Host-Derived Microvesicles Carrying Bacterial Pore-Forming Toxins Deliver Signals to Macrophages: A Novel Mechanism of Shaping Immune Responses

Biodelivery of therapeutic extracellular vesicles: should mononuclear phagocytes always be feared?

At present, extracellular vesicles (EVs) are considered key candidates for cell-free therapies, including treatment of allergic and autoimmune diseases. However, their therapeutic effectiveness, dependent on proper targeting to the desired cells, is significantly limited due to the reduced bioavailability resulting from their rapid clearance by the cells of the mononuclear phagocyte system (MPS). Thus, developing strategies to avoid EV elimination is essential when applying them in clinical practice. On the other hand, malfunctioning MPS contributes to various immune-related pathologies. Therapeutic reversal of these effects with EVs would be beneficial and could be achieved, for example, by modulating the macrophage phenotype or regulating antigen presentation by dendritic cells. Additionally, intended targeting of EVs to MPS macrophages for replication and repackaging of their molecules into new vesicle subtype can allow for their specific targeting to appropriate populations of acceptor cells. Herein, we briefly discuss the under-explored aspects of the MPS-EV interactions that undoubtedly require further research in order to accelerate the therapeutic use of EVs.

Coagulation Disorders in Sepsis and COVID-19-Two Sides of the Same Coin? A Review of Inflammation-Coagulation Crosstalk in Bacterial Sepsis and COVID-19

Sepsis is a major cause of morbidity and mortality worldwide. Sepsis-associated coagulation disorders are involved in the pathogenesis of multiorgan failure and lead to a subsequently worsening prognosis. Alongside the global impact of the COVID-19 pandemic, a great number of research papers have focused on SARS-CoV-2 pathogenesis and treatment. Significant progress has been made in this regard and coagulation disturbances were once again found to underlie some of the most serious adverse outcomes of SARS-CoV-2 infection, such as acute lung injury and multiorgan dysfunction. In the attempt of untangling the mechanisms behind COVID-19-associated coagulopathy (CAC), a series of similarities with sepsis-induced coagulopathy (SIC) became apparent. Whether they are, in fact, the same disease has not been established yet. The clinical picture of CAC shows the unique feature of an initial phase of intravascular coagulation confined to the respiratory system. Only later on, patients can develop a clinically significant form of systemic coagulopathy, possibly with a consumptive pattern, but, unlike SIC, it is not a key feature. Deepening our understanding of CAC pathogenesis has to remain a major goal for the research community, in order to design and validate accurate definitions and classification criteria.

Cells Responding to Closely Related Cholesterol-Dependent Cytolysins Release Extracellular Vesicles with a Common Proteomic Content Including Membrane Repair Proteins

The plasma membrane (PM) protects cells from extracellular threats and supports cellular homeostasis. Some pathogens produce pore-forming toxins (PFTs) that disrupt PM integrity by forming transmembrane pores. High PFT concentrations cause massive damage leading to cell death and facilitating infection. Sub-lytic PFT doses activate repair mechanisms to restore PM integrity, support cell survival and limit disease. Shedding of extracellular vesicles (EVs) has been proposed as a key mechanism to eliminate PFT pores and restore PM integrity. We show here that cholesterol-dependent cytolysins (CDCs), a specific family of PFTs, are at least partially eliminated through EVs release, and we hypothesize that proteins important for PM repair might be included in EVs shed by cells during repair. To identify new PM repair proteins, we collected EVs released by cells challenged with sub-lytic doses of two different bacterial CDCs, listeriolysin O and pneumolysin, and determined the EV proteomic repertoire by LC-MS/MS. Intoxicated cells release similar EVs irrespectively of the CDC used. Also, they release more and larger EVs than non-intoxicated cells. A cluster of 70 proteins including calcium-binding proteins, molecular chaperones, cytoskeletal, scaffold and membrane trafficking proteins, was detected enriched in EVs collected from intoxicated cells. While some of these proteins have well-characterized roles in repair, the involvement of others requires further study. As proof of concept, we show here that Copine-1 and Copine-3, proteins abundantly detected in EVs released by intoxicated cells, are required for efficient repair of CDC-induced PM damage. Additionally, we reveal here new proteins potentially involved in PM repair and give new insights into common mechanisms and machinery engaged by cells in response to PM damage.

