Protective activity of group C anticapsular antibodies elicited in two-year-olds by an investigational quadrivalent Neisseria meningitidis-diphtheria toxoid conjugate vaccine

Persistence of bactericidal antibodies 4 years after a booster dose of quadrivalent meningococcal diphtheria toxoid conjugate vaccine (MenACWY-D)

One dose of quadrivalent meningococcal conjugate vaccine (MenACWY) was first recommended for US adolescents (ages 11-12 years) in 2005 to protect against invasive meningococcal disease (IMD). In 2010, after evidence emerged about waning protection within 5 years after MenACWY vaccination, the US Advisory Committee on Immunization Practices (ACIP) recommended a MenACWY booster at age 16 years. We used a serum bactericidal assay with human complement (hSBA) to evaluate antibody persistence after a MenACWY-D booster in a sample of 110 participants who received the booster 4 years earlier in a phase 2 study. High proportions (89.9-98.2%) of participants maintained hSBA titers (≥1:4) associated with protection against IMD; a majority (81.7-97.2%) also had hSBA titers ≥1:8, a more conservative threshold. These findings support ACIP recommendations regarding MenACWY booster vaccination, which are aimed at protecting adolescents and young adults throughout the period in which they are at increased risk of IMD.

Effect of Vaccine-Elicited Antibodies on Colonization of Neisseria meningitidis Serogroup B and C Strains in a Human Bronchial Epithelial Cell Culture Model

Capsular polysaccharide-protein conjugate vaccines protect individuals from invasive disease and decrease carriage, which reduces spread of the organism in the population. In contrast, antibodies elicited by plain polysaccharide or protein antigen-based meningococcal (Men) vaccines have little or no effect on decreasing carriage. In this study, we investigated the mechanism by which vaccine-induced human immunoglobulin G (IgG) antibodies affect colonization by meningococcal serogroup B (MenB) or C (MenC) strains using a human bronchial epithelial cell culture model (16HBE14o-). Fluorescence microscopy showed that bacteria colonizing the apical side of 16HBE14o- monolayers had decreased capsular polysaccharide on the bacterial surface that resulted from shedding the capsule and not decreased production of polysaccharide. Capsular polysaccharide shedding depended on the presence of 16HBE14o- cells and bacteria but not direct adherence of the bacteria to the cells. Treatment of bacteria and cells with postimmunization MenC-conjugate IgG or murine anti-MenB polysaccharide monoclonal antibodies (MAbs) inhibited capsule shedding, microcolony dispersal, and invasion of the 16HBE14o- cell monolayer. In contrast, the IgG responses elicited by immunization with MenC polysaccharide (PS), MenB outer membrane vesicle (OMV)-based, or factor H binding protein (FHbp)-based vaccines were not different than preimmune IgG or no-treatment response. The results provide new insights on the mechanism by which high-avidity anticapsular antibodies elicited by polysaccharide-conjugate vaccines affect meningococcal colonization. The data also suggest that any effect on colonization by IgG elicited by OMV- or FHbp-based vaccines may involve a different mechanism.

Safety and immunogenicity of a booster dose of meningococcal (groups A, C, W, and Y) polysaccharide diphtheria toxoid conjugate vaccine

BACKGROUND

Quadrivalent meningococcal conjugate vaccines (MenACWY) were developed to offer long-term protection against invasive disease caused by serogroups A, C, W, and Y. Reduced MenACWY effectiveness within 5 years after primary vaccination (likely due to declining bactericidal antibody titers) has been described, particularly with respect to C and Y disease in the United States. We evaluated the safety and immunogenicity of a single booster dose of quadrivalent meningococcal polysaccharide diphtheria toxoid conjugate vaccine (MenACWY-D) in adolescents and adults who received a previous dose 4-6 years earlier.

METHODS

This phase 2, open-label, multicenter study of 834 persons was conducted in the United States. Participants received a single 0.5-mL booster dose of MenACWY-D. Serogroup-specific bactericidal antibody geometric mean titers (GMTs) were measured with a serum bactericidal antibody assay using human complement (hSBA). Proportions of participants achieving antibody titers of ⩾1:8 for each vaccine serogroup on Days 6 and 28 were determined. Rates of adverse events (AEs), including serious adverse events (SAEs), were also assessed.

