Protection of infant mice from challenge with Streptococcus pneumoniae type 19F by immunization with a type 19F polysaccharide--pneumolysoid conjugate

Recent progress in pneumococcal protein vaccines

Pneumococcal infections continue to pose a significant global health concern, necessitating the development of effective vaccines. Despite the progress shown by pneumococcal polysaccharide and conjugate vaccines, their limited coverage and the emergence of non-vaccine serotypes have highlighted the need for alternative approaches. Protein-based pneumococcal vaccines, targeting conserved surface proteins of Streptococcus pneumoniae, have emerged as a promising strategy. In this review, we provide an overview of the advancements made in the development of pneumococcal protein vaccines. We discuss the key protein vaccine candidates, highlight their vaccination results in animal studies, and explore the challenges and future directions in protein-based pneumococcal vaccine.

Pneumococcal Vaccines

Streptococcus pneumoniae is a Gram-Positive pathogen that is a major causative agent of pneumonia, otitis media, sepsis and meningitis across the world. The World Health Organization estimates that globally over 500,000 children are killed each year by this pathogen. Vaccines offer the best protection against S. pneumoniae infections. The current polysaccharide conjugate vaccines have been very effective in reducing rates of invasive pneumococcal disease caused by vaccine type strains. However, the effectiveness of these vaccines have been somewhat diminished by the increasing numbers of cases of invasive disease caused by non-vaccine type strains, a phenomenon known as serotype replacement. Since, there are currently at least 98 known serotypes of S. pneumoniae, it may become cumbersome and expensive to add many additional serotypes to the current 13-valent vaccine, to circumvent the effect of serotype replacement. Hence, alternative serotype independent strategies, such as vaccination with highly cross-reactive pneumococcal protein antigens, should continue to be investigated to address this problem. This chapter provides a comprehensive discussion of pneumococcal vaccines past and present, protein antigens that are currently under investigation as vaccine candidates, and other alternatives, such as the pneumococcal whole cell vaccine, that may be successful in reducing current rates of disease caused by S. pneumoniae.

Human pathogenic streptococcal proteomics and vaccine development

Gram-positive streptococci are non-motile, chain-forming bacteria commonly found in the normal oral and bowel flora of warm-blooded animals. Over the past decade, a proteomic approach combining 2-DE and MS has been used to systematically map the cellular, surface-associated and secreted proteins of human pathogenic streptococcal species. The public availability of complete streptococcal genomic sequences and the amalgamation of proteomic, genomic and bioinformatic technologies have recently facilitated the identification of novel streptococcal vaccine candidate antigens and therapeutic agents. The objective of this review is to examine the constituents of the streptococcal cell wall and secreted proteome, the mechanisms of transport of surface and secreted proteins, and describe the current methodologies employed for the identification of novel surface-displayed proteins and potential vaccine antigens.

Pneumococcal vaccination in adults

Pneumococci remain the most common etiology of community-acquired pneumonia in adults, with significant attendant mortality in the elderly. With the recognition of increasing rates of drug-resistant Streptococcus pneumoniae in recent years, efforts to prevent disease through vaccination have gained greater impetus. The 23-valent pneumococcal vaccine is used widely in the United States and provides effective protection against bacteremic pneumococcal disease, particularly in the immunocompetent host. The 7-valent pneumococcal conjugate vaccine, licensed in the United States in 2000, has had a dramatic impact on pneumococcal disease in the pediatric population, and its use in children has had effects on incidence rates in nonimmunized adults as well. Future directions include efforts to improve vaccination coverage in targeted populations and the development of more immunogenic and efficacious vaccines for high-risk groups.

