Pore-forming toxin-mediated ion dysregulation leads to death receptor-independent necroptosis of lung epithelial cells during bacterial pneumonia

Targeting necroptosis: a promising avenue for respiratory disease treatment

Respiratory diseases are a growing concern in public health because of their potential to endanger the global community. Cell death contributes critically to the pathophysiology of respiratory diseases. Recent evidence indicates that necroptosis, a unique form of programmed cell death (PCD), plays a vital role in the molecular mechanisms underlying respiratory diseases, distinguishing it from apoptosis and conventional necrosis. Necroptosis is a type of inflammatory cell death governed by receptor-interacting serine/threonine protein kinase 1 (RIPK1), RIPK3, and mixed-lineage kinase domain-like protein (MLKL), resulting in the release of intracellular contents and inflammatory factors capable of initiating an inflammatory response in adjacent tissues. These necroinflammatory conditions can result in significant organ dysfunction and long-lasting tissue damage within the lungs. Despite evidence linking necroptosis to various respiratory diseases, there are currently no specific alternative treatments that target this mechanism. This review provides a comprehensive overview of the most recent advancements in understanding the significance and mechanisms of necroptosis. Specifically, this review emphasizes the intricate association between necroptosis and respiratory diseases, highlighting the potential use of necroptosis as an innovative therapeutic approach for treating these conditions.

Blocking HXA3-mediated neutrophil elastase release during S. pneumoniae lung infection limits pulmonary epithelial barrier disruption and bacteremia

Streptococcus pneumoniae (Sp), a leading cause of community-acquired pneumonia, can spread from the lung into the bloodstream to cause septicemia and meningitis, with a concomitant threefold increase in mortality. Limitations in vaccine efficacy and a rise in antimicrobial resistance have spurred searches for host-directed therapies that target pathogenic immune processes. Polymorphonuclear leukocytes (PMNs) are essential for infection control but can also promote tissue damage and pathogen spread. The major Sp virulence factor, pneumolysin, triggers acute inflammation by stimulating the 12-lipoxygenase (12-LOX) eicosanoid synthesis pathway in epithelial cells. This pathway is required for systemic spread in a mouse pneumonia model and produces a number of bioactive lipids, including hepoxilin A3 (HXA3), a hydroxy epoxide PMN chemoattractant that has been hypothesized to facilitate breach of mucosal barriers. To understand how 12-LOX-dependent inflammation promotes dissemination during Sp lung infection and dissemination, we utilized bronchial stem cell-derived air-liquid interface cultures that lack this enzyme to show that HXA3 methyl ester (HXA3-ME) is sufficient to promote basolateral-to-apical PMN transmigration, monolayer disruption, and concomitant Sp barrier breach. In contrast, PMN transmigration in response to the non-eicosanoid chemoattractant N-formyl-L-methionyl-L-leucyl-phenylalanine (fMLP) did not lead to epithelial disruption or bacterial translocation. Correspondingly, HXA3-ME but not fMLP increased the release of neutrophil elastase (NE) from Sp-infected PMNs. Pharmacologic blockade of NE secretion or activity diminished epithelial barrier disruption and bacteremia after pulmonary challenge of mice. Thus, HXA3 promotes barrier-disrupting PMN transmigration and NE release, pathological events that can be targeted to curtail systemic disease following pneumococcal pneumonia.IMPORTANCEStreptococcus pneumoniae (Sp), a leading cause of pneumonia, can spread from the lung into the bloodstream to cause systemic disease. Limitations in vaccine efficacy and a rise in antimicrobial resistance have spurred searches for host-directed therapies that limit pathologic host immune responses to Sp. Excessive polymorphonuclear leukocyte (PMN) infiltration into Sp-infected airways promotes systemic disease. Using stem cell-derived respiratory cultures that reflect bona fide lung epithelium, we identified eicosanoid hepoxilin A3 as a critical pulmonary PMN chemoattractant that is sufficient to drive PMN-mediated epithelial damage by inducing the release of neutrophil elastase. Inhibition of the release or activity of this protease in mice limited epithelial barrier disruption and bacterial dissemination, suggesting a new host-directed treatment for Sp lung infection.

The role of RIPK1 in chronic obstructive pulmonary disease

Necroptosis has emerged as one of the crucial pathological processes involved in the regulation of cell death and inflammation in chronic obstructive pulmonary disease (COPD). Airway epithelial necroptosis is closely linked to COPD pathogenesis. Necroptotic lung cells can release damage-associated molecular patterns (DAMPs) that can initiate a robust inflammatory response. However, the underlying mechanism of necroptosis in COPD is still not clearly understood. Therefore, we aimed to explore the roles and mechanisms of receptor-interacting serine/threonine-protein kinase 1 (RIPK1)-mediated necroptosis in the regulation of inflammatory responses in COPD to provide insights into RIPK1-inhibitor drug discovery efforts and their therapeutic benefits in COPD.

Blocking HXA3-mediated neutrophil elastase release during S. pneumoniae lung infection limits pulmonary epithelial barrier disruption and bacteremia

Streptococcus pneumoniae (Sp), a leading cause of community-acquired pneumonia, can spread from the lung into the bloodstream to cause septicemia and meningitis, with a concomitant three-fold increase in mortality. Limitations in vaccine efficacy and a rise in antimicrobial resistance have spurred searches for host-directed therapies that target pathogenic immune processes. Polymorphonuclear leukocytes (PMNs) are essential for infection control but can also promote tissue damage and pathogen spread. The major Sp virulence factor, pneumolysin (PLY), triggers acute inflammation by stimulating the 12-lipoxygenase (12-LOX) eicosanoid synthesis pathway in epithelial cells. This pathway is required for systemic spread in a mouse pneumonia model and produces a number of bioactive lipids, including hepoxilin A3 (HXA3), a hydroxy epoxide PMN chemoattractant that has been hypothesized to facilitate breach of mucosal barriers. To understand how 12-LOX-dependent inflammation promotes dissemination during Sp lung infection and dissemination, we utilized bronchial stem cell-derived air-liquid interface (ALI) cultures that lack this enzyme to show that HXA3 methyl ester (HXA3-ME) is sufficient to promote basolateral-to-apical PMN transmigration, monolayer disruption, and concomitant Sp barrier breach. In contrast, PMN transmigration in response to the non-eicosanoid chemoattractant fMLP did not lead to epithelial disruption or bacterial translocation. Correspondingly, HXA3-ME but not fMLP increased release of neutrophil elastase (NE) from Sp-infected PMNs. Pharmacologic blockade of NE secretion or activity diminished epithelial barrier disruption and bacteremia after pulmonary challenge of mice. Thus, HXA3 promotes barrier disrupting PMN transmigration and NE release, pathological events that can be targeted to curtail systemic disease following pneumococcal pneumonia.

Story of Pore-Forming Proteins from Deadly Disease-Causing Agents to Modern Applications with Evolutionary Significance

Animal venoms are a complex mixture of highly specialized toxic molecules. Among them, pore-forming proteins (PFPs) or toxins (PFTs) are one of the major disease-causing toxic elements. The ability of the PFPs in defense and toxicity through pore formation on the host cell surface makes them unique among the toxin proteins. These features made them attractive for academic and research purposes for years in the areas of microbiology as well as structural biology. All the PFPs share a common mechanism of action for the attack of host cells and pore formation in which the selected pore-forming motifs of the host cell membrane-bound protein molecules drive to the lipid bilayer of the cell membrane and eventually produces water-filled pores. But surprisingly their sequence similarity is very poor. Their existence can be seen both in a soluble state and also in transmembrane complexes in the cell membrane. PFPs are prevalent toxic factors that are predominately produced by all kingdoms of life such as virulence bacteria, nematodes, fungi, protozoan parasites, frogs, plants, and also from higher organisms. Nowadays, multiple approaches to applications of PFPs have been conducted by researchers both in basic as well as applied biological research. Although PFPs are very devastating for human health nowadays researchers have been successful in making these toxic proteins into therapeutics through the preparation of immunotoxins. We have discussed the structural, and functional mechanism of action, evolutionary significance through dendrogram, domain organization, and practical applications for various approaches. This review aims to emphasize the PFTs to summarize toxic proteins together for basic knowledge as well as to highlight the current challenges, and literature gap along with the perspective of promising biotechnological applications for their future research.

