Pneumococci: immunology of the innate host response

Global transcriptional responses of pneumococcus to human blood components and cerebrospinal fluid

Streptococcus pneumoniae (pneumococcus) is a leading cause of severe invasive infectious diseases such as sepsis and meningitis. Understanding how pneumococcus adapts and survive in the human bloodstream environment and cerebrospinal fluid (CSF) is important for development of future treatment strategies. This study investigates the global transcriptional response of pneumococcus to human blood components and CSF acquired from discarded and anonymized patient samples. Extensive transcriptional changes to human blood components were observed during early stages of interaction. Plasma-specific responses were primarily related to metabolic components and include strong downregulation of fatty acid biosynthesis genes, and upregulation of nucleotide biosynthesis genes. No transcriptional responses specific to the active plasma proteins (e.g., complement proteins) were observed during early stages of interaction as demonstrated by a differential expression analysis between plasma and heat-inactivated plasma. The red blood cell (RBC)-specific response was far more complex, and included activation of the competence system, differential expression of several two-component systems, phosphotransferase systems and transition metal transporter genes. Interestingly, most of the changes observed for CSF were also observed for plasma. One of the few CSF-specific responses, not observed for plasma, was a strong downregulation of the iron acquisition system piuBCDA. Intriguingly, this transcriptomic analysis also uncovers significant differential expression of more than 20 small non-coding RNAs, most of them in response to RBCs, including small RNAs from uncharacterized type I toxin-antitoxin systems. In summary, this transcriptomic study identifies key pneumococcal metabolic pathways and regulatory genes involved with adaptation to human blood and CSF. Future studies should uncover the potential involvement of these factors with virulence in-vivo.

Alternative Complement Pathway Inhibition Abrogates Pneumococcal Opsonophagocytosis in Vaccine-Naïve, but Not in Vaccinated Individuals

To assess the relative contribution of opsonisation by antibodies, classical and alternative complement pathways to pneumococcal phagocytosis, we analyzed killing of pneumococci by human blood leukocytes collected from vaccine-naïve and PCV13-vaccinated subjects. With serotype 4 pneumococci as model, two different physiologic opsonophagocytosis assays based on either hirudin-anticoagulated whole blood or on washed cells from EDTA-anticoagulated blood reconstituted with active serum, were compared. Pneumococcal killing was measured in the presence of inhibitors targeting the complement components C3, C5, MASP-2, factor B or factor D. The two assay formats yielded highly consistent and comparable results. They highlighted the importance of alternative complement pathway activation for efficient opsonophagocytic killing in blood of vaccine-naïve subjects. In contrast, alternative complement pathway inhibition did not affect pneumococcal killing in PCV13-vaccinated individuals. Independent of amplification by the alternative pathway, even low capsule-specific antibody concentrations were sufficient to efficiently trigger classical pathway mediated opsonophagocytosis. In heat-inactivated or C3-inhibited serum, high concentrations of capsule-specific antibodies were required to trigger complement-independent opsonophagocytosis. Our findings suggest that treatment with alternative complement pathway inhibitors will increase susceptibility for invasive pneumococcal infection in non-immune subjects, but it will not impede pneumococcal clearance in vaccinated individuals.

Celiac disease and complement activation in response to Streptococcus pneumoniae

Individuals with celiac disease (CD) are at increased risk of invasive pneumococcal disease (IPD). The aim of this study was to explore whether the complement response to Streptococcus pneumoniae differed according to CD status, and could serve as an explanation for the excess risk of IPD in CD. Twenty-two children with CD and 18 controls, born 1999-2008, were included at Kalmar County Hospital, Sweden. The degree of complement activation was evaluated by comparing levels of activation products C3a and sC5b-9 in plasma incubated for 30 min with Streptococcus pneumoniae and in non-incubated plasma. Complement analyses were performed with enzyme-linked immunosorbent assay (ELISA). Pneumococcal stimulation caused a statistically significant increase in C3a as well as sC5b-9 in both children with CD and controls but there was no difference in response between the groups. After incubation, C3a increased on average 4.6 times and sC5b-9 22 times in both the CD and the control group (p = 0.497 and p = 0.724 respectively).Conclusion: Complement response to Streptococcus pneumoniae seems to be similar in children with and without CD and is thus unlikely to contribute to the increased susceptibility to invasive pneumococcal disease in CD.What is Known:• An excess risk of pneumococcal infections has been demonstrated in individuals with celiac disease.• Infectious complications can depend on hyposplenism but alternative mechanisms are sparsely examined.What is New:• Complement activation in response to Streptococcus pneumoniae was examined in children with and without celiac disease but no differences could be demonstrated.

