The changing and dynamic epidemiology of meningococcal disease

The epidemiology of invasive meningococcal disease continues to change rapidly, even in the three years since the first Meningococcal Exchange Meeting in 2008. Control of disease caused by serogroup C has been achieved in countries that have implemented meningococcal C or quadrivalent meningococcal ACWY conjugate vaccines. Initiation of mass immunization programs with meningococcal A conjugate vaccines across the meningitis belt of Africa may lead to the interruption of cyclical meningococcal epidemics. A meningococcal B vaccination program in New Zealand has led to a decreased incidence of high rates of endemic serogroup B disease. Increases in serogroup Y disease have been observed in certain Nordic countries which, if they persist, may require consideration of use of a multiple serogroup vaccine. The imminent availability of recombinant broadly protective serogroup B vaccines may provide the tools for further control of invasive meningococcal disease in areas where serogroup B disease predominates. Continued surveillance of meningococcal disease is essential; ongoing global efforts to improve the completeness of reporting are required.     

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.     

B群脑膜炎球菌疫苗研究进展

脑膜炎奈瑟菌（Neisseria Meningitidis,Nm）感染仍然严重威胁人群健康,接种疫苗是预防该类疾病的有效手段。根据荚膜多糖的特征,Nm分为A、B、C、W135、Y等不同的血清群。A、C、W135、Y群Nm多糖疫苗以及多糖-蛋白结合疫苗,已经得到广泛应用并证明了其有效性。由于B群Nm菌株的荚膜多糖免疫原性较低,并且与人体神经组织具有同源性,因此B群Nm多糖不能用于疫苗抗原成分。近年来,国际上开展了大量B群Nm蛋白疫苗研究工作,以疫苗外膜蛋白为基础的疫苗研究,以及反向疫苗学技术在B群疫苗研究中的应用,使B群Nm疫苗的研究取得了长足进展。某些B群疫苗显示出良好的免疫原性和有效性,并且已经有B群蛋白疫苗获得许可并规模化应用。     

The concept of "tailor-made", protein-based, outer membrane vesicle vaccines against meningococcal disease

Protein-based, outer membrane vesicle (OMV) vaccines have previously proven to be efficacious against serogroup B meningococcal disease in Norway and Cuba. Currently, a public health intervention is going on in order to control a serogroup B epidemic in New Zealand. The scale-up and standardization of vaccine production required for controlling the New Zealand epidemic has allowed the establishment of large-scale GMP manufacturing for OMV vaccines. The outcome of this will be licensing of the vaccine in New Zealand and possibly other countries. The availability of licensed OMV vaccines raises the question of whether such vaccines may provide the opportunity to control other outbreaks and epidemics. For instance, such a vaccine could control a localised outbreak of group B meningococci in Normandy, France. "Tailor-made" vaccines, focusing on the sub-capsular antigens may also be considered for use in sub-Saharan Africa for the prevention of the recurrent outbreaks by serogroups A and W135 meningococci. This assumption is based on the epidemiological observation that meningococcal outbreaks in Africa are clonal and are strikingly stable regarding their phenotypic characteristics.     

Considerations for controlling invasive meningococcal disease in high income countries

The development of conjugate vaccines has enabled the prevention and control of Neisseria meningitidis caused by serogroups A, C, W-135 and Y. Vaccines that provide protection against a broad number of serogroup B strains likely will be available soon to enable greater control of meningococcal disease in high income countries. We present an argument for adequate post-marketing surveillance to monitor epidemiological shifts and to provide a context for the safety and reactogenicity of serogroup B vaccines, including the newer recombinant vaccines. We also offer a series of recommendations to address possible concerns about vaccine safety.     

Meningococcal factor H-binding protein vaccines with decreased binding to human complement factor H have enhanced immunogenicity in human factor H transgenic mice

Factor H-binding protein (fHbp) is a component of a meningococcal vaccine recently licensed in Europe for prevention of serogroup B disease, and a second vaccine in clinical development. The protein specifically binds human factor H (fH), which down-regulates complement activation and enhances resistance to bactericidal activity. There are conflicting data from studies in human fH transgenic mice on whether binding of human fH to fHbp vaccines decreases immunogenicity, and whether mutant fHbp vaccines with decreased fH binding have enhanced immunogenicity. fHbp can be classified into two sub-families based on sequence divergence and immunologic cross-reactivity. Previous studies of mutant fHbp vaccines with low fH binding were from sub-family B, which account for approximately 60% of serogroup B case isolates. In the present study, we evaluated the immunogenicity of two mutant sub-family A fHbp vaccines containing single substitutions, T221A or D211A, which resulted in 15- or 30-fold lower affinity for human fH, respectively, than the corresponding control wild-type fHbp vaccine. In transgenic mice with high serum concentrations of human fH, both mutant vaccines elicited significantly higher IgG titers and higher serum bactericidal antibody responses than the control fHbp vaccine that bound human fH. Thus, mutations introduced into a sub-family A fHbp antigen to decrease fH binding can increase protective antibody responses in human fH transgenic mice. Collectively the data suggest that mutant fHbp antigens with decreased fH binding will result in superior vaccines in humans.     

