Dual function of pneumolysin in the early pathogenesis of murine pneumococcal pneumonia

Pore-forming activity of S. pneumoniae pneumolysin disrupts the paracellular localization of the epithelial adherens junction protein E-cadherin

Streptococcus pneumoniae, a common cause of community-acquired bacterial pneumonia, can cross the respiratory epithelial barrier to cause lethal septicemia and meningitis. S. pneumoniae pore-forming toxin pneumolysin (PLY) triggers robust neutrophil (PMN) infiltration that promotes bacterial transepithelial migration in vitro and disseminated disease in mice. Apical infection of polarized respiratory epithelial monolayers by S. pneumoniae at a multiplicity of infection (MOI) of 20 resulted in recruitment of PMNs, loss of 50% of the monolayer, and PMN-dependent bacterial translocation. Reducing the MOI to 2 decreased PMN recruitment two-fold and preserved the monolayer, but apical-to-basolateral translocation of S. pneumoniae remained relatively efficient. At both MOI of 2 and 20, PLY was required for maximal PMN recruitment and bacterial translocation. Co-infection by wild-type S. pneumoniae restored translocation by a PLY-deficient mutant, indicating that PLY can act in trans. Investigating the contribution of S. pneumoniae infection on apical junction complexes in the absence of PMN transmigration, we found that S. pneumoniae infection triggered the cleavage and mislocalization of the adherens junction (AJ) protein E-cadherin. This disruption was PLY-dependent at MOI of 2 and was recapitulated by purified PLY, requiring its pore-forming activity. In contrast, at MOI of 20, E-cadherin disruption was independent of PLY, indicating that S. pneumoniae encodes multiple means to disrupt epithelial integrity. This disruption was insufficient to promote bacterial translocation in the absence of PMNs. Thus, S. pneumoniae triggers cleavage and mislocalization of E-cadherin through PLY-dependent and -independent mechanisms, but maximal bacterial translocation across epithelial monolayers requires PLY-dependent neutrophil transmigration.

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.

The unremarkable alveolar epithelial glycocalyx: a thorium dioxide-based electron microscopic comparison after heparinase or pneumolysin treatment

Recent investigations analyzed in depth the biochemical and biophysical properties of the endothelial glycocalyx. In comparison, this complex cell-covering structure is largely understudied in alveolar epithelial cells. To better characterize the alveolar glycocalyx ultrastructure, unaffected versus injured human lung tissue explants and mouse lungs were analyzed by transmission electron microscopy. Lung tissue was treated with either heparinase (HEP), known to shed glycocalyx components, or pneumolysin (PLY), the exotoxin of Streptococcus pneumoniae not investigated for structural glycocalyx effects so far. Cationic colloidal thorium dioxide (cThO₂) particles were used for glycocalyx glycosaminoglycan visualization. The level of cThO₂ particles orthogonal to apical cell membranes (≙ stained glycosaminoglycan height) of alveolar epithelial type I (AEI) and type II (AEII) cells was stereologically measured. In addition, cThO₂ particle density was studied by dual-axis electron tomography (≙ stained glycosaminoglycan density in three dimensions). For untreated samples, the average cThO₂ particle level was ≈ 18 nm for human AEI, ≈ 17 nm for mouse AEI, ≈ 44 nm for human AEII and ≈ 35 nm for mouse AEII. Both treatments, HEP and PLY, resulted in a significant reduction of cThO₂ particle levels on human and mouse AEI and AEII. Moreover, a HEP- and PLY-associated reduction in cThO₂ particle density was observed. The present study provides quantitative data on the differential glycocalyx distribution on AEI and AEII based on cThO₂ and demonstrates alveolar glycocalyx shedding in response to HEP or PLY resulting in a structural reduction in both glycosaminoglycan height and density. Future studies should elucidate the underlying alveolar epithelial cell type-specific distribution of glycocalyx subcomponents for better functional understanding.

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.

Pneumolysin suppresses the initial macrophage pro-inflammatory response to Streptococcus pneumoniae

Published data for the Streptococcus pneumoniae virulence factor Pneumolysin (Ply) show contradictory effects on the host inflammatory response to infection. Ply has been shown to activate the inflammasome, but also can bind to MRC-1 resulting in suppression of dendritic cell inflammatory responses. We have used an in vitro infection model of human monocyte-derived macrophages (MDM), and a mouse model of pneumonia to clarify whether pro- or anti-inflammatory effects dominate the effects of Ply on the initial macrophage inflammatory response to S. pneumoniae, and the consequences during early lung infection. We found that infection with S. pneumoniae expressing Ply suppressed tumour necrosis factor (TNF) and interleukin-6 production by MDMs compared to cells infected with ply-deficient S. pneumoniae. This effect was independent of bacterial effects on cell death. Transcriptional analysis demonstrated S. pneumoniae expressing Ply caused a qualitatively similar but quantitatively lower MDM transcriptional response to S. pneumoniae compared to ply-deficient S. pneumoniae, with reduced expression of TNF and type I IFN inducible genes. Reduction of the MDM inflammatory response was prevented by inhibition of SOCS1. In the early lung infection mouse model, the TNF response to ply-deficient S. pneumoniae was enhanced and bacterial clearance increased compared to infection with wild-type S. pneumoniae. Overall, these data show Ply inhibits the initial macrophage inflammatory response to S. pneumoniae, probably mediated through SOCS1, and this was associated with improved immune evasion during early lung infection.

Procoagulant Activity of Blood and Microvesicles Is Disturbed by Pneumococcal Pneumolysin, Which Interacts with Coagulation Factors

The coagulation and contact systems are parts of the innate immune system as they prevent bleeding and dissemination of pathogens and also contribute to microbial killing by inflammatory reactions and the release of antimicrobial peptides. Here, we investigated the influence of Streptococcus pneumoniae on the coagulation and contact system. S. pneumoniae (pneumococci), but no other investigated streptococcal species, impairs coagulation of blood by autolysis and release of pneumolysin. Defective blood coagulation results from the lysis of tissue factor-producing mononuclear cells and their procoagulant microvesicles, which are the main trigger for blood coagulation during sepsis. In addition, pneumolysin binds coagulation and contact system factors, but this does not result in activation. Thus, pneumococci modulate activation of the coagulation system by releasing pneumolysin, which could potentiate lung injury during pneumonia.

Neutrophil Recruitment in Pneumococcal Pneumonia

Streptococcus pneumoniae (Spn) is the primary agent of community-acquired pneumonia. Neutrophils are innate immune cells that are essential for bacterial clearance during pneumococcal pneumonia but can also do harm to host tissue. Neutrophil migration in pneumococcal pneumonia is therefore a major determinant of host disease outcomes. During Spn infection, detection of the bacterium leads to an increase in proinflammatory signals and subsequent expression of integrins and ligands on both the neutrophil as well as endothelial and epithelial cells. These integrins and ligands mediate the tethering and migration of the neutrophil from the bloodstream to the site of infection. A gradient of host-derived and bacterial-derived chemoattractants contribute to targeted movement of neutrophils. During pneumococcal pneumonia, neutrophils are rapidly recruited to the pulmonary space, but studies show that some of the canonical neutrophil migratory machinery is dispensable. Investigation of neutrophil migration is necessary for us to understand the dynamics of pneumococcal infection. Here, we summarize what is known about the pathways that lead to migration of the neutrophil from the capillaries to the lung during pneumococcal infection.