Protective immunity and gene expression related to pneumococcal glycoconjugate

The immunogenicity of most polysaccharides (PSs) contained in the pneumococcal vaccine is low in children less than 2 years of age. Enhancement of immune response in early life can be induced by immunization of pneumococcal glycoconjugate as well as plasma DNA coding for cell-surface protein antigen. Pneumococcal type 19F PS conjugated with inactivated pneumolysin (Ply) induced in mice remarkable antibody responses to both type 19F PS and the protein carrier. In addition, the conjugate was administered to pregnant mice during gestation and/or lactation, and to their offspring during early infancy. When the young mice were challenged with type 19F pneumococci, the bacteria were cleared more rapidly from the blood of immunized mice than from that of the control group. The mortality rate of young mice from immunized mothers was also significantly lower than the control group. These results indicate that the effective protective immunity against pneumococcal infection can be induced in young mice by the maternal immunization with the glycoconjugate during gestation and at early infancy. Studies have been conducted to express type 19A pneumolysin gene (ply) in mammalian cells. Ply DNA was inserted into the cloning site of a vector containing CMV promoter. The recombinant plasmid DNA containing ply was transfected in human rhabdomyosarcoma cells and the gene expression was confirmed by immunoblot. Injection of mice three times 50-100 ug per dose ply DNA produced high serum levels of Ply IgG and IgM antibodies and showed rapid bacterial clearance from the blood.

Development and evaluation of pneumococcal conjugate vaccines: clinical trials and control tests

Streptococcus pneumoniae is a major cause of pneumonia, meningitis, and otitis media and is responsible for disease in young children, the elderly, and immunocompromised individuals. Emerging high-level resistance to penicillin, multiple antibiotics, and tolerance to vancomycin emphasizes the importance of preventing pneumococcal infection by alternative methods such as immunization. The development of pneumococcal conjugate vaccines using the same carrier proteins as those used in Hemophilus influenzae type b vaccines has enhanced the immune response in infants and children compared with polysaccharide vaccines and has significantly improved the ability to prevent pneumococcal disease in this population worldwide. Here we review the clinical trials of multivalent pneumococcal conjugate vaccines under evaluation, identify potential carrier proteins considered for development of future pneumococcal conjugate vaccines, discuss issues regarding licensure of new candidate vaccines from a clinical trial and quality control perspective, and alternative vaccine strategies for the prevention of pneumococcal disease.

Immunogenicity in a mouse model of a conjugate vaccine made with a synthetic single repeating unit of type 14 pneumococcal polysaccharide coupled to CRM197

Oligosaccharides (OSs) related to the pneumococcal type 14 capsular polysaccharide (Pn14PS) were studied for their ability to inhibit the binding between anti-PS14 antisera and native PS14. A synthetic tetrasaccharide corresponding to the repeating unit of the Pn14PS, a hexasaccharide mimic, and an octasaccharide fragment obtained by Pn14PS depolymerization were good inhibitors. CRM197 conjugates of the tetrasaccharide and an octasaccharide mimic were prepared by using either adipic acid diester or diethyl squarate linkers. The conjugate with the tetrasaccharide chains induced anti-Pn14PS antibodies when injected subcutaneously into mice, as determined by an enzyme-linked immunosorbent assay, and antibody titers increased with oligosaccharide loading. The adipic acid-linked tetrasaccharide conjugates elicited higher antibody titers than those prepared with a squarate spacer. The lower anti-Pn14PS antibody response of the octasaccharide mimic conjugate indicates the importance of the backbone galactose residue for an appropriate antibody response. The OS-CRM197 conjugate prepared from a single repeat unit of the Pn14PS is a potential vaccine candidate.

