Construction and immunological characterization of a novel nontoxic protective pneumolysin mutant for use in future pneumococcal vaccines

Detoxified pneumolysin derivative ΔA146Ply inhibits triple- negative breast cancer metastasis mainly via mannose receptor-mediated autophagy inhibition

The detoxified pneumolysin derivative ΔA146Ply has been proven to have a direct anti-triple negative breast cancer effect by our group, but its work model remains unclear. In this study, we focused on its ability to inhibit triple-negative breast cancer metastasis. We found that ΔA146Ply suppressed the migration and invasion of triple-negative breast cancer cells by activating mannose receptor and toll-like receptor 4. Their activation triggers the activation of the mammalian target of rapamycin signalling, sequentially leading to autophagy, transforming growth factor-β1, and epithelial-mesenchymal transition inhibition. Furthermore, the combination of doxorubicin and ΔA146Ply significantly inhibited triple-negative breast cancer progression and prolonged survival in tumour-bearing mice. Taken together, our study provides an alternative microbiome-based mannose receptor-targeted therapy for triple-negative breast cancer and a novel theoretical and experimental basis for the downstream signalling pathway of the mannose receptor.

A Streptococcus pneumoniae endolysin mutant protein ΔA146Ply elicits rapid broad-spectrum mucosal protection in mice via upregulation of GPX4 through TLR4/IRG1/NRF2 to alleviate macrophage ferroptosis

Innovative solutions for rapid protection against broad-spectrum infections are very important in dealing with complex infection environments. We utilized a functionally inactive mutated endolysin protein of Streptococcus pneumoniae (ΔA146Ply) to immunize mice against pneumonic infections by multidrug-resistant bacteria, Candida albicans and influenza virus type A. ΔA146Ply protection relied on both immunized tissue-resident and monocyte-derived alveolar macrophages and inhibited infection induced ferroptosis that upregulated expression of GPX4 (glutathione peroxidase) in alveolar macrophages. Ferroptosis resistance endowed macrophages with enhanced phagocytosis by inhibiting lipid peroxidation during infection. Moreover, we demonstrated ΔA146Ply upregulated GPX4 through the TLR4/IRG1/NRF2 pathway. ΔA146Ply also induced ferroptosis inhibition and phagocytosis improvement in human monocytes. This mode of action is a novel and potentially prophylactic and rapid broad-spectrum anti-infection mechanism. Our study provides new insights into protective interventions that act by regulating ferroptosis to improve multiple pathogen resistance via GPX4 targeting.

Transcriptional regulation of TacL-mediated lipoteichoic acids biosynthesis by ComE during competence impacts pneumococcal transformation

Competence development is essential for bacterial transformation since it enables bacteria to take up free DNA from the surrounding environment. The regulation of teichoic acid biosynthesis is tightly controlled during pneumococcal competence; however, the mechanism governing this regulation and its impact on transformation remains poorly understood. We demonstrated that a defect in lipoteichoic acid ligase (TacL)-mediated lipoteichoic acids (LTAs) biosynthesis was associated with impaired pneumococcal transformation. Using a fragment of tacL regulatory probe as bait in a DNA pulldown assay, we successfully identified several regulatory proteins, including ComE. Electrophoretic mobility shift assays revealed that phosphomimetic ComE, but not wild-type ComE, exhibited specific binding to the probe. DNase I footprinting assays revealed the specific binding sequences encompassing around 30 base pairs located 31 base pairs upstream from the start codon of tacL. Expression of tacL was found to be upregulated in the ΔcomE strain, and the addition of exogenous competence-stimulating peptide repressed the tacL transcription in the wild-type strain but not the ΔcomE mutant, indicating that ComE exerted a negative regulatory effect on the transcription of tacL. Mutation in the JH2 region of tacL upstream regulatory sequence led to increased LTAs abundance and displayed higher transformation efficiency. Collectively, our work identified the regulatory mechanisms that control LTAs biosynthesis during competence and thereby unveiled a repression mechanism underlying pneumococcal transformation.

NAD+ metabolism is a key modulator of bacterial respiratory epithelial infections

Lower respiratory tract infections caused by Streptococcus pneumoniae (Spn) are a leading cause of death globally. Here we investigate the bronchial epithelial cellular response to Spn infection on a transcriptomic, proteomic and metabolic level. We found the NAD+ salvage pathway to be dysregulated upon infection in a cell line model, primary human lung tissue and in vivo in rodents, leading to a reduced production of NAD+. Knockdown of NAD+ salvage enzymes (NAMPT, NMNAT1) increased bacterial replication. NAD+ treatment of Spn inhibited its growth while growth of other respiratory pathogens improved. Boosting NAD+ production increased NAD+ levels in immortalized and primary cells and decreased bacterial replication upon infection. NAD+ treatment of Spn dysregulated the bacterial metabolism and reduced intrabacterial ATP. Enhancing the bacterial ATP metabolism abolished the antibacterial effect of NAD+. Thus, we identified the NAD+ salvage pathway as an antibacterial pathway in Spn infections, predicting an antibacterial mechanism of NAD+.

Naturally-occurring serotype 3 Streptococcus pneumoniae strains that lack functional pneumolysin and autolysin have attenuated virulence but induce localized protective immune responses

Streptococcus pneumoniae is an important cause of fatal pneumonia in humans. These bacteria express virulence factors, such as the toxins pneumolysin and autolysin, that drive host inflammatory responses. In this study we confirm loss of pneumolysin and autolysin function in a group of clonal pneumococci that have a chromosomal deletion resulting in a pneumolysin-autolysin fusion gene Δ(lytA'-ply')593. The Δ(lytA'-ply')593 pneumococci strains naturally occur in horses and infection is associated with mild clinical signs. Here we use immortalized and primary macrophage in vitro models, which include pattern recognition receptor knock-out cells, and a murine acute pneumonia model to show that a Δ(lytA'-ply')593 strain induces cytokine production by cultured macrophages, however, unlike the serotype-matched ply+lytA+ strain, it induces less tumour necrosis factor α (TNFα) and no interleukin-1β production. The TNFα induced by the Δ(lytA'-ply')593 strain requires MyD88 but, in contrast to the ply+lytA+ strain, is not reduced in cells lacking TLR2, 4 or 9. In comparison to the ply+lytA+ strain in a mouse model of acute pneumonia, infection with the Δ(lytA'-ply')593 strain resulted in less severe lung pathology, comparable levels of interleukin-1α, but minimal release of other pro-inflammatory cytokines, including interferon-γ, interleukin-6 and TNFα. These results suggest a mechanism by which a naturally occurring Δ(lytA'-ply')593 mutant strain of S. pneumoniae that resides in a non-human host has reduced inflammatory and invasive capacity compared to a human S. pneumoniae strain. These data probably explain the relatively mild clinical disease in response to S. pneumoniae infection seen in horses in comparison to humans.

