A 43-kilodalton pneumococcal surface protein, PspA: isolation, protective abilities, and structural analysis of the amino-terminal sequence

Streptococcus pneumoniae binds to host GAPDH on dying lung epithelial cells worsening secondary infection following influenza

Streptococcus pneumoniae (Spn) alone and during co-infection with influenza A virus (IAV) can result in severe pneumonia with mortality. Pneumococcal surface protein A (PspA) is an established virulence factor required for Spn evasion of lactoferricin and C-reactive protein-activated complement-mediated killing. Herein, we show that PspA functions as an adhesin to dying host cells. We demonstrate that PspA binds to host-derived glyceraldehyde-3-phosphate dehydrogenase (GAPDH) bound to outward-flipped phosphatidylserine residues on dying host cells. PspA-mediated adhesion was to apoptotic, pyroptotic, and necroptotic cells, but not healthy lung cells. Using isogenic mutants of Spn, we show that PspA-GAPDH-mediated binding to lung cells increases pneumococcal localization in the lower airway, and this is enhanced as a result of pneumolysin exposure or co-infection with IAV. PspA-mediated binding to GAPDH requires amino acids 230-281 in its α-helical domain with intratracheal inoculation of this PspA fragment alongside the bacteria reducing disease severity in an IAV/Spn pneumonia model.

PspA facilitates evasion of pneumococci from bactericidal activity of neutrophil extracellular traps (NETs)

Pneumococcal strains are variably resistant to killing by neutrophil extracellular traps (NETs). We hypothesize that this variability in resistance is due to heterogeneity in pneumococcal surface protein A (PspA), a structurally diverse virulence factor of Streptococcus pneumoniae. Pneumococcal strains showed variability in induction of NETs and in susceptibility to killing by NETs. The variability in susceptibility to NETs-mediated killing of pneumococcal strains is attributed to PspA, as strains lacking the surface expression of PspA were significantly more sensitive to NETs-mediated killing compared to the wild-type strains. Using pspA switch mutants we were further able to demonstrate that NETs induction and killing by NETs is a function of PspA as mutants with switch PspA demonstrated donor phenotype. Antibody to PspA alone showed an increase in induction of NETs, and NETs thus generated were able to trap and kill pneumococci. Pneumococci opsonized with antibody to PspA showed increase adherence to NETs but a decrease susceptibility to killing by NETs. In conclusion we demonstrate a novel role for pneumococcal PspA in resisting NETs mediated killing and allowing the bacteria to escape containment by blocking binding of pneumococci to NETs.

Human pleural fluid is a potent growth medium for Streptococcus pneumoniae

Empyema is defined by the presence of bacteria and/or pus in pleural effusions. However, the biology of bacteria within human pleural fluid has not been studied. Streptococcus pneumoniae is the most common cause of pediatric and frequent cause of adult empyema. We investigated whether S. pneumoniae can proliferate within human pleural fluid and if growth is affected by the cellular content of the fluid and/or characteristics of pneumococcal surface proteins. Invasive S. pneumoniae isolates (n = 24) and reference strain recovered from human blood or empyema were inoculated (1.5×10⁶CFU/mL) into sterile human malignant pleural fluid samples (n = 11). All S. pneumoniae (n = 25) strains proliferated rapidly, increasing by a median of 3009 (IQR 1063–9846) from baseline at 24hrs in all pleural effusions tested. Proliferation was greater than in commercial pneumococcal culture media and concentrations were maintained for 48hrs without autolysis. A similar magnitude of proliferation was observed in pleural fluid before and after removal of its cellular content, p = 0.728. S. pneumoniae (D39 strain) wild-type, and derivatives (n = 12), each with mutation(s) in a different gene required for full virulence were inoculated into human pleural fluid (n = 8). S. pneumoniae with pneumococcal surface antigen A (ΔpsaA) mutation failed to grow (2207-fold lower than wild-type), p<0.001, however growth was restored with manganese supplementation. Growth of other common respiratory pathogens (n = 14) across pleural fluid samples (n = 7) was variable and inconsistent, with some strains failing to grow. We establish for the first time that pleural fluid is a potent growth medium for S. pneumoniae and proliferation is dependent on the PsaA surface protein and manganese.

Conjugation of PspA4Pro with Capsular Streptococcus pneumoniae Polysaccharide Serotype 14 Does Not Reduce the Induction of Cross-Reactive Antibodies

Current pneumococcal vaccines are composed of bacterial polysaccharides as antigens, plain or conjugated to carrier proteins. While efficacious against vaccine serotypes, epidemiologic data show an increasing incidence of infections caused by nonvaccine serotypes of Streptococcus pneumoniae The use of pneumococcal surface protein A (PspA) as a carrier protein in a conjugate vaccine could help prevent serotype replacement by increasing vaccine coverage and reducing selective pressure of S. pneumoniae serotypes. PspA is present in all pneumococcal strains, is highly immunogenic, and is known to induce protective antibodies. Based on its sequence, PspA has been classified into three families and six clades. A PspA fragment derived from family 2, clade 4 (PspA4Pro), was shown to generate antibodies with a broad range of cross-reactivity, across clades and families. Here, PspA4Pro was modified and conjugated to capsular polysaccharide serotype 14 (PS14). We investigated the impact of conjugation on the immune response induced to PspA4Pro and PS14. Mice immunized with the PS14-mPspA4Pro conjugate produced higher titers of anti-PS14 antibodies than the animals that received coadministered antigens. The conjugate induced antibodies with opsonophagocytic activity against PS14-carrying strains, as well as against a panel of strains bearing PspAs from five clades (encompassing families 1 and 2) bearing a non-PS14 serotype. Furthermore, mice immunized with PS14-mPspA4Pro were protected against nasal colonization with a nonrelated S. pneumoniae strain bearing PspA from clade 1, serotype 6B. These results demonstrate that the cross-reactivity mediated by PspA4Pro is retained following conjugation, supporting the use of PspA4 as a carrier protein in order to enhance pneumococcal vaccine coverage and encourage its further investigation as a candidate in future vaccine designs.

