Dying in self-defence: a comparative overview of immunogenic cell death signalling in animals and plants

Cytoplasmic calcium influx mediated by plant MLKLs confers TNL-triggered immunity

The plant homolog of vertebrate necroptosis inducer mixed-lineage kinase domain-like (MLKL) contributes to downstream steps in Toll-interleukin-1 receptor domain NLR (TNL)-receptor-triggered immunity. Here, we show that Arabidopsis MLKL1 (AtMLKL1) clusters into puncta at the plasma membrane upon TNL activation and that this sub-cellular reorganization is dependent on the TNL signal transducer, EDS1. We find that AtMLKLs confer TNL-triggered immunity in parallel with RPW8-type HeLo-domain-containing NLRs (RNLs) and that the AtMLKL N-terminal HeLo domain is indispensable for both immunity and clustering. We show that the AtMLKL HeLo domain mediates cytoplasmic Ca2+ ([Ca2+]cyt) influx in plant and human cells, and AtMLKLs are responsible for sustained [Ca2+]cyt influx during TNL-triggered, but not CNL-triggered, immunity. Our study reveals parallel immune signaling functions of plant MLKLs and RNLs as mediators of [Ca2+]cyt influx and a potentially common role of the HeLo domain fold in the Ca2+-signal relay of diverse organisms.

The conceptual foundations of innate immunity: Taking stock 30 years later

While largely neglected over decades during which adaptive immunity captured most of the attention, innate immune mechanisms have now become central to our understanding of immunology. Innate immunity provides the first barrier to infection in vertebrates, and it is the sole mechanism of host defense in invertebrates and plants. Innate immunity also plays a critical role in maintaining homeostasis, shaping the microbiota, and in disease contexts such as cancer, neurodegeneration, metabolic syndromes, and aging. The emergence of the field of innate immunity has led to an expanded view of the immune system, which is no longer restricted to vertebrates and instead concerns all metazoans, plants, and even prokaryotes. The study of innate immunity has given rise to new concepts and language. Here, we review the history and definition of the core concepts of innate immunity, discussing their value and fruitfulness in the long run.

Plant cathepsin B, a versatile protease

Plant proteases are essential enzymes that play key roles during crucial phases of plant life. Some proteases are mainly involved in general protein turnover and recycle amino acids for protein synthesis. Other proteases are involved in cell signalling, cleave specific substrates and are key players during important genetically controlled molecular processes. Cathepsin B is a cysteine protease that can do both because of its exopeptidase and endopeptidase activities. Animal cathepsin B has been investigated for many years, and much is known about its mode of action and substrate preferences, but much remains to be discovered about this potent protease in plants. Cathepsin B is involved in plant development, germination, senescence, microspore embryogenesis, pathogen defence and responses to abiotic stress, including programmed cell death. This review discusses the structural features, the activity of the enzyme and the differences between the plant and animal forms. We discuss its maturation and subcellular localisation and provide a detailed overview of the involvement of cathepsin B in important plant life processes. A greater understanding of the cell signalling processes involving cathepsin B is needed for applied discoveries in plant biotechnology.

Endosperm cell death: roles and regulation in angiosperms

Double fertilization in angiosperms results in the formation of a second zygote, the fertilized endosperm. Unlike its embryo sibling, the endosperm is a transient structure that eventually undergoes developmentally controlled programmed cell death (PCD) at specific time points of seed development or germination. The nature of endosperm PCD exhibits a considerable diversity, both across different angiosperm taxa and within distinct endosperm tissues. In endosperm-less species, PCD might cause central cell degeneration as a mechanism preventing the formation of a fertilized endosperm. In most other angiosperms, embryo growth necessitates the elimination of surrounding endosperm cells. Nevertheless, complete elimination of the endosperm is rare, and in most cases, specific endosperm tissues persist. In mature seeds, these persisting cells may be dead, such as the starchy endosperm in cereals, or remain alive to die only during germination, like the cereal aleurone or the endosperm of castor beans. In this review, we explore the current knowledge surrounding the cellular, molecular, and genetic aspects of endosperm PCD, and the influence environmental stresses have on PCD processes. Overall, this review provides an exhaustive overview of endosperm PCD processes in angiosperms, shedding light on its diverse mechanisms and its significance in seed development and seedling establishment.