Protein-Based Adjuvants for Vaccines as Immunomodulators of the Innate and Adaptive Immune Response: Current Knowledge, Challenges, and Future Opportunities

New-generation vaccines, formulated with subunits or nucleic acids, are less immunogenic than classical vaccines formulated with live-attenuated or inactivated pathogens. This difference has led to an intensified search for additional potent vaccine adjuvants that meet safety and efficacy criteria and confer long-term protection. This review provides an overview of protein-based adjuvants (PBAs) obtained from different organisms, including bacteria, mollusks, plants, and humans. Notably, despite structural differences, all PBAs show significant immunostimulatory properties, eliciting B-cell- and T-cell-mediated immune responses to administered antigens, providing advantages over many currently adopted adjuvant approaches. Furthermore, PBAs are natural biocompatible and biodegradable substances that induce minimal reactogenicity and toxicity and interact with innate immune receptors, enhancing their endocytosis and modulating subsequent adaptive immune responses. We propose that PBAs can contribute to the development of vaccines against complex pathogens, including intracellular pathogens such as Mycobacterium tuberculosis, those with complex life cycles such as Plasmodium falciparum, those that induce host immune dysfunction such as HIV, those that target immunocompromised individuals such as fungi, those with a latent disease phase such as Herpes, those that are antigenically variable such as SARS-CoV-2 and those that undergo continuous evolution, to reduce the likelihood of outbreaks.

Pneumococcal Surface Proteins as Virulence Factors, Immunogens, and Conserved Vaccine Targets

Streptococcus pneumoniae is an opportunistic pathogen that causes over 1 million deaths annually despite the availability of several multivalent pneumococcal conjugate vaccines (PCVs). Due to the limitations surrounding PCVs along with an evolutionary rise in antibiotic-resistant and unencapsulated strains, conserved immunogenic proteins as vaccine targets continue to be an important field of study for pneumococcal disease prevention. In this review, we provide an overview of multiple classes of conserved surface proteins that have been studied for their contribution to pneumococcal virulence. Furthermore, we discuss the immune responses observed in response to these proteins and their promise as vaccine targets.

The Yin and Yang of Pneumolysin During Pneumococcal Infection

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

IL-4 plays an essential role in DnaJ-ΔA146Ply-mediated immunoprotection against Streptococcus pneumoniae in mice

The fusion protein DnaJ-ΔA146Ply is protective against pneumococcal infections in mice. However, we found that immunized IL-4-/- mice showed significant lower survival rates and higher bacterial loads than did wild-type (WT) mice after being challenged. We explored the role of IL-4 in the protective immunity conferred by DnaJ-ΔA146Ply. Our results showed that there were no significant differences in antibody titers between immunized WT mice and IL-4-/- mice. The bacterial loads of passively immunized IL-4-/- mice were significantly higher than those of WT mice, while mice immunized with anti-DnaJ-ΔA146Ply serum from WT and IL-4-/- mice showed similar capacity for bacterial clearance. DnaJ-ΔA146Ply-dependent phagocytosis of IL-4-/- neutrophils was significant decreased compared with that of WT neutrophils. The levels of Syk and phosphor-Syk in IL-4-/- neutrophils were decreased compared with those in WT neutrophils. Additionally, Splenocytes in IL-4-/- mice triggered significantly higher levels of IFN-γ and IL-17A than did splenocytes in WT mice. Taken together, our findings illustrate that IL-4 deficiency does not influence the antibody production or antibody effect, but change the cellular immune response induced by DnaJ-ΔA146Ply. Additionally, IL-4 can enhance the antibody-dependent phagocytosis of neutrophils partially by activating Syk and participate in the protective immunity induced by DnaJ-ΔA146Ply.

Corrected and Republished from: "A Novel, Multiple-Antigen Pneumococcal Vaccine Protects against Lethal Streptococcus pneumoniae Challenge"

Current vaccination against Streptococcus pneumoniae uses vaccines based on capsular polysaccharides from selected serotypes and has led to nonvaccine serotype replacement disease. We have investigated an alternative serotype-independent approach, using multiple-antigen vaccines (MAV) prepared from S. pneumoniae TIGR4 lysates enriched for surface proteins by a chromatography step after culture under conditions that induce expression of heat shock proteins (Hsp; thought to be immune adjuvants). Proteomics and immunoblot analyses demonstrated that, compared to standard bacterial lysates, MAV was enriched with Hsps and contained several recognized protective protein antigens, including pneumococcal surface protein A (PspA) and pneumolysin (Ply). Vaccination of rodents with MAV induced robust antibody responses to multiple serotypes, including nonpneumococcal conjugate vaccine serotypes. Homologous and heterologous strains of S. pneumoniae were opsonized after incubation in sera from vaccinated rodents. In mouse models, active vaccination with MAV significantly protected against pneumonia, while passive transfer of rabbit serum from MAV-vaccinated rabbits significantly protected against sepsis caused by both homologous and heterologous S. pneumoniae strains. Direct comparison of MAV preparations made with or without the heat shock step showed no clear differences in protein antigen content and antigenicity, suggesting that the chromatography step rather than Hsp induction improved MAV antigenicity. Overall, these data suggest that the MAV approach may provide serotype-independent protection against S. pneumoniae.

Subcutaneous immunization with the fusion protein ΔA146Ply-SP0148 confers protection against Streptococcus pneumoniae infection

Pneumococcal SP0148 and pneumolysin (Ply) derivatives are important vaccine candidates. SP0148 is a conserved lipoprotein with high immunogenicity produced by Streptococcus pneumoniae. We have previously demonstrated that SP0148 can confer protection against fatal infections caused by S. pneumoniae. ΔA146Ply is a noncytotoxic mutant of Ply that retains the TLR4 agonistic effect and has mucosal and subcutaneous adjuvant activities suggested to induce protective immunity against S. pneumoniae infection. In this study, we constructed the fusion protein ΔA146Ply-SP0148, composed of ΔA146Ply and SP0148, and evaluated the immunoprotective effect of the fusion protein. When mice were subcutaneously immunized with the fusion protein ΔA146Ply-SP0148, high levels of anti-ΔA146Ply and anti-SP0148 IgG antibodies were induced in the serum. Specific antibodies can bind to a variety of different serotypes of S. pneumoniae. Compared with mice immunized with ΔA146Ply and SP0148 alone, mice immunized subcutaneously with the fusion protein ΔA146Ply-SP0148 with Al(OH)₃ had a higher survival rate when challenged by a lethal dose of S. pneumoniae, and they also had significantly lower lung bacterial loads and milder lung inflammation. In addition, mice immunized subcutaneously with the fusion protein ΔA146Ply-SP0148 stimulated strong Th1, Th2, and Th17 cell responses. In summary, these results suggest that subcutaneous immunization with the ΔA146Ply-SP0148 fusion protein can protect mice against fatal pneumococcal infection and lung infection. The fusion protein ΔA146ply-SP0148 can be a new pneumococcal vaccine target.