Choline Binding Proteins from Streptococcus pneumoniae: A Dual Role as Enzybiotics and Targets for the Design of New Antimicrobials

Streptococcus pneumoniae (pneumococcus) is an important pathogen responsible for acute invasive and non-invasive infections such as meningitis, sepsis and otitis media, being the major cause of community-acquired pneumonia. The fight against pneumococcus is currently hampered both by insufficient vaccine coverage and by rising antimicrobial resistances to traditional antibiotics, making necessary the research on novel targets. Choline binding proteins (CBPs) are a family of polypeptides found in pneumococcus and related species, as well as in some of their associated bacteriophages. They are characterized by a structural organization in two modules: a functional module (FM), and a choline-binding module (CBM) that anchors the protein to the choline residues present in the cell wall through non-covalent interactions. Pneumococcal CBPs include cell wall hydrolases, adhesins and other virulence factors, all playing relevant physiological roles for bacterial viability and virulence. Moreover, many pneumococcal phages also make use of hydrolytic CBPs to fulfill their infectivity cycle. Consequently, CBPs may play a dual role for the development of novel antipneumococcal drugs, both as targets for inhibitors of their binding to the cell wall and as active cell lytic agents (enzybiotics). In this article, we review the current state of knowledge about host- and phage-encoded pneumococcal CBPs, with a special focus on structural issues, together with their perspectives for effective anti-infectious treatments.

New adhesin functions of surface-exposed pneumococcal proteins

Background

Streptococcus pneumoniae is a widely distributed commensal Gram-positive bacteria of the upper respiratory tract. Pneumococcal colonization can progress to invasive disease, and thus become lethal, reason why antibiotics and vaccines are designed to limit the dramatic effects of the bacteria in such cases. As a consequence, pneumococcus has developed efficient antibiotic resistance, and the use of vaccines covering a limited number of serotypes such as Pneumovax^® and Prevnar^® results in the expansion of non-covered serotypes. Pneumococcal surface proteins represent challenging candidates for the development of new therapeutic targets against the bacteria. Despite the number of described virulence factors, we believe that the majority of them remain to be characterized. This is the reason why pneumococcus invasion processes are still largely unknown.

Results

Availability of genome sequences facilitated the identification of pneumococcal surface proteins bearing characteristic motifs such as choline-binding proteins (Cbp) and peptidoglycan binding (LPXTG) proteins. We designed a medium throughput approach to systematically test for interactions between these pneumococcal surface proteins and host proteins (extracellular matrix proteins, circulating proteins or immunity related proteins). We cloned, expressed and purified 28 pneumococcal surface proteins. Interactions were tested in a solid phase assay, which led to the identification of 23 protein-protein interactions among which 20 are new.

Conclusions

We conclude that whether peptidoglycan binding proteins do not appear to be major adhesins, most of the choline-binding proteins interact with host proteins (elastin and C reactive proteins are the major Cbp partners). These newly identified interactions open the way to a better understanding of host-pneumococcal interactions.

Versatility of choline metabolism and choline-binding proteins in Streptococcus pneumoniae and commensal streptococci

The pneumococcal choline-containing teichoic acids are targeted by cholinebinding proteins (CBPs), major surface components implicated in the interaction with host cells and bacterial cell physiology. CBPs also occur in closely related commensal species, Streptococcus oralis and Streptococcus mitis, and many strains of these species contain choline in their cell wall. Physiologically relevant CBPs including cell wall lytic enzymes are highly conserved between Streptococcus pneumoniae and S. mitis. In contrast, the virulence-associated CBPs, CbpA, PspA and PcpA, are S. pneumoniae specific and are thus relevant for the characteristic properties of this species.

Streptococcus pneumoniaeProtein Vaccine Candidates: Properties, Activities and Animal Studies

Streptococcus pneumoniae is a causative agent for community acquired pneumonia, bacteremia, acute otitis media, and meningitis. Recent emergence of multi-drug resistant clinical isolates prompts the need of effective vaccine for the prevention of disease. The licensed polysaccharide-based pneumococcal vaccines only elicit protective antibodies against the infection of serotypes that are included in the vaccine. To broaden the protection, the use of pneumococcal proteins will be a feasible and preferable alternative. This communication provides a review on the biochemical properties of these protein candidates, their immunization results in animal studies, and perspectives on the development of protein-based pneumococcal vaccine.