Examining the Complex Relationship Between Tuberculosis and Other Infectious Diseases in Children

Millions of children are exposed to tuberculosis (TB) each year, many of which become infected with Mycobacterium tuberculosis. Most children can immunologically contain or eradicate the organism without pathology developing. However, in a minority, the organism overcomes the immunological constraints, proliferates and causes TB disease. Each year a million children develop TB disease, with a quarter dying. While it is known that young children and those with immunodeficiencies are at increased risk of progression from TB infection to TB disease, our understanding of risk factors for this transition is limited. The most immunologically disruptive process that can happen during childhood is infection with another pathogen and yet the impact of co-infections on TB risk is poorly investigated. Many diseases have overlapping geographical distributions to TB and affect similar patient populations. It is therefore likely that infection with viruses, bacteria, fungi and protozoa may impact on the risk of developing TB disease following exposure and infection, although disentangling correlation and causation is challenging. As vaccinations also disrupt immunological pathways, these may also impact on TB risk. In this article we describe the pediatric immune response to M. tuberculosis and then review the existing evidence of the impact of co-infection with other pathogens, as well as vaccination, on the host response to M. tuberculosis. We focus on the impact of other organisms on the risk of TB disease in children, in particularly evaluating if co-infections drive host immune responses in an age-dependent way. We finally propose priorities for future research in this field. An improved understanding of the impact of co-infections on TB could assist in TB control strategies, vaccine development (for TB vaccines or vaccines for other organisms), TB treatment approaches and TB diagnostics.

Immunomodulatory Effects of Pneumococcal Extracellular Vesicles on Cellular and Humoral Host Defenses

Gram-positive bacteria, including the major respiratory pathogen Streptococcus pneumoniae, were recently shown to produce extracellular vesicles (EVs) that likely originate from the plasma membrane and are released into the extracellular environment. EVs may function as cargo for many bacterial proteins, however, their involvement in cellular processes and their interactions with the innate immune system are poorly understood. Here, EVs from pneumococci were characterized and their immunomodulatory effects investigated. Pneumococcal EVs were protruding from the bacterial surface and released into the medium as 25 to 250 nm lipid stained vesicles containing a large number of cytosolic, membrane, and surface-associated proteins. The cytosolic pore-forming toxin pneumolysin was significantly enriched in EVs compared to a total bacterial lysate but was not required for EV formation. Pneumococcal EVs were internalized into A549 lung epithelial cells and human monocyte-derived dendritic cells and induced proinflammatory cytokine responses irrespective of pneumolysin content. EVs from encapsulated pneumococci were recognized by serum proteins, resulting in C3b deposition and formation of C5b-9 membrane attack complexes as well as factor H recruitment, depending on the presence of the choline binding protein PspC. Addition of EVs to human serum decreased opsonophagocytic killing of encapsulated pneumococci. Our data suggest that EVs may act in an immunomodulatory manner by allowing delivery of vesicle-associated proteins and other macromolecules into host cells. In addition, EVs expose targets for complement factors in serum, promoting pneumococcal evasion of humoral host defense.

Streptococcus pneumoniae is a major contributor to morbidity and mortality worldwide, being the major cause of milder respiratory tract infections such as otitis and sinusitis and of severe infections such as community-acquired pneumonia, with or without septicemia, and meningitis. More knowledge is needed on how pneumococci interact with the host, deliver virulence factors, and activate immune defenses. Here we show that pneumococci form extracellular vesicles that emanate from the plasma membrane and contain virulence properties, including enrichment of pneumolysin. We found that pneumococcal vesicles can be internalized into epithelial and dendritic cells and bind complement proteins, thereby promoting pneumococcal evasion of complement-mediated opsonophagocytosis. They also induce pneumolysin-independent proinflammatory responses. We suggest that these vesicles can function as a mechanism for delivery of pneumococcal proteins and other immunomodulatory components into host cells and help pneumococci to avoid complement deposition and phagocytosis-mediated killing, thereby possibly contributing to the symptoms found in pneumococcal infections.

The Role of Cadaverine Synthesis on Pneumococcal Capsule and Protein Expression

Invasive infections caused by Streptococcus pneumoniae, a commensal in the nasopharynx, pose significant risk to human health. Limited serotype coverage by the available polysaccharide-based conjugate vaccines coupled with increasing incidence of antibiotic resistance complicates therapeutic strategies. Bacterial physiology and metabolism that allows pathogens to adapt to the host are a promising avenue for the discovery of novel therapeutics. Intracellular polyamine concentrations are tightly regulated by biosynthesis, transport and degradation. We previously reported that deletion of cadA, a gene that encodes for lysine decarboxylase, an enzyme that catalyzes cadaverine synthesis results in an attenuated phenotype. Here, we report the impact of cadA deletion on pneumococcal capsule and protein expression. Our data show that genes for polyamine biosynthesis and transport are downregulated in ∆cadA. Immunoblot assays show reduced capsule in ∆cadA. Reduced capsule synthesis could be due to reduced transcription and availability of precursors for synthesis. The capsule is the predominant virulence factor in pneumococci and is critical for evading opsonophagocytosis and its loss in ∆cadA could explain the reported attenuation in vivo. Results from this study show that capsule synthesis in pneumococci is regulated by polyamine metabolism, which can be targeted for developing novel therapies.