Cross-protective immunity of mice induced by oral immunization with pneumococcal surface adhesin a encapsulated in microspheres

The global use of a capsular polysaccharide-based pneumococcal vaccine has been limited because of serotype-specific protection and poor effectiveness in individuals with low immunocompetency. The mucosal immune system develops earlier in infants and lasts longer in the elderly than does the systemic immune system. Furthermore, mucosal immunization is beneficial for AIDS patients, because human immunodeficiency virus-infected subjects can develop normal mucosal antibody responses even in late clinical phases. For these reasons, we evaluated recombinant pneumococcal surface adhesin A (rPsaA) of Streptococcus pneumoniae in terms of cross-protective immune responses after oral delivery. Encapsulated rPsaA provided higher immunogenicity than naked rPsaA. Coencapsulation or codelivery of the cholera toxin (CT) B subunit (CTB) and CT also increased the immunogenicity of rPsaA. Cross-protective immunities against five strains of S. pneumoniae (types 4, 6B, 14, 19F, and 23F) were induced after oral immunization with microencapsulated rPsaA. Lung colonization and septicemia caused by the five serotypes were significantly inhibited by oral immunization with microencapsulated rPsaA. These results suggest that rPsaA coencapsulated with CTB can be used as an oral vaccine to induce cross-protective immunity for the prevention of pneumococcal infection.

PsaA (pneumococcal surface adhesin A) and PspA (pneumococcal surface protein A) DNA vaccines induce humoral and cellular immune responses against Streptococcus pneumoniae

Streptococcus pneumoniae is one of the most important human pathogens and improvement of the currently used polysaccharide vaccines is being pursued. We constructed DNA vaccine vectors containing either the full-length psaA (pneumococcal surface adhesin A) or a truncated pspA (pneumococcal surface protein A--pspA') gene. Both constructs showed transient expression of the antigens in vertebrate cells and induced significant antibody response to the pneumococcal antigens in BALB/c mice injected intramuscularly (i.m.). Fusion with an N-terminal cytoplasmatic SV40 T-antigen (CT-Ag), which was previously shown to stabilize poorly expressed antigens through association with Hsp73, also induced anti-PspA antibody response. The induction of antibodies with a low IgG1:IgG2a ratio and elevated gamma interferon (IFN-gamma) production by spleen cells elicited by DNA vaccination indicate preferential priming of Th1 immunity. Since induction of antibodies against both PsaA and PspA was previously shown to correlate with protection against fatal infection with S. pneumoniae and cell-mediated immune responses could contribute to protection, further evaluation of PsaA and PspA as antigens for a DNA vaccine against S. pneumoniae could be promising.

Immunogenicity in mice of pneumococcal glycoconjugate vaccines using pneumococcal protein carriers

Antibody response and protective immunity were evaluated in mice immunized with pneumococcal glycoconjugate vaccines using two pneumococcal protein carriers. Mice injected with type 9V polysaccharide (PS) conjugated to inactivated pneumolysin (Ply) or autolysin (Aly) produced high levels of IgG and IgM antibodies to both the PS and the protein carrier. Higher PS antibody titers to the pneumococcal PS conjugates were measured by ELISA using PS-Ply or PS-tetanus toxoid (TT) conjugate as a coating antigen compared with PS mixed with methylated human serum albumin. Type 9V PS (10 microg) inhibited most of the 9V IgM and IgG antibody binding to the 9V-TT coated plate. In contrast, absorption with 19F PS did not inhibit 9V antibody binding. The avidity index of IgG antibodies in the 9V PS-Ply serum was 55.5 +/- 0.9, compared with 47.8 +/- 1.4 for 9V PS-Aly serum. Thus, high avidity of serum antibodies in conjugate-immunized mice can provide more effective functional activity for protection against pneumococcal infection. Mice immunized with these glycoconjugates exhibited rapid bacterial clearance from blood and provided cross-protection against challenge with heterologous serotypes of virulent pneumococci. These results reveal that conjugates using pneumococcal protein carriers can induce opsonophagocytic activity to destroy homologous and heterologous pneumococci, indicating that such conjugates can confer broader protective immunity than conjugates using non-pneumococcal proteins.

