved in uninfected Ripk1^-/-Casp8^-/-Ripk3^-/- mice. Thus, CASP8 naturally regulates the magnitude of NK cell responses in response to infection where strong activation signals depend on another key regulator of death signaling, RIPK1. In addition, the strong NK cell response promotes survival of effector CD8 T cells during their expansion. Thus, hyperaccumulation of NK cells and crosstalk with T cells becomes amplified in the absence of extrinsic cell death machinery.

Caspase-8-dependent control of NK- and T cell responses during cytomegalovirus infection

Caspase-8 (CASP8) impacts antiviral immunity in expected as well as unexpected ways. Mice with combined deficiency in CASP8 and RIPK3 cannot support extrinsic apoptosis or RIPK3-dependent programmed necrosis, enabling studies of CASP8 function without complications of unleashed necroptosis. These extrinsic cell death pathways are naturally targeted by murine cytomegalovirus (MCMV)-encoded cell death suppressors, showing they are key to cell-autonomous host defense. Remarkably, Casp8^-/-Ripk3^-/-, Ripk1^-/-Casp8^-/-Ripk3^-/- and Casp8^-/-Ripk3^K51A/K51A mice mount robust antiviral T cell responses to control MCMV infection. Studies in Casp8^-/-Ripk3^-/- mice show that CASP8 restrains expansion of MCMV-specific natural killer (NK) and CD8 T cells without compromising contraction or immune memory. Infected Casp8^-/-Ripk3^-/- or Casp8^-/-Ripk3^K51A/K51A mice have higher levels of virus-specific NK cells and CD8 T cells compared to matched RIPK3-deficient littermates or WT mice. CASP8, likely acting downstream of Fas death receptor, dampens proliferation of CD8 T cells during expansion. Importantly, contraction proceeds unimpaired in the absence of extrinsic death pathways owing to intact Bim-dependent (intrinsic) apoptosis. CD8 T cell memory develops in Casp8^-/-Ripk3^-/- mice, but memory inflation characteristic of MCMV infection is not sustained in the absence of CASP8 function. Despite this, Casp8^-/-Ripk3^-/- mice are immune to secondary challenge. Interferon (IFN)γ is recognized as a key cytokine for adaptive immune control of MCMV. Ifngr^-/-Casp8^-/-Ripk3^-/- mice exhibit increased lifelong persistence in salivary glands as well as lungs compared to Ifngr^-/- and Casp8^-/-Ripk3^-/- mice. Thus, mice deficient in CASP8 and RIPK3 are more dependent on IFNγ mechanisms for sustained T cell immune control of MCMV. Overall, appropriate NK- and T cell immunity to MCMV is dependent on host CASP8 function independent of RIPK3-regulated pathways.

Select sequencing of clonally expanded CD8+ T cells reveals limits to clonal expansion

T cells are a central component of our immune system. They express a T cell receptor (TCR) on their surface, which detects pathogens and stimulates the T cell to initiate an immune response. Upon exposure to a specific pathogen, some T cells are activated and clonally expand. These pathogen-specific clonally expanded T cells, however, are generally rare and difficult to isolate. We have developed a technology (SELECT-seq) to isolate this rare population and to analyze their transcriptome and TCR composition. Therefore, our work may be the key to achieving a better understanding of pathogen-specific T cell clonality and function.

To permit the recognition of antigens, T cells generate a vast diversity of T cell receptor (TCR) sequences. Upon binding of the TCR to an antigen–MHC complex, T cells clonally expand to establish an immune response. To study antigen-specific T cell clonality, we have developed a method that allows selection of rare cells, based on RNA expression, before in-depth scRNA-seq (named SELECT-seq). We applied SELECT-seq to collect both TCR sequences and then transcriptomes from single cells of peripheral blood lymphocytes activated by a Mycobacterium tuberculosis (Mtb) lysate. TCR sequence analysis allowed us to preferentially select expanded conventional CD8⁺ T cells as well as invariant natural killer T (iNKT) cells and mucosal-associated invariant T (MAIT) cells. The iNKT and MAIT cells have a highly similar transcriptional pattern, indicating that they carry out similar immunological functions and differ considerably from conventional CD8⁺ T cells. While there is no relationship between expression profiles and clonal expansion in iNKT or MAIT cells, highly expanded conventional CD8⁺ T cells down-regulate the interleukin 2 (IL-2) receptor alpha (IL2RA, or CD25) protein and show signs of senescence. This suggests inherent limits to clonal expansion that act to diversify the T cell response repertoire.