Protease activated receptor 4 limits bacterial growth and lung pathology during late stage Streptococcus pneumoniae induced pneumonia in mice

Streptococcus pneumoniae is a common causative pathogen of pneumonia and sepsis. Pneumonia and sepsis are associated with enhanced activation of coagulation, resulting in the production of several host-derived proteases at the primary site of infection and in the circulation. Serine proteases cleave protease activated receptors (PARs), which form a molecular link between coagulation and inflammation. PAR4 is one of four subtypes of PARs and is widely expressed by multiple cell types in the respiratory tract implicated in pulmonary inflammation, by immune cells and by platelets. In mice, mouse (m)PAR4 is the only thrombin receptor expressed by platelets. We here sought to determine the contribution of mPAR4 to the host response during pneumococcal pneumonia. Pneumonia was induced by intranasal inoculation with S. pneumoniae in mPAR4-deficient (par4-/- ) and wild-type mice. Mice were sacrificed after 6, 24 or 48 hours (h). Blood, lungs, liver and spleen were collected for analyses. Ex vivo stimulation assays were performed with S. pneumoniae and mPAR4 activating peptides. At 48 h after infection, higher bacterial loads were found in the lungs and blood of par4-/- mice (p < 0.05), accompanied by higher histopathology scores and increased cytokine levels (p < 0.05) in the lungs. Ex vivo, co-stimulation with mPAR4 activating peptide enhanced the whole blood cytokine response to S. pneumoniae. Thrombin inhibition resulted in decreased cytokine release after S. pneumoniae stimulation in human whole blood. Our findings suggest that mPAR4 contributes to antibacterial defence during murine pneumococcal pneumonia.

Inhibition of Phosphodiesterase-4 during Pneumococcal Pneumonia Reduces Inflammation and Lung Injury in Mice

Pneumococcal pneumonia is a leading cause of mortality worldwide. The inflammatory response to bacteria is necessary to control infection, but it may also contribute to tissue damage. Phosphodiesterase-4 inhibitors, such as rolipram (ROL), effectively reduce inflammation. Here, we examined the impact of ROL in a pneumococcal pneumonia murine model. Mice were infected intranasally with 10(5)-10(6) CFU of Streptococcus pneumoniae, treated with ROL in a prophylactic or therapeutic schedule in combination, or not, with the antibiotic ceftriaxone. Inflammation and bacteria counts were assessed, and ex vivo phagocytosis assays were performed. ROL treatment during S. pneumoniae infection decreased neutrophil recruitment into lungs and airways and reduced lung injury. Prophylactic ROL treatment also decreased cytokine levels in the airways. Although modulation of inflammation by ROL ameliorated pneumonia, bacteria burden was not reduced. On the other hand, antibiotic therapy reduced bacteria without reducing neutrophil infiltration, cytokine level, or lung injury. Combined ROL and ceftriaxone treatment decreased lethality rates and was more efficient in reducing inflammation, by increasing proresolving protein annexin A1 (AnxA1) expression, and bacterial burden by enhancing phagocytosis. Lack of AnxA1 increased inflammation and lethality induced by pneumococcal infection. These data show that immunomodulatory effects of phosphodiesterase-4 inhibitors are useful during severe pneumococcal pneumonia and suggest their potential benefit as adjunctive therapy during infectious diseases.

Bacterial Pneumonia in Patients with Cancer: Novel Risk Factors and Management

Bacterial pneumonias exact unacceptable morbidity on patients with cancer. Although the risk is often most pronounced among patients with treatment-induced cytopenias, the numerous contributors to life-threatening pneumonias in cancer populations range from derangements of lung architecture and swallow function to complex immune defects associated with cytotoxic therapies and graft-versus-host disease. These structural and immunologic abnormalities often make the diagnosis of pneumonia challenging in patients with cancer and impact the composition and duration of therapy. This article addresses host factors that contribute to pneumonia susceptibility, summarizes diagnostic recommendations, and reviews current guidelines for management of bacterial pneumonia in patients with cancer.

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.

Efferocytosis-induced prostaglandin E2 production impairs alveolar macrophage effector functions during Streptococcus pneumoniae infection

Alveolar macrophages (AMs) are multitasking cells that maintain lung homeostasis by clearing apoptotic cells (efferocytosis) and performing antimicrobial effector functions. Different PRRs have been described to be involved in the binding and capture of non-opsonized Streptococcus pneumoniae, such as TLR-2, mannose receptor (MR) and scavenger receptors (SRs). However, the mechanism by which the ingestion of apoptotic cells negatively influences the clearance of non-opsonized S. pneumoniae remains to be determined. In this study, we evaluated whether the prostaglandin E2 (PGE₂) produced during efferocytosis by AMs inhibits the ingestion and killing of non-opsonized S. pneumoniae. Resident AMs were pre-treated with an E prostanoid (EP) receptor antagonist, inhibitors of cyclooxygenase and protein kinase A (PKA), incubated with apoptotic Jurkat T cells, and then challenged with S. pneumoniae. Efferocytosis slightly decreased the phagocytosis of S. pneumoniae but greatly inhibited bacterial killing by AMs in a manner dependent on PGE₂ production, activation of the EP2-EP4/cAMP/PKA pathway and inhibition of H₂O₂ production. Our data suggest that the PGE₂ produced by AMs during efferocytosis inhibits H₂O₂ production and impairs the efficient clearance non-opsonized S. pneumoniae by EP2-EP4/cAMP/PKA pathway.

