Evaluation of De-O-acetylated meningococcal C polysaccharide-tetanus toxoid conjugate vaccine in infancy: reactogenicity, immunogenicity, immunologic priming, and bactericidal activity against O-acetylated and De-O-acetylated serogroup C strains

Meningococcal polysaccharide-protein conjugate vaccines

It is now 5 years since the UK became the first country to introduce the serogroup C meningococcal polysaccharide-protein conjugate vaccines (MenC) into its routine immunisation schedule. This article reviews the global use of MenC with particular reference to the range of immunisation strategies used internationally. To date, concerns that MenC may result in an increase in meningococcal disease due to non-C serogroups have not been realised. The vaccine has proved to be highly safe and effective; however, concerns have arisen regarding the duration of vaccine effectiveness. Although booster doses of MenC may potentially extend the duration of protection offered by the vaccine, there are, as yet, no studies assessing this option. Clinical trials are underway to assess new combination conjugate vaccines (containing A, C, Y, and W polysaccharides), and it is probable that these more broadly protective vaccines will become available in the near future.

Meningococcal vaccines

Meningococcal disease is one of the most feared and serious infections in the young and its prevention by vaccination is an important goal. The high degree of antigenic variability of the organism makes the meningococcus a challenging target for vaccine prevention. Meningococcal polysaccharide vaccines against serogroup A and C are efficacious and have been widely used, often in combination with serogroup Y and W135 components. Their relative lack of immunogenicity in young children and infants can be overcome by conjugation to a protein carrier. The effectiveness of serogroup C glycoconjugate vaccines in children of all ages has been demonstrated and they have now been introduced into routine vaccination schedules. Conjugate vaccines against other serogroups, including A, Y, and W135 will soon be available and it is hoped they may emulate this success. Prevention of serogroup B disease has proven more elusive. Several serogroup B vaccines based on outer membrane vesicles have been shown to be immunogenic and reasonably effective in adults and older children, but the protection offered by them is chiefly strain-specific. Multivalent recombinant PorA vaccines have been developed to broaden the protective effect, but no efficacy data are available as yet. Intensive efforts have been directed at other outer membrane protein vaccine candidates and lipopolysaccharide, and some of these have been shown to offer protection in experimental animal models. Nonpathogenic Neisseriae spp. such as Neisseria lactamica are also possible vaccine candidates. Previously unknown proteins have been identified from in silico analysis of the meningococcal genome and their vaccine potential explored. However, none of these has yet been presented as the 'universal' protective antigen and work in this field continues to be held back by our limited knowledge concerning the mechanisms of natural protection against serogroup B meningococci.

The Neisseria meningitidis serogroup A capsular polysaccharide O-3 and O-4 acetyltransferase

Neisseria meningitidis serogroup A capsular polysaccharide (CPS) is composed of a homopolymer of O-acetylated, alpha1-->6-linked ManNAc 1-phosphate that is distinct from the capsule structures of the other meningococcal disease-causing serogroups, B, C, Y, and W-135. The serogroup A capsule biosynthetic genetic cassette consists of four open reading frames, mynA-D (sacA-D), that are specific to serogroup A, but the functions of these genes have not been well characterized. mynC was found to encode an inner membrane-associated acetyltransferase that is responsible for the O-acetylation of the CPS of serogroup A. The wild-type CPS as revealed by 1H NMR had 60-70% O-acetylated ManNAc residues that contained acetyl groups at O-3, with some species acetylated at O-4 and at both O-3 and O-4. A non-polar mynC mutant generated by introducing an aphA-3 kanamycin resistance cassette produced CPS with no O-acetylation. A serogroup A capsule-specific monoclonal antibody was shown to recognize the wild-type O-acetylated CPS, but not the CPS of the mynC mutant, which lacked O-acetylation. MynC was C-terminally His-tagged and overexpressed in Escherichia coli to obtain the predicted approximately 26-kDa protein. The acetyltransferase activity of purified MynC was demonstrated in vitro using [14C]acetyl-CoA. MynC O-acetylated the O-acetylated CPS of the mynC mutant and further acetylated the wild-type CPS of serogroup A meningococci, but not the CPS of serogroup B or C meningococci. Genetic complementation of the mynC mutant confirmed the function of MynC as the serogroup A CPS O-3 and O-4 acetyltransferase. MynC represents a new subclass of O-acetyltransferases that utilize acetyl-CoA to decorate the D-mannosamine capsule of N. meningitidis serogroup A.