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

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

Protective efficacy of mucosal and subcutaneous immunization with DnaJ-ΔA146Ply against influenza and Streptococcus pneumoniae co-infection in mice

Respiratory tract coinfections, specifically involving influenza A virus (IAV) and Streptococcus pneumoniae (S. pneumoniae), remain a major health problem worldwide. Secondary bacterial pneumonia is a common complication and an important cause of mortality related to seasonal and pandemic influenza infections. Vaccination is a basic control strategy against influenza and S. pneumoniae. The fusion protein DnaJ-ΔA146Ply is a vaccine candidate which can induce immune responses against pneumococcal infections via mucosal and subcutaneous immunization in mice. In the present study, we established a co-infection model using mouse-adapted laboratory strains of IAV (PR8) and S. pneumoniae (19F) in mice intranasally and subcutaneously immunized with DnaJ-ΔA146Ply. Our results showed that vaccinated mice suffered decreased weight loss compared with control mice. The survival rates were higher in intranasally and subcutaneously immunized mice than in control mice. In addition, the bacterial loads in nasal washes and lung homogenates were lower in vaccinated mice than in control mice. Furthermore, lung damage was alleviated in vaccinated mice compared with control mice, with less broken alveoli and less proinflammatory cytokine production. Taken together, these results indicate that vaccination with DnaJ-ΔA146Ply shows protective potential against influenza and S. pneumoniae co-infection in mice.

Combination of Detoxified Pneumolysin Derivative ΔA146Ply and Berbamine as a Treatment Approach for Breast Cancer

Increasing evidence demonstrates that microorganisms and their products can modulate host responses to cancer therapies and contribute to tumor shrinkage via various mechanisms, including intracellular signaling pathways modulation and immunomodulation. Detoxified pneumolysin derivative ΔA146Ply is a pneumolysin mutant lacking hemolytic activity. To determine the antitumor activity of ΔA146Ply, the combination of ΔA146Ply and berbamine, a well-established antitumor agent, was used for breast cancer therapy, especially for triple-negative breast cancer. The efficacy of the combination therapy was evaluated in vitro using four breast cancer cell lines and in vivo using a synergistic mouse tumor model. We demonstrated that in vitro, the combination therapy significantly inhibited cancer cell proliferation, promoted cancer cell apoptosis, caused cancer cell-cycle arrest, and suppressed cancer cell migration and invasion. In vivo, the combination therapy significantly suppressed tumor growth and prolonged the median survival time of tumor-bearing mice partially through inhibiting tumor cell proliferation, promoting tumor cell apoptosis, and activating systemic antitumor immune responses. The safety analysis demonstrated that the combination therapy showed no obvious liver and kidney toxicity to tumor-bearing mice. Our study provides a new treatment option for breast cancer and lays the experimental basis for the development of ΔA146Ply as an antitumor agent.

Relative Contributions of Extracellular and Internalized Bacteria to Early Macrophage Proinflammatory Responses to Streptococcus pneumoniae

Both intracellular immune sensing and extracellular innate immune sensing have been implicated in initiating macrophage proinflammatory cytokine responses to Streptococcus pneumoniae The S. pneumoniae capsule, a major virulence determinant, prevents phagocytosis, and we hypothesized that this would reduce activation of host innate inflammatory responses by preventing activation of intracellular proinflammatory signaling pathways. We investigated this hypothesis in human monocyte-derived macrophages stimulated with encapsulated or isogenic unencapsulated mutant S. pneumoniae Unexpectedly, despite strongly inhibiting bacterial internalization, the capsule resulted in enhanced inflammatory cytokine production by macrophages infected with S. pneumoniae Experiments using purified capsule material and a Streptococcus mitis mutant expressing an S. pneumoniae serotype 4 capsule indicated these differences required whole bacteria and were not due to proinflammatory effects of the capsule itself. Transcriptional profiling demonstrated relatively few differences in macrophage gene expression profiles between infections with encapsulated S. pneumoniae and those with unencapsulated S. pneumoniae, largely limited to reduced expression of proinflammatory genes in response to unencapsulated bacteria, predicted to be due to reduced activation of the NF-κB family of transcription factors. Blocking S. pneumoniae internalization using cytochalasin D had minimal effects on the inflammatory response to S. pneumoniae Experiments using murine macrophages indicated that the affected genes were dependent on Toll-like receptor 2 (TLR2) activation, although not through direct stimulation of TLR2 by capsule polysaccharide. Our data demonstrate that the early macrophage proinflammatory response to S. pneumoniae is mainly dependent on extracellular bacteria and reveal an unexpected proinflammatory effect of encapsulated S. pneumoniae that could contribute to disease pathogenesis.IMPORTANCE Multiple extra- and intracellular innate immune receptors have been identified that recognize Streptococcus pneumoniae, but the relative contributions of intra- versus extracellular bacteria to the inflammatory response were unknown. We have shown that intracellular S. pneumoniae contributes surprisingly little to the inflammatory responses, with production of important proinflammatory cytokines largely dependent on extracellular bacteria. Furthermore, although we expected the S. pneumoniae polysaccharide capsule to block activation of the host immune system by reducing bacterial internalization and therefore activation of intracellular innate immune receptors, there was an increased inflammatory response to encapsulated compared to unencapsulated bacteria, which is likely to contribute to disease pathogenesis.

A Novel, Multiple-Antigen Pneumococcal Vaccine Protects against Lethal Streptococcus pneumoniae Challenge

Current vaccination against Streptococcus pneumoniae uses vaccines based on capsular polysaccharides from selected serotypes and has led to nonvaccine serotype replacement disease. We have investigated an alternative serotype-independent approach, using multiple-antigen vaccines (MAV) prepared from S. pneumoniae TIGR4 lysates enriched for surface proteins by a chromatography step after culture under conditions that induce expression of heat shock proteins (Hsp; thought to be immune adjuvants).

Current vaccination against Streptococcus pneumoniae uses vaccines based on capsular polysaccharides from selected serotypes and has led to nonvaccine serotype replacement disease. We have investigated an alternative serotype-independent approach, using multiple-antigen vaccines (MAV) prepared from S. pneumoniae TIGR4 lysates enriched for surface proteins by a chromatography step after culture under conditions that induce expression of heat shock proteins (Hsp; thought to be immune adjuvants). Proteomics and immunoblot analyses demonstrated that, compared to standard bacterial lysates, MAV was enriched with Hsps and contained several recognized protective protein antigens, including pneumococcal surface protein A (PspA) and pneumolysin (Ply). Vaccination of rodents with MAV induced robust antibody responses to multiple serotypes, including nonpneumococcal conjugate vaccine serotypes. Homologous and heterologous strains of S. pneumoniae were opsonized after incubation in sera from vaccinated rodents. In mouse models, active vaccination with MAV significantly protected against pneumonia, while passive transfer of rabbit serum from MAV-vaccinated rabbits significantly protected against sepsis caused by both homologous and heterologous S. pneumoniae strains. Direct comparison of MAV preparations made with or without the heat shock step showed no clear differences in protein antigen content and antigenicity, suggesting that the chromatography step rather than Hsp induction improved MAV antigenicity. Overall, these data suggest that the MAV approach may provide serotype-independent protection against S. pneumoniae.

