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.

Asthma and antibodies to pneumococcal virulence proteins

PURPOSE

We previously reported that asthmatics had lower anti-serotype-specific pneumococcal polysaccharide antibody levels than non-asthmatics, and the T-helper 2 (Th2) immune profile was associated with suboptimal pneumococcal polysaccharide antibody. Our objective was to determine the influence of asthma status on anti-pneumococcal protein antigen antibody levels.

METHODS

We conducted a cross-sectional study, which enrolled 16 children and adults with asthma and 14 subjects without asthma. Asthma was ascertained by predetermined criteria. Serum IgG antibody levels to pneumococcal surface protein A (PspA), pneumococcal surface protein C (PspC), pneumococcal choline-binding protein A (PcpA), and pneumolysin (PLY) were measured by enzyme-linked immunosorbent assays (ELISA). These antibody levels were compared between asthmatics and non-asthmatics. The Th2 immune profile was determined by IL-5 secretion from PBMCs cultured with house dust mite (HDM) and staphylococcal enterotoxin B (SEB) at day 7. The correlation between the anti-pneumococcal antibody levels and the Th2-HDM and SEB-responsive immune profile was assessed.

RESULTS

Of the 30 subjects, 16 (53%) were male and the median age was 26 years. There were no significant differences in anti-PspA, anti-PspC, anti-PcpA, and anti-PLY antibody levels between asthmatics and non-asthmatics. The Th2 immune profile was inversely correlated with the anti-PspC antibody levels (r = -0.53, p = 0.003). This correlation was significantly modified by asthma status (r = -0.74, p = 0.001 for asthmatics vs. r = -0.06, p = 0.83 for non-asthmatics). Other pneumococcal protein antibodies were not correlated with the Th2 immune profile.

CONCLUSION

No significant differences in the anti-pneumococcal protein antigen antibody levels between asthmatics and non-asthmatics were found. Asthma status is an important effect modifier determining the negative influence of the Th2 immune profile on anti-PspC antibody levels.

Prospects for use of interleukin-12 as a mucosal adjuvant for vaccination of humans to protect against respiratory pneumococcal infection

Mucosal vaccination against pneumococcal disease offers potential protection against otitis media, pneumonia and invasive disease, including providing herd benefit by reducing pathogen carriage. The major obstacle, however, remains the lack of a suitable adjuvant for use in humans. Animal models have demonstrated success of interleukin-12 (IL-12) as an adjuvant for mucosal vaccines using recombinant pneumococcal protein antigens. This review examines the biology of the IL-12 cytokine family, the toxicity of IL-12 in human studies and suggests approaches by which IL-12 could be developed as a mucosal adjuvant with pneumococcal protein based vaccines, for use in humans.

Pneumococcal surface protein A (PspA) is effective at eliciting T cell-mediated responses during invasive pneumococcal disease in adults

Humoral immune response is essential for protection against invasive pneumococcal disease and this property is the basis of the polysaccharide-based anti-pneumococcal vaccines. Pneumococcal surface protein A (PspA), a cell-wall-associated surface protein, is a promising component for the next generation of pneumococcal vaccines. This PspA antigen has been shown to stimulate an antibody-based immunity. In the present study, we evaluated the capacity of PspA to stimulate CD4+ T cells which are needed for the correct development of a B cell based immune response in humans. Cellular immunity to PspA was evaluated by whole-blood culture with different pneumococcal antigens, followed by flow cytometric detection of activated CD4+CD25+ T cells. T cell-mediated immune responses to recombinant PspA proteins were assessed in acute-phase and convalescent blood from adults with invasive pneumococcal disease and in blood from healthy subjects. All cases had detectable antibodies against PspA on admission. We found that invasive pneumococcal disease induced transient T cell depletion but adaptive immune responses strengthened markedly during convalescence. The increased production of both interleukin (IL)-10 and interferon (IFN)-gamma during convalescence suggests that these cytokines may be involved in modulating antibody-based immunity to pneumococcal disease. We demonstrated that PspA is efficient at eliciting T cell immune responses and antibodies to PspA. This study broadens the applicability of recombinant PspA as potent pneumococcal antigen for vaccination against S. pneumoniae.

Antibodies to the pneumococcal surface protein A, PspA, can be produced in splenectomized and can protect splenectomized mice from infection with Streptococcus pneumoniae

Asplenic individuals have increased susceptibility to septicemia caused by encapsulated bacteria. Streptococcus pneumoniae, a pathogen carried in the nasal passages of many humans without complication, is responsible for a large proportion of infections seen in asplenic individuals. Our studies have evaluated the efficacy of antibodies to pneumococcal surface protein A (PspA) in protection of asplenic mice. In passive immunity studies, pneumococci were more completely cleared from the blood of splenectomized mice receiving passive antiserum to PspA than those receiving normal rabbit serum. From active mucosal (intranasal) and systemic (subcutaneous) immunizations with rPspA, we determined that the levels of PspA antibodies produced in splenectomized mice were not significantly different from levels seen in mock-splenectomized animals. This active immunity to PspA was able to protect splenectomized mice against death following infection with live pneumococci. Our results suggest that PspA immunization may also protect asplenic humans from pneumococcal infections.

PspA family typing and PCR-based DNA fingerprinting with BOX A1R primer of pneumococci from the blood of patients in the USA with and without sickle cell disease

Disease and mortality rates for Streptococcus pneumoniae infections are much higher in patients with sickle cell disease (SCD) than in age-matched patients without SCD. Pneumococcal surface protein A (PspA) has been proposed as a component in human vaccines against S. pneumoniae to provide greater breadth of coverage than can be obtained with the 7-valent conjugate vaccine. The cross-reactivity of PspA is associated with the 'PspA family' structure. In this study we examined strains of S. pneumoniae from patients with and without SCD to determine whether the strains infecting the hypersusceptible population of SCD patients were limited to the same two PspA families already known to comprise over 95% of strains infecting non-SCD patients. Each strain was also evaluated according to the presence or absence of specific PCR fragments based on repetitive BOX elements to screen for possible SCD-associated clonal structure. Strains from SCD and non-SCD patients were similarly dispersed among the most common BOX PCR groups and strains from both groups expressed a similar distribution of PspA variants. Thus, a PspA vaccine designed for the population at large should also be appropriate for patients with SCD.