Programmed Necrosis and Disease:We interrupt your regular programming to bring you necroinflammation

Compared to the tidy and immunologically silent death during apoptosis, necrosis seems like a chaotic and unorganized demise. However, we now recognize that there is a method to its madness, as many forms of necrotic cell death are indeed programmed and function beyond lytic cell death to support homeostasis and immunity. Inherently more immunogenic than their apoptotic counterpart, programmed necrosis, such as necroptosis, pyroptosis, ferroptosis, and NETosis, releases inflammatory cytokines and danger-associated molecular patterns (DAMPs), skewing the milieu to a pro-inflammatory state. Moreover, impaired clearance of dead cells often leads to inflammation. Importantly, these pathways have all been implicated in inflammatory and autoimmune diseases, therefore careful understanding of their molecular mechanisms can have long lasting effects on how we interpret their role in disease and how we translate these mechanisms into therapy.

Remarkably Robust Antiviral Immune Response despite Combined Deficiency in Caspase-8 and RIPK3

Caspase-8 (Casp8)-mediated signaling triggers extrinsic apoptosis while suppressing receptor-interacting protein kinase (RIPK) 3-dependent necroptosis. Although Casp8 is dispensable for the development of innate and adaptive immune compartments in mice, the importance of this proapoptotic protease in the orchestration of immune response to pathogens remains to be fully explored. In this study, Casp8^-/-Ripk3^-/- C57BL/6 mice show robust innate and adaptive immune responses to the natural mouse pathogen, murine CMV. When young, these mice lack lpr-like lymphoid hyperplasia and accumulation of either B220 ⁺ CD3⁺ or B220^-CD3⁺CD4⁺ and CD8⁺ T cells with increased numbers of immature myeloid cells that are evident in older mice. Dendritic cell activation and cytokine production drive both NK and T cell responses to control viral infection in these mice, suggesting that Casp8 is dispensable to the generation of antiviral host defense. Curiously, NK and T cell expansion is amplified, with greater numbers observed by 7 d postinfection compared with either Casp8^+/-Ripk3^-/- or wild type (Casp8^+/+Ripk3^+/+ ) littermate controls. Casp8 and RIPK3 are natural targets of virus-encoded cell death suppressors that prevent infected cell apoptosis and necroptosis, respectively. It is clear from the current studies that the initiation of innate immunity and the execution of cytotoxic lymphocyte functions are all preserved despite the absence of Casp8 in responding cells. Thus, Casp8 and RIPK3 signaling is completely dispensable to the generation of immunity against this natural herpesvirus infection, although the pathways driven by these initiators serve as a crucial first line for host defense within virus-infected cells.

Necroptosis in development and diseases

This review by Shan et al. discusses necroptosis, a form of regulated necrotic cell death mediated by RIPK1 kinase activity, RIPK3, and MLKL, which can be activated under apoptosis-deficient conditions. Both necroptosis and apoptosis can be activated in response to various mutations that result in the abortion of defective embryos and during human inflammatory and neurodegenerative pathologies.

Necroptosis, a form of regulated necrotic cell death mediated by RIPK1 (receptor-interacting protein kinase 1) kinase activity, RIPK3, and MLKL (mixed-lineage kinase domain-like pseudokinase), can be activated under apoptosis-deficient conditions. Modulating the activation of RIPK1 by ubiquitination and phosphorylation is critical to control both necroptosis and apoptosis. Mutant mice with kinase-dead RIPK1 or RIPK3 and MLKL deficiency show no detrimental phenotype in regard to development and adult homeostasis. However, necroptosis and apoptosis can be activated in response to various mutations that result in the abortion of the defective embryos and human inflammatory and neurodegenerative pathologies. RIPK1 inhibition represents a key therapeutic strategy for treatment of diseases where blocking both necroptosis and apoptosis can be beneficial.

Necroptosis in tumorigenesis, activation of anti-tumor immunity, and cancer therapy

While the mechanisms underlying apoptosis and autophagy have been well characterized over recent decades, another regulated cell death event, necroptosis, remains poorly understood. Elucidating the signaling networks involved in the regulation of necroptosis may allow this form of regulated cell death to be exploited for diagnosis and treatment of cancer, and will contribute to the understanding of the complex tumor microenvironment. In this review, we have summarized the mechanisms and regulation of necroptosis, the converging and diverging features of necroptosis in tumorigenesis, activation of anti-tumor immunity, and cancer therapy, as well as attempts to exploit this newly gained knowledge to provide therapeutics for cancer.

The in vivo evidence for regulated necrosis

Necrosis is a hallmark of several widespread diseases or their direct complications. In the past decade, we learned that necrosis can be a regulated process that is potentially druggable. RIPK3- and MLKL-mediated necroptosis represents by far the best studied pathway of regulated necrosis. During necroptosis, the release of damage-associated molecular patterns (DAMPs) drives a phenomenon referred to as necroinflammation, a common consequence of necrosis. However, most studies of regulated necrosis investigated cell lines in vitro in a cell autonomous manner, which represents a non-physiological situation. Conclusions based on such work might not necessarily be transferrable to disease states in which synchronized, non-cell autonomous effects occur. Here, we summarize the current knowledge of the pathophysiological relevance of necroptosis in vivo, and in light of this understanding, we reassess the morphological classification of necrosis that is generally used by pathologists. Along these lines, we discuss the paucity of data implicating necroptosis in human disease. Finally, the in vivo relevance of non-necroptotic forms of necrosis, such as ferroptosis, is addressed.

