Streptococcus pneumoniae surface-exposed glutamyl tRNA synthetase, a putative adhesin, is able to induce a partially protective immune response in mice

Streptococcus pneumoniae Cell Wall-Localized Trigger Factor Elicits a Protective Immune Response and Contributes to Bacterial Adhesion to the Host

Trigger factor (TF) has a known cytoplasmic function as a chaperone. In a previous study we showed that pneumococcal TF is also cell-wall localized and this finding combined with the immunogenic characteristic of TF, has led us to determine the vaccine potential of TF and decipher its involvement in pneumococcal pathogenesis. Bioinformatic analysis revealed that TF is conserved among pneumococci and has no human homologue. Immunization of mice with recombinant (r)TF elicited a protective immune response against a pneumococcal challenge, suggesting that TF contributes to pneumococcal pathogenesis. Indeed, rTF and an anti-rTF antiserum inhibited bacterial adhesion to human lung derived epithelial cells, indicating that TF contributes to the bacterial adhesion to the host. Moreover, bacteria lacking TF demonstrated reduced adhesion, in vitro, to lung-derived epithelial cells, neural cells and glial cells. The reduced adhesion could be restored by chromosomal complementation. Furthermore, bacteria lacking TF demonstrated significantly reduced virulence in a mouse model. Taken together, the ability of rTF to elicit a protective immune response, involvement of TF in bacterial adhesion, conservation of the protein among pneumococcal strains and the lack of human homologue, all suggest that rTF can be considered as a future candidate vaccine with a much broader coverage as compared to the currently available pneumococcal vaccines.

Identification of Clostridium difficile Immunoreactive Spore Proteins of the Epidemic Strain R20291

PURPOSE

Clostridium difficile infections are the leading cause of diarrhea associated with the use of antibiotics. During infection, C. difficile initiates a sporulation cycle leading to the persistence of C. difficile spores in the host and disease dissemination. The development of vaccine and passive immunization therapies against C. difficile has focused on toxins A and B. In this study, an immunoproteome-based approach to identify immunogenic proteins located on the outer layers of C. difficile spores as potential candidates for the development of immunotherapy and/or diagnostic methods against this devastating infection is used.

EXPERIMENTAL DESIGN

To identify potential immunogenic proteins on the surface of C. difficile R20291, spore coat/exosporium extracts are separated by 2D electrophoresis (2-DE) and analyzed for reactivity against C. difficile spore-specific goat sera. Finally, the selected spots are in-gel digested with chymotrypsin, peptides generated are separated by nanoUPLC followed by MS/MS using Quad-TOF-MS, corroborated by Ultimate 3000RS-nano-UHPLC coupled to Q-Exactive-Plus-Orbitrap MS.

RESULTS

The analysis identify five immunoreactive proteins: spore coat proteins CotE, CotA, and CotCB; exosporium protein CdeC; and a cytosolic methyltransferase.

CONCLUSION

This data provides a list of spore surface protein candidates as antigens for vaccine development against C. difficile infections.

Novel Immunoprotective Proteins of Streptococcus pneumoniae Identified by Opsonophagocytosis Killing Screen

The success of polysaccharide conjugate vaccines represents a major advance in the prevention of pneumococcal disease, but the power of these vaccines is limited by partial spectrum of coverage and high cost. Vaccines using immunoprotective proteins are a promising alternative type of pneumococcal vaccines. In this study, we constructed a library of antisera against conserved pneumococcal proteins predicted to be associated with cell surface or virulence using a combination of bioinformatic prediction and immunization of rabbits with recombinant proteins. Screening of the library by an opsonophagocytosis killing (OPK) assay identified the OPK-positive antisera, which represented 15 (OPK-positive) proteins. Further tests showed that virtually all of these OPK-positive antisera conferred passive protection against lethal infection of virulent pneumococci. More importantly, immunization with recombinant forms of three OPK-positive proteins (SP148, PBP2b, and ScpB), alone or in combination, conferred significant protection against lethal challenge of pneumococcal strains representing capsular serotypes 3, 4, and 6A in a mouse sepsis model. To our best knowledge, this work represents the first example in which novel vaccine candidates are successfully identified by the OPK screening. Our data have also provided further confirmation that the OPK activity may serve as a reliable in vitro surrogate for evaluating vaccine efficacy of pneumococcal proteins.

