Pneumolysin activates the NLRP3 inflammasome and promotes proinflammatory cytokines independently of TLR4

Age-related differences in IL-1 signaling and capsule serotype affect persistence of Streptococcus pneumoniae colonization

Young age is a risk factor for prolonged colonization by common pathogens residing in their upper respiratory tract (URT). Why children present with more persistent colonization is unknown and there is relatively little insight into the host-pathogen interactions that contribute to persistent colonization. To identify factors permissive for persistent colonization during infancy, we utilized an infant mouse model of Streptococcus pneumoniae colonization in which clearance from the mucosal surface of the URT requires many weeks to months. Loss of a single bacterial factor, the pore-forming toxin pneumolysin (Ply), and loss of a single host factor, IL-1α, led to more persistent colonization. Exogenous administration of Ply promoted IL-1 responses and clearance, and intranasal treatment with IL-1α was sufficient to reduce colonization density. Major factors known to affect the duration of natural colonization include host age and pneumococcal capsular serotype. qRT-PCR analysis of the uninfected URT mucosa showed reduced baseline expression of genes involved in IL-1 signaling in infant compared to adult mice. In line with this observation, IL-1 signaling was important in initiating clearance in adult mice but had no effect on early colonization of infant mice. In contrast to the effect of age, isogenic constructs of different capsular serotype showed differences in colonization persistence but induced similar IL-1 responses. Altogether, this work underscores the importance of toxin-induced IL-1α responses in determining the outcome of colonization, clearance versus persistence. Our findings about IL-1 signaling as a function of host age may provide an explanation for the increased susceptibility and more prolonged colonization during early childhood.

During early childhood, opportunistic pathogens are often carried in the upper respiratory tract (URT) for prolonged periods of time. Why young children experience more persistent carriage is unclear and there is little understanding of host-bacteria interactions that affect persistence, especially in infants. Here, we utilized an infant mouse model of Streptococcus pneumoniae colonization, a common pathogen of the infant URT, that persists for several months. We identified that clearance is dictated by bacterial expression of a single pneumococcal toxin, pneumolysin, and by the host response via a single cytokine, IL-1α, that activates IL-1 signaling. Absence of either of these factors led to increased persistence of S. pneumoniae. We discovered that the infant URT shows repression of IL-1 signaling compared to adults. Our study presents new insight into the importance of IL-1 signaling in clearance of persistent URT carriage and may provide an explanation why infants present with more persistent carriage by common URT pathogens.

Tolerance and Innate Immunity Shape the Development of Postpartum Uterine Disease and the Impact of Endometritis in Dairy Cattle

Bacteria are ubiquitous in the bovine uterus after parturition, but 50 years ago, cows tolerated these bacteria and few animals developed uterine disease. Now, up to 40% of dairy cattle develop postpartum uterine disease. Uterine disease causes infertility by compromising the function of not only the endometrium but also the ovary. Animals defend themselves against pathogens using tolerance and resistance mechanisms. Tolerance is the ability to limit the disease severity induced by a given pathogen burden. Resistance is the ability to limit the pathogen burden and is usually the function of immunity. Endometrial cells contribute to tolerance and have roles in innate immunity and the inflammatory response to pathogens. However, failures in endometrial tolerance and the character of the inflammatory response shape postpartum uterine disease. We propose that uterine health is more dependent on the ability of the endometrium to tolerate pathogens than the ability to resist invading bacteria.

Role of Inflammatory Risk Factors in the Pathogenesis of Streptococcus pneumoniae

Streptococcus pneumoniae (Spn) is a colonizer of the human nasopharynx (NP), causing a variety of infections in humans including otitis media, pneumonia, sepsis, and meningitis. The NP is an immune permissive site which allows for the persistence of commensal bacteria. Acute or chronic respiratory airway inflammation constitutes a significant risk factor for the manifestation of Spn infections. The inflammatory conditions caused by an upper respiratory viral infection or respiratory conditions such as allergic asthma and chronic obstructive pulmonary disorders (COPDs) are implicated in the dysregulation of airway inflammation and tissue damage, which compromise the respiratory barrier integrity. These immune events promote bacterial outgrowth leading to Spn dissemination and invasion into the bloodstream. Therefore, suppression of inflammation and restoration of respiratory barrier integrity could contain Spn infections manifesting in the backdrop of an inflammatory disease condition. The gained knowledge could be harnessed in the design of novel therapeutic interventions to circumvent Spn bacterial infections.

Cytosolic Recognition of Microbes and Pathogens: Inflammasomes in Action

Infection is a dynamic biological process underpinned by a complex interplay between the pathogen and the host. Microbes from all domains of life, including bacteria, viruses, fungi, and protozoan parasites, have the capacity to cause infection. Infection is sensed by the host, which often leads to activation of the inflammasome, a cytosolic macromolecular signaling platform that mediates the release of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18 and cleavage of the pore-forming protein gasdermin D, leading to pyroptosis. Host-mediated sensing of the infection occurs when pathogens inject or carry pathogen-associated molecular patterns (PAMPs) into the cytoplasm or induce damage that causes cytosolic liberation of danger-associated molecular patterns (DAMPs) in the host cell. Recognition of PAMPs and DAMPs by inflammasome sensors, including NLRP1, NLRP3, NLRC4, NAIP, AIM2, and Pyrin, initiates a cascade of events that culminate in inflammation and cell death. However, pathogens can deploy virulence factors capable of minimizing or evading host detection. This review presents a comprehensive overview of the mechanisms of microbe-induced activation of the inflammasome and the functional consequences of inflammasome activation in infectious diseases. We also explore the microbial strategies used in the evasion of inflammasome sensing at the host-microbe interaction interface.

SUMO-mediated regulation of NLRP3 modulates inflammasome activity

The NLRP3 inflammasome responds to infection and tissue damage, and rapidly escalates the intensity of inflammation by activating interleukin (IL)-1β, IL-18 and cell death by pyroptosis. How the NLRP3 inflammasome is negatively regulated is poorly understood. Here we show that NLRP3 inflammasome activation is suppressed by sumoylation. NLRP3 is sumoylated by the SUMO E3-ligase MAPL, and stimulation-dependent NLRP3 desumoylation by the SUMO-specific proteases SENP6 and SENP7 promotes NLRP3 activation. Defective NLRP3 sumoylation, either by NLRP3 mutation of SUMO acceptor lysines or depletion of MAPL, results in enhanced caspase-1 activation and IL-1β release. Conversely, depletion of SENP7 suppresses NLRP3-dependent ASC oligomerisation, caspase-1 activation and IL-1β release. These data indicate that sumoylation of NLRP3 restrains inflammasome activation, and identify SUMO proteases as potential drug targets for the treatment of inflammatory diseases.

NLRP3 Inflammasome Participates in Host Response to Neospora caninum Infection

Neospora caninum is an intracellular protozoan parasite closely related to Toxoplasma gondii that mainly infects canids as the definitive host and cattle as the intermediate host, resulting in abortion in cattle and leading to financial losses worldwide. Commercial vaccines or drugs are not available for the prevention and treatment of bovine neosporosis. Knowledge about the hallmarks of the immune response to this infection could form the basis of important prevention strategies. The innate immune system first responds to invading parasite and subsequently initiates the appropriate adaptive immune response against this parasite. Upon infection, activation of host pattern-recognition receptors expressed by immune cells triggers the innate immune response. Toll-like receptors, NOD-like receptors, and C-type lectin receptors play key roles in recognizing protozoan parasite. Therefore, we aimed to explore the role of the NLRP3 inflammasome during the acute period of N. caninum infection. In vitro results showed that N. caninum infection of murine bone marrow-derived macrophages activated the NLRP3 inflammasome, accompanied by the release of IL-1β and IL-18, cleavage of caspase-1, and induction of cell death. K⁺ efflux induced by N. caninum infection participated in the activation of the inflammasome. Infection of mice deficient in NLRP3, ASC, and caspase-1/11 resulted in decreased production of IL-18 and reduced IFN-γ in serum. Elevated numbers of monocytes/macrophages and neutrophils were found at the initial infection site, but they failed to limit N. caninum replication. These findings suggest that the NLRP3 inflammasome is involved in the host response to N. caninum infection at the acute stage and plays an important role in limiting parasite growth, and it may enhance Th1 response by inducing production of IFN-γ. These findings may help devise protocols for controlling neosporosis.