C-type Lectin Mincle Recognizes Glucosyl-diacylglycerol of Streptococcus pneumoniae and Plays a Protective Role in Pneumococcal Pneumonia

Among various innate immune receptor families, the role of C-type lectin receptors (CLRs) in lung protective immunity against Streptococcus pneumoniae (S. pneumoniae) is not fully defined. We here show that Mincle gene expression was induced in alveolar macrophages and neutrophils in bronchoalveolar lavage fluids of mice and patients with pneumococcal pneumonia. Moreover, S. pneumoniae directly triggered Mincle reporter cell activation in vitro via its glycolipid glucosyl-diacylglycerol (Glc-DAG), which was identified as the ligand recognized by Mincle. Purified Glc-DAG triggered Mincle reporter cell activation and stimulated inflammatory cytokine release by human alveolar macrophages and alveolar macrophages from WT but not Mincle KO mice. Mincle deficiency led to increased bacterial loads and decreased survival together with strongly dysregulated cytokine responses in mice challenged with focal pneumonia inducing S. pneumoniae, all of which was normalized in Mincle KO mice reconstituted with a WT hematopoietic system. In conclusion, the Mincle-Glc-DAG axis is a hitherto unrecognized element of lung protective immunity against focal pneumonia induced by S. pneumoniae.

Pneumococcal Vaccination Strategies. An Update and Perspective

Streptococcus pneumoniae is an important global pathogen that causes a wide range of clinical disease in children and adults. Pneumococcal pneumonia is by far the common presentation of noninvasive and invasive pneumococcal disease and affects the young, the elderly, and the immunocompromised disproportionately. Patients with chronic pulmonary diseases are also at higher risk for pneumococcal infections. Substantial progress over the century has been made in the understanding of pneumococcal immunobiology and the prevention of invasive pneumococcal disease through vaccination. Currently, two pneumococcal vaccines are available for individuals at risk of pneumococcal disease: the 23-valent pneumococcal polysaccharide vaccine (PPV23) and the 13-valent pneumococcal protein-conjugate vaccine (PCV13). The goal of pneumococcal vaccination is to stimulate effective antipneumococcal antibody and mucosal immunity response and immunological memory. Vaccination of infants and young children with pneumococcal conjugate vaccine has led to significant decrease in nasal carriage rates and pneumococcal disease in all age groups. Recent pneumococcal vaccine indication and schedule recommendations on the basis of age and risk factors are outlined in this Focused Review. As new pneumococcal vaccine recommendations are being followed, continued efforts are needed to address the vaccine efficacy in the waning immunity of the ever-aging population, the implementation of vaccines using two different vaccines under very specific schedules and their real world clinical and cost effectiveness, and the development of next generation pneumococcal vaccines.

Glucocorticoid-Augmented Efferocytosis Inhibits Pulmonary Pneumococcal Clearance in Mice by Reducing Alveolar Macrophage Bactericidal Function

Inhaled corticosteroids (ICS) increase community-acquired pneumonia (CAP) incidence in patients with chronic obstructive pulmonary disease (COPD) by unknown mechanisms. Apoptosis is increased in the lungs of COPD patients. Uptake of apoptotic cells (ACs) ("efferocytosis") by alveolar macrophages (AMøs) reduces their ability to combat microbes, including Streptococcus pneumoniae, the most common cause of CAP in COPD patients. Having shown that ICS significantly increase AMø efferocytosis, we hypothesized that this process, termed glucocorticoid-augmented efferocytosis, might explain the association of CAP with ICS therapy in COPD. To test this hypothesis, we studied the effects of fluticasone, AC, or both on AMøs of C57BL/6 mice in vitro and in an established model of pneumococcal pneumonia. Fluticasone plus AC significantly reduced TLR4-stimulated AMø IL-12 production, relative to either treatment alone, and decreased TNF-α, CCL3, CCL5, and keratinocyte-derived chemoattractant/CXCL1, relative to AC. Mice treated with fluticasone plus AC before infection with viable pneumococci developed significantly more lung CFUs at 48 h. However, none of the pretreatments altered inflammatory cell recruitment to the lungs at 48 h postinfection, and fluticasone plus AC less markedly reduced in vitro mediator production to heat-killed pneumococci. Fluticasone plus AC significantly reduced in vitro AMø killing of pneumococci, relative to other conditions, in part by delaying phagolysosome acidification without affecting production of reactive oxygen or nitrogen species. These results support glucocorticoid-augmented efferocytosis as a potential explanation for the epidemiological association of ICS therapy of COPD patients with increased risk for CAP, and establish murine experimental models to dissect underlying molecular mechanisms.