Flavin Reductase Contributes to Pneumococcal Virulence by Protecting from Oxidative Stress and Mediating Adhesion and Elicits Protection Against Pneumococcal Challenge

Pneumococcal flavin reductase (FlaR) is known to be cell-wall associated and possess age dependent antigenicity in children. This study aimed at characterizing FlaR and elucidating its involvement in pneumococcal physiology and virulence. Bioinformatic analysis of FlaR sequence identified three-conserved cysteine residues, suggesting a transition metal-binding capacity. Recombinant FlaR (rFlaR) bound Fe²⁺ and exhibited FAD-dependent NADP-reductase activity, which increased in the presence of cysteine or excess Fe²⁺ and inhibited by divalent-chelating agents. flaR mutant was highly susceptible to H₂O₂ compared to its wild type (WT) and complemented strains, suggesting a role for FlaR in pneumococcal oxidative stress resistance. Additionally, flaR mutant demonstrated significantly decreased mice mortality following intraperitoneal infection. Interestingly, lack of FlaR did not affect the extent of phagocytosis by primary mouse peritoneal macrophages but reduced adhesion to A549 cells compared to the WT and complemented strains. Noteworthy are the findings that immunization with rFlaR elicited protection in mice against intraperitoneal lethal challenge and anti-FlaR antisera neutralized bacterial virulence. Taken together, FlaR's roles in pneumococcal physiology and virulence, combined with its lack of significant homology to human proteins, point towards rFlaR as a vaccine candidate.

Enhancing the Biological Relevance of Machine Learning Classifiers for Reverse Vaccinology

Reverse vaccinology (RV) is a bioinformatics approach that can predict antigens with protective potential from the protein coding genomes of bacterial pathogens for subunit vaccine design. RV has become firmly established following the development of the BEXSERO^® vaccine against Neisseria meningitidis serogroup B. RV studies have begun to incorporate machine learning (ML) techniques to distinguish bacterial protective antigens (BPAs) from non-BPAs. This research contributes significantly to the RV field by using permutation analysis to demonstrate that a signal for protective antigens can be curated from published data. Furthermore, the effects of the following on an ML approach to RV were also assessed: nested cross-validation, balancing selection of non-BPAs for subcellular localization, increasing the training data, and incorporating greater numbers of protein annotation tools for feature generation. These enhancements yielded a support vector machine (SVM) classifier that could discriminate BPAs (n = 200) from non-BPAs (n = 200) with an area under the curve (AUC) of 0.787. In addition, hierarchical clustering of BPAs revealed that intracellular BPAs clustered separately from extracellular BPAs. However, no immediate benefit was derived when training SVM classifiers on data sets exclusively containing intra- or extracellular BPAs. In conclusion, this work demonstrates that ML classifiers have great utility in RV approaches and will lead to new subunit vaccines in the future.

Adhesion and invasion of Streptococcus pneumoniae to primary and secondary respiratory epithelial cells

The interaction between Streptococcus pneumoniae (S. pneumoniae) and the mucosal epithelial cells of its host is a prerequisite for pneumococcal disease development, yet the specificity of this interaction between different respiratory cells is not fully understood. In the present study, three areas were examined: i) The capability of the encapsulated S. pneumoniae serotype 3 strain (WU2) to adhere to and invade primary nasal‑derived epithelial cells in comparison to primary oral‑derived epithelial cells, A549 adenocarcinoma cells and BEAS‑2B viral transformed bronchial cells; ii) the capability of the unencapsulated 3.8DW strain (a WU2 derivative) to adhere to and invade the same cells over time; and iii) the ability of various genetically‑unrelated encapsulated and unencapsulated S. pneumoniae strains to adhere to and invade A549 lung epithelial cells. The results of the present study demonstrated that the encapsulated WU2 strain adhesion to and invasion of primary nasal epithelial cells was greatest, followed by BEAS‑2B, A549 and primary oral epithelial cells. By contrast, the unencapsulated 3.8‑DW strain invaded oral epithelial cells significantly more efficiently when compared to the nasal epithelial cells. In addition, unencapsulated S. pneumoniae strains adhered to and invaded the A459 cells significantly more efficiently than the encapsulated strains; this is consistent with previously published data. In conclusion, the findings presented in the current study indicated that the adhesion and invasion of the WU2 strain to primary nasal epithelial cells was more efficient compared with the other cultured respiratory epithelial cells tested, which corresponds to the natural course of S. pneumoniae infection and disease development. The target cell preference of unencapsulated strains was different from that of the encapsulated strains, which may be due to the exposure of cell wall proteins.