Streptococcus pneumoniae surface protein PcpA elicits protection against lung infection and fatal sepsis

Previous studies have suggested that pneumococcal choline binding protein A (PcpA) is important for the full virulence of Streptococcus pneumoniae, and its amino acid sequence suggests that it may play a role in cellular adherence. PcpA is under the control of a manganese-dependent regulator and is only expressed at low manganese concentrations, similar to those found in the blood and lungs. PcpA expression is repressed under high manganese concentrations, similar to those found in secretions. In this study, we have demonstrated that PcpA elicits statistically significant protection in murine models of pneumonia and sepsis. In the model of pneumonia with each of four challenge strains, statistically fewer S. pneumoniae cells were recovered from the lungs of mice immunized with PcpA and alum versus mice immunized with alum only. The immunizations reduced the median CFU by 4- to 400-fold (average of 28-fold). In the model of sepsis using strain TIGR4, PcpA expression resulted in shorter times to become moribund and subcutaneous immunization with PcpA increased survival times of mice infected with wild-type PcpA-expressing pneumococci.

Enhanced protective immunity against pneumococcal infection with PspA DNA and protein

The effect of priming and boosting with pspA/EF5668 and purified recombinant PspA/EF5668 was examined. With this strategy CBA/N mice were protected against fatal challenge with Streptococcus pneumoniae EF5668. Anti-PspA antibody titers were elevated, and Western analysis with the immune serum demonstrated cross-reactivity with PspA from several different pneumococcal isolates, representing different PspA clades. Immune serum localized cross-reactive epitopes to the alpha-helical domain of PspA/Rx1 and PspA/EF5668. We demonstrated that DNA/protein prime-boost immunizations can enhance protective immunity against pneumococcal challenge.

Multifunctional role of choline binding protein G in pneumococcal pathogenesis

Members of the choline binding protein (Cbp) family are noncovalently bound to phosphorylcholine residues on the surface of Streptococcus pneumoniae. It has been suggested that CbpG plays a role in adherence and increase virulence both at the mucosal surface and in the bloodstream, but the function of this protein has been unclear. A new sequence analysis indicated that CbpG is a possible member of the S1 family of multifunctional surface-associated serine proteases. Clinical isolates contained two alleles of cbpG, and one-third of the strains expressed a truncated protein lacking the C-terminal, cell wall-anchoring choline binding domain. CbpG on the surface of pneumococci (full length) or released into the supernatant (truncated) showed proteolytic activity for fibronectin and casein, as did CbpG expressed on lactobacilli or as a purified full-length or truncated recombinant protein. Recombinant CbpG (rCbpG)-coated beads adhered to eukaryotic cells, and TIGR4 mutants lacking CbpG or having a truncated CbpG protein showed decreased adherence in vitro and attenuation of disease in mouse challenge models of colonization, pneumonia, and bacteremia. Immunization with rCbpG was protective in an animal model of colonization and sepsis. We propose that CbpG is a multifunctional surface protein that in the cell-attached or secreted form cleaves host extracellular matrix and in the cell-attached form participates in bacterial adherence. This is the first example of distinct functions in virulence that are dependent on natural variation in expression of a choline binding domain.

Novel vaccine strategies with protein antigens of Streptococcus pneumoniae

Infections caused by Streptococcus pneumoniae (pneumococcus) are a major cause of mortality throughout the world. This organism is primarily a commensal in the upper respiratory tract of humans, but can cause pneumonia in high-risk persons and disseminate from the lungs by invasion of the bloodstream. Currently, prevention of pneumococcal infections is by immunization with vaccines which contain capsular polysaccharides from the most common serotypes causing invasive disease. However, there are more than 90 antigenically distinct serotypes and there is concern that serotypes not included in the vaccines may become more prevalent in the face of continued use of polysaccharide vaccines. Also, certain high-risk groups have poor immunological responses to some of the polysaccharides in the vaccine formulations. Protein antigens that are conserved across all capsular serotypes would induce more effective and durable humoral immune responses and could potentially protect against all clinically relevant pneumococcal capsular types. This review provides a summary of work on pneumococcal proteins that are being investigated as components for future generations of improved pneumococcal vaccines.

In vivo characterization of the psa genes from Streptococcus pneumoniae in multiple models of infection

Differential fluorescence induction technology was used to identify promoters of Streptococcus pneumoniae genes that are expressed during lung infection of the mouse. Among the promoter clones that were identified multiple times was the psa promoter, which drives expression of the psaBCA operon. These genes have been identified previously and shown to encode a manganese permease system as well as play a role in the virulence of this organism. Mutations in psaB, psaC or psaA result in growth limitation in low manganese. The expression of the psa operon was examined in vivo and the virulence of deletion mutants of psaB, psaC, psaA and psaBCA was assessed in four different animal models of infection. The psa promoter was induced more than ten-fold in vivo using an intraperitoneal chamber implant model. The psaB, psaC and psaA mutants were completely attenuated in systemic, respiratory tract and otitis media infections. In addition, these mutants were unable to grow in an implanted peritoneal chamber, but growth was restored by the addition of manganese to the chambers.