Bacterial cholesterol-dependent cytolysins and their interaction with the human immune response

PURPOSE OF REVIEW

Many cholesterol-dependent cytolysin (CDC)-producing pathogens pose a significant threat to human health. Herein, we review the pore-dependent and -independent properties CDCs possess to assist pathogens in evading the host immune response.

RECENT FINDINGS

Within the last 5 years, exciting new research suggests CDCs can act to inhibit important immune functions, disrupt critical cell signaling pathways, and have tissue-specific effects. Additionally, recent studies have identified a key region of CDCs that generates robust immunity, providing resources for the development of CDC-based vaccines.

SUMMARY

This review provides new information on how CDCs alter host immune responses to aid bacteria in pathogenesis. These studies can assist in the design of more efficient vaccines and therapeutics against CDCs that will enhance the immune response to CDC-producing pathogens while mitigating the dampening effects CDCs have on the host immune response.

NLRP3 knockout in mice provided protection against Serratia marcescens-induced acute pneumonia by decreasing PD-L1 and PD-1 expression in macrophages

Antibiotic-resistant Serratia marcescens (Sm) is known to cause bloodstream infections, pneumonia, etc. The nod-like receptor family, pyrin domain-containing 3 (NLRP3), has been implicated in various lung infections. Yet, its role in Sm-induced pneumonia was not well understood. In our study, we discovered that deletion of Nlrp3 in mice significantly improved Sm-induced survival rates, reduced bacterial loads in the lungs, bronchoalveolar lavage fluid (BALF), and bloodstream, and mitigated the severity of acute lung injury (ALI) compared to wild-type (WT) mice. Mechanistically, we observed that 24 h post-Sm infection, NLRP3 inflammasome activation occurred, leading to gasdermin D NH2-terminal (GSDMD-NT)-induced pyroptosis in macrophages and IL-1β secretion. The NLRP3 or NLRP3 inflammasome influenced the expression PD-L1 and PD-1, as well as the count of PD-L1 or PD-1-expressing macrophages, alveolar macrophages, interstitial macrophages, PD-L1-expressing neutrophils, and the count of macrophage receptors with collagenous structure (MARCO)-expressing macrophages, particularly MARCO+ alveolar macrophages. The frequency of MARCO+ alveolar macrophages, PD-1 expression, particularly PD-1+ interstitial macrophages were negatively or positively correlated with the Sm load, respectively. Additionally, IL-1β levels in BALF correlated with three features of acute lung injury: histologic score, protein concentration and neutrophil count in BALF. Consequently, our findings suggest that Nlrp3 deletion offers protection agaisnt acute Sm pneumonia in mice by inhibiting inflammasome activation and reducing Sm infection-induced PD-L1/PD-1 or MARCO expression, particularly in macrophages. This highlights potential therapeutic targets for Sm and other gram-negative bacteria-induced acute pneumonia.

Membrane Dynamics and Cation Handling in Ferroptosis

Ferroptosis, a regulated cell death hallmarked by excessive lipid peroxidation, is implicated in various (patho)physiological contexts. During ferroptosis, lipid peroxidation leads to a diverse change in membrane properties and the dysregulation of ion homeostasis via the cation channels, ultimately resulting in plasma membrane rupture. This review illuminates cellular membrane dynamics and cation handling in ferroptosis regulation.

Necroptosis in Pneumonia: Therapeutic Strategies and Future Perspectives

Pneumonia remains a major global health challenge, necessitating the development of effective therapeutic approaches. Recently, necroptosis, a regulated form of cell death, has garnered attention in the fields of pharmacology and immunology for its role in the pathogenesis of pneumonia. Characterized by cell death and inflammatory responses, necroptosis is a key mechanism contributing to tissue damage and immune dysregulation in various diseases, including pneumonia. This review comprehensively analyzes the role of necroptosis in pneumonia and explores potential pharmacological interventions targeting this cell death pathway. Moreover, we highlight the intricate interplay between necroptosis and immune responses in pneumonia, revealing a bidirectional relationship between necrotic cell death and inflammatory signaling. Importantly, we assess current therapeutic strategies modulating necroptosis, encompassing synthetic inhibitors, natural products, and other drugs targeting key components of the programmed necrosis pathway. The article also discusses challenges and future directions in targeting programmed necrosis for pneumonia treatment, proposing novel therapeutic strategies that combine antibiotics with necroptosis inhibitors. This review underscores the importance of understanding necroptosis in pneumonia and highlights the potential of pharmacological interventions to mitigate tissue damage and restore immune homeostasis in this devastating respiratory infection.

Cell type-specific molecular mechanisms and implications of necroptosis in inflammatory respiratory diseases

Necroptosis is generally considered as an inflammatory cell death form. The core regulators of necroptotic signaling are receptor-interacting serine-threonine protein kinases 1 (RIPK1) and RIPK3, and the executioner, mixed lineage kinase domain-like pseudokinase (MLKL). Evidence demonstrates that necroptosis contributes profoundly to inflammatory respiratory diseases that are common public health problem. Necroptosis occurs in nearly all pulmonary cell types in the settings of inflammatory respiratory diseases. The influence of necroptosis on cells varies depending upon the type of cells, tissues, organs, etc., which is an important factor to consider. Thus, in this review, we briefly summarize the current state of knowledge regarding the biology of necroptosis, and focus on the key molecular mechanisms that define the necroptosis status of specific cell types in inflammatory respiratory diseases. We also discuss the clinical potential of small molecular inhibitors of necroptosis in treating inflammatory respiratory diseases, and describe the pathological processes that engage cross talk between necroptosis and other cell death pathways in the context of respiratory inflammation. The rapid advancement of single-cell technologies will help understand the key mechanisms underlying cell type-specific necroptosis that are critical to effectively treat pathogenic lung infections and inflammatory respiratory diseases.

Lipopolysaccharide and Glycolipoprotein Coordinately Triggered Necroptosis Contributes to the Pathogenesis of Leptospira Infection in Mice

Leptospirosis is a recurring but neglected zoonotic disease caused by pathogenic Leptospira. The explicit underlying mechanism of necroptosis and its role in Leptospira infection have not yet been elucidated. Here we reported that leptospiral pathogen-associated molecular patterns, lipopolysaccharide, and glycolipoprotein activate the necroptotic RIPK1-RIPK3-MLKL cascade through the TLR4 signaling pathway in mouse macrophages. Using the murine acute leptospirosis model, we reveal that abolition of necroptosis exhibited significantly improved outcomes in acute phases, with enhanced eradication of Leptospira from liver, mild clinical symptoms, and decreased cytokine production. RIPK3 was also found to exert a necroptosis-independent function in CXCL1 production and neutrophil recruitment, with the consequence of improved Leptospira control. These findings improve our understanding of the mechanism of Leptospira-macrophage interactions, indicating potential therapeutic values by targeting necroptosis signaling pathways.

Necroptosis in apical periodontitis: A programmed cell death with multiple roles

Programmed cell death (PCD) has been a research focus for decades and different mechanisms of cell death, such as necroptosis, pyroptosis, ferroptosis, and cuproptosis have been discovered. Necroptosis, a form of inflammatory PCD, has gained increasing attention in recent years due to its critical role in disease progression and development. Unlike apoptosis, which is mediated by caspases and characterized by cell shrinkage and membrane blebbing, necroptosis is mediated by mixed lineage kinase domain-like protein (MLKL) and characterized by cell enlargement and plasma membrane rupture. Necroptosis can be triggered by bacterial infection, which on the one hand represents a host defense mechanism against the infection, but on the other hand can facilitate bacterial escape and worsen inflammation. Despite its importance in various diseases, a comprehensive review on the involvement and roles of necroptosis in apical periodontitis is still lacking. In this review, we tried to provide an overview of recent progresses in necroptosis research, summarized the pathways involved in apical periodontitis (AP) activation, and discussed how bacterial pathogens induce and regulated necroptosis and how necroptosis would inhibit bacteria. Furthermore, the interplay between various types of cell death in AP and the potential treatment strategy for AP by targeting necroptosis were also discussed.

A programmed cell death-related model based on machine learning for predicting prognosis and immunotherapy responses in patients with lung adenocarcinoma

Background

lung adenocarcinoma (LUAD) remains one of the most common and lethal malignancies with poor prognosis. Programmed cell death (PCD) is an evolutionarily conserved cell suicide process that regulates tumorigenesis, progression, and metastasis of cancer cells. However, a comprehensive analysis of the role of PCD in LUAD is still unavailable.