The Effects of Dendritic Cell Hypersensitivity on Persistent Viral Infection

Caspase-8 (CASP8) is known as an executioner of apoptosis, but more recent studies have shown that it participates in the regulation of necroptosis and innate immunity. In this study, we show that CASP8 negatively regulates retinoic acid-inducible gene I (RIG-I) signaling such that, in its absence, stimulation of the RIG-I pathway in dendritic cells (DCs) produced modestly enhanced activation of IFN regulatory factor 3 with correspondingly greater amounts of proinflammatory cytokines. In addition, mice lacking DC-specific CASP8 (dcCasp8^-/- mice) develop age-dependent symptoms of autoimmune disease characterized by hyperactive DCs and T cells, spleen and liver immunopathology, and the appearance of Th1-polarized CD4⁺ T cells. Such mice infected with chronic lymphocytic choriomeningitis virus, an RNA virus detected by RIG-I, mounted an enhanced lymphocytic choriomeningitis virus-specific immune response as measured by increased proportions of Ag-specific CD4⁺ T cells and multicytokine-producing CD4⁺ and CD8⁺ T cells. These results show that CASP8 subtly modulates DC maturation, which controls the spontaneous appearance of autoimmune T cells while simultaneously attenuating the acquired immune system and its potential to control a persistent viral infection.

Activation of concurrent apoptosis and necroptosis by SMAC mimetics for the treatment of refractory and relapsed ALL

More precise treatment strategies are urgently needed to decrease toxicity and improve outcomes for treatment-refractory leukemia. We used ex vivo drug response profiling of high-risk, relapsed, or refractory acute lymphoblastic leukemia (ALL) cases and identified a subset with exquisite sensitivity to small-molecule mimetics of the second mitochondria-derived activator of caspases (SMAC) protein. Potent ex vivo activity of the SMAC mimetic (SM) birinapant correlated with marked in vivo antileukemic effects, as indicated by delayed engraftment, decreased leukemia burden, and prolonged survival of xenografted mice. Antileukemic activity was dependent on simultaneous execution of apoptosis and necroptosis, as demonstrated by functional genomic dissection with a multicolored lentiCRISPR approach to simultaneously disrupt multiple genes in patient-derived ALL. SM specifically targeted receptor-interacting protein kinase 1 (RIP1)-dependent death, and CRISPR-mediated disruption of RIP1 completely blocked SM-induced death yet had no impact on the response to standard antileukemic agents. Thus, SM compounds such as birinapant circumvent escape from apoptosis in leukemia by activating a potent dual RIP1-dependent apoptotic and necroptotic cell death, which is not exploited by current therapy. Ex vivo drug activity profiling could provide important functional diagnostic information to identify patients who may benefit from targeted treatment with birinapant in early clinical trials.

The caspase-8/RIPK3 signaling axis in antigen presenting cells controls the inflammatory arthritic response

Background

Caspase-8 is a well-established initiator of apoptosis and suppressor of necroptosis, but maintains functions beyond cell death that involve suppression of receptor-interacting serine-threonine kinases (RIPKs). A genome-wide association study meta-analysis revealed an SNP associated with risk of rheumatoid arthritis (RA) development within the locus containing the gene encoding for caspase-8. Innate immune cells, like macrophages and dendritic cells, are gaining momentum as facilitators of autoimmune disease pathogenesis, and, in particular, RA. Therefore, we examined the involvement of caspase-8 within these antigen-presenting cell populations in the pathogenesis of an arthritis model that resembles the RA effector phase.

Methods

Cre^LysMCasp8^flox/flox and Cre^CD11cCasp8^flox/flox mice were bred via a cross between Casp8^flox/flox and Cre^LysM or Cre^CD11c mice. RIPK3^–/–Cre^LysMCasp8^flox/flox and RIPK3^–/–Cre^CD11cCasp8^flox/flox mice were generated to assess RIPK3 contribution. Mice were subjected to K/BxN serum-transfer-induced arthritis. Luminex-based assays were used to measure cytokines/chemokines. Histological analyses were utilized to examine joint damage. Mixed bone marrow chimeras were generated to assess synovial cell survival. Flow cytometric analysis was employed to characterize cellular distribution. For arthritis, differences between the groups were assessed using two-way analysis of variance (ANOVA) for repeated measurements. All other data were compared by the Mann-Whitney test.