Inhibition of bacterial toxin recognition of membrane components as an anti-virulence strategy

Over recent years, the development of new antibiotics has not kept pace with the rate at which bacteria develop resistance to these drugs. For this reason, many research groups have begun to design and study alternative therapeutics, including molecules to specifically inhibit the virulence of pathogenic bacteria. Because many of these pathogenic bacteria release protein toxins, which cause or exacerbate disease, inhibition of the activity of bacterial toxins is a promising anti-virulence strategy. In this review, we describe several approaches to inhibit the initial interactions of bacterial toxins with host cell membrane components. The mechanisms by which toxins interact with the host cell membrane components have been well-studied over the years, leading to the identification of therapeutic targets, which have been exploited in the work described here. We review efforts to inhibit binding to protein receptors and essential membrane lipid components, complex assembly, and pore formation. Although none of these molecules have yet been demonstrated in clinical trials, the in vitro and in vivo results presented here demonstrate their promise as novel alternatives and/or complements to traditional antibiotics.

Membrane deformation and layer-by-layer peeling of giant vesicles induced by the pore-forming toxin pneumolysin

Membranes under attack by the pore-forming toxin pneumolysin reveal a hitherto unknown layer-by-layer peeling mechanism and disclose the multilamellar structure.

Pharmacological Targeting of Pore-Forming Toxins as Adjunctive Therapy for Invasive Bacterial Infection

For many of the most important human bacterial infections, invasive disease severity is fueled by the cell damaging and pro-inflammatory effects of secreted pore-forming toxins (PFTs). Isogenic PFT-knockout mutants, e.g., Staphylococcus aureus lacking α-toxin or Streptococcus pneumoniae deficient in pneumolysin, show attenuation in animal infection models. This knowledge has inspired multi-model investigations of strategies to neutralize PFTs or counteract their toxicity as a novel pharmacological approach to ameliorate disease pathogenesis in clinical disease. Promising examples of small molecule, antibody or nanotherapeutic drug candidates that directly bind and neutralize PFTs, block their oligomerization or membrane receptor interactions, plug establishment membrane pores, or boost host cell resiliency to withstand PFT action have emerged. The present review highlights these new concepts, with a special focus on β-PFTs produced by leading invasive human Gram-positive bacterial pathogens. Such anti-virulence therapies could be applied as an adjunctive therapy to antibiotic-sensitive and -resistant strains alike, and further could be free of deleterious effects that deplete the normal microflora.

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

Bacterial infectious diseases are a leading cause of death. Pore-forming toxins (PFTs) are important virulence factors of Gram-positive pathogens, which disrupt the plasma membrane of host cells and can lead to cell death. Yet, host defense and cell membrane repair mechanisms have been identified: i.e., PFTs can be eliminated from membranes as microvesicles, thus limiting the extent of cell damage. Released into an inflammatory environment, these host-derived PFTs-carrying microvesicles encounter innate immune cells as first-line defenders. This study investigated the impact of microvesicle- or liposome-sequestered PFTs on human macrophage polarization in vitro. We show that microvesicle-sequestered PFTs are phagocytosed by macrophages and induce their polarization into a novel CD14⁺MHCII^lowCD86^low phenotype. Macrophages polarized in this way exhibit an enhanced response to Gram-positive bacterial ligands and a blunted response to Gram-negative ligands. Liposomes, which were recently shown to sequester PFTs and so protect mice from lethal bacterial infections, show the same effect on macrophage polarization in analogy to host-derived microvesicles. This novel type of polarized macrophage exhibits an enhanced response to Gram-positive bacterial ligands. The specific recognition of their cargo might be of advantage in the efficiency of targeted bacterial clearance.

Production and efficacy of a low-cost recombinant pneumococcal protein polysaccharide conjugate vaccine

Streptococcus pneumoniae is the leading cause of bacterial pneumonia. Although this is a vaccine preventable disease, S. pneumoniae still causes over 1 million deaths per year, mainly in children under the age of five. The biggest disease burden is in the developing world, which is mainly due to unavailability of vaccines due to their high costs. Protein polysaccharide conjugate vaccines are given routinely in the developed world to children to induce a protective antibody response against S. pneumoniae. One of these vaccines is Prevnar13, which targets 13 of the 95 known capsular types. Current vaccine production requires growth of large amounts of the 13 serotypes, and isolation of the capsular polysaccharide that is then chemically coupled to a protein, such as the diphtheria toxoid CRM197, in a multistep expensive procedure. In this study, we design, purify and produce novel recombinant pneumococcal protein polysaccharide conjugate vaccines in Escherichia coli, which act as mini factories for the low-cost production of conjugate vaccines. Recombinant vaccine efficacy was tested in a murine model of pneumococcal pneumonia; ability to protect against invasive disease was compared to that of Prevnar13. This study provides the first proof of principle that protein polysaccharide conjugate vaccines produced in E. coli can be used to prevent pneumococcal infection. Vaccines produced in this manner may provide a low-cost alternative to the current vaccine production methodology.

Immunogenicity and mechanisms of action of PnuBioVax, a multi-antigen serotype-independent prophylactic vaccine against infection with Streptococcus pneumoniae

Streptococcus pneumoniae has multiple protein antigens on the surface in addition to the serotype specific polysaccharide capsule antigen. Whilst the capsule antigen is the target of the polysaccharide vaccines, bacterial proteins can also act as targets for the immune system. PnuBioVax (PBV) is being developed as a multi-antigen, serotype-independent prophylactic vaccine against S. pneumoniae disease. In this study we have sought to elucidate the immune response to PBV in immunised rabbits. Sera from PBV immunised rabbits contained high levels of IgG antibodies to the PBV vaccine, and pneumococcal antigens PspA, Ply, PsaA and PiuA which are components of PBV, when compared with control sera. The PBV sera supported killing of the vaccine strain TIGR4 in an opsonophagocytic killing assay and heterologous strains 6B, 19F and 15B. In addition, incubation in PBV sera led to agglutination of several strains of pneumococci, inhibition of Ply-mediated lysis of erythrocytes and reduced bacterial invasion of lung epithelial cells in vitro. These data suggest that PBV vaccination generates sera that has multiple mechanisms of action that may provide effective protection against pneumococcal infection and give broader strain coverage than the current polysaccharide based vaccines.

Pneumolysin-damaged cells benefit from non-homogeneous toxin binding to cholesterol-rich membrane domains

Nucleated cells eliminate lesions induced by bacterial pore-forming toxins, such as pneumolysin via shedding patches of damaged plasmalemma into the extracellular milieu. Recently, we have shown that the majority of shed pneumolysin is present in the form of inactive pre-pores. This finding is surprising considering that shedding is triggered by Ca²⁺-influx following membrane perforation and therefore is expected to positively discriminate for active pores versus inactive pre-pores. Here we provide evidence for the existence of plasmalemmal domains that are able to attract pneumolysin at high local concentrations. Within such a domain an immediate plasmalemmal perforation induced by a small number of pneumolysin pores would be capable of triggering the elimination of a large number of not yet active pre-pores/monomers and thus pre-empt more frequent and perilous perforation events. Our findings provide further insights into the functioning of the cellular repair machinery which benefits from an inhomogeneous plasmalemmal distribution of pneumolysin.