Inflammasomes as regulators of mechano-immunity

Mechano-immunity, the intersection between cellular or tissue mechanics and immune cell function, is emerging as an important factor in many inflammatory diseases. Mechano-sensing defines how cells detect mechanical changes in their environment. Mechano-response defines how cells adapt to such changes, e.g. form synapses, signal or migrate. Inflammasomes are intracellular immune sensors that detect changes in tissue and cell homoeostasis during infection or injury. We and others recently found that mechano-sensing of tissue topology (swollen tissue), topography (presence and distribution of foreign solid implant) or biomechanics (stiffness), alters inflammasome activity. Once activated, inflammasomes induce the secretion of inflammatory cytokines, but also change cellular mechanical properties, which influence how cells move, change their shape, and interact with other cells. When overactive, inflammasomes lead to chronic inflammation. This clearly places inflammasomes as important players in mechano-immunity. Here, we discuss a model whereby inflammasomes integrate pathogen- and tissue-injury signals, with changes in tissue mechanics, to shape the downstream inflammatory responses and allow cell and tissue mechano-adaptation. We will review the emerging evidence that supports this model.

Amplifying "eat me signal" by immunogenic cell death for potentiating cancer immunotherapy

Immunogenic cell death (ICD) is a unique mode of cell death, which can release immunogenic damage-associated molecular patterns (DAMPs) and tumor-associated antigens to trigger long-term protective antitumor immune responses. Thus, amplifying "eat me signal" during tumor ICD cascade is critical for cancer immunotherapy. Some therapies (radiotherapy, photodynamic therapy (PDT), photothermal therapy (PTT), etc.) and inducers (chemotherapeutic agents, etc.) have enabled to initiate and/or facilitate ICD and activate antitumor immune responses. Recently, nanostructure-based drug delivery systems have been synthesized for inducing ICD through combining treatment of chemotherapeutic agents, photosensitizers for PDT, photothermal transformation agents for PTT, radiosensitizers for radiotherapy, etc., which can release loaded agents at an appropriate dosage in the designated place at the appropriate time, contributing to higher efficiency and lower toxicity. Also, immunotherapeutic agents in combination with nanostructure-based drug delivery systems can produce synergetic antitumor effects, thus potentiating immunotherapy. Overall, our review outlines the emerging ICD inducers, and nanostructure drug delivery systems loading diverse agents to evoke ICD through chemoradiotherapy, PDT, and PTT or combining immunotherapeutic agents. Moreover, we discuss the prospects and challenges of harnessing ICD induction-based immunotherapy, and highlight the significance of multidisciplinary and interprofessional collaboration to promote the optimal translation of this treatment strategy.

How plants manage pathogen infection

To combat microbial pathogens, plants have evolved specific immune responses that can be divided into three essential steps: microbial recognition by immune receptors, signal transduction within plant cells, and immune execution directly suppressing pathogens. During the past three decades, many plant immune receptors and signaling components and their mode of action have been revealed, markedly advancing our understanding of the first two steps. Activation of immune signaling results in physical and chemical actions that actually stop pathogen infection. Nevertheless, this third step of plant immunity is under explored. In addition to immune execution by plants, recent evidence suggests that the plant microbiota, which is considered an additional layer of the plant immune system, also plays a critical role in direct pathogen suppression. In this review, we summarize the current understanding of how plant immunity as well as microbiota control pathogen growth and behavior and highlight outstanding questions that need to be answered.