Results

We show that intact caspase-8 signaling maintains opposing roles in lysozyme-M- and CD11c-expressing cells in the joint; namely, caspase-8 is crucial in CD11c-expressing cells to delay arthritis induction, while caspase-8 in lysozyme M-expressing cells hinders arthritis resolution. Caspase-8 is also implicated in the maintenance of synovial tissue-resident macrophages that can limit arthritis. Global loss of RIPK3 in both caspase-8 deletion constructs causes the response to arthritis to revert back to control levels via a mechanism potentially independent of cell death. Mixed bone marrow chimeric mice demonstrate that caspase-8 deficiency does not confer preferential expansion of synovial macrophage and dendritic cell populations, nor do caspase-8-deficient synovial populations succumb to RIPK3-mediated necroptotic death.

Conclusions

These data demonstrate that caspase-8 functions in synovial antigen-presenting cells to regulate the response to inflammatory stimuli by controlling RIPK3 action, and this delicate balance maintains homeostasis within the joint.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-017-1436-4) contains supplementary material, which is available to authorized users.

Preservative solution that stabilizes erythrocyte morphology and leukocyte viability under ambient conditions

The deterioration of whole blood ex vivo represents a logistical hurdle in clinical and research settings. Here, a cocktail preservative is described that stabilizes leukocyte viability and erythrocyte morphology in whole blood under ambient storage. Neutrophil biostabilization was explored using a sophisticated microfluidic assay to examine the effectiveness of caspase inhibition to stabilize purified neutrophils. Following 72 h ambient storage, neutrophils remained fully functional to migrate towards chemical cues and maintained their ability to undergo NETosis after stimulation. Furthermore, stored neutrophils exhibited improved CD45 biomarker retention and reduced apoptosis and mortality compared to untreated controls. To stabilize erythrocyte morphology, a preservative solution was formulated using Taguchi methods of experimental design, and combined with the caspase inhibitor to form a whole blood cocktail solution, CS_WB. CS_WB was evaluated in blood from healthy donors and from women with metastatic breast cancer stored under ambient conditions for 72 h. CS_WB-treated samples showed a significant improvement in erythrocyte morphology compared to untreated controls. Leukocytes in CS_WB-treated blood exhibited significantly higher viability and CD45 biomarker retention compared to untreated controls. This 72 h shelf life under ambient conditions represents an opportunity to transport isolates or simply ease experimental timelines where blood degradation is problematic.

Initiation and execution mechanisms of necroptosis: an overview

Necroptosis is a form of regulated cell death, which is induced by ligand binding to TNF family death domain receptors, pattern recognizing receptors and virus sensors. The common feature of these receptor systems is the implication of proteins, which contain a receptor interaction protein kinase (RIPK) homology interaction motif (RHIM) mediating recruitment and activation of receptor-interacting protein kinase 3 (RIPK3), which ultimately activates the necroptosis executioner mixed lineage kinase domain-like (MLKL). In case of the TNF family members, the initiator is the survival- and cell death-regulating RIPK1 kinase, in the case of Toll-like receptor 3/4 (TLR3/4), a RHIM-containing adaptor, called TRIF, while in the case of Z-DNA-binding protein ZBP1/DAI, the cytosolic viral sensor itself contains a RHIM domain. In this review, we discuss the different protein complexes that serve as nucleation platforms for necroptosis and the mechanism of execution of necroptosis. Transgenic models (knockout, kinase-dead knock-in) and pharmacologic inhibition indicate that RIPK1, RIPK3 or MLKL are implicated in many inflammatory, degenerative and infectious diseases. However, the conclusion of necroptosis being solely involved in the etiology of diseases is blurred by the pleiotropic roles of RIPK1 and RIPK3 in other cellular processes such as apoptosis and inflammasome activation.

Necroptosis: Mechanisms and Relevance to Disease

Necroptosis is a form of regulated cell death that critically depends on receptor-interacting serine-threonine kinase 3 (RIPK3) and mixed lineage kinase domain-like (MLKL) and generally manifests with morphological features of necrosis. The molecular mechanisms that underlie distinct instances of necroptosis have just begun to emerge. Nonetheless, it has already been shown that necroptosis contributes to cellular demise in various pathophysiological conditions, including viral infection, acute kidney injury, and cardiac ischemia/reperfusion. Moreover, human tumors appear to obtain an advantage from the downregulation of key components of the molecular machinery for necroptosis. Although such an advantage may stem from an increased resistance to adverse microenvironmental conditions, accumulating evidence indicates that necroptosis-deficient cancer cells are poorly immunogenic and hence escape natural and therapy-elicited immunosurveillance. Here, we discuss the molecular mechanisms and relevance to disease of necroptosis.