Methods

We analyzed multi-omic variations in PCD-related genes (PCDRGs) for LUAD. We used cross-validation of 10 machine learning algorithms (101 combinations) to synthetically develop and validate an optimal prognostic cell death score (CDS) model based on the PCDRGs expression profile. Patients were classified based on their median CDS values into the high and low-CDS groups. Next, we compared the differences in the genomics, biological functions, and tumor microenvironment of patients between both groups. In addition, we assessed the ability of CDS for predicting the response of patients from the immunotherapy cohort to immunotherapy. Finally, functional validation of key genes in CDS was performed.

Results

We constructed CDS based on four PCDRGs, which could effectively and consistently stratify patients with LUAD (patients with high CDS had poor prognoses). The performance of our CDS was superior compared to 77 LUAD signatures that have been previously published. The results revealed significant genetic alterations like mutation count, TMB, and CNV were observed in patients with high CDS. Furthermore, we observed an association of CDS with immune cell infiltration, microsatellite instability, SNV neoantigens. The immune status of patients with low CDS was more active. In addition, CDS could be reliable to predict therapeutic response in multiple immunotherapy cohorts. In vitro experiments revealed that high DNA damage inducible transcript 4 (DDIT4) expression in LUAD cells mediated protumor effects.

Conclusion

CDS was constructed based on PCDRGs using machine learning. This model could accurately predict patients' prognoses and their responses to therapy. These results provide new promising tools for clinical management and aid in designing personalized treatment strategies for patients with LUAD.

The Global Burden of Community-Acquired Pneumonia in Adults, Encompassing Invasive Pneumococcal Disease and the Prevalence of Its Associated Cardiovascular Events, with a Focus on Pneumolysin and Macrolide Antibiotics in Pathogenesis and Therapy

Despite innovative advances in anti-infective therapies and vaccine development technologies, community-acquired pneumonia (CAP) remains the most persistent cause of infection-related mortality globally. Confronting the ongoing threat posed by Streptococcus pneumoniae (the pneumococcus), the most common bacterial cause of CAP, particularly to the non-immune elderly, remains challenging due to the propensity of the elderly to develop invasive pneumococcal disease (IPD), together with the predilection of the pathogen for the heart. The resultant development of often fatal cardiovascular events (CVEs), particularly during the first seven days of acute infection, is now recognized as a relatively common complication of IPD. The current review represents an update on the prevalence and types of CVEs associated with acute bacterial CAP, particularly IPD. In addition, it is focused on recent insights into the involvement of the pneumococcal pore-forming toxin, pneumolysin (Ply), in subverting host immune defenses, particularly the protective functions of the alveolar macrophage during early-stage disease. This, in turn, enables extra-pulmonary dissemination of the pathogen, leading to cardiac invasion, cardiotoxicity and myocardial dysfunction. The review concludes with an overview of the current status of macrolide antibiotics in the treatment of bacterial CAP in general, as well as severe pneumococcal CAP, including a consideration of the mechanisms by which these agents inhibit the production of Ply by macrolide-resistant strains of the pathogen.

Acute organ injury and long-term sequelae of severe pneumococcal infections

Streptococcus pneumoniae (Spn) is a major public health problem, as it is a main cause of otitis media, community-acquired pneumonia, bacteremia, sepsis, and meningitis. Acute episodes of pneumococcal disease have been demonstrated to cause organ damage with lingering negative consequences. Cytotoxic products released by the bacterium, biomechanical and physiological stress resulting from infection, and the corresponding inflammatory response together contribute to organ damage accrued during infection. The collective result of this damage can be acutely life-threatening, but among survivors, it also contributes to the long-lasting sequelae of pneumococcal disease. These include the development of new morbidities or exacerbation of pre-existing conditions such as COPD, heart disease, and neurological impairments. Currently, pneumonia is ranked as the 9^th leading cause of death, but this estimate only considers short-term mortality and likely underestimates the true long-term impact of disease. Herein, we review the data that indicates damage incurred during acute pneumococcal infection can result in long-term sequelae which reduces quality of life and life expectancy among pneumococcal disease survivors.

An Atypical, Staged Cell Death Pathway Induced by Depletion of SNARE-Proteins MUNC18-1 or Syntaxin-1

The presynaptic proteins MUNC18-1, syntaxin-1, and SNAP25 drive SNARE-mediated synaptic vesicle fusion and are also required for neuronal viability. Their absence triggers rapid, cell-autonomous, neuron-specific degeneration, unrelated to synaptic vesicle deficits. The underlying cell death pathways remain poorly understood. Here, we show that hippocampi of munc18-1 null mice (unknown sex) express apoptosis hallmarks cleaved caspase 3 (CC-3) and phosphorylated p53, and have condensed nuclei. However, side-by-side in vitro comparison with classical apoptosis induced by camptothecin uncovered striking differences to syntaxin-1 and MUNC18-1 depleted neurons. First, live-cell imaging revealed consecutive neurite retraction hours before cell death in MUNC18-1 or syntaxin-1 depleted neurons, whereas all neurites retracted at once, directly before cell death in classical apoptosis. Second, CC-3 activation was observed only after loss of all neurites and cellular breakdown, whereas CC-3 is activated before any neurite loss in classical apoptosis. Third, a pan-caspase inhibitor and a p53 inhibitor both arrested classical apoptosis, as expected, but not cell death in MUNC18-1 or syntaxin-1 depleted neurons. Neuron-specific cell death, consecutive neurite retraction, and late CC-3 activation were conserved in syntaxin-1 depleted human neurons. Finally, no indications were observed for involvement of other established cell death pathways, including necroptosis, Wallerian degeneration, autophagic cell death, and pyroptosis. Together, these data show that depletion of presynaptic proteins MUNC18-1 or syntaxin-1 triggers an atypical, staged cell death pathway characterized by consecutive neurite retraction, ultimately leading to, but not driven by, apoptosis.SIGNIFICANCE STATEMENT Neuronal cell death can occur via a multitude of pathways and plays an important role in the developing nervous system as well as neurodegenerative diseases. One poorly understood pathway to neuronal cell death takes place on depletion of presynaptic SNARE proteins syntaxin-1, SNAP25, or MUNC18-1. The current study demonstrates that MUNC18-1 or syntaxin-1 depleted neurons show a new, atypical, staged cell death that does not resemble any of the established cell death pathways in neurons. Cell death on MUNC18-1 or syntaxin-1 depletion is characterized by consecutive neurite retraction, ultimately involving, but not driven by, classical apoptosis.

Meningitis-associated pneumococcal serotype 8, ST 53, strain is hypervirulent in a rat model and has non-haemolytic pneumolysin which can be attenuated by liposomes

Introduction

Streptococcus pneumoniae bacteria cause life-threatening invasive pneumococcal disease (IPD), including meningitis. Pneumococci are classified into serotypes, determined by differences in capsular polysaccharide and both serotype and pneumolysin toxin are associated with disease severity. Strains of serotype 8, ST 53, are increasing in prevalence in IPD in several countries.

Methods

Here we tested the virulence of such an isolate in a rat model of meningitis in comparison with a serotype 15B and a serotype 14 isolate. All three were isolated from meningitis patients in South Africa in 2019, where serotype 8 is currently the most common serotype in IPD.

Results and Discussion

Only the serotype 8 isolate was hypervirulent causing brain injury and a high mortality rate. It induced a greater inflammatory cytokine response than either the serotype 15B or 14 strain in the rat model and from primary mixed-glia cells isolated from mouse brains. It had the thickest capsule of the three strains and produced non-haemolytic pneumolysin. Pneumolysin-sequestering liposomes reduced the neuroinflammatory cytokine response in vitro indicating that liposomes have the potential to be an effective adjuvant therapy even for hypervirulent pneumococcal strains with non-haemolytic pneumolysin.