Th17 activation by dendritic cells stimulated with gamma-irradiated Streptococcus pneumoniae

Dendritic cells (DCs) play an important role in antigen presentation, which is an essential step for the induction of antigen-specific adaptive immunity. Inactivated bacterial whole cell vaccines have been widely used to prevent many bacterial infections because they elicit good immunogenicity due to the presence of various antigens and are relatively inexpensive and easy to manufacture. Recently, gamma-irradiated whole cells of nonencapsulated Streptococcus pneumoniae were developed as a broad-spectrum and serotype-independent multivalent vaccine. In the present study, we generated gamma-irradiated S. pneumoniae (r-SP) and investigated its capacity to stimulate mouse bone marrow-derived DCs (BM-DCs) in comparison with heat-inactivated and formalin-inactivated S. pneumoniae (h-SP and f-SP, respectively). r-SP showed an attenuated binding and internalization level to BM-DCs when compared to h-SP or f-SP. r-SP weakly induced the expression of CD80, CD83, CD86, MHC class I, and PD-L2 compared with h-SP or f-SP. Furthermore, r-SP less potently induced IL-6, TNF-α, and IL-23 expression than h-SP or f-SP but more potently induced IL-1β expression than h-SP or f-SP in BM-DCs. Since Th17-mediated immune responses are known to be important for the protection against pneumococcal infections, r-SP-primed DCs were co-cultured with splenocytes or splenic CD4⁺ T cells. Interestingly, r-SP-sensitized BM-DCs markedly induced IL-17A⁺ CD4⁺ T cells whereas h-SP- or f-SP-sensitized BM-DCs weakly induced them. Collectively, these results suggest that r-SP could be an effective pneumococcal vaccine candidate eliciting Th17-mediated immune responses by stimulation of DCs.

Targeting the NLRP3 inflammasome in inflammatory diseases

Danger signals are a hallmark of many common inflammatory diseases, and these stimuli can function to activate the cytosolic innate immune signalling receptor NLRP3 (NOD-, LRR- and pyrin domain-containing 3). Once activated, NLRP3 nucleates the assembly of an inflammasome, leading to caspase 1-mediated proteolytic activation of the interleukin-1β (IL-1β) family of cytokines, and induces an inflammatory, pyroptotic cell death. Pharmacological inhibition of NLRP3 activation results in potent therapeutic effects in a wide variety of rodent models of inflammatory diseases, effects that are mirrored by genetic ablation of NLRP3. Although these findings highlight the potential of NLRP3 as a drug target, an understanding of NLRP3 structure and activation mechanisms is incomplete, which has hampered the discovery and development of novel therapeutics against this target. Here, we review recent advances in our understanding of NLRP3 activation and regulation, highlight the evolving landscape of NLRP3 modulators and discuss opportunities for pharmacologically targeting NLRP3 with novel small molecules.

Streptococcus pneumoniae's Virulence and Host Immunity: Aging, Diagnostics, and Prevention

Streptococcus pneumoniae is an infectious pathogen responsible for millions of deaths worldwide. Diseases caused by this bacterium are classified as pneumococcal diseases. This pathogen colonizes the nasopharynx of its host asymptomatically, but overtime can migrate to sterile tissues and organs and cause infections. Pneumonia is currently the most common pneumococcal disease. Pneumococcal pneumonia is a global health concern and vastly affects children under the age of five as well as the elderly and individuals with pre-existing health conditions. S. pneumoniae has a large selection of virulence factors that promote adherence, invasion of host tissues, and allows it to escape host immune defenses. A clear understanding of S. pneumoniae's virulence factors, host immune responses, and examining the current techniques available for diagnosis, treatment, and disease prevention will allow for better regulation of the pathogen and its diseases. In terms of disease prevention, other considerations must include the effects of age on responses to vaccines and vaccine efficacy. Ongoing work aims to improve on current vaccination paradigms by including the use of serotype-independent vaccines, such as protein and whole cell vaccines. Extending our knowledge of the biology of, and associated host immune response to S. pneumoniae is paramount for our improvement of pneumococcal disease diagnosis, treatment, and improvement of patient outlook.

Neutrophils drive alveolar macrophage IL-1β release during respiratory viral infection

BACKGROUND

Alveolar macrophages are sentinels of the airways that must exhibit immune restraint to innocuous antigens but elicit a robust inflammatory response to pathogenic threats. How distinction between these dichotomous functions is controlled is poorly defined.Neutrophils are the first responders to infection, and we hypothesised that they may free alveolar macrophages from their hyporesponsive state, promoting their activation. Activation of the inflammasome and interleukin (IL)-1β release is a key early inflammatory event that must be tightly regulated. Thus, the role of neutrophils in defining inflammasome activation in the alveolar macrophage was assessed.

METHODS

Mice were infected with the X31 strain of influenza virus and the role of neutrophils in alveolar macrophage activation established through administration of a neutrophil-depleting (1A8) antibody.

RESULTS

Influenza elicited a robust IL-1β release that correlated (r=0.6849; p<0.001) with neutrophil infiltrate and was ablated by neutrophil depletion. Alveolar macrophages were shown to be the prominent source of IL-1β during influenza infection, and virus triggered the expression of Nod-like receptor protein 3 (NLRP3) inflammasome and pro-IL-1β in these cells. However, subsequent activation of the inflammasome complex and release of mature IL-1β from alveolar macrophages were critically dependent on the provision of a secondary signal, in the form of antimicrobial peptide mCRAMP, from infiltrating neutrophils.

CONCLUSIONS

Neutrophils are critical for the activation of the NLRP3 inflammasome in alveolar macrophages during respiratory viral infection. Accordingly, we rationalise that neutrophils are recruited to the lung to confront a viable pathogenic threat and subsequently commit alveolar macrophages to a pro-inflammatory phenotype to combat infection.

Decreased NLRP3 inflammasome expression in aged lung may contribute to increased susceptibility to secondary Streptococcus pneumoniae infection

Post-viral pneumococcal pneumonia is a leading morbidity and mortality in older patients (≥65years of age). The goal of our current study is to understand the impact of chronological aging on innate immune responses to a secondary, post viral infection with Streptococcus pneumoniae, a causative agent of bacterial pneumonia. Using aged murine models of infection, our findings demonstrate increased morbidity and mortality in aged mice within 48h post-secondary S. pneumoniae infection. Increased susceptibility of aged mice was associated with decreased TLR1, TLR6, and TLR9 mRNA expression and diminished IL1β mRNA expression. Examination of NLRP3 inflammasome expression illustrated decreased NLRP3 mRNA expression and decreased IL1β production in aged lung in response to secondary S. pneumoniae infection.