Natural materno-fetal transfer of antibodies to PspA and to PsaA

PspA and PsaA are Streptococcus pneumoniae surface proteins and potential pneumococcal vaccine antigens. The aim of this study was to characterize the transplacental transfer of antibodies to PspA and to PsaA. Paired mother and cord blood sera were obtained at delivery from 28 women. Concentrations of antibodies against PspA, PsaA, tetanus toxoid (vaccine-induced antibodies) and P6-outer membrane protein (OMP) of nontypeable Haemophilus influenzae were determined by ELISA. Antibodies to PspA of the IgG, IgG1 and IgG2 antibodies were also determined. The geometric mean percentage (GM%) of the paired infant:mother antibody were calculated.

RESULTS

The GM% of the infant:mother antibody concentrations against PspA, PsaA and P6-OMP antibodies were 64.7% (3.3 micro g/ml in infants vs. 5.1 micro g/ml in mothers), 50.4% (6.8 micro g/ml vs. 13.5 micro g/ml) and 66.7% (5.6 micro g/ml vs. 8.4 micro g/ml), respectively; the GM% of antibodies against tetanus toxoid was 104.5% (4.6 micro g/ml vs. 4.4 micro g/ml). Transplacental transfer of IgG1 was more efficient than that of IgG2 (approximately 120%vs. 65%). A transplacental transfer of antibodies to PspA and to PsaA exist. Moreover, these data suggest an active placental transfer of IgG1 antibodies to PspA since the concentration of these antibodies were consistently higher in cord sera than in the mother's sera.

Pneumococcal surface protein A is expressed in vivo, and antibodies to PspA are effective for therapy in a murine model of pneumococcal sepsis

Pneumococcal surface protein A (PspA) is an immunogenic protein expressed on the surface of all strains of Streptococcus pneumoniae (pneumococcus) and induces antibodies which protect against invasive infection in mice. Pneumococci used for infectious challenge in protection studies are typically collected from cultures grown in semisynthetic medium in vitro. The purpose of these studies is to confirm that PspA is expressed by pneumococci during growth in vivo at a level sufficient for antibodies to PspA to be protective. Mice were actively immunized with purified PspA or by passive transfer of monoclonal antibody (MAb) and challenged with a capsular type 3 strain in diluted whole blood from bacteremic mice. All were protected against challenge with 10 times the 50% lethal dose (LD(50)), and mice challenged with 1,000 times the LD(50) had increased survival compared with controls. Additionally, nonimmune mice treated with MAbs to PspA or PspA immune serum at 6 and 12 h after infection with 10 times the LD(50) also showed increased survival. Northern blot analysis of RNA from pneumococci grown either in vitro or in vivo showed similar levels of PspA mRNA. These results demonstrate that PspA is expressed in vivo in a mouse model and that immunization with PspA induces antibodies to an antigen which is expressed during the course of invasive infection. Immunotherapy with antibodies to PspA may have some utility in treating pneumococcal infections in humans.

Intranasal vaccination with pneumococcal surface protein A and interleukin-12 augments antibody-mediated opsonization and protective immunity against Streptococcus pneumoniae infection

Streptococcus pneumoniae is a major pathogen in humans that enters the host primarily through the respiratory tract. Targeting mucosal surfaces directly may therefore be an optimal approach for vaccination to prevent bacterial colonization and invasive disease. We have previously demonstrated the effectiveness of interleukin-12 (IL-12) delivered intransally (i.n.) as an antiviral respiratory adjuvant. In this study, we examined the effects of i.n. IL-12 treatment on induction of protective humoral immunity against S. pneumoniae. Immunization i.n. with pneumococcal surface protein A (PspA) and IL-12 resulted in enhanced lung IL-10 mRNA expression and marked augmentation of respiratory and systemic immunoglobulin G1 (IgG1), IgG2a, and IgA antibody levels compared to those in animals receiving PspA alone. In addition, i.n. vaccination with PspA and IL-12 provided increased protection against nasopharyngeal carriage. Flow cytometric analysis revealed a threefold increase in antibody-mediated, complement-independent opsonic activity in the sera of PspA- and IL-12-treated animals, which was mainly contributed by IgG2a and, to a lesser extent, IgA. Passive transfer of these immune sera conferred complete protection from death upon systemic pneumococcal challenge. These findings demonstrate the effectiveness of combining PspA and IL-12 at mucosal sites to achieve optimal antibody-mediated opsonization and killing of S. pneumoniae.

Pneumococcal pulmonary infection, septicaemia and survival in young zinc-depleted mice

The aim of the present study was to explore whether mice fed a diet low in Zn (2.0 mg Zn/kg diet) for a relatively short period of time were more prone to severe Streptococcus pneumoniae infection than mice fed a normal diet (25 mg elemental Zn/kg). The Zn-deficient mice were compared with mice in two Zn-adequate control groups; one pair-fed and another with free access to the diet. After 2 weeks feeding, the mice were infected intranasally under anaesthesia with a suspension containing about 10(7) pneumococci. Clinical status was observed every day and blood samples were examined for S. pneumoniae every second day for a week. All infected mice examined carried the infecting strain intranasally. The survival time and time before positive blood culture were significantly shorter in the Zn-depleted group than in the pair-fed Zn-adequate group (hazard ratios 15.6 and 3.2, and respectively). At the end of the observation period, ten of the twelve mice in the Zn-deficient group were dead while one of twelve and two of twelve were dead in the two Zn-adequate control groups. This study shows that even acutely-induced Zn deficiency dramatically increases the risk of serious pneumococcal infection in mice.

Clones of Streptococcus pneumoniae isolated from nasopharyngeal carriage and invasive disease in young children in central Tennessee

To determine whether nasopharyngeal carriage isolates of Streptococcus pneumoniae are of the same genetic background as isolates that caused invasive disease in one community, IS1167 and boxA genotypes were obtained for 182 pneumococcal isolates from children living in central Tennessee. The isolates represented 70 combined IS1167-boxA genotypes. The genotypic diversity of the invasive isolates was significantly less than that of the total population (P=.003). Most of the carriage isolates belonged to genotypes unique to carriage, whereas most of the invasive isolates belonged to genotypes common to carriage and disease (P=.02). Monte Carlo simulations showed a greater number of genotypes unique to carriage than can be explained by chance (P<.05 in all cases). Two genotypes were identified by multilocus sequence typing as members of globally disseminated clones, and one such genotype that was strictly carriage in this sample caused disease in other studies. Thus, clones can have different propensities for carriage and invasion.

