Validation of a multiplex pneumococcal serotyping assay with clinical samples

We have recently developed a rapid pneumococcal serotyping method called "multibead assay" (J. Yu et al., J. Clin. Microbiol. 43:156-162, 2005) based on a multiplexed immunoassay for capsular polysaccharides in lysates of pneumococcal cultures. The multibead assay can identify 36 serotypes (1, 2, 3, 4, 5, 6A, 6B, 7A/7F, 8, 9L/9N, 9V, 10A/10B/39/33C, 11A/11D/11F, 12A/12B/12F, 14, 15B/5C, 17F, 18C, 19A, 19F, 20, 22A/22F, 23F, and 33A/33F). More than 90% of the U.S. isolates express one of these serotypes (J. B. Robbins et al., J. Infect. Dis. 148:1136-1159, 1983). To validate the new assay, we examined 495 clinical isolates of pneumococci obtained in Brazil, Denmark, and Mexico. Pneumococci were serotyped by the Neufeld test in their countries of origin, and lysates of each strain were coded and mailed to the United States for the multibead assay at ambient temperature without any thermal protection. After breaking the code, 54 discrepancies (11% of samples) were noted, but 46 were due to nonreproducible technical problems or insufficient growth of the pneumococci. All of the isolates grew well for a second test, and therefore, the culture medium used for the multibead assay is adequate. The discrepancies persisted for eight isolates, involving the 6A, 11A, and 18C serotypes. Additional studies of the eight isolates showed that the discrepancies were due to differences in the reagents used in the multibead or Neufeld tests for these three serotypes. For instance, five isolates were typed as 6A with the Neufeld test but as nontypeable by the multibead assay. Selection of another new monoclonal antibody (Hyp6AG1) for the multibead assay resulted in all five discrepant isolates typing as 6A. This finding indicates the validity of the multibead assay and emphasizes the need to validate any new pneumococcal serotyping assay with a large number of clinical isolates from different locations. It also suggests the presence of serological subtypes among isolates expressing the 6A serotype.

Lipoteichoic acid-induced nitric oxide production depends on the activation of platelet-activating factor receptor and Jak2

NO production by macrophages in response to lipoteichoic acid (LTA) and a synthetic lipopeptide (Pam3CSK4) was investigated. LTA and Pam3CSK4 induced the production of both TNF-alpha and NO. Inhibitors of platelet-activating factor receptor (PAFR) blocked LTA- or Pam3CSK4-induced production of NO but not TNF-alpha. Jak2 tyrosine kinase inhibition blocked LTA-induced production of NO but not TNF-alpha. PAFR inhibition blocked phosphorylation of Jak2 and STAT1, a key factor for expressing inducible NO synthase. In addition, LTA did not induce IFN-beta expression, and p38 mitogen-activated protein serine kinase was necessary for LTA-induced NO production but not for TNF-alpha production. These findings suggest that Gram-positive bacteria induce NO production using a PAFR signaling pathway to activate STAT1 via Jak2. This PAFR/Jak2/STAT1 signaling pathway resembles the IFN-beta, type I IFNR/Jak/STAT1 pathway described for LPS. Consequently, Gram-positive and Gram-negative bacteria appear to have different but analogous mechanisms for NO production.

Anti-idiotypic antibody as a potential candidate vaccine for Neisseria meningitidis serogroup B