Effect of sequence variation in meningococcal PorA outer membrane protein on the effectiveness of a hexavalent PorA outer membrane vesicle vaccine in toddlers and school children

Though meningococcal conjugate vaccines are effective against serogroup C, there is currently no vaccine solution for serogroup B disease. PorA outer membrane protein (OMP) is a potential serogroup B vaccine candidate. A hexavalent PorA outer membrane vesicle (OMV) vaccine has been evaluated in phase I and II trials with promising results. However, considerable sequence variation occurs in the variable regions (VRs) encoding these serosubtypes. By using five wild type P1.19,15 variant strains we examined the serum bactericidal antibody (SBA) titres from sera collected from toddlers and school children pre- and post-vaccination. The numbers of subjects with SBA titres of <4, 4 and > or = 8 varied greatly between the different strains. This was also reflected when > or = 4-fold rises in SBA titres were examined. This finding in sera from toddlers and school children may have implications for PorA based vaccines.     

The challenge of post-implementation surveillance for novel meningococcal vaccines

Novel serogroup B meningococcal vaccines are currently in late stage development and may be used in mass immunisation campaigns over the coming years. This represents an exciting development in the prevention of childhood meningitis, however monitoring the impact of these vaccines on meningococcal disease epidemiology will provide significant challenges. Although designed to prevent serogroup B meningococcal disease the vaccine antigens are not serogroup specific, creating the potential for multiple definitions of vaccine effectiveness and vaccine failure.     

Preclinical studies on new proteins as carrier for glycoconjugate vaccines

Glycoconjugate vaccines are made of carbohydrate antigens covalently bound to a carrier protein to enhance their immunogenicity. Among the different carrier proteins tested in preclinical and clinical studies, five have been used so far for licensed vaccines: Diphtheria and Tetanus toxoids, the non-toxic mutant of diphtheria toxin CRM₁₉₇, the outer membrane protein complex of Neisseria meningitidis serogroup B and the Protein D derived from non-typeable Haemophilus influenzae. Availability of novel carriers might help to overcome immune interference in multi-valent vaccines containing several polysaccharide-conjugate antigens, and also to develop vaccines which target both protein as well saccharide epitopes of the same pathogen. Accordingly we have conducted a study to identify new potential carrier proteins. Twenty-eight proteins, derived from different bacteria, were conjugated to the model polysaccharide Laminarin and tested in mice for their ability in inducing antibodies against the carbohydrate antigen and eight of them were subsequently tested as carrier for serogroup meningococcal C oligosaccharides. Four out of these eight were able to elicit in mice satisfactory anti meningococcal serogroup C titers. Based on immunological evaluation, the Streptococcus pneumoniae protein spr96/2021 was successfully evaluated as carrier for serogroups A, C, W, Y and X meningococcal capsular saccharides.     

Priorities for research on meningococcal disease and the impact of serogroup A vaccination in the African meningitis belt

For over 100 years, large epidemics of meningococcal meningitis have occurred every few years in areas of the African Sahel and sub-Sahel known as the African meningitis belt. Until recently, the main approach to the control of these epidemics has been reactive vaccination with a polysaccharide vaccine after an outbreak has reached a defined threshold and provision of easy access to effective treatment but this approach has not prevented the occurrence of new epidemics. Meningococcal conjugate vaccines, which can prevent meningococcal carriage and thus interrupt transmission, may be more effective than polysaccharide vaccines at preventing epidemics. Because the majority of African epidemics have been caused by serogroup A meningococci, a serogroup A polysaccharide/tetanus toxoid protein conjugate vaccine (PsA–TT) has recently been developed. Results from an initial evaluation of the impact of this vaccine on meningococcal disease and meningococcal carriage in Burkina Faso have been encouraging.     

Diversity of factor H-binding protein in Neisseria meningitidis carriage isolates

Several meningococcal vaccines under development for prevention of serogroup B disease target the factor H-binding protein (FHbp), an immunogenic lipoprotein expressed on the surface of Neisseria meningitidis. Based upon sequence and phylogenetic analyses, FHbp can be classified into 3 protein variants (1, 2 or 3) or 2 subfamilies (A or B). The potential effect of FHbp-containing vaccines on meningococcal carriage is not known. We determined the diversity of FHbp among a population of carriage isolates obtained from Georgia and Maryland high school students in 1998 and 2006-2007. Analysis of the fHbp gene sequence from 408 carriage isolates identified 30 different FHbp protein sequences. The majority of carriage isolates harbored FHbp proteins belonging to variant 2/subfamily A. Association between FHbp proteins and genetic lineage was observed among the carriage isolates. However, split decomposition analysis, together with tests of linkage disequilibrium and pairwise homoplasy suggest recombination at fHbp contribute to allelic diversity. Of note, the FHbp proteins in serogroup B vaccines under development are either absent or not well represented in this carriage population. The FHbp genetic repertoire observed in carriage isolate populations will be useful in understanding the potential impact of FHbp-containing vaccines on meningococcal carriage.     