Surface plasmon resonance analysis of antipolysaccharide antibody specificity: responses to meningococcal group C conjugate vaccines and bacteria

Antibody (Ab) responses to polysaccharides (PS), such as Neisseria meningitidis group C PS (MCPS), are characterized as being thymus independent and are restricted with regard to clonotype and isotype expression. PS conjugated to proteins, e.g., MCPS coupled with tetanus toxoid or the diphtheria toxin derivative CRM197, elicit thymus-dependent responses. The present study developed a surface plasmon resonance approach to evaluate Ab responses to MCPS conjugate vaccines, including either O-acetylated (OAc+) or de-O-acetylated (OAc-) forms of the PS. The results were generally consistent with those obtained by enzyme-linked immunosorbent assay and showed that sera from mice immunized with conjugate vaccines contain Abs that bind more effectively to OAc+ and OAc- MCPS than sera from mice immunized with fixed bacteria. The data suggest a critical shared or overlapping epitope recognized by all the conjugate vaccine immune sera and strategies for assessing polyclonal Ab avidity.

Immune response to meningococcal serogroup C conjugate vaccine in asplenic individuals

Asplenic individuals are known to be at increased risk of infection with encapsulated bacteria. Recent United Kingdom recommendations stated that this at-risk group should receive one dose of the meningococcal serogroup C conjugate (MCC) vaccine. However, the immune response of asplenic individuals to MCC vaccine is unknown. The immune response of asplenics (n = 130) to immunization with the MCC vaccine was investigated. Asplenic individuals had a significantly lower geometric mean titer (GMT) (157.8; 95% confidence interval [CI], 94.5 to 263.3) of bactericidal antibody in serum (SBA) than an age-matched control group (n = 48) (1448.2; 95% CI, 751.1 to 2792.0). However, 80% of asplenic individuals achieved the proposed protective SBA titer of > or =8. No differences were observed between the two groups in the serogroup C-specific immunoglobulin G geometric mean concentration. A significant reduction in SBA GMT or the number of responders achieving an SBA titer of > or =8 was observed if the reason for splenectomy was a medical cause or if MCC vaccination occurred <10 years after splenectomy. Individuals (n = 29) who did not achieve an SBA titer of > or =16 were offered a second dose of MCC vaccine. Analysis of the SBA response revealed that 61% (14 of 23) of the individuals who received a second dose achieved a protective titer. In total, 93% of asplenic individuals achieved a titer of > or =8 following MCC vaccination (one or two doses combined). We recommend that, following vaccination of asplenics, either the level of functional antibody should be determined, with a second dose of MCC vaccine offered to nonresponders, or two doses of MCC vaccine should be routinely offered.

Scientific challenges for the quality control and production of group C meningococcal conjugate vaccines

Recommendations (formerly known as requirements) for meningococcal polysaccharide vaccines were adopted by the World Health Organisation (WHO) Expert Committee on Biological Standardisation in 1976 and amended in 1978 and 1981. In clinical studies, these vaccines have been shown to have efficacy of at least 90% and have proved to be highly effective in vaccination programmes. Nevertheless, their inability to elicit protective responses in young infants or to induce good immunological memory has prevented their implementation in national infant immunisation schedules. Following the successful introduction of the Haemophilus influenzae type b conjugate (Hib) vaccines, considerable progress has been made in the development of similar conjugate vaccines based on meningococcal group C capsular polysaccharide. Controlled clinical trials have demonstrated that they induce protective levels of antibodies to group C polysaccharide in all age groups and, as T-cell dependent antigens, induce immunological memory and affinity maturation of anti-capsular antibodies. Such vaccines have been shown to offer protective immunity following the introduction of group C conjugates in the UK. The World Health Organisation has produced recommendations for the production and control of these new vaccines.

Bacterial polysaccharide-protein conjugate vaccines

Following demonstration that chemical conjugation of polysaccharide antigens to proteins could enhance their immunogenicity in the 1920s, interest in this approach to primary prevention of bacterial infections waned with the development and widespread use of antibiotics. Emergence of resistant bacteria rekindled interest in the late 20th century, which saw extremely rapid development and implementation of several vaccines which are already rapidly changing the epidemiology of childhood infections with Haemophilus influenzae type b, Streptococcus pneumoniae and Neisseria meningitidis. Others such as Group B streptococcus and Salmonella typhi infections may soon follow. However, several important questions about the immunology of these antigens remain unanswered and the long-term implications of reducing or eliminating the circulation of organisms which are more commonly nasopharyngeal commensals than pathogenic invaders are uncertain.