RESULTS

Before booster vaccination, 38.7-68.5% of participants had an hSBA titer ⩾1:8, depending on vaccine serogroup. By Day 6 post-vaccination, 98.2-99.1% of participants had hSBA titers ⩾1:8. By Day 28, >99% of participants achieved this threshold and the primary hypothesis (lower limit of the one-sided 95% confidence limit ⩾85% for each serogroup) was met. The GMT ratios (post-vaccination divided by pre-vaccination) at Day 28 ranged from 47.2 (serogroup A) to 209.1 (serogroup Y). Rates of AEs, including SAEs, were similar to those observed among adolescents and adults who received a primary dose of MenACWY-D in previous studies. There were no study discontinuations due to an AE and no deaths.

CONCLUSIONS

Booster vaccination with MenACWY-D was safe and induced robust bactericidal antibody responses, consistent with immune memory, among adolescents and adults 4-6 years after primary vaccination. ClinicalTrials.gov registration: NCT01442675.

Inhibition of the alternative pathway of nonhuman infant complement by porin B2 contributes to virulence of Neisseria meningitidis in the infant rat model

Neisseria meningitidis utilizes capsular polysaccharide, lipooligosaccharide (LOS) sialic acid, factor H binding protein (fHbp), and neisserial surface protein A (NspA) to regulate the alternative pathway (AP) of complement. Using meningococcal mutants that lacked all four of the above-mentioned molecules (quadruple mutants), we recently identified a role for PorB2 in attenuating the human AP; inhibition was mediated by human fH, a key downregulatory protein of the AP. Previous studies showed that fH downregulation of the AP via fHbp or NspA is specific for human fH. Here, we report that PorB2-expressing quadruple mutants also regulate the AP of baby rabbit and infant rat complement. Blocking a human fH binding region on PorB2 of the quadruple mutant of strain 4243 with a chimeric protein that comprised human fH domains 6 and 7 fused to murine IgG Fc enhanced AP-mediated baby rabbit C3 deposition, which provided evidence for an fH-dependent mechanism of nonhuman AP regulation by PorB2. Using isogenic mutants of strain H44/76 that differed only in their PorB molecules, we confirmed a role for PorB2 in resistance to killing by infant rat serum. The PorB2-expressing strain also caused higher levels of bacteremia in infant rats than its isogenic PorB3-expressing counterpart, thus providing a molecular basis for increased survival of PorB2 isolates in this model. These studies link PorB2 expression with infection of infant rats, which could inform the choice of meningococcal strains for use in animal models, and reveals, for the first time, that PorB2-expressing strains of N. meningitidis regulate the AP of baby rabbits and rats.

Protein carriers of conjugate vaccines: characteristics, development, and clinical trials

The immunogenicity of polysaccharides as human vaccines was enhanced by coupling to protein carriers. Conjugation transformed the T cell-independent polysaccharide vaccines of the past to T cell-dependent antigenic vaccines that were much more immunogenic and launched a renaissance in vaccinology. This review discusses the conjugate vaccines for prevention of infections caused by Hemophilus influenzae type b, Streptococcus pneumoniae, and Neisseria meningitidis. Specifically, the characteristics of the proteins used in the construction of the vaccines including CRM, tetanus toxoid, diphtheria toxoid, Neisseria meningitidis outer membrane complex, and Hemophilus influenzae protein D are discussed. The studies that established differences among and key features of conjugate vaccines including immunologic memory induction, reduction of nasopharyngeal colonization and herd immunity, and antibody avidity and avidity maturation are presented. Studies of dose, schedule, response to boosters, of single protein carriers with single and multiple polysaccharides, of multiple protein carriers with multiple polysaccharides and conjugate vaccines administered concurrently with other vaccines are discussed along with undesirable consequences of conjugate vaccines. The clear benefits of conjugate vaccines in improving the protective responses of the immature immune systems of young infants and the senescent immune systems of the elderly have been made clear and opened the way to development of additional vaccines using this technology for future vaccine products.