Pneumococcal pneumonia and bacteremia model in mice for the analysis of protective antibodies

Pneumococci cause infection by colonizing the nasopharynx and invading the mucosal surfaces. Infection models in mice, where the natural route of infection is mimicked, may be useful to study antibody mediated protection against pneumococcal pneumonia and bacteremia. We have established a pneumococcal pneumonia and bacteremia model in mice and investigated the protective capacity of human antibodies. Intranasal challenge with serotypes 1, 3, 6A and 8 caused lung infection and bacteremia which was lethal. Serotype 6B caused low, but detectable, infection and other serotypes tested were not virulent. Passive immunization with a human IgG preparation i.p. protected mice in a dose dependent manner against bacteremia caused by the virulent serotypes (except serotype 3) and partially or completely cleared pneumococci from the lungs of mice infected with serotypes 1, 6A and 8. Adsorption of antibodies with homologous capsular polysaccharides eliminated protection against disease but adsorption with cell wall polysaccharides (CWPS) did not. Furthermore, a good correlation was observed between protection of sera in vivo and opsonic activity in vitro. The results indicate that the model may be useful to analyse the levels, isotypes, specificity and other characteristics of human antibodies which protect against pneumococcal infection and to evaluate the protective potential of pneumococcal vaccine candidates.

Immunization of mice with combinations of pneumococcal virulence proteins elicits enhanced protection against challenge with Streptococcus pneumoniae

The vaccine potential of a combination of three pneumococcal virulence proteins was evaluated in an active-immunization-intraperitoneal-challenge model in BALB/c mice, using very high challenge doses of Streptococcus pneumoniae. The proteins evaluated were a genetic toxoid derivative of pneumolysin (PdB), pneumococcal surface protein A (PspA), and a 37-kDa metal-binding lipoprotein referred to as PsaA. Mice immunized with individual proteins or combinations thereof were challenged with high doses of virulent type 2 or type 4 pneumococci. The median survival times for mice immunized with combinations of proteins, particularly PdB and PspA, were significantly longer than those for mice immunized with any of the antigens alone. A similar effect was seen in a passive protection model. Thus, combinations of pneumococcal proteins may provide the best non-serotype-dependent protection against S. pneumoniae.

Preparation of pneumococcal capsular polysaccharide-protein conjugate vaccines utilizing new fragmentation and conjugation technologies

There is a global urgent need for a new efficient and inexpensive vaccine to combat pneumococcal disease, which should also be affordable in developing countries. In view of this need a simple low-cost technique to prepare such a vaccine was developed. The preparation of serotype 14 and 23F pneumococcal capsular polysaccharide (PnPS)-protein conjugates to be included in a forthcoming multivalent PnPS conjugate vaccine is described. Commercial lots of PnPSs produced according to Good Manufacturing Practice from Streptococcus pneumoniae serotype 14 (PS14) and 23F (PS23F) were partially depolymerized by sonication or irradiation in an electron beam accelerator. The PnPS fragments were conjugated to tetanus toxoid (TT) using a recently developed conjugation chemistry. The application of these new simple, efficient and inexpensive fragmentation and conjugation technologies allowed the synthesis of several PnPS-protein conjugates containing PnPS fragments of preselected sizes and differing in the degree of substitution. The PS14TT and PS23FTT conjugate vaccine candidates were characterized chemically and their immunogenicity was evaluated in rabbits and mice. All PnPS conjugate vaccines, unlike the corresponding plain polysaccharides, produced high IgG titres in both animal species. The PS14TT conjugates tended to be more immunogenic than the PS23FTT conjugates. The immune response to the PS14TT conjugates, but not to the PS23FTT conjugates, was related to the size of the conjugated polysaccharide hapten. Both types of conjugates elicited strong booster effects upon secondary immunizations, resulting in high IgG1, IgG2a and IgG2b titres.