Characterization of binding of human lactoferrin to pneumococcal surface protein A

Human lactoferrin is an iron-binding glycoprotein that is particularly prominent in exocrine secretions and leukocytes and is also found in serum, especially during inflammation. It is able to sequester iron from microbes and has immunomodulatory functions, including inhibition of both complement activation and cytokine production. This study used mutants lacking pneumococcal surface protein A (PspA) and PspC to demonstrate that the binding of human lactoferrin to the surface of Streptococcus pneumoniae was entirely dependent on PspA. Lactoferrin bound both family 1 and family 2 PspAs. Binding of lactoferrin to PspA was shown by surface colocalization with PspA and was verified by the lack of binding to PspA-negative mutants. Lactoferrin was expressed on the body of the cells but was largely absent from the poles. PspC showed exactly the same distribution on the pneumococcal surface as PspA but did not bind lactoferrin. PspA's binding site for lactoferrin was mapped using recombinant fragments of PspA of families 1 and 2. Binding of human lactoferrin was detected primarily in the C-terminal half of the alpha-helical domain of PspA (amino acids 167 to 288 of PspA/Rx1), with no binding to the N-terminal 115 amino acids in either strain. The interaction was highly specific. As observed previously, bovine lactoferrin bound poorly to PspA. Human transferrin did not bind PspA at all. The binding of lactoferrin to S. pneumoniae might provide a way for the bacteria to interfere with host immune functions or to aid in the acquisition of iron at the site of infection.

The autolytic enzyme LytA of Streptococcus pneumoniae is not responsible for releasing pneumolysin

It was previously proposed that autolysin's primary role in the virulence of pneumococci was to release pneumolysin to an extracellular location. This interpretation came into question when pneumolysin was observed to be released in significant amounts from some pneumococci during log-phase growth, because autolysis was not believed to occur at this time. We have reexamined this phenomenon in detail for one such strain, WU2. This study found that the extracellular release of pneumolysin from WU2 was not dependent on autolysin action. A mutant lacking autolysin showed the same pattern of pneumolysin release as the wild-type strain. Addition of mitomycin C to a growing WU2 culture did not induce lysis, indicating the absence of resident bacteriophages that could potentially harbor lytA-like genes. Furthermore, release of pneumolysin was unaltered by growth in 2% choline, a condition which is reported to inactivate autolysin, as well as most known pneumococcal phage lysins. Profiles of total proteins in the cytoplasm and in the supernatant media supported the hypothesis that release of pneumolysin is independent of pneumococcal lysis. Finally, under some infection conditions, mutations in pneumolysin and autolysin had different effects on virulence.

IL-12-mediated increases in protection elicited by pneumococcal and meningococcal conjugate vaccines

Interleukin-12 (IL-12) may be a beneficial adjuvant for augmenting vaccine efficacy against encapsulated bacteria such as Streptococcus pneumoniae and Neisseria meningitidis since it can stimulate production of interferon-gamma (IFN-gamma) and secretion of antibody isotypes that are efficient at mediating complement fixation and opsonophagocytosis. In this study, we demonstrate the ability of IL-12 to enhance murine antibody responses, particularly IgG2a levels, to both pneumococcal and meningococcal conjugate vaccines. Transfer of immune serum from mice immunized with the meningococcal conjugate vaccine and IL-12 resulted in increased survival times, whereas transfer of serum from mice immunized with the pneumococcal conjugate and IL-12 resulted in protection from death upon bacterial challenge. Although treatment with vaccine and IL-12 increased levels of IFN-gamma mRNA, IL-12-mediated enhancement of antibody responses still occurred in IFN-gamma(-/-) mice. The results demonstrate the effectiveness of IL-12 as an adjuvant for polysaccharide conjugate vaccines, especially the pneumococcal conjugate vaccine.

Streptococcus pneumoniae: new tools for an old pathogen

The pneumococcus is one of the longest-known pathogens. It has been instrumental to our understanding of biology in many ways, such as in the discovery of the Gram strain and the identification of nucleic acid as the hereditary material. Despite major advances in our understanding of pneumococcal pathogenesis, the need for vaccines and antibiotics to combat this pathogen is still vital. Genomics is beginning to uncover new virulence factors to advance this process, and it is enabling the development of DNA chip technology, which will permit the analysis of gene expression in specific tissues and in virulence regulatory circuits.

Pneumococcal surface protein A of invasive Streptococcus pneumoniae isolates from Colombian children

Pneumococcal surface protein A (PspA) elicits protection in mice against fatal bacteremia and sepsis caused by genetically diverse pneumococci and protects against carriage and lung infection. We determined the PspA families of invasive isolates of Streptococcus pneumoniae recovered from Colombian children <5 years of age. That 97.5% of Colombian isolates belong to PspA families 1 and 2 supports the hypothesis that a human PspA vaccine covering a few PspA families could be broadly effective.

The potential for using protein vaccines to protect against otitis media caused by Streptococcus pneumoniae

Potential vaccine strategies against otitis media are to prevent (1) symptomatic infections in the middle ear and/or (2) carriage of pneumococci and thereby subsequent middle ear infections. The possibility of using immunity to virulence proteins of pneumococci to elicit immunity against pneumococci has been examined. PspA has been found to have efficacy against otitis media in animals. Vaccination with a mixture of PsaA and PspA has been observed to offer better protection against nasal carriage in mice, than vaccination with either protein alone. PspA and pneumolysin have been shown to elicit protection against invasive infections. The inclusion of a few of these proteins into the polysaccharide-protein conjugate vaccines may be able to enhance their efficacy against otitis media and might be able to constitute a successful all-protein pneumococcal vaccine.