Stimuli-responsive nanodelivery systems for amplifying immunogenic cell death in cancer immunotherapy

Immunogenic cell death (ICD) is a special pattern of tumor cell death, enabling to elicit tumor-specific immune response via the release of damage-associated molecular patterns and tumor-associated antigens in the tumor microenvironment. ICD-induced immunotherapy holds the promise for completely eliminating tumors and long-term protective antitumor immune response. Increasing ICD inducers have been discovered for boosting antitumor immunity via evoking ICD. Nonetheless, the utilization of ICD inducers remains insufficient owing to serious toxic reactions, low localization efficiency within the tumor microenvironmental niche, etc. For overcoming such limitations, stimuli-responsive multifunctional nanoparticles or nanocomposites with ICD inducers have been developed for improving immunotherapeutic efficiency via lowering toxicity, which represent a prospective scheme for fostering the utilization of ICD inducers in immunotherapy. This review outlines the advances in near-infrared (NIR)-, pH-, redox-, pH- and redox-, or NIR- and tumor microenvironment-responsive nanodelivery systems for ICD induction. Furthermore, we discuss their clinical translational potential. The progress of stimuli-responsive nanoparticles in clinical settings depends upon the development of biologically safer drugs tailored to patient needs. Moreover, an in-depth comprehending of ICD biomarkers, immunosuppressive microenvironment, and ICD inducers may accelerate the advance in smarter multifunctional nanodelivery systems to further amplify ICD.

Plant NLR immunity activation and execution: a biochemical perspective

Plants deploy cell-surface and intracellular receptors to detect pathogen attack and trigger innate immune responses. Inside host cells, families of nucleotide-binding/leucine-rich repeat (NLR) proteins serve as pathogen sensors or downstream mediators of immune defence outputs and cell death, which prevent disease. Established genetic underpinnings of NLR-mediated immunity revealed various strategies plants adopt to combat rapidly evolving microbial pathogens. The molecular mechanisms of NLR activation and signal transmission to components controlling immunity execution were less clear. Here, we review recent protein structural and biochemical insights to plant NLR sensor and signalling functions. When put together, the data show how different NLR families, whether sensors or signal transducers, converge on nucleotide-based second messengers and cellular calcium to confer immunity. Although pathogen-activated NLRs in plants engage plant-specific machineries to promote defence, comparisons with mammalian NLR immune receptor counterparts highlight some shared working principles for NLR immunity across kingdoms.

Combination of MCL-1 and BCL-2 inhibitors is a promising approach for a host-directed therapy for tuberculosis

Tuberculosis (TB) accounts for 1.6 million deaths annually and over 25% of deaths due to antimicrobial resistance. Mycobacterium tuberculosis (M.tb) drives MCL-1 expression (family member of anti-apoptotic BCL-2 proteins) to limit apoptosis and grow intracellularly in human macrophages. The feasibility of re-purposing specific MCL-1 and BCL-2 inhibitors to limit M.tb growth, using inhibitors that are in clinical trials and FDA-approved for cancer treatment has not be tested previously. We show that specifically inhibiting MCL-1 and BCL-2 induces apoptosis of M.tb-infected macrophages, and markedly reduces M.tb growth in human and murine macrophages, and in a pre-clinical model of human granulomas. MCL-1 and BCL-2 inhibitors limit growth of drug resistant and susceptible M.tb in macrophages and act in additive fashion with the antibiotics isoniazid and rifampicin. This exciting work uncovers targeting the intrinsic apoptosis pathway as a promising approach for TB host-directed therapy. Since safety and activity studies are underway in cancer clinics for MCL-1 and BCL-2 inhibitors, we expect that re-purposing them for TB treatment should translate more readily and rapidly to the clinic. Thus, the work supports further development of this host-directed therapy approach to augment current TB treatment.