Diverse Mechanisms of Protective Anti-Pneumococcal Antibodies

The gram-positive bacterium Streptococcus pneumoniae is a leading cause of pneumonia, otitis media, septicemia, and meningitis in children and adults. Current prevention and treatment efforts are primarily pneumococcal conjugate vaccines that target the bacterial capsule polysaccharide, as well as antibiotics for pathogen clearance. While these methods have been enormously effective at disease prevention and treatment, there has been an emergence of non-vaccine serotypes, termed serotype replacement, and increasing antibiotic resistance among these serotypes. To combat S. pneumoniae, the immune system must deploy an arsenal of antimicrobial functions. However, S. pneumoniae has evolved a repertoire of evasion techniques and is able to modulate the host immune system. Antibodies are a key component of pneumococcal immunity, targeting both the capsule polysaccharide and protein antigens on the surface of the bacterium. These antibodies have been shown to play a variety of roles including increasing opsonophagocytic activity, enzymatic and toxin neutralization, reducing bacterial adherence, and altering bacterial gene expression. In this review, we describe targets of anti-pneumococcal antibodies and describe antibody functions and effectiveness against S. pneumoniae.

Matcha Green Tea Exhibits Bactericidal Activity against Streptococcus pneumoniae and Inhibits Functional Pneumolysin

Streptococcus pneumoniae is a causative pathogen of several human infectious diseases including community-acquired pneumonia. Pneumolysin (PLY), a pore-forming toxin, plays an important role in the pathogenesis of pneumococcal pneumonia. In recent years, the use of traditional natural substances for prevention has drawn attention because of the increasing antibacterial drug resistance of S. pneumoniae. According to some studies, green tea exhibits antibacterial and antitoxin activities. The polyphenols, namely the catechins epigallocatechin gallate (EGCG), epigallocatechin (EGC), epicatechin gallate (ECG), and epicatechin (EC) are largely responsible for these activities. Although matcha green tea provides more polyphenols than green tea infusions, its relationship with pneumococcal pneumonia remains unclear. In this study, we found that treatment with 20 mg/mL matcha supernatant exhibited significant antibacterial activity against S. pneumoniae regardless of antimicrobial resistance. In addition, the matcha supernatant suppressed PLY-mediated hemolysis and cytolysis by inhibiting PLY oligomerization. Moreover, the matcha supernatant and catechins inhibited PLY-mediated neutrophil death and the release of neutrophil elastase. These findings suggest that matcha green tea reduces the virulence of S. pneumoniae in vitro and may be a promising agent for the treatment of pneumococcal infections.

The Role of Neutrophils and Neutrophil Elastase in Pneumococcal Pneumonia

Streptococcus pneumoniae, also known as pneumococcus, is a Gram-positive diplococcus and a major human pathogen. This bacterium is a leading cause of bacterial pneumonia, otitis media, meningitis, and septicemia, and is a major cause of morbidity and mortality worldwide. To date, studies on S. pneumoniae have mainly focused on the role of its virulence factors including toxins, cell surface proteins, and capsules. However, accumulating evidence indicates that in addition to these studies, knowledge of host factors and host-pathogen interactions is essential for understanding the pathogenesis of pneumococcal diseases. Recent studies have demonstrated that neutrophil accumulation, which is generally considered to play a critical role in host defense during bacterial infections, can significantly contribute to lung injury and immune subversion, leading to pneumococcal invasion of the bloodstream. Here, we review bacterial and host factors, focusing on the role of neutrophils and their elastase, which contribute to the progression of pneumococcal pneumonia.

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

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

Mapping of Recognition Sites of Monoclonal Antibodies Responsible for the Inhibition of Pneumolysin Functional Activity

The pathogenicity of many bacteria, including Streptococcus pneumoniae, depends on pore-forming toxins (PFTs) that cause host cell lysis by forming large pores in cholesterol-containing cell membranes. Therefore, PFTs-neutralising antibodies may provide useful tools for reducing S. pneumoniae pathogenic effects. This study aimed at the development and characterisation of monoclonal antibodies (MAbs) with neutralising activity to S. pneumoniae PFT pneumolysin (PLY). Five out of 10 produced MAbs were able to neutralise the cytolytic activity of PLY on a lung epithelial cell line. Epitope mapping with a series of recombinant overlapping PLY fragments revealed that neutralising MAbs are directed against PLY loops L1 and L3 within domain 4. The epitopes of MAbs 3A9, 6E5 and 12F11 located at L1 loop (aa 454–471) were crucial for PLY binding to the immobilised cholesterol. In contrast, the MAb 12D10 recognising L3 (aa 403–423) and the MAb 3F3 against the conformational epitope did not interfere with PLY-cholesterol interaction. Due to conformation-dependent binding, the approach to use overlapping peptides for fine epitope mapping of the neutralising MAbs was unsuccessful. Therefore, the epitopes recognised by the MAbs were analysed using computational methods. This study provides new data on PLY sites involved in functional activity.

Pneumolysin: Pathogenesis and Therapeutic Target

Streptococcus pneumoniae is an opportunistic pathogen responsible for widespread illness and is a major global health issue for children, the elderly, and the immunocompromised population. Pneumolysin (PLY) is a cholesterol-dependent cytolysin (CDC) and key pneumococcal virulence factor involved in all phases of pneumococcal disease, including transmission, colonization, and infection. In this review we cover the biology and cytolytic function of PLY, its contribution to S. pneumoniae pathogenesis, and its known interactions and effects on the host with regard to tissue damage and immune response. Additionally, we review statins as a therapeutic option for CDC toxicity and PLY toxoid as a vaccine candidate in protein-based vaccines.

Increased pathogenicity of pneumococcal serotype 1 is driven by rapid autolysis and release of pneumolysin

Streptococcus pneumoniae serotype 1 is the predominant cause of invasive pneumococcal disease in sub-Saharan Africa, but the mechanism behind its increased invasiveness is not well understood. Here, we use mouse models of lung infection to identify virulence factors associated with severe bacteraemic pneumonia during serotype-1 (ST217) infection. We use BALB/c mice, which are highly resistant to pneumococcal pneumonia when infected with other serotypes. However, we observe 100% mortality and high levels of bacteraemia within 24 hours when BALB/c mice are intranasally infected with ST217. Serotype 1 produces large quantities of pneumolysin, which is rapidly released due to high levels of bacterial autolysis. This leads to substantial levels of cellular cytotoxicity and breakdown of tight junctions between cells, allowing a route for rapid bacterial dissemination from the respiratory tract into the blood. Thus, our results offer an explanation for the increased invasiveness of serotype 1.

