Immunization with polyamine transport protein PotD protects mice against systemic infection with Streptococcus pneumoniae

Converso T. Role of the polyamine transporter PotABCD during biofilm formation by Streptococcus pneumoniae

Streptococcus pneumoniae is a bacterium of great global importance, responsible for more than one million deaths per year. This bacterium is commonly acquired in the first years of life and colonizes the upper respiratory tract asymptomatically by forming biofilms that persist for extended times in the nasopharynx. However, under conditions that alter the bacterial environment, such as viral infections, pneumococci can escape from the biofilm and invade other niches, causing local and systemic disease of varying severity. The polyamine transporter PotABCD is required for optimal survival of the organism in the host. Immunization of mice with recombinant PotD can reduce subsequent bacterial colonization. PotD has also been suggested to be involved in pneumococcal biofilm development. Therefore, in this study we aimed to elucidate the role of PotABCD and polyamines in pneumococcal biofilm formation. First, the formation of biofilms was evaluated in the presence of exogenous polyamines-the substrate transported by PotABCD-added to culture medium. Next, a potABCD-negative strain was used to determine biofilm formation in different model systems using diverse levels of complexity from abiotic surface to cell substrate to in vivo animal models and was compared with its wild-type strain. The results showed that adding more polyamines to the medium stimulated biofilm formation, suggesting a direct correlation between polyamines and biofilm formation. Also, deletion of potABCD operon impaired biofilm formation in all models tested. Interestingly, more differences between wild-type and mutant strains were observed in the more complex model, which emphasizes the significance of employing more physiological models in studying biofilm formation.

Polyamine and Ethanolamine Metabolism in Bacteria as an Important Component of Nitrogen Assimilation for Survival and Pathogenicity

Nitrogen is an essential element required for bacterial growth. It serves as a building block for the biosynthesis of macromolecules and provides precursors for secondary metabolites. Bacteria have developed the ability to use various nitrogen sources and possess two enzyme systems for nitrogen assimilation involving glutamine synthetase/glutamate synthase and glutamate dehydrogenase. Microorganisms living in habitats with changeable availability of nutrients have developed strategies to survive under nitrogen limitation. One adaptation is the ability to acquire nitrogen from alternative sources including the polyamines putrescine, cadaverine, spermidine and spermine, as well as the monoamine ethanolamine. Bacterial polyamine and monoamine metabolism is not only important under low nitrogen availability, but it is also required to survive under high concentrations of these compounds. Such conditions can occur in diverse habitats such as soil, plant tissues and human cells. Strategies of pathogenic and non-pathogenic bacteria to survive in the presence of poly- and monoamines offer the possibility to combat pathogens by using their capability to metabolize polyamines as an antibiotic drug target. This work aims to summarize the knowledge on poly- and monoamine metabolism in bacteria and its role in nitrogen metabolism.

Fine-tuning spermidine binding modes in the putrescine binding protein PotF

A profound understanding of the molecular interactions between receptors and ligands is important throughout diverse research, such as protein design, drug discovery, or neuroscience. What determines specificity and how do proteins discriminate against similar ligands? In this study, we analyzed factors that determine binding in two homologs belonging to the well-known superfamily of periplasmic binding proteins, PotF and PotD. Building on a previously designed construct, modes of polyamine binding were swapped. This change of specificity was approached by analyzing local differences in the binding pocket as well as overall conformational changes in the protein. Throughout the study, protein variants were generated and characterized structurally and thermodynamically, leading to a specificity swap and improvement in affinity. This dataset not only enriches our knowledge applicable to rational protein design but also our results can further lay groundwork for engineering of specific biosensors as well as help to explain the adaptability of pathogenic bacteria.

The Effect of Impaired Polyamine Transport on Pneumococcal Transcriptome

Infections due to Streptococcus pneumoniae, a commensal in the nasopharynx, still claim a significant number of lives worldwide. Genome plasticity, antibiotic resistance, and limited serotype coverage of the available polysaccharide-based conjugate vaccines confounds therapeutic interventions to limit the spread of this pathogen. Pathogenic mechanisms that allow successful adaption and persistence in the host could be potential innovative therapeutic targets. Polyamines are ubiquitous polycationic molecules that regulate many cellular processes. We previously reported that deletion of polyamine transport operon potABCD, which encodes a putrescine/spermidine transporter (ΔpotABCD), resulted in an unencapsulated attenuated phenotype. Here, we characterize the transcriptome, metabolome, and stress responses of polyamine transport-deficient S. pneumoniae. Compared with the wild-type strain, the expression of genes involved in oxidative stress responses and the nucleotide sugar metabolism was reduced, while expression of genes involved in the Leloir, tagatose, and pentose phosphate pathways was higher in ΔpotABCD. A metabolic shift towards the pentose phosphate pathway will limit the synthesis of precursors of capsule polysaccharides. Metabolomics results show reduced levels of glutathione and pyruvate in the mutant. Our results also show that the potABCD operon protects pneumococci against hydrogen peroxide and nitrosative stress. Our findings demonstrate the importance of polyamine transport in pneumococcal physiology that could impact in vivo fitness. Thus, polyamine transport in pneumococci represents a novel target for therapeutic interventions.

The expansive effects of polyamines on the metabolism and virulence of Streptococcus pneumoniae

Polyamines are common intracellular metabolites of nearly all cells, and their conservation across a vast diversity of cells suggests critical roles for these compounds in cellular physiology. Most intracellular polyamines are associated with RNA and, subsequently, polyamines have significant effects on transcription and translation. Putrescine and spermidine are the most common polyamines in bacteria. Intracellular polyamine pools in bacteria are tightly controlled by both de novo synthesis and transport. Polyamine homeostasis is emerging as a critical parameter of multiple pathways and physiology with substantial impact on bacterial pathogenesis, including the important human pathogen Streptococcus pneumoniae. Modulation of polyamine metabolism in pneumococci is an important regulator of central metabolism. It has broad effects on virulence factors such as capsule as well as stress responses that ultimately impact the survival of pneumococcus in a host. Polyamine transport protein as a single antigen or in combination with other pneumococcal proteins is shown to be an efficacious immunogen that protects against nasopharyngeal colonization, and invasive disease. A comprehensive description of polyamine metabolic pathways and their intersection with pneumococcal pathogenesis will undoubtedly point to novel approaches for treatment and prevention of pneumococcal disease.