Role of novel choline binding proteins in virulence of Streptococcus pneumoniae

The choline binding proteins (CBPs) are a family of surface proteins noncovalently bound to the phosphorylcholine moiety of the cell wall of Streptococcus pneumoniae by a conserved choline binding domain. Six new members of this family were identified, and these six plus two recently described cell wall hydrolases, LytB and LytC, were characterized for their roles in virulence. CBP-deficient mutants were constructed and tested for adherence to eukaryotic cells, colonization of the rat nasopharynx, and ability to cause sepsis. Five CBP mutants, CbpD, CbpE, CbpG, LytB, and LytC, showed significantly reduced colonization of the nasopharynx. For CbpE and -G this was attributable to a decreased ability to adhere to human cells. CbpG, a putative serine protease, also played a role in sepsis, the first observation of a pneumococcal virulence determinant strongly operative both on the mucosal surface and in the bloodstream.

The putative proteinase maturation protein A of Streptococcus pneumoniae is a conserved surface protein with potential to elicit protective immune responses

Surface-exposed proteins often play an important role in the interaction between pathogenic bacteria and their host. We isolated a pool of hydrophobic, surface-associated proteins of Streptococcus pneumoniae. The opsonophagocytic activity of hyperimmune serum raised against this protein fraction was high and species specific. Moreover, the opsonophagocytic activity was independent of the capsular type and chromosomal genotype of the pneumococcus. Since the opsonophagocytic activity is presumed to correlate with in vivo protection, these data indicate that the protein fraction has the potential to elicit species-specific immune protection with cross-protection against various pneumococcal strains. Individual proteins in the extract were purified by two-dimensional gel electrophoresis. Antibodies raised against three distinct proteins contributed to the opsonophagocytic activity of the serum. The proteins were identified by mass spectrometry and N-terminal amino acid sequencing. Two proteins were the previously characterized pneumococcal surface protein A and oligopeptide-binding lipoprotein AmiA. The third protein was the recently identified putative proteinase maturation protein A (PpmA), which showed homology to members of the family of peptidyl-prolyl cis/trans isomerases. Immunoelectron microscopy demonstrated that PpmA was associated with the pneumococcal surface. In addition, PpmA was shown to elicit species-specific opsonophagocytic antibodies that were cross-reactive with various pneumococcal strains. This antibody cross-reactivity was in line with the limited sequence variation of ppmA. The importance of PpmA in pneumococcal pathogenesis was demonstrated in a mouse pneumonia model. Pneumococcal ppmA-deficient mutants showed reduced virulence. The properties of PpmA reported here indicate its potential for inclusion in multicomponent protein vaccines.

Molecular analysis of putative pneumococcal virulence proteins

Although the polysaccharide capsule has been recognized as a sine qua non of virulence, recent attention has focused on the role of pneumococcal proteins in pathogenesis, particularly in view of their potential as vaccine antigens. The contribution of pneumolysin, two distinct neuraminidases, autolysin, hyaluronidase, and the 37 kDa pneumococcal surface adhesin A has been examined by specifically mutagenizing the respective genes in the pneumococcal chromosome and examining the impact on virulence in animal models. The vaccine potential of these proteins has also been assessed by immunization of mice with purified antigens, followed by challenge with virulent pneumococci.

Immunization of healthy adults with a single recombinant pneumococcal surface protein A (PspA) variant stimulates broadly cross-reactive antibodies to heterologous PspA molecules

Pneumococcal surface protein A (PspA) is a highly variable protein found on all strains of pneumococci. To be successful, a PspA-based vaccine for S. pneumoniae must induce antibodies that are broadly cross-reactive. To address whether cross-reactive antibodies could be induced in man, we evaluated serum from adults immunized with recombinant clade 2 PspA from strain Rx1. Immunization with 5-125 microg rPspA lead to a significant increase in circulating anti-PspA antibodies, as well as antibodies reactive to heterologous rPspA molecules. Increased binding of post-immune sera to 37 pneumococcal strains expressing a variety of PspA and capsule types was observed, versus pre-immune sera. The extent of cross-clade reactivity of human anti-rPspA followed roughly the amount of sequence homology to the non-clade 2 antigens. It is hypothesized that priming of humans by natural exposure to S. pneumoniae contributes to the breadth of the cross-reactivity of antibody to PspA.

Truncated surface protective antigen (SpaA) of Erysipelothrix rhusiopathiae serotype 1a elicits protection against challenge with serotypes 1a and 2b in pigs

Erysipelothrix rhusiopathiae is a causal agent of swine erysipelas, which is of economic importance in the swine industry by virtue of causing acute septicemia, chronic arthritis, and endocarditis. However, little is known about the genetic properties of its protective antigens. Recently, a surface protective antigen (SpaA) gene was identified from serotype 2 in a mouse model. We cloned spaA from virulent strain Fujisawa (serotype 1a) and determined that the N-terminal 342 amino acids without C-terminal repeats of 20 amino acids have the ability to elicit protection in mice. Fusions of 342 amino acids of Fujisawa SpaA and histidine hexamer (HisSpa1.0) protected pigs against challenge with both serotype 1 and serotype 2, the most important serotypes in the swine industry. Pigs immunized with HisSpa1.0 reacted well with both HisSpa1.0 and intact SpaA by enzyme-linked immunosorbent assay and immunoblotting. Serum collected at the time of challenge from a pig immunized with HisSpa1. 0 markedly enhanced the in vitro phagocytic and killing activity of pig neutrophils against the bacteria. DNA sequences of protective regions of spaA genes from five strains of serotypes 1 and 2 were almost identical. The full DNA sequences also seemed to be conserved among strains of all 12 serotype reference strains harboring the spaA gene by restriction fragment length polymorphism analysis of PCR products. These results indicates that SpaA is a common protective antigen of serotypes 1 and 2 of E. rhusiopathiae in swine and will be a useful tool for development of new types of vaccines and diagnostic tools for effective control of the disease.