Platelet Versus Megakaryocyte: Who Is the Real Bandleader of Thromboinflammation in Sepsis?

Platelets are mainly known for their key role in hemostasis and thrombosis. However, studies over the last two decades have shown their strong implication in mechanisms associated with inflammation, thrombosis, and the immune system in various neoplastic, inflammatory, autoimmune, and infectious diseases. During sepsis, platelets amplify the recruitment and activation of innate immune cells at the site of infection and contribute to the elimination of pathogens. In certain conditions, these mechanisms can lead to thromboinflammation resulting in severe organ dysfunction. Here, we discuss the interactions of platelets with leukocytes, neutrophil extracellular traps (NETs), and endothelial cells during sepsis. The intrinsic properties of platelets that generate an inflammatory signal through the NOD-like receptor family, pyrin domain-containing 3 (NLRP3) inflammasome are discussed. As an example of immunothrombosis, the implication of platelets in vaccine-induced immune thrombotic thrombocytopenia is documented. Finally, we discuss the role of megakaryocytes (MKs) in thromboinflammation and their adaptive responses.

The Yin and Yang of Pneumolysin During Pneumococcal Infection

Pneumolysin (PLY) is a pore-forming toxin produced by the human pathobiont Streptococcus pneumoniae, the major cause of pneumonia worldwide. PLY, a key pneumococcal virulence factor, can form transmembrane pores in host cells, disrupting plasma membrane integrity and deregulating cellular homeostasis. At lytic concentrations, PLY causes cell death. At sub-lytic concentrations, PLY triggers host cell survival pathways that cooperate to reseal the damaged plasma membrane and restore cell homeostasis. While PLY is generally considered a pivotal factor promoting S. pneumoniae colonization and survival, it is also a powerful trigger of the innate and adaptive host immune response against bacterial infection. The dichotomy of PLY as both a key bacterial virulence factor and a trigger for host immune modulation allows the toxin to display both "Yin" and "Yang" properties during infection, promoting disease by membrane perforation and activating inflammatory pathways, while also mitigating damage by triggering host cell repair and initiating anti-inflammatory responses. Due to its cytolytic activity and diverse immunomodulatory properties, PLY is integral to every stage of S. pneumoniae pathogenesis and may tip the balance towards either the pathogen or the host depending on the context of infection.

Neutrophil-Derived Extracellular Vesicles Activate Platelets after Pneumolysin Exposure

Pneumolysin (PLY) is a pore-forming toxin of Streptococcus pneumoniae that contributes substantially to the inflammatory processes underlying pneumococcal pneumonia and lung injury. Host responses against S. pneumoniae are regulated in part by neutrophils and platelets, both individually and in cooperative interaction. Previous studies have shown that PLY can target both neutrophils and platelets, however, the mechanisms by which PLY directly affects these cells and alters their interactions are not completely understood. In this study, we characterize the effects of PLY on neutrophils and platelets and explore the mechanisms by which PLY may induce neutrophil–platelet interactions. In vitro studies demonstrated that PLY causes the formation of neutrophil extracellular traps (NETs) and the release of extracellular vesicles (EVs) from both human and murine neutrophils. In vivo, neutrophil EV (nEV) levels were increased in mice infected with S. pneumoniae. In platelets, treatment with PLY induced the cell surface expression of P-selectin (CD62P) and binding to annexin V and caused a significant release of platelet EVs (pl-EVs). Moreover, PLY-induced nEVs but not NETs promoted platelet activation. The pretreatment of nEVs with proteinase K inhibited platelet activation, indicating that the surface proteins of nEVs play a role in this process. Our findings demonstrate that PLY activates neutrophils and platelets to release EVs and support an important role for neutrophil EVs in modulating platelet functions in pneumococcal infections.