Sepsis and meningitis caused by Neisseria meningitidis serogroup B (NMGB) are serious diseases in infants and young adults, but no effective vaccine is available. The capsular polysaccharide (PS) of NMGB has poor immunogenicity and a structural similarity to polysialic acid (PSA) on neuronal tissue that may elicit autoantibodies. Using HmenB3, a protective and nonautoreactive monoclonal antibody (MAb) to NMGB capsular PS, we produced an anti-idiotypic MAb, Naid60, which mimics the capsular PS of NMGB. We produced an anti-anti-idiotypic MAb, MoB34, by using the immunogenic site on Naid60 responsible for inducing the anti-NMGB PS antibody response. MoB34 elicited the complement-mediated killing of representative strains of serogroup B meningococci. MoB34 did not bind to CHP-134, a neuroblastoma cell line expressing alpha(2-8) PSA, or to mouse brain cryosections at a high concentration. Naid60-keyhole limpet hemocyanin immunization inhibited the growth of live NMGB in intraperitoneally challenged mice; in contrast, three of five control mice developed bacteremia. Thus, Naid60 has an immunogenic site that elicits antibodies with bactericidal activity against NMGB and no autoimmunity to PSA. We suggest that the immunogenic region of Naid60 is a candidate for the development of a new vaccine against NMGB.

Safety of varying dosages of 7-valent pneumococcal protein conjugate vaccine in seniors previously vaccinated with 23-valent pneumococcal polysaccharide vaccine

In a phase I/II dose escalation study, varying volumes (0.1 ml, 0.5 ml, 1.0 ml and 2.0 ml) of 7-valent pneumococcal conjugate vaccine (PCV) (Prevnar or 0.5 ml of 23-valent pneumococcal polysaccharide vaccine (PPV) were administered to 220 adults 70 through 79 years of age previously vaccinated with 0.5 ml PPV at age 65 years or above and at least 5 years previously. Fever was uncommon and did not vary by study group. The rate of local reactions increased with higher volumes of PCV and the rate following 2.0 ml of PCV was comparable to that following 0.5 ml PPV.

Simplified method to automatically count bacterial colony forming unit

Bacterial colony counting is a significant technical hurdle for vaccine studies as well as various microbiological studies. We now show that an automated colony counter can process images obtained with a digital camera or document scanner and that any laboratory can efficiently have bacterial colonies enumerated by sending the images to a laboratory with a colony counter via internet.

Monoacyl lipoteichoic acid from pneumococci stimulates human cells but not mouse cells

We have developed a method for obtaining pneumococcal lipoteichoic acid (LTA) with none, one, or two acyl chains. Anion-exchange chromatography at pH 9.5 yields pneumococcal LTA (labeled LTA-9.5) that has a mass spectrum identical to that of pre-ion-exchange LTA and loses 500 mass units after deacylation by alkali hydrolysis. Anion exchange at pH 10.5 produces LTA (labeled LTA-10.5) with mass peaks that are 264 mass units lower than those of pre-ion-exchange LTA, and deacylation of LTA-10.5 by alkali hydrolysis reduces the mass by only 239 mass units. This result indicates that LTA-10.5 has lost one of the two acyl chains, whereas LTA-9.5 has both acyl chains. When the biological properties of LTA-9.5 and LTA-10.5 are examined with mouse cells, only LTA-9.5 (and not LTA-10.5) is able to stimulate mouse cells to produce tumor necrosis factor alpha, interleukin-1beta, and nitric oxide. In contrast, both LTA-9.5 and LTA-10.5 can stimulate human cells. LTA became inactive when both acyl chains were removed. Thus, acyl chains are critical for LTA function, and small variations in acyl chains can alter biological properties of LTA.

Rapid multiplex assay for serotyping pneumococci with monoclonal and polyclonal antibodies