Gut Microbiota Contributes to Resistance Against Pneumococcal Pneumonia in Immunodeficient Rag-/- Mice

Streptococcus pneumoniae causes infection-related mortality worldwide. Immunocompromised individuals, including young children, the elderly, and those with immunodeficiency, are especially vulnerable, yet little is known regarding S. pneumoniae-related pathogenesis and protection in immunocompromised hosts. Recently, strong interest has emerged in the gut microbiota's impact on lung diseases, or the "gut-lung axis." However, the mechanisms of gut microbiota protection against gut-distal lung diseases like pneumonia remain unclear. We investigated the role of the gut commensal, segmented filamentous bacteria (SFB), against pneumococcal pneumonia in immunocompetent and immunocompromised mouse models. For the latter, we chose the Rag-/- model, with adaptive immune deficiency. Immunocompetent adaptive protection against S. pneumoniae infection is based on antibodies against pneumococcal capsular polysaccharides, prototypical T cell independent-II (TI-II) antigens. Although SFB colonization enhanced TI-II antibodies in C57BL/6 mice, our data suggest that SFB did not further protect these immunocompetent animals. Indeed, basal B cell activity in hosts without SFB is sufficient for essential protection against S. pneumoniae. However, in immunocompromised Rag-/- mice, we demonstrate a gut-lung axis of communication, as SFB influenced lung protection by regulating innate immunity. Neutrophil resolution is crucial to recovery, since an unchecked neutrophil response causes severe tissue damage. We found no early neutrophil recruitment differences between hosts with or without SFB; however, we observed a significant drop in lung neutrophils in the resolution phase of S. pneumoniae infection, which corresponded with lower CD47 expression, a molecule that inhibits phagocytosis of apoptotic cells, in SFB-colonized Rag-/- mice. SFB promoted a shift in lung neutrophil phenotype from inflammatory neutrophils expressing high levels of CD18 and low levels of CD62L, to pro-resolution neutrophils with low CD18 and high CD62L. Blocking CD47 in SFB(-) mice increased pro-resolution neutrophils, suggesting CD47 down-regulation may be one neutrophil-modulating mechanism SFB utilizes. The SFB-induced lung neutrophil phenotype remained similar with heat-inactivated S. pneumoniae treatment, indicating these SFB-induced changes in neutrophil phenotype during the resolution phase are not simply secondary to better bacterial clearance in SFB(+) than SFB(-) mice. Together, these data demonstrate that the gut commensal SFB may provide much-needed protection in immunocompromised hosts in part by promoting neutrophil resolution post lung infection.

Multifaceted Role of Pneumolysin in the Pathogenesis of Myocardial Injury in Community-Acquired Pneumonia

Pneumolysin (PLY), a member of the family of Gram-positive bacterial, cholesterol-dependent, β-barrel pore-forming cytolysins, is the major protein virulence factor of the dangerous respiratory pathogen, Streptococcus pneumoniae (pneumococcus). PLY plays a major role in the pathogenesis of community-acquired pneumonia (CAP), promoting colonization and invasion of the upper and lower respiratory tracts respectively, as well as extra-pulmonary dissemination of the pneumococcus. Notwithstanding its role in causing acute lung injury in severe CAP, PLY has also been implicated in the development of potentially fatal acute and delayed-onset cardiovascular events, which are now recognized as being fairly common complications of this condition. This review is focused firstly on updating mechanisms involved in the immunopathogenesis of PLY-mediated myocardial damage, specifically the direct cardiotoxic and immunosuppressive activities, as well as the indirect pro-inflammatory/pro-thrombotic activities of the toxin. Secondly, on PLY-targeted therapeutic strategies including, among others, macrolide antibiotics, natural product antagonists, cholesterol-containing liposomes, and fully humanized monoclonal antibodies, as well as on vaccine-based preventive strategies. These sections are preceded by overviews of CAP in general, the role of the pneumococcus as the causative pathogen, the occurrence and types of CAP-associated cardiac complication, and the structure and biological activities of PLY.

Immunodominance in T cell responses elicited against different domains of detoxified pneumolysin PlyD1

Detoxified pneumolysin, PlyD1, is a protein vaccine candidate that induces protection against infections with Streptococcus pneumoniae in mouse models. Despite extensive knowledge on antibody responses against PlyD1, limited information is available about PlyD1 induced T cell recognition. Here we interrogated epitope breadth and functional characteristics of the T cell response to PlyD1 in two mouse strains. BALB/c (H-2^d) and C57BL/6 (H-2^b) mice were vaccinated with Al(OH)₃-adjuvanted or non-adjuvanted PlyD1, or placebo, on day 0, 21 and 42 and were sacrificed at day 56 for collection of sera and spleens. Vaccination with adjuvanted and non-adjuvanted PlyD1 induced anti-pneumolysin IgG antibodies with neutralizing capacity in both mouse strains. Adjuvantation of PlyD1 enhanced the serological responses in both strains. In vitro restimulation of splenocytes with PlyD1 and 18-mer synthetic peptides derived from pneumolysin revealed specific proliferative and cytokine responses. For both mouse strains, one immunodominant and three subdominant natural epitopes were identified. Overlap between H-2^d and H-2^b restricted T cell epitopes was limited, yet similarities were found between epitopes processed in mice and predicted to be immunogenic in humans. H-2^d restricted T cell epitopes were localized in pneumolysin domains 2 and 3, whereas H-2^b epitopes were scattered over the protein. Cytokine responses show mostly a Th2 profile, with low levels of Th1 cytokines, in both mouse strains. In conclusion, PlyD1 evokes T cell responses in mice directed against multiple epitope regions, that is dependent on Major Histocompatibility Complex (MHC) background. These results are important to understand human PlyD1 T cell immunogenicity, to guide cell mediated immunity studies in the context of vaccine development.

Interleukin-1 Blockade: An Update on Emerging Indications

Interleukin (IL)-1 is a pro-inflammatory cytokine that induces local and systemic inflammation aimed to eliminate microorganisms and tissue damage. However, an increasing number of clinical conditions have been identified in which IL-1 production is considered inappropriate and IL-1 is part of the disease etiology. In autoinflammatory diseases, gout, Schnitzler's syndrome, and adult-onset Still's disease, high levels of inappropriate IL-1 production have been shown to be a key process in the etiology of the disease. In these conditions, blocking IL-1 has proven very effective in clinical studies. In other diseases, IL-1 has shown to be present in disease process but is not the central driving force of inflammation. In these conditions, including type 1 and 2 diabetes mellitus, acute coronary syndrome, amyotrophic lateral sclerosis, and several neoplastic diseases, the benefits of IL-1 blockade are minimal or absent.

The cGAS/STING Pathway Detects Streptococcus pneumoniae but Appears Dispensable for Antipneumococcal Defense in Mice and Humans

Streptococcus pneumoniae is a frequent colonizer of the upper respiratory tract and a leading cause of bacterial pneumonia. The innate immune system senses pneumococcal cell wall components, toxin, and nucleic acids, which leads to production of inflammatory mediators to initiate and control antibacterial defense. Here, we show that the cGAS (cyclic GMP-AMP [cGAMP] synthase)-STING pathway mediates detection of pneumococcal DNA in mouse macrophages to primarily stimulate type I interferon (IFN) responses. Cells of human individuals carrying HAQ TMEM173, which encodes a common hypomorphic variant of STING, were largely or partly defective in inducing type I IFNs and proinflammatory cytokines upon infection. Subsequent analyses, however, revealed that STING was dispensable for restricting S. pneumoniae during acute pneumonia in mice. Moreover, explorative analyses did not find differences in the allele frequency of HAQ TMEM173 in nonvaccinated pneumococcal pneumonia patients and healthy controls or an association of HAQ TMEM173 carriage with disease severity. Together, our results indicate that the cGAS/STING pathway senses S. pneumoniae but plays no major role in antipneumococcal immunity in mice and humans.