Immunogenicity of Streptococcus pneumoniae type 6B and 14 polysaccharide-tetanus toxoid conjugates and the effect of uncoupled polysaccharide on the antigen-specific immune response

The immunogenicity of two types of Streptococcus pneumoniae capsular polysaccharide-tetanus toxoid conjugates (PS6BTT and PS14TT) was evaluated in mice. Both conjugates induced high titres of high avidity type-specific anti-PS IgG, which include all IgG isotypes except IgG2a. Repeated immunization resulted in booster responses in both cases. The antibodies induced exhibited opsonic activity, as measured in an in vitro opsonophagocytosis assay, using the mouse macrophage cell line RAW-264. Furthermore, the influence of spiking PS6BTT with free PS6B of either 1000 kDa (native) or 37 kDa was investigated. The results indicate that not only the amount but also the molecular weight of the free PS6B present in the conjugate vaccine affect the anti-PS6B immune response. Large amounts of free PS6B of both molecular weights decrease each anti-PS6B IgG isotype response. However, unlike admixture of the low molecular weight PS6B, addition of the high molecular weight PS6B leads to a rather persistent state of unresponsiveness.

Multivalent pneumococcal capsular polysaccharide conjugate vaccines employing genetically detoxified pneumolysin as a carrier protein

A genetically detoxified pneumolysin, pneumolysoid (PLD), was investigated as a carrier protein for pneumococcal capsular polysaccharide (CPS). Such a CPS-PLD conjugate might provide additional protection against pneumococcal infections and resultant tissue damage. A single point mutant of pneumolysin was selected, which lacked measurable haemolytic activity, but exhibited the overall structural and immunological properties of the wild type. PLD conjugates were prepared from CPS serotypes 6B, 14, 19F, and 23F by reductive amination. The structural features of free PLD, as well as the corresponding CPS-PLD, as assessed by circular dichroism spectroscopy, were virtually indistinguishable from the wild type counterpart. Each of the CPS monovalent and tetravalent conjugate formulations were examined for immunogenicity in mice at both 0.5 and 2.0 micrograms CPS per dose. Tetanus toxoid (TT) conjugates were similarly created and used for comparison. The resultant conjugate vaccines elicited high levels of CPS-specific IgG that was opsonophagocytic for all serotypes tested. Opsonophagocytic titres, expressed as reciprocal dilutions resulting in 50% killing using HL-60 cells, ranged from 100 to 30,000, depending on the serotype and formulation. In general, the lower dose and tetravalent formulations yielded the best responses for all serotypes (i.e., either equivalent or better than the higher dose and monovalent formulations). The PLD conjugates were also generally equivalent to or better in CPS-specific responses than the TT conjugates. In particular, both the PLD conjugate and the tetravalent formulations induced responses for type 23F CPS that were approximately an order of magnitude greater than that of the corresponding TT conjugate and monovalent formulations. In addition, all the PLD conjugates elicited high levels of pneumolysin-specific IgG which were shown to neutralize pneumolysin-induced haemolytic activity in vitro. As a result of these findings, PLD appears to provide an advantageous alternative to conventional carrier proteins for pneumococcal multivalent CPS conjugate vaccines.

Standardization of an opsonophagocytic assay for the measurement of functional antibody activity against Streptococcus pneumoniae using differentiated HL-60 cells