Immunization of humans with recombinant pneumococcal surface protein A (rPspA) elicits antibodies that passively protect mice from fatal infection with Streptococcus pneumoniae bearing heterologous PspA

Pneumococcal surface protein A (PspA), a cross-reactive protein expressed by all pneumococci, is known to elicit an antibody in animals that can passively protect mice from infection with Streptococcus pneumoniae. A phase I trial with recombinant PspA showed the protein to be immunogenic in humans. Pre- and postimmune serum samples from this trial were examined, and human antibody to PspA could protect mice from pneumococcal infection. The serum samples of subjects immunized twice with 125 microg of PspA had >100 times as much antibody per milliliter as was required to consistently protect mice from fatal infection (1.3 microg/dose). At least 98% of PspAs fall into PspA sequence/serologic families 1 or 2. Human antibodies elicited by a family 1 PspA protected against infection with S. pneumoniae expressing either family 1 or 2 PspAs and with strains of all 3 capsular types tested: 3, 6A, and 6B. These studies suggest that PspA may have efficacy as a human vaccine.

Diversity of PspA: mosaic genes and evidence for past recombination in Streptococcus pneumoniae

Pneumococcal surface protein A (PspA) is a serologically variable protein of Streptococcus pneumoniae. Twenty-four diverse alleles of the pspA gene were sequenced to investigate the genetic basis for serologic diversity and to evaluate the potential of diversity to have an impact on PspA's use in human vaccination. The 24 pspA gene sequences from unrelated strains revealed two major allelic types, termed "families," subdivided into clades. A highly mosaic gene structure was observed in which individual mosaic sequence blocks in PspAs diverged from each other by over 20% in many cases. This level of divergence exceeds that observed for blocks in the penicillin-binding proteins of S. pneumoniae or in many cross-species comparisons of gene loci. Conversely, because the mosaic pattern is so complex, each pair of pspA genes also has numerous shared blocks, but the position of conserved blocks differs from gene pair to gene pair. A central region of pspA, important for eliciting protective antibodies, was found in six clades, which each diverge from the other clades by >20%. Sequence relationships among the 24 alleles analyzed over three windows were discordant, indicating that intragenic recombination has occurred within this locus. The extensive recombination which generated the mosaic pattern seen in the pspA locus suggests that natural selection has operated in the history of this gene locus and underscores the likelihood that PspA may be important in the interaction between the pneumococcus and its human host.

Natural development of antibodies to pneumococcal surface protein A, pneumococcal surface adhesin A, and pneumolysin in relation to pneumococcal carriage and acute otitis media

Pneumococcal surface protein A (PspA), pneumococcal surface adhesin A (PsaA), and pneumolysin (Ply) are common to virtually all Streptococcus pneumoniae isolates. They are immunogenic and protective against pneumococcal challenge in animals and are the major candidates for a protein-based pneumococcal vaccine for humans. However, little is known of the natural development of antibodies to these proteins in humans. The objective of this study was to evaluate the natural development of antibodies to PspA, PsaA, and Ply in relation to pneumococcal infection and carriage in young children. Serum antibodies to these proteins were measured by EIA in children at ages 6, 12, 18, and 24 months and in their mothers. All age groups were capable of producing antibodies to the 3 proteins. The antibody concentrations increased with age and were strongly associated with pneumococcal exposure, whether by carriage or infection (acute otitis media).

Competition among Streptococcus pneumoniae for intranasal colonization in a mouse model

Widespread use of conjugate vaccines against Streptococcus pneumoniae, by reducing carriage of S. pneumoniae serotypes included in the vaccine, may result in an increase in nasopharyngeal carriage of - and disease from - nonvaccine serotypes of the same species. Mathematical models predict that the extent of such replacement will depend positively on the degree to which carriage of vaccine-type S. pneumoniae inhibits acquisition of nonvaccine-type pneumococci, and may depend negatively on the inhibition of vaccine-type pneumococci by nonvaccine-type pneumococci. We used a mouse model of intranasal carriage of pneumococci to test whether such inhibition occurs between different pneumococcal strains. Mice carrying a streptomycin-resistant derivative of S. pneumoniae BG9163 (serotype 6B) as a resident strain showed reduced levels of colonization when challenged intranasally by optochin-resistant derivatives of the same strain and of a serotype 23F pneumococcus, BG8826. Inhibition could be overcome by increasing the dose of the challenge strain. Carriage of optochin-resistant BG9163 did not inhibit acquisition of the streptomycin-resistant variant. Colonization by a challenge strain did not significantly affect the level of colonization with the resident strain. These results provide evidence that is consistent with several hitherto untested assumptions of mathematical models of serotype replacement and suggest that a biological mechanism exists that could account for serotype replacement that is observed in clinical trials. The findings provide a basis for further studies of in vivo interactions between strains of S. pneumoniae.

Development of a multi-specificity opsonophagocytic killing assay

The opsonophagocytic-killing assay (OPKA) is one of the primary surrogate assays for evaluating the pneumococcal capsular polysaccharide-protein conjugates under development as vaccines. Because each vaccine contains seven or more different conjugates, multiple OPKA must be performed on each serum. Moreover, the large number of assays can deplete serum samples from infants. To reduce the amount of serum and effort required to conduct OPKA we developed a multi-specificity OPKA using antibiotic resistant pneumococci. Equal numbers of optochin-resistant serotype 6B and streptomycin-resistant 19F pneumococci were used as the target bacteria. Surviving bacteria of each serotype were enumerated by plating on agar containing the appropriate antibiotic. In an examination of 25 immune sera the results obtained with this new assay correlated well with those obtained when bacterial targets were examined individually. By using additional antibiotic resistance markers, more than two specificities can be examined in a single assay.

A latex bead-based flow cytometric immunoassay capable of simultaneous typing of multiple pneumococcal serotypes (Multibead assay)

A simple and rapid method of simultaneously determining 15 Streptococcus pneumoniae serotypes was developed. Fifteen latex beads of different sizes and different red fluorescence levels were coated with 1 of 15 serotypes (1, 3, 4, 5, 6A, 6B, 7F, 9N, 9V, 14, 18C, 19A, 19F, 22F, and 23F) of pneumococcal capsular polysaccharide (PS). The bead mixture was incubated with individual pneumococcal lysate, a pool of rabbit antisera capable of binding the 15 serotypes, and fluorescein (green fluorescence)-conjugated anti-rabbit antibody. Bead size, red fluorescence, and green fluorescence were measured in a single flow cytometer run. The green fluorescence of the beads was inhibited only when there was a serotypic match between PS on the bead and PS in the pneumococcal lysate. This method distinguished cross-reactive serotypes and correctly identified the serotypes in 100% of 86 pneumococcal isolates tested.

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.

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.