The mechanisms behind the high invasiveness of Streptococcus pneumoniae serotype 1 are unclear. Here, Jacques et al. show that this feature is due to overproduction and rapid release of pneumolysin, which induces cytotoxicity and breakdown of tight junctions, allowing rapid bacterial dissemination from the respiratory tract into the blood.

Disruption of the cpsE and endA Genes Attenuates Streptococcus pneumoniae Virulence: Towards the Development of a Live Attenuated Vaccine Candidate

The majority of deaths due to Streptococcus pneumoniae infections are in developing countries. Although polysaccharide-based pneumococcal vaccines are available, newer types of vaccines are needed to increase vaccine affordability, particularly in developing countries, and to provide broader protection across all pneumococcal serotypes. To attenuate pneumococcal virulence with the aim of engineering candidate live attenuated vaccines (LAVs), we constructed knockouts in S. pneumoniae D39 of one of the capsular biosynthetic genes, cpsE that encodes glycosyltransferase, and the endonuclease gene, endA, that had been implicated in the uptake of DNA from the environment as well as bacterial escape from neutrophil-mediated killing. The cpsE gene knockout significantly lowered peak bacterial density, BALB/c mice nasopharyngeal (NP) colonisation but increased biofilm formation when compared to the wild-type D39 strain as well as the endA gene knockout mutant. All constructed mutant strains were able to induce significantly high serum and mucosal antibody response in BALB/c mice. However, the cpsE-endA double mutant strain, designated SPEC, was able to protect mice from high dose mucosal challenge of the D39 wild-type. Furthermore, SPEC showed 23-fold attenuation of virulence compared to the wild-type. Thus, the cpsE-endA double-mutant strain could be a promising candidate for further development of a LAV for S. pneumoniae.

Impact of Bacterial Toxins in the Lungs

Bacterial toxins play a key role in the pathogenesis of lung disease. Based on their structural and functional properties, they employ various strategies to modulate lung barrier function and to impair host defense in order to promote infection. Although in general, these toxins target common cellular signaling pathways and host compartments, toxin- and cell-specific effects have also been reported. Toxins can affect resident pulmonary cells involved in alveolar fluid clearance (AFC) and barrier function through impairing vectorial Na⁺ transport and through cytoskeletal collapse, as such, destroying cell-cell adhesions. The resulting loss of alveolar-capillary barrier integrity and fluid clearance capacity will induce capillary leak and foster edema formation, which will in turn impair gas exchange and endanger the survival of the host. Toxins modulate or neutralize protective host cell mechanisms of both the innate and adaptive immunity response during chronic infection. In particular, toxins can either recruit or kill central players of the lung's innate immune responses to pathogenic attacks, i.e., alveolar macrophages (AMs) and neutrophils. Pulmonary disorders resulting from these toxin actions include, e.g., acute lung injury (ALI), the acute respiratory syndrome (ARDS), and severe pneumonia. When acute infection converts to persistence, i.e., colonization and chronic infection, lung diseases, such as bronchitis, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF) can arise. The aim of this review is to discuss the impact of bacterial toxins in the lungs and the resulting outcomes for pathogenesis, their roles in promoting bacterial dissemination, and bacterial survival in disease progression.

Pneumolysin Induces 12-Lipoxygenase-Dependent Neutrophil Migration during Streptococcus pneumoniae Infection

Streptococcus pneumoniae is a major cause of pneumonia, wherein infection of respiratory mucosa drives a robust influx of neutrophils. We have previously shown that S. pneumoniae infection of the respiratory epithelium induces the production of the 12-lipoxygenase (12-LOX)-dependent lipid inflammatory mediator hepoxilin A3, which promotes recruitment of neutrophils into the airways, tissue damage, and lethal septicemia. Pneumolysin (PLY), a member of the cholesterol-dependent cytolysin (CDC) family, is a major S. pneumoniae virulence factor that generates ∼25-nm diameter pores in eukaryotic membranes and promotes acute inflammation, tissue damage, and bacteremia. We show that a PLY-deficient S. pneumoniae mutant was impaired in triggering human neutrophil transepithelial migration in vitro. Ectopic production of PLY endowed the nonpathogenic Bacillus subtilis with the ability to trigger neutrophil recruitment across human-cultured monolayers. Purified PLY, several other CDC family members, and the α-toxin of Clostridium septicum, which generates pores with cross-sectional areas nearly 300 times smaller than CDCs, reproduced this robust neutrophil transmigration. PLY non-pore-forming point mutants that are trapped at various stages of pore assembly did not recruit neutrophils. PLY triggered neutrophil recruitment in a 12-LOX-dependent manner in vitro. Instillation of wild-type PLY but not inactive derivatives into the lungs of mice induced robust 12-LOX-dependent neutrophil migration into the airways, although residual inflammation induced by PLY in 12-LOX-deficient mice indicates that 12-LOX-independent pathways also contribute to PLY-triggered pulmonary inflammation. These data indicate that PLY is an important factor in promoting hepoxilin A3-dependent neutrophil recruitment across pulmonary epithelium in a pore-dependent fashion.

Pneumococcal Vaccines

Streptococcus pneumoniae is a Gram-Positive pathogen that is a major causative agent of pneumonia, otitis media, sepsis and meningitis across the world. The World Health Organization estimates that globally over 500,000 children are killed each year by this pathogen. Vaccines offer the best protection against S. pneumoniae infections. The current polysaccharide conjugate vaccines have been very effective in reducing rates of invasive pneumococcal disease caused by vaccine type strains. However, the effectiveness of these vaccines have been somewhat diminished by the increasing numbers of cases of invasive disease caused by non-vaccine type strains, a phenomenon known as serotype replacement. Since, there are currently at least 98 known serotypes of S. pneumoniae, it may become cumbersome and expensive to add many additional serotypes to the current 13-valent vaccine, to circumvent the effect of serotype replacement. Hence, alternative serotype independent strategies, such as vaccination with highly cross-reactive pneumococcal protein antigens, should continue to be investigated to address this problem. This chapter provides a comprehensive discussion of pneumococcal vaccines past and present, protein antigens that are currently under investigation as vaccine candidates, and other alternatives, such as the pneumococcal whole cell vaccine, that may be successful in reducing current rates of disease caused by S. pneumoniae.

Animal Models of Pneumococcal pneumonia

Streptococcus pneumoniae remains the most common bacterial pathogen causing lower respiratory tract infections and is a leading cause of morbidity and mortality worldwide, especially in children and the elderly. Another important aspect related to pneumococcal infections is the persistent rate of penicillin and macrolide resistance. Therefore, animal models have been developed to better understand the pathogenesis of pneumococcal disease and test new therapeutic agents and vaccines. This narrative review will focus on the characteristics of the different animal pneumococcal pneumonia models. The assessment of the different animal models will include considerations regarding pneumococcal strains, microbiology properties, procedures used for bacterial inoculation, pathogenesis, clinical characteristics, diagnosis, treatment, and preventive approaches.