SP_0916 Is an Arginine Decarboxylase That Catalyzes the Synthesis of Agmatine, Which Is Critical for Capsule Biosynthesis in Streptococcus pneumoniae

The global burden of invasive pneumococcal diseases, including pneumonia and sepsis, caused by Streptococcus pneumoniae, a Gram-positive bacterial pathogen, remains a major global health risk. The success of pneumococcus as a pathogen can be attributed to its ability to regulate the synthesis of capsular polysaccharide (CPS) during invasive disease. We previously reported that deletion of a putative lysine decarboxylase (LDC; ΔSP_0916) in pneumococcal serotype 4 (TIGR4) results in reduced CPS. SP_0916 locus is annotated as either an arginine or a LDC in pneumococcal genomes. In this study, by biochemical characterization of the recombinant SP_0916, we determined the substrate specificity of SP_0916 and show that it is an arginine decarboxylase (speA/ADC). We also show that deletion of the polyamine transporter (potABCD) predicted to import putrescine and spermidine results in reduced CPS, while deletion of spermidine synthase (speE) for the conversion of putrescine to spermidine had no impact on the capsule. Targeted metabolomics identified a correlation between reduced levels of agmatine and loss of capsule in ΔspeA and ΔpotABCD, while agmatine levels were comparable between the encapsulated TIGR4 and ΔspeE. Exogenous supplementation of agmatine restored CPS in both ΔpotABCD and ΔspeA. These results demonstrate that agmatine is critical for regulating the CPS, a predominant virulence factor in pneumococci.

Genome-wide screening of lipoproteins in Actinobacillus pleuropneumoniae identifies three antigens that confer protection against virulent challenge

Actinobacillus pleuropneumoniae is an important veterinary pathogen that causes porcine pleuropneumonia. Lipoproteins of bacterial pathogens play pleiotropic roles in the infection process. In addition, many bacterial lipoproteins are antigenic and immunoprotective. Therefore, characterization of lipoproteins is a promising strategy for identification of novel vaccine candidates or diagnostic markers. We cloned 58 lipoproteins from A. pleuropneumoniae JL03 (serovar 3) and expressed them in Escherichia coli. Five proteins with strong positive signals in western blotting analysis were used to immunize mice. These proteins elicited significant antibody responses, and three of them (APJL_0922, APJL_1380 and APJL_1976) generated efficient immunoprotection in mice against lethal heterologous challenge with A. pleuropneumoniae 4074 (serovar 1), both in the active and passive immunization assays. Then immunogenicity of these three lipoproteins (APJL_0922, APJL_1380 and APJL_1976) were further tested in pigs. Results showed that these proteins elicited considerable humoral immune responses and effective protective immunity against virulent A. pleuropneumoniae challenge. Our findings suggest that these three novel lipoproteins could be potential subunit vaccine candidates.

Polyamine Transport Protein PotD Protects Mice against Haemophilus parasuis and Elevates the Secretion of Pro-Inflammatory Cytokines of Macrophage via JNK-MAPK and NF-κB Signal Pathways through TLR4

The potD gene, belonging to the well-conserved ABC (ATP-binding cassette) transport system potABCD, encodes the bacterial substrate-binding subunit of the polyamine transport system. In this study, we found PotD in Haemophilus (Glaesserella) parasuis could actively stimulate both humoral immune and cellular immune responses and elevate lymphocyte proliferation, thus eliciting a Th1-type immune response in a murine immunity and infection model. Stimulation of Raw 264.7 macrophages with PotD validated that Toll-like receptor 4, rather than 2, participated in the positive transcription and expression of pro-inflammatory cytokines IL–1β, IL–6, and TNF–α using qPCR and ELISA. Blocking signal-regulated JNK–MAPK and RelA(p65) pathways significantly decreased PotD-induced pro-inflammatory cytokine production. Overall, we conclude that vaccination of PotD could induce both humoral and cellular immune responses and provide immunoprotection against H. parasuis challenge. The data also suggest that Glaesserella PotD is a novel pro-inflammatory mediator and induces TLR4-dependent pro-inflammatory activity in Raw 264.7 macrophages through JNK–MAPK and RelA(p65) pathways.

Polyamine Synthesis Effects Capsule Expression by Reduction of Precursors in Streptococcus pneumoniae

Streptococcus pneumoniae (pneumococcus, Spn) colonizes the human nasopharynx asymptomatically but can cause infections such as otitis media, and invasive pneumococcal disease such as community-acquired pneumonia, meningitis, and sepsis. Although the success of Spn as a pathogen can be attributed to its ability to synthesize and regulate capsular polysaccharide (CPS) for survival in the host, the mechanisms of CPS regulation are not well-described. Recent studies from our lab demonstrate that deletion of a putative polyamine biosynthesis gene (ΔcadA) in Spn TIGR4 results in the loss of the capsule. In this study, we characterized the transcriptome and metabolome of ΔcadA and identified specific mechanisms that could explain the regulatory role of polyamines in pneumococcal CPS biosynthesis. Our data indicate that impaired polyamine synthesis impacts galactose to glucose interconversion via the Leloir pathway which limits the availability of UDP-galactose, a precursor of serotype 4 CPS, and UDP-N-acetylglucosamine (UDP-GlcNAc), a nucleotide sugar precursor that is at the intersection of CPS and peptidoglycan repeat unit biosynthesis. Reduced carbon flux through glycolysis, coupled with altered fate of glycolytic intermediates further supports impaired synthesis of UDP-GlcNAc. A significant increase in the expression of transketolases indicates a potential shift in carbon flow toward the pentose phosphate pathway (PPP). Higher PPP activity could constitute oxidative stress responses in ΔcadA which warrants further investigation. The results from this study clearly demonstrate the potential of polyamine synthesis, targeted for cancer therapy in human medicine, for the development of novel prophylactic and therapeutic strategies for treating bacterial infections.