Beyond pore formation: reorganization of the plasma membrane induced by pore-forming proteins

Pore-forming proteins (PFPs) are a heterogeneous group of proteins that are expressed and secreted by a wide range of organisms. PFPs are produced as soluble monomers that bind to a receptor molecule in the host cell membrane. They then assemble into oligomers that are incorporated into the lipid membrane to form transmembrane pores. Such pore formation alters the permeability of the plasma membrane and is one of the most common mechanisms used by PFPs to destroy target cells. Interestingly, PFPs can also indirectly manipulate diverse cellular functions. In recent years, increasing evidence indicates that the interaction of PFPs with lipid membranes is not only limited to pore-induced membrane permeabilization but is also strongly associated with extensive plasma membrane reorganization. This includes lateral rearrangement and deformation of the lipid membrane, which can lead to the disruption of target cell function and finally death. Conversely, these modifications also constitute an essential component of the membrane repair system that protects cells from the lethal consequences of pore formation. Here, we provide an overview of the current knowledge on the changes in lipid membrane organization caused by PFPs from different organisms.

Bacterial Toxins Are a Never-Ending Source of Surprises: From Natural Born Killers to Negotiators

The idea that bacterial toxins are not only killers but also execute more sophisticated roles during bacteria-host interactions by acting as negotiators has been highlighted in the past decades. Depending on the toxin, its cellular target and mode of action, the final regulatory outcome can be different. In this review, we have focused on two families of bacterial toxins: genotoxins and pore-forming toxins, which have different modes of action but share the ability to modulate the host's immune responses, independently of their capacity to directly kill immune cells. We have addressed their immuno-suppressive effects with the perspective that these may help bacteria to avoid clearance by the host's immune response and, concomitantly, limit detrimental immunopathology. These are optimal conditions for the establishment of a persistent infection, eventually promoting asymptomatic carriers. This immunomodulatory effect can be achieved with different strategies such as suppression of pro-inflammatory cytokines, re-polarization of the immune response from a pro-inflammatory to a tolerogenic state, and bacterial fitness modulation to favour tissue colonization while preventing bacteraemia. An imbalance in each of those effects can lead to disease due to either uncontrolled bacterial proliferation/invasion, immunopathology, or both.

Microvesicles released from pneumolysin-stimulated lung epithelial cells carry mitochondrial cargo and suppress neutrophil oxidative burst

Microvesicles (MVs) are cell-derived extracellular vesicles that have emerged as markers and mediators of acute lung injury (ALI). One of the most common pathogens in pneumonia-induced ALI is Streptococcus pneumoniae (Spn), but the role of MVs during Spn lung infection is largely unknown. In the first line of defense against Spn and its major virulence factor, pneumolysin (PLY), are the alveolar epithelial cells (AEC). In this study, we aim to characterize MVs shed from PLY-stimulated AEC and explore their contribution in mediating crosstalk with neutrophils. Using in vitro cell and ex vivo (human lung tissue) models, we demonstrated that Spn in a PLY-dependent manner stimulates AEC to release increased numbers of MVs. Spn infected mice also had higher levels of epithelial-derived MVs in their alveolar compartment compared to control. Furthermore, MVs released from PLY-stimulated AEC contain mitochondrial content and can be taken up by neutrophils. These MVs then suppress the ability of neutrophils to produce reactive oxygen species, a critical host-defense mechanism. Taken together, our results demonstrate that AEC in response to pneumococcal PLY release MVs that carry mitochondrial cargo and suggest that these MVs regulate innate immune responses during lung injury.

Plasma membrane integrity: implications for health and disease

Plasma membrane integrity is essential for cellular homeostasis. In vivo, cells experience plasma membrane damage from a multitude of stressors in the extra- and intra-cellular environment. To avoid lethal consequences, cells are equipped with repair pathways to restore membrane integrity. Here, we assess plasma membrane damage and repair from a whole-body perspective. We highlight the role of tissue-specific stressors in health and disease and examine membrane repair pathways across diverse cell types. Furthermore, we outline the impact of genetic and environmental factors on plasma membrane integrity and how these contribute to disease pathogenesis in different tissues.