Tight Junctions, the Epithelial Barrier, and Toll-like Receptor-4 During Lung Injury

Lung epithelium is constantly exposed to the environment and is critically important for the orchestration of initial responses to infectious organisms, toxins, and allergic stimuli, and maintenance of normal gaseous exchange and pulmonary function. The integrity of lung epithelium, fluid balance, and transport of molecules is dictated by the tight junctions (TJs). The TJs are formed between adjacent cells. We have focused on the topic of the TJ structure and function in lung epithelial cells. This review includes a summary of the last twenty years of literature reports published on the disrupted TJs and epithelial barrier in various lung conditions and expression and regulation of specific TJ proteins against pathogenic stimuli. We discuss the molecular signaling and crosstalk among signaling pathways that control the TJ structure and function. The Toll-like receptor-4 (TLR4) recognizes the pathogen- and damage-associated molecular patterns released during lung injury and inflammation and coordinates cellular responses. The molecular aspects of TLR4 signaling in the context of TJs or the epithelial barrier are not fully known. We describe the current knowledge and possible networking of the TLR4-signaling with cellular and molecular mechanisms of TJs, lung epithelial barrier function, and resistance to treatment strategies.

A multiomics analysis of direct interkingdom dynamics between influenza A virus and Streptococcus pneumoniae uncovers host-independent changes to bacterial virulence fitness

BACKGROUND

For almost a century, it has been recognized that influenza A virus (IAV) infection can promote the development of secondary bacterial infections (SBI) mainly caused by Streptococcus pneumoniae (Spn). Recent observations have shown that IAV is able to directly bind to the surface of Spn. To gain a foundational understanding of how direct IAV-Spn interaction alters bacterial biological fitness we employed combinatorial multiomic and molecular approaches.

RESULTS

Here we show IAV significantly remodels the global transcriptome, proteome and phosphoproteome profiles of Spn independently of host effectors. We identified Spn surface proteins that interact with IAV proteins (hemagglutinin, nucleoprotein, and neuraminidase). In addition, IAV was found to directly modulate expression of Spn virulence determinants such as pneumococcal surface protein A, pneumolysin, and factors associated with antimicrobial resistance among many others. Metabolic pathways were significantly altered leading to changes in Spn growth rate. IAV was also found to drive Spn capsule shedding and the release of pneumococcal surface proteins. Released proteins were found to be involved in evasion of innate immune responses and actively reduced human complement hemolytic and opsonizing activity. IAV also led to phosphorylation changes in Spn proteins associated with metabolism and bacterial virulence. Validation of proteomic data showed significant changes in Spn galactose and glucose metabolism. Furthermore, supplementation with galactose rescued bacterial growth and promoted bacterial invasion, while glucose supplementation led to enhanced pneumolysin production and lung cell apoptosis.

CONCLUSIONS

Here we demonstrate that IAV can directly modulate Spn biology without the requirement of host effectors and support the notion that inter-kingdom interactions between human viruses and commensal pathobionts can promote bacterial pathogenesis and microbiome dysbiosis.

RIPK1 and RIPK3 in antibacterial defence

Upon sensing pathogenic bacterial infection, host cells activate a multitude of inflammatory and immunogenic responses to promote bacterial clearance and restore tissue homeostasis. RIPK1 and RIPK3 are two key players in antimicrobial defence, by either driving inflammatory signalling or inducing programmed cell death activation, ranging from apoptosis, pyroptosis to necroptosis. In this review, we first discuss the mechanisms by which RIPK1 and RIPK3 promote the assembly of death-inducing complexes and how these cell death pathways are activated as host responses to counteract pathogenic bacteria. We further outline the immunological importance of cell death in antibacterial defence and highlight outstanding questions in the field.

A necroptosis-related prognostic model for predicting prognosis, immune landscape, and drug sensitivity in hepatocellular carcinoma based on single-cell sequencing analysis and weighted co-expression network

Background: Hepatocellular carcinoma (HCC) is a highly lethal cancer and is the second leading cause of cancer-related deaths worldwide. Unlike apoptosis, necroptosis (NCPS) triggers an immune response by releasing damage-related molecular factors. However, the clinical prognostic features of necroptosis-associated genes in HCC are still not fully explored.

Methods: We analyzed the single-cell datasets GSE125449 and GSE151530 from the GEO database and performed weighted co-expression network analysis on the TCGA data to identify the necroptosis genes. A prognostic model was built using COX and Lasso regression. In addition, we performed an analysis of survival, immunity microenvironment, and mutation. Furthermore, the hub genes and pathways associated with HCC were localized within the single-cell atlas.

Results: Patients with HCC in the TCGA and ICGC cohorts were classified using a necroptosis-related model with significant differences in survival times between high- and low-NCPS groups (p < 0.05). High-NCPS patients expressed more immune checkpoint-related genes, suggesting immunotherapy and some chemotherapies might prove beneficial to them. In addition, a single-cell sequencing approach was conducted to investigate the expression of hub genes and associated signaling pathways in different cell types.

Conclusion: Through the analysis of single-cell and bulk multi-omics sequencing data, we constructed a prognostic model related to necroptosis and explored the relationship between high- and low-NCPS groups and immune cell infiltration in HCC. This provides a new reference for further understanding the role of necroptosis in HCC.

Enterococcus faecalis-Induced Macrophage Necroptosis Promotes Refractory Apical Periodontitis

The persistence of residual bacteria, particularly Enterococcus faecalis, contributes to refractory periapical periodontitis, which still lacks effective therapy. The role of receptor-interacting protein kinase 3 (RIPK3)- and mixed lineage kinase domain-like protein (MLKL)-mediated necroptosis, a highly proinflammatory form of regulated cell death, has recently drawn much attention. However, the role of necroptosis in the pathogenesis of refractory periapical periodontitis remains unclear. We investigated whether the RIPK3/MLKL signaling pathway was activated in periapical lesion specimens obtained from patients diagnosed with refractory periapical periodontitis. RIPK3-deficient mice were then used to determine the role of necroptosis under this condition in vivo. We found that the phosphorylation levels of RIPK3 and MLKL were elevated in periapical lesion specimens of patients with refractory periapical periodontitis. In addition, necroptosis was induced in an E. faecalis-infected refractory periapical periodontitis mouse model, in which inhibition of necroptosis by RIPK3 deficiency could markedly alleviate inflammation and bone destruction. Moreover, double-labeling immunofluorescence suggested that macrophage necroptosis may be involved in the development of refractory periapical periodontitis. Then, we established an in vitro macrophage infection model with E. faecalis. E. faecalis infection was found to induce necroptotic cell death in macrophages through the RIPK3/MLKL signaling pathway, which was markedly alleviated by the RIPK3- or MLKL-specific inhibitor. Our study revealed that RIPK3/MLKL-mediated macrophage necroptosis contributes to the development of refractory periapical periodontitis and suggests that inhibitors or treatments targeting necroptosis represent a plausible strategy for the management of refractory periapical periodontitis. IMPORTANCE Oral infectious diseases represent a major neglected global population health challenge, imposing an increasing burden on public health and economy. Refractory apical periodontitis (RAP), mainly caused by Enterococcus faecalis, is a representative oral infectious disease with considerable therapeutic challenges. The interplay between E. faecalis and the host often leads to the activation of programmed cell death. This study identifies an important role of macrophage necroptosis induced by E. faecalis in the pathogenesis of RAP. Manipulating RIPK3/MLKL-mediated necroptosis may represent novel therapeutic targets, not only for RAP but also for other E. faecalis-associated infectious diseases.

Staphylococcus aureus-Induced Necroptosis Promotes Mitochondrial Damage in Goat Endometrial Epithelial Cells

Simple Summary

The death of endometrial cells induced by bacterial infections can result in damage to the endometrial function. In this study, we investigated the potential role of necroptosis in Staphylococcus aureus-induced goat endometrial epithelial cell (gEEC) death. We found that S. aureus induced gEECs RIPK1/RIPK3/MLKL-mediated necroptosis, triggered mainly by membrane disruption and ion imbalance. Moreover, gEEC necroptosis contributed to the regulation of reactive oxygen species generation and mitochondrial damage. These provide evidence of the involvement of necroptosis in the S. aureus-induced gEEC death.