A Nontoxic Adjuvant for Mucosal Immunity to Pneumococcal Surface Protein A

In this study, we demonstrated that pneumococcal surface protein A (PspA) nasally administered with a nontoxic A subunit mutant of cholera toxin (mCT) S61F elicited a protective immune response. Immunization with PspA and mCT elicited higher levels of PspA-specific IgG and IgA Abs in serum and of IgG and IgA anti-PspA Ab-forming cells in spleens, cervical lymph nodes (CLN), and lung tissue when compared to nonimmunized mice. Furthermore, significant PspA-specific IgA Abs were induced in saliva and nasal secretions. These responses were dependent on the use of mCT as a mucosal adjuvant. The PspA-specific Ab responses induced by mCT S61F were comparable with those induced by native CT (nCT). Analysis of cytokine responses showed that nasal PspA plus mCT S61F enhanced the induction of PspA-specific CD4+ T cells producing IL-4 but not IFN-γ in CLN at both the protein and mRNA levels. Importantly, significant numbers of mice intranasally immunized with PspA plus mCT S61F were protected from lethal challenge with capsular serotype 3 Streptococcus pneumoniae A66. These results show that intranasal administration of PspA together with mCT S61F is an effective mucosal vaccine against pneumococcal infection and induces CD4+ Th2-type cells, which provide help for both mucosal and systemic Ab responses.

Pneumococcal diversity: considerations for new vaccine strategies with emphasis on pneumococcal surface protein A (PspA)

Streptococcus pneumoniae is a problematic infectious agent, whose seriousness to human health has been underscored by the recent rise in the frequency of isolation of multidrug-resistant strains. Pneumococcal pneumonia in the elderly is common and often fatal. Young children in the developing world are at significant risk for fatal pneumococcal respiratory disease, while in the developed world otitis media in children results in substantial economic costs. Immunocompromised patients are extremely susceptible to pneumococcal infection. With 90 different capsular types thus far described, the diversity of pneumococci contributes to the challenges of preventing and treating S. pneumoniae infections. The current capsular polysaccharide vaccine is not recommended for use in children younger than 2 years and is not fully effective in the elderly. Therefore, innovative vaccine strategies to protect against this agent are needed. Given the immunogenic nature of S. pneumoniae proteins, these molecules are being investigated as potential vaccine candidates. Pneumococcal surface protein A (PspA) has been evaluated for its ability to elicit protection against S. pneumoniae infection in mouse models of systemic and local disease. This review focuses on immune system responsiveness to PspA and the ability of PspA to elicit cross-protection against heterologous strains. These parameters will be critical to the design of broadly protective pneumococcal vaccines.

A live recombinant avirulent oral Salmonella vaccine expressing pneumococcal surface protein A induces protective responses against Streptococcus pneumoniae

A live oral recombinant Salmonella vaccine strain expressing pneumococcal surface protein A (PspA) was developed. The strain was attenuated with Deltacya Deltacrp mutations. Stable expression of PspA was achieved by the use of the balanced-lethal vector-host system, which employs an asd deletion in the host chromosome to impose an obligate requirement for diaminopimelic acid. The chromosomal Deltaasd mutation was complemented by a plasmid vector possessing the asd+ gene. A portion of the pspA gene from Streptococcus pneumoniae Rx1 was cloned onto a multicopy Asd+ vector. After oral immunization, the recombinant Salmonella-PspA vaccine strain colonized the Peyer's patches, spleens, and livers of BALB/cByJ and CBA/N mice and stimulated humoral and mucosal antibody responses. Oral immunization of outbred New Zealand White rabbits with the recombinant Salmonella strain induced significant anti-PspA immunoglobulin G titers in serum and vaginal secretions. Polyclonal sera from orally immunized mice detected PspA on the S. pneumoniae cell surface as revealed by immunofluorescence. Oral immunization of BALB/cJ mice with the PspA-producing Salmonella strain elicited antibody to PspA and resistance to challenge by the mouse-virulent human clinical isolate S. pneumoniae WU2. Immune sera from orally immunized mice conferred passive protection against otherwise lethal intraperitoneal or intravascular challenge with strain WU2.

Oligonucleotides identify conserved and variable regions of pspA and pspA-like sequences of Streptococcus pneumoniae

Pneumococcal surface protein A (PspA) is an immunogenic surface protein of Streptococcus pneumoniae. PspA of S. pneumoniae strain Rx1 is a 65-kDa protein composed of an alpha-helical N-terminus of 288 amino acids followed by an 82-amino-acid proline-rich region, 10 repeats of 20 amino acids each, and a 17-amino-acid C-terminus. It has been demonstrated that the 3'-half of pspA is relatively conserved among unrelated pneumococcal isolates and the 5'-half of the gene is highly variable. Additionally, nearly all pneumococcal strains contain at least one other locus with sequence homology to pspA. In this study oligonucleotides derived from the DNA sequence of pspA of Rx1 were used both as hybridization probes and as primers in the polymerase chain reaction (PCR) to investigate genetic variation within domains of pspA and in the pspA-like sequences from 18 strains representing 12 capsule and 9 PspA serotypes. Sequences encoding the leader peptide, the proline-rich region, and the repeat region are highly conserved among pspA and pspA-like sequences. The alpha-helical coding domain is highly diverse among pspA and pspA-like sequences of different strains.