Host protection against pneumococcal disease i primarily mediated by phagocytosis. We developed and standardized an opsonophagocytic assay using HL-60 cells (human promyelocytic leukemia cells). Fifty-five serum samples were analyzed for the presence of functional antibody against seven pneumococcal serogroups or serotypes (4, 6B, 9V, 14, 18C, 19F, and 23F) by using differentiated HL-60 cells (granulocytes) and peripheral blood leukocytes (PBLs). Six of the 55 serum samples were from unvaccinated adult volunteers, 31 serum samples were from adults who received one dose of the 14-valent or the 23-valent polysaccharide vaccine, and 18 serum samples were from 16-month-old infants who received four doses of an investigational 7-valent polysaccharide-protein conjugate vaccine. The results of an opsonophagocytic assay with HL-60 cells correlated highly with those of an assay with PBLs as effector cells (median r for seven serotypes = 0.87: P < 0.01). Opsonophagocytic titers were compared with the immunoglobulin G antibody concentrations determined by enzyme-linked immunosorbent assay (ELISA). The r values for serogroups or serotypes 4, 6B, 9V, 14, 18C, 19F, and 23F were 0.61, 0.60, 0.67 0.90, 0.61, 0.39, and 0.57, respectively, when HL-60 cells were used as effector cells and 0.56, 0.47, 0.61, 0.90, 0.71, 0.31, and 0.62, respectively, when PBLs were used. The assay requires small amounts of serum (40 microliters per serotype), making this test suitable for assaying infant sera. Culturable cells aid in assay standardization and likely reduce donor-to-donor variability. This standardized assay, in combination with the standardized ELISA, can be used to evaluate current and developing pneumococcal vaccines, in which functional opsonophagocytic antibody activity may correlate with protection against 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.

Production, binding characteristics and protective immunity of monoclonal antibody to pneumococcal type-9V conjugate

A monoclonal antibody (MAb) to pneumococcal type-9V polysaccharide (PS) was produced using PS conjugated to inactivated pneumolysin as the immunogen. The MAb to 9V PS was of the IgG1 subclass. The antigen-antibody reaction increased rapidly at low concentrations and reached a plateau at 10 micrograms PS/ml as measured by nephelometry of the group 9 PS against 9V MAb binding. In contrast, the binding of group 9 PS against rabbit 9V antiserum (AS) increased proportionally and continued to increase up to the highest concentration of PS tested (20 micrograms PS/ml). The 9V MAb reacted with all group 9 PSs (9A, 9L, 9N and 9V) by immunodiffusion. In the homologous 9V Ag-MAb reaction, there were marked differences in the inhibition of binding by the cross-reactive 9L PS (19.2% inhibition) and the 9N PS (0.2%). In contrast, inhibition of the homologous 9V Ag-rabbit AS binding by cross-reactive 9L and 9N PSs ranged from 57.8 to 62.7%. Removal of the O-acetyl group from 9V PS by alkali hydrolysis resulted in decreased binding with rabbit 9V AS. However, the binding reaction with 9V MAb was less affected by the loss of O-acetyl content. The 9V MAb was both opsonic and passively protected young mice against challenge with type-9V pneumococci.

Roles of autolysin and pneumolysin in middle ear inflammation caused by a type 3 Streptococcus pneumoniae strain in the chinchilla otitis media model

Streptococcus pneumoniae cell wall and pneumolysin are important contributors to pneumococcal pathogenicity in some animal models. To further explore these factors in middle ear inflammation caused by pneumococci, penicillin-induced inflammatory acceleration was studied by using three closely related pneumococcal strains: a wild-type 3 strain (WT3), its pneumolysin-negative derivative (P-1), and into autolysin-negative derivative (A-1). Both middle ears of chinchillas were inoculated with one of the three pneumococcal strains. During the first 12 h, all three strains grew in vivo at the same rate, and all three strains induced similar inflammatory cell responses in middle ear fluid (MEF). Procaine penicillin G was given as 12 h to one-half of the animals in each group, and all treated chinchillas had sterile MEF at 24 h. Penicillin significantly accelerated MEF inflammatory cell influx into WT3-and P-1-infected ears at 18 and 24 h in comparison with the rate for penicillin-treated A-1-infected ears. Inflammatory cell influx was slightly, but not significantly, greater after treatment of WT3 infection than after treatment of P-1 infection. Interleukin (IL)-1beta and IL-6, but not IL-8, concentrations in MEF at 24 h reflected the penicillin effect on MEF inflammatory cells; however, differences between treatment groups were not significant. Results suggest that pneumococcal otitis media pathogenesis is triggered principally by the inflammatory effects of intact and lytic cell wall products in the middle ear, with at most a modes additional pneumolysin effect. Investigation strategies that limit the release of these products or neutralize them warrant further investigation.