Identification of SP1683 as a pneumococcal protein that is protective against nasopharyngeal colonization

Serotype-independent protein-based pneumococcal vaccines represent attractive alternatives to capsular polysaccharide-based vaccines. The aim of this study was to identify novel immunogenic proteins from Streptococcus pneumoniae that may be used in protein-based pneumococcal vaccine. An immunoproteomics approach and a humanized severe combined immunodeficient mouse model were used to identify S. pneumoniae proteins that are immunogenic for the human immune system. Among the several proteins identified, SP1683 was selected, recombinantly produced, and infection and colonization murine models were used to evaluate the capacity of SP1683 to elicit protective responses, in comparison to known pneumococcal immunogenic proteins (PhtD and detoxified pneumolysin, dPly). Immunisation with SP1683 elicited a weaker antibody response than immunisation with PhtD and did not provide protection in the model of invasive disease. However, similar to PhtD, it was able to significantly reduce colonization in the mouse model of nasopharyngeal carriage. Treatment with anti-IL17A and anti-IL17F antibodies abolished the protection against colonization elicited by SP1683 or PhtD + dPly, which indicated that the protection afforded in this model was Th17-dependent. In conclusion, intranasal immunization with the pneumococcal protein SP1683 conferred IL17-dependent protection against nasopharyngeal carriage in mice, but systemic immunization did not protect against invasive disease. These results do not support the use of SP1683 as an isolated pneumococcal vaccine antigen. Nevertheless, SP1683 could be used as a first line of defence in formulations combining several proteins.

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.

Polyamine transporter potABCD is required for virulence of encapsulated but not nonencapsulated Streptococcus pneumoniae

Streptococcus pneumoniae is commonly found in the human nasopharynx and is the causative agent of multiple diseases. Since invasive pneumococcal infections are associated with encapsulated pneumococci, the capsular polysaccharide is the target of licensed pneumococcal vaccines. However, there is an increasing distribution of non-vaccine serotypes, as well as nonencapsulated S. pneumoniae (NESp). Both encapsulated and nonencapsulated pneumococci possess the polyamine oligo-transport operon (potABCD). Previous research has shown inactivation of the pot operon in encapsulated pneumococci alters protein expression and leads to a significant reduction in pneumococcal murine colonization, but the role of the pot operon in NESp is unknown. Here, we demonstrate deletion of potD from the NESp NCC1 strain MNZ67 does impact expression of the key proteins pneumolysin and PspK, but it does not inhibit murine colonization. Additionally, we show the absence of potD significantly increases biofilm production, both in vitro and in vivo. In a chinchilla model of otitis media (OM), the absence of potD does not significantly affect MNZ67 virulence, but it does significantly reduce the pathogenesis of the virulent encapsulated strain TIGR4 (serotype 4). Deletion of potD also significantly reduced persistence of TIGR4 in the lungs but increased persistence of PIP01 in the lungs. We conclude the pot operon is important for the regulation of protein expression and biofilm formation in both encapsulated and NCC1 nonencapsulated Streptococcus pneumoniae. However, in contrast to encapsulated pneumococcal strains, polyamine acquisition via the pot operon is not required for MNZ67 murine colonization, persistence in the lungs, or full virulence in a model of OM. Therefore, NESp virulence regulation needs to be further established to identify potential NESp therapeutic targets.

Polyamine transporter in Streptococcus pneumoniae is essential for evading early innate immune responses in pneumococcal pneumonia

Streptococcus pneumoniae is the most common bacterial etiology of pneumococcal pneumonia in adults worldwide. Genomic plasticity, antibiotic resistance and extreme capsular antigenic variation complicates the design of effective therapeutic strategies. Polyamines are ubiquitous small cationic molecules necessary for full expression of pneumococcal virulence. Polyamine transport system is an attractive therapeutic target as it is highly conserved across pneumococcal serotypes. In this study, we compared an isogenic deletion strain of S. pneumoniae TIGR4 in polyamine transport operon (ΔpotABCD) with the wild type in a mouse model of pneumococcal pneumonia. Our results show that the wild type persists in mouse lung 24 h post infection while the mutant strain is cleared by host defense mechanisms. We show that intact potABCD is required for survival in the host by providing resistance to neutrophil killing. Comparative proteomics analysis of murine lungs infected with wild type and ΔpotABCD pneumococci identified expression of proteins that could confer protection to wild type strain and help establish infection. We identified ERM complex, PGLYRP1, PTPRC/CD45 and POSTN as new players in the pathogenesis of pneumococcal pneumonia. Additionally, we found that deficiency of polyamine transport leads to up regulation of the polyamine synthesis genes speE and cad in vitro.