Single priming dose of meningococcal group C conjugate vaccine (NeisVac-C®) in infants

Since the introduction of the meningococcal C conjugate (MCC) vaccine in the pediatric population in 1999, numerous clinical studies have confirmed the immunogenicity and safety of the NeisVac-C(®) vaccine, and several have observed a strong immune response after a single priming dose, which could be successfully boosted. Maximizing protection of infants with as few vaccine doses as possible would increase the general acceptability of the immunization strategies and support broader coverage without increasing vaccination costs. This was a randomized feasibility study of a single priming NeisVac-C(®) vaccine dose administered at 4 or 6 months of age, compared to the currently licensed two dose priming at 2 and 4 months of age, followed by a booster vaccination at 12-13 months of age. High seroprotection rates and serum bactericidal antibody (rSBA) titers were observed in all study groups, whether a single or two dose priming vaccination was administered, at all time points investigated: one month after the priming vaccination(s) (>99% of subjects rSBA≥8), prior to booster vaccination (>65% of subjects with rSBA≥8, with the lowest titers and GMTs seen in the two dose priming group), as well as after booster vaccination administration (99% with rSBA≥128 in all three study groups, with the highest GMT of 2472 seen in the 4 month single dose group). This study confirmed trends seen in previous reports that a single-dose priming vaccination at 4 or 6 months of age can be considered a valuable alternative to the currently licensed two-dose priming vaccination schedule.

Meningococcal conjugate vaccines: optimizing global impact

Meningococcal conjugate vaccines have several advantages over polysaccharide vaccines, including the ability to induce greater antibody persistence, avidity, immunologic memory, and herd immunity. Since 1999, meningococcal conjugate vaccine programs have been established across the globe. Many of these vaccination programs have resulted in significant decline in meningococcal disease in several countries. Recent introduction of serogroup A conjugate vaccine in Africa offers the potential to eliminate meningococcal disease as a public health problem in Africa. However, the duration of immune response and the development of widespread herd immunity in the population remain important questions for meningococcal vaccine programs. Because of the unique epidemiology of meningococcal disease around the world, the optimal vaccination strategy for long-term disease prevention will vary by country.

WITHDRAWN: Conjugate vaccines for preventing meningococcal C meningitis and septicaemia

BACKGROUND

Meningococcal polysaccharide (MPLS) vaccines protect against Serogroup C disease, but do not produce an immune response in infants less than two years of age. This limitation can be overcome by linking C polysaccharide to carrier proteins ('conjugating'), to create meningococcal serogroup C conjugate (MCC) vaccines. In the absence of trial data, the immune response to vaccination has been considered to be a reasonable surrogate for vaccine protection.

OBJECTIVES

To assess the immunogenicity, safety and efficacy of MCC vaccines for preventing meningitis and septicaemia.

SEARCH STRATEGY

We searched the Cochrane Central Register Controlled Trials (CENTRAL) (The Cochrane Library 2005, issue 3); MEDLINE (1966 to September, Week 1 2005); and EMBASE (1990 to June 2005) and references of studies.

SELECTION CRITERIA

Randomised controlled trials (RCTs) and controlled clinical trials (CCTs) in humans comparing MCC vaccines against a control vaccine or none. In the absence of any trials on vaccine efficacy, population-based observational studies about effectiveness were included.

DATA COLLECTION AND ANALYSIS

Two authors independently screened the results of the literature searches, selected eligible studies, extracted the data and evaluated the quality of them.

MAIN RESULTS

The studies showed that MCC vaccine was highly immunogenic in infants after two and three doses, in toddlers after one and two doses and in older age groups after one dose. In general higher titres were generated after MCC than after MPLS vaccines. Immunological hypo-responsiveness seen after repeated doses of MPLS vaccine may be overcome with MCC. Observational studies have documented a significant decline in meningococcal C disease in countries where MCC vaccines have been widely used. The timing of the vaccinations schedules, the specific conjugate used, and the vaccines given concomitantly or combined, may be important.

AUTHORS' CONCLUSIONS

The MCC vaccine appears to be safe, immunogenic and able to induce immunological memory in all age groups. Observational studies strongly suggest that MCC is clinically effective.

Impaired antibody response to conjugated meningococcal serogroup C vaccine in asplenic patients

The purpose of this study was to determine the quantity and quality of antibodies against the meningococcal serogroup C (MenC) conjugated vaccine in asplenic patients. In 116 asplenic patients, antibody concentrations (IgG) were measured against meningococcal serogroup C before and after immunisation. Of MenC-specific IgG, both antibody avidity and subclasses of IgG1 and IgG2 were determined. The mean MenC IgG concentration rose from 0.16 μg/mL prior to vaccination to 3.69 μg/mL 3 weeks post-vaccination, with 67% of patients reaching the threshold of ≥ 2.0 μg/mL. The mean IgG concentration at 35 weeks post-vaccination was 3.10 μg/mL. IgG2 concentrations increased more than IgG1. Marginal avidity maturation was seen. Hypo-responders to the first MenC vaccine (IgG anti-MenC ≤ 2.0 μg/mL) were offered a booster dose. After revaccination, 59% reached the chosen IgG threshold. The IgG concentration rose from 0.29 to 1.12 μg/mL, with an increase in the IgG1/IgG2 ratio. Avidity indices remained below 33%. In asplenic patients, the quantity and quality of antibodies produced after one dose of conjugated MenC vaccination is lower than that observed in previous studies in healthy adults. Booster vaccination does, indeed, lead to a rise in IgG geometric mean concentrations (GMCs), but does not lead to higher avidity of antibodies.