Purification and characterization of Streptococcus pneumoniae palmitoylated pneumococcal surface adhesin A expressed in Escherichia coli

All Streptococcus pneumoniae isolates tested to date express a species-common lipoprotein designated as pneumococcal surface adhesin A (PsaA). This protein is cell-associated, hydrophobic, immunogenic, and genetically conserved. It is currently under investigation as a potential component in third-generation pneumococcal vaccine formulations. To overcome the problem of low-level expression of native hydrophobic PsaA in S. pneumoniae, and also of the recombinant PsaA (rPsaA) in Escherichia coli, we generated a stable E. coli construct expressing functional palmitoylated rPsaA ( approximately 10 mg/l of fermentation culture) using Borrelia burgdorferi outer surface protein A (OspA, a hydrophobic lipoprotein) signal peptide. By Western blot analysis, the chimeric rPsaA ( approximately 34 kDa) was detected in the cell lysate using anti-PsaA antibodies. It was partially purified by extracting the cell pellet with PBS/Triton X(R)-114 buffers, followed by anion exchange filter chromatography. A trypsin digestion profile of rPsaA closely resembled that of the native protein, as revealed by SDS-PAGE/silver staining. Lipidation of rPsaA was confirmed by labeling recombinant E. coli cells with [(3)H] palmitic acid and analyzing the labeled E. coli cells by Western blotting coupled with autoradiography. Further, analysis of purified rPsaA by mass spectrometry (MALDI-TOF) revealed a heterogenous spectrum with a major peak (M+H)(+1) of mass 33,384 Da (theoretical mass of palmitoylated rPsaA=33,361 Da). Purified rPsaA was immunogenic in CBA/NCAHN-XID female mice following intranasal immunization with or without adjuvant, as determined by measurement of anti-PsaA serum IgG levels. These anti-PsaA antibodies reacted with both native and rPsaA polypeptides. Our data strongly suggest that E. coli-expressed rPsaA is palmitoylated and closely resembles the native protein in structure and immunogenicity. It was also observed to elicit measurable protection against nasopharyngeal carriage with S. pneumoniae.

The potential to use PspA and other pneumococcal proteins to elicit protection against pneumococcal infection

Pneumococcal proteins, alone, in combination with each other, or in combination with capsular polysaccharide-protein conjugates may be useful pneumococcal vaccine components. Four proteins with a potential for use in vaccines are PspA, pneumolysin, PsaA, and PspC. In a mouse model of carriage, PsaA and PspC were the most efficacious vaccine proteins. Of these, PsaA was the best at eliciting protection against carriage. However, a combination of PspA and pneumolysin may elicit stronger immunity to pulmonary infection and possibly sepsis than either protein alone. Recently, a phase one trial of a recombinant family 1 PspA was completed in man. PspA was observed to be safe and immunogenic. Injection of 0.1 ml of immune serum diluted to 1/400 was able to protect mice from fatal infection with S. pneumoniae. Under these conditions, pre-immune serum was not protective. The immune human serum protected mice from infections with pneumococci expressing either of the major PspA families (1 and 2) and both of the pneumococcal capsular types tested: 3 and 6.

Acquired, but not innate, immune responses to Streptococcus pneumoniae are compromised by neutralization of CD40L

Streptococcus pneumoniae is a significant pathogen of young children and the elderly. Systemic infection by pneumococci is a complex process involving several bacterial and host factors. We have investigated the role of CD40L in host defense against pneumococcal infection. Treatment of mice with MR-1 antibody (anti-CD154/CD40L) markedly reduced antibody responses to the pneumococcal protein PspA, elicited by immunization of purified protein or whole bacteria. In mice immunized with whole bacteria, MR-1 treatment reduced antibody responses to capsular polysaccharides but not cell wall polysaccharides. MR-1 did not suppress antibody responses to isolated capsular polysaccharides but did reduce the production of antibody to a capsular polysaccharide-protein conjugate, indicating that when presented in the context of whole bacteria, the humoral response to capsular polysaccharides is partially T-cell dependent. Despite the reduction of the protective humoral responses to pneumococcal infection, administration of MR-1 had no effect on sepsis, lung infection, or nasal carriage in nonimmune mice inoculated with virulent pneumococci. Thus, short-term neutralization of CD40L does not compromise innate host defenses against pneumococcal invasion.

Intranasal immunization of mice with a mixture of the pneumococcal proteins PsaA and PspA is highly protective against nasopharyngeal carriage of Streptococcus pneumoniae

Acquisition of pneumococci is generally from carriers rather than from infected individuals. Therefore, to induce herd immunity against Streptococcus pneumoniae it will be necessary to elicit protection against carriage. Capsular polysaccharide-protein conjugates, PspA, and PsaA are known to elicit some protection against nasopharyngeal carriage of pneumococci but do not always completely eliminate carriage. In this study, we observed that PsaA elicited better protection than did PspA against carriage. Pneumolysin elicited no protection against carriage. Immunization with a mixture of PsaA and PspA elicited the best protection against carriage. These results indicate that PspA and PsaA may be useful for the elicitation of herd immunity in humans. As PspA and pneumolysin are known to elicit immunity to bacteremia and pneumonia, their inclusion in a mucosal vaccine may enable such a vaccine to prevent invasive disease as well as carriage.

Human antibodies to pneumococcal surface protein A in health and disease

BACKGROUND

Diseases caused by Streptococcus pneumoniae have a high impact in young children whose ability to mount antibodies to capsular polysaccharides is impaired. Pneumococcal surface protein A (PspA) is a potential vaccine candidate for this age group.

METHODS

We used Western blot analysis and enzyme immunoassay to study human sera of healthy adults from Alabama (n = 20) and from Finland (n = 21), healthy children from Finland (n = 20) and ill children from Finland, those with pneumococcal invasive infection (n = 26) and those with nonpneumococcal invasive infection (n = 26).

RESULTS

Human antibodies to PspA exhibited strong cross-reactivity among different pneumococcal strains. The geometric mean titer of IgG antibody to PspA in sera from 21 healthy adults was 4,040, from ten 3-year-old healthy children 1,080 and from ten 2-month-old healthy children 1,650. The geometric mean titer of PspA antibody of acute phase sera of children with invasive pneumococcal disease was 140, significantly (P < 0.001) lower than the respective value, 1,020, for children with infection caused by other bacteria.