Characterization of Pneumococcal Genes Involved in Bloodstream Invasion in a Mouse Model

Streptococcus pneumoniae (the pneumococcus) continues to account for significant morbidity and mortality worldwide, causing life-threatening diseases such as pneumonia, bacteremia and meningitis, as well as less serious infections such as sinusitis, conjunctivitis and otitis media. Current polysaccharide vaccines are strictly serotype-specific and also drive the emergence of non-vaccine serotype strains. In this study, we used microarray analysis to compare gene expression patterns of either serotype 4 or serotype 6A pneumococci in the nasopharynx and blood of mice, as a model to identify genes involved in invasion of blood in the context of occult bacteremia in humans. In this manner, we identified 26 genes that were significantly up-regulated in the nasopharynx and 36 genes that were significantly up-regulated in the blood that were common to both strains. Gene Ontology classification revealed that transporter and DNA binding (transcription factor) activities constitute the significantly different molecular functional categories for genes up-regulated in the nasopharynx and blood. Targeted mutagenesis of selected genes from both niches and subsequent virulence and pathogenesis studies identified the manganese-dependent superoxide dismutase (SodA) as most likely to be essential for colonization, and the cell wall-associated serine protease (PrtA) as important for invasion of blood. This work extends our previous analyses and suggests that both PrtA and SodA warrant examination in future studies aimed at prevention and/or control of pneumococcal disease.

Spermidine biosynthesis and transport modulate pneumococcal autolysis

Polyamines are small cationic molecules that have far-reaching roles in biology. In the case of pathogenic bacteria, these functions include those central to their pathogenesis. Streptococcus pneumoniae is a major bacterial pathogen, causing a diverse range of diseases that account for significant morbidity and mortality worldwide. In this work, we characterize the polyamine biosynthetic pathway of S. pneumoniae, demonstrating that this organism produces spermidine from arginine. The synthesis of spermidine was found to be nonessential for growth in a polyamine-free chemically defined medium. However, mutant strains lacking the ability to synthesize or transport spermidine displayed a significant delay in the onset of autolysis. We provide evidence for a model in which spermidine modulates the activity of the major autolysin LytA in the pneumococcal cell wall compartment via interactions with negatively charged molecules, such as teichoic acids.

Comparative proteomic analysis of the membrane proteins of two Haemophilus parasuis strains to identify proteins that may help in habitat adaptation and pathogenesis

Background

Haemophilus parasuis is the causative agent of Glässer’s disease characterized by polyserositis, arthritis, and meningitis in pig, leading to serious economic loss. Despite many years of study, virulence factors and the mechanisms of the entire infection process remain largely unclear. So two-dimensional gel electrophoresis and mass spectrometry were used to search for distinctions at the membrane protein expression level between two H. parasuis isolates aimed at uncovering some proteins potentially involved in habitat adaption and pathogenesis.

Results

A comparative proteomic approach combining two-dimensional gel electrophoresis with mass spectrometry and tandem mass spectrometry was employed to explore the differences among membrane proteomes of a virulent Haemophilus parasuis strain isolated from the lung of a diseased pig and an avirulent strain isolated from the nasal swab of a healthy pig. Differentially expressed protein spots identified by mass spectrometry were annotated and analyzed by bioinformatic interpretation. The mRNA level was determined by quantitative real-time PCR. Proteins representing diverse functional activities were identified. Among them, the tonB-dependent siderophore receptor was a new discovery highlighted for its activity in iron uptake. In addition, periplasmic serine protease and putrescine/spermidine ABC transporter substrate-binding protein were given focus because of their virulence potential. This study revealed that the differentially expressed proteins were important in either the habitat adaption or pathogenesis of H. parasuis.

Conclusions

The outcome demonstrated the presence of some proteins which raise the speculation for their importance in helping in habitat adaption or pathogenesis within the host.

Identification of potential new protein vaccine candidates through pan-surfomic analysis of pneumococcal clinical isolates from adults

Purified polysaccharide and conjugate vaccines are widely used for preventing infections in adults and in children against the Gram-positive bacterium Streptococcus pneumoniae, a pathogen responsible for high morbidity and mortality rates, especially in developing countries. However, these polysaccharide-based vaccines have some important limitations, such as being serotype-dependent, being subjected to losing efficacy because of serotype replacement and high manufacturing complexity and cost. It is expected that protein-based vaccines will overcome these issues by conferring a broad coverage independent of serotype and lowering production costs. In this study, we have applied the “shaving” proteomic approach, consisting of the LC/MS/MS analysis of peptides generated by protease treatment of live cells, to a collection of 16 pneumococcal clinical isolates from adults, representing the most prevalent strains circulating in Spain during the last years. The set of unique proteins identified in all the isolates, called “pan-surfome”, consisted of 254 proteins, which included most of the protective protein antigens reported so far. In search of new candidates with vaccine potential, we identified 32 that were present in at least 50% of the clinical isolates analyzed. We selected four of them (Spr0012, Spr0328, Spr0561 and SP670_2141), whose protection capacity has not yet been tested, for assaying immunogenicity in human sera. All of them induced the production of IgM antibodies in infected patients, thus indicating that they could enter the pipeline for vaccine studies. The pan-surfomic approach shows its utility in the discovery of new proteins that can elicit protection against infectious microorganisms.

Polyamines: emerging players in bacteria-host interactions

Polyamines are small polycationic molecules found in almost all cells and associated with a wide variety of physiological processes. In recent years it has become increasingly clear that, in addition to core physiological functions, polyamines play a crucial role in bacterial pathogenesis. Considerable evidence has built up that bacteria have evolved mechanisms to turn these molecules to their own advantage and a novel standpoint to look at host-bacterium interactions emerges from the interplay among polyamines, host cells and infecting bacteria. In this review, we highlight how human bacterial pathogens have developed their own resourceful strategies to exploit polyamines or manipulate polyamine-related processes to optimize their fitness within the host. Besides contributing to a better understanding of the complex relationship between a pathogen and its host, acquisitions in this field have a significant potential towards the development of novel antibacterial therapeutic approaches.