Relative importance of complement-mediated bactericidal and opsonic activity for protection against meningococcal disease

Killing of Neisseria meningitidis can result from complement-mediated serum bactericidal activity (SBA) or opsonophagocytosis (OPA), or a combination of the two mechanisms. While SBA titers > or =1:4 confer protection, recent evidence suggests that this threshold titer may not be required. For example, the incidence of meningococcal disease declines between ages 1 and 4 years without evidence of acquisition of SBA titers > or =1:4. Meningococcal polysaccharide vaccination also elicited OPA and lowered the risk of disease in patients with late complement component deficiencies whose sera did not support SBA. Sera from healthy adults immunized with an outer membrane vesicle vaccine showed OPA killing of N. meningitidis with C6-depleted complement, and whole blood from complement-sufficient non-immunized adults with SBA titers <1:4 also frequently had killing activity. Collectively the data indicate that SBA titers <1:4 and/or vaccine-induced OPA can confer protection against meningococcal disease.

Meningococcal A, C, Y and W-135 polysaccharide-protein conjugate vaccines

Serogroup C meningococcal conjugate vaccines, first launched in the UK in 1999, have been used successfully in Australia, Canada and several other European countries. Combination conjugate vaccines, containing more than one meningococcal polysaccharide, have been developed to broaden protection against the disease. A tetravalent meningococcal A, C, Y and W-135 conjugate vaccine was licensed for use in 11-55 year old adolescents and adults in the US in January 2005, and subsequently also in 2-11 year old children in Canada in May 2006. This article discusses the different glycoconjugate meningococcal vaccines which have been developed and the potential for their use to control disease caused by serogroups A, C, Y and W-135 of Neisseria meningitidis.

Reconsideration of the use of meningococcal polysaccharide vaccine

In 2005, a quadrivalent meningococcal conjugate vaccine was licensed in the United States for persons aged 11-55 years of age. For children aged 2-10 years with underlying diseases associated with increased risk of meningococcal disease, unconjugated meningococcal polysaccharide (MPS) vaccination is still recommended. This article reviews the increasing evidence that MPS vaccination impairs serum anticapsular antibody responses to subsequent injections of MPS or meningococcal conjugate vaccines (antibody hyporesponsiveness). Administering MPS as a probe to assess conjugate vaccine-induced immunologic memory also can extinguish subsequent memory anticapsular antibody responses, whereas conjugate vaccination regenerates memory B cells. Whether induction of antibody hyporesponsiveness or loss of immunologic memory increase the risk of acquiring meningococcal disease remains speculative. However, for children at increased risk of meningococcal disease, immunization with meningococcal quadrivalent conjugate vaccine off-label instead of MPS vaccine should be considered. Requirements for licensure of new glycoconjugate vaccines that include performing comparative clinical trials to demonstrate noninferiority with MPS vaccine, or use of a MPS challenge to assess conjugate-induced immunologic memory also should be modified because there are safer approaches for obtaining the same information.