Cardiotoxicity during invasive pneumococcal disease

Streptococcus pneumoniae is the leading cause of community-acquired pneumonia and sepsis, with adult hospitalization linked to approximately 19% incidence of an adverse cardiac event (e.g., heart failure, arrhythmia, infarction). Herein, we review the specific host-pathogen interactions that contribute to cardiac dysfunction during invasive pneumococcal disease: (1) cell wall-mediated inhibition of cardiomyocyte contractility; (2) the new observation that S. pneumoniae is capable of translocation into the myocardium and within the heart, forming discrete, nonpurulent, microscopic lesions that are filled with pneumococci; and (3) the bacterial virulence determinants, pneumolysin and hydrogen peroxide, that are most likely responsible for cardiomyocyte cell death. Pneumococcal invasion of heart tissue is dependent on the bacterial adhesin choline-binding protein A that binds to laminin receptor on vascular endothelial cells and binding of phosphorylcholine residues on pneumococcal cell wall to platelet-activating factor receptor. These are the same interactions responsible for pneumococcal translocation across the blood-brain barrier during the development of meningitis. We discuss these interactions and how their neutralization, either with antibody or therapeutic agents that modulate platelet-activating factor receptor expression, may confer protection against cardiac damage and meningitis. Considerable collagen deposition was observed in hearts of mice that had recovered from invasive pneumococcal disease. We discuss the possibility that cardiac scar formation after severe pneumococcal infection may explain why individuals who are hospitalized for pneumonia are at greater risk for sudden death up to 1 year after infection.

The α-tocopherol form of vitamin E reverses age-associated susceptibility to streptococcus pneumoniae lung infection by modulating pulmonary neutrophil recruitment

Streptococcus pneumoniae infections are an important cause of morbidity and mortality in older patients. Uncontrolled neutrophil-driven pulmonary inflammation exacerbates this disease. To test whether the α-tocopherol (α-Toc) form of vitamin E, a regulator of immunity, can modulate neutrophil responses as a preventive strategy to mitigate the age-associated decline in resistance to S. pneumoniae, young (4 mo) and old (22-24 mo) C57BL/6 mice were fed a diet containing 30-PPM (control) or 500-PPM (supplemented) α-Toc for 4 wk and intratracheally infected with S. pneumoniae. Aged mice fed a control diet were exquisitely more susceptible to S. pneumoniae than young mice. At 2 d postinfection, aged mice suffered 1000-fold higher pulmonary bacterial burden, 2.2-fold higher levels of neutrophil recruitment to the lung, and a 2.25-fold higher rate of lethal septicemia. Strikingly, α-Toc supplementation of aged mice resulted in a 1000-fold lower bacterial lung burden and full control of infection. This α-Toc-induced resistance to pneumococcal challenge was associated with a 2-fold fewer pulmonary neutrophils, a level comparable to S. pneumoniae-challenged, conventionally fed young mice. α-Toc directly inhibited neutrophil egress across epithelial cell monolayers in vitro in response to pneumococci or hepoxilin-A3, an eicosanoid required for pneumococcus-elicited neutrophil trans-epithelial migration. α-Toc altered expression of multiple epithelial and neutrophil adhesion molecules involved in migration, including CD55, CD47, CD18/CD11b, and ICAM-1. These findings suggest that α-Toc enhances resistance of aged mice to bacterial pneumonia by modulating the innate immune response, a finding that has potential clinical significance in combating infection in aged individuals through nutritional intervention.

Epicutaneous immunization with phosphorylcholine conjugated to bovine serum albumin (PC-BSA) and TLR9 ligand CpG alleviates pneumococcal pneumonia in mice

BACKGROUND

Epicutaneous (EC) immunization is a potential new strategy of a needle-free and self-administrable immunization by using a topically applied patch to deliver vaccine. We have previously shown that EC immunization with various protein antigens inhibits both Th1- and Tc1-mediated contact hypersensitivity (CHS) in mice. Our further work showed that maneuver of EC immunization with an antigen and Toll-like receptor (TLR) ligands prior to hapten sensitization reverses skin-induced suppression.

METHODS

Animal model of pneumococcal pneumonia was used to study efficacy of EC induced immunopotentiation.

RESULTS

Current work showed that EC immunization with phosphorylcholine conjugated to bovine serum albumin (PC-BSA) and CpG prior to Streptococcus pneumoniae infection results in smaller decrease of body weight when compared to PBS treated mice. Consistent with the behavioral observations and body weight, smaller numbers of bacteria were quantitated in lung homogenates of mice patched with PC-BSA and CpG prior inoculation with S. pneumonia when compared to mice patched with PBS alone. In vitro experiments showed that lymph node cells and spleen cells from mice EC immunized with PC-BSA plus CpG produced high levels of IFN-γ and IL-17A when compared to PBS or PC-BSA or CpG treated mice.

CONCLUSION

This work shows that EC immunization with PC-BSA plus TLR9 ligand CpG may be a potential tool to boost immunity to S. pneumoniae.