Streptococcus pneumoniae: Invasion and Inflammation

Streptococcus pneumoniae (the pneumoccus) is the leading cause of otitis media, community-acquired pneumonia, and bacterial meningitis. The success of the pneumococcus stems from its ability to persist in the population as a commensal and avoid killing by immune system. This chapter first reviews the molecular mechanisms that allow the pneumococcus to colonize and spread from one anatomical site to the next. Then, it discusses the mechanisms of inflammation and cytotoxicity during emerging and classical pneumococcal infections.

Effectiveness of combination therapy versus monotherapy with a third-generation cephalosporin in bacteraemic pneumococcal pneumonia: A propensity score analysis

OBJECTIVE

Combining a macrolide or a fluoroquinolone to beta-lactam regimens in the treatment of patients with moderate to severe community-acquired pneumonia is recommended by the international guidelines. However, the information in patients with bacteraemic pneumococcal pneumonia is limited.

METHODS

A propensity score technique was used to analyze prospectively collected data from all patients with bacteraemic pneumococcal pneumonia admitted from 2000 to 2015 in our institution, who had received empirical treatment with third-generation cephalosporin in monotherapy or plus macrolide or fluoroquinolone.

RESULTS

We included 69 patients in the monotherapy group and 314 in the combination group. After adjustment by PS for receiving monotherapy, 30-day mortality (OR 2.89; 95% CI 1.07-7.84) was significantly higher in monotherapy group. A higher 30-day mortality was observed in monotherapy group in both 1:1 and 1:2 matched samples although it was statistically significant only in 1:2 sample (OR: 3.50 (95% CI 1.03-11.96), P = 0.046).

CONCLUSIONS

Our study suggests that in bacteraemic pneumococcal pneumonia, empirical therapy with a third-generation cephalosporin plus a macrolide or a fluoroquinolone is associated with a lower mortality rate than beta-lactams in monotherapy. These results support the recommendation of combination therapy in patients requiring admission with moderate to severe disease.

Biphasic lung injury during Streptococcus pneumoniae infection in a murine model

OBJECTIVES

Streptococcus pneumoniae is the leading cause of community-acquired pneumonia. We aimed to analyze the epithelial response to S. pneumoniae-induced lung injury.

METHODS

Using an in vitro model with 16HBE cells and experimental in vivo murine model of acute lung injury, we analyzed the epithelial response to S. pneumoniae. Lung epithelial cell monolayers were exposed to S. pneumoniae and permeability was assessed by transepithelial resistance (TER) measurement and organization and expression of junction proteins. Functional consequences were studied with an in vivo murine model measuring alveolar permeability, distal alveolar fluid clearance (DAFC), and the alveolar inflammatory response.

RESULTS

In vitro, S. pneumoniae induced a dose-dependent decrease in transepithelial resistance, which was associated with significant modifications in the organization of junction proteins assessed by immunofluorescence staining and expression after 6hours of exposure. In vivo, S. pneumoniae induced a transient increase in alveolar permeability with an adequate increase in DAFC 6hours post infection. In a second phase, a permanent increased permeability was associated with a major decrease in DAFC.

CONCLUSION

Overall, the epithelial response to S. pneumoniae followed a biphasic pattern with an initial reversible increase in permeability related to the alteration of tight and adherens junctions and a second phase associated with an epithelial injury with a major increase in permeability with a decreased DAFC reflecting an injured alveolar capillary barrier.

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.

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.

Preclinical evaluation of a chemically detoxified pneumolysin as pneumococcal vaccine antigen

The use of protein antigens able to protect against the majority of Streptococcus pneumoniae serotypes is envisaged as stand-alone and/or complement to the current capsular polysaccharide-based pneumococcal vaccines. Pneumolysin (Ply) is a key virulence factor that is highly conserved in amino acid sequence across pneumococcal serotypes, and therefore may be considered as a vaccine target. However, native Ply cannot be used in vaccines due to its intrinsic cytolytic activity. In the present work a completely, irreversibly detoxified pneumolysin (dPly) has been generated using an optimized formaldehyde treatment. Detoxi-fication was confirmed by dPly challenge in mice and histological analysis of the injection site in rats. Immunization with dPly elicited Ply-specific functional antibodies that were able to inhibit Ply activity in a hemolysis assay. In addition, immunization with dPly protected mice against lethal intranasal challenge with Ply, and intranasal immunization inhibited nasopharyngeal colonization after intranasal challenge with homologous or heterologous pneumococcal strain. Our findings supported dPly as a valid candidate antigen for further pneumococcal vaccine development.

Statin-conferred enhanced cellular resistance against bacterial pore-forming toxins in airway epithelial cells

Statins are widely used to prevent cardiovascular disease. In addition to their inhibitory effects on cholesterol synthesis, statins have beneficial effects in patients with sepsis and pneumonia, although molecular mechanisms have mostly remained unclear. Using human airway epithelial cells as a proper in vitro model, we show that prior exposure to physiological nanomolar serum concentrations of simvastatin (ranging from 10-1,000 nM) confers significant cellular resistance to the cytotoxicity of pneumolysin, a pore-forming toxin and the main virulence factor of Streptococcus pneumoniae. This protection could be demonstrated with a different statin, pravastatin, or on a different toxin, α-hemolysin. Furthermore, through the use of gene silencing, pharmacological inhibitors, immunofluorescence microscopy, and biochemical and metabolic rescue approaches, we demonstrate that the mechanism of protection conferred by simvastatin at physiological nanomolar concentrations could be different from the canonical mevalonate pathways seen in most other mechanistic studies conducted with statins at micromolar levels. All of these data are integrated into a protein synthesis-dependent, calcium-dependent model showing the interconnected pathways used by statins in airway epithelial cells to elicit an increased resistance to pore-forming toxins. This research fills large gaps in our understanding of how statins may confer host cellular protection against bacterial infections in the context of airway epithelial cells without the confounding effect from the presence of immune cells. In addition, our discovery could be potentially developed into a host-centric strategy for the adjuvant treatment of pore-forming toxin associated bacterial infections.