Bacterial meningitis: the impact of vaccination

Acute bacterial meningitis remains an important cause of morbidity and mortality in children. Children <2 years of age are particularly susceptible to infection with encapsulated bacteria due to their immature response to polysaccharide antigens. Conjugate vaccines, which induce T cell memory, can provide immunological protection for these children. The Haemophilus influenzae type b (Hib) conjugate vaccine was the first such vaccine to become available. The efficacy of the vaccine has been quoted as being 98%. Its introduction was followed by a dramatic decrease in the incidence of all invasive Hib disease, including meningitis. This reduction was in part due to the ability of these vaccines to reduce nasopharyngeal carriage of the organism and thereby induce herd immunity. Different Hib vaccines use a variety of protein carriers and differ in their immunogenicity and efficacy. The most suitable vaccine needs to be determined according to the local epidemiology of Hib disease. Commercial combination vaccines may lead to lower antibody levels. A recent increase in the incidence of Hib disease in the UK highlights the importance of continued surveillance and the need for booster vaccinations to ensure continued protection. Conjugate vaccines to Streptococcus pneumoniae and Neisseria meningitidis have been developed. The introduction of a pneumococcal conjugate vaccine in the US has led to a decrease in the rate of infection by nearly 60% in children <5 years of age. A reduction in pneumococcal carriage may also modify disease epidemiology. The UK introduced the conjugate meningococcal C vaccine into its infant schedule with a corresponding reduction in N. meningitidis group C disease. A recent decrease in the effectiveness of the vaccine, however, suggests a booster may be necessary in the future. Our present understanding of the immunology of conjugate vaccines is far from complete. Developed countries have introduced conjugate vaccines into their immunisation schedules to prevent bacterial meningitis; however, their high cost precludes their use in many developing countries. Progress needs to be made in order to get these highly effective vaccines to those areas that need them.

Avidity of the immunoglobulin G response to a Neisseria meningitidis group C polysaccharide conjugate vaccine as measured by inhibition and chaotropic enzyme-linked immunosorbent assays

Antibody avidity, the strength of the multivalent interaction between antibodies and their antigens, is an important characteristic of protective immune responses. We have developed an inhibition enzyme-linked immunosorbent assay (ELISA) to measure antibody avidity for the capsular polysaccharide (PS) of Neisseria meningitidis group C (MnC) and determined the avidity constants (K(D)s) for 100 sera from children immunized with an MnC PS conjugate vaccine. The avidity constants were compared to the avidity indices (AI) obtained for the same sera using a chaotropic ELISA protocol. After the primary immunization series, the geometric mean (GM) K(D) was 674 nM and did not change in the months following immunization. However, the GM avidity did increase after the booster dose (GM K(D), 414 nM 1 month after booster immunization). In contrast, the GM AI increased from an initial value of 118 after the primary immunization series to 147 6 months after the completion of the primary immunization series and then further increased to 178 after booster immunization. At the individual subject level, the avidity constant and AI correlated after the primary immunization series and after booster immunization but not prior to boosting. This work suggests that the AI, as measured by the chaotropic ELISA, in contrast to the K(D), reflects changes that render antibody populations less susceptible to disruption by chaotropic agents without directly affecting the strength of the binding interactions.

Conjugate vaccines for preventing meningococcal C meningitis and septicaemia

BACKGROUND

Meningococcal polysaccharide (MPLS) vaccines protect against Serogroup C disease, but do not produce an immune response in infants less than two years of age. This limitation can be overcome by linking C polysaccharide to carrier proteins ('conjugating'), to create meningococcal serogroup C conjugate (MCC) vaccines. In the absence of trial data, the immune response to vaccination has been considered to be a reasonable surrogate for vaccine protection.

OBJECTIVES

To assess the immunogenicity, safety and efficacy of MCC vaccines for preventing meningitis and septicaemia.

SEARCH STRATEGY

We searched the Cochrane Central Register Controlled Trials (CENTRAL) (The Cochrane Library Issue 3, 2005); MEDLINE (1966 to September, Week 1 2005); and EMBASE (1990 to June 2005) and references of studies.

SELECTION CRITERIA

Randomised controlled trials (RCTs) and controlled clinical trials (CCTs) in humans comparing MCC vaccines against a control vaccine or none. In the absence of any trials on vaccine efficacy, population-based observational studies about effectiveness were included.

DATA COLLECTION AND ANALYSIS

Two authors independently screened the results of the literature searches, selected eligible studies, extracted the data and evaluated the quality of them.

MAIN RESULTS

The studies showed that MCC vaccine was highly immunogenic in infants after two and three doses, in toddlers after one and two doses and in older age groups after one dose. In general higher titres were generated after MCC than after MPLS vaccines. Immunological hypo-responsiveness seen after repeated doses of MPLS vaccine may be overcome with MCC. Observational studies have documented a significant decline in meningococcal C disease in countries where MCC vaccines have been widely used. The timing of the vaccinations schedules, the specific conjugate used, and the vaccines given concomitantly or combined, may be important.

AUTHORS' CONCLUSIONS

The MCC vaccine appears to be safe, immunogenic and able to induce immunological memory in all age groups. Observational studies strongly suggest that MCC is clinically effective.