Genetics of capsule O-acetylation in serogroup C, W-135 and Y meningococci

Capsular polysaccharides of serogroup C, W-135 and Y meningococci were previously reported to be O-acetylated at the sialic acid residues. There is evidence that O-acetylation affects the immunogenicity of polysaccharide vaccines. We identified genes indispensable for O-acetylation of serogroup C, W-135 and Y meningococci downstream of the capsule synthesis genes siaA-D. The genes were co-transcribed with the sia operon as shown by reverse transcription polymerase chain reaction analysis. The putative capsular polysaccharide O-acetyltransferases were designated OatC and OatWY. The protein OatWY of serogroups W-135 and Y showed sequence homologies to members of the NodL-LacA-CysE family of bacterial acetyltransferases, whereas no sequence homology with any known protein in the different databases was found for the serogroup C protein OatC. In serogroup W-135 and Y meningococci, several clonal lineages either lacked OatWY or OatWY was inactivated by insertion of IS1301. For serogroup C meningococci, we observed in vitro phase variation of O-acetylation, which resulted from slipped-strand mispairing in homopolymeric tracts. This finding explains the observation of naturally occurring de-O-acetylated serogroup C meningococci. Our report is the first description of sequences of sialic acid O-acetyltransferase genes that have not been cloned from either other bacterial or mammalian organisms.

Immunogenicity of, and immunologic memory to, a reduced primary schedule of meningococcal C-tetanus toxoid conjugate vaccine in infants in the United kingdom

It has been previously shown that one of the three meningococcal C conjugate (MCC) vaccines introduced in the United Kingdom proved highly immunogenic after the first dose of a three-dose schedule, with evidence of immune memory after dose 3. Thus, in infants a one- or two-dose schedule of this MCC vaccine, conjugated to tetanus toxoid (TT), may suffice. Healthy infants (n = 586) were randomized to receive either one (group 1), two (group 2), or three (group 3) doses of MCC-TT vaccine with a 10- micro g polysaccharide booster given at 13 to 14 months of age. Serum bactericidal antibody (SBA) levels were measured by utilizing rabbit complement (rSBA), meningococcal C-specific immunoglobulin G (IgG), and avidity indices (AIs). For groups 1, 2, and 3, the percentages of infants with an rSBA level of > or =8 against strain C11 were 98.4, 100, and 99.4%, respectively. Infants in group 1 with prevaccination rSBA titers of > or =8 had post-primary MCC rSBA geometric mean titers (GMTs) significantly lower than those infants with prevaccination rSBA titers of <8. One dose of MCC-TT vaccine given to infants at 2 months of age yielded significantly lower SBA GMTs and geometric mean AIs (GMAIs) than two or three doses but elicited a significantly greater response after boosting, as reflected by rSBA levels and GMAI. This study provides the first evidence that the number of doses of MCC-TT used in infant immunization schedules could be decreased.

Monoclonal antibodies reactive with immunorecessive epitopes of glucuronoxylomannan, the major capsular polysaccharide of Cryptococcus neoformans

Cryptococcus neoformans is surrounded by an antiphagocytic capsule whose primary constituent is glucuronoxylomannan (GXM). An epitope shared by GXM serotypes A, B, C, and D is immunodominant when mice are immunized with serotype A GXM. In contrast, an epitope shared only by serotypes A and D is immunodominant when mice are immunized with serotype D. Hybridomas secreting antibodies reactive with subdominant epitopes were identified through a positive-negative screening procedure in which antibody-secreting colonies were characterized by reactivity with both the immunizing polysaccharide and GXMs from each of the four major serotypes. In this manner, a monoclonal antibody (MAb) that was reactive with an epitope shared only by serotypes A and B was identified and designated F10F5. Such an epitope has not been described previously. Immunization of mice with de-O-acetylated serotype A GXM generated a hybridoma that secreted an antibody, designated F12D2, that was reactive with all four serotypes. Unlike previously described monoclonal and polyclonal panspecific antibodies, the reactivity of MAb F12D2 was not altered by de-O-acetylation of GXM. These results indicate that there are at least two panspecific GXM epitopes; one epitope is dependent on O acetylation for antibody reactivity, and the other is independent of O acetylation. This study identifies strategies for production of MAbs that are reactive with subdominant or cryptic GXM epitopes and provides new information regarding the antigenic makeup and the humoral immune response to GXM, an essential virulence factor that is a target for active and passive immunization.