Effect of sequence variation in meningococcal PorA outer membrane protein on the effectiveness of a hexavalent PorA outer membrane vesicle vaccine

Though meningococcal serogroup C conjugate vaccines have been introduced into the UK infant immunisation schedule, there is currently no vaccine solution for serogroup B disease. PorA outer membrane protein (OMP) is a potential serogroup B vaccine candidate. A hexavalent PorA outer membrane vesicle (OMV) vaccine has been evaluated in phase I and II trials with promising results. This vaccine contains six different PorA OMPs each representing a different serosubtype. However, considerable sequence variation occurs in the variable regions (VRs) encoding these serosubtypes. By using recombinant P1.5,10 PorA variants we have demonstrated that the killing of this particular serosubtype combination was due mainly to the induction of antibody to the VR2 (P1.10) epitope region, and that after three or four doses of vaccine there was a significant reduction in the killing of variants P1.10a (three doses, p<0.0001; four doses, p = 0.003) and P1.10f (three doses, p<0.0001; four doses, p = 0.002), as compared to responses to the P1.10 strain, when the P1.10 serosubtype was used as the immunogen. Since large numbers of serosubtype variants are known to exist, this finding may have implications for the use of PorA as a meningococcal serogroup B vaccine.     

Avidity maturation following vaccination with a meningococcal recombinant hexavalent PorA OMV vaccine in UK infants

To date, there are no data assessing the utility of avidity indices as a surrogate marker for the induction of immunological memory following meningococcal serogroup B outer membrane vesicle (OMV) vaccination. We studied infants who had been immunized with three doses of a recombinant hexavalent PorA OMV vaccine at ages 2-4 months, together with a fourth dose at age 12-18 months. A control group had received a single dose of the same vaccine at age 12-18 months. As previously reported, serum bactericidal antibody (SBA) titres increased after each of the first three doses, with a significant increase observed from 6 months post third dose to 1 month post fourth dose. The geometric mean avidity indices (GMAI), against strain H44/76 OMVs, increased from 1 month post first dose to 1 month post third dose. Significant increases in GMAI were observed at 6 months post third dose and again following the fourth dose. At 32-42 months of age, though the SBA titres had returned to post first dose levels, the GMAI remained elevated. No increase in avidity was observed in the control group. Antibody avidity indices are useful laboratory markers for the priming of immunological memory following vaccination with meningococcal serogroup B OMV vaccines.     

TLR4-dependent adjuvant activity of Neisseria meningitidis lipid A

The adjuvant activity of Neisseria meningitidis serogroup B lipopoly(oligo)saccharide (LOS) from wild-type and genetically defined LOS mutants and unglycosylated meningococcal lipid A was assessed in C3H/HeN and C3H/HeJ mice. Meningococcal lipid A, a weak agonist for TLR4/MD-2 in human macrophages, was found to have adjuvant activity similar to that of wild-type and KDO(2)-lipid A LOS in C3H/HeN mice. All meningococcal LOS structures as adjuvants induced high titers of IgG1, IgG2a and IgG2b but very little IgG3 to OMP compared to no adjuvant PBS controls. In addition, induced OMP antibodies were shown to have high bactericidal activity against serogroup B meningococci. Purified LOS and lipid A structures failed to induce any adjuvant activity in C3H/HeJ mice indicating that meningococcal LOS as an adjuvant was TLR4-dependent. Unglycosylated meningococcal lipid A because of its weak agonist activity for human macrophages and retention of adjuvant activity may be a candidate for use in serogroup B meningococcal OMP and OMV vaccines and for use as an adjuvant in other vaccines.     

Serum bactericidal antibody assays – The role of complement in infection and immunity

Complement is an essential component of the immune system and human pathogenic organisms have developed various mechanisms for evading complement mediated serum killing. The “gold standard” for measuring the ability of vaccine-induced antibody to kill Neisseria meningitidis is the serum bactericidal antibody (SBA) assay which measures complement mediated killing via antibody. This assay requires active complement, either intrinsic from the serum being tested or the addition of exogenous complement, either from a human or from another species such as rabbit. For serogroup C, an SBA titre of ≥4 was established as the correlate of protection when using human complement and ≥8 as the threshold when using rabbit complement, based on comparative assay results. Licensure of meningococcal vaccines, including polysaccharide protein conjugate vaccines and serogroup B vaccines has been based on the immune responses measured with the SBA assay, thus on a surrogate of vaccine efficacy. This review examines the use of complement and the SBA assay to assess immunity to meningococcal infection, and provides examples of vaccine trials in different age groups where various assays have been used.     