Pneumolysin plays a key role at the initial step of establishing pneumococcal nasal colonization

Nasopharyngeal colonization by Streptococcus pneumoniae is an important initial step for the subsequent development of pneumococcal infections. Pneumococci have many virulence factors that play a role in colonization. Pneumolysin (PLY), a pivotal pneumococcal virulence factor for invasive disease, causes severe tissue damage and inflammation with disruption of epithelial tight junctions. In this study, we evaluated the role of PLY in nasal colonization of S. pneumoniae using a mouse colonization model. A reduction of numbers of PLY-deficient pneumococci recovered from nasal tissue, as well as nasal wash, was observed at days 1 and 2 post-intranasal challenges, but not later. The findings strongly support an important role for PLY in the initial establishment nasal colonization. PLY-dependent invasion of local nasal mucosa may be required to establish nasal colonization with S. pneumoniae. The data help provide a rationale to explain why an organism that exists as an asymptomatic colonizer has evolved virulence factors that enable it to occasionally invade and kill its hosts. Thus, the same pneumococcal virulence factor, PLY that can contribute to killing the host, may also play a role early in the establishment of nasopharynx carriage.

Serotype 1 and 8 Pneumococci Evade Sensing by Inflammasomes in Human Lung Tissue

Streptococcus pneumoniae is a major cause of pneumonia, sepsis and meningitis. The pore-forming toxin pneumolysin is a key virulence factor of S. pneumoniae, which can be sensed by the NLRP3 inflammasome. Among the over 90 serotypes, serotype 1 pneumococci (particularly MLST306) have emerged across the globe as a major cause of invasive disease. The cause for its particularity is, however, incompletely understood. We therefore examined pneumococcal infection in human cells and a human lung organ culture system mimicking infection of the lower respiratory tract. We demonstrate that different pneumococcal serotypes differentially activate inflammasome-dependent IL-1β production in human lung tissue and cells. Whereas serotype 2, 3, 6B, 9N pneumococci expressing fully haemolytic pneumolysins activate NLRP3 inflammasome-dependent responses, serotype 1 and 8 strains expressing non-haemolytic toxins are poor activators of IL-1β production. Accordingly, purified haemolytic pneumolysin but not serotype 1-associated non-haemolytic toxin activates strong IL-1β production in human lungs. Our data suggest that the evasion of inflammasome-dependent innate immune responses by serotype 1 pneumococci might contribute to their ability to cause invasive diseases in humans.

The role of macrophages in the innate immune response to Streptococcus pneumoniae and Staphylococcus aureus: mechanisms and contrasts

Macrophages are critical mediators of innate immune responses against bacteria. The Gram-positive bacteria Streptococcus pneumoniae and Staphylococcus aureus express a range of virulence factors, which challenge macrophages' immune competence. We review how macrophages respond to this challenge. Macrophages employ a range of strategies to phagocytose and kill each pathogen. When the macrophages capacity to clear bacteria is overwhelmed macrophages play important roles in orchestrating the inflammatory response through pattern recognition receptor-mediated responses. Macrophages also ensure the inflammatory response is tightly constrained, to avoid tissue damage, and play an important role in downregulating the inflammatory response once initial bacterial replication is controlled.

Pneumolysin activates macrophage lysosomal membrane permeabilization and executes apoptosis by distinct mechanisms without membrane pore formation

Intracellular killing of Streptococcus pneumoniae is complemented by induction of macrophage apoptosis. Here, we show that the toxin pneumolysin (PLY) contributes both to lysosomal/phagolysosomal membrane permeabilization (LMP), an upstream event programing susceptibility to apoptosis, and to apoptosis execution via a mitochondrial pathway, through distinct mechanisms. PLY is necessary but not sufficient for the maximal induction of LMP and apoptosis. PLY's ability to induce both LMP and apoptosis is independent of its ability to form cytolytic pores and requires only the first three domains of PLY. LMP involves TLR (Toll-like receptor) but not NLRP3/ASC (nucleotide-binding oligomerization domain [Nod]-like receptor family, pyrin domain-containing protein 3/apoptosis-associated speck-like protein containing a caspase recruitment domain) signaling and is part of a PLY-dependent but phagocytosis-independent host response that includes the production of cytokines, including interleukin-1 beta (IL-1β). LMP involves progressive and selective permeability to 40-kDa but not to 250-kDa fluorescein isothiocyanate (FITC)-labeled dextran, as PLY accumulates in the cytoplasm. In contrast, the PLY-dependent execution of apoptosis requires phagocytosis and is part of a host response to intracellular bacteria that also includes NO generation. In cells challenged with PLY-deficient bacteria, reconstitution of LMP using the lysomotrophic detergent LeuLeuOMe favored cell necrosis whereas PLY reconstituted apoptosis. The results suggest that PLY contributes to macrophage activation and cytokine production but also engages LMP. Following bacterial phagocytosis, PLY triggers apoptosis and prevents macrophage necrosis as a component of a broad-based antimicrobial strategy. This illustrates how a key virulence factor can become the focus of a multilayered and coordinated innate response by macrophages, optimizing pathogen clearance and limiting inflammation. Importance: Streptococcus pneumoniae, the commonest cause of bacterial pneumonia, expresses the toxin pneumolysin, which can make holes in cell surfaces, causing tissue damage. Macrophages, resident immune cells essential for responses to bacteria in tissues, activate a program of cell suicide called apoptosis, maximizing bacterial clearance and limiting harmful inflammation. We examined pneumolysin's role in activating this response. We demonstrate that pneumolysin did not directly form holes in cells to trigger apoptosis and show that pneumolysin has two distinct roles which require only part of the molecule. Pneumolysin and other bacterial factors released by bacteria that have not been eaten by macrophages activate macrophages to release inflammatory factors but also make the cell compartment containing ingested bacteria leaky. Once inside the cell, pneumolysin ensures that the bacteria activate macrophage apoptosis, rather than necrosis, enhancing bacterial killing and limiting inflammation. This dual response to pneumolysin is critical for an effective immune response to S. pneumoniae.

PKC-dependent phosphorylation of eNOS at T495 regulates eNOS coupling and endothelial barrier function in response to G+ -toxins

Gram positive (G⁺) infections make up ∼50% of all acute lung injury cases which are characterized by extensive permeability edema secondary to disruption of endothelial cell (EC) barrier integrity. A primary cause of increased permeability are cholesterol-dependent cytolysins (CDCs) of G⁺-bacteria, such as pneumolysin (PLY) and listeriolysin-O (LLO) which create plasma membrane pores, promoting Ca²⁺-influx and activation of PKCα. In human lung microvascular endothelial cells (HLMVEC), pretreatment with the nitric oxide synthase (NOS) inhibitor, ETU reduced the ability of LLO to increase microvascular cell permeability suggesting an endothelial nitric oxide synthase (eNOS)-dependent mechanism. LLO stimulated superoxide production from HLMVEC and this was prevented by silencing PKCα or NOS inhibition suggesting a link between these pathways. Both LLO and PLY stimulated eNOS T495 phosphorylation in a PKC-dependent manner. Expression of a phosphomimetic T495D eNOS (human isoform) resulted in increased superoxide and diminished nitric oxide (NO) production. Transduction of HLMVEC with an active form of PKCα resulted in the robust phosphorylation of T495 and increased peroxynitrite production, indicative of eNOS uncoupling. To determine the mechanisms underlying eNOS uncoupling, HLMVEC were stimulated with LLO and the amount of hsp90 and caveolin-1 bound to eNOS determined. LLO stimulated the dissociation of hsp90, and in particular, caveolin-1 from eNOS. Both hsp90 and caveolin-1 have been shown to influence eNOS uncoupling and a peptide mimicking the scaffolding domain of caveolin-1 blocked the ability of PKCα to stimulate eNOS-derived superoxide. Collectively, these results suggest that the G⁺ pore-forming toxins promote increased EC permeability via activation of PKCα, phosphorylation of eNOS-T495, loss of hsp90 and caveolin-1 binding which collectively promote eNOS uncoupling and the production of barrier disruptive superoxide.