Activity of Uncleaved Caspase-8 Controls Anti-bacterial Immune Defense and TLR-Induced Cytokine Production Independent of Cell Death

Caspases regulate cell death programs in response to environmental stresses, including infection and inflammation, and are therefore critical for the proper operation of the mammalian immune system. Caspase-8 is necessary for optimal production of inflammatory cytokines and host defense against infection by multiple pathogens including Yersinia, but whether this is due to death of infected cells or an intrinsic role of caspase-8 in TLR-induced gene expression is unknown. Caspase-8 activation at death signaling complexes results in its autoprocessing and subsequent cleavage and activation of its downstream apoptotic targets. Whether caspase-8 activity is also important for inflammatory gene expression during bacterial infection has not been investigated. Here, we report that caspase-8 plays an essential cell-intrinsic role in innate inflammatory cytokine production in vivo during Yersinia infection. Unexpectedly, we found that caspase-8 enzymatic activity regulates gene expression in response to bacterial infection as well as TLR signaling independently of apoptosis. Using newly-generated mice in which caspase-8 autoprocessing is ablated (Casp8^DA/DA), we now demonstrate that caspase-8 enzymatic activity, but not autoprocessing, mediates induction of inflammatory cytokines by bacterial infection and a wide variety of TLR stimuli. Because unprocessed caspase-8 functions in an enzymatic complex with its homolog cFLIP, our findings implicate the caspase-8/cFLIP heterodimer in control of inflammatory cytokines during microbial infection, and provide new insight into regulation of antibacterial immune defense.

TLR signaling induces expression of key inflammatory cytokines and pro-survival factors that facilitate control of microbial infection. TLR signaling can also engage cell death pathways through activation of enzymes known as caspases. Caspase-8 activates apoptosis in response to infection by pathogens that interfere with NF-κB signaling, including Yersinia, but has also recently been linked to control of inflammatory gene expression. Pathogenic Yersinia can cause severe disease ranging from gastroenteritis to plague. While caspase-8 mediates cell death in response to Yersinia infection as well as other signals, its precise role in gene expression and host defense during in vivo infection is unknown. Here, we show that caspase-8 activity promotes cell-intrinsic cytokine expression, independent of its role in cell death in response to Yersinia infection. Our studies further demonstrate that caspase-8 enzymatic activity plays a previously undescribed role in ensuring optimal TLR-induced gene expression by innate cells during bacterial infection. This work sheds new light on mechanisms that regulate essential innate anti-bacterial immune defense.

Differences and Similarities in TRAIL- and Tumor Necrosis Factor-Mediated Necroptotic Signaling in Cancer Cells

Recently, a type of regulated necrosis (RN) called necroptosis was identified to be involved in many pathophysiological processes and emerged as an alternative method to eliminate cancer cells. However, only a few studies have elucidated components of TRAIL-mediated necroptosis useful for anticancer therapy. Therefore, we have compared this type of cell death to tumor necrosis factor (TNF)-mediated necroptosis and found similar signaling through acid and neutral sphingomyelinases, the mitochondrial serine protease HtrA2/Omi, Atg5, and vacuolar H(+)-ATPase. Notably, executive mechanisms of both TRAIL- and TNF-mediated necroptosis are independent of poly(ADP-ribose) polymerase 1 (PARP-1), and depletion of p38α increases the levels of both types of cell death. Moreover, we found differences in signaling between TNF- and TRAIL-mediated necroptosis, e.g., a lack of involvement of ubiquitin carboxyl hydrolase L1 (UCH-L1) and Atg16L1 in executive mechanisms of TRAIL-mediated necroptosis. Furthermore, we discovered indications of an altered involvement of mitochondrial components, since overexpression of the mitochondrial protein Bcl-2 protected Jurkat cells from TRAIL- and TNF-mediated necroptosis, and overexpression of Bcl-XL diminished only TRAIL-induced necroptosis in Colo357 cells. Furthermore, TRAIL does not require receptor internalization and endosome-lysosome acidification to mediate necroptosis. Taken together, pathways described for TRAIL-mediated necroptosis and differences from those for TNF-mediated necroptosis might be unique targets to increase or modify necroptotic signaling and eliminate tumor cells more specifically in future anticancer approaches.

The Pseudokinase MLKL and the Kinase RIPK3 Have Distinct Roles in Autoimmune Disease Caused by Loss of Death-Receptor-Induced Apoptosis

The kinases RIPK1 and RIPK3 and the pseudo-kinase MLKL have been identified as key regulators of the necroptotic cell death pathway, although a role for MLKL within the whole animal has not yet been established. Here, we have shown that MLKL deficiency rescued the embryonic lethality caused by loss of Caspase-8 or FADD. Casp8(-/-)Mlkl(-/-) and Fadd(-/-)Mlkl(-/-) mice were viable and fertile but rapidly developed severe lymphadenopathy, systemic autoimmune disease, and thrombocytopenia. These morbidities occurred more rapidly and with increased severity in Casp8(-/-)Mlkl(-/-) and Fadd(-/-)Mlkl(-/-) mice compared to Casp8(-/-)Ripk3(-/-) or Fadd(-/-)Ripk3(-/-) mice, respectively. These results demonstrate that MLKL is an essential effector of aberrant necroptosis in embryos caused by loss of Caspase-8 or FADD. Furthermore, they suggest that RIPK3 and/or MLKL may exert functions independently of necroptosis. It appears that non-necroptotic functions of RIPK3 contribute to the lymphadenopathy, autoimmunity, and excess cytokine production that occur when FADD or Caspase-8-mediated apoptosis is abrogated.