Oral immunization with PspA elicits protective humoral immunity against Streptococcus pneumoniae infection

Streptococcus pneumoniae is a major respiratory mucosal pathogen affecting infants and children. Although a polysaccharide-based vaccine has been useful in adult populations, it does not elicit protective immunity in infants and young children. Pneumococcal surface protein A (PspA) is a highly immunogenic surface protein produced by all strains of Streptococcus pneumoniae. Previous studies have shown that systemic immunization of mice with PspA can elicit protective immunity against fatal pneumococcal infection. In this study, we demonstrated that oral immunization with PspA could elicit protective immune responses against pneumococcal infection. When mice were orally immunized with PspA alone, low levels of PspA-specific immunoglobulin G (IgG) responses were induced in serum; none was induced in secretion. On the other hand, when PspA was given orally with the mucosal adjuvant cholera toxin (CT), significant levels of IgG and IgA anti-PspA responses were induced in serum. The major IgG subclass was IgG1, followed by IgG2b, a profile of antibody response supported by Th2-type cells. In addition, all mice orally immunized with PspA and CT were protected from the lethal challenge with capsular serotype 3 S. pneumoniae A66. These results suggested that an oral PspA vaccine may be a useful means of preventing pneumococcal disease.

Sequence heterogeneity of PsaA, a 37-kilodalton putative adhesin essential for virulence of Streptococcus pneumoniae

psaA encodes a 37-kDa putative pneumococcal surface adhesin. Although its complete nucleotide sequence has been determined, its contribution to the pathogenicity of Streptococcus pneumoniae has not previously been assessed. In this study, we used a PCR-amplified internal fragment of the psaA gene from S. pneumoniae type 2 strain D39 cloned in pVA891, to direct the construction of D39 derivatives in which the psaA gene had been specifically interrupted, by insertion-duplication mutagenesis. Two independent D39 psaA mutants (PsaA-(1) and PsaA-(2)) were significantly less virulent (as judged by intranasal or intraperitoneal challenge of mice) than either the wild-type D39 strain or a derivative of PsaA-(1) in which the psaA gene had been reconstituted by back-transformation with an intact copy of the cloned gene. pVA891-directed mutagenesis of an open reading frame (designated ORF3) immediately 3' to psaA or insertion of pVA891 between psaA and ORF3 had no impact on intranasal virulence. However, a small but significant difference in virulence was observed between these two derivatives and the parental D39 strain in a low-dose intraperitoneal challenge model, suggesting that the ORF3 product may also contribute to pathogenesis. Adherence of PsaA-(1) to A549 cells (type II pneumocytes) was only 9% of that for D39, while the ORF3-negative strain exhibited intermediate adherence (23%). This is the first functional evidence that PsaA is an adhesin. Sequence analysis of the psaA gene from D39 indicated significant deviation from that previously published for the homolog from S. pneumoniae R36A. The deduced amino acid sequences of mature PsaA from the two strains had only 81% homology, with the bulk of the variation occurring in the amino-terminal portion.

Systemic and mucosal protective immunity to pneumococcal surface protein A

To date our studies demonstrate that PspA is a highly immunogenic molecule in mice and that it can elicit immunity to otherwise fatal infections following iv, ip, in, and it challenge. Although the molecule is serologically variable, it is sufficiently cross-reactive so that immunization with a single PspA can protect against strains of highly diverse serotypes. It is anticipated that a vaccine composed of a mixture of carefully chosen PspA molecules will be able to elicit protective immunity to virtually all pneumococci. If this vaccine proved efficacious in man, it would provide a more simple and less costly means of immunizing against pneumococcal infection than using recombinant vaccines. This could be especially important in the developing world where the cost of successful vaccines must be no more than pennies per dose. If PspA is found to be less efficacious than capsular polysaccharides, it may be valuable as a protein component of a PS-protein conjugate vaccine. In this capacity, PspA might expand the breath of protection elicited by a vaccine composed of only a few polysaccharide-protein conjugates representing capsule types most commonly associated with infectious pneumococci.

Streptococcus pneumoniae: virulence factors, pathogenesis, and vaccines

Although pneumococcal conjugate vaccines are close to being licensed, a more profound knowledge of the virulence factors responsible for the morbidity and mortality caused by Streptococcus pneumoniae is necessary. This review deals with the major structures of pneumococci involved in the pathogenesis of pneumococcal disease and their interference with the defense mechanisms of the host. It is well known that protection against S. pneumoniae is the result of phagocytosis of invading pathogens. For this process, complement and anticapsular polysaccharide antibodies are required. Besides, relatively recent experimental data suggest that protection is also mediated by the removal of disintegrating pneumococci and their degradation products (cell wall, pneumolysin). These structures seem to be major contributors to illness and death caused by pneumococci. An effective conjugate vaccine should therefore preferably include the capsular polysaccharide and at least one of these inflammatory factors.