We have developed and characterized a rapid semiautomated pneumococcal serotyping system incorporating a pneumococcal lysate preparation protocol and a multiplex serotyping assay. The lysate preparation incorporates a bile solubility test to confirm pneumococcal identification that also enhances assay specificity. The multiplex serotyping assay consists of 24 assays specific for 36 serotypes: serotypes 1, 2, 3, 4, 5, 6A, 6B, 7A/7F, 8, 9L/9N, 9V, 10A/10B/39/(33C), 11A/11D/11F, 12A/12B/12F, 14, 15B/(15C), 17F, 18C, 19A, 19F, 20, 22A/22F, 23F, and 33A/33F. The multiplex assay requires a flow cytometer, two sets of latex particles coated with pneumococcal polysaccharides, and serotype-specific antibodies. Fourteen newly developed monoclonal antibodies specific for common serotypes and a pool of polyclonal rabbit sera for some of the less-common serotypes are used. The two monoclonal antibodies specific for serotypes 18C and 23F recognize serotype-specific epitopes that have not been previously described. These monoclonal antibodies make the identification of the 14 common serotypes invariant. The specificity of the serotyping assay is fully characterized with pneumococci of all known (i.e., 90) serotypes. The assay is sensitive enough to use bacterial lysates diluted 20 fold. Our serotyping system can identify not only all the serotypes in pneumococcal vaccines but also most (>90%) of clinical isolates. This system should be very useful in serotyping clinical isolates for evaluating pneumococcal vaccine efficacy.

A new murine model of humoral immuno-deficiency specifically affects class switching to T-independent antigens

Immunoglobulin (Ig) isotype deficiencies are among the most common and least characterized humoral immunodeficiencies. A thorough understanding of their immunological and genetic features has been hampered by their extreme heterogeneity and the paucity of suitable animal models. Here, we report the initial characterization of a new mouse model with selective Ig deficiency. SENCARA mice display low serum IgG3 levels as well as severely deficient IgG3 responses to T cell-independent (TI) type 1 and 2 antigens. However, despite the significant block in class switching, expression of activation-induced deaminase and gamma3 germ-line transcription after TI antigen immunization are normal. IgG3 production in response to in vitro LPS stimulation was also normal, ruling out a specific defect in the Cgamma3 switch machinery. A decrease in the number of peritoneal B1a cells and enlarged splenic marginal zones were observed. The immunodeficiency is inherited as an autosomal, semi-dominant, essentially monogenic trait in SENCARA x C57BL/6 crosses. The SENCARA humoral immunodeficiency constitutes a novel immune phenotype, resembling human conditions such as IgG2 deficiency. This new mouse model will be of interest for the understanding of mechanisms involved in TI immune responses and may provide new insights into the molecular basis of human Ig deficiencies.

Development of experimental carbohydrate-conjugate vaccines composed of Streptococcus pneumoniae capsular polysaccharides and the universal helper T-lymphocyte epitope (PADRE®)

Experimental carbohydrate-conjugate vaccines composed of the 13 amino acid universal Pan HLA-DR Epitope (PADRE) and Streptococcus pneumoniae capsular polysaccharides from serotypes 14, 6B and 9V were produced. Simple carbodiimide-mediated condensation chemistry was used to conjugate the PADRE synthetic peptide to the three chemically different capsular polysaccharides in a 1:1 molar ratio. The immunogenicity of the PADRE peptide component of the conjugate vaccines was confirmed by the induction of PADRE-specific CD4+ helper T cell (HTL) responses following immunization of C57BL/6 mice. High titer antibody responses specific for polysaccharides of S. pneumoniae serotypes 14, 6B and 9V were induced using Complete Freund's Adjuvant (CFA) and alhydrogel Al(OH)3 formulations. The HTL, or carrier, effect of the PADRE synthetic peptide was only evident using the PADRE-polysaccharide conjugates; simple mixtures of the PADRE peptide and polysaccharides were essentially nonimmunogenic. The functional or potential protective value of the polysaccharide-specific antibodies was measured as a function of opsonophagocytic activity for the 6B serotype. High titers of opsonophagocytic activity were measured in sera from mice immunized with formulations containing both adjuvants. These data demonstrate that the PADRE synthetic peptide can induce the HTL responses needed to support the development of antibodies specific for bacterial carbohydrates used in conjugate vaccines.