MLKL and FADD Are Critical for Suppressing Progressive Lymphoproliferative Disease and Activating the NLRP3 Inflammasome

MLKL, a key component downstream of RIPK3, is suggested to be a terminal executor of necroptosis. Genetic studies have revealed that Ripk3 ablation rescues embryonic lethality in Fadd- or Caspase-8-deficient mice. Given that RIPK3 has also been implicated in non-necroptotic pathways including apoptosis and inflammatory signaling, it remains unclear whether the lethality in Fadd(-/-) mice is indeed caused by necropotosis. Here, we show that genetic deletion of Mlkl rescues the developmental defect in Fadd-deficient mice and that Fadd(-/-)Mlkl(-/-) mice are viable and fertile. Mlkl(-/-)Fadd(-/-) mice display significantly accelerated lymphoproliferative disease characterized by lymphadenopathy and splenomegaly when compared to Ripk3(-/-)Fadd(-/-) mice. Mlkl(-/-)Fadd(-/-) bone-marrow-derived macrophages and dendritic cells have impaired NLRP3 inflammasome activation associated with defects in ASC speck formation and NF-κB-dependent NLRP3 transcription. Our findings reveal that MLKL and FADD play critical roles in preventing lymphoproliferative disease and activating the NLRP3 inflammasome.

Protease signaling in animal and plant-regulated cell death

This review aims to highlight the proteases required for regulated cell death mechanisms in animals and plants. The aim is to be incisive, and not inclusive of all the animal proteases that have been implicated in various publications. The review also aims to focus on instances when several publications from disparate groups have demonstrated the involvement of an animal protease, and also when there is substantial biochemical, mechanistic and genetic evidence. In doing so, the literature can be culled to a handful of proteases, covering most of the known regulated cell death mechanisms: apoptosis, regulated necrosis, necroptosis, pyroptosis and NETosis in animals. In plants, the literature is younger and not as extensive as for mammals, although the molecular drivers of vacuolar death, necrosis and the hypersensitive response in plants are becoming clearer. Each of these death mechanisms has at least one proteolytic component that plays a major role in controlling the pathway, and sometimes they combine in networks to regulate cell death/survival decision nodes. Some similarities are found among animal and plant cell death proteases but, overall, the pathways that they govern are kingdom-specific with very little overlap.

An outline of necrosome triggers

Necroptosis was initially identified as a backup cell death program when apoptosis is blocked. However, it is now recognized as a cellular defense mechanism against infections and is presumed to be a detrimental factor in several pathologies driven by cell death. Necroptosis is a prototypic form of regulated necrosis that depends on activation of the necrosome, which is a protein complex in which receptor interacting protein kinase (RIPK) 3 is activated. The RIP homotypic interaction motif (RHIM) is the core domain that regulates activation of the necrosome. To date, three RHIM-containing proteins have been reported to activate the kinase activity of RIPK3 within the necrosome: RIPK1, Toll/IL-1 receptor domain-containing adaptor inducing IFN-β (TRIF), and DNA-dependent activator of interferon regulatory factors (DAI). Here, we review and discuss commonalities and differences of the increasing number of activators of the necrosome. Since the discovery that activation of mixed lineage kinase domain-like (MLKL) by RIPK3 kinase activity is crucial in necroptosis, interest has increased in monitoring and therapeutically targeting their activation. The availability of new phospho-specific antibodies, pharmacologic inhibitors, and transgenic models will allow us to further document the role of necroptosis in degenerative, inflammatory and infectious diseases.

Cancer therapy in the necroptosis era

Necroptosis is a caspase-independent form of regulated cell death executed by the receptor-interacting protein kinase 1 (RIP1), RIP3, and mixed lineage kinase domain-like protein (MLKL). Recently, necroptosis-based cancer therapy has been proposed to be a novel strategy for antitumor treatment. However, a big controversy exists on whether this type of therapy is feasible or just a conceptual model. Proponents believe that because necroptosis and apoptosis use distinct molecular pathways, triggering necroptosis could be an alternative way to eradicate apoptosis-resistant cancer cells. This hypothesis has been preliminarily validated by recent studies. However, some skeptics doubt this strategy because of the intrinsic or acquired defects of necroptotic machinery observed in many cancer cells. Moreover, two other concerns are whether or not necroptosis inducers are selective in killing cancer cells without disturbing the normal cells and whether it will lead to inflammatory diseases. In this review, we summarize current studies surrounding this controversy on necroptosis-based antitumor research and discuss the advantages, potential issues, and countermeasures of this novel therapy.