Immunization of mice with pneumolysin toxoid confers a significant degree of protection against at least nine serotypes of Streptococcus pneumoniae

Pneumolysin is the thiol-activated cytolysin produced by Streptococcus pneumoniae. Mice were immunized with a genetically engineered toxoid version of pneumolysin, which was derived from a serotype 2 pneumococcus. The toxoid carried the mutation Trp-433-->Phe. Alum was used as the adjuvant. Immunized mice had significantly increased levels of anti-pneumolysin antibodies, principally immunoglobulin G1. Mice were challenged intraperitoneally or intranasally with 12 strains covering capsular serotypes 1 to 6, 7F, 8, and 18C. Following challenge, the survival rate and/or the time of death of nonsurvivors (survival time) was significantly greater than that of sham-immunized mice for all nine serotypes. However, differences in the degree of protection were noted between different strains. The route of challenge also appeared to influence the degree of protection. Nevertheless, the significant, albeit in some cases partial, protection provided against all nine pneumococcal serotypes supports the conclusion that pneumolysin toxoids warrant consideration for inclusion in a human vaccine.

Protective humoral response against pneumococcal infection in mice elicited by recombinant bacille Calmette-Guérin vaccines expressing pneumococcal surface protein A

Pneumococcal surface protein A (PspA), a cell-surface protein present on all strains of pneumococci, has been shown to elicit protective antibody responses in mice in the absence of capsular polysaccharide. Whereas PspA is polymorphic, considerable cross-reactivity and cross-protection have been demonstrated among PspA proteins of pneumococci exhibiting different capsular and PspA serotypes. A gene segment encoding the nonrepetitive variable NH2-terminal portion of PspA has been cloned into three distinct recombinant Bacille Calmette-Guérin (rBCG) vectors, allowing for expression of PspA as a cytoplasmic or secreted protein, or a chimeric exported membrane-associated lipoprotein. All rBCG-PspA strains elicited comparable anti-PspA ELISA titers, ranging from 10(4) to 10(5) (reciprocal titers) in both BALB/c and C3H/HeJ mice. However, protective responses were observed only in animals immunized with the rBCG-PspA vaccines expressing PspA as a secreted protein or chimeric exported lipoprotein. In addition, anti-PspA immune sera elicited by the rBCG vaccines passively protected X-linked immunodeficient mice from lethal challenge with the highly virulent, encapsulated WU2 strain of Streptococcus pneumoniae and two additional virulent strains exhibiting heterologous PspA and capsular serotypes. These studies confirm previous PspA immunization studies showing cross-protection against heterologous serotypes of S. pneumoniae and demonstrate a potential for rBCG-based PspA vaccines to elicit protective humoral responses against pneumococcal disease in humans.

Novel surface attachment mechanism of the Streptococcus pneumoniae protein PspA

Pneumococcal surface protein A (PspA) of Streptococcus pneumoniae has been found to utilize a novel mechanism for anchoring to the bacterial cell surface. In contrast to that of surface proteins from other gram-positive bacteria, PspA anchoring required choline-mediated interactions between the membrane-associated lipoteichoic acid and the C-terminal repeat region of PspA. Release of PspA from the cell surface could be effected by deletion of 5 of the 10 C-terminal repeat units, by high concentrations of choline, or by growth in choline-deficient medium. Other pneumococcal proteins, including autolysin, which has a similar C-terminal repeat region, were not released by these treatments. The attachment mechanism utilized by PspA thus appears to be uniquely adapted to exploit the unusual structure of the pneumococcal cell surface. Further, it has provided the means for rapid and simple isolation of immunogenic PspA from S. pneumoniae.

Analysis of pneumococcal PspA microheterogeneity in SDS polyacrylamide gels and the association of PspA with the cell membrane

Pneumococcal surface protein A (PspA) is a protection-eliciting surface protein found on all pneumococci. Although highly cross-reactive, it displays interstrain variation in its size and in the expression of individual antibody reactive epitopes. PspA was not released in significant amounts from pneumococcal membranes treated with sodium carbonate, but was solubilized with SDS. Thus, PspA is either an integral membrane protein or is attached to an integral membrane component. By SDS-PAGE and immunoblot analysis, we found two predominant molecular sizes of PspA in each strain examined. The smaller band was about the size expected from the inferred amino acid sequence of PspA and the larger band appeared to be a dimer of the monomer PspA. When higher concentrations of lysate were run on SDS gels, it was also possible to detect many additional high molecular weight components that reacted with antibodies to PspA. These multiple high molecular weight PspA bands were not due to the attachment of PspA to peptidoglycan or teichoic acids, did not appear to be composed of degraded PspA and most likely resulted from non-covalent polymerization or aggregation of PspA.

Molecular localization of variable and conserved regions of pspA and identification of additional pspA homologous sequences in Streptococcus pneumoniae

PspA is anchored to the surface of all pneumococci by the C-terminal end of the molecule. The N-terminal half of PspA is known to be serologically variable and to be able to elicit protective immune responses. Molecular analysis with DNA probes spanning different regions of pspA was carried out to identify homologous sequences among pneumococcal isolates. At high stringency, DNA probes derived from the 3'-half of pspA (encoding the C-terminal half of PspA) hybridized to all of 37 pneumococcal isolates tested, representing 20 capsular serotypes and 12 PspA serotypes. Most strains had two sequences highly homologous to this region of pspA. Using derivatives of strain Rx1, with insertion mutations in pspA, it was possible to identify the functional pspA sequence. At 50% stringency, the 3' pspA probes also detected lytA and additional sequences. lytA encodes autolysin and shares homology with the 3' portion of pspA. A probe derived from the 5'-half of pspA (encoding the N-terminal half of PspA) hybridized with only 75% of strains and generally detected only one of the two sequences recognized by the 3' probes. Thus, the 3'-half of pspA appears to contain more highly conserved sequences than the 5'-half of pspA and shares homology with several additional sequences, suggesting that the pneumococcus might make several proteins that interact with the surface by the same mechanism as PspA.