Pneumococcal lipoteichoic acid (LTA) is not as potent as staphylococcal LTA in stimulating Toll-like receptor 2

Streptococcus pneumoniae is a leading cause of gram-positive sepsis, and lipoteichoic acid (LTA) may be important in causing gram-positive bacterial septic shock. Even though pneumococcal LTA is structurally distinct from the LTA of other gram-positive bacteria, the immunological properties of pneumococcal LTA have not been well characterized. We have investigated the ability of LTAs to stimulate human monocytes by using highly pure and structurally intact preparations of pneumococcal LTA and its two structural variants. The variants were pneumococcal LTA with only one acyl chain (LTA-1) and completely deacylated LTA (LTA-0). The target cells used in the study were peripheral blood mononuclear cells (PBMCs) and two model cell lines (CHO/CD14/TLR2 and CHO/CD14/TLR4) that express human CD25 protein in response to Toll-like receptor 2 (TLR2) and TLR4 stimulation, respectively. Intact pneumococcal LTA and LTA-1 stimulated PBMC and CHO/CD14/TLR2 cells in a dose-dependent manner but did not stimulate CHO/CD14/TLR4 cells. Pneumococcal LTA was about 100-fold less potent than Staphylococcus aureus LTA in stimulating the CHO/CD14/TLR2 cells and PBMCs. LTA-0 (or pneumococcal teichoic acid) stimulated neither CHO/CD14/TLR2 nor CHO/CD14/TLR4 cells even at high concentrations. Excess teichoic acid, LTA-0, antibodies to phosphocholine, or antibodies to TLR4 did not inhibit the LTA-induced TLR2 stimulation. However, antibodies to CD14, TLR1, or TLR2 suppressed tumor necrosis factor alpha (TNF-alpha) production by PBMCs in response to LTA or LTA-1. These results suggest that pneumococcal LTA with one or both acyl chains stimulates PBMCs primarily via TLR2 with the help of CD14 and TLR1.

Multilaboratory evaluation of a viability assay for measurement of opsonophagocytic antibodies specific to the capsular polysaccharides of Streptococcus pneumoniae

Opsonophagocytosis is a correlate of protection that measures the functional activity of vaccine-induced antibodies. A standardized opsonophagocytosis assay (OPA) should be used as part of the evaluation of current and future pneumococcal (Pnc) polysaccharide (Ps)-based vaccines. We enrolled five laboratories to evaluate a previously standardized viability OPA. Each laboratory was provided with a detailed OPA protocol, seven target Pnc strains (serotypes 4, 6B, 9V, 14, 18C, 19F, and 23F), two quality control sera and 12 paired sera (blinded) from adult donors who received one dose of the 23-valent Pnc Ps vaccine. Laboratories sent their results to the Centers for Disease Control and Prevention for analysis. Sera were tested in duplicate (single run), and the results were averaged to yield a single OPA titer (> or = 50% killing) for each serum sample. The percentage of sera within one or two dilutions of the calculated median OPA titer was determined for each laboratory and for each serotype. In general, laboratories were capable of detecting OPA titers within one or two dilutions of the median for at least 75 and 88%, respectively, of the sera tested. The level of agreement with the median OPA titers varied depending on the participating laboratory (overall agreement = 0.8 [99% confidence interval = 0.75 to 0.85]). All OPA median titers reported for quality control sera were within one dilution of the expected titer. We conclude that this OPA can be done in multiple laboratories with a high degree of interlaboratory reproducibility.

The influence of maternal immunization on light chain response to Haemophilus influenzae type b vaccine

The repertoire of human antibodies to Haemophilus influenzae type b polysaccharide (Hib PS) has been extensively characterized. In response to Hib PS vaccines, both IgG1 and IgG2 antibodies are produced. Based on the Vkappa gene used for the antibodies, heamophilus antibodies can be divided into A2 and non-A2 antibodies. A2 antibodies account for the majority of heamophilus antibodies, are generally more functional, and are specific for heamophilus polysaccharide. In contrast, non-A2 antibodies cross-react with Escherichia coli K100. Studies of placental transfer showed that IgG1 antibodies cross the placenta better than IgG2 antibodies and the A2 antibodies better than non-A2 antibodies. Babies born to mothers who were immunized with a heamophilus vaccine had a high level of antibodies to Hib PS but the antibody level decreased during the first 6 months. The children also responded to a heamophilus conjugate vaccine.