The msbB mutant of Neisseria meningitidis strain NMB has a defect in lipooligosaccharide assembly and transport to the outer membrane

A deletion-insertion mutation in msbB, a gene that encodes a lipid A acyltransferase, was introduced into encapsulated Neisseria meningitidis serogroup B strain NMB and an acapsular mutant of the same strain. These mutants were designated NMBA11K3 and NMBA11K3cap-, respectively. Neither lipooligosaccharide (LOS) nor lipid A could be isolated from NMBA11K3 although a number of techniques were tried, but both were easily extracted from NMBA11K3cap-. Immunoelectron microscopy using monoclonal antibody (MAb) 6B4, which recognizes the terminal Galbeta1-4GlcNAc of LOS, demonstrated that NMB, NMBcap-, and NMBA11K3cap- expressed LOS circumferentially, while MAb 6B4 did not bind to the surface of NMBA11K3. However, cytoplasmic staining of NMBA11K3 with MAb 6B4 was a consistent observation. Mass-spectrometric analyses demonstrated that the relative amounts of the lipid A-specific C12:0 3-OH and C14:0 3-OH present in the membrane preparations (MP) from NMBA11K3 were substantially decreased (25- and 23-fold, respectively) compared to the amount in MP from its parent strain, NMB. Western blot analyses of MP from NMBA11K3 demonstrated that the levels of porin in the outer membrane of NMBA11K3 were also substantially decreased. These studies suggest that the lipid A acylation defect in encapsulated NMBA11K3 influences the assembly of the lipid A and consequently the incorporation of porin in the outer membrane.

Effect of O acetylation of Neisseria meningitidis serogroup A capsular polysaccharide on development of functional immune responses

The importance of O-acetyl groups to the immunogenicity of Neisseria meningitidis serogroup A polysaccharide (PS) was examined in studies using human sera and mouse immunization. In 17 of 18 postimmunization human sera, inhibition enzyme-linked immunosorbent assay indicated that the majority of antibodies binding to serogroup A PS were specific for epitopes involving O-acetyl groups. Studies with mice also showed an essential role for O-acetyl groups, where serum bactericidal titers following immunization with de-O-acetylated (de-O-Ac) conjugate vaccine were at least 32-fold lower than those following immunization with O-Ac PS-conjugate vaccine and 4-fold lower than those following immunization with native capsular PS. Inhibition studies using native and de-O-Ac PS confirmed the specificity of murine antibodies to native PS. The dramatic reduction in immunogenicity associated with removal of O-acetyl groups indicates that O acetylation is essential to the immunogenic epitopes of serogroup A PS. Since levels of bactericidal antibodies are correlated with protection against disease, O-acetyl groups appear to be important in protection.

O-Acetylation status of the capsular polysaccharides of serogroup Y and W135 meningococci isolated in the UK

At a time when tetravalent conjugate vaccines for meningococcal serogroups A/C/Y/W135 are being formulated the O-acetylation status of their respective capsular polysaccharides has not previously been studied in the UK for all components. Although this has been elucidated for serogroup C, little is known about the O-acetylation status of serogroups W135 and Y. Meningococcal serogroup W135 (n=181) and Y (n=90) isolates submitted to the PHLS Meningococcal Reference Unit in 1996, 2000 and 2001 were investigated for O-acetylation capsular status by dot blot assay. Eight per cent of W135 and 79% of Y isolates respectively were found to be O-acetylated with a similar distribution found in both carrier and case isolates. An increase in O-acetylated W135 isolates was noted between 2000 (0%) and 2001 (21%) which was not due to the introduction of the Hajj associated W135 (ET 37 complex; serosubtype P1.5,2) isolates, all of which were de-O-acetylated. Although the biological relevance of O-acetylation status is unknown for these serogroups, an understanding of O-acetylation status of the respective polysaccharides may provide useful insights into the optimal vaccine formulation.

Vaccine prevention of meningococcal disease, coming soon?

The past century has seen the use of a number of vaccines for prevention and control of meningococcal disease with varied success. The use of polysaccharide vaccines for the control of outbreaks of serogroup C infections in teenagers and young adults and epidemic serogroup A disease has been established for 30 years and an effective protein-polysaccharide conjugate vaccine against serogroup C was introduced into the UK infant immunisation schedule in 2000. The next generation of these glycoconjugate vaccines will be on the shelf soon, eventually offering the prospect of eradication of serogroups A, C, Y and W135 through routine infant immunisation. Despite these exciting prospects, serogroup B meningococci still account for a majority of infections in industrialised nations but development of safe, immunogenic and effective serogroup B meningococcal vaccines has been an elusive goal. Outer membrane vesicle vaccines for B disease are already used in some countries, and will likely be used more widely in the next few years, but efficacy for endemic disease in children has so far been disappointing. However, the innovations arising from the availability of the meningococcal genome sequence, public and scientific interest in the disease and recent pharmaceutical company investment in development of serogroup B vaccines may have started the countdown to the end of meningococcal infection in children.