CONCLUSIONS

We demonstrate for the first time the existence of antibodies to PspA in human sera in health and disease. The findings in ill children suggest that antibodies to PspA might play a role in protection against pneumococcal disease.

Production and characterization of the functional fragment of pneumococcal surface protein A

Pneumococcal surface protein A (PspA) is present on the cell wall of Streptococcus pneumoniae pathogen and has an antigenetically variable N-terminal domain. This aminoterminal domain is essential for full pneumococcal virulence, and monoclonal antibodies raised against it protect mice against pneumococcal infections. We have cloned and expressed a 34-kDa N-terminal fragment of PspA in Escherichia coli in a soluble form using the T7 RNA polymerase pET-20b vector system. Nickel chelate affinity purification followed by size exclusion and anion exchange chromatography yielded large amounts of pure and homogeneous protein. Analytical ultracentrifugation sedimentation velocity band and boundary studies showed that the molecule was present in aqueous solutions in a monomeric form with an axial shape ratio of approximately 1:12, typical of fibrous proteins. Sequence analyses indicated an alpha-helical coiled-coil structure for this monomeric molecule with only few loop-type breaks in helicity. The mostly alpha-helical structure of this PspA construct was consistent with circular dichroism spectroscopy data. Based on the ultracentrifugation studies, the circular dichroism spectra, and the PspA's sequence analyses, two structural models for the amino-terminal part of the PspA molecule are proposed. The evident highly charged and polar character of the surface of the modeled structures suggests functional properties of PspA that are related to the prevention of S. pneumoniae interactions with the host complement system.

The pspC gene of Streptococcus pneumoniae encodes a polymorphic protein, PspC, which elicits cross-reactive antibodies to PspA and provides immunity to pneumococcal bacteremia

PspC is one of three designations for a pneumococcal surface protein whose gene is present in approximately 75% of all Streptococcus pneumoniae strains. Under the name SpsA, the protein has been shown to bind secretory immunoglobulin A (S. Hammerschmidt, S. R. Talay, P. Brandtzaeg, and G. S. Chhatwal, Mol. Microbiol. 25:1113-1124, 1997). Under the name CbpA, the protein has been shown to interact with human epithelial and endothelial cells (C. Rosenow et al., Mol. Microbiol. 25:819-829, 1997). The gene is paralogous to the pspA gene in S. pneumoniae and was thus called pspC (A. Brooks-Walter, R. C. Tart, D. E. Briles, and S. K. Hollingshead, Abstracts of the 97th General Meeting of the American Society for Microbiology 1997). Sequence comparisons of five published and seven new alleles reveal that this gene has a mosaic structure, and modular domains have contributed to gene diversity during evolution. Two major clades exist: clade A alleles are larger and contain an extra module that is shared with many pspA alleles; clade B alleles are smaller and lack this pspA-like domain. All alleles have a proline-rich domain and a choline-binding repeat domain that show 0% divergence from similar domains in the PspA protein. Immunization of a rabbit with a recombinant clade B PspC molecule produced antiserum that cross-reacted with both PspC and PspA from 15 pneumococcal isolates. The cross-reactive antibodies afforded cross-protection in a mouse model system. Mice immunized with PspC were protected against challenge with a strain that expressed PspA but not PspC. The PspA- and PspC-cross-reactive antibodies were directed to the proline-rich domain present in both molecules.

Pneumococcal surface protein A inhibits complement activation by Streptococcus pneumoniae

Pneumococcal surface protein A (PspA) is a surface-exposed protein virulence factor for Streptococcus pneumoniae. In this study, no significant depletion of serum complement was observed for the serum of mice infected with pneumococci that express PspA. In contrast, in mice infected with an isogenic strain of pneumococci lacking PspA, significant activation of serum complement was detected within 30 min after infection. Also, the PspA-deficient strain but not the PspA-expressing strain was cleared from the blood within 6 h. The contribution of PspA to pneumococcal virulence was further investigated by using mice deficient for C5, C3, or factor B. In mice deficient for C3 or factor B, PspA-negative pneumococci became fully virulent. In contrast, in C5-deficient mice as in wild-type mice, PspA-deficient pneumococci were avirulent. These in vivo data suggest that, in nonimmune mice infected with pneumococci, PspA interferes with complement-dependent host defense mechanisms mediated by factor B. Immunoblots of pneumococci opsonized in vitro suggested that more C3b was deposited on PspA-negative than on PspA-positive pneumococci. This was observed with and without anticapsular antibody. Furthermore, processing of the alpha chain of C3b was reduced in the presence of PspA. We propose that PspA exerts its virulence function by interfering with deposition of C3b onto pneumococci and/or by inhibiting formation of a fully functional alternative pathway C3 convertase. By blocking recruitment of the alternative pathway, PspA reduces the amount of C3b deposited onto pneumococci, thereby reducing the effectiveness of complement receptor-mediated pathways of clearance.

In vivo polysaccharide-specific IgG isotype responses to intact Streptococcus pneumoniae are T cell dependent and require CD40- and B7-ligand interactions

In vivo Ig responses to soluble, haptenated polysaccharide (PS) Ags are T cell independent and do not require CD40 ligand (CD40L). However, little is known regarding the regulation of in vivo PS-specific Ig responses to intact bacteria. We immunized mice with a nonencapsulated, type 2 Streptococcus pneumoniae (R36A) and compared the parameters that regulated in vivo Ig isotype responses to the bacterial cell wall C-PS determinant, phosphorylcholine (PC), relative to Ig responses to the cell wall protein, pneumococcal surface protein A. Consistent with previous reports using soluble PS and protein Ags, the anti-PC and anti-pneumococcal surface protein A responses differed in that the anti-PC response was induced more rapidly, had a distinctive Ig isotype profile, and failed to demonstrate boosting upon secondary challenge with R36A. However, in contrast to previous studies, the IgG anti-PC response was TCR-alphabeta+ T cell dependent, required CD40L, and was blocked by administration of CTLA4 Ig. The nature of the T cell help for the anti-PC response had distinct features in that it was only partially blocked by CTLA4 Ig and was dependent upon both CD4+ and CD8+ T cells. Surprisingly, whereas the IgM anti-PC response was largely T cell independent, a strong requirement for CD40L was still observed, suggesting the possibility of an in vivo T cell-independent source for CD40L-dependent help. These data suggest that the regulatory parameters that govern in vivo Ig responses to purified, soluble PS Ags may not adequately account for PS-specific Ig responses to intact bacteria.