NLRP3 inflammasome activation in aged macrophages is diminished during Streptococcus pneumoniae infection

Pneumococcal infections are the eigth leading cause of death in the United States, and it is estimated that older patients (≥65 yr of age) account for the most serious cases. The goal of our current study is to understand the impact of biological aging on innate immune responses to Streptococcus pneumoniae, a causative agent of bacterial pneumonia. With the use of in vitro and in vivo aged murine models, our findings demonstrate that age-enhanced unfolded protein responses (UPRs) contribute to diminished inflammasome assembly and activation during S. pneumoniae infection. Pretreatment of aged mice with endoplasmic reticulum chaperone and the stress-reducing agent tauroursodeoxycholic acid (TUDCA) decreased mortality in aged hosts that was associated with increased NLRP3 inflammasome activation, improved pathogen clearance, and decreased pneumonitis during infection. Taken together, our data provide new evidence as to why older persons are more susceptible to S. pneumoniae and provide a possible therapeutic target to decrease morbidity and mortality in this population.

Inflammasome Involvement in Alzheimer's Disease

Inflammasomes are responsible for the maturation of pro-inflammatory cytokines such as interleukin (IL)-1β, IL-18, and IL-33 and activation of inflammatory cell death, pyroptosis. They assemble in response to cellular infection and stress or to tissue damage, promote inflammatory reactions, and are important in regulating innate immunity particularly by acting as platforms for activation of caspase proteases. They appear to be involved in several pathological processes activated by microbes including Alzheimer's disease (AD). Best characterized in microbial pathogenesis is the nucleotide-binding domain and leucine-rich repeat (NLR)-protein 3 (NLRP3) inflammasome. AD is a neurodegenerative condition in which the neuropathological hallmarks are the deposition of amyloid-β (Aβ) and hyperphosphorylated tau protein coated neurofibrillary tangles. For decades, the role of the innate immune system in the etiology of AD was considered less important, but the recently discovered inflammatory genes by genome-wide association studies driving inflammation in this disease has changed this view. Innate immune inflammatory activity in the AD brain can result from the pathological hallmark protein Aβ as well as from specific bacterial infections that tend to possess weak immunostimulatory responses for peripheral blood myeloid cell recruitment to the brain. The weak immunostimulatory activity is a consequence of their immune evasion strategies and survival. In this review we discuss the possibility that inflammasomes, particularly via the NLR family of proteins NLRP3 are involved in the pathogenesis of AD. In addition, we discuss the plausible contribution of specific bacteria playing a role in influencing the activity of the NLRP3 inflammasome to AD progression.

Baicalin modulates NF-κB and NLRP3 inflammasome signaling in porcine aortic vascular endothelial cells Infected by Haemophilus parasuis Causing Glässer's disease

Haemophilus parasuis (H. parasuis) can cause vascular inflammatory injury, but the molecular basis of this effect remains unclear. In this study,we investigated the effect of the anti-inflammatory, anti-microbial and anti-oxidant agent, baicalin, on the nuclear factor (NF)-κB and NLRP3 inflammasome signaling pathway in pig primary aortic vascular endothelial cells. Activation of the NF-κB and NLRP3 inflammasome signaling pathway was induced in H. parasuis-infected cells. However, baicalin reduced the production of reactive oxygen species, apoptosis, and activation of the NF-κB and NLRP3 inflammasome signaling pathway in infected cells. These results revealed that baicalin can inhibit H. parasuis-induced inflammatory responses in porcine aortic vascular endothelial cells, and may thus offer a novel strategy for controlling and treating H. parasuis infection. Furthermore, the results suggest that piglet primary aortic vascular endothelial cells may provide an experimental model for future studies of H. parasuis infection.

Pneumolysin induced mitochondrial dysfunction leads to release of mitochondrial DNA

Streptococcus pneumoniae (S.pn.) is the most common bacterial pathogen causing community acquired pneumonia. The pore-forming toxin pneumolysin (PLY) is the major virulence factor of S.pn. and supposed to affect alveolar epithelial cells thereby activating the immune system by liberation of danger-associated molecular patterns (DAMP). To test this hypothesis, we established a novel live-cell imaging based assay to analyse mitochondrial function and associated release of mitochondrial DNA (mtDNA) as DAMP in real-time. We first revealed that bacterially released PLY caused significant changes of the cellular ATP homeostasis and led to morphologic alterations of mitochondria in human alveolar epithelial cells in vitro and, by use of spectral live-tissue imaging, in human alveoli. This was accompanied by strong mitochondrial calcium influx and loss of mitochondrial membrane potential resulting in opening of the mitochondrial permeability transition pore and mtDNA release without activation of intrinsic apoptosis. Moreover, our data indicate cellular mtDNA liberation via microvesicles, which may contribute to S.pn. related pro-inflammatory immune activation in the human alveolar compartment.

Inflammasome in the Pathogenesis of Pulmonary Diseases

Lung diseases are common and significant causes of illness and death around the world. Inflammasomes have emerged as an important regulator of lung diseases. The important role of IL-1 beta and IL-18 in the inflammatory response of many lung diseases has been elucidated. The cleavage to turn IL-1 beta and IL-18 from their precursors into the active forms is tightly regulated by inflammasomes. In this chapter, we structurally review current evidence of inflammasome-related components in the pathogenesis of acute and chronic lung diseases, focusing on the "inflammasome-caspase-1-IL-1 beta/IL-18" axis.

Comparative Genomic Analysis and In Vivo Modeling of Streptococcus pneumoniae ST3081 and ST618 Isolates Reveal Key Genetic and Phenotypic Differences Contributing to Clonal Replacement of Serotype 1 in The Gambia

Streptococcus pneumoniae serotype 1 is one of the leading causes of invasive pneumococcal disease (IPD) in West Africa, with ST618 being the dominant cause of IPD in The Gambia. Recently however, a rare example of clonal replacement was observed, where the ST3081 clone of serotype 1 replaced the predominant ST618 clone as the main cause of IPD. In the current study, we sought to find the reasons for this unusual replacement event. Using whole-genome sequence analysis and clinically relevant models of in vivo infection, we identified distinct genetic and phenotypic characteristics of the emerging ST3081 clone. We show that ST3081 is significantly more virulent than ST618 in models of invasive pneumonia, and is carried at higher densities than ST618 during nasopharyngeal carriage. We also observe sequence type-specific accessory genes and a unique sequence type-specific fixed mutation in the pneumococcal toxin pneumolysin, which is associated with increased hemolytic activity in ST3081 and may contribute to increased virulence in this clone. Our study provides evidence that, within the same serotype 1 clonal complex, biological properties differ significantly from one clone to another in terms of virulence and host invasiveness, and that these differences may be the result of key genetic differences within the genome.

ASC and NLRP3 impair host defense during lethal pneumonia caused by serotype 3 Streptococcus pneumoniae in mice

Streptococcus (S.) pneumoniae is the most common cause of community-acquired pneumonia. The Nod-like receptor family pyrin domain containing 3 (NLRP3) inflammasome, consisting of NLRP3, ASC (the adaptor apoptosis-associated speck-like protein containing a CARD) and caspase-1, has been implicated in protective immunity during pneumonia induced by high doses of S. pneumoniae serotype 2. Here we investigated the role of the NLRP3 inflammasome in the host response during lethal airway infection with a low dose of serotype 3 S. pneumoniae. Mice were euthanized at predefined endpoints for analysis or observed in survival studies. In additional studies, Tlr2^-/- /Tlr4^-/- mice and Myd88^-/- mice incapable of Toll-like receptor signaling were studied. In stark contrast with existing literature, both Nlrp3^-/- and Asc^-/- mice showed a strongly improved host defense, as reflected by a markedly reduced mortality rate accompanied by diminished bacterial growth and dissemination. Host defense was unaltered in Tlr2^-/- /Tlr4^-/- mice and Myd88^-/- mice. These results show that the NLRP3 inflammasome impairs host defense during lethal pneumonia caused by serotype 3 S. pneumoniae. Our findings challenge the current paradigm that proximal innate detection systems are indispensable for an adequate host immune response against bacteria.