Safety of a new conjugate meningococcal C vaccine in infants

BACKGROUND

Group C conjugate meningococcal vaccines (Men C) were introduced into the UK primary immunisation schedule in November 1999. There has been extensive professional and public interest in their efficacy and safety.

AIM

To determine the occurrence of at least one uncommon adverse event in infants related to the administration of the Chiron Men C vaccine.

METHODS

A total of 2796 infants aged approximately 2 months were recruited into the study from areas in and around Sheffield and from Scotland. They were vaccinated with the Chiron Men C vaccine at 2, 3, and 4 months along with routine immunisations. Data on adverse events occurring one month after each dose were collected actively and prospectively and reviewed for possible relation to the vaccine.

RESULTS

There were no deaths. There were no serious adverse events considered definitely or probably caused by the vaccine. Four infants developed serious adverse events (hypotonia, screaming syndrome, maculopapular rash, and agitation, respectively) that were considered possibly related to the vaccine. All recovered completely. Adverse events were seen in 1804 children but were considered possibly related to the vaccine in only 49 (1.8%). On subsequent immunisation there were no recurrences of adverse events considered to be possibly related to the vaccine.

Planning, registration, and implementation of an immunisation campaign against meningococcal serogroup C disease in the UK: a success story

The introduction of meningococcal C conjugate (MCC) vaccine in the UK in November 1999 as a routine 3 dose infant immunisation course, with a single catch-up dose for all children aged between 12 months and 17 years, was the result of an intensive 5 year collaborative research programme funded by the Department of Health for England and involving public bodies, academia and vaccine manufacturers. The research programme established the safety and immunogenicity of MCC vaccines in infants, toddlers, pre-school and school-aged children. The nature and frequency of common adverse events in school-aged children was similar to that after a booster dose of diphtheria and tetanus vaccine given to the same age groups. The recommendation that a single dose was adequate for children aged 12 months and above was based on antibody levels measured by serum bactericidal assay and evidence of induction of immunological memory as shown by maturation of antibody avidity. Licensure by the Medicines Control Agency was based on serological criteria alone without direct evidence of efficacy and has set a precedent for other meningococcal conjugate polysaccharide vaccines. Vaccine coverage of around 85% was achieved in the targeted age groups and has resulted in a drop in the incidence of serogroup C disease in these groups of over 80% within 18 months of the start of the vaccination programme. Early post-licensure efficacy estimates for toddlers and teenagers (88 and 96%, respectively, in the first 16 months after vaccination) validate the serological criteria used for licensure. Surveillance of the prevalent serogroups and serosubtypes among invasive case isolates has shown no evidence of any capsular switching to serogroup B during the first 18 months of the MCC vaccination programme.

Prospects offered by genome studies for combating meningococcal disease by vaccination

Meningococcal disease was first recognised and Neisseria meningitidis isolated as the causative agent over 100 years ago, but despite more than a century of research, attempts to eliminate this distressing illness have so far been thwarted. The main problem lies in the fact that N. meningitidis usually exists as a harmless commensal inhabitant of the human nasopharynx, the pathogenic state being the exception rather than the norm. As man is its only host, the meningococcus is uniquely adapted to this ecological niche and has evolved an array of mechanisms for evading clearance by the human immune response. Progress has been made in combating the disease by developing vaccines that target specific pathogenic serogroups of meningococci. However, a fully comprehensive vaccine that protects against all pathogenic strains is still just beyond reach. The publication of the genome sequences of two meningococcal strains, one each from serogroups A and B and the imminent completion of a third illustrates the extent of the problems to be overcome, namely the vast array of genetic mechanisms for the generation of meningococcal diversity. Fortunately, genome studies also provide new hope for solutions to these problems in the potential for a greater understanding of meningococcal pathogenesis and possibilities for the identification of new vaccine candidates. This review describes some of the approaches that are currently being used to exploit the information from meningococcal genome sequences and seeks to identify future prospects for combating meningococcal disease.