Pneumococcal IgA1 protease subverts specific protection by human IgA1

Bacterial immunoglobulin A1 (IgA1) proteases may sabotage the protective effects of IgA. In vitro, both exogenous and endogenously produced IgA1 protease inhibited phagocytic killing of Streptococcus pneumoniae by capsule-specific IgA1 human monoclonal antibodies (hMAbs) but not IgA2. These IgA1 proteases cleaved and reduced binding of the the effector Fcα1 heavy chain but not the antigen-binding F(ab)/light chain to pneumococcal surfaces. In vivo, IgA1 protease-resistant IgA2, but not IgA1 protease-sensitive IgA1, supported 60% survival in mice infected with wild-type S. pneumoniae. IgA1 hMAbs protected mice against IgA1 protease-deficient but not -producing pneumococci. Parallel mouse sera with human IgA2 showed more efficient complement-mediated reductions in pneumococci with neutrophils than did IgA1, particularly with protease-producing organisms. After natural human pneumococcal bacteremia, purified serum IgG inhibited IgA1 protease activity in 7 of 11 patients (64%). These observations provide the first evidence in vivo that IgA1 protease can circumvent killing of S. pneumoniae by human IgA. Acquisition of IgA1 protease-neutralizing IgG after infection directs attention to IgA1 protease both as a determinant of successful colonization and infection and as a potential vaccine candidate.

Tailoring the Immune Response via Customization of Pathogen Gene Expression

The majority of studies focused on the construction and reengineering of bacterial pathogens have mainly relied on the knocking out of virulence factors or deletion/mutation of amino acid residues to then observe the microbe's phenotype and the resulting effect on the host immune response. These knockout bacterial strains have also been proposed as vaccines to combat bacterial disease. Theoretically, knockout strains would be unable to cause disease since their virulence factors have been removed, yet they could induce a protective memory response. While knockout strains have been valuable tools to discern the role of virulence factors in host immunity and bacterial pathogenesis, they have been unable to yield clinically relevant vaccines. The advent of synthetic biology and enhanced user-directed gene customization has altered this binary process of knockout, followed by observation. Recent studies have shown that a researcher can now tailor and customize a given microbe's gene expression to produce a desired immune response. In this commentary, we highlight these studies as a new avenue for controlling the inflammatory response as well as vaccine development.

The biology of pneumolysin

Cholesterol dependent cytolysins are important in the ability of some bacteria to cause disease in man and animals. Pneumolysin (PLY) plays a key role in the diseases caused by Streptococcus pneumoniae (the pneumococcus). This chapter describes the role of PLY in some of the key process in disease. These include induction of cell death by pore formation and toxin-induced apoptosis as well as more subtle effects on gene expression of host cells including epigenetic effects of the toxin. The use of bacterial mutants that either do not express the toxin or express altered versions in biological systems is described. Use of isolated tissue and whole animal systems to dissect the structure/function relationships of the toxin as well as the role played by different activities in the pathogenesis of infection are described. The role of PLY in meningitis and the associated deafness is discussed as well as the role of the toxin in promoting increased lung permeability and inflammation during pneumococcal pneumonia. Different clinical strains of the pneumococcus produce different forms of PLY and the impact of this on disease caused by these strains is discussed. Finally, the impact of this knowledge on the development of treatment and prevention strategies for pneumococcal disease is discussed.

Intermedilysin Is Essential for the Invasion of Hepatoma HepG2 Cells byStreptococcus intermedius

Streptococcus intermedius causes endogenous infections leading to abscesses. This species produces intermedilysin (ILY), a human-specific cytolysin. Because of the significant correlation between higher ILY production levels by S. intermedius and deep-seated abscesses, we constructed ily knockout mutant UNS38 B3 and complementation strain UNS38 B3R1 in order to investigate the role of ILY in deep-seated infections. Strain UNS38 reduced the viability of human liver cell line HepG2 at infection but not of rat liver cell line BRL3A. Isogenic mutant strain UNS38 B3 was not cytotoxic in either cell line. Quantification of S. intermedius revealed that in infected HepG2 cells UNS38 but not UNS38 B3 increased intracellularly concomitantly with increasing cell damage. This difference between UNS38 and UNS38 B3 was not observed with UNS38 B3R1. Invasion and proliferation in BRL3A cells was not observed. Masking UNS38 or UNS38 B3R1 with ILY antibody drastically decreased adherence and invasion of HepG2. Moreover, coating strain UNS38 B3 with ILY partially restored adherence to HepG2 but without subsequent bacterial growth. At 1 day post-infection, many intact UNS38 were detected in the damaged phagosomes of HepG2 with bacterial proliferation observed in the cytoplasm of dead HepG2 after an additional 2 day incubation. These results indicate that surface-bound ILY on S. intermedius is an important factor for invasion of human cells by this bacterium and that secretion of ILY within host cells is essential for subsequent host cell death. These data strongly implicate ILY as an important factor in the pathogenesis of abscesses in vivo by this streptococcus.

Pneumococcal immune evasion: ZmpC inhibits neutrophil influx

Neutrophil recruitment is essential in clearing pneumococcal infections. The first step in neutrophil extravasation involves the interaction between P-selectin on activated endothelium and P-Selectin Glycoprotein 1 (PSGL-1) on neutrophils. Here, we identify pneumococcal Zinc metalloproteinase C as a potent inhibitor of PSGL-1. ZmpC degrades the N-terminal domain of PSGL-1, thereby disrupting the initial rolling of neutrophils on activated human umbilical vein endothelial cells. Furthermore, mice infected with wild-type strain in the model of pneumococcal pneumonia showed lower lungs neutrophil infiltration compare to animals infected with ZmpC mutant. In addition, we confirmed the association of zmpC with serotype 8 and 11A and found it to be associated with serotype 33F as well. In conclusion, wereport PSGL-1 as a novel target for ZmpC and show that ZmpC inhibits neutrophil extravasation during pneumococcal pneumonia.