How Inflammasomes Inform Adaptive Immunity

An immune response consists of a finely orchestrated interplay between initial recognition of potential microbial threats by the innate immune system and subsequent licensed adaptive immune neutralization. The initial recognition integrates environmental cues derived from pathogen-associated molecular patterns and cell-intrinsic damage-associated molecular patterns to contextualize the insult and inform a tailored adaptive response via T and B lymphocytes. While there are much data to support the role of transcriptional responses downstream of pattern recognition receptors in informing the adaptive immune response, markedly less attention has been paid to the role of post-translational responses to pathogen-associated molecular pattern and damage-associated molecular pattern recognition by the innate immune system, and how this may influence adaptive immunity. A well-characterized post-translational consequence of pattern recognition receptor signaling is the assembly of a multimeric signaling platform, termed the inflammasome, by members of the nucleotide-binding oligomerization domain (Nod), leucine-rich repeat-containing receptors (NLRs), and pyrin and HIN domain (PYHIN) families. Inflammasomes assemble in response to cytosolic perturbations, such as mitochondrial dysfunction and aberrant ion fluxes in the case of the canonical NLRP3 inflammasome or the presence of bacterial lipopolysaccharides in the case of the non-canonical inflammasome. Assembly of the inflammasome allows for the cleavage and activation of inflammatory caspases. These activated inflammatory caspases in turn cleave pro-form inflammatory cytokines into their mature bioactive species and lead to unconventional protein secretion and lytic cell death. In this review, we discuss evidence for inflammasome-mediated instruction and contextualization of infectious and sterile agents to the adaptive immune system.

Review manuscript: Mechanisms of platelet activation by the pneumococcus and the role of platelets in community-acquired pneumonia

There is increasing recognition of the involvement of platelets in orchestrating inflammatory responses, driving the activation of neutrophils, monocytes and vascular endothelium, which, if poorly controlled, may lead to microvascular dysfunction. Importantly, hyperreactivity of platelets has been implicated in the pathogenesis of myocardial injury and the associated particularly high prevalence of acute cardiovascular events in patients with severe community-acquired pneumonia (CAP), of which Streptococcus pneumoniae (pneumococcus) is the most commonly encountered aetiologic agent. In this context, it is noteworthy that a number of studies have documented various mechanisms by which the pneumococcus may directly promote platelet aggregation and activation. The major contributors to platelet activation include several different types of pneumococcal adhesin, the pore-forming toxin, pneumolysin, and possibly pathogen-derived hydrogen peroxide, which collectively represent a major focus of the current review. This is followed by an overview of the limited experimental studies together with a larger series of clinical studies mainly focused on all-cause CAP, which have provided evidence in support of associations between alterations in circulating platelet counts, most commonly thrombocytopenia, and a poor clinical outcome. The final section of the review covers, albeit briefly, systemic biomarkers of platelet activation which may have prognostic potential.

Guanylate Binding Proteins Regulate Inflammasome Activation in Response to Hyperinjected Yersinia Translocon Components

Gram-negative bacterial pathogens utilize virulence-associated secretion systems to inject, or translocate, effector proteins into host cells to manipulate cellular processes and promote bacterial replication. However, translocated bacterial products are sensed by nucleotide binding domain and leucine-rich repeat-containing proteins (NLRs), which trigger the formation of a multiprotein complex called the inflammasome, leading to secretion of interleukin-1 (IL-1) family cytokines, pyroptosis, and control of pathogen replication. Pathogenic Yersinia bacteria inject effector proteins termed Yops, as well as pore-forming proteins that comprise the translocon itself, into target cells. The Yersinia translocation regulatory protein YopK promotes bacterial virulence by limiting hyperinjection of the translocon proteins YopD and YopB into cells, thereby limiting cellular detection of Yersinia virulence activity. How hyperinjection of translocon proteins leads to inflammasome activation is currently unknown. We found that translocated YopB and YopD colocalized with the late endosomal/lysosomal protein LAMP1 and that the frequency of YopD and LAMP1 association correlated with the level of caspase-1 activation in individual cells. We also observed colocalization between YopD and Galectin-3, an indicator of endosomal membrane damage. Intriguingly, YopK limited the colocalization of Galectin-3 with YopD, suggesting that YopK limits the induction or sensing of endosomal membrane damage by components of the type III secretion system (T3SS) translocon. Furthermore, guanylate binding proteins (GBPs) encoded on chromosome 3 (Gbp^Chr3 ), which respond to pathogen-induced damage or alteration of host membranes, were necessary for inflammasome activation in response to hyperinjected YopB/-D. Our findings indicate that lysosomal damage by Yersinia translocon proteins promotes inflammasome activation and implicate GBPs as key regulators of this process.

Streptococcus pneumoniae in the heart subvert the host response through biofilm-mediated resident macrophage killing

For over 130 years, invasive pneumococcal disease has been associated with the presence of extracellular planktonic pneumococci, i.e. diplococci or short chains in affected tissues. Herein, we show that Streptococcus pneumoniae that invade the myocardium instead replicate within cellular vesicles and transition into non-purulent biofilms. Pneumococci within mature cardiac microlesions exhibited salient biofilm features including intrinsic resistance to antibiotic killing and the presence of an extracellular matrix. Dual RNA-seq and subsequent principal component analyses of heart- and blood-isolated pneumococci confirmed the biofilm phenotype in vivo and revealed stark anatomical site-specific differences in virulence gene expression; the latter having major implications on future vaccine antigen selection. Our RNA-seq approach also identified three genomic islands as exclusively expressed in vivo. Deletion of one such island, Region of Diversity 12, resulted in a biofilm-deficient and highly inflammogenic phenotype within the heart; indicating a possible link between the biofilm phenotype and a dampened host-response. We subsequently determined that biofilm pneumococci released greater amounts of the toxin pneumolysin than did planktonic or RD12 deficient pneumococci. This allowed heart-invaded wildtype pneumococci to kill resident cardiac macrophages and subsequently subvert cytokine/chemokine production and neutrophil infiltration into the myocardium. This is the first report for pneumococcal biofilm formation in an invasive disease setting. We show that biofilm pneumococci actively suppress the host response through pneumolysin-mediated immune cell killing. As such, our findings contradict the emerging notion that biofilm pneumococci are passively immunoquiescent.

Pneumolysin-Dependent Calpain Activation and Interleukin-1α Secretion in Macrophages Infected with Streptococcus pneumoniae

Pneumolysin (PLY), a major virulence factor of Streptococcus pneumoniae, is a pore-forming cytolysin that modulates host innate responses contributing to host defense against and pathogenesis of pneumococcal infections. Interleukin-1α (IL-1α) has been shown to be involved in tissue damage in a pneumococcal pneumonia model; however, the mechanism by which this cytokine is produced during S. pneumoniae infection remains unclear. In this study, we examined the role of PLY in IL-1α production. Although the strains induced similar levels of pro-IL-1α expression, wild-type S. pneumoniae D39, but not a deletion mutant of the ply gene (Δply), induced the secretion of mature IL-1α from host macrophages, suggesting that PLY is critical for the maturation and secretion of IL-1α during S. pneumoniae infection. Further experiments with calcium chelators and calpain inhibitors indicated that extracellular calcium ions and calpains (calcium-dependent proteases) facilitated the maturation and secretion of IL-1α from D39-infected macrophages. Moreover, we found that PLY plays a critical role in calcium influx and calpain activation, as elevated intracellular calcium levels and the degradation of the calpain substrate α-fodrin were detected in macrophages infected with D39 but not the Δply strain. These results suggested that PLY induces the influx of calcium in S. pneumoniae-infected macrophages, followed by calpain activation and subsequent IL-1α maturation and secretion.