Ensembling and filtering: an effective and rapid in silico multitarget drug-design strategy to identify RIPK1 and RIPK3 inhibitors

Necroptosis, a programmed necrosis pathway, is witnessed in diverse human diseases and is primarily regulated by receptor-interacting serine/threonine protein kinase 1 (RIPK1) and RIPK3. Ablation or inhibition of these individual proteins, or both, has been shown to be protective in various in vitro and in vivo disease models involving necroptosis. In this study, we propose an effective and rapid virtual screening strategy to identify multitarget inhibitors of both RIPK1 and RIPK3. It involves ensemble pharmacophore-based screening (EPS) of a compound database, post-EPS filtration (PEPSF) of the ligand hits, and multiple dockings. Structurally diverse inhibitors were identified through ensemble pharmacophore features, and the speed of this process was enhanced by filtering out the compounds containing cross-features. The stability of these inhibitors with both of the proteins was verified by means of molecular dynamics (MD) simulation. Graphical Abstract A generalized workflow employed in this study. Subsequent utilization of EPS and PEPSF might lead to reduced computational time and load.

Die another way--non-apoptotic mechanisms of cell death

Regulated, programmed cell death is crucial for all multicellular organisms. Cell death is essential in many processes, including tissue sculpting during embryogenesis, development of the immune system and destruction of damaged cells. The best-studied form of programmed cell death is apoptosis, a process that requires activation of caspase proteases. Recently it has been appreciated that various non-apoptotic forms of cell death also exist, such as necroptosis and pyroptosis. These non-apoptotic cell death modalities can be either triggered independently of apoptosis or are engaged should apoptosis fail to execute. In this Commentary, we discuss several regulated non-apoptotic forms of cell death including necroptosis, autophagic cell death, pyroptosis and caspase-independent cell death. We outline what we know about their mechanism, potential roles in vivo and define outstanding questions. Finally, we review data arguing that the means by which a cell dies actually matters, focusing our discussion on inflammatory aspects of cell death.

Deletion of FADD in macrophages and granulocytes results in RIP3- and MyD88-dependent systemic inflammation

Myeloid cells, which include monocytes, macrophages, and granulocytes, are important innate immune cells, but the mechanism and downstream effect of their cell death on the immune system is not completely clear. Necroptosis is an alternate form of cell death that can be triggered when death receptor-mediated apoptosis is blocked, for example, in stimulated Fas-associated Death Domain (FADD) deficient cells. We report here that mice deficient for FADD in myeloid cells (mFADD^-/-) exhibit systemic inflammation with elevated inflammatory cytokines and increased levels of myeloid and B cell populations while their dendritic and T cell numbers are normal. These phenotypes were abolished when RIP3 deficiency was introduced, suggesting that systemic inflammation is caused by RIP3-dependent necroptotic and/or inflammatory activity. We further found that loss of MyD88 can rescue the systemic inflammation observed in these mice. These phenotypes are surprisingly similar to that of dendritic cell (DC)-specific FADD deficient mice with the exception that DC numbers are normal in mFADD^-/- mice. Together these data support the notion that innate immune cells are constantly being stimulated through the MyD88-dependent pathway and aberrations in their cell death machinery can result in systemic effects on the immune system.

Activation-induced and damage-induced cell death in aging human T cells

In multicellular organisms the proper system functionality is ensured by the balance between cell division, differentiation, senescence and death. This balance is changed during aging. Immunosenescence plays a crucial role in aging and leads to the shrinkage of T cell repertoire and the propensity to apoptosis. The elimination of expanded T cells at the end of immune response is crucial to maintain homeostasis and avoid any uncontrolled inflammation. Resting mature T lymphocytes, when activated via their antigen-specific receptor (TCR) and CD28 co-receptor, start to proliferate and then undergo the so called activation induced cell death (AICD), which mechanistically is triggered by the death receptor and leads to apoptosis. T lymphocytes, like other cells, are also exposed to damage, which can trigger the so called damage-induced cell death (DICD). It was hypothesized that oxidative stress and chronic antigenic load increasing with age reduced lymphocyte susceptibility to DICD and enhanced a proinflamatory status leading to increased AICD. However, data collected so far are inconsistent and does not support this assumption. Systematic and comprehensive studies are still needed for conclusive elucidation of the role of AICD and DICD in human immunosenescence, including the role of autophagy and necroptosis in the processes.