Streptococcus pneumoniae Cell-Wall-Localized Phosphoenolpyruvate Protein Phosphotransferase Can Function as an Adhesin: Identification of Its Host Target Molecules and Evaluation of Its Potential as a Vaccine

In Streptococcus pneumonia, phosphoenolpyruvate protein phosphotransferase (PtsA) is an intracellular protein of the monosaccharide phosphotransferase systems. Biochemical and immunostaining methods were applied to show that PtsA also localizes to the bacterial cell-wall. Thus, it was suspected that PtsA has functions other than its main cytoplasmic enzymatic role. Indeed, recombinant PtsA and anti-rPtsA antiserum were shown to inhibit adhesion of S. pneumoniae to cultured human lung adenocarcinoma A549 cells. Screening of a combinatorial peptide library expressed in a filamentous phage with rPtsA identified epitopes that were capable of inhibiting S. pneumoniae adhesion to A549 cells. The insert peptides in the phages were sequenced, and homologous sequences were found in human BMPER, multimerin1, protocadherin19, integrinβ4, epsin1 and collagen type VIIα1 proteins, all of which can be found in A549 cells except the latter. Six peptides, synthesized according to the homologous sequences in the human proteins, specifically bound rPtsA in the micromolar range and significantly inhibited pneumococcal adhesion in vitro to lung- and tracheal-derived cell lines. In addition, the tested peptides inhibited lung colonization after intranasal inoculation of mice with S. pneumoniae. Immunization with rPtsA protected the mice against a sublethal intranasal and a lethal intravenous pneumococcal challenge. In addition, mouse anti rPtsA antiserum reduced bacterial virulence in the intravenous inoculation mouse model. These findings showed that the surface-localized PtsA functions as an adhesin, PtsA binding peptides derived from its putative target molecules can be considered for future development of therapeutics, and rPtsA should be regarded as a candidate for vaccine development.

Immunoproteomics: the key to discovery of new vaccine antigens against bacterial respiratory infections

The increase in antibiotic resistance and the shortage of new antimicrobials to prevent difficult bacterial infections underlines the importance of prophylactic therapies to prevent infection by bacterial pathogens. Vaccination has reduced the incidence of many serious diseases, including respiratory bacterial infections. However, there are many pathogens for which no vaccine is available and some vaccines are not effective among all age groups or among immunocompromised individuals. Immunoproteomics is a powerful technique which has been used to identify potential vaccine candidates to protect against pathogenic bacteria. The combination of proteomics with the detection of immunoreactive antigens using serum highlights immunogenic proteins that are expressed during infection. This is particularly useful when patient serum is used as the antigens that promote a humoral response during human infection are identified. This review outlines examples of vaccine candidates that have been identified using immunoproteomics and have successfully protected animals against challenge when tested in immunisation studies. Many immunoreactive proteins are common to several unrelated pathogens, however some of these are not always protective in animal immunisation and challenge studies. Furthermore, examples of well-established immunogens, including Bordetella pertussis antigen FHA were not detected in immunoproteomics studies, indicating that this technology may underrepresent the immunoreactive proteins in a pathogen. Although only one step in the pathway towards an efficacious approved vaccine, immunoproteomics is an important technology in the identification of novel vaccine antigens.