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

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

Safety and immunogenicity of a novel multiple antigen pneumococcal vaccine in adults: A Phase 1 randomised clinical trial

BACKGROUND

Pneumococcal vaccines, combining multiple protein antigens, provide an alternative approach to currently marketed vaccines and may provide broader protection against pneumococcal disease. This trial evaluated the safety and immunogenicity of a novel vaccine candidate PnuBioVax in healthy young adults.

METHODS

In a Phase 1 double-blind study, 36 subjects (18-40 years) were randomised to receive 3 doses of PnuBioVax, 28 days apart, at one of three dose levels (50, 200, 500 µg) or placebo. Safety assessments included rates of emergent adverse events (AEs), injection site and systemic reactions. Immunogenicity endpoints included antibody titre against PnuBioVax and selected pneumococcal antigens.

RESULTS

In the placebo (n=9) and PnuBioVax (n=27) vaccinated subjects, there were 15 and 72, reported TEAEs, respectively. The majority of TEAEs were classified as common vaccine related AEs. There were no serious AEs. Common vaccine-related AEs occurred in 13 PnuBioVax (48%) and 2 placebo (22%) subjects and were all headaches (mild and moderate). Injection site reactions, mostly pain and tenderness (graded mild or moderate) were reported, in particular in the 200 µg and 500 µg PnuBioVax groups. There were no clinically significant changes in vital signs, ECG or blood chemistries. Subjects receiving the higher dose (200 and 500 μg) demonstrated a greater fold increase in IgG titre compared with the starting dose (50 μg) or the placebo group. The fold-increase was statistically significantly higher for 200 and 500µg PnuBioVax vs 50µg PnuBioVax and placebo at each timepoint post-immunisation. Most subjects receiving 200 and 500 µg PnuBioVax demonstrated a ≥2-fold increase in antibody against pneumolysin (Ply), Pneumococcal surface antigen (PsaA), PiaA (Pneumococcal iron acquisition), PspA (Pneumococcal surface protein A) and pilus proteins (RrgB and RrgA).

CONCLUSIONS

All dose levels were considered safe and well tolerated. There was a statistically significant increase in anti-PnuBioVax IgG titres at the 200 and 500 µg dose levels compared to 50 µg and placebo.

TRIAL REGISTRATION NUMBER

NCT02572635https://www.clinicaltrials.gov.

A Novel Biomimetic Nanosponge Protects the Retina from the Enterococcus faecalis Cytolysin

Endophthalmitis is a serious, potentially blinding infection that can result in vision loss, leaving a patient with only the ability to count fingers, or it may require enucleation of the globe. The incidence of postoperative endophthalmitis has markedly increased over the past 2 decades, paralleling the rise in ocular surgeries and intravitreal therapies. E. faecalis is a leading cause of infection following ocular procedures, and such infections are increasingly difficult to treat due to multidrug resistance. Cytolysin is the primary virulence factor responsible for retinal tissue damage in E. faecalis eye infections. Treatment of these infections with antibiotics alone does not impede ocular damage and loss of visual function. Pore-forming toxins (PFTs) have been established as major virulence factors in endophthalmitis caused by several bacterial species. These facts establish a critical need for a novel therapy to neutralize bacterial PFTs such as cytolysin. Here, we demonstrate that biomimetic nanosponges neutralize cytolysin, protect the retina, preserve vision, and may provide an adjunct detoxification therapy for bacterial infections.

Intraocular infections are a potentially blinding complication of common ocular surgeries and traumatic eye injuries. Bacterial toxins synthesized in the eye can damage intraocular tissue, often resulting in poor visual outcomes. Enteroccocus faecalis causes blinding infections and is responsible for 8 to 17% of postoperative endophthalmitis cases. These infections are increasingly difficult to treat due to the emergence of multidrug-resistant strains. Virulent E. faecalis isolates secrete a pore-forming bicomponent cytolysin that contributes to retinal tissue damage during endophthalmitis. We hypothesized that a biomimetic nanosponge, which mimics erythrocytes, might adsorb subunits of the cytolysin and reduce retinal damage, protecting vision. To test the efficacy of nanosponges in neutralizing the cytolysin in vitro, hemoglobin release assays were performed on culture supernatants from cytolysin-producing E. faecalis with and without preincubation with nanosponges. Treatment with nanosponges for 30 min reduced hemolytic activity by ~70%. To determine whether nanosponges could neutralize the cytolysin in vivo, electroretinography was performed on mice 24 h after intravitreal injection with cytolysin-containing supernatants treated with nanosponges. Pretreatment of cytolysin-containing supernatants with nanosponges increased the A-wave retention from 12.2% to 65.5% and increased the B-wave retention from 21.0% to 77.0%. Histology revealed that in nanosponge-treated eyes, retinas remained intact and attached, with little to no damage. Rabbit nanosponges were also nontoxic and noninflammatory when injected into mouse eyes. In an experimental murine model of E. faecalis endophthalmitis, injection of nanosponges into the vitreous 6 h after infection with a wild-type cytolysin-producing strain increased A-wave retention from 5.9% to 31% and increased B-wave retention from 12.6% to 27.8%. Together, these results demonstrated that biomimetic nanosponges neutralized cytolysin activity and protected the retinas from damage. These results suggest that this novel strategy might also protect eyes from the activities of pore-forming toxins of other virulent ocular bacterial pathogens.

IMPORTANCE Endophthalmitis is a serious, potentially blinding infection that can result in vision loss, leaving a patient with only the ability to count fingers, or it may require enucleation of the globe. The incidence of postoperative endophthalmitis has markedly increased over the past 2 decades, paralleling the rise in ocular surgeries and intravitreal therapies. E. faecalis is a leading cause of infection following ocular procedures, and such infections are increasingly difficult to treat due to multidrug resistance. Cytolysin is the primary virulence factor responsible for retinal tissue damage in E. faecalis eye infections. Treatment of these infections with antibiotics alone does not impede ocular damage and loss of visual function. Pore-forming toxins (PFTs) have been established as major virulence factors in endophthalmitis caused by several bacterial species. These facts establish a critical need for a novel therapy to neutralize bacterial PFTs such as cytolysin. Here, we demonstrate that biomimetic nanosponges neutralize cytolysin, protect the retina, preserve vision, and may provide an adjunct detoxification therapy for bacterial infections.

Subcutaneous Immunization with Fusion Protein DnaJ-ΔA146Ply without Additional Adjuvants Induces both Humoral and Cellular Immunity against Pneumococcal Infection Partially Depending on TLR4

Subunit vaccines that are poorly immunogenic are often combined with adjuvants for immunization. Our previous research identified a pneumolysin variant (ΔA146Ply), a Toll-like receptor 4 agonist, that was an effective adjuvant in the protection of fusion protein DnaJ-ΔA146Ply against mucosal Streptococcus pneumoniae infections. For pneumococcal vaccines, World Health Organization recommend injection as a regular vaccination approach. Subcutaneous immunization is a common and effective method of injection, so we explored the immunity mechanism of subcutaneous immunization with DnaJ-ΔA146Ply. We found that mice immunized subcutaneously with fusion proteins ΔA146Ply-DnaJ and DnaJ-ΔA146Ply produced a higher anti-DnaJ IgG titer than when DnaJ alone was administered. DnaJ-ΔA146Ply induced both B-cell and T-cell-dependent protection against both colonization and lethal pneumococcal infections. Levels of IFN-γ, IL-4, and IL-17A were also elevated in DnaJ-ΔA146Ply immunized mice. However, all these effects were negated in TLR4^-/- mice compared to WT mice immunized with DnaJ-ΔA146Ply. B-cell-deficient μMT mice, nude mice, IFN-γ^-/-, and IL-4^-/- mice immunized with DnaJ-ΔA146Ply could not resist infection with pneumococci. IL-17A^-/- and TLR4^-/- mice did not benefit from DnaJ-ΔPly immunization in colonization experiments although their survival was not impaired compared with WT mice. Collectively, our data indicated that ΔA146Ply can be a potential subcutaneous adjuvant, and the DnaJ-ΔA146Ply fusion protein induces both humoral and cellular immune response to resist S. pneumoniae infection. The protective effect of colonization also depends on TLR4.