Abstract

Endometrial cell death is induced by bacterial infection, resulting in damage to the physical barriers and immune function. An in-depth understanding of the mechanisms of endometrial epithelial cell necroptosis might provide new insights into the treatment of uterine diseases. In the present study, we investigated the effect of Staphylococcus aureus on goat endometrial epithelial cell (gEEC) necroptosis, and the underlying molecular mechanism. We found that S. aureus induced significant necroptosis in gEECs by increasing the expression of key proteins of the RIPK1/RIPK3/MLKL axis; importantly, this effect was alleviated by inhibitors of RIPK1, RIPK3, and MLKL. Moreover, we found that the main triggers of gEEC necroptosis induced by S. aureus were not the toll-like receptors (TLRs) and tumor necrosis factor receptor (TNFR), but membrane disruption and ion imbalance. Moreover, we observed a significant decrease in the mitochondrial membrane potential, indicating mitochondrial damage, in addition to increased cytochrome c levels and reactive oxygen species (ROS) generation in S. aureus-infected gEECs; these, effects were also suppressed by the inhibitors of RIPK1, RIPK3, and MLKL. Taken together, these data revealed the molecular mechanism of S. aureus-induced gEEC necroptosis and provided potential new targeted therapies for clinical intervention in bacterial infections.

C. elegans monitor energy status via the AMPK pathway to trigger innate immune responses against bacterial pathogens

Pathogen recognition and the triggering of host innate immune system are critical to understanding pathogen-host interaction. Cellular surveillance systems have been identified as an important strategy for the identification of microbial infection. In the present study, using Bacillus thuringiensis-Caenorhabditis elegans as a model, we found an approach for surveillance systems to sense pathogens. We report that Bacillus thuringiensis Cry5Ba, a typical pore-forming toxin, caused mitochondrial damage and energy imbalance by triggering potassium ion leakage, instead of directly targeting mitochondria. Interestingly, we find C. elegans can monitor intracellular energy status to trigger innate immune responses via AMP-activated protein kinase (AMPK), secreting multiple effectors to defend against pathogenic attacks. Our study indicates that the imbalance of energy status is a prevalent side effect of pathogen infection. Furthermore, the AMPK-dependent surveillance system may serve as a practicable strategy for the host to recognize and defense against pathogens.

Bacillus thuringiensis toxin Cry5Ba triggers potassium ion leakage, causing mitochondrial damage and energy imbalance. C. elegans can monitor this intracellular energy imbalance via AMP-activated protein kinase to trigger innate immune responses.

Reprogramming of Cell Death Pathways by Bacterial Effectors as a Widespread Virulence Strategy

The modulation of programmed cell death (PCD) processes during bacterial infections is an evolving arms race between pathogens and their hosts. The initiation of apoptosis, necroptosis, and pyroptosis pathways are essential to immunity against many intracellular and extracellular bacteria. These cellular self-destructive mechanisms are used by the infected host to restrict and eliminate bacterial pathogens. Without a tight regulatory control, host cell death can become a double-edged sword. Inflammatory PCDs contribute to an effective immune response against pathogens, but unregulated inflammation aggravates the damage caused by bacterial infections. Thus, fine-tuning of these pathways is required to resolve infection while preserving the host immune homeostasis. In turn, bacterial pathogens have evolved secreted virulence factors or effector proteins that manipulate PCD pathways to promote infection. In this review, we discuss the importance of controlled cell death in immunity to bacterial infection. We also detail the mechanisms employed by type 3 secreted bacterial effectors to bypass these pathways and their importance in bacterial pathogenesis.

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.

Interferon-γ Preferentially Promotes Necroptosis of Lung Epithelial Cells by Upregulating MLKL

Necroptosis, a form of programmed lytic cell death, has emerged as a driving factor in the pathogenesis of acute lung injury (ALI). As ALI is often associated with a cytokine storm, we determined whether pro-inflammatory cytokines modulate the susceptibility of lung cells to necroptosis and which mediators dominate to control necroptosis. In this study, we pretreated/primed mouse primary lung epithelial and endothelial cells with various inflammatory mediators and assessed cell type-dependent responses to different necroptosis inducers and their underlying mechanisms. We found that interferon-γ (IFNγ) as low as 1 ng/mL preferentially promoted necroptosis and accelerated the release of damage-associated molecular patterns from primary alveolar and airway epithelial cells but not lung microvascular endothelial cells. Type-I IFNα was about fifty-fold less effective than IFNγ. Conversely, TNFα or agonists of Toll-like receptor-3 (TLR3), TLR4, TLR7 and TLR9 had a minor effect. The enhanced necroptosis in IFNγ-activated lung epithelial cells was dependent on IFNγ signaling and receptor-interacting protein kinase-3. We further showed that necroptosis effector mixed lineage kinase domain-like protein (MLKL) was predominantly induced by IFNγ, contributing to the enhanced necroptosis in lung epithelial cells. Collectively, our findings indicate that IFNγ is a potent enhancer of lung epithelial cell susceptibility to necroptosis.

Engineered Liposomes Protect Immortalized Immune Cells from Cytolysins Secreted by Group A and Group G Streptococci

The increasing antibiotic resistance of bacterial pathogens fosters the development of alternative, non-antibiotic treatments. Antivirulence therapy, which is neither bacteriostatic nor bactericidal, acts by depriving bacterial pathogens of their virulence factors. To establish a successful infection, many bacterial pathogens secrete exotoxins/cytolysins that perforate the host cell plasma membrane. Recently developed liposomal nanotraps, mimicking the outer layer of the targeted cell membranes, serve as decoys for exotoxins, thus diverting them from attacking host cells. In this study, we develop a liposomal nanotrap formulation that is capable of protecting immortalized immune cells from the whole palette of cytolysins secreted by Streptococcus pyogenes and Streptococcus dysgalactiae subsp. equisimilis—important human pathogens that can cause life-threatening bacteremia. We show that the mixture of cholesterol-containing liposomes with liposomes composed exclusively of phospholipids is protective against the combined action of all streptococcal exotoxins. Our findings pave the way for further development of liposomal antivirulence therapy in order to provide more efficient treatment of bacterial infections, including those caused by antibiotic resistant pathogens.

Delivery of Ultrasmall Nanoparticles to the Cytosolic Compartment of Pyroptotic J774A.1 Macrophages via GSDMDNterm Membrane Pores

Endosome capture is a major physiological barrier to the successful delivery of nanomedicine. Here, we found a strategy to deliver ultrasmall nanoparticles (<10 nm) to the cytosolic compartment of pyroptotic cells with spontaneous endosomal escape. To mimic pathological pyroptotic cells, J774A.1 macrophages were stimulated with lipopolysaccharide (LPS) plus nigericin (Nig) or adenosine triphosphate (ATP) to form specific gasdermin D protein-driven membrane pores at an N-terminal domain (GSDMD^Nterm). Through GSDMD^Nterm membrane pores, both anionic and cationic nanoparticles (NPs) with diameters less than 10 nm were accessed into the cytosolic compartment of pyroptotic cells in an energy- and receptor-independent manner, while NPs larger than the size of GSDMD^Nterm membrane pores failed to enter pyroptotic cells. NPs pass through GSDMD^Nterm membrane pores via free diffusion and then access into the cytoplasm of pyroptotic cells in a microtubule-independent manner. Interestingly, we found that LPS-primed NPs may act as Trojan horse, deliver extracellular LPS into normal cells through endocytosis, and in turn induce GSDMD^Nterm membrane pores, which facilitate further internalization of NPs. This study presented a straightforward method of distinguishing normal and pyroptotic cells through GSDMD membrane pores, implicating their potential application in monitoring the delivery of desired nanomedicines in pyroptosis-related diseases and conditions.

Necroptosis in Pulmonary Diseases: A New Therapeutic Target

In the past decades, apoptosis has been the most well-studied regulated cell death (RCD) that has essential functions in tissue homeostasis throughout life. However, a novel form of RCD called necroptosis, which requires receptor-interacting protein kinase-3 (RIPK3) and mixed-lineage kinase domain-like pseudokinase (MLKL), has recently been receiving increasing scientific attention. The phosphorylation of RIPK3 enables the recruitment and phosphorylation of MLKL, which oligomerizes and translocates to the plasma membranes, ultimately leading to plasma membrane rupture and cell death. Although apoptosis elicits no inflammatory responses, necroptosis triggers inflammation or causes an innate immune response to protect the body through the release of damage-associated molecular patterns (DAMPs). Increasing evidence now suggests that necroptosis is implicated in the pathogenesis of several human diseases such as systemic inflammation, respiratory diseases, cardiovascular diseases, neurodegenerative diseases, neurological diseases, and cancer. This review summarizes the emerging insights of necroptosis and its contribution toward the pathogenesis of lung diseases.

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.