NADH oxidase functions as an adhesin in Streptococcus pneumoniae and elicits a protective immune response in mice

The initial event in disease caused by S. pneumoniae is adhesion of the bacterium to respiratory epithelial cells, mediated by surface expressed molecules including cell-wall proteins. NADH oxidase (NOX), which reduces free oxygen to water in the cytoplasm, was identified in a non-lectin enriched pneumococcal cell-wall fraction. Recombinant NOX (rNOX) was screened with sera obtained longitudinally from children and demonstrated age-dependent immunogenicity. NOX ablation in S. pneumoniae significantly reduced bacterial adhesion to A549 epithelial cells in vitro and their virulence in the intranasal or intraperitoneal challenge models in mice, compared to the parental strain. Supplementation of Δnox WU2 with the nox gene restored its virulence. Saturation of A549 target cells with rNOX or neutralization of cell-wall residing NOX using anti-rNOX antiserum decreased adhesion to A549 cells. rNOX-binding phages inhibited bacterial adhesion. Moreover, peptides derived from the human proteins contactin 4, chondroitin 4 sulfotraferase and laminin5, homologous to the insert peptides in the neutralizing phages, inhibited bacterial adhesion to the A549 cells. Furthermore, rNOX immunization of mice elicited a protective immune response to intranasal or intraperitoneal S. pneumoniae challenge, whereas pneumococcal virulence was neutralized by anti-rNOX antiserum prior to intraperitoneal challenge. Our results suggest that in addition to its enzymatic activity, NOX contributes to S. pneumoniae virulence as a putative adhesin and thus peptides derived from its target molecules may be considered for the treatment of pneumococcal infections. Finally, rNOX elicited a protective immune response in both aerobic and anaerobic environments, which renders NOX a candidate for future pneumococcal vaccine.

Immune response and protective efficacy of S9 ribosomal protein of Streptococcus pneumoniae in a model of sepsis

Vaccination is the most promising strategy to reduce the incidence of pneumococcal infection. Although there are vaccines available, all of them are based on polysaccharide antigens (conjugated or not). In addition to their high cost, those vaccines do not cover all serotypes. To overcome these hindrances, we evaluated the immunogenicity and the protective efficacy of the S9 ribosomal protein of Streptococcus pneumoniae with the aim of developing a protein-based vaccine in the future. The gene encoding the S9 ribosomal protein was cloned in pET21-a expression vector, and the recombinant S9 protein was used to immunize mice. Significantly higher levels of anti-S9 immunoglobulin G were achieved (with predominance of immunoglobulin G1) in comparison with the control. Antibodies elicited against S. pneumoniae protein extract in rabbit recognized the recombinant S9 protein by Western blot, thus demonstrating its immunogenicity. Moreover, mice immunized with recombinant S9 protein and challenged with a virulent strain of S. pneumoniae presented a significant reduction of bacteremia after 24 h of infection as compared with the control. However, in the S9-immunized mice the onset of death was insignificantly delayed, but all of them died by the fourth day postinfection.

Mucosal immunization with caseinolytic protease X elicited cross-protective immunity against pneumococcal infection in mice

Streptococcus pneumoniae resides on the mucosal surface of the upper respiratory tract and is ready to spread and trigger clinical diseases. Hence the vaccine that can eliminate the nasopharyngeal colonization was thought to be an ideal protective strategy against pneumococcal invasive diseases. Caseinolytic protease X (ClpX), a pneumococcal caseinolytic protease ATPase subunit, was shown to be a non-transmembrane protein by bioinformatics analysis. Consistent with the in silico prediction, the secretory expression of ClpX, instead of surface exposure, was further confirmed by flow cytometry and Western blot. Furthermore, ClpX was highly conserved in nine different serotypes of S. pneumoniae at both gene and protein concentrations. In addition, the anti-ClpX IgG antibody levels in human serum samples were much higher in healthy children, compared with pediatric patients, and displayed an age-related increase. Finally, ClpX protein antigen was introduced to BALB/c mice through a mucosal route, and protection against nasopharyngeal colonization and lethal infection caused by different S. pneumoniae serotypes was successfully elicited. Our findings suggest that ClpX is a potential candidate antigen that could be incorporated in pneumococcal protein vaccines.

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.