Development of Burkholderia mallei and pseudomallei vaccines

Burkholderia mallei and Burkholderia pseudomallei are Gram-negative bacteria that cause glanders and melioidosis, respectively. Inhalational infection with either organism can result in severe and rapidly fatal pneumonia. Inoculation by the oral and cutaneous routes can also produce infection. Chronic infection may develop after recovery from acute infection with both agents, and control of infection with antibiotics requires prolonged treatment. Symptoms for both meliodosis and glanders are non-specific, making diagnosis difficult. B. pseudomallei can be located in the environment, but in the host, B. mallei and B. psedomallei are intracellular organisms, and infection results in similar immune responses to both agents. Effective early innate immune responses are critical to controlling the early phase of the infection. Innate immune signaling molecules such as TLR, NOD, MyD88, and pro-inflammatory cytokines such as IFN-γ and TNF-α play key roles in regulating control of infection. Neutrophils and monocytes are critical cells in the early infection for both microorganisms. Both monocytes and macrophages are necessary for limiting dissemination of B. pseudomallei. In contrast, the role of adaptive immune responses in controlling Burkholderia infection is less well understood. However, T cell responses are critical for vaccine protection from Burkholderia infection. At present, effective vaccines for prevention of glanders or meliodosis have not been developed, although recently development of Burkholderia vaccines has received renewed attention. This review will summarize current and past approaches to develop B. mallei and B. pseudomalllei vaccines, with emphasis on immune mechanisms of protection and the challenges facing the field. At present, immunization with live attenuated bacteria provides the most effective and durable immunity, and it is important therefore to understand the immune correlates of protection induced by live attenuated vaccines. Subunit vaccines have typically provided less robust immunity, but are safer to administer to a wider variety of people, including immune compromised individuals because they do not reactivate or cause disease. The challenges facing B. mallei and B. pseudomalllei vaccine development include identification of broadly protective antigens, design of efficient vaccine delivery and adjuvant systems, and a better understanding of the correlates of protection from both acute and chronic infection.

The effects of methionine acquisition and synthesis on Streptococcus pneumoniae growth and virulence

Bacterial pathogens need to acquire nutrients from the host, but for many nutrients their importance during infection remain poorly understood. We have investigated the importance of methionine acquisition and synthesis for Streptococcus pneumoniae growth and virulence using strains with gene deletions affecting a putative methionine ABC transporter lipoprotein (Sp_0149, metQ) and/or methionine biosynthesis enzymes (Sp_0585 - Sp_0586, metE and metF). Immunoblot analysis confirmed MetQ was a lipoprotein and present in all S. pneumoniae strains investigated. However, vaccination with MetQ did not prevent fatal S. pneumoniae infection in mice despite stimulating a strong specific IgG response. Tryptophan fluorescence spectroscopy and isothermal titration calorimetry demonstrated that MetQ has both a high affinity and specificity for L-methionine with a K_D of ∼25 nM, and a ΔmetQ strain had reduced uptake of C₁₄-methionine. Growth of the ΔmetQ/ΔmetEF strain was greatly impaired in chemically defined medium containing low concentrations of methionine and in blood but was partially restored by addition of high concentrations of exogenous methionine. Mixed infection models showed no attenuation of the ΔmetQ, ΔmetEF and ΔmetQ/ΔmetEF strains in their ability to colonise the mouse nasopharnyx. In a mouse model of systemic infection although significant infection was established in all mice, there were reduced spleen bacterial CFU after infection with the ΔmetQ/ΔmetEF strain compared to the wild-type strain. These data demonstrate that Sp_0149 encodes a high affinity methionine ABC transporter lipoprotein and that Sp_0585 – Sp_0586 are likely to be required for methionine synthesis. Although Sp_0149 and Sp_0585-Sp_0586 make a contribution towards full virulence, neither was essential for S. pneumoniae survival during infection.

Protection against pneumococcal infection elicited by immunization with glutamyl tRNA synthetase, polyamine transport protein D and sortase A

Protein-based vaccines are considered to be the next-generation of pneumococcal vaccines. Here we evaluated the protection elicited by immunization with recombinant glutamyl tRNA synthetase (Gts), polyamine transport protein D (PotD) and sortase A (SrtA) antigens in preclinical mouse models. In mucosal immunization studies, intranasal immunization with either Gts, PotD or SrtA could significantly reduce pneumococcal nasopharyngeal and lung colonization and significantly increase mice survival times following invasive pneumococcal challenge, and combinations of these antigens could enhance this protection. In systemic immunization studies, intraperitoneal immunization with multiple protein antigens also provided better protection against pneumococcal sepsis caused by different pneumococcal strains. Finally, passive immunization studies showed an additive effect by using multiple anti-sera when compared to single anti-sera. Therefore, a multicomponent protein-based pneumococcal vaccine composed of Gts, PotD or SrtA could confer protection against pneumococcal colonization as well as invasive infections in terms of efficacy of protection and serotype coverage.

Active Immunization with Pneumolysin versus 23-Valent Polysaccharide Vaccine for Streptococcus pneumoniae Keratitis

PURPOSE

The purpose of this study was to determine whether active immunization against pneumolysin (PLY), or polysaccharide capsule, protects against the corneal damage associated with Streptococcus pneumoniae keratitis.

METHODS

New Zealand White rabbits were actively immunized with Freund's adjuvant mixed with pneumolysin toxoid (ψPLY), Pneumovax 23 (PPSV23; Merck, Whitehouse Station, NJ), or phosphate-buffered saline (PBS), before corneal infection with 10⁵ colony-forming units (CFU) of S. pneumoniae. Serotype-specific rabbit polyclonal antisera or mock antisera were passively administered to rabbits before either intravenous infection with 10¹¹ CFU S. pneumoniae or corneal infection with 10⁵ CFU of S. pneumoniae.