Meningococcal surrogates of protection--serum bactericidal antibody activity

Despite the availability of anti-microbial agents effective against Neisseria meningitidis, meningococcal disease continues to be a major global health problem, particularly in the very young. Serogroup A meningococci cause large epidemics in sub-Saharan Africa, whilst serogroups B and C organisms are responsible for sporadic cases and localised outbreaks of disease world-wide. For measuring functional activity, the serum bactericidal antibody (SBA) assay is the most important method. It is mediated by antibody and complement resulting in lysis of the bacterial cells. To date the SBA has proved to be the best surrogate of protection for all serogroups. For serogroup C, an SBA titre of either >/ or =4 or > or =8 has being utilised for putatively indicating protection when using either human or baby rabbit complement, respectively. For serogroup B, the proportions of vaccines with > or =4-fold rises in SBA pre- to post-vaccination or SBA titres > or =4 have been correlated with clinical efficacy in trials of outer membrane vesicle (OMV) vaccines in Cuba, Brazil and Norway. SBA activity as a correlate of protection for evaluating the immune response to meningococcal vaccines is described in this review.     

Lipoprotein NMB0928 from Neisseria meningitidis serogroup B as a novel vaccine candidate

Polysaccharide-based vaccines for serogroup B Neisseria meningitidis have failed to induce protective immunity. As a result, efforts to develop vaccines for serogroup B meningococcal disease have mostly focused on outer membrane proteins (OMP). Vaccine candidates based on meningococcal OMP have emerged in the form of outer membrane vesicles (OMVs) or, more recently, purified recombinant proteins, as alternative strategies for serogroup B vaccine development. In our group, the protein composition of the Cuban OMVs-based vaccine VA-MENGOC-BC was elucidated using two-dimensional gel electrophoresis and mass spectrometry. The proteomic map of this product allowed the identification of new putative protective proteins not previously reported as components of an antimeningococcal vaccine. In the present study, we have determined the immunogenicity and protective capacity of NMB0928, one of those proteins present in the OMVs. The antigen was obtained as a recombinant protein in Escherichia coli, purified and used to immunize mice. The antiserum produced against the protein was capable to recognize the natural protein in different meningococcal strains by whole-cell ELISA and Western blotting. After immunization, recombinant NMB0928 induced bactericidal antibodies, and when the protein was administered inserted into liposomes, the elicited antibodies were protective in the infant rat model. These results suggest that NMB0928 is a novel antigen worth to be included in a broadly protective meningococcal vaccine.     

A phase III observer-blind randomized,controlled study to evaluate the immune response and the correlation with nasopharyngeal carriage after immunization of university students with a quadrivalent meningococcal ACWY glycoconjugate or serogroup B meningococcal vaccine

BackgroundUniversity students have high rates of pharyngeal carriage of Neisseria meningitidis. Interruption of carriage acquisition is an important mechanism of vaccines for inducing herd protection. 4CMenB and MenACWY-CRM vaccines have been shown to be immunogenic against meningococcal serogroups B and ACWY respectively in younger age groups, and also to elicit a modest impact on meningococcal carriage in vaccinated students. However, vaccine responses in university students and the impact of serum bactericidal antibody (SBA) titers on meningococcal carriage are undetermined.MethodsImmunogenicity of two 4CMenB doses or one MenACWY-CRM dose was measured in university students at Months 2, 4, 6 and 12 post-first vaccination. Immunogenicity of one MenACWY-CRM dose in students with previous meningococcal serogroup C conjugate vaccination was also assessed. Immune responses were measured with an SBA assay using human complement (hSBA) against three reference strains for serogroup B and against one strain for each for serogroups C and Y. Correlations between hSBA titers and meningococcal carriage were analyzed.ResultsAll subjects demonstrated robust functional antibody responses to both vaccines at Month 2 and a high proportion maintained protective hSBA titers up to Month 12. At baseline, carriage of disease-associated serogroup B strains and serogroups C and Y were higher in subjects with already-protective hSBA titers. Post-vaccination, while both 4CMenB and MenACWY-CRM elicited robust immunogenicity in students, significant correlations between post-vaccination hSBA titers and carriage of disease-associated serogroups were not observed.Conclusions4CMenB and MenACWY-CRM were both highly immunogenic. There was no correlation between carriage and post-vaccination hSBA titers.     

Cross-protective immunity conferred by a DNA adenine methylase deficient Salmonella enterica serovar Typhimurium vaccine in calves challenged with Salmonella serovar Newport

Intensive livestock production and management systems are associated with increased fecal-oral pathogen transmission and a