Truncated forms of PspA that are secreted from Streptococcus pneumoniae and their use in functional studies and cloning of the pspA gene

Insertion-duplication mutagenesis was used to generate mutants of Streptococcus pneumoniae that produced truncated forms of PspA (pneumococcal surface protein A). The truncated products, representing from 20 to 80% of the complete PspA molecule, were all secreted from the cell and could be detected in unconcentrated culture medium. Analysis of the truncated molecules showed that the antigenic variability known to be associated with PspA is located in the alpha-helical N-terminal half of the molecule. This region was also found to contain immunogenic and protection-eliciting epitopes and to define the maximum region of the molecule that is likely to be surface exposed. The apparent molecular weight variability seen for PspA molecules of different S. pneumoniae strains was localized to both the N- and C-terminal halves of the protein. Attachment of PspA to S. pneumoniae was found to require regions located carboxy to the fifth repeat unit in the C-terminal end of the molecule. From the insertion-duplication mutants, the complete pspA gene was cloned and expressed in Escherichia coli. Differences in apparent molecular weight were observed when the same cloned product was expressed in E. coli and S. pneumoniae, suggesting that PspA is modified differently in the two hosts.

Molecular, immunological and functional characterization of the major surface adhesin of Streptococcus mutans

In the 15 years since the last major NIH conference that dealt with anti-caries vaccines, we have learned much. Certainly, whole bacteria or bacterial fractions may not be proper immunogens due to the possibility of inducing tissue cross-reactivity. Our own experience (van de Rijn et al., 1976) illustrates that pitfall. But even in the era of genetically engineered vaccines, we first must understand the biological functions of our chosen immunogen before employing that pure protein in a vaccine. Our recent work (Brady et al., 1991c) indicates that antigen P1, a ubiquitous protein found on several oral streptococci, may possess different, but possibly overlapping, functional domains influencing reactions with fluid-phase salivary agglutinin (aggregation) versus fixed agglutinin (adherence). A proper vaccine would induce antibodies against the latter domain(s) thereby retarding colonization. An improper vaccine that induces antibodies against aggregation-related domains on P1 would lessen the host's ability to clear those bacteria from the oral cavity. After carefully identifying appropriate functional domains and obtaining sub-clones of the larger gene that yield truncated polypeptides typical of adherence-specific regions that are also immunogenic, we may be in a position to create the most effective vaccine. In studies employing the polymerase chain reaction (PCR) and standard cloning procedures, we have already begun to produce such polypeptides. Once a library of polypeptides is assembled, they may be tested for functional activity and for lack of induction of cross-reactivity with nonpathogenic streptococci (i.e., S. gordonii). Certain of these recombinant-specified polypeptides could serve as the basis for an anti-caries vaccine. Alternatively, peptides may be synthesized that resemble these sub-molecular regions for use in a vaccine or as competitive inhibitors of adherence but not aggregation. Clearly, a vaccine against dental caries remains a real possibility for the future.

Structural properties and evolutionary relationships of PspA, a surface protein of Streptococcus pneumoniae, as revealed by sequence analysis

Analysis of the sequence for the gene encoding PspA (pneumococcal surface protein A) of Streptococcus pneumoniae revealed the presence of four distinct domains in the mature protein. The structure of the N-terminal half of PspA was highly consistent with that of an alpha-helical coiled-coil protein. The alpha-helical domain was followed by a proline-rich domain (with two regions in which 18 of 43 and 5 of 11 of the residues are prolines) and a repeat domain consisting of 10 highly conserved 20-amino-acid repeats. A fourth domain consisting of a hydrophobic region too short to serve as a membrane anchor and a poorly charged region followed the repeats and preceded the translation stop codon. The C-terminal region of PspA did not possess features conserved among numerous other surface proteins, suggesting that PspA is attached to the cell by a mechanism unique among known surface proteins of gram-positive bacteria. The repeat domain of PspA was found to have significant homology with C-terminal repeat regions of proteins from Streptococcus mutans, Streptococcus downei, Clostridium difficile, and S. pneumoniae. Comparisons of these regions with respect to functions and homologies suggested that, through evolution, the repeat regions may have lost or gained a mechanism for attachment to the bacterial cell.

Identification of monoclonal antibody-binding domains within antigen P1 of Streptococcus mutans and cross-reactivity with related surface antigens of oral streptococci

Eleven monoclonal antibodies (MAbs) specific for P1, the major protein surface antigen of Streptococcus mutans serotype c, were characterized by Western blot (immunoblot) analysis and by radioimmunoassay using whole bacterial cells. The approximate binding domains of the MAbs were determined by using full-length and truncated P1 polypeptides. The accessibility of these binding sites on the surfaces of intact bacteria was determined by radioimmunoassay. The ability of each MAb to cross-react with related proteins from strains of S. mutans serotypes e and f, S. sanguis, and S. sobrinus serotype g is also reported.