Genetic disruption of the murine complement C3 promoter region generates deficient mice with extrahepatic expression of C3 mRNA

Genetic deficiencies of the complement protein C3 occur naturally in humans and animal models and have been induced in mice by targeted deletion of the C3 gene. The study of these deficiencies has provided evidence for C3 functions in immune responses. C3 deficient mice were generated by replacing the 5'-flanking region of the C3 gene with the neomycin-resistance (neo) gene. Serum from these mice had no detectable C3 protein or complement activity. Challenge with Streptococcus pneumoniae revealed approximately 2000-fold increase in bacteremia as compared to littermate controls. C3 mRNA was absent in the liver, but it was detected in the lung, kidney, fat tissue, heart and spleen. Metabolic labeling of the lung tissue and peritoneal macrophages showed synthesis of pro-C3, but no post-synthetic intracellular processing of the protein and no secretion of mature C3. cDNA analysis at the cap site indicated that extrahepatic transcription of the targeted gene was initiated in the neo cassette, close to the C3/neo junction and predicted a primary translation product lacking the leader peptide. The data indicate that these mice provide a good animal model for the study of complete C3 deficiencies and a potential probe for tissue-specific C3 gene regulatory elements.

Molecular characterization of a globally distributed lineage of serotype 12F Streptococcus pneumoniae causing invasive disease

These studies have identified a major genetic lineage of capsule serotype 12F Streptococcus pneumoniae, which has maintained two different types of the pneumococcal surface protein A (PspA) virulence factor and caused invasive disease in geographically disjoint locations. Twenty outbreak strains from a Texas jail and Maryland day care center and 16 reference strains from Texas, Maryland, Washington, Michigan, Oklahoma, Missouri, Alaska, and Australia were examined. Although the Texas and Maryland outbreak strains were indistinguishable by IS1167 and boxA genotyping procedures, all strains examined were members of a genetically similar lineage. The microevolutionary history of pspA differed from that of the overall genetic background of the strains. Taken together, these findings suggested that the Texas and Maryland outbreaks were caused by different clones of a major genetic lineage of serotype 12F pneumococci, within which at least one PspA has been acquired via localized genetic recombination.

Tumor necrosis factor alpha receptor I is important for survival from Streptococcus pneumoniae infections

Tumor necrosis factor alpha (TNF-alpha) is important in resistance to various microorganisms and provides signals to the target cells through two different receptors, TNF-alpha receptor I (TNFRI) (p55 receptor) and TNFRII (p75 receptor). To delineate the significance of the two different signaling pathways in resisting infections with extracellular bacteria, we examined the resistance of mice to Streptococcus pneumoniae (serotype 6B). TNF-alpha needs to be present early in infections, since one injection of wild-type mice with anti-TNF-alpha leads to an increased susceptibility of these mice to S. pneumoniae. TNF-alpha signaling through the p55 receptor (but not the p75 receptor) is crucial in resisting S. pneumoniae infections, because intraperitoneal injection of 100 CFU/mouse killed p55-deficient mice by day 2 of infection, whereas 1,000,000 CFU/mouse was needed to kill half of the control mice. p55-deficient mice do not show evidence of a deficient acute-phase response. All three types of mice (p55 deficient, p75 deficient, and normal) showed comparable rises in the levels of two acute-phase proteins (serum amyloid P and C3) at 24, 48, and 72 h after the experimental infections, and all of the mice showed comparable influxes of neutrophils to the site of infection. Finally, it was demonstrated that p55-deficient mice can be protected from the lethal effects of S. pneumoniae infection by injection of antibodies specific for S. pneumoniae polysaccharide capsule.

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-gamma 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.

Comparison of the PspA sequence from Streptococcus pneumoniae EF5668 to the previously identified PspA sequence from strain Rx1 and ability of PspA from EF5668 to elicit protection against pneumococci of different capsular types

PspA (pneumococcal surface protein A) is a serologically varied virulence factor of Streptococcus pneumoniae. In mice, PspA has been shown to elicit an antibody response that protects against fatal challenge with encapsulated S. pneumoniae, and the protection-eliciting residues have been mapped to the alpha-helical N-terminal half of the protein. To date, a published DNA sequence for pspA is available only for S. pneumoniae Rx1, a laboratory strain. PspA/EF5668 (EF5668 indicates the strain of origin of the PspA) is serologically distinct from PspA/Rx1. Sequencing of the gene encoding PspA/EF5668 revealed 71% identity with that of PspA/Rx1. The greatest amount of divergence between the two proteins was seen in their alpha-helical portions, which are surface exposed and probably under selective pressure to diversify serologically. In spite of the diversity within the alpha-helical regions of PspAs, we have observed that recombinant PspA (rPspA)/EF5668, like rPspA/Rx1, can elicit cross-protection against pneumococci of different capsular and PspA serological types.

The IS1167 insertion sequence is a phylogenetically informative marker among isolates of serotype 6B Streptococcus pneumoniae

The phylogenetic utility of the IS1167 insertion sequence was examined with restriction fragment length polymorphism (RFLP) analyses of a sample of 50, predominantly invasive, capsular serotype 6B Streptococcus pneumoniae isolates previously characterized by multilocus enzyme electrophoresis (MLEE). The strains represented a genetically diverse assemblage of 34 distinct clonotypes composed of 26 restriction fragment types and 23 multilocus enzyme types. All isolates carried the IS1167 insertion sequence, with an average of 9.5 copies. The cross-classification of isolates based on RFLP and MLEE typing schemes was 81% concordant. Phylogenetic analyses demonstrated a significant (P < 0.0001) association between strains of a given RFLP lineage with those of a given MLEE lineage. A significant correlation (P < 0.00004) was also found between the proportion of restriction fragments shared by any given pair of isolates and their genetic distances estimated from the MLEE data. Parity between the two genetic markers implied that the sampled isolates were in linkage disequilibrium. The existence of nonrandom associations among genetic loci was confirmed by Monte Carlo analyses of the MLEE data. These studies, thus, demonstrated that invasive pneumococcal isolates of a single capsule type recovered on a regional scale can retain a largely clonal population structure over a period of 8 years. The ability to detect linkage disequilibrium and generate relatively congruent dendrograms based on distance and parsimony methods suggested that the restriction fragment data were robust to phylogenetic analysis.