Immunization with DnaJ (hsp40) could elicit protection against nasopharyngeal colonization and invasive infection caused by different strains of Streptococcus pneumoniae

Increasing mortality, morbidity and economic costs have been paid to pneumococcal diseases every year. Currently, vaccination is the most promising strategy to reduce the occurrence of pneumococcal infection. In this study, we investigated the protective efficacy of immunization with recombinant DnaJ (hsp40) protein against infections of different serotypes of Streptococcus pneumoniae. We demonstrated that mucosal immunization with DnaJ antigen could induce both systemic and mucosal antibodies for DnaJ and stimulate the release of high levels of IL-10, IFN-γ and IL-17A. Moreover, this mucosal vaccination could reduce nasal or lung colonization of pneumococcus and elicit protection against different serotypes of invasive pneumococcal infections. As well, we found that intraperitoneal immunization with DnaJ could also protect against invasive infections caused by different serotypes of pneumococcus, and passive immunization with antibodies specific for DnaJ confirmed that this protection was antibody-mediated. Our results therefore support the potential of DnaJ as a conserved pneumococcal protein vaccine.

Immunization with a ZmpB-based protein vaccine could protect against pneumococcal diseases in mice

Zinc metalloprotease B (ZmpB) is present in all isolated pneumococcal strains and contributes to the pathogenesis of pneumococcal infection. In this study, recombinant ZmpB was cloned and expressed in Escherichia coli. The expression of ZmpB by different pneumococcal strains was detectable by Western blotting with antisera raised to recombinant ZmpB. Flow cytometry analysis demonstrated that anti-ZmpB polyclonal antibodies could bind to the cell surface of the pneumococcal strains analyzed. Both recombinant ZmpB protein and anti-ZmpB polyclonal antibodies significantly inhibited the adhesion of Streptococcus pneumoniae D39 to A549 cells. In mouse models, mucosal immunization with recombinant ZmpB could significantly reduce pneumococcal lung colonization caused by S. pneumoniae serotypes 19F and 14 and significantly increase mice survival times following invasive pneumococcal challenge with different pneumococcal strains, including serotypes 2, 3, 6B, and 14. Furthermore, intraperitoneal immunization with recombinant ZmpB in combination with the recombinant pneumolysin mutant (DeltaA146 Ply) and heat shock protein 40 (DnaJ) could enhance the protection against pneumococcal infection compared to protection provided by single-protein antigens. Passive immunization with hyperimmune antisera against these three antigens also demonstrated that the combination of three hyperimmune antisera could provide better protection than single antisera. Taken together, our results suggest that ZmpB is a good candidate pneumococcal vaccine antigen.

Adhesion determinants of the Streptococcus species

Streptococci are clinically important Gram-positive bacteria that are capable to cause a wide variety of diseases in humans and animals. Phylogenetic analyses based on 16S rRNA sequences of the streptococcal species reveal a clustering pattern, reflecting, with a few exceptions, their pathogenic potential and ecological preferences. Microbial adhesion to host tissues is the initial critical event in the pathogenesis of most infections. Streptococci use multiple adhesins to attach to the epithelium, and their expression is regulated in response to environmental and growth conditions. Bacterial adhesins recognize and bind cell surface molecules and extracellular matrix components through specific domains that for certain adhesin families have been well defined and found conserved across the streptococcal species. In this review, we present the different streptococcal adhesin families categorized on the basis of their adhesive properties and structural characteristics, and, when available, we focus the attention on conserved functional domains.

CD4(+) T lymphocytes mediated protection against invasive pneumococcal infection induced by mucosal immunization with ClpP and CbpA

Intranasal delivery of pneumococcal protein vaccines would be a promising way to prevent invasive pneumococcal infection. Using an invasive infection model by intranasal inoculation of pneumococci, we demonstrated that immunizing mice intranasally with a mixture of ClpP (the caseinolytic protease) and CbpA (Choline binding protein A) elicited better protection than that of immunizing either single ClpP or CbpA. Anti-ClpP or anti-CbpA hyperimmune sera from intranasal-immunized mice significantly inhibited the adhesion of Streptococcus pneumoniae to A549 cells and combination of two antisera resulted in an additive effect. Both of two antisera could also kill S. pneumoniae by polymorphonuclear leukocytes in a complement-dependent way. The anti-infection activity and production of hyperimmune antibodies induced by mucosal immunization with ClpP and CbpA could be abrogated by the depletion of CD4(+) T lymphocytes. Our data therefore indicated a critical role for CD4(+) T lymphocytes in developing mucosal protein-based vaccines against invasive pneumococcal infection.