Systemic and mucosal immune responses elicited by intranasal immunization with a pneumococcal bacterium-like particle-based vaccine displaying pneumolysin mutant Plym2

Pneumolysin (Ply) is an important virulence factor in pneumococcal infection and a conserved cholesterol-binding cytotoxin expressed by all serotypes of Streptococcus pneumoniae. We previously developed a highly detoxified Ply mutant designated Plym2 by replacement of two amino acids (C428G and W433F), which lost cytotoxicity but retained the ability to induce neutralizing antibodies. In the present work, we applied bacterium-like particles (BLPs) as a carrier and immunostimulant for the development of a Plym2 intranasal vaccine, in which the Plym2 protein was displayed on the surface of BLPs. Intranasal immunization of mice with BLP-Plym2 not only induced a high level of serum IgG antibodies but also a high level of mucosal SIgA antibodies in lung lavages. Antiserum induced by the BLP-Plym2 vaccine elicited high-titer neutralization activity which could inhibit the hemolysis of wild-type Ply. In conclusion, the BLP-Plym2 vaccine was demonstrated to be a promising strategy for intranasal immunization to enhance both systemic and mucosal immune responses.

CryoEM structures of membrane pore and prepore complex reveal cytolytic mechanism of Pneumolysin

Many pathogenic bacteria produce pore-forming toxins to attack and kill human cells. We have determined the 4.5 Å structure of the ~2.2 MDa pore complex of pneumolysin, the main virulence factor of Streptococcus pneumoniae, by cryoEM. The pneumolysin pore is a 400 Å ring of 42 membrane-inserted monomers. Domain 3 of the soluble toxin refolds into two ~85 Å β-hairpins that traverse the lipid bilayer and assemble into a 168-strand β-barrel. The pore complex is stabilized by salt bridges between β-hairpins of adjacent subunits and an internal α-barrel. The apolar outer barrel surface with large sidechains is immersed in the lipid bilayer, while the inner barrel surface is highly charged. Comparison of the cryoEM pore complex to the prepore structure obtained by electron cryo-tomography and the x-ray structure of the soluble form reveals the detailed mechanisms by which the toxin monomers insert into the lipid bilayer to perforate the target membrane.

Otitis-Prone Children Produce Functional Antibodies to Pneumolysin and Pneumococcal Polysaccharides

The pneumococcus is a major otitis media (OM) pathogen, but data are conflicting regarding whether otitis-prone children have impaired humoral immunity to pneumococcal antigens. We and others have shown that otitis-prone and healthy children have similar antibody titers to pneumococcal proteins and polysaccharides (vaccine and nonvaccine types); however, the quality of antibodies from otitis-prone children has not been investigated. Antibody function, rather than titer, is considered to be a better correlate of protection from pneumococcal disease. Therefore, we compared the capacities of antibodies from otitis-prone (cases) and healthy (controls) children to neutralize pneumolysin, the pneumococcal toxin currently in development as a vaccine antigen, and to opsonize pneumococcal vaccine and nonvaccine serotypes. A pneumolysin neutralization assay was conducted on cholesterol-depleted complement-inactivated sera from 165 cases and 61 controls. A multiplex opsonophagocytosis assay (MOPA) was conducted on sera from 20 cases and 20 controls. Neutralizing and opsonizing titers were calculated with antigen-specific IgG titers to determine antibody potency for pneumolysin, pneumococcal conjugate vaccine (PCV) polysaccharides, and non-PCV polysaccharides. There was no significant difference in antibody potencies between cases and controls for the antigens tested. Antipneumolysin neutralizing titers increased with the number of episodes of acute OM, but antibody potency did not. Pneumolysin antibody potency was lower in children colonized with pneumococci than in noncarriers, and this was significant for the otitis-prone group (P < 0.05). The production of functional antipneumococcal antibodies in otitis-prone children demonstrates that they respond to the current PCV and are likely to respond to pneumolysin-based vaccines as effectively as healthy children.

Pneumolysin mediates heterotypic aggregation of neutrophils and platelets in vitro

OBJECTIVES

Platelets orchestrate the inflammatory activities of neutrophils, possibly contributing to pulmonary and myocardial damage during severe pneumococcal infection. This study tested the hypothesis that the pneumococcal toxin, pneumolysin (Ply), activates production of platelet-activating factor (PAF) and thromboxane A₂ (TxA₂) by neutrophils, these bioactive lipids being potential mediators of neutrophil:platelet (NP) networking.

METHODS

The effects of recombinant Ply (10-80 ng mL^-1) on the production of PAF and TxA₂ by isolated neutrophils were measured using ELISA procedures, and NP aggregation by flow cytometry.

RESULTS

Exposure of neutrophils to Ply induced production of PAF and, to a lesser extent, TxA₂, achieving statistical significance at ≥20 ng mL^-1 of the toxin. In the case of NP interactions, Ply promoted heterotypic aggregation which was dependent on upregulation of P-selectin (CD62P) and activation of protease-activated receptor 1 (PAR1), attaining statistical significance at ≥10 ng mL^-1 of the toxin, but did not involve either PAF or TxA₂.

CONCLUSION

Ply induces synthesis of PAF and TxA_2, by human neutrophils, neither of which appears to contribute to the formation of NP heterotypic aggregates in vitro, a process which is seemingly dependent on CD62P and PAR1. These pro-inflammatory activities of Ply may contribute to the pathogenesis of pulmonary and myocardial injury during severe pneumococcal infection.

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

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

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.

Active release of pneumolysin prepores and pores by mammalian cells undergoing a Streptococcus pneumoniae attack

BACKGROUND

Streptococcus pneumoniae is a potent human pathogen. Its pore-forming exotoxin pneumolysin is instrumental for breaching the host's epithelial barrier and for the incapacitation of the immune system.

METHODS AND RESULTS

Using a combination of life imaging and cryo-electron microscopy we show that pneumolysin, released by cultured bacteria, is capable of permeabilizing the plasmalemma of host cells. However, such permeabilization does not lead to cell lysis since pneumolysin is actively removed by the host cells. The process of pore elimination starts with the formation of pore-bearing plasmalemmal nanotubes and proceeds by the shedding of pores that are embedded in the membrane of released microvesicles. Pneumolysin prepores are likewise removed. The protein composition of the toxin-induced microvesicles, assessed by mass spectrometry, is suggestive of a Ca(2+)-triggered mechanism encompassing the proteins of the annexin family and members of the endosomal sorting complex required for transport (ESCRT) complex.