Mucin 1 protects against severe Streptococcus pneumoniae infection

Streptococcus pneumoniae is a bacterial pathogen that commonly resides in the human nasopharynx, typically without causing any disease. However, in some cases these bacteria migrate from the nasopharynx to other sites of the body such as the lungs and bloodstream causing pneumonia and sepsis, respectively. This study used a mouse model of infection to investigate the potential role of Mucin 1 (MUC1), a cell membrane-associated glycoprotein known for playing a key barrier role at mucosal surfaces, in regulating this process. Wildtype (WT) and MUC1-deficient (Muc1^-/-) mice were infected intranasally with an invasive strain of S. pneumoniae and bacterial loads in the nasopharynx, lungs, and blood were analyzed. Lungs were graded histologically for inflammation and cytokine profiles in the lungs analyzed by ELISA. While there was no difference in pneumococcal colonization of the nasopharynx between WT and Muc1^-/- mice, infected Muc1^-/- mice showed high pneumococcal loads in their lungs 16 hours post-infection, as well as bacteremia. In contrast, infected WT mice cleared the pneumococci from their lungs and remained asymptomatic. Infection in Muc1^-/- mice was associated with an elevation in lung inflammation, with cellular recruitment especially of monocytes/macrophages. While MUC1-deficiency has been shown to increase phagocytosis of Pseudomonas aeruginosa, macrophages from Muc1^-/- mice exhibited a reduced capacity to phagocytose S. pneumoniae indicating diverse and bacterial-specific effects. In conclusion, these findings indicate that MUC1 plays an important role in protection against severe pneumococcal disease, potentially mediated by facilitating macrophage phagocytosis.

Neutrophilic NLRP3 inflammasome-dependent IL-1β secretion regulates the γδT17 cell response in respiratory bacterial infections

Traditionally regarded as simple foot soldiers of the innate immune response limited to the eradication of pathogens, neutrophils recently emerged as more complex cells endowed with a set of immunoregulatory functions. Using a model of invasive pneumococcal disease, we highlighted an unexpected key role for neutrophils as accessory cells in innate interleukin (IL)-17A production by lung resident Vγ6Vδ1⁺ T cells via nucleotide-binding oligomerization domain receptor, pyrin-containing 3 (NLRP3) inflammasome-dependent IL-1β secretion. In vivo activation of the NLRP3 inflammasome in neutrophils required both host-derived and bacterial-derived signals. Elaborately, it relies on (i) alveolar macrophage-secreted TNF-α for priming and (ii) subsequent exposure to bacterial pneumolysin for activation. Interestingly, this mechanism can be translated to human neutrophils. Our work revealed the cellular and molecular dynamic events leading to γδT17 cell activation, and highlighted for the first time the existence of a fully functional NLRP3 inflammasome in lung neutrophils. This immune axis thus regulates the development of a protective host response to respiratory bacterial infections.

Natural Killer Cell Functions during the Innate Immune Response to Pathogenic Streptococci

Dendritic cells (DCs) and NK cells play a crucial role in the first phase of host defense against infections. Group B Streptococcus (GBS) and Streptococcus suis are encapsulated streptococci causing severe systemic inflammation, leading to septicemia and meningitis. Yet, the involvement of NK cells in the innate immune response to encapsulated bacterial infection is poorly characterized. Here, it was observed that these two streptococcal species rapidly induce the release of IFN-γ and that NK cells are the major cell type responsible for this production during the acute phase of the infection. Albeit S. suis capacity to activate NK cells was lower than that of GBS, these cells partially contribute to S. suis systemic infection; mainly through amplification of the inflammatory loop. In contrast, such a role was not observed during GBS systemic infection. IFN-γ release by NK cells required the presence of DCs, which in turn had a synergistic effect on DC cytokine production. These responses were mainly mediated by direct DC-NK cell contact and partially dependent on soluble factors. Though IL-12 and LFA-1 were shown to be critical in S. suis-mediated activation of the DC-NK cell crosstalk, different or redundant molecular pathways modulate DC-NK interactions during GBS infection. The bacterial capsular polysaccharides also differently modulated NK cell activation. Together, these results demonstrated a role of NK cells in the innate immune response against encapsulated streptococcal infections; yet the molecular pathways governing NK activation seem to differ upon the pathogen and should not be generalized when studying bacterial infections.

Inflammasomes in Pneumococcal Infection: Innate Immune Sensing and Bacterial Evasion Strategies

Streptococcus pneumoniae frequently colonizes the upper respiratory tract of healthy individuals, but also commonly causes severe invasive infections such as community-acquired pneumonia and meningitis. One of the key virulence factors of pneumococci is the pore-forming toxin pneumolysin which stimulates cell death and is involved in the evasion of some defense mechanisms. The immune system, however, employs different inflammasomes to sense pneumolysin-induced pore formation, cellular membrane damage, and/or subsequent leakage of bacterial nucleic acid into the host cell cytosol. Canonical inflammasomes are cytosolic multiprotein complexes consisting of a receptor molecule such as NLRP3 or AIM2, the adapter ASC, and caspase-1. NLRP3 and AIM2 inflammasomes mediate cell death and production of important IL-1 family cytokines to recruit leukocytes and defend against S. pneumoniae. Here, we review recent evidence that highlights inflammasomes as critical sensors of S. pneumoniae-induced cellular perturbations, summarize their role in pneumococcal infections, and discuss potential evasion strategies of some emerging pneumococcal strains.

Bacterial Secretant from Pseudomonas aeruginosa Dampens Inflammasome Activation in a Quorum Sensing-Dependent Manner

Inflammasome signaling can contribute to host innate immune defense against bacterial pathogens such as Pseudomonas aeruginosa. However, bacterial evasion of host inflammasome activation is still poorly elucidated. Quorum sensing (QS) is a bacterial communication mechanism that promotes coordinated adaptation by triggering expression of a wide range of genes. QS is thought to strongly contribute to the virulence of P. aeruginosa, but the molecular impact of bacterial QS on host inflammasome defense is completely unknown. Here, we present evidence that QS-related factors of the bacterial secretant (BS) from P. aeruginosa can dampen host inflammasome signaling in mouse bone marrow-derived macrophages. We found that BS from QS-defective ΔlasR/rhlR mutant, but not from wild-type (WT) P. aeruginosa, induces robust activation of the NLRC4 inflammasome. P. aeruginosa-released flagellin mediates this inflammasome activation by ΔlasR/rhlR secretant, but QS-regulated bacterial proteases in the WT BS impair extracellular flagellin to attenuate NLRC4 inflammasome activation. P. aeruginosa-secreted proteases also degrade inflammasome components in the extracellular space to inhibit the propagation of inflammasome-mediated responses. Furthermore, QS-regulated virulence factor pyocyanin and QS autoinducer 3-oxo-C12-homoserine lactone directly suppressed NLRC4- and even NLRP3-mediated inflammasome assembly and activation. Taken together, our data indicate that QS system of P. aeruginosa facilitates bacteria to evade host inflammasome-dependent sensing machinery.

Recombinant BCG expressing a PspA-PdT fusion protein protects mice against pneumococcal lethal challenge in a prime-boost strategy

Pneumococcal proteins have been evaluated as genetically-conserved potential vaccine candidates. We have previously demonstrated that a fragment of PspA in fusion with PdT (rPspA-PdT) induced protective immune responses in mice. However, purified proteins have shown poor immunogenicity and often require the combination with strong adjuvants and booster doses. Here, we investigated the use of a Bacillus Calmette-Guérin (BCG) strain, a well-established prophylactic vaccine for tuberculosis with known adjuvant properties, for delivery of the PspA-PdT fusion protein. Immunization of mice in a prime-boost strategy, using rPspA-PdT as a boost, demonstrated that rBCG PspA-PdT/rPspA-PdT was able to induce an antibody response against both proteins, promoting an IgG1 to IgG2 antibody isotype shift. Sera from rBCG PspA-PdT/rPspA-PdT immunized mice showed antibodies able to bind to the pneumococcal surface and promoted higher complement deposition when compared with WT-BCG/rPspA-PdT or a single dose of rPspA-PdT. In addition, these antisera inhibited the cytolytic activity of Ply. Production of interleukin-6 (IL-6), gamma interferon (IFN-γ), and tumor necrosis factor alpha (TNF-α) was increased in splenocytes culture. Furthermore, a higher expression of CD69 early activation molecule was observed on splenic CD4⁺ T cells from mice immunized with rBCG PspA-PdT before and after the protein booster dose. Finally, immunization with rBCG PspA-PdT/rPspA-PdT protected mice against pneumococcal lethal challenge. These results support the further investigation of recombinant BCG strains to express pneumococcal proteins, which could be administered in early stages of life and lead to protective pneumococcal immunity in infants and children.