Efficiency of a pneumococcal opsonophagocytic killing assay improved by multiplexing and by coloring colonies

For evaluating pneumococcal vaccines, the opsonophagocytic killing assay (OPKA) is useful as a supplement to the pneumococcal antibody enzyme-linked immunosorbent assay (ELISA). However, evaluations of pneumococcal vaccines require the determination of antibody responses to 7 to 11 serotypes, and the OPKA is tedious to perform and requires more serum than the ELISA. Consequently, the OPKA is infrequently used for evaluating pneumococcal vaccines. To overcome these limitations, we have developed a simple multiplexed (double-serotype) OPKA by using antibiotic-resistant pneumococci for nine serotypes. Serotype 6B, 9V, 19A, and 23F strains were made streptomycin resistant, and serotype 4, 6A, 14, 18C, and 19F strains were made optochin resistant. The multiplexed OPKA was the same as the single-serotype OPKA except for two changes. First, the target bacteria were a mixture of one streptomycin-resistant strain and one optochin-resistant strain. Second, the surviving bacteria of each serotype were enumerated by plating on Todd-Hewitt agar plates with yeast extract and an agar overlay containing the appropriate antibiotics and 2,3,5-triphenyl tetrazolium chloride. The performance of the multiplexed OPKA was evaluated by analyzing 28 serum samples from adults immunized with a 23-valent polysaccharide vaccine by using the single-serotype OPKA and the multiplexed OPKA. The multiplexed OPKA was specific for the desired serotypes. The multiplexed and conventional OPKAs had comparable assay sensitivities and produced results that were highly correlated (r(2) values ranging from 0.92 to 0.98) for all nine serotypes. A simple modification of the conventional OPKA produces a multiplexed assay that greatly reduces effort, reagents, and the necessary amount of serum.

Central role of complement in passive protection by human IgG1 and IgG2 anti-pneumococcal antibodies in mice

Streptococcus pneumoniae is an important cause of morbitity and mortality worldwide. Capsule-specific IgG1 and IgG2 Abs are induced upon vaccination with polysaccharide-based vaccines that mediate host protection. We compared the protective capacity of human recombinant serogroup 6-specific IgG1 and IgG2 Abs in mice deficient for either leukocyte FcR or complement factors. Human IgG1 was found to interact with mouse leukocyte FcR in vitro, whereas human IgG2 did not. Both subclasses induced complement activation, resulting in C3c deposition on pneumococcal surfaces. Passive immunization of C57BL/6 mice with either subclass before intranasal challenge with serotype 6A induced similar degrees of protection. FcgammaRI- and III-deficient mice, as well as the combined FcgammaRI, II, and III knockout mice, were protected by passive immunization, indicating FcR not to be essential for protection. C1q or C2/factor B knockout mice, however, were not protected by passive immunization. Passively immunized C2/factor B(-/-) mice displayed higher bacteremic load than C1q(-/-) mice, supporting an important protective role of the alternative complement pathway. Spleens from wild-type and C1q(-/-) mice showed hyperemia and thrombotic vessel occlusion, as a result of septicemic shock. Notably, thrombus formation was absent in spleens of C2/factor B(-/-) mice, suggesting that the alternative complement pathway contributes to shock-induced intravascular coagulation. These studies demonstrate complement to play a central role in Ab-mediated protection against pneumococcal infection in vivo, as well as in bacteremia-associated thrombotic complications.