The pneumococcal polysaccharide capsule and pneumolysin differentially affect CXCL8 and IL-6 release from cells of the upper and lower respiratory tract

The polysaccharide capsule and pneumolysin toxin are major virulence factors of the human bacterial pathogen Streptococcus pneumoniae. Colonization of the nasopharynx is asymptomatic but invasion of the lungs can result in invasive pneumonia. Here we show that the capsule suppresses the release of the pro-inflammatory cytokines CXCL8 (IL-8) and IL-6 from the human pharyngeal epithelial cell line Detroit 562. Release of both cytokines was much less from human bronchial epithelial cells (iHBEC) but levels were also affected by capsule. Pneumolysin stimulates CXCL8 release from both cell lines. Suppression of CXCL8 homologue (CXCL2/MIP-2) release by the capsule was also observed in vivo during intranasal colonization of mice but was only discernable in the absence of pneumolysin. When pneumococci were administered intranasally to mice in a model of long term, stable nasopharyngeal carriage, encapsulated S. pneumoniae remained in the nasopharynx whereas the nonencapsulated pneumococci disseminated into the lungs. Pneumococcal capsule plays a role not only in protection from phagocytosis but also in modulation of the pro-inflammatory immune response in the respiratory tract.

Streptococcus pneumoniae serine protease HtrA, but not SFP or PrtA, is a major virulence factor in pneumonia

Streptococcus (S.) pneumoniae is a common causative pathogen in pneumonia. Serine protease orthologs expressed by a variety of bacteria have been found of importance for virulence. Previous studies have identified two serine proteases in S. pneumoniae, HtrA (high-temperature requirement A) and PrtA (cell wall-associated serine protease A), that contributed to virulence in models of pneumonia and intraperitoneal infection respectively. We here sought to identify additional S. pneumoniae serine proteases and determine their role in virulence. The S. pneumoniae D39 genome contains five putative serine proteases, of which HtrA, Subtilase Family Protein (SFP) and PrtA were selected for insertional mutagenesis because they are predicted to be secreted and surface exposed. Mutant D39 strains lacking serine proteases were constructed by in-frame insertion deletion mutagenesis. Pneumonia was induced by intranasal infection of mice with wild-type or mutant D39. After high dose infection, only D39ΔhtrA showed reduced virulence, as reflected by strongly reduced bacterial loads, diminished dissemination and decreased lung inflammation. D39ΔprtA induced significantly less lung inflammation together with smaller infiltrated lung surface, but without influencing bacterial loads. After low dose infection, D39ΔhtrA again showed strongly reduced bacterial loads; notably, pneumococcal burdens were also modestly lower in lungs after infection with D39Δsfp. These data confirm the important role for HtrA in S. pneumoniae virulence. PrtA contributes to lung damage in high dose pneumonia; it does not however contribute to bacterial outgrowth in pneumococcal pneumonia. SFP may facilitate S. pneumoniae growth after low dose infection.

Childhood epilepsy and maternal antibodies to microbial and tissue antigens during pregnancy

INTRODUCTION

Several epidemiologic studies show associations between mother's infections during pregnancy and an increased risk of mental and neurological disorders in the offspring. Such associations could be due to the direct or indirect effects of infectious agents, including immune responses to infectious agents that display molecular mimicry with host antigens. We measured a range of antigen-specific maternal IgG antibodies to examine if any were associated with risk for childhood epilepsy in offspring.

METHODS

We used a case-cohort design within the Danish National Birth Cohort (DNBC) to examine maternal IgG antibodies to 25 microbial and tissue antigens during pregnancy and their association with the risk of epilepsy in offspring. The source population of this study was 68,250 live born singletons with up to 10 years of follow up. We randomly identified a sample of 282 children as a subcohort and included 275 children with a verified diagnosis of epilepsy as cases. Maternal antibodies were categorized into 6 groups (<50, 50-59, 60-69, 70-79, 80-89, ≥90 percentile) according to the level in the subcohort. We used a Prentice-weighted Cox regression model to estimate the hazard ratio (HR) and 95% confidence interval (CI) for epilepsy according to measured antibodies.

RESULTS

Higher levels of maternal antibodies against herpes simples virus type 1 (anti-HSV1) were associated with a slightly higher risk of childhood epilepsy (HR for trend=1.09, 95% CI: 0.99-1.21), while higher levels of maternal antibodies against pneumococcal polysaccharide 18 (anti-PnPS18) were associated with a lower risk of childhood epilepsy (HR for trend=0.90, 95% CI: 0.81-1.01). Among the subtypes, a significantly higher risk associated with anti-HSV1 antibodies was seen for childhood absence epilepsy (HR for trend=2.08, 95% CI: 1.12-3.85) and for epileptic encephalopathies (HR for trend=1.49, 95% CI: 1.01-2.22). The significantly lower risk associated with anti-PnPS18 antibodies was observed for infantile spasms (HR for trend=0.47, 95% CI: 0.27-0.83).

CONCLUSIONS

Maternal anti-HSV1and anti-PnPS18 antibodies during pregnancy may be associated with the risk of epilepsy in offspring, but any potential etiologic and preventative implications of these associations warrant further exploration.