Protein kinase C-α and arginase I mediate pneumolysin-induced pulmonary endothelial hyperpermeability

Antibiotics-induced release of the pore-forming virulence factor pneumolysin (PLY) in patients with pneumococcal pneumonia results in its presence days after lungs are sterile and is a major factor responsible for the induction of permeability edema. Here we sought to identify major mechanisms mediating PLY-induced endothelial dysfunction. We evaluated PLY-induced endothelial hyperpermeability in human lung microvascular endothelial cells (HL-MVECs) and human lung pulmonary artery endothelial cells in vitro and in mice instilled intratracheally with PLY. PLY increases permeability in endothelial monolayers by reducing stable and dynamic microtubule content and modulating VE-cadherin expression. These events, dependent upon an increased calcium influx, are preceded by protein kinase C (PKC)-α activation, perturbation of the RhoA/Rac1 balance, and an increase in myosin light chain phosphorylation. At later time points, PLY treatment increases the expression and activity of arginase in HL-MVECs. Arginase inhibition abrogates and suppresses PLY-induced endothelial barrier dysfunction by restoring NO generation. Consequently, a specific PKC-α inhibitor and the TNF-derived tonoplast intrinsic protein peptide, which blunts PLY-induced PKC-α activation, are able to prevent activation of arginase in HL-MVECs and to reduce PLY-induced endothelial hyperpermeability in mice. Arginase I (AI)(+/-)/arginase II (AII)(-/-) C57BL/6 mice, displaying a significantly reduced arginase I expression in the lungs, are significantly less sensitive to PLY-induced capillary leak than their wild-type or AI(+/+)/AII(-/-) counterparts, indicating an important role for arginase I in PLY-induced endothelial hyperpermeability. These results identify PKC-α and arginase I as potential upstream and downstream therapeutic targets in PLY-induced pulmonary endothelial dysfunction.

Neutralizing antibodies elicited by a novel detoxified pneumolysin derivative, PlyD1, provide protection against both pneumococcal infection and lung injury

Streptococcus pneumoniae pneumolysin (PLY) is a virulence factor that causes toxic effects contributing to pneumococcal pneumonia. To date, deriving a PLY candidate vaccine with the appropriate detoxification and immune profile has been challenging. A pneumolysin protein that is appropriately detoxified and that retains its immunogenicity is a desirable vaccine candidate. In this study, we assessed the protective efficacy of our novel PlyD1 detoxified PLY variant and investigated its underlying mechanism of protection. Results have shown that PlyD1 immunization protected mice against lethal intranasal (i.n.) challenge with pneumococci and lung injury mediated by PLY challenge. Protection was associated with PlyD1-specific IgG titers and in vitro neutralization titers. Pretreatment of PLY with PlyD1-specific rat polyclonal antiserum prior to i.n. delivery of toxin reduced PLY-mediated lung lesions, interleukin-6 (IL-6) production, and neutrophil infiltration into lungs, indicating that protection from lung lesions induced by PLY is antibody mediated. Preincubation of PLY with a neutralizing monoclonal PLY antibody also specifically reduced the cytotoxic effects of PLY after i.n. inoculation in comparison to nonneutralizing monoclonal antibodies. These results indicate that the induction of neutralizing antibodies against PLY can contribute to protection against bacterial pneumonia by preventing the development of PLY-induced lung lesions and inflammation. Our detoxified PlyD1 antigen elicits such PLY neutralizing antibodies, thus serving as a candidate vaccine antigen for the prevention of pneumococcal pneumonia.

A Streptococcus pneumoniae infection model in larvae of the wax moth Galleria mellonella

The bacterium Streptococcus pneumoniae is a leading human opportunistic pathogen. The limitations of the current vaccine have led to increased recognition of the need to understand bacterial behaviour and competitive dynamics using in vivo models of infection. Here, we investigate the potential application of the larvae of the wax moth Galleria mellonella as an informative infection model. Larvae were challenged with a range of doses of S. pneumoniae isolates differing in known virulence factors to determine the LD(50) values. Infection dynamics were determined by obtaining bacterial counts from larvae over a time course. Differences in virulence between serotypes could be distinguished in this host. Infection with strains differing in known virulence factors demonstrated predicted differences in virulence. Acapsulate and pneumolysin-negative strains were less virulent than their respective wild types. A large reduction in virulence was seen in strains lacking cell wall D-alanylation. The mortality of G. mellonella larvae is attributable to bacterial growth within larvae, while surviving larvae are able to clear infections by reducing bacterial numbers. These data demonstrate that G. mellonella larvae represent an in vivo infection model with applications for investigating aspects of bacterial-host interactions such as the role of antimicrobial peptide activity and resistance.

Streptococcus pneumoniae in biofilms are unable to cause invasive disease due to altered virulence determinant production

It is unclear whether Streptococcus pneumoniae in biofilms are virulent and contribute to development of invasive pneumococcal disease (IPD). Using electron microscopy we confirmed the development of mature pneumococcal biofilms in a continuous-flow-through line model and determined that biofilm formation occurred in discrete stages with mature biofilms composed primarily of dead pneumococci. Challenge of mice with equal colony forming units of biofilm and planktonic pneumococci determined that biofilm bacteria were highly attenuated for invasive disease but not nasopharyngeal colonization. Biofilm pneumococci of numerous serotypes were hyper-adhesive and bound to A549 type II pneumocytes and Detroit 562 pharyngeal epithelial cells at levels 2 to 11-fold greater than planktonic counterparts. Using genomic microarrays we examined the pneumococcal transcriptome and determined that during biofilm formation S. pneumoniae down-regulated genes involved in protein synthesis, energy production, metabolism, capsular polysaccharide (CPS) production, and virulence. We confirmed these changes by measuring CPS by ELISA and immunoblotting for the toxin pneumolysin and the bacterial adhesins phosphorylcholine (ChoP), choline-binding protein A (CbpA), and Pneumococcal serine-rich repeat protein (PsrP). We conclude that biofilm pneumococci were avirulent due to reduced CPS and pneumolysin production along with increased ChoP, which is known to bind C-reactive protein and is opsonizing. Likewise, biofilm pneumococci were hyper-adhesive due to selection for the transparent phase variant, reduced CPS, and enhanced production of PsrP, CbpA, and ChoP. These studies suggest that biofilms do not directly contribute to development of IPD and may instead confer a quiescent mode of growth during colonization.