RESULTS

After active immunization, clinical scores of corneas of the rabbits immunized with ψPLY and Freund's adjuvant were significantly lower than scores of the rabbits that were mock immunized with PBS and Freund's adjuvant or with PPSV23 and Freund's adjuvant at 48 hours after infection (P ≤ 0.0010), whereas rabbits immunized with PPSV23 and Freund's adjuvant failed to show differences in clinical scores compared with those in mock-immunized rabbits (P = 1.00) at 24 and 48 hours after infection. Antisera from rabbits actively immunized with PPSV23 and Freund's adjuvant were nonopsonizing. Bacterial loads recovered from infected corneas were higher for the ψPLY- and PPSV23-immunized rabbits after infection with WU2, when compared with the mock-immunized rabbits (P ≤ 0.007). Conversely, after infection with K1443, the ψPLY-immunized rabbits had lower bacterial loads than the control rabbits (P = 0.0008). Quantitation of IgG, IgA, and IgM in the sera of ψPLY-immunized rabbits showed high concentrations of PLY-specific IgG. Furthermore, anti-PLY IgG purified from ψPLY-immunized rabbits neutralized the cytolytic effects of PLY on human corneal epithelial cells. Passive administration of serotype-specific antisera capable of opsonizing and killing S. pneumoniae protected against pneumococcal bacteremia (P ≤ 0.05), but not against keratitis (P ≥ 0.476).

CONCLUSIONS

Active immunization with pneumococcal capsular polysaccharide and Freund's adjuvant fails to produce opsonizing antibodies, and passive administration of serotype specific opsonizing antibodies offers no protection against pneumococcal keratitis in the rabbit, whereas active immunization with the conserved protein virulence factor PLY and Freund's adjuvant is able to reduce corneal inflammation associated with pneumococcal keratitis, but has variable effects on bacterial loads in the cornea.

Recombinant polyamine-binding protein of Synechocystis sp. PCC 6803 specifically binds to and is induced by polyamines

His-tagged Synechocystis sp. PCC 6803 PotD protein (rPotD) involved in polyamine transport was overexpressed in Escherichia coli. The purified rPotD showed saturable binding kinetics with radioactively labeled polyamines. The rPotD exhibited a similar binding characteristic for three polyamines, with putrescine having less preference. The K(d) values for putrescine, spermine, and spermidine were 13.2, 8.3, and 7.8 µM, respectively. Binding of rPotD with polyamines was maximal at pH 8.0. Docking of these polyamines into the homology model of Synechocystis PotD showed that all three polyamines are able to interact with Synechocystis PotD. The binding modes of the docked putrescine and spermidine in Synechocystis are similar to those of PotF and PotD in E. coli, respectively. Competition experiments showed specific binding of rPotD with polyamines. The presence of putrescine and spermidine in the growth medium could induce an increase in PotD contents, suggesting the role of PotD in mediating the transport of polyamine in Synechocystis sp. PCC 6803.

Next generation pneumococcal vaccines

Currently licensed pneumococcal vaccines are based on the generation of antibodies to the pneumococcal polysaccharide, of which there are more than 90 different types. While these vaccines are highly effective against the serotypes included, their high cost and limited serotype coverage limit their usefulness worldwide, particularly in low resource areas. Thus alternative or adjunctive options are being actively pursued. This review will present these various approaches, including variations of the polysaccharide-protein conjugate strategy, protein-based strategies, and whole cell pneumococcal vaccines. The immunological basis for these different approaches is discussed as well.

Cloning and optimization of induction conditions for mature PsaA (pneumococcal surface adhesin A) expression in Escherichia coli and recombinant protein stability during long-term storage

The gene corresponding to mature PsaA from Streptococcus pneumoniae serotype 14 was cloned into a plasmid with kanamycin resistance and without a purification tag in Escherichia coli to express high levels of the recombinant protein for large-scale production as a potential vaccine candidate or as a carrier for polysaccharide conjugation at Bio-Manguinhos/Fiocruz. The evaluation of induction conditions (IPTG concentration, temperature and time) in E. coli was accomplished by experimental design techniques to enhance the expression level of mature recombinant PsaA (rPsaA). The optimization of induction process conditions led us to perform the recombinant protein induction at 25°C for 16 h, with 0.1mM IPTG in Terrific Broth medium. At these conditions, the level of mature rPsaA expression obtained in E. coli BL21 (DE3) Star by pET28a induction with IPTG was in the range of 0.8 g/L of culture medium, with a 10-fold lower concentration of inducer than usually employed, which contributes to a less expensive process. Mature rPsaA expressed in E. coli BL21 (DE3) Star accounted for approximately 30-35% of the total protein. rPsaA purification by ion exchange allowed the production of high-purity recombinant protein without fusion tags. The results presented in this work confirm that the purified recombinant protein maintains its stability and integrity for long periods of time in various storage conditions (temperatures of 4 or -70°C using different cryoprotectors) and for at least 3 years at 4 or -70°C in PBS. The conformation of the stored protein was confirmed using circular dichroism. Mature rPsaA antigenicity was proven by anti-rPsaA mouse serum recognition through western blot analysis, and no protein degradation was detected after long periods of storage.