Immunosuppressive property within the Streptococcus pneumoniae cell wall that inhibits generation of T follicular helper, germinal center, and plasma cell response to a coimmunized heterologous protein

We previously demonstrated that intact, inactivated Streptococcus pneumoniae (unencapsulated strain R36A) inhibits IgG responses to a number of coimmunized soluble antigens (Ags). In this study, we investigated the mechanism of this inhibition and whether other extracellular bacteria exhibited similar effects. No inhibition was observed if R36A was given 24 h before or after immunization with soluble chicken ovalbumin (cOVA), indicating that R36A acts transiently during the initiation of the immune response. Using transgenic cOVA-specific CD4(+) T cells, we observed that R36A had no significant effect on T-cell activation (24 h) or generation of regulatory T cells (day 7) and only a modest effect on T-cell proliferation (48 to 96 h) in response to cOVA. However, R36A mediated a significant reduction in the formation of Ag-specific splenic germinal center T follicular helper (GC Tfh) and GC B cells and antibody-secreting cells in the spleen and bone marrow in response to cOVA or cOVA conjugated to 4-hydroxy-3-nitrophenylacetyl hapten (NP-cOVA). Of note, the inhibitory effect of intact R36A on the IgG anti-cOVA response could be reproduced using R36A-derived cell walls. In contrast to R36A, neither inactivated, unencapsulated, intact Neisseria meningitidis nor Streptococcus agalactiae inhibited the OVA-specific IgG response. These results suggest a novel immunosuppressive property within the cell wall of Streptococcus pneumoniae.

Limited anti-inflammatory role for interleukin-1 receptor like 1 (ST2) in the host response to murine postinfluenza pneumococcal pneumonia

Interleukin-1 receptor like 1 (ST2) is a negative regulator of Toll-like receptor (TLR) signaling. TLRs are important for host defense during respiratory tract infections by both influenza and Streptococcus (S.) pneumoniae. Enhanced susceptibility to pneumococcal pneumonia is an important complication following influenza virus infection. We here sought to determine the role of ST2 in primary influenza A infection and secondary pneumococcal pneumonia. ST2 knockout (st2^−/−) and wild-type (WT) mice were intranasally infected with influenza A virus; in some experiments mice were infected 2 weeks later with S. pneumoniae. Both mouse strains cleared the virus similarly during the first 14 days of influenza infection and had recovered their weights equally at day 14. Overall st2^−/− mice tended to have a stronger pulmonary inflammatory response upon infection with influenza; especially 14 days after infection modest but statistically significant elevations were seen in lung IL-6, IL-1β, KC, IL-10, and IL-33 concentrations and myeloperoxidase levels, indicative of enhanced neutrophil activity. Interestingly, bacterial lung loads were higher in st2^−/− mice during the later stages of secondary pneumococcal pneumonia, which was associated with relatively increased lung IFN-γ levels. ST2 deficiency did not impact on gross lung pathology in either influenza or secondary S. pneumoniae pneumonia. These data show that ST2 plays a limited anti-inflammatory role during both primary influenza and postinfluenza pneumococcal pneumonia.

Chitinase 3-like-1 promotes Streptococcus pneumoniae killing and augments host tolerance to lung antibacterial responses

Host antibacterial responses include mechanisms that kill bacteria, but also those that protect or tolerize the host to potentially damaging antibacterial effects. We determined that Chitinase 3-like-1 (Chi3l1), a conserved prototypic chitinase-like protein, is induced by Streptococcus pneumoniae and plays central roles in promoting bacterial clearance and mediating host tolerance. S. pneumoniae-infected Chi3l1 null mice exhibit exaggerated lung injury, inflammation and hemorrhage, more frequent bacterial dissemination, decreased bacterial clearance, and enhanced mortality compared to controls. Chi3l1 augments macrophage bacterial killing by inhibiting caspase-1-dependent macrophage pyroptosis and augments host tolerance by controlling inflammasome activation, ATP accumulation, expression of ATP receptor P2X7R, and production of thymic stromal lymphopoietin and type 1, type 2, and type 17 cytokines. These data demonstrate that Chi3l1 is induced during infection, where it promotes bacterial clearance while simultaneously augmenting host tolerance, and that these roles likely contributed to the retention of Chi3l1 over species and evolutionary time.

Inhibitors of Streptococcus pneumoniae surface endonuclease EndA discovered by high-throughput screening using a PicoGreen fluorescence assay

The human commensal pathogen Streptococcus pneumoniae expresses a number of virulence factors that promote serious pneumococcal diseases, resulting in significant morbidity and mortality worldwide. These virulence factors may give S. pneumoniae the capacity to escape immune defenses, resist antimicrobial agents, or a combination of both. Virulence factors also present possible points of therapeutic intervention. The activities of the surface endonuclease, EndA, allow S. pneumoniae to establish invasive pneumococcal infection. EndA's role in DNA uptake during transformation contributes to gene transfer and genetic diversification. Moreover, EndA's nuclease activity degrades the DNA backbone of neutrophil extracellular traps (NETs), allowing pneumococcus to escape host immune responses. Given its potential impact on pneumococcal pathogenicity, EndA is an attractive target for novel antimicrobial therapy. Herein, we describe the development of a high-throughput screening assay for the discovery of nuclease inhibitors. Nuclease-mediated digestion of double-stranded DNA was assessed using fluorescence changes of the DNA dye ligand, PicoGreen. Under optimized conditions, the assay provided robust and reproducible activity data (Z'= 0.87) and was used to screen 4727 small molecules against an imidazole-rescued variant of EndA. In total, six small molecules were confirmed as novel EndA inhibitors, some of which may have utility as research tools for understanding pneumococcal pathogenesis and for drug discovery.