A Staphylococcus aureus pore-forming toxin subverts the activity of ADAM10 to cause lethal infection in mice

Staphylococcus aureus is a major cause of human disease, responsible for half a million infections and approximately 20,000 deaths per year in the United States alone ^1,2. This pathogen secretes α-hemolysin, a pore-forming cytotoxin that contributes to the pathogenesis of pneumonia ^3–5. α-hemolysin injures epithelial cells by interacting with its receptor, the zinc-dependent metalloprotease ADAM10 ⁶. We show that mice harboring a conditional disruption of the Adam10 gene in lung epithelium are resistant to lethal pneumonia. Investigation of the molecular mechanism of toxin-receptor function revealed that α-hemolysin upregulates ADAM10 metalloprotease activity in alveolar epithelial cells, resulting in cleavage of the adherens junction protein E-cadherin. Cleavage is associated with disruption of epithelial barrier function, contributing to the pathogenesis of lethal acute lung injury. A metalloprotease inhibitor of ADAM10 prevents E-cadherin cleavage; similarly, E-cadherin proteolysis and barrier disruption is attenuated in ADAM10 knockout mice. Together, these data attest to the function of ADAM10 as the cellular receptor for α-hemolysin. The observation that Hla can usurp the metalloprotease activity of its receptor reveals a novel mechanism of pore-forming cytotoxin action in which pathologic insults are not solely the result of irreversible membrane injury, and defines ADAM10 inhibition as a strategy for disease modification.

A comparison of pneumolysin activity and concentration in vitro and in vivo in a rabbit endophthalmitis model

The purpose of this study was to determine whether the in vitro activity and concentration of Streptococcus pneumoniae pneumolysin correlated to the pathogenesis of S. pneumoniae endophthalmitis. Five S. pneumoniae clinical endophthalmitis strains were grown in media to similar optical densities (OD), and extracellular milieu was tested for pneumolysin activity by hemolysis of rabbit red blood cells. Pneumolysin concentration was determined using a sandwich ELISA. Rabbit vitreous was injected with 10² colony-forming units (CFU) of 1 of 2 different strains with low hemolytic activity (n = 10 and 12 for strains 4 and 5, respectively) or 1 of 3 different strains with high hemolytic activity (n = 12 per strain). Pathogenesis of endophthalmitis infection was graded by slit lamp examination (SLE) at 24 hours post-infection. Bacteria were recovered from infected vitreous and quantitated. The SLE scores of eyes infected with strains having high hemolytic activity were significantly higher than the scores of those infected with strains having low hemolytic activity (P < 0.05). Pneumolysin concentration in vitro, however, did not correlate with hemolysis or severity of endophthalmitis. Bacterial concentrations from the vitreous infected with 4 of the strains were not significantly different (P > 0.05). These data suggest that pneumolysin hemolytic activity in vitro directly correlates to the pathogenesis of S. pneumoniae endophthalmitis. The protein concentration of pneumolysin, however, is not a reliable indicator of pneumolysin activity.

Detection of large numbers of pneumococcal virulence genes in streptococci of the mitis group

Seven streptococcal isolates from the mitis group were analyzed for the presence of pneumococcal gene homologues by comparative genomic hybridization studies with microarrays based on open reading frames from the genomes of Streptococcus pneumoniae TIGR4 and R6. The diversity of pneumolysin (ply) and neuraminidase A (nanA) gene sequences was explored in more detail in a collection of 14 S. pseudopneumoniae and 29 mitis group isolates, respectively. The mitis group isolates used in the microarray experiments included a type strain (NCTC 12261), two S. mitis isolates from the nasopharynxes of children, one S. mitis isolate from a case of infective endocarditis, one S. mitis isolate from a dental abscess, and one S. oralis isolate and one S. pseudopneumoniae isolate from the nasopharynxes of children. The results of the microarray study showed that the 5 S. mitis isolates had homologues to between 67 and 82% of pneumococcal virulence genes, S. oralis hybridized to 83% of pneumococcal virulence genes, and S. pseudopneumoniae hybridized to 92% of identified pneumococcal virulence genes. Comparison of the pneumolysin, mitilysin (mly), and newly identified pseudopneumolysin (pply) gene sequences revealed that mly and pply genes are more closely related to each other than either is to ply. In contrast, the nanA gene sequences in the pneumococcus and streptococci from the mitis group are closely clustered together, sharing 99.4 to 99.7% sequence identity with pneumococcal nanA alleles.

Phosphatase-dependent regulation of epithelial mitogen-activated protein kinase responses to toxin-induced membrane pores

Diverse bacterial species produce pore-forming toxins (PFT) that can puncture eukaryotic cell membranes. Host cells respond to sublytic concentrations of PFT through conserved intracellular signaling pathways, including activation of mitogen-activated protein kinases (MAPK), which are critical to cell survival. Here we demonstrate that in respiratory epithelial cells p38 and JNK MAPK were phosphorylated within 30 min of exposure to pneumolysin, the PFT from Streptococcus pneumoniae. This activation was tightly regulated, and dephosphorylation of both MAPK occurred within 60 min following exposure. Pretreatment of epithelial cells with inhibitors of cellular phosphatases, including sodium orthovanadate, calyculin A, and okadaic acid, prolonged and intensified MAPK activation. Specific inhibition of MAPK phosphatase-1 did not affect the kinetics of MAPK activation in PFT-exposed epithelial cells, but siRNA-mediated knockdown of serine/threonine phosphatases PP1 and PP2A were potent inhibitors of MAPK dephosphorylation. These results indicate an important role for PP1 and PP2A in termination of epithelial responses to PFT and only a minor contribution of dual-specificity phosphatases, such as MAPK phosphatase-1, which are the major regulators of MAPK signals in other cell types. Epithelial regulation of MAPK signaling in response to membrane disruption involves distinct pathways and may require different strategies for therapeutic interventions.

Human alpha-defensins inhibit hemolysis mediated by cholesterol-dependent cytolysins

Many pathogenic gram-positive bacteria release exotoxins that belong to the family of cholesterol-dependent cytolysins. Here, we report that human alpha-defensins HNP-1 to HNP-3 acted in a concentration-dependent manner to protect human red blood cells from the lytic effects of three of these exotoxins: anthrolysin O (ALO), listeriolysin O, and pneumolysin. HD-5 was very effective against listeriolysin O but less effective against the other toxins. Human alpha-defensins HNP-4 and HD-6 and human beta-defensin-1, -2, and -3 lacked protective ability. HNP-1 required intact disulfide bonds to prevent toxin-mediated hemolysis. A fully linearized analog, in which all six cysteines were replaced by aminobutyric acid (Abu) residues, showed greatly reduced binding and protection. A partially unfolded HNP-1 analog, in which only cysteines 9 and 29 were replaced by Abu residues, showed intact ALO binding but was 10-fold less potent in preventing hemolysis. Surface plasmon resonance assays revealed that HNP-1 to HNP-3 bound all three toxins at multiple sites and also that solution-phase HNP molecules could bind immobilized HNP molecules. Defensin concentrations that inhibited hemolysis by ALO and listeriolysin did not prevent these toxins from binding either to red blood cells or to cholesterol. Others have shown that HNP-1 to HNP-3 inhibit lethal toxin of Bacillus anthracis, toxin B of Clostridium difficile, diphtheria toxin, and exotoxin A of Pseudomonas aeruginosa; however, this is the first time these defensins have been shown to inhibit pore-forming toxins. An "ABCDE mechanism" that can account for the ability of HNP-1 to HNP-3 to inhibit so many different exotoxins is proposed.