Programmed Cell Death in Unicellular Versus Multicellular Organisms

Programmed cell death (self-induced) is intrinsic to all cellular life forms, including unicellular organisms. However, cell death research has focused on animal models to understand cancer, degenerative disorders, and developmental processes. Recently delineated suicidal death mechanisms in bacteria and fungi have revealed ancient origins of animal cell death that are intertwined with immune mechanisms, allaying earlier doubts that self-inflicted cell death pathways exist in microorganisms. Approximately 20 mammalian death pathways have been partially characterized over the last 35 years. By contrast, more than 100 death mechanisms have been identified in bacteria and a few fungi in recent years. However, cell death is nearly unstudied in most human pathogenic microbes that cause major public health burdens. Here, we consider how the current understanding of programmed cell death arose through animal studies and how recently uncovered microbial cell death mechanisms in fungi and bacteria resemble and differ from mechanisms of mammalian cell death.

Intricate guard-guardee interplay in plant immune signaling

Plant helper NLRs are immune signal transducers. In this issue of Cell Host & Microbe, Wang et al. report that the ADR1 subfamily of helper NLRs in Arabidopsis thaliana is functionally diversified to cope with the perturbation by bacterial pathogen effector and is guarded by the NLR protein SNC1.

Combination of transcriptomic, proteomic, and degradomic profiling reveals common and distinct patterns of pathogen-induced cell death in maize

Regulated cell death (RCD) is crucial for plant development, as well as in decision-making in plant-microbe interactions. Previous studies revealed components of the molecular network controlling RCD, including different proteases. However, the identity, the proteolytic network as well as molecular components involved in the initiation and execution of distinct plant RCD processes, still remain largely elusive. In this study, we analyzed the transcriptome, proteome, and N-terminome of Zea mays leaves treated with the Xanthomonas effector avrRxo1, the mycotoxin Fumonisin B1 (FB1), or the phytohormone salicylic acid (SA) to dissect plant cellular processes related to cell death and plant immunity. We found highly distinct and time-dependent biological processes being activated on transcriptional and proteome levels in response to avrRxo1, FB1, and SA. Correlation analysis of the transcriptome and proteome identified general, as well as trigger-specific markers for cell death in Zea mays. We found that proteases, particularly papain-like cysteine proteases, are specifically regulated during RCD. Collectively, this study characterizes distinct RCD responses in Z. mays and provides a framework for the mechanistic exploration of components involved in the initiation and execution of cell death.

A dominant-negative avirulence effector of the barley powdery mildew fungus provides mechanistic insight into barley MLA immune receptor activation

Nucleotide-binding leucine-rich repeat receptors (NLRs) recognize pathogen effectors to mediate plant disease resistance often involving host cell death. Effectors escape NLR recognition through polymorphisms, allowing the pathogen to proliferate on previously resistant host plants. The powdery mildew effector AVRA13-1 is recognized by the barley NLR MLA13 and activates host cell death. We demonstrate here that a virulent form of AVRA13, called AVRA13-V2, escapes MLA13 recognition by substituting a serine for a leucine residue at the C-terminus. Counterintuitively, this substitution in AVRA13-V2 resulted in an enhanced MLA13 association and prevented the detection of AVRA13-1 by MLA13. Therefore, AVRA13-V2 is a dominant-negative form of AVRA13 and has probably contributed to the breakdown of Mla13 resistance. Despite this dominant-negative activity, AVRA13-V2 failed to suppress host cell death mediated by the MLA13 autoactive MHD variant. Neither AVRA13-1 nor AVRA13-V2 interacted with the MLA13 autoactive variant, implying that the binding moiety in MLA13 that mediates association with AVRA13-1 is altered after receptor activation. We also show that mutations in the MLA13 coiled-coil domain, which were thought to impair Ca2+ channel activity and NLR function, instead resulted in MLA13 autoactive cell death. Our results constitute an important step to define intermediate receptor conformations during NLR activation.