Pore-forming toxins of foodborne pathogens

Pore-forming toxins (PFTs) are water-soluble molecules that have been identified as the most crucial virulence factors during bacterial pathogenesis. PFTs disrupt the host cell membrane to internalize or to deliver other bacterial or virulence factors for establishing infections. Disruption of the host cell membrane by PFTs can lead to uncontrollable exchanges between the extracellular and the intracellular matrix, thereby disturbing the cellular homeostasis. Recent studies have provided insights into the molecular mechanism of PFTs during pathogenesis. Evidence also suggests the activation of several signal transduction pathways in the host cell on recognition of PFTs. Additionally, numerous distinctive host defense mechanisms as well as membrane repair mechanisms have been reported; however, studies reveal that PFTs aid in host immune evasion of the bacteria through numerous pathways. PFTs have been primarily associated with foodborne pathogens. Infection and death from diseases by consuming contaminated food are a constant threat to public health worldwide, affecting socioeconomic development. Moreover, the emergence of new foodborne pathogens has led to the rise of bacterial antimicrobial resistance affecting the population. Hence, this review focuses on the role of PFTs secreted by foodborne pathogens. The review highlights the molecular mechanism of foodborne bacterial PFTs, assisting bacterial survival from the host immune responses and understanding the downstream mechanism in the activation of various signaling pathways in the host upon PFT recognition. PFT research is a remarkable and an important field for exploring novel and broad applications of antimicrobial compounds as therapeutics.

Pore-forming toxins in infection and immunity

The integrity of the plasma membranes is extremely crucial for the survival and proper functioning of the cells. Organisms from all kingdoms of life employ specialized pore-forming proteins and toxins (PFPs and PFTs) that perforate cell membranes, and cause detrimental effects. PFPs/PFTs exert their damaging actions by forming oligomeric pores in the membrane lipid bilayer. PFPs/PFTs play important roles in diverse biological processes. Many pathogenic bacteria secrete PFTs for executing their virulence mechanisms. The immune system of the higher vertebrates employs PFPs to kill pathogen-infected cells and transformed cancer cells. The most obvious consequence of membrane pore-formation by the PFPs/PFTs is the killing of the target cells due to the disruption of the permeability barrier function of the plasma membranes. PFPs/PFTs can also activate diverse cellular processes that include activation of the stress-response pathways, induction of programmed cell death, and inflammation. Upon attack by the PFTs, host cells may also activate pathways to repair the injured membranes, restore cellular homeostasis, and trigger inflammatory immune responses. In this article, we present an overview of the diverse cellular responses that are triggered by the PFPs/PFTs, and their implications in the process of pathogen infection and immunity.

Membrane Microvesicles as Potential Vaccine Candidates

The prevention and control of infectious diseases is crucial to the maintenance and protection of social and public healthcare. The global impact of SARS-CoV-2 has demonstrated how outbreaks of emerging and re-emerging infections can lead to pandemics of significant public health and socio-economic burden. Vaccination is one of the most effective approaches to protect against infectious diseases, and to date, multiple vaccines have been successfully used to protect against and eradicate both viral and bacterial pathogens. The main criterion of vaccine efficacy is the induction of specific humoral and cellular immune responses, and it is well established that immunogenicity depends on the type of vaccine as well as the route of delivery. In addition, antigen delivery to immune organs and the site of injection can potentiate efficacy of the vaccine. In light of this, microvesicles have been suggested as potential vehicles for antigen delivery as they can carry various immunogenic molecules including proteins, nucleic acids and polysaccharides directly to target cells. In this review, we focus on the mechanisms of microvesicle biogenesis and the role of microvesicles in infectious diseases. Further, we discuss the application of microvesicles as a novel and effective vaccine delivery system.

Exosomes released from Shiga toxin 2a-treated human macrophages modulate inflammatory responses and induce cell death in toxin receptor expressing human cells

Shiga toxins (Stxs) produced by Stx-producing Escherichia coli are the primarily virulence factors of hemolytic uremic syndrome and central nervous system (CNS) impairment. Although the precise mechanisms of toxin dissemination remain unclear, Stxs bind to extracellular vesicles (EVs). Exosomes, a subset of EVs, may play a key role in Stx-mediated renal injury. To test this hypothesis, we isolated exosomes from monocyte-derived macrophages in the presence of Stx2a or Stx2 toxoids. Macrophage-like differentiated THP-1 cells treated with Stxs secreted Stx-associated exosomes (Stx-Exo) of 90-130 nm in diameter, which induced cytotoxicity in recipient cells in a toxin receptor globotriaosylceramide (Gb₃ )-dependent manner. Stx2-Exo engulfed by Gb₃ -positive cells were translocated to the endoplasmic reticulum in the human proximal tubule epithelial cell line HK-2. Stx2-Exo contained pro-inflammatory cytokine mRNAs and proteins and induced more severe inflammation than purified Stx2a accompanied by greater death of target cells such as human renal or retinal pigment epithelial cells. Blockade of exosome biogenesis using the pharmacological inhibitor GW4869 reduced Stx2-Exo-mediated human renal cell death. Stx2-Exo isolated from human primary monocyte-derived macrophages activated caspase 3/7 and resulted in significant cell death in primary human renal cortical epithelial cells. Based on these results, we speculate that Stx-containing exosomes derived from macrophages may exacerbate cytotoxicity and inflammation and trigger cell death in toxin-sensitive cells. Therapeutic interventions targeting Stx-containing exosomes may prevent or ameliorate Stx-mediated acute vascular dysfunction.