Genetic relatedness of levofloxacin-nonsusceptible Streptococcus pneumoniae isolates from North America

We characterized 32 levofloxacin-nonsusceptible Streptococcus pneumoniae (LNSP) isolates obtained from a broad geographic region of North America over a 5-year period by using capsular serotypes, antimicrobial susceptibility profiles, BOX-PCR, multilocus sequence typing (MLST), and pulsed-field gel electrophoresis (PFGE). Sixteen international clones identified by the Pneumococcal Molecular Epidemiology Network also were included for comparison. Fifteen serotypes were represented, with serogroups 6, 9, 14, 19, and 23 accounting for 63% of isolates. Among isolates whose quinolone resistance-determining regions were sequenced, all contained gyrA and parC point mutations. Sixty-three percent were penicillin susceptible, and 84% were erythromycin susceptible. BOX-PCR analysis identified 39 different band patterns among 32 LNSP and 16 international clones and grouped 16 isolates, including 2 international clones, into seven unrelated groups of 2 to 4 isolates each. PFGE analysis identified 35 different band patterns among 32 LNSP and 16 international clones and grouped 21 isolates, including 3 international clones, into eight unrelated groups of 2 to 6 isolates each. MLST performed on 10 isolates identified five allelic profiles and separated 9 isolates into four groups of 2 to 3 isolates each. Overall, each typing method indicated that the LNSP were heterogeneous and that resistance to fluoroquinolones was not closely associated with a particular serotype or with coresistance to other antimicrobial classes and suggests that LNSP have likely arisen through independent mutational events as a result of selective pressure. However, seven LNSP were found to be related to three international clones by PFGE.

Immunogenic protein contaminants in pneumococcal vaccines

Currently available pneumococcal vaccines were examined for contamination by pneumococcal surface protein A (PspA) and pneumococcal surface adhesin A (PsaA). PspA could be detected in some (but not all) lots of 23-valent polysaccharide vaccine. Many lots of pneumococcal vaccines, including the heptavalent conjugate vaccine, were found to elicit small (but variable) antibody responses to PspA, PsaA, or both. The degree of contamination was highly variable, and this should be considered in pneumococcal vaccine evaluations or when capsular polysaccharide is used for pneumococcal antibody assays.

Peptide mimotopes as prototypic templates of broad-spectrum surrogates of carbohydrate antigens

Peptide mimetics of carbohydrate antigens can function as templates to exploit immune mechanisms to augment vaccine design strategies as they are T cell dependent antigens. In this study we evaluate a peptide mimetic (peptide 105) of the Pneumococcal capsular polysaccharide type 14 (Pn14) as a model antigen to explore differences in antigenicity and immunogenicity of peptide mimotopes. The multiple antigenic peptide (MAP) form, by ELISA, competes with native Pn14 in a concentration-dependent manner for binding to an anti-Pn14 monoclonal antibody. It was observed that peptide priming with a conjugated form (105-BSA) and boosting with Pn14 produced higher levels of Pn14-reactive IgG1, IgG2a, IgG2b and IgG3 than priming and boosting with Pn14. This serum also displayed reactivity with multiple serotypes, as assessed by ELISA. However, when compared with serum from humans immunized with the 23-valent pneumococcal vaccines, mimetic-induced mouse serum did not display a significant ability to mediate opsonophagocytic killing of pneumococci. These results suggest the feasibility of designing mimotopes to render effective humoral responses not only to a single serotype of Streptococcus pneumoniae, but to multiple serotypes at once. Such peptides would simplify currently available vaccine approaches, yet highlights the requirement of more extensive polymerization to fully emulate native antigen.

Peptide mimotopes of pneumococcal capsular polysaccharide of 6B serotype: a peptide mimotope can bind to two unrelated antibodies