Prospects for vaccine prevention of meningococcal infection

Neisseria meningitidis is the leading cause of bacterial meningitis in the United States and worldwide. A serogroup A/C/W-135/Y polysaccharide meningococcal vaccine has been licensed in the United States since 1981 but has not been used universally outside of the military. On 14 January 2005, a polysaccharide conjugate vaccine that covers meningococcal serogroups A, C, W-135, and Y was licensed in the United States for 11- to 55-year-olds and is now recommended for the routine immunization of adolescents and other high-risk groups. This review covers the changing epidemiology of meningococcal disease in the United States, issues related to vaccine prevention, and recommendations on the use of the new vaccine.

Combined conjugate vaccines: enhanced immunogenicity with the N19 polyepitope as a carrier protein

The N19 polyepitope, consisting of a sequential string of universal human CD4(+)-T-cell epitopes, was tested as a carrier protein in a formulation of combined glycoconjugate vaccines containing the capsular polysaccharides (PSs) of Neisseria meningitidis serogroups A, C, W-135, and Y. Good antibody responses to all four polysaccharides were induced by one single immunization of mice with N19-based conjugates. Two immunizations with N19 conjugates elicited anti-MenACWY antibody titers comparable to those induced after three doses of glycoconjugates containing CRM197 as carrier protein. Compared to cross-reacting material (CRM)-based constructs, lower amounts of N19-MenACWY conjugates still induced high bactericidal titers to all four PSs. Moreover, N19-MenACWY-conjugated constructs induced faster and higher antibody avidity maturation against meningococcal C PS than CRM-based conjugates. Very importantly, N19-specific antibodies did not cross-react with the parent protein from which N19 epitopes were derived, e.g., tetanus toxoid and influenza virus hemagglutinin. Finally, T helper epitopes of the N19 carrier protein were effectively generated both in vivo (after immunization with the N19 itself) and in vitro (after restimulation of epitope-specific spleen cells). Taken together, these data show that the N19 polyepitope represents a strong and valid option for the generation of improved or new combined glycoconjugate vaccines.

Immunogenicity of an investigational quadrivalent Neisseria meningitidis-diphtheria toxoid conjugate vaccine in 2-year old children

BACKGROUND

One dose of a quadrivalent meningococcal conjugate (MC-4) vaccine elicits higher group C bactericidal responses in 2-year olds than a U.S.-licensed quadrivalent polysaccharide (MPS-4) vaccine [Granoff DM, Harris SL. Protective activity of group C anticapsular antibodies elicited in 2-year olds by an investigational quadrivalent Neisseria meningitidis-diptheria toxoid conjugate vaccine. Pediatr Infect Dis J, 2004;23:490-7].

OBJECTIVE

Assess immunogenicity and persistence of serum antibody to capsular groups A, Y and W-135 in 2-year old children immunized with MC-4 vaccine.

DESIGN

Measurement of antibody responses in sera from a multicenter randomized trial comparing MC-4 and MPS-4 vaccines.

RESULTS

At 1 month, the group A serum bactericidal GMT was higher in the MC-4 than the MPS-4 group (P < 0.001) but there were no significant differences at 6 months. At 6 months, the respective Y and W-135 GMTs were higher in the MC-4 group (P < 0.05) but there were no differences at 1 month. The higher W-135 bactericidal titers reflected higher antibody avidity in the MC-4 group (P < 0.0001) and avidity maturation between 1 and 6 months (P < 0.05). At 2-year post-vaccination, the proportion of MC-4 immunized children with bactericidal titers > or =1:4 (presumed to be protective) was 15.0% (group A), 32.5% (group Y) and 45% (group W-135), as compared with 2.5, 15.4 and 17.5%, respectively, in unvaccinated children (P < 0.01 for group W-135; P < 0.05 for group A, and P = 0.07 for Y).

CONCLUSIONS

One dose of the MC-4 vaccine in 2-year olds elicits higher A, Y and W-135 bactericidal titers than MPS-4 vaccine. Compared with unvaccinated children, serum antibody titers remain elevated for 2 years after MC-4 vaccination. However, many vaccinated children have titers <1:4 and may require a booster dose for sustained protection.

Carbohydrate moieties as vaccine candidates

Carbohydrate epitopes or glycotopes are structurally diverse, occur in a variety of chemical contexts, and are present on the surfaces of cells in the body and on the surfaces of pathogens. These various structures and modes of presentation affect how they are perceived and processed by the body and dictate the outcome of the immune response directed against them. This review focuses on mechanisms of carbohydrate immunity, with an emphasis on carbohydrate vaccines that have been or are being developed for protection against encapsulated bacterial pathogens. We discuss the cellular basis of carbohydrate immunity, newly identified glycotope processing pathways and recognition capabilities, and the synthetic and microarray technologies that are being developed that will permit new experimental approaches to carbohydrate vaccine development and the exploration of the interaction of the immune system with self and nonself glycans.