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.

Enhanced protective antibody responses to PspA after intranasal or subcutaneous injections of PspA genetically fused to granulocyte-macrophage colony-stimulating factor or interleukin-2

Antibody to pneumococcal surface protein A (PspA) has been shown to be protective for Streptococcus pneumoniae infections in mice. In an attempt to define a model for inducing protective antibody to PspA in the absence of adjuvant, we designed two genetic fusions, PspA-interleukin-2 [IL-2]) and PspA-granulocyte-macrophage colony-stimulating factor (GM-CSF). These constructs maintained high cytokine function in vitro, as tested by their activity on IL-2 or GM-CSF-dependent cell lines. While intranasal immunization with PspA induced no detectable anti-PspA response, both PspA-IL-2 and PspA-GM-CSF stimulated high immunoglobulin G1 (IgG1) antibody responses. Interestingly, only the PspA-IL-2, not the PspA-GM-CSF, construct stimulated IgG2a antibody responses, suggesting that this construct directed the response along a TH1-dependent pathway. Comparable enhancement of the anti-PspA response with similar isotype profiles was observed after subcutaneous immunization as well. The enhancement observed with PspA-IL-2 was dependent on IL-2 activity in that it was not seen in IL-2 receptor knockout mice, while PspA in alum induced high-titer antibody in these mice. The antibody was tested for its protective activity in a mouse lethality model using S. pneumoniae WU-R2. Passive transfer of 1:90 dilutions of sera from mice immunized with PspA-IL-2 and PspA-GM-CSF elicited protection of CBA/N mice against intravenous challenge with over 170 50% lethal doses of capsular type 3 strain WU2. Only 0.17 microg or less of IgG antibody to PspA was able to provide passive protection against otherwise fatal challenge with S. pneumoniae. The data demonstrate that designing protein-cytokine fusions may be a useful approach for mucosal immunization and can induce high-titer systemic protective antibody responses.

Role of tumor necrosis factor alpha in the host response of mice to bacteremia caused by pneumolysin-deficient Streptococcus pneumoniae

Pneumolysin-deficient mutant strains of Streptococcus pneumoniae are known to cause less-severe sepsis than wild-type pneumococcal strains that produce pneumolysin. This difference is associated with greater host resistance in mice infected with the pneumolysin-deficient strains. These studies show that the host resistance developed during the first 1 to 2 days after infection with a pneumolysin-deficient mutant strain is dependent on tumor necrosis factor alpha but is apparently independent of interleukin 1beta (IL-1beta) or IL-6. Survival beyond 5 days appeared to depend on the ability of the mice to produce IL-1beta.

Development of a model of low-inoculum Streptococcus pneumoniae intrapulmonary infection in infant rats

We have developed a model of low-inoculum Streptococcus pneumoniae infection in infant rats. We challenged 4-day-old Sprague-Dawley pups via intraperitoneal or intrapulmonary injection of S. pneumoniae serotypes 1, 3, 4, 5, 6b, 7f, 9v, 14, 19f, and 23f. To achieve bacteremia with low inocula, it was necessary to passage the isolates in rats. Inocula of the 10 S. pneumoniae serotypes producing bacteremia in 50% or more animals ranged from 1 to 400 CFU. Virulence was similar by intraperitoneal and intrapulmonary routes. Lung specimens from animals challenged by the intrapulmonary route grew S. pneumoniae and demonstrated histologic evidence of focal infection. Meningitis was detected in 20 to 50% of bacteremic animals, and mortality invariably followed bacteremia within 24 to 48 h. This model of intrapulmonary infection uses low inocula of S. pneumoniae and results in bacteremia, meningitis, and death in infant rats.

The hemolytic and complement-activating properties of pneumolysin do not contribute individually to virulence in a pneumococcal bacteremia model

The virulence of pneumococcal capsular type 2 strain D39 and derivatives with mutations in the pneumolysin gene were examined in a mouse bacteremia model. In CBA/N-XID mice D39 is known to exhibit exponential growth in the blood until the death of the mice at 24 to 36 h. In contrast, PLN, a pneumolysin-deficient derivative of D39, reaches a plateau in growth that is maintained for several days. The growth patterns of D39 and PLN observed in CBA/N-XID mice were also observed in C3H/HeJ and C3H/HeOuJ mice, but not in 129/SvJ and C57BL/6J mice. These results demonstrate that the effect of pneumolysin on bacteremia is dependent on the genetic background of the mice. D39 derivatives with point mutations which abolish the cytotoxic or complement-activating properties of pneumolysin did not have major individual effects on virulence in CBA/N- XID and C3H/HeOuJ mice. A derivative with mutations affecting both the cytotoxic and complement- activating properties resulted in a modest, yet statistically significant, increase in survival time of i.v. challenged CBA/N-XID mice. However, the effect was less marked than that seen with PLN. These findings suggest that the virulence effects of pneumolysin in bacteremia must be due in part to properties other than hemolysis and complement fixation.

Establishment of a Streptococcus pneumoniae nasopharyngeal colonization model in adult mice

Human nasopharyngeal carriage of Streptococcus pneumoniae constitutes the major natural reservoir of pneumococci and is thought to be the prelude to virtually all pneumococcal disease. If carriage could be greatly reduced, pneumococcal transmission and disease could be largely eliminated. To facilitate the studies of mechanisms important in carriage and to identify immunogens that can elicit protection against carriage, we characterized an adult mouse model of nasopharyngeal carriage. Non-anaesthetized mice were inoculated intranasally with pneumococci in 10 microl of fluid. Nasopharyngeal carriage was observed with strains of capsular types 3, 4, 6A, 6B, 14, 19, and 23. Carriage was stable over time, and the numbers of pneumococci carried were relatively independent of inoculation dose; findings which indicate that the recovery of pneumococci from 1 day to 2 weeks post inoculation was dependent on colonization, rather than just temporary contamination. To ensure carriage in the largest percentage of mice, without causing sepsis or death, inoculations of 10(7) colony forming units (cfu) should be used. In this model, carriage was generally observed without concomitant bacteremia or sepsis and carriage was observed even with strains that were avirulent when injected i.v. The model should be useful for the identification of protection-eliciting antigens, since intranasal immunization with heat-killed pneumococci or lysates of pneumococci protected against carriage.