Two groups of bacteriophage clones displaying the antigenic properties of serotype 6B pneumococcal capsular polysaccharide (PS) were obtained from different phage libraries expressing random heptameric peptides. One group, biopanned with a mouse mAb (Hyp6BM1), is comprised of 17 phage clones expressing 10 unique sequences of linear peptides. The other group, selected with another mAb (Hyp6BM8), contained six clones, all of which expressed the identical circular peptide. Phage clones expressing the linear peptides (e.g., PhaM1L3) bound only to Hyp6BM1, but not other 6B PS-specific mAb, and their binding could be inhibited with pneumococcal capsular type 6B PS only. In contrast, a phage clone expressing the circular peptide (PhaM8C1) cross-reacted with several other 6B PS-specific mAbs, and their binding could be inhibited with pneumococcal capsular PS of 6A and 6B serotypes. Two short peptides, PepM1L3 and PepM8C1, reflecting the peptide inserts of the corresponding phage clones, could inhibit the binding of the two clones to their respective mAb. Interestingly, the peptide insert in PhaM8C1 was identical to that in PhaB3C4, a previously reported mimotope of alpha(2-->8) polysialic acid, Neisseria meningitidis group B PS. Indeed, PhaM8C1 bound to HmenB3 (a meningococcal Ab), and their association could be inhibited with alpha(2-8) polysialic acid, but not with 6B PS. Conversely, alpha(2-8) polysialic acid could not inhibit the binding of PhaM8C1 to Hyp6BM8. The two-dimensional nuclear magnetic resonance studies indicate that PepM8C1 peptide can assume several conformations in solution. The ability of this peptide to assume multiple conformations might account for its ability to mimic more than one Ag type.

Type III group B streptococcal polysaccharide induces antibodies that cross-react with Streptococcus pneumoniae type 14

Covalent linkage of a bacterial polysaccharide to a protein greatly enhances the carbohydrate's immunogenicity and its binding to solid surfaces in immunoassays. These findings have spurred the development of glycoconjugate vaccines to prevent serious bacterial infections as well as the use of glycoconjugates as coating antigens in bioassays. We evaluated sera from women immunized with unconjugated group B streptococcal (GBS) type III (GBS III) polysaccharide (IIIPS) or with IIIPS covalently linked to tetanus toxoid to assess specificity, sensitivity, and parallelism in dilution curves in two GBS III enzyme-linked immunosorbent assays (ELISAs). One assay used IIIPS mixed with methylated human serum albumin (IIIPS + mHSA) as the coating antigen, and the other used IIIPS covalently linked to HSA (III-HSA). Each coating antigen was associated with a highly specific GBS III bioassay. The sensitivity was higher in the III-HSA ELISA, in which conjugated IIIPS is bound to the plates. Parallelism in titration curves was observed in the III-HSA but not in the IIIPS + mHSA ELISA. The excellent correlation between the concentrations of GBS IIIPS-specific immunoglobulin G (IgG) and the opsonophagocytic activity of these antibodies indicated that the III-HSA assay can predict functionality of vaccine-induced IgG against GBS III disease. The structure of the repeating unit of the capsular polysaccharide of GBS III differs from that of Streptococcus pneumoniae type 14 (Pn14 PS) only by the presence on GBS III of a sialic acid residue at the end of the side chain. The majority of healthy adults responding to GBS III vaccines with a fourfold or greater increase in GBS III-specific IgG antibodies developed antibodies cross-reacting with Pn14 PS (i.e., desialylated GBS IIIPS). The proportion of GBS vaccine responders who developed IgG to the desialylated IIIPS did not depend on whether IIIPS was given in the unconjugated or conjugated form. When present, these vaccine-induced cross-reacting antibodies conferred in vitro antibody-mediated opsonophagocytosis and killing of both GBS III and Pn14, two pathogens that cause invasive disease in young infants.

Maternal immunization with pneumococcal polysaccharide vaccine in the third trimester of gestation

In a randomized, double blinded study, 23-valent pneumococcal polysaccharide vaccine (PSV) or conjugate Haemophilus influenzae type b (HbOC) vaccine was administered to 60 healthy women in the third trimester of gestation. Total IgG, IgG1, and IgG2 antibodies to pneumococcal serotypes 6B, 14, 19F and 23F were measured by ELISA in mothers prior to immunization, at delivery and 7 months after delivery, and in infants at birth (cord blood), 2 and 7 months after delivery. IgA was evaluated in breast milk at 2 and 7 months, and opsonophagocytic activity in cord blood. PSV was safe and immunogenic in pregnant women. Transplacental transmission of vaccine-specific antibodies was efficient. Maternal immunization with PSV resulted in significantly higher concentrations of pneumococcal antibodies in infants at birth and at 2 months of age, and greater functional opsonophagocytic activity of passively acquired IgG antibody.