The molecular relationships between apoptosis, autophagy and necroptosis

Cells are constantly subjected to a vast range of potentially lethal insults, which may activate specific molecular pathways that have evolved to kill the cell. Cell death pathways are defined partly by their morphology, and more specifically by the molecules that regulate and enact them. As these pathways become more thoroughly characterized, interesting molecular links between them have emerged, some still controversial and others hinting at the physiological and pathophysiological roles these death pathways play. We describe specific molecular programs controlling cell death, with a focus on some of the distinct features of the pathways and the molecular links between them.

Synchronized renal tubular cell death involves ferroptosis

Receptor-interacting protein kinase 3 (RIPK3)-mediated necroptosis is thought to be the pathophysiologically predominant pathway that leads to regulated necrosis of parenchymal cells in ischemia-reperfusion injury (IRI), and loss of either Fas-associated protein with death domain (FADD) or caspase-8 is known to sensitize tissues to undergo spontaneous necroptosis. Here, we demonstrate that renal tubules do not undergo sensitization to necroptosis upon genetic ablation of either FADD or caspase-8 and that the RIPK1 inhibitor necrostatin-1 (Nec-1) does not protect freshly isolated tubules from hypoxic injury. In contrast, iron-dependent ferroptosis directly causes synchronized necrosis of renal tubules, as demonstrated by intravital microscopy in models of IRI and oxalate crystal-induced acute kidney injury. To suppress ferroptosis in vivo, we generated a novel third-generation ferrostatin (termed 16-86), which we demonstrate to be more stable, to metabolism and plasma, and more potent, compared with the first-in-class compound ferrostatin-1 (Fer-1). Even in conditions with extraordinarily severe IRI, 16-86 exerts strong protection to an extent which has not previously allowed survival in any murine setting. In addition, 16-86 further potentiates the strong protective effect on IRI mediated by combination therapy with necrostatins and compounds that inhibit mitochondrial permeability transition. Renal tubules thus represent a tissue that is not sensitized to necroptosis by loss of FADD or caspase-8. Finally, ferroptosis mediates postischemic and toxic renal necrosis, which may be therapeutically targeted by ferrostatins and by combination therapy.

Caspase-8 modulates dectin-1 and complement receptor 3-driven IL-1β production in response to β-glucans and the fungal pathogen, Candida albicans

Inflammasomes are central mediators of host defense to a wide range of microbial pathogens. The nucleotide-binding domain and leucine-rich repeat containing family (NLR), pyrin domain-containing 3 (NLRP3) inflammasome plays a key role in triggering caspase-1-dependent IL-1β maturation and resistance to fungal dissemination in Candida albicans infection. β-Glucans are major components of fungal cell walls that trigger IL-1β secretion in both murine and human immune cells. In this study, we sought to determine the contribution of β-glucans to C. albicans-induced inflammasome responses in mouse dendritic cells. We show that the NLRP3-apoptosis-associated speck-like protein containing caspase recruitment domain protein-caspase-1 inflammasome is absolutely critical for IL-1β production in response to β-glucans. Interestingly, we also found that both complement receptor 3 (CR3) and dectin-1 play a crucial role in coordinating β-glucan-induced IL-1β processing as well as a cell death response. In addition to the essential role of caspase-1, we identify an important role for the proapoptotic protease caspase-8 in promoting β-glucan-induced cell death and NLRP3 inflammasome-dependent IL-1β maturation. A strong requirement for CR3 and caspase-8 also was found for NLRP3-dependent IL-1β production in response to heat-killed C. albicans. Taken together, these results define the importance of dectin-1, CR3, and caspase-8, in addition to the canonical NLRP3 inflammasome, in mediating β-glucan- and C. albicans-induced innate responses in dendritic cells. Collectively, these findings establish a novel link between β-glucan recognition receptors and the inflammatory proteases caspase-8 and caspase-1 in coordinating cytokine secretion and cell death in response to immunostimulatory fungal components.

Necroptosis, in vivo detection in experimental disease models

Over the last decade, our picture of cell death signals involved in experimental disease models totally shifted. Indeed, in addition to apoptosis, multiple forms of regulated necrosis have been associated with an increasing number of pathologies such as ischemia-reperfusion injury in brain, heart and kidney, inflammatory diseases, sepsis, retinal disorders, neurodegenerative diseases and infectious disorders. Especially necroptosis is currently attracting the attention of the scientific community. However, the in vivo identification of ongoing necroptosis in experimental disease conditions remains troublesome, mainly due to the lack of specific biomarkers. Initially, Receptor-Interacting Protein Kinase 1 (RIPK1) and RIPK3 kinase activity were uniquely associated with induction of necroptosis, however recent evidence suggests pleiotropic functions in cell death, inflammation and survival, obscuring a clear picture. In this review, we will present the last methodological advances for in vivo necroptosis identification and discuss past and recent data to provide an update of the so-called "necroptosis-associated pathologies".