Role of Streptococcus pneumoniae Proteins in Evasion of Complement-Mediated Immunity

The complement system plays a central role in immune defense against Streptococcus pneumoniae. In order to evade complement attack, pneumococci have evolved a number of mechanisms that limit complement mediated opsonization and subsequent phagocytosis. This review focuses on the strategies employed by pneumococci to circumvent complement mediated immunity, both in vitro and in vivo. At last, since many of the proteins involved in interactions with complement components are vaccine candidates in different stages of validation, we explore the use of these antigens alone or in combination, as potential vaccine approaches that aim at elimination or drastic reduction in the ability of this bacterium to evade complement.

Role of neural barriers in the pathogenesis and outcome of Streptococcus pneumoniae meningitis

Bacterial meningitis is an inflammatory disease of the meninges of the central nervous system (CNS). Streptococcus pneumoniae (S. pneumoniae), Neisseria meningitidis, and Haemophilus influenzae are the major bacterial pathogens causing meningitis with S. pneumoniae being responsible for two thirds of meningitis cases in the developed world. To reach the CNS following nasopharyngeal colonization and bacteraemia, the bacteria traverse from the circulation across the blood brain barrier (BBB) and choroid plexus. While the BBB has a protective role in healthy individuals by shielding the CNS from neurotoxic substances circulating in the blood and maintaining the homeostasis within the brain environment, dysfunction of the BBB is associated with the pathophysiology of numerous neurologic disorders, including bacterial meningitis. Inflammatory processes, including release of a broad range of cytokines and free radicals, further increase vascular permeability and contribute to the excessive neural damage observed. Injury to the cerebral microvasculature and loss of blood flow auto-regulation promote increased intracranial pressure and may lead to vascular occlusion. Other common complications commonly associated with meningitis include abnormal neuronal hyper-excitability (e.g., seizures) and loss of hearing. Despite the existence of antibiotic treatment and adjuvant therapy, the relatively high mortality rate and the severe outcomes among survivors of pneumococcal meningitis in developing and developed countries increase the urgency in the requirement of discovering novel biomarkers for the early diagnosis as well as novel treatment approaches. The present review aimed to explore the changes in the brain vascular barriers, which allow S. pneumoniae to invade the CNS, and describe the resultant brain injuries following bacterial meningitis.

Inflammasomes in the lung

Innate immune responses act as first line defences upon exposure to potentially noxious stimuli. The innate immune system has evolved numerous intracellular and extracellular receptors that undertake surveillance for potentially damaging particulates. Inflammasomes are intracellular innate immune multiprotein complexes that form and are activated following interaction with these stimuli. Inflammasome activation leads to the cleavage of pro-IL-1β and release of the pro-inflammatory cytokine, IL-1β, which initiates acute phase pro-inflammatory responses, and other responses are also involved (IL-18, pyroptosis). However, excessive activation of inflammasomes can result in chronic inflammation, which has been implicated in a range of chronic inflammatory diseases. The airways are constantly exposed to a wide variety of stimuli. Inflammasome activation and downstream responses clears these stimuli. However, excessive activation may drive the pathogenesis of chronic respiratory diseases such as severe asthma and chronic obstructive pulmonary disease. Thus, there is currently intense interest in the role of inflammasomes in chronic inflammatory lung diseases and in their potential for therapeutic targeting. Here we review the known associations between inflammasome-mediated responses and the development and exacerbation of chronic lung diseases.

Cord blood Streptococcus pneumoniae-specific cellular immune responses predict early pneumococcal carriage in high-risk infants in Papua New Guinea

In areas where Streptococcus pneumoniae is highly endemic, infants experience very early pneumococcal colonization of the upper respiratory tract, with carriage often persisting into adulthood. We aimed to explore whether newborns in high‐risk areas have pre‐existing pneumococcal‐specific cellular immune responses that may affect early pneumococcal acquisition. Cord blood mononuclear cells (CBMC) of 84 Papua New Guinean (PNG; high endemic) and 33 Australian (AUS; low endemic) newborns were stimulated in vitro with detoxified pneumolysin (dPly) or pneumococcal surface protein A (PspA; families 1 and 2) and compared for cytokine responses. Within the PNG cohort, associations between CBMC dPly and PspA‐induced responses and pneumococcal colonization within the first month of life were studied. Significantly higher PspA‐specific interferon (IFN)‐γ, tumour necrosis factor (TNF)‐α, interleukin (IL)‐5, IL‐6, IL‐10 and IL‐13 responses, and lower dPly‐IL‐6 responses were produced in CBMC cultures of PNG compared to AUS newborns. Higher CBMC PspA‐IL‐5 and PspA‐IL‐13 responses correlated with a higher proportion of cord CD4 T cells, and higher dPly‐IL‐6 responses with a higher frequency of cord antigen‐presenting cells. In the PNG cohort, higher PspA‐specific IL‐5 and IL‐6 CBMC responses were associated independently and significantly with increased risk of earlier pneumococcal colonization, while a significant protective effect was found for higher PspA‐IL‐10 CBMC responses. Pneumococcus‐specific cellular immune responses differ between children born in pneumococcal high versus low endemic settings, which may contribute to the higher risk of infants in high endemic settings for early pneumococcal colonization, and hence disease.

Streptococcus pneumoniae induces pyroptosis through the regulation of autophagy in murine microglia

Streptococcus pneumoniae is responsible for significant mortality and morbidity worldwide and causes invasive pneumococcal diseases including pneumococcal meningitis. Pyroptosis is caspase-1-dependent inflammatory cell death and is known to be induced by various microbial infections. In the present study, we investigated the molecular mechanisms that regulate pyroptosis induced by S. pneumoniae in microglia. Our results revealed that S. pneumoniae induced pyroptosis through caspase-1 activation and IL-1β production. We also found that the activation of caspase-1 and the maturation of IL-1β and IL-18 in the S. pneumoniae-triggered pyroptotic cell death process were mediated by NLRP3 inflammasome. In addition, pneumococcal infection increased the expression of autophagy-related genes and induced autophagosome formation. We also showed that the inhibition of autophagy promoted pneumococcus-induced pyroptosis. Furthermore, ROS was generated by pneumococcal infection and inhibited caspase-1 activation within 4 h of infection. However, in the late phase of infection, IL-1β secretion and caspase-1-dependent cell death were induced by ROS. These results suggest that autophagy induction transiently delay pyroptosis induced by S. pneumoniae in microglia. Our study also revealed that the activation of caspase-1 and the production of IL-1β were induced by pneumolysin and that pneumolysin triggered pyroptosis in microglial cells. Similar to the in vitro results, S. pneumoniae induced caspase-1 activation and caspase-1-dependent cytokine maturation in the mouse meningitis model. Thus, the present data demonstrate that S. pneumoniae induces pyroptosis in murine microglia and that NLRP3 inflammasome is critical for caspase-1 activation during the process. Furthermore, the induction of autophagy could transiently protect microglia from pyroptosis.