RIPK3 Activates MLKL-mediated Necroptosis and Inflammasome Signaling during Streptococcus Infection

Community-acquired pneumonia is the most common type of pneumonia and remains a leading cause of morbidity and mortality worldwide. Although many different pathogens can contribute to pneumonia, Streptococcus pneumoniae is one of the common bacterial pathogens that underlie community-acquired pneumonia. RIPK3 (receptor-interacting protein kinase 3) is widely recognized as a key modulator of inflammation and cell death. To elucidate a potential role of RIPK3 in pneumonia, we examined plasma from healthy control subjects and patients positive for streptococcal pneumonia. In human studies, RIPK3 protein concentrations were significantly elevated and were identified as a potential plasma marker of pneumococcal pneumonia. To expand these findings, we used an in vivo murine model of pneumococcal pneumonia to demonstrate that RIPK3 deficiency leads to reduced bacterial clearance, severe pathological damage, and high mortality. Our results illustrated that RIPK3 forms a complex with RIPK1, MLKL (mixed-lineage kinase domain-like protein), and MCU (mitochondrial calcium uniporter) to induce mitochondrial calcium uptake and mitochondrial reactive oxygen species(mROS) production during S. pneumoniae infection. In macrophages, RIPK3 initiated necroptosis via the mROS-mediated mitochondrial permeability transition pore opening and NLRP3 inflammasome activation via the mROS-AKT pathway to protect against S. pneumoniae. In conclusion, our study demonstrated a mechanism by which RIPK3-initiated necroptosis is essential for host defense against S. pneumoniae.

Streptococcus pneumoniae binds to host GAPDH on dying lung epithelial cells worsening secondary infection following influenza

Streptococcus pneumoniae (Spn) alone and during co-infection with influenza A virus (IAV) can result in severe pneumonia with mortality. Pneumococcal surface protein A (PspA) is an established virulence factor required for Spn evasion of lactoferricin and C-reactive protein-activated complement-mediated killing. Herein, we show that PspA functions as an adhesin to dying host cells. We demonstrate that PspA binds to host-derived glyceraldehyde-3-phosphate dehydrogenase (GAPDH) bound to outward-flipped phosphatidylserine residues on dying host cells. PspA-mediated adhesion was to apoptotic, pyroptotic, and necroptotic cells, but not healthy lung cells. Using isogenic mutants of Spn, we show that PspA-GAPDH-mediated binding to lung cells increases pneumococcal localization in the lower airway, and this is enhanced as a result of pneumolysin exposure or co-infection with IAV. PspA-mediated binding to GAPDH requires amino acids 230-281 in its α-helical domain with intratracheal inoculation of this PspA fragment alongside the bacteria reducing disease severity in an IAV/Spn pneumonia model.

Effect of cefoperazone sulbactam sodium combined with meropenem on the immune function in the treatment of neonatal pneumonia caused by multidrug-resistant bacteria

OBJECTIVE

To explore the effect of cefoperazone sulbactam sodium combined with meropenem on the immune function in the treatment of neonatal multi-drug resistant pneumonia.

METHODS

Altogether 130 children with pneumonia caused by multi-drug resistant bacteria admitted to our hospital from January 2016 to January 2019 were recruited as the study cohort. The children were randomly divided into a combined group (n=80, combined therapy) and a control group (n=50, cefoperazone sulbactam sodium therapy). Their clinical indexes and their pulmonary function indexes, their serum heparin-binding protein (HBP) 1,25-dihydroxy vitamin D3 [1,25-(OH)2D3] levels, and their inflammatory factors and immune indexes were observed. The bacterial eradication rates, total effective rates, and adverse reaction rates of the two groups were investigated.

RESULTS

Compared with the control group, the cough disappearance times, the antipyretic times, the pulmonary rales disappearance times, and the hospital stay lengths in the combination group were shorter, the FEV1% Pred (the percentage of forced expiratory volume in one second compared to the predicted value) and the FEV1/Fvc% (the percentage of forced expiratory volume in one second compared to the forced vital capacity) were higher, the HBP levels and the inflammatory factor CRP and IL-6 levels were lower, the 1,25-(OH)2D3, and the immune index gA, IgG, and C4 levels were higher, and the bacterial eradication rates and the total effective rates were higher, and the incidence of adverse reactions was lower.

CONCLUSION

Cefoperazone sulbactam sodium combined with meropenem can improve the immune function of newborn children with multi-drug resistant pneumonia.

Influenza-Induced Oxidative Stress Sensitizes Lung Cells to Bacterial-Toxin-Mediated Necroptosis

Pneumonias caused by influenza A virus (IAV) co- and secondary bacterial infections are characterized by their severity and high mortality rate. Previously, we have shown that bacterial pore-forming toxin (PFT)-mediated necroptosis is a key driver of acute lung injury during bacterial pneumonia. Here, we evaluate the impact of IAV on PFT-induced acute lung injury during co- and secondary Streptococcus pneumoniae (Spn) infection. We observe that IAV synergistically sensitizes lung epithelial cells for PFT-mediated necroptosis in vitro and in murine models of Spn co-infection and secondary infection. Pharmacoelogical induction of oxidative stress without virus sensitizes cells for PFT-mediated necroptosis. Antioxidant treatment or inhibition of necroptosis reduces disease severity during secondary bacterial infection. Our results advance our understanding on the molecular basis of co- and secondary bacterial infection to influenza and identify necroptosis inhibition and antioxidant therapy as potential intervention strategies.

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.

Streptococcus pneumoniae Binds to Host Lactate Dehydrogenase via PspA and PspC To Enhance Virulence

Pneumococcal surface protein A (PspA) and pneumococcal surface protein C (PspC, also called CbpA) are major virulence factors of Streptococcus pneumoniae (Spn). These surface-exposed choline-binding proteins (CBPs) function independently to inhibit opsonization, neutralize antimicrobial factors, or serve as adhesins. PspA and PspC both carry a proline-rich domain (PRD) whose role, other than serving as a flexible connector between the N-terminal and C-terminal domains, was up to this point unknown. Herein, we demonstrate that PspA binds to lactate dehydrogenase (LDH) released from dying host cells during infection. Using recombinant versions of PspA and isogenic mutants lacking PspA or specific domains of PspA, this property was mapped to a conserved 22-amino-acid nonproline block (NPB) found within the PRD of most PspAs and PspCs. The NPB of PspA had specific affinity for LDH-A, which converts pyruvate to lactate. In a mouse model of pneumonia, preincubation of Spn carrying NPB-bearing PspA with LDH-A resulted in increased bacterial titers in the lungs. In contrast, incubation of Spn carrying a version of PspA lacking the NPB with LDH-A or incubation of wild-type Spn with enzymatically inactive LDH-A did not enhance virulence. Preincubation of NPB-bearing Spn with lactate alone enhanced virulence in a pneumonia model, indicating exogenous lactate production by Spn-bound LDH-A had an important role in pneumococcal pathogenesis. Our observations show that lung LDH, released during the infection, is an important binding target for Spn via PspA/PspC and that pneumococci utilize LDH-A derived lactate for their benefit in vivoIMPORTANCEStreptococcus pneumoniae (Spn) is the leading cause of community-acquired pneumonia. PspA and PspC are among its most important virulence factors, and these surface proteins carry the proline-rich domain (PRD), whose role was unknown until now. Herein, we show that a conserved 22-amino-acid nonproline block (NPB) found within most versions of the PRD binds to host-derived lactate dehydrogenase A (LDH-A), a metabolic enzyme which converts pyruvate to lactate. PspA-mediated binding of LDH-A increased Spn titers in the lungs and this required LDH-A enzymatic activity. Enhanced virulence was also observed when Spn was preincubated with lactate, suggesting LDH-A-derived lactate is a vital food source. Our findings define a role for the NPB of the PRD and show that Spn co-opts host enzymes for its benefit. They advance our understanding of pneumococcal pathogenesis and have key implications on the susceptibility of individuals with preexisting airway damage that results in LDH-A release.

M1 Macrophages Are More Susceptible to Necroptosis

Macrophages play a crucial role in host innate immune defense against infection and tissue injury. Although macrophage activation and polarization has been well studied, we know less regarding the role of macrophage activation/polarization in inflammation-associated necrotic cell death. By using bone marrow-derived macrophages, we have recently demonstrated that M1 macrophages are much more susceptible than M0 and M2 subtypes of macrophages to necrotic cell death. Moreover, we showed that the enhanced necroptosis in M1 macrophages is dependent on the kinase activity of receptor-interacting protein kinase-3 (RIPK3) and may involve the upregulation of key necroptosis signaling molecules including RIPK3, mixed lineage kinase domain-like protein, and Z-DNA/ RNA binding protein 1. Our findings provide novel insights into the mechanisms of M1 macrophage engagement in inflammation and tissue injury.