NLR immune receptors: structure and function in plant disease resistance

Nucleotide-binding and leucine-rich repeat receptors (NLRs) are a diverse family of intracellular immune receptors that play crucial roles in recognizing and responding to pathogen invasion in plants. This review discusses the overall model of NLR activation and provides an in-depth analysis of the different NLR domains, including N-terminal executioner domains, the nucleotide-binding oligomerization domain (NOD) module, and the leucine-rich repeat (LRR) domain. Understanding the structure-function relationship of these domains is essential for developing effective strategies to improve plant disease resistance and agricultural productivity.

New Biochemical Principles for NLR Immunity in Plants

While working for the United States Department of Agriculture on the North Dakota Agricultural College campus in Fargo, North Dakota, in the 1940s and 1950s, Harold H. Flor formulated the genetic principles for coevolving plant host-pathogen interactions that govern disease resistance or susceptibility. His 'gene-for-gene' legacy runs deep in modern plant pathology and continues to inform molecular models of plant immune recognition and signaling. In this review, we discuss recent biochemical insights to plant immunity conferred by nucleotide-binding domain/leucine-rich-repeat (NLR) receptors, which are major gene-for-gene resistance determinants in nature and cultivated crops. Structural and biochemical analyses of pathogen-activated NLR oligomers (resistosomes) reveal how different NLR subtypes converge in various ways on calcium (Ca2+) signaling to promote pathogen immunity and host cell death. Especially striking is the identification of nucleotide-based signals generated enzymatically by plant toll-interleukin 1 receptor (TIR) domain NLRs. These small molecules are part of an emerging family of TIR-produced cyclic and noncyclic nucleotide signals that steer immune and cell-death responses in bacteria, mammals, and plants. A combined genetic, molecular, and biochemical understanding of plant NLR activation and signaling provides exciting new opportunities for combatting diseases in crops. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Effect of respiratory tract co-colonizers on initial attachment of Neisseria meningitidis

Respiratory tract is a complex system comprising of unique microbiota inhabitants. Neisseria meningitidis, Staphylococcus aureus, Streptococcus pyogenes, Pseudomonas aeruginosa and Klebsiella pneumoniae are few prevalent bacteria in the community composition during lung infections. Although, N. meningitidis resides asymptomatically in nasopharynx of the human host, it can cause fatal infections like meningitis. However, factors affecting transit from carriage to symptomatic infection are not well understood. Various host metabolites and environmental conditions affect the virulence of bacteria. Here, we report that presence of co-colonizers significantly reduces the initial attachment of N. meningitidis to A549 nasopharyngeal epithelial cells. Further, significant decrease in invasion to A549 nasopharyngeal epithelial cells was observed. Moreover, survival in J774A.1 murine macrophage also increases significantly when conditioned media (CM) from S. pyogenes and L. rhamnosus is used for culturing N. meningitidis. The increase in survival could be attributed to increased capsule synthesis. The gene expression studies revealed increased expression of siaC and ctrB in CM prepared from the growth S. pyogenes and L. rhamnosus. Overall, the results suggest change in the virulence of N. meningitidis is assisted by lung microbiota.

Emergence of the fungal immune system

Investigation of fungal biology has been frequently motivated by the fact that many fungal species are important plant and animal pathogens. Such efforts have contributed significantly toward our understanding of fungal pathogenic lifestyles (virulence factors and strategies) and the interplay with host immune systems. In parallel, work on fungal allorecognition systems leading to the characterization of fungal regulated cell death determinants and pathways, has been instrumental for the emergent concept of fungal immunity. The uncovered evolutionary trans-kingdom parallels between fungal regulated cell death pathways and innate immune systems incite us to reflect further on the concept of a fungal immune system. Here, I briefly review key findings that have shaped the fungal immunity paradigm, providing a perspective on what I consider its most glaring knowledge gaps. Undertaking to fill such gaps would establish firmly the fungal immune system inside the broader field of comparative immunology.