Community-acquired bacterial meningitis

Meningitis is an inflammation of the meninges and subarachnoid space that can also involve the brain cortex and parenchyma. It can be acquired spontaneously in the community - community-acquired bacterial meningitis - or in the hospital as a complication of invasive procedures or head trauma (nosocomial bacterial meningitis). Despite advances in treatment and vaccinations, community-acquired bacterial meningitis remains one of the most important infectious diseases worldwide. Streptococcus pneumoniae and Neisseria meningitidis are the most common causative bacteria and are associated with high mortality and morbidity; vaccines targeting these organisms, which have designs similar to the successful vaccine that targets Haemophilus influenzae type b meningitis, are now being used in many routine vaccination programmes. Experimental and genetic association studies have increased our knowledge about the pathogenesis of bacterial meningitis. Early antibiotic treatment improves the outcome, but the growing emergence of drug resistance as well as shifts in the distribution of serotypes and groups are fuelling further development of new vaccines and treatment strategies. Corticosteroids were found to be beneficial in high-income countries depending on the bacterial species. Further improvements in the outcome are likely to come from dampening the host inflammatory response and implementing preventive measures, especially the development of new vaccines.

IL-17A attracts inflammatory cells in murine lung infection with P. aeruginosa

IL-17A-dependent immunity is of importance in the protection against extracellular bacterial pathogens. However, IL-17A is also suggested to mediate the pathogenesis of lung diseases, such as acute respiratory distress syndrome. Here, we studied the role of IL-17A in a mouse model of acute pneumonia. IL-17A mediated the expression of keratinocyte-derived chemokine (KC) and the recruitment of inflammatory cells in mice infected with a sub-lethal dose of Pseudomonas aeruginosa. IL-17A deficiency protected mice from lethal P. aeruginosa lung infection. A sub-lethal infection with Streptococcus pneumoniae resulted in increased bacterial burden associated with increased pulmonary inflammation. Thus, the type of infectious bacteria seemed to influence the way in which IL-17A functions during pulmonary infection. Reducing pulmonary inflammation by targeting IL-17A may be a therapeutic option in acute P. aeruginosa pneumonia.

Pathogenic, immunologic, and clinical aspects of sepsis - update 2016

INTRODUCTION

Sepsis is a major cause of death worldwide but its orchestrating components remain incompletely understood. On the one hand, development of sepsis results from an infectious focus that cannot be controlled by the immune system, but on the other, responding immune cells that can eliminate the infection inflict damage to the host by contributing to complications such as endothelial leakage, septic shock, and multiorgan failure.

AREAS COVERED

In this review we give a comprehensive overview of how sepsis occurs, which exogenous and endogenous factors might affect the immune-pathophysiological course of sepsis and finally how this knowledge translates into up-to-date definitions and therapeutic approaches. Expert commentary: Although new immunological mechanisms altering the course of sepsis have been identified recently, future research needs to address the limitations of experimental approaches, redirect the research focus into translational approaches, and finally evaluate personalized treatment strategies.

NOX4-dependent fatty acid oxidation promotes NLRP3 inflammasome activation in macrophages

Altered metabolism has been implicated in the pathogenesis of inflammatory diseases. NADPH oxidase 4 (NOX4), a source of cellular superoxide anions, has multiple biological functions that may be of importance in inflammation and in the pathogenesis of human metabolic diseases, including diabetes. However, the mechanisms by which NOX4-dependent metabolic regulation affect the innate immune response remain unclear. Here we show that deficiency of NOX4 resulted in reduced expression of carnitine palmitoyltransferase 1A (CPT1A), which is a key mitochondrial enzyme in the fatty acid oxidation (FAO) pathway. The reduced FAO resulted in less activation of the nucleotide-binding domain, leucine-rich-repeat-containing receptor (NLR), pyrin-domain-containing 3 (NLRP3) inflammasome in human and mouse macrophages. In contrast, NOX4 deficiency did not inhibit the activation of the NLR family, CARD-domain-containing 4 (NLRC4), the NLRP1 or the absent in melanoma 2 (AIM2) inflammasomes. We also found that inhibition of FAO by etomoxir treatment suppressed NLRP3 inflammasome activation. Furthermore, Nox4-deficient mice showed substantial reduction in caspase-1 activation and in interleukin (IL)-1β and IL-18 production, and there was improved survival in a mouse model of NLRP3-mediated Streptococcus pneumoniae infection. The pharmacologic inhibition of NOX4 by either GKT137831, which is currently in phase 2 clinical trials, or VAS-2870 attenuated NLRP3 inflammasome activation. Our results suggest that NOX4-mediated FAO promotes NLRP3 inflammasome activation.

NLRP3 protects alveolar barrier integrity by an inflammasome-independent increase of epithelial cell adherence

Bacterial pneumonia is a major cause of acute lung injury and acute respiratory distress syndrome, characterized by alveolar barrier disruption. NLRP3 is best known for its ability to form inflammasomes and to regulate IL-1β and IL-18 production in myeloid cells. Here we show that NLRP3 protects the integrity of the alveolar barrier in a mouse model of Streptococcus pneumoniae-induced pneumonia, and ex vivo upon treatment of isolated perfused and ventilated lungs with the purified bacterial toxin, pneumolysin. We reveal that the preserving effect of NLRP3 on the lung barrier is independent of inflammasomes, IL-1β and IL-18. NLRP3 improves the integrity of alveolar epithelial cell monolayers by enhancing cellular adherence. Collectively, our study uncovers a novel function of NLRP3 by demonstrating that it protects epithelial barrier function independently of inflammasomes.

Clinical Streptococcus pneumoniae isolates induce differing CXCL8 responses from human nasopharyngeal epithelial cells which are reduced by liposomes

Background

Streptococcus pneumoniae causes several human diseases, including pneumonia and meningitis, in which pathology is associated with an excessive inflammatory response. A major inducer of this response is the cholesterol dependent pneumococcal toxin, pneumolysin. Here, we measured the amount of inflammatory cytokine CXCL8 (interleukin (IL)-8) by ELISA released by human nasopharyngeal epithelial (Detroit 562) cells as inflammatory response to a 24 h exposure to different pneumococcal strains.

Results

We found pneumolysin to be the major factor influencing the CXCL8 response. Cholesterol and sphingomyelin-containing liposomes designed to sequester pneumolysin were highly effective at reducing CXCL8 levels from epithelial cells exposed to different clinical pneumococcal isolates. These liposomes also reduced CXCL8 response from epithelial cells exposed to pneumolysin knock-out mutants of S. pneumoniae indicating that they also reduce the CXCL8-inducing effect of an unidentified pneumococcal virulence factor, in addition to pneumolysin.

Conclusion

The results indicate the potential of liposomes in attenuating excessive inflammation as a future adjunctive treatment of pneumococcal diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0777-5) contains supplementary material, which is available to authorized users.

Brief Report: Severe Inflammation Following Vaccination Against Streptococcus pneumoniae in Patients With Cryopyrin-Associated Periodic Syndromes

OBJECTIVE

Pneumococcal vaccination is recommended for patients receiving immunosuppressive drugs. We describe unusually severe adverse reactions to pneumococcal vaccination in each of 7 consecutive patients with cryopyrin-associated periodic syndromes (CAPS).

METHODS

Seven consecutive patients with CAPS were vaccinated with pneumococcal polysaccharide or conjugate vaccines. Clinical information was collected retrospectively.

RESULTS

Within a few hours after the vaccination, all 7 patients developed severe local reactions at the injection site. Two patients had to be hospitalized for systemic reactions including fever. All symptoms resolved in a period of 3-17 days.

CONCLUSION

Our findings indicate that pneumococcal vaccines can trigger a severe local and systemic inflammatory reaction in patients with CAPS and possibly patients with other autoinflammatory diseases. Careful consideration is warranted when implementing current European League Against Rheumatism immunization guidelines in this patient population.