Chromogenic assay measuring opsonophagocytic killing capacities of antipneumococcal antisera

Assays measuring opsonophagocytic killing capacity of immune sera are good surrogate assays for assessing pneumococcal vaccine responses, but they are tedious to perform primarily because the enumeration of surviving bacteria requires the counting of individual bacterial colonies. To overcome this limitation, we have developed a simple and rapid chromogenic assay for estimating the number of surviving bacteria. In this method, the conventional opsonophagocytic killing assays were performed in microtiter wells with differentiated HL-60 cells as phagocytes. At the end of the assay the reaction mixture was cultured for an additional 4.5 h to increase the number of bacteria. After the short culture, XTT (3,3'-[1[(phenylamino)carbonyl]-3,4-tetrazolium]-bis[4-methoxy-6-nitro] benzene sulfonic acid hydrate) and coenzyme Q were added to the wells and the optical density at 450 nm was measured. Our study shows that changes in the optical density were proportional to the number of CFU of live bacteria in the wells. Also, the number of bacteria at the end of the 4.5-h culture was found to be proportional to the original number of bacteria in the wells. When the performance of the chromogenic assay was evaluated by measuring the opsonizing titers of Streptococcus pneumoniae serotypes 6B and 19F, the sensitivity and precision of the new method were similar to those of the conventional opsonization assay employing the colony counting method. Furthermore, the results of this chromogenic assay obtained with 33 human sera correlate well with those obtained with the conventional colony counting method (R > 0.90) for the two serotypes (6B and 19F). Thus, this simple chromogenic assay would be useful in rapidly measuring the capacities of antisera to opsonize pneumococci.

Monoclonal antibodies specific for Neisseria meningitidis group B polysaccharide and their peptide mimotopes

From five mice immunized with Escherichia coli K1 bacteria, we produced 12 immunoglobulin M hybridomas secreting monoclonal antibodies (MAbs) that bind to Neisseria meningitidis group B (NMGB). The 12 MAbs also bound the capsular polysaccharide (PS) of E. coli K1 [which, like NMGB, is alpha(2-8)-linked polysialic acid (PSA)] and bound to EV36, a nonpathogenic E. coli K-12 strain producing alpha(2-8) PSA. Except for HmenB5, which cross-reacted with N. meningitidis group C, none of the MAbs bound to N. meningitidis groups A, C, and Y. Of the 12 MAbs, 6 were autoantibodies as defined by binding to CHP-134, a neuroblastoma cell line expressing short-chain alpha(2-8) PSA; five of these MAbs killed NMGB in the presence of rabbit complement, and two also killed NMGB with human complement. The other six MAbs, however, were nonautoreactive; all killed NMGB with rabbit complement, and five killed NMGB with human complement. To obtain peptide mimotopes of NMGB PS, four of the nonautoreactive MAbs (HmenB2, HmenB3, HmenB13, and HmenB14) were used to screen two types of phage libraries, one with a linear peptide of 7 amino acids and the other with a circular peptide of 7 amino acids inserted between two linked cysteines. We obtained 86 phage clones that bound to the screening MAb in the absence but not in the presence of E. coli K1 PSA in solution. The clones contained 31 linear and 4 circular mimotopes expressing unique sequences. These mimotopes nonrandomly expressed amino acids and were different from previously described mimotopes for NMGB PS. The new mimotopes may be useful in producing a vaccine(s) capable of eliciting anti-NMGB antibodies not reactive with neuronal tissue.