New vaccines for the prevention of pneumococcal infections

Streptococcus pneumoniae is a major cause of acute otitis media, pneumonia, bacteremia, and meningitis. Because in recent years antibiotic-resistant pneumococcal strains have been emerging throughout the world, vaccination against pneumococcal infections has become more urgent. The capsular polysaccharide vaccine that has been available is neither immunogenic nor protective in young children and other immunocompromised patients. Several pneumococcal proteins have been proposed as candidate vaccines, but no human studies associated with them have been reported. Clinical trials of first-generation pneumococcal conjugate vaccines have shown that covalent coupling of pneumococcal capsular polysaccharides to protein carriers improves the immunogenicity of the polysaccharides. The protective efficacy of the conjugate vaccines against carriage, acute otitis media, and invasive infections is being studied.

Capsular polysaccharide-protein conjugate vaccines of carbotype 1 Vibrio vulnificus: construction, immunogenicity, and protective efficacy in a murine model

Vibrio vulnificus causes septicemia and wound infections in immunocompromised humans. The capsular polysaccharide of Vibrio vulnificus (VvPS) is critical for virulence. We synthesized conjugate vaccines of carbotype 1 VvPS under conditions and in formulations suitable for human use. Purified VvPS was conjugated to tetanus toxoid (TT) or to inactivated V. vulnificus cytolysin or elastase by two different schemes. All conjugates elicited elevated anticapsular immunoglobulin G (IgG) and IgM and antiprotein IgG responses in mice compared with saline placebo. The conjugates prepared through caboxyl activation of VvPS (VvPS-TTa, VvPS-cytolysin, and VvPS-elastase) were more immunogenic than the one prepared through hydroxyl activation (VvPS-TTb). The protective efficacy of conjugated and unconjugated formulations of VvPS and that of protein carriers were evaluated in a mouse septicemia model. Eighty percent of mice actively immunized with VvPS-TTa vaccine survived challenge with carbotype 1 V. vulnificus, while VvPS-cytolysin and VvPS-elastase conjugates conferred 44 and 40% protection, respectively. Control mice immunized with VvPS, cytolysin, or elastase alone, or saline only, showed 70 to 100% mortality. VvPS-TTa vaccine is nontoxic, immunogenic, and protective in mice.

Characterization of a recombinant pneumolysin and its use as a protein carrier for pneumococcal type 18C conjugate vaccines

Pneumolysin from Streptococcus pneumoniae was expressed in Escherichia coli as a glutathione S-transferase fusion protein and purified by affinity and hydroxylapatite chromatography. The purified recombinant pneumolysin (rPL), with a molecular mass of 53 kDa, had a specific activity of 3 x 10(5) hemolytic units per mg of protein on rabbit erythrocytes and reacted identically in immunodiffusion with the antisera against native pneumolysin. The rPL was used as a protein carrier to prepare conjugate vaccine with pneumococcal type 18C polysaccharide (PS18C). The PS18C was directly coupled to rPL by reductive animation or was indirectly coupled to rPL via a spacer molecule, adipic acid dihydrazide. The conjugates were nontoxic for mice and guinea pigs at 100 micrograms per dose. The immunogenicity and protective efficacy of both conjugates were tested in mice. A single dose of either of the vaccines elicited a rise in immunoglobulin G antibody production; after two booster injections of the vaccines, statistically significant booster responses (P < 0.001) to both rPL and PS18C were produced. The sera containing the antibodies to rPL were capable of neutralizing the hemolytic activity of rPL to rabbit erythrocytes and the cytotoxicity of rPL to bovine pulmonary endothelial cells. Immunization with the conjugate vaccines conferred statistically significant protection in mice against lethal challenge with type 18C pneumococci.