CONCLUSIONS

S. pneumoniae releases sufficient amounts of pneumolysin to perforate the plasmalemma of host cells, however, the immediate cell lysis, which is frequently reported as a result of treatment with purified and artificially concentrated toxin, appears to be an unlikely event in vivo since the toxin pores are efficiently eliminated by microvesicle shedding. Therefore the dysregulation of cellular homeostasis occurring as a result of transient pore formation/elimination should be held responsible for the damaging toxin action.

GENERAL SIGNIFICANCE

We have achieved a comprehensive view of a general plasma membrane repair mechanism after injury by a major bacterial toxin.

Pneumolysin Mediates Platelet Activation In Vitro

This study has explored the role of the pneumococcal toxin, pneumolysin (Ply), in activating human platelets. Following exposure to Ply (10-80 ng/ml), platelet activation and cytosolic Ca(2+) concentrations were measured flow cytometrically according to the level of expression of CD62P (P-selectin) and spectrofluorimetrically, respectively. Exposure to Ply resulted in marked upregulation of expression of platelet CD62P, achieving statistical significance at concentrations of 40 ng/ml and higher (P < 0.05), in the setting of increased influx of Ca(2+). These potentially pro-thrombotic actions of Ply were attenuated by depletion of Ca(2+) from the extracellular medium or by exposure of the cells to a pneumolysoid devoid of pore-forming activity. These findings are consistent with a mechanism of Ply-mediated platelet activation involving sub-lytic pore formation, Ca(2+) influx, and mobilization of CD62P-expressing α-granules, which, if operative in vivo, may contribute to the pathogenesis of associated acute lung and myocardial injury during invasive pneumococcal disease.

Red Blood Cell Susceptibility to Pneumolysin: CORRELATION WITH MEMBRANE BIOCHEMICAL AND PHYSICAL PROPERTIES

This study investigated the effect of the biochemical and biophysical properties of the plasma membrane as well as membrane morphology on the susceptibility of human red blood cells to the cholesterol-dependent cytolysin pneumolysin, a key virulence factor of Streptococcus pneumoniae, using single cell studies. We show a correlation between the physical properties of the membrane (bending rigidity and surface and dipole electrostatic potentials) and the susceptibility of red blood cells to pneumolysin-induced hemolysis. We demonstrate that biochemical modifications of the membrane induced by oxidative stress, lipid scrambling, and artificial cell aging modulate the cell response to the toxin. We provide evidence that the diversity of response to pneumolysin in diabetic red blood cells correlates with levels of glycated hemoglobin and that the mechanical properties of the red blood cell plasma membrane are altered in diabetes. Finally, we show that diabetic red blood cells are more resistant to pneumolysin and the related toxin perfringolysin O relative to healthy red blood cells. Taken together, these studies indicate that the diversity of cell response to pneumolysin within a population of human red blood cells is influenced by the biophysical and biochemical status of the plasma membrane and the chemical and/or oxidative stress pre-history of the cell.

Pneumolysin activates neutrophil extracellular trap formation

The primary objective of the current study was to investigate the potential of the pneumococcal toxin, pneumolysin (Ply), to activate neutrophil extracellular trap (NET) formation in vitro. Isolated human blood neutrophils were exposed to recombinant Ply (5-20 ng ml(-1) ) for 30-90 min at 37°C and NET formation measured using the following procedures to detect extracellular DNA: (i) flow cytometry using Vybrant® DyeCycle™ Ruby; (ii) spectrofluorimetry using the fluorophore, Sytox(®) Orange (5 μM); and (iii) NanoDrop(®) technology. These procedures were complemented by fluorescence microscopy using 4', 6-diamino-2-phenylindole (DAPI) (nuclear stain) in combination with anti-citrullinated histone monoclonal antibodies to visualize nets. Exposure of neutrophils to Ply resulted in relatively rapid (detected within 30-60 min), statistically significant (P < 0·05) dose- and time-related increases in the release of cellular DNA impregnated with both citrullinated histone and myeloperoxidase. Microscopy revealed that NETosis appeared to be restricted to a subpopulation of neutrophils, the numbers of NET-forming cells in the control and Ply-treated systems (10 and 20 ng ml(-1) ) were 4·3 (4·2), 14.3 (9·9) and 16·5 (7·5), respectively (n = 4, P < 0·0001 for comparison of the control with both Ply-treated systems). Ply-induced NETosis occurred in the setting of retention of cell viability, and apparent lack of involvement of reactive oxygen species and Toll-like receptor 4. In conclusion, Ply induces vital NETosis in human neutrophils, a process which may either contribute to host defence or worsen disease severity, depending on the intensity of the inflammatory response during pneumococcal infection.

Streptococcal toxins: role in pathogenesis and disease

Group A Streptococcus (Streptococcus pyogenes), group B Streptococcus (Streptococcus agalactiae) and Streptococcus pneumoniae (pneumococcus) are host-adapted bacterial pathogens among the leading infectious causes of human morbidity and mortality. These microbes and related members of the genus Streptococcus produce an array of toxins that act against human cells or tissues, resulting in impaired immune responses and subversion of host physiological processes to benefit the invading microorganism. This toxin repertoire includes haemolysins, proteases, superantigens and other agents that ultimately enhance colonization and survival within the host and promote dissemination of the pathogen.

Cell Membrane-Coated Nanoparticles As an Emerging Antibacterial Vaccine Platform

Nanoparticles have demonstrated unique advantages in enhancing immunotherapy potency and have drawn increasing interest in developing safe and effective vaccine formulations. Recent technological advancement has led to the discovery and development of cell membrane-coated nanoparticles, which combine the rich functionalities of cellular membranes and the engineering flexibility of synthetic nanomaterials. This new class of biomimetic nanoparticles has inspired novel vaccine design strategies with strong potential for modulating antibacterial immunity. This article will review recent progress on using cell membrane-coated nanoparticles for antibacterial vaccination. Specifically, two major development strategies will be discussed, namely (i) vaccination against virulence factors through bacterial toxin sequestration; and (ii) vaccination against pathogens through mimicking bacterial antigen presentation.

Crystal structure of Streptococcus pneumoniae pneumolysin provides key insights into early steps of pore formation

Pore-forming proteins are weapons often used by bacterial pathogens to breach the membrane barrier of target cells. Despite their critical role in infection important structural aspects of the mechanism of how these proteins assemble into pores remain unknown. Streptococcus pneumoniae is the world’s leading cause of pneumonia, meningitis, bacteremia and otitis media. Pneumolysin (PLY) is a major virulence factor of S. pneumoniae and a target for both small molecule drug development and vaccines. PLY is a member of the cholesterol-dependent cytolysins (CDCs), a family of pore-forming toxins that form gigantic pores in cell membranes. Here we present the structure of PLY determined by X-ray crystallography and, in solution, by small-angle X-ray scattering. The crystal structure reveals PLY assembles as a linear oligomer that provides key structural insights into the poorly understood early monomer-monomer interactions of CDCs at the membrane surface.