Inhibition of MLKL-dependent necroptosis via downregulating interleukin-1R1 contributes to neuroprotection of hypoxic preconditioning in transient global cerebral ischemic rats

Background

Our previous study indicated that hypoxic preconditioning reduced receptor interacting protein (RIP) 3-mediated necroptotic neuronal death in hippocampal CA1 of adult rats after transient global cerebral ischemia (tGCI). Although mixed lineage kinase domain-like (MLKL) has emerged as a crucial molecule for necroptosis induction downstream of RIP3, how MLKL executes necroptosis is not yet well understood. In this study, we aim to elucidate the molecular mechanism underlying hypoxic preconditioning that inactivates MLKL-dependent neuronal necroptosis after tGCI.

Methods

Transient global cerebral ischemia was induced by the four-vessel occlusion method. Twenty-four hours before ischemia, rats were exposed to systemic hypoxia with 8% O₂ for 30 min. Western blotting was used to detect the expression of MLKL and interleukin-1 type 1 receptor (IL-1R1) in CA1. Immunoprecipitation was used to assess the interactions among IL-1R1, RIP3, and phosphorylated MLKL (p-MLKL). The concentration of intracellular free calcium ion (Ca²⁺) was measured using Fluo-4 AM. Silencing and overexpression studies were used to study the role of p-MLKL in tGCI-induced neuronal death.

Results

Hypoxic preconditioning decreased the phosphorylation of MLKL both in neurons and microglia of CA1 after tGCI. The knockdown of MLKL with siRNA decreased the expression of p-MLKL and exerted neuroprotective effects after tGCI, whereas treatment with lentiviral delivery of MLKL showed opposite results. Mechanistically, hypoxic preconditioning or MLKL siRNA attenuated the RIP3-p-MLKL interaction, reduced the plasma membrane translocation of p-MLKL, and blocked Ca²⁺ influx after tGCI. Furthermore, hypoxic preconditioning downregulated the expression of IL-1R1 in CA1 after tGCI. Additionally, neutralizing IL-1R1 with its antagonist disrupted the interaction between IL-1R1 and the necrosome, attenuated the expression and the plasma membrane translocation of p-MLKL, thus alleviating neuronal death after tGCI.

Conclusions

These data support that the inhibition of MLKL-dependent neuronal necroptosis through downregulating IL-1R1 contributes to neuroprotection of hypoxic preconditioning against tGCI.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02141-y.

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.

Inhibition of Necroptosis to Prevent Long-term Cardiac Damage During Pneumococcal Pneumonia and Invasive Disease

BACKGROUND

Streptococcus pneumoniae infection can result in bacteremia with devastating consequences including heart damage. Necroptosis is a proinflammatory form of cell death instigated by pore-forming toxins such as S. pneumoniae pneumolysin. Necroptosis-inhibiting drugs may lessen organ damage during invasive pneumococcal disease (IPD).

METHODS

In vitro experiments were carried out with human and mouse cardiomyocytes. Long-term cardiac damage was assessed using high-resolution echocardiography in ampicillin-rescued mice 3 months after challenge with S. pneumoniae. Ponatinib, a necroptosis-inhibiting and Food and Drug Administration-approved drug for lymphocytic leukemia treatment, was administered intraperitoneally alongside ampicillin to test its therapeutic efficacy. Histology of heart sections included hematoxylin-eosin staining for overt damage, immunofluorescence for necroptosis, and Sirius red/fast green staining for collagen deposition.

RESULTS

Cardiomyocyte death and heart damage was due to pneumolysin-mediated necroptosis. IPD leads to long-term cardiac damage, as evidenced by de novo collagen deposition in mouse hearts and a decrease in fractional shortening. Adjunct necroptosis inhibition reduced the number of S. pneumoniae foci observed in hearts of acutely infected mice and serum levels of troponin I. Ponatinib reduced collagen deposition and protected heart function in convalescence.

CONCLUSIONS

Acute and long-term cardiac damage incurred during IPD is due in part to cardiomyocyte necroptosis. Necroptosis inhibitors may be a viable adjunct therapy.

Cross Kingdom Immunity: The Role of Immune Receptors and Downstream Signaling in Animal and Plant Cell Death

Both plants and animals are endowed with sophisticated innate immune systems to combat microbial attack. In these multicellular eukaryotes, innate immunity implies the presence of cell surface receptors and intracellular receptors able to detect danger signal referred as damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs). Membrane-associated pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), C-type lectin receptors (CLRs), receptor-like kinases (RLKs), and receptor-like proteins (RLPs) are employed by these organisms for sensing different invasion patterns before triggering antimicrobial defenses that can be associated with a form of regulated cell death. Intracellularly, animals nucleotide-binding and oligomerization domain (NOD)-like receptors or plants nucleotide-binding domain (NBD)-containing leucine rich repeats (NLRs) immune receptors likely detect effectors injected into the host cell by the pathogen to hijack the immune signaling cascade. Interestingly, during the co-evolution between the hosts and their invaders, key cross-kingdom cell death-signaling macromolecular NLR-complexes have been selected, such as the inflammasome in mammals and the recently discovered resistosome in plants. In both cases, a regulated cell death located at the site of infection constitutes a very effective mean for blocking the pathogen spread and protecting the whole organism from invasion. This review aims to describe the immune mechanisms in animals and plants, mainly focusing on cell death signaling pathways, in order to highlight recent advances that could be used on one side or the other to identify the missing signaling elements between the perception of the invasion pattern by immune receptors, the induction of defenses or the transmission of danger signals to other cells. Although knowledge of plant immunity is less advanced, these organisms have certain advantages allowing easier identification of signaling events, regulators and executors of cell death, which could then be exploited directly for crop protection purposes or by analogy for medical research.

Multi-Valent Protein Hybrid Pneumococcal Vaccines: A Strategy for the Next Generation of Vaccines

Streptococcus pneumoniae (Spn) is a bacterial pathogen known to colonize the upper respiratory tract and cause serious opportunistic diseases such as pneumonia, bacteremia, sepsis and meningitis. As a consequence, millions of attributable deaths occur annually, especially among infants, the elderly and immunocompromised individuals. Although current vaccines, composed of purified pneumococcal polysaccharide in free form or conjugated to a protein carrier, are widely used and have been demonstrated to be effective in target groups, Spn has continued to colonize and cause life-threatening disease in susceptible populations. This lack of broad protection highlights the necessity of improving upon the current "gold standard" pneumococcal vaccines to increase protection both by decreasing colonization and reducing the incidence of sterile-site infections. Over the past century, most of the pneumococcal proteins that play an essential role in colonization and pathogenesis have been identified and characterized. Some of these proteins have the potential to serve as antigens in a multi-valent protein vaccine that confers capsule independent protection. This review seeks to summarize the benefits and limitations of the currently employed vaccine strategies, describes how leading candidate proteins contribute to pneumococcal disease development, and discusses the potential of these proteins as protective antigens-including as a hybrid construct.

Influenza Causes MLKL-Driven Cardiac Proteome Remodeling During Convalescence

RATIONALE

Patients with and without cardiovascular diseases have been shown to be at risk of influenza-mediated cardiac complications. Recent clinical reports support the notion of a direct link between laboratory-confirmed influenza virus infections and adverse cardiac events.

OBJECTIVE

Define the molecular mechanisms underlying influenza virus-induced cardiac pathogenesis after resolution of pulmonary infection and the role of necroptosis in this process.

METHODS AND RESULTS

Hearts from wild-type and necroptosis-deficient (MLKL [mixed lineage kinase domain-like protein]-KO) mice were dissected 12 days after initial influenza A virus (IAV) infection when viral titers were undetectable in the lungs. Immunofluorescence microscopy and plaque assays showed presence of viable IAV particles in the myocardium without generation of interferon responses. Global proteome and phosphoproteome analyses using high-resolution accurate mass-based LC-MS/MS and label-free quantitation showed that the global proteome as well as the phosphoproteome profiles were significantly altered in IAV-infected mouse hearts in a strain-independent manner. Necroptosis-deficient mice had increased survival and reduced weight loss post-IAV infection, as well as increased antioxidant and mitochondrial function, indicating partial protection to IAV infection. These findings were confirmed in vitro by pretreatment of human and rat myocytes with antioxidants or necroptosis inhibitors, which blunted oxidative stress and mitochondrial damage after IAV infection.