Interaction of Macrophages and Cholesterol-Dependent Cytolysins: The Impact on Immune Response and Cellular Survival

Cholesterol-dependent cytolysins (CDCs) are key virulence factors involved in many lethal bacterial infections, including pneumonia, necrotizing soft tissue infections, bacterial meningitis, and miscarriage. Host responses to these diseases involve myeloid cells, especially macrophages. Macrophages use several systems to detect and respond to cholesterol-dependent cytolysins, including membrane repair, mitogen-activated protein (MAP) kinase signaling, phagocytosis, cytokine production, and activation of the adaptive immune system. However, CDCs also promote immune evasion by silencing and/or destroying myeloid cells. While there are many common themes between the various CDCs, each CDC also possesses specific features to optimally benefit the pathogen producing it. This review highlights host responses to CDC pathogenesis with a focus on macrophages. Due to their robust plasticity, macrophages play key roles in the outcome of bacterial infections. Understanding the unique features and differences within the common theme of CDCs bolsters new tools for research and therapy.

Bacterial pore-forming toxin pneumolysin: Cell membrane structure and microvesicle shedding capacity determines differential survival of cell types

Bacterial infectious diseases can lead to death or to serious illnesses. These outcomes are partly the consequence of pore-forming toxins, which are secreted by the pathogenic bacteria (eg, pneumolysin of Streptococcus pneumoniae). Pneumolysin binds to cholesterol within the plasma membrane of host cells and assembles to form trans-membrane pores, which can lead to Ca²⁺ influx and cell death. Membrane repair mechanisms exist that limit the extent of damage. Immune cells which are essential to fight bacterial infections critically rely on survival mechanisms after detrimental pneumolysin attacks. This study investigated the susceptibility of different immune cell types to pneumolysin. As a model system, we used the lymphoid T-cell line Jurkat, and myeloid cell lines U937 and THP-1. We show that Jurkat T cells are highly susceptible to pneumolysin attack. In contrast, myeloid THP-1 and U937 cells are less susceptible to pneumolysin. In line with these findings, human primary T cells are shown to be more susceptible to pneumolysin attack than monocytes. Differences in susceptibility to pneumolysin are due to (I) preferential binding of pneumolysin to Jurkat T cells and (II) cell type specific plasma membrane repair capacity. Myeloid cell survival is mostly dependent on Ca²⁺ induced expelling of damaged plasma membrane areas as microvesicles. Thus, in myeloid cells, first-line defense cells in bacterial infections, a potent cellular repair machinery ensures cell survival after pneumolysin attack. In lymphoid cells, which are important at later stages of infections, less efficient repair mechanisms and enhanced toxin binding renders the cells more sensitive to pneumolysin.

Mechanisms protecting host cells against bacterial pore-forming toxins

Pore-forming toxins (PFTs) are key virulence determinants produced and secreted by a variety of human bacterial pathogens. They disrupt the plasma membrane (PM) by generating stable protein pores, which allow uncontrolled exchanges between the extracellular and intracellular milieus, dramatically disturbing cellular homeostasis. In recent years, many advances were made regarding the characterization of conserved repair mechanisms that allow eukaryotic cells to recover from mechanical disruption of the PM membrane. However, the specificities of the cell recovery pathways that protect host cells against PFT-induced damage remain remarkably elusive. During bacterial infections, the coordinated action of such cell recovery processes defines the outcome of infected cells and is, thus, critical for our understanding of bacterial pathogenesis. Here, we review the cellular pathways reported to be involved in the response to bacterial PFTs and discuss their impact in single-cell recovery and infection.