Meningococcal conjugate vaccine in adolescents and children

Disease caused by Neisseria meningitidis is associated with high mortality rates and significant sequelae. Quadrivalent (A, C, Y, W-135) polysaccharide meningococcal vaccine has been available for more than 20 years, and although used widely for adolescents entering college, it has a number of limitations, including short duration of immunity and lack of a herd effect. A quadrivalent meningococcal conjugate vaccine has recently been licensed and endorsed for use in adolescents 11 to 12 years of age, at age 15, or at college entry. Among the key features of the new meningococcal conjugate vaccine are stimulation of both B-cell-dependent and T-cell-dependent immune responses, induction of immunologic memory and booster effects, long-term protection, reduction of nasopharyngeal carriage of N. meningitidis, and herd immunity. Large randomized, double-blind studies in adolescents and 2- to 10-year olds have demonstrated the immunogenicity as well as the safety and tolerability of the new meningococcal conjugate vaccine formulation.

Protective efficacy and immunogenicity of Escherichia coli K13 diphtheria toxoid conjugate against experimental ascending pyelonephritis

In the present study, protective efficacy of Escherichia coli capsular antigen, K13, was evaluated in a mouse model of pyelonephritis. Unconjugated capsular polysaccharide failed to provide any protection. However, coupling of K13 to diphtheria toxoid (DT) enhanced its immunogenicity and led to significant production of anticapsular antibodies in mice. Immunization of animals with K13-DT conjugate also caused significant improvement in cell-mediated immune response as indicated by an increase in lymphoblastogenic response and in the CD4+/CD8+ cell ratio of splenic lymphocytes. Significant decrease in bacterial load and renal severity scores were observed in K13-DT immunized animals. Suitability of K13-DT conjugate as an effective vaccine candidate against urinary tract infections caused by E. coli has been discussed.

Naturally-acquired immunity to Neisseria meningitidis group A

Group A meningococcal disease is epidemic in Sudan, less common in Uganda, a country bordering the "meningitis belt," and rare in North America. The basis of naturally-acquired group A immunity is unknown but in North America protection has been attributed to a high prevalence of serum anticapsular antibodies elicited by cross-reacting bacteria. We measured group A anticapsular antibody concentrations and bactericidal titers in sera from 236 adults (47 from the Sudan obtained at the height of a group A epidemic, 57 from Uganda, and 132 from North America). Anticapsular antibody concentrations were higher in Sudanese sera than in North American or Ugandan sera (geometric mean of 31.5 versus 5.4 and 5.3 microg/ml, respectively, P < 0.0001). Bactericidal titers of > or =1:4 (presumed to be a protective titer when measured with human complement) were detected in 66% of Sudanese sera as compared with 27 and 23%, respectively, of North American and Ugandan sera (P < 0.0001). Bactericidal activity was inhibited by group A polysaccharide in 58% of the Sudanese bactericidal sera as compared to 17 and 6% of North America and Ugandan bactericidal sera (P < 0.0005). Approximately 50% of non-bactericidal Sudanese sera had high IgA anticapsular antibody concentrations, which were rare in bactericidal Sudanese sera. Thus, serum anticapsular antibodies and bactericidal activity are prevalent in Sudanese exposed to a group A epidemic. Cross-reacting group A anticapsular antibodies are prevalent in North American and Ugandan sera, but bactericidal activity is infrequent and when present is largely directed at non-capsular antigens.

Persistence of group C anticapsular antibodies two to three years after immunization with an investigational quadrivalent Neisseria meningitidis-diphtheria toxoid conjugate vaccine

BACKGROUND

An investigational quadrivalent (A, C, Y and W-135) meningococcal conjugate (MC-4) vaccine was reported to be more immunogenic in 2-year-olds than the currently licensed meningococcal polysaccharide vaccine, but persistence of serum antibody beyond 6 months after conjugate vaccination is unknown.

OBJECTIVE

Determine persistence and the immunologic basis of protective activity of group C anticapsular antibodies in sera obtained 2-3 years after MC-4 vaccination.