Polyamine biosynthesis and transport mechanisms are crucial for fitness and pathogenesis of Streptococcus pneumoniae

Polyamines such as cadaverine, putrescine and spermidine are polycationic molecules that have pleiotropic effects on cells via their interaction with nucleic acids. Streptococcus pneumoniae (the pneumococcus) is a Gram-positive pathogen capable of causing pneumonia, septicaemia, otitis media and meningitis. Pneumococci have a polyamine transport operon (potABCD) responsible for the binding and transport of putrescine and spermidine, and can synthesize cadaverine and spermidine using their lysine decarboxylase (cad) and spermidine synthase (speE) enzymes. Previous studies from our laboratory have shown that an increase in PotD expression is seen following exposure to various stresses, while during infection, potD inactivation significantly attenuates pneumococcal virulence, and anti-PotD immune responses are protective in mice. In spite of their relative importance, not much is known about the global contribution of polyamine biosynthesis and transport pathways to pneumococcal disease. Mutants deficient in polyamine biosynthesis (ΔspeE or Δcad) or transport genes (ΔpotABCD) were constructed and were found to be attenuated in murine models of pneumococcal colonization and pneumonia, either alone or in competition with the wild-type strain. The ΔspeE mutant was also attenuated during invasive disease, while the potABCD and cad genes seemed to be dispensable. HPLC analyses showed reduced intracellular polyamine levels in all mutant strains compared with wild-type bacteria. High-throughput proteomic analyses indicated reduced expression of growth, replication and virulence factors in mutant strains. Thus, polyamine biosynthesis and transport mechanisms are intricately linked to the fitness, survival and pathogenesis of the pneumococcus in host microenvironments, and may represent important targets for prophylactic and therapeutic interventions.

Pooled protein immunization for identification of cell surface antigens in Streptococcus sanguinis

Background

Available bacterial genomes provide opportunities for screening vaccines by reverse vaccinology. Efficient identification of surface antigens is required to reduce time and animal cost in this technology. We developed an approach to identify surface antigens rapidly in Streptococcus sanguinis, a common infective endocarditis causative species.

Methods and Findings

We applied bioinformatics for antigen prediction and pooled antigens for immunization. Forty-seven surface-exposed proteins including 28 lipoproteins and 19 cell wall-anchored proteins were chosen based on computer algorithms and comparative genomic analyses. Eight proteins among these candidates and 2 other proteins were pooled together to immunize rabbits. The antiserum reacted strongly with each protein and with S. sanguinis whole cells. Affinity chromatography was used to purify the antibodies to 9 of the antigen pool components. Competitive ELISA and FACS results indicated that these 9 proteins were exposed on S. sanguinis cell surfaces. The purified antibodies had demonstrable opsonic activity.

Conclusions

The results indicate that immunization with pooled proteins, in combination with affinity purification, and comprehensive immunological assays may facilitate cell surface antigen identification to combat infectious diseases.

ATP-Binding Cassette Systems of Brucella

Brucellosis is a prevalent zoonotic disease and is endemic in the Middle East, South America, and other areas of the world. In this study, complete inventories of putative functional ABC systems of five Brucella species have been compiled and compared. ABC systems of Brucella melitensis 16M, Brucella abortus 9-941, Brucella canis RM6/66, Brucella suis 1330, and Brucella ovis 63/290 were identified and aligned. High numbers of ABC systems, particularly nutrient importers, were found in all Brucella species. However, differences in the total numbers of ABC systems were identified (B. melitensis, 79; B. suis, 72; B. abortus 64; B. canis, 74; B. ovis, 59) as well as specific differences in the functional ABC systems of the Brucella species. Since B. ovis is not known to cause human brucellosis, functional ABC systems absent in the B. ovis genome may represent virulence factors in human brucellosis.

Immunoproteomic analysis of outer membrane proteins and extracellular proteins of Actinobacillus pleuropneumoniae JL03 serotype 3

Background

Actinobacillus pleuropneumoniae is the causative agent of porcine contagious pleuropneumonia, a highly contagious respiratory infection in pigs, and all the 15 serotypes are able to cause disease. Current vaccines including subunit vaccines could not provide satisfactory protection against A. pleuropneumoniae. In this study, the immunoproteomic approach was applied to the analysis of extracellular and outer membrane proteins of A. pleuropneumoniae JL03 serotype 3 for the identification of novel immunogenic proteins for A. pleuropneumoniae.

Results

A total of 30 immunogenic proteins were identified from outer membrane and extracellular proteins of JL03 serotype 3, of which 6 were known antigens and 24 were novel immunogenic proteins for A. pleuropneumoniae.

Conclusion

These data provide information about novel immunogenic proteins for A. pleuropneumoniae serotype 3, and are expected to aid in development of novel vaccines against A. pleuropneumoniae.

Identification of conserved antigens for early serodiagnosis of relapsing fever Borrelia

Borrelia hermsii is a blood-borne pathogen transmitted by the argasid tick Ornithodoros hermsi. Since spirochaete clearance in mice is associated with an IgM-mediated response, an immunoproteomic analysis was used to identify proteins reactive with IgM. We report that IgM from both mice and human patients infected with B. hermsii not only reacted with the previously identified variable membrane proteins but also identified candidate antigens including heat-shock proteins, an adhesin protein, ABC transporter proteins, flagellar proteins, housekeeping proteins, an immune evasion protein, and proteins with unknown function. Furthermore, IgM reactivity to recombinant glycerophosphodiester phosphodiesterase was detected during early spirochaete infection and prior to a detectable IgG response. Lastly, a conserved hypothetical protein was produced in Escherichia coli and tested with immune serum against B. hermsii and Borrelia recurrentis. These results identify a much larger set of immunoreactive proteins, and could help in the early serodiagnosis of this tick-borne infection.

Microarray-based comparative genomic profiling of reference strains and selected Canadian field isolates of Actinobacillus pleuropneumoniae

Background

Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia, is a highly contagious respiratory pathogen that causes severe losses to the swine industry worldwide. Current commercially-available vaccines are of limited value because they do not induce cross-serovar immunity and do not prevent development of the carrier state. Microarray-based comparative genomic hybridizations (M-CGH) were used to estimate whole genomic diversity of representative Actinobacillus pleuropneumoniae strains. Our goal was to identify conserved genes, especially those predicted to encode outer membrane proteins and lipoproteins because of their potential for the development of more effective vaccines.