CONCLUSIONS

This study provides the first evidence that the cardiac proteome and phosphoproteome are significantly altered post-pulmonary influenza infection. Moreover, viral particles can persist in the heart after lung clearance, altering mitochondrial function and promoting cell death without active replication and interferon responses. Finally, our findings show inhibition of necroptosis or prevention of mitochondrial damage as possible therapeutic interventions to reduce cardiac damage during influenza infections. Graphic Abstract: A graphic abstract is available for this article.

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.

Role of astroglial Connexin 43 in pneumolysin cytotoxicity and during pneumococcal meningitis

Streptococcus pneumoniae or pneumococcus (PN) is a major causative agent of bacterial meningitis with high mortality in young infants and elderly people worldwide. The mechanism underlying PN crossing of the blood brain barrier (BBB) and specifically, the role of non-endothelial cells of the neurovascular unit that control the BBB function, remains poorly understood. Here, we show that the astroglial connexin 43 (aCx43), a major gap junctional component expressed in astrocytes, plays a predominant role during PN meningitis. Following intravenous PN challenge, mice deficient for aCx43 developed milder symptoms and showed severely reduced bacterial counts in the brain. Immunofluorescence analysis of brain slices indicated that PN induces the aCx43–dependent destruction of the network of glial fibrillary acid protein (GFAP), an intermediate filament protein specifically expressed in astrocytes and up-regulated in response to brain injury. PN also induced nuclear shrinkage in astrocytes associated with the loss of BBB integrity, bacterial translocation across endothelial vessels and replication in the brain cortex. We found that aCx4-dependent astrocyte damages could be recapitulated using in vitro cultured cells upon challenge with wild-type PN but not with a ply mutant deficient for the pore-forming toxin pneumolysin (Ply). Consistently, we showed that purified Ply requires Cx43 to promote host cell plasma membrane permeabilization in a process involving the Cx43-dependent release of extracellular ATP and prolonged increase of cytosolic Ca²⁺ in host cells. These results point to a critical role for astrocytes during PN meningitis and suggest that the cytolytic activity of the major virulence factor Ply at concentrations relevant to bacterial infection requires co-opting of connexin plasma membrane channels.

The role of non-endothelial cells constituting the neurovascular unit during infectious meningitis is poorly appreciated despite their key regulatory functions on the blood-brain barrier integrity. Here, we show that Streptococcus pneumoniae or pneumococcus, a major causative agent of bacterial meningitis, targets astroglial cells to translocate across brain endothelial vessels. We found that astroglial connexin 43, a gap junctional component, played a major role during PN meningitis in mice. PN translocation and replication in the brain cortex were associated with connexin-dependent fragmentation of astrocytic the GFAP network, a process associated with brain injury. These findings were recapitulated and extended in vitro using cultured primary astrocytes and the major PN virulence determinant Pneumolysin. Ply-mediated cytotoxicity was linked to Ca²⁺ increase and required aCx43, arguing against a direct toxin activity. The results reveal a key role for astroglial signaling during PN crossing of the BBB and shed light on the mechanism of Ply-mediated cytotoxicity during meningitis.

Pore-forming proteins: From defense factors to endogenous executors of cell death

Pore-forming proteins (PFPs) and small antimicrobial peptides (AMPs) represent a large family of molecules with the common ability to punch holes in cell membranes to alter their permeability. They play a fundamental role as infectious bacteria's defensive tools against host's immune system and as executors of endogenous machineries of regulated cell death in eukaryotic cells. Despite being highly divergent in primary sequence and 3D structure, specific folds of pore-forming domains have been conserved. In fact, pore formation is considered an ancient mechanism that takes place through a general multistep process involving: membrane partitioning and insertion, oligomerization and pore formation. However, different PFPs and AMPs assemble and form pores following different mechanisms that could end up either in the formation of protein-lined or protein-lipid pores. In this review, we analyze the current findings in the mechanism of action of different PFPs and AMPs that support a wide role of membrane pore formation in nature. We also provide the newest insights into the development of state-of-art techniques that have facilitated the characterization of membrane pores. To understand the physiological role of these peptides/proteins or develop clinical applications, it is essential to uncover the molecular mechanism of how they perforate membranes.

Necroptotic movers and shakers: cell types, inflammatory drivers and diseases

The necroptotic cell death pathway has received significant attention for its ability to trigger inflammatory responses and its potential involvement in related conditions. Recent insights into the essential membrane damaging necroptotic pseudokinase effector, Mixed lineage kinase domain like (MLKL), have revealed a number of diverse MLKL functions that contribute to the inflammatory nature of necroptosis. Here we review distinct MLKL signalling roles and document the immunogenic molecules released by necroptosis. We discuss specific in vivo MLKL-driven responses, the activation of inflammasome complexes and innate lymphoid cells, which have been documented to drive disease. Finally, we list necroptotic competent cell types and their involvement in MLKL-driven cell death-associated and inflammatory-associated conditions.

Structural insights into loss of function of a pore forming toxin and its role in pneumococcal adaptation to an intracellular lifestyle

The opportunistic pathogen Streptococcus pneumoniae has dual lifestyles: one of an asymptomatic colonizer in the human nasopharynx and the other of a deadly pathogen invading sterile host compartments. The latter triggers an overwhelming inflammatory response, partly driven via pore forming activity of the cholesterol dependent cytolysin (CDC), pneumolysin. Although pneumolysin-induced inflammation drives person-to-person transmission from nasopharynx, the primary reservoir for pneumococcus, it also contributes to high mortality rates, creating a bottleneck that hampers widespread bacterial dissemination, thus acting as a double-edged sword. Serotype 1 ST306, a widespread pneumococcal clone, harbours a non-hemolytic variant of pneumolysin (Ply-NH). Performing crystal structure analysis of Ply-NH, we identified Y150H and T172I as key substitutions responsible for loss of its pore forming activity. We uncovered a novel inter-molecular cation-π interaction, governing formation of the transmembrane β-hairpins (TMH) in the pore state of Ply, which can be extended to other CDCs. H150 in Ply-NH disrupts this interaction, while I172 provides structural rigidity to domain-3, through hydrophobic interactions, inhibiting TMH formation. Loss of pore forming activity enabled improved cellular invasion and autophagy evasion, promoting an atypical intracellular lifestyle for pneumococcus, a finding that was corroborated in in vivo infection models. Attenuation of inflammatory responses and tissue damage promoted tolerance of Ply-NH-expressing pneumococcus in the lower respiratory tract. Adoption of this altered lifestyle may be necessary for ST306 due to its limited nasopharyngeal carriage, with Ply-NH, aided partly by loss of its pore forming ability, facilitating a benign association of SPN in an alternative, intracellular host niche.

Central residues of the amphipathic β-hairpin loop control the properties of Clostridium perfringens epsilon-toxin channel

Clostridium perfringens epsilon toxin (ETX) is a heptameric pore-forming toxin of the aerolysin toxin family. ETX is the most potent toxin of this toxin family and the third most potent bacterial toxin with high cytotoxic and lethal activities in animals. In addition, ETX shows a demyelinating activity in nervous tissue leading to devastating multifocal central nervous system white matter disease in ruminant animals. Pore formation in target cell membrane is most likely the initial critical step in ETX biological activity. Eight single to quadruple ETX mutants were generated by replacement of polar residues (serine, threonine, glutamine) in middle positions of the β-strands forming the β-barrel and facing the channel lumen with charged glutamic residues. Channel activity and ion selectivity were monitored in artificial lipid monolayer membranes and cytotoxicity was investigated in MDCK cells by the viability MTT test and propidium iodide entry. All the mutants formed channels with similar conductance in artificial lipid membranes and increasing cation selectivity for increasing number of mutations. Here, we show that residues in the central position of each β-strand of the amphipathic β-hairpin loop that forms the transmembrane pore, control the size and ion selectivity of the channel. While the highest cationic ETX mutants were not cytotoxic, no strict correlation was observed between ion selectivity and cytotoxicity.