Engineered nanoparticles mimicking cell membranes for toxin neutralization

Protein toxins secreted from pathogenic bacteria and venomous animals rely on multiple mechanisms to overcome the cell membrane barrier to inflict their virulence effect. A promising therapeutic concept toward developing a broadly applicable anti-toxin platform is to administer cell membrane mimics as decoys to sequester these virulence factors. As such, lipid membrane-based nanoparticulates are an ideal candidate given their structural similarity to cellular membranes. This article reviews the virulence mechanisms employed by toxins at the cell membrane interface and highlights the application of cell-membrane mimicking nanoparticles as toxin decoys for systemic detoxification. In addition, the implication of particle/toxin nanocomplexes in the development of toxoid vaccines is discussed.

Genetic conjugation of components in two pneumococcal fusion protein vaccines enhances paediatric mucosal immune responses

Streptococcus pneumoniae colonises the upper respiratory tract and can cause pneumonia, meningitis and otitis media. Existing pneumococcal conjugate vaccines are expensive to produce and only protect against 13 of the 90+ pneumococcal serotypes; hence there is an urgent need for the development of new vaccines. We have shown previously in mice that pneumolysin (Ply) and a non-toxic variant (Δ6Ply) enhance antibody responses when genetically fused to pneumococcal surface adhesin A (PsaA), a potentially valuable effect for future vaccines. We investigated this adjuvanticity in human paediatric mucosal primary immune cell cultures. Adenoidal mononuclear cells (AMNC) from children aged 0-15 years (n=46) were stimulated with conjugated, admixed or individual proteins, cell viability and CD4+ T-cell proliferative responses were assessed using flow cytometry and cytokine secretion was measured using multiplex technology. Proliferation of CD4+ T-cells in response to PsaAPly, was significantly higher than responses to individual or admixed proteins (p=0.002). In contrast, an enhanced response to PsaAΔ6Ply compared to individual or admixed proteins only occurred at higher concentrations (p<0.01). Evaluation of cytotoxicity suggested that responses occurred when Ply-induced cytolysis was inhibited, either by fusion or mutation, but importantly an additional toxicity independent immune enhancing effect was also apparent as a result of fusion. Responses were MHC class II dependent and had a Th1/Th17 profile. Genetic fusion of Δ6Ply to PsaA significantly modulates and enhances pro-inflammatory CD4+ T-cell responses without the cytolytic effects of some other pneumolysoids. Membrane binding activity of such proteins may confer valuable adjuvant properties as fusion may assist Δ6Ply to deliver PsaA to the APC surface effectively, contributing to the initiation of anti-pneumococcal CD4+ T-cell immunity.

The impact of pneumolysin on the macrophage response to Streptococcus pneumoniae is strain-dependent

Streptococcus pneumoniae is the world's leading cause of pneumonia, bacteremia, meningitis and otitis media. A major pneumococcal virulence factor is the cholesterol-dependent cytolysin, which has the defining property of forming pores in cholesterol-containing membranes. In recent times a clinically significant and internationally successful serotype 1 ST306 clone has been found to express a non-cytolytic variant of Ply (Ply₃₀₆). However, while the pneumococcus is a naturally transformable organism, strains of the ST306 clonal group have to date been virtually impossible to transform, severely restricting efforts to understand the role of non-cytolytic Ply in the success of this clone. In this study isogenic Ply mutants were constructed in the D39 background and for the first time in the ST306 background (A0229467) to enable direct comparisons between Ply variants for their impact on the immune response in a macrophage-like cell line. Strains that expressed cytolytic Ply were found to induce a significant increase in IL-1β release from macrophage-like cells compared to the non-cytolytic and Ply-deficient strains in a background-independent manner, confirming the requirement for pore formation in the Ply-dependent activation of the NLRP3 inflammasome. However, cytolytic activity in the D39 background was found to induce increased expression of the genes encoding GM-CSF (CSF2), p19 subunit of IL-23 (IL23A) and IFNβ (IFNB1) compared to non-cytolytic and Ply-deficient D39 mutants, but had no effect in the A0229467 background. The impact of Ply on the immune response to the pneumococcus is highly dependent on the strain background, thus emphasising the importance of the interaction between specific virulence factors and other components of the genetic background of this organism.

Contributions to protection from Streptococcus pneumoniae infection using the monovalent recombinant protein vaccine candidates PcpA, PhtD, and PlyD1 in an infant murine model during challenge

A vaccine consisting of several conserved proteins with different functions directing the pathogenesis of pneumonia and sepsis would be preferred for protection against infection by Streptococcus pneumoniae. Infants will be the major population targeted for next-generation pneumococcal vaccines. Here, we investigated the potential efficacy provided by three recombinant pneumococcal vaccine candidate proteins--pneumococcal histidine triad D (PhtD), detoxified pneumolysin derivative (PlyD1), and pneumococcal choline-binding protein A (PcpA)--for reducing pneumonia and sepsis in an infant mouse vaccine model. We found vaccination with PhtD and PcpA provided high IgG antibody titers after vaccination in infant mice, similar to adult mice comparators. PlyD1-specific total IgG was significantly lower in infant mice, with minimal boosting with the second and third vaccinations. Similar isotypes of IgG for PhtD and PlyD1 were generated in infant compared to adult mice. Although lower total specific IgG to all three proteins was elicited in infant than in adult mice, the infant mice were protected from bacteremic pneumonia and sepsis mortality (PlyD1) and had lower lung bacterial burdens (PcpA and PhtD) after challenge. The observed immune responses coupled with bacterial reductions elicited by each of the monovalent proteins support further testing in human infant clinical trials.

Activation of memory Th17 cells by domain 4 pneumolysin in human nasopharynx-associated lymphoid tissue and its association with pneumococcal carriage

Pneumococcal carriage is common in children that may account for the high incidence of disease in this age group. Recent studies in animals suggest an important role for CD4+ T cells, T helper type 17 (Th17) cells in particular, in pneumococcal clearance. Whether this Th17-mediated mechanism operates in humans and what pneumococcal components activate Th17 are unknown. We investigated the ability of domain 4 pneumolysin (D4Ply) to activate CD4+ T cells including Th17 in human nasopharynx-associated lymphoid tissue (NALT) and peripheral blood. We show that D4Ply elicited a prominent CD4+ T-cell proliferative response. More importantly, D4Ply elicited a significant memory Th17 response in NALT, and a moderate response in peripheral blood mononuclear cells (PBMCs). This D4Ply-elicited memory Th17 response was more marked in carriage- than in carriage+ children in both NALT and PBMCs. In contrast, no difference was shown in D4Ply-induced Th1 response between the two groups. We also show D4Ply activated human monocytes and murine macrophages that was in part dependent on Toll-like receptor 4 (TLR-4). Our results support a protective role of Th17 against pneumococcal carriage in human nasopharynx, and identify a novel property of D4Ply to activate Th17 in NALT that may offer an attractive vaccine candidate in intranasal immunization against pneumococcal infection.