Results

Using hierarchical clustering, our M-CGH results showed that the majority of the genes in the genome of the serovar 5 A. pleuropneumoniae L20 strain were conserved in the reference strains of all 15 serovars and in representative field isolates. Fifty-eight conserved genes predicted to encode for outer membrane proteins or lipoproteins were identified. As well, there were several clusters of diverged or absent genes including those associated with capsule biosynthesis, toxin production as well as genes typically associated with mobile elements.

Conclusion

Although A. pleuropneumoniae strains are essentially clonal, M-CGH analysis of the reference strains of the fifteen serovars and representative field isolates revealed several classes of genes that were divergent or absent. Not surprisingly, these included genes associated with capsule biosynthesis as the capsule is associated with sero-specificity. Several of the conserved genes were identified as candidates for vaccine development, and we conclude that M-CGH is a valuable tool for reverse vaccinology.

Mucosal immunization with polyamine transport protein D (PotD) protects mice against nasopharyngeal colonization with Streptococcus pneumoniae

Streptococcus pneumoniae is an encapsulated pathogen that can cause invasive disease following colonization of the nasopharynx. Targeting colonization of mucosal surfaces may, therefore, be the best approach for vaccination to prevent pneumococcal invasive disease. Previous studies in our laboratory have shown that immunization with recombinant polyamine transport protein D (PotD) protects mice against systemic pneumococcal infections. In this study we investigated the efficacy of mucosal immunization with rPotD to protect against pneumococcal carriage and invasion in a murine model. Mice were intranasally immunized with either rPotD and cholera toxin B subunit (CTB) or CTB alone. Significantly less pneumococci were recovered from the nasopharynx of immunized mice compared to the control animals following intranasal challenge with either EF3030 (serotype 19F) (P < 0.05) or an invasive serotype 4 isolate (TIGR4) (P < 0.05). PotD immunized mice also had lesser bacteria in their sinus tissues (P < 0.05), brains (P < 0.05), lungs and olfactory bulbs following intranasal challenge with TIGR4. ELISA analysis demonstrated the presence of IgG antibodies to PotD in the serum and IgA antibodies in the saliva. These results indicate that mucosal immunization with PotD generates both mucosal and systemic immune responses and prevents establishment of nasopharyngeal carriage by multiple pneumococcal serotypes. Thus, PotD is a potentially important antigen for development of a pneumococcal protein vaccine.

An immunoproteomic approach for identification of clinical biomarkers of Whipple's disease

Whipple's disease (WD) is a chronic multisystemic infection, caused by the bacterium Tropheryma whipplei. The main clinical presentations are classic WD (CWD) with histologic lesions in the gastrointestinal tract, endocarditis, and isolated neurologic infection. The current strategy for diagnosis remains invasive.The present study aimed to select the protein candidates for serological diagnosis of WD. The first step was to identify candidate proteins by an immunoproteomic approach combining 2-DE using a total extract of a T. whipplei, immunoblotting, and MS. The second step was to validate the discovered biomarkers using a recombinant protein-based ELISA. Serum samples from 18 patients with WD and from 54 control individuals were tested. A sugar ABC transporter, TWT328 (sensitivity (Se) 61%, specificity (Sp) 87%, positive predictive value (PPV) 61%, negative predictive value (NPV) 87%, and positive likelihood ratio (PLR) 4.69) was the best marker for development of serodiagnosis for CWD. We also obtained a reproducible immunoreactive protein pattern for patients with isolated neurological infection due to T. whipplei (Se 100%, Sp 93%, PPV 55.5%, NPV 100%, and PLR 13.51) as an encouraging step towards noninvasive diagnosis of this particular manifestation. Nine recombinant candidates have been successfully screened with serum samples. Results from these ELISA assays skewed with those obtained with immunoblots.

Physiological polyamines: simple primordial stress molecules

Physiological polyamines are ubiquitous polycations with pleiotropic biochemical activities, including regulation of gene expression, cell proliferation and modulation of cell signalling. Reports that the polyamines with cytoprotective activities were induced by diverse stresses raised the hypothesis that physiological polyamines may play a role in inducing stress response. In a wide range of organisms, physiological polyamines were not only induced by diverse stresses, such as reactive oxygen species (ROS), heat, ultraviolet (UV) and psychiatric stress but were able to confer beneficial effects for survival. Recent biochemical and genetic evidences show that polyamines can function as an ROS scavenger, acid tolerance factor and chemical chaperone, and positive regulators for expression of stress response genes which may explain their protective functions against diverse stresses. Taken together, these data suggest that physiological polyamines can function as primordial stress molecules in bacteria, plants and mammals, and may play an essential role in regulation of pathogen-host interactions.

Identification of a LolC homologue in Burkholderia pseudomallei, a novel protective antigen for melioidosis

Melioidosis is an emerging disease of humans in Southeast Asia and tropical Australia. The bacterium causing this disease, Burkholderia pseudomallei, is also considered a bioterrorism agent, and as yet there is no licensed vaccine for preventing B. pseudomallei infection. In this study, we evaluated selected proteins (LolC, PotF, and OppA) of the ATP-binding cassette systems of B. pseudomallei as candidate vaccine antigens. Nonmembrane regions of the B. pseudomallei proteins were expressed and purified from Escherichia coli and then evaluated as vaccine candidates in an established mouse model of B. pseudomallei infection. When delivered with the monophosphoryl lipid A-trehalose dicorynomycolate adjuvant, the proteins stimulated antigen-specific humoral and cellular immune responses. Immunization with LolC or PotF protein domains afforded significant protection against a subsequent challenge with B. pseudomallei. The most promising vaccine candidate, LolC, provided a greater level of protection when it was administered with immune-stimulating complexes complexed with CpG oligodeoxynucleotide 10103. Immunization with LolC also protected against a subsequent challenge with a heterologous strain of B. pseudomallei, demonstrating the potential utility of this protein as a vaccine antigen for melioidosis.