DESIGN

Group C antibody concentrations, bactericidal activity and passive protective activity were measured in sera from 48 children, ages 4-5 years, who had been immunized 2-3 years earlier with an MC-4 vaccine and from 47 children who had not been previously vaccinated.

RESULTS

Serum antibody concentrations were higher in the vaccinated than the unvaccinated children (geometric means, 0.30 and 0.09 mug/mL, respectively, P < 0.0001). Bactericidal titers > or =1/4 (considered protective) were infrequent in both vaccinated and unvaccinated children (14.6 and 6.4%, respectively, P = 0.3). Passive protective activity against bacteremia in the infant rat model was more frequent in sera from vaccinated (37.5%) than sera from unvaccinated children (12.5%, P < 0.02). The proportion of sera with passive protective activity increased with increasing anticapsular antibody concentrations (P < 0.0001).

INTERPRETATION

Serum group C antibody concentrations remained elevated for 2-3 years after MC-4 vaccination, and passive protective activity was more frequent in vaccinated than unvaccinated children. However, serum antibody concentrations in many vaccinated children were no longer sufficient to activate complement-mediated bacteriolysis in vitro or to confer passive protection against experimental group C disease.

Naturally acquired passive protective activity against Neisseria meningitidis Group C in the absence of serum bactericidal activity

The hallmark of immunity to meningococcal disease is a bactericidal titer in serum of > or =1:4 measured with human complement, but this threshold titer may underestimate the extent of protection. We used the infant rat model of meningococcal bacteremia to measure group C passive protective activity in serum samples from 91 unimmunized adults living in California. A total of 35 sera (38.5%) had passive protective activity. Sera with complement-mediated bactericidal titers of > or =1:4 were 3.4-fold more likely to confer protection (89%) than nonbactericidal sera (26%; P < 0.0001). Thus, bactericidal titers of > or =1:4 are a marker of protection, but this threshold lacks sensitivity for predicting protective activity. We investigated the 73 sera with bactericidal titers of <1:4 to determine the basis of protective activity. The 19 sera with protective activity had a higher geometric mean group C anticapsular antibody concentration (0.72 microg/ml) than the 54 sera that lacked protective activity (0.16 microg/ml; P < 0.001). Thus, protective activity in the absence of bactericidal activity was associated with higher concentrations of anticapsular antibodies, but not all sera with anticapsular antibodies conferred protection. Of 18 nonbactericidal sera with anticapsular antibody concentrations between 0.31 and 0.99 microg/ml, the 11 sera that conferred protection had a higher mean antibody avidity constant (21.9 nM(-1)) than the 7 nonprotective sera (14.6 nM(-1); P < 0.03). Thus, in sera with titers of <1:4, protective activity is associated with higher-avidity group C anticapsular antibodies, which are present in concentrations insufficient to elicit complement-mediated bacteriolysis in vitro but sufficient to confer protection in an in vivo bacteremia model.

Serum group a anticapsular antibodies in a Sudanese population immunized with meningococcal polysaccharide vaccine during a group A epidemic

BACKGROUND

Vaccination during group A meningococcal epidemics is reported to decrease the number of new cases of disease. However, implementing mass vaccination is often delayed, and little information is available on whether immunity increases in a population vaccinated during an epidemic when exposure to the epidemic strain is common.

METHODS

A convenience sample of 134 previously unimmunized persons, ages 3-49 years, were immunized with a meningococcal polysaccharide vaccine. Their serum group A antibody responses were compared with those of 26 adults immunized in California with no known group A exposure.

RESULTS

Before immunization, serum anticapsular antibody concentrations were 10-fold higher in Sudanese adults and 4-fold higher in Sudanese, ages 3-17 years, than in North American adults (geometric means, 29 and 13 microg/ml, respectively, versus 3 microg/ml, P < 0.001). Seventy-five percent of the Sudanese had serum bactericidal titers that correlate with protection (> or 1/128). Nearly all Sudanese with low bactericidal titers before vaccination developed protective bactericidal antibody responses after vaccination, and the magnitude of the anticapsular antibody responses of the Sudanese was similar to that of the immunized North Americans.

INTERPRETATION

The high titers of naturally acquired antibody in the Sudanese may reflect widespread exposure to the epidemic strain and underscore difficulties of instituting immunization before exposure occurs. Also epidemics in sub-Saharan Africans may not abate even if 75% of the population is immune to disease as long as the organism is transmitted widely among both immune and susceptible persons.