Altered regulation of Toll-like receptor responses impairs antibacterial immunity in the allergic lung

The lung is colonized by commensal bacteria, some of which are associated with asthma exacerbations. Using the intranasal house-dust mite-sensitized mouse model of allergic airway disease, we show an imbalance in novel antibacterial pathways that culminates in a reduction in neutrophil recruitment to the airspaces and leads to bacterial invasion and dissemination. The expression of TREM (Triggering Receptor Expressed on Myeloid cells)-1 that amplifies Toll-like receptor (TLR) signaling and TREM-2 that inhibits this process is reversed. Furthermore, endogenous TLR inhibitors (A20, Tollip, SOCS1, and IRAK-M) and proteins involved in receptor recycling (TRIAD3) are raised. Consequently, the production of neutrophil chemoattractants is reduced. Intranasal administration of either chemokine restores the ability to recruit neutrophils, which prevents bacterial invasion. A background of allergic airway disease therefore exacerbates bacterial infection by altering key antibacterial innate immune pathways that are amenable to therapeutic intervention.

T regulatory cells control susceptibility to invasive pneumococcal pneumonia in mice

Streptococcus pneumoniae is an important human pathogen responsible for a spectrum of diseases including pneumonia. Immunological and pro-inflammatory processes induced in the lung during pneumococcal infection are well documented, but little is known about the role played by immunoregulatory cells and cytokines in the control of such responses. We demonstrate considerable differences in the immunomodulatory cytokine transforming growth factor (TGF)-β between the pneumococcal pneumonia resistant BALB/c and susceptible CBA/Ca mouse strains. Immunohistochemistry and flow cytometry reveal higher levels of TGF-β protein in BALB/c lungs during pneumococcal pneumonia that correlates with a rapid rise in lung Foxp3⁺Helios⁺ T regulatory cells. These cells have protective functions during pneumococcal pneumonia, because blocking their induction with an inhibitor of TGF-β impairs BALB/c resistance to infection and aids bacterial dissemination from lungs. Conversely, adoptive transfer of T regulatory cells to CBA/Ca mice, prior to infection, prolongs survival and decreases bacterial dissemination from lungs to blood. Importantly, strong T regulatory cell responses also correlate with disease-resistance in outbred MF1 mice, confirming the importance of immunoregulatory cells in controlling protective responses to the pneumococcus. This study provides exciting new evidence for the importance of immunomodulation during pulmonary pneumococcal infection and suggests that TGF-β signalling is a potential target for immunotherapy or drug design.

Streptococcus pneumoniae is a major human bacterial pathogen that causes a wide range of diseases including pneumonia, meningitis, sepsis and ear infections. The bacterium is responsible for around 1.2 million deaths per year, mostly in high-risk groups such as children, the elderly and those with a weakened immune system. Infection with the pneumococcus can induce a wide-variety of immune responses and disease symptoms and it is not known why some people are more resistant to infection than others. Here, we identify an important role in natural resistance against pneumococcal pneumonia for a group of cells – known as T regulatory cells – that control the immune response to pneumococcal infection. In mice, strong T regulatory cell responses correlate with resistance to invasive pneumococcal pneumonia. Disease-resistance can be boosted by administering T regulatory cells to highly susceptible mice or inhibited by blocking the activity of these cells in resistant mice. These results advance our understanding of the host immunity differences that underpin resistance to pneumococcal pneumonia and offer hope that in the future we might boost resistance in susceptible individuals through modulation of their immune system.

Host Defense and the Airway Epithelium: Frontline Responses That Protect against Bacterial Invasion and Pneumonia

Airway epithelial cells are the first line of defense against invading microbes, and they protect themselves through the production of carbohydrate and protein matrices concentrated with antimicrobial products. In addition, they act as sentinels, expressing pattern recognition receptors that become activated upon sensing bacterial products and stimulate downstream recruitment and activation of immune cells which clear invading microbes. Bacterial pathogens that successfully colonize the lungs must resist these mechanisms or inhibit their production, penetrate the epithelial barrier, and be prepared to resist a barrage of inflammation. Despite the enormous task at hand, relatively few virulence factors coordinate the battle with the epithelium while simultaneously providing resistance to inflammatory cells and causing injury to the lung. Here we review mechanisms whereby airway epithelial cells recognize pathogens and activate a program of antibacterial pathways to prevent colonization of the lung, along with a few examples of how bacteria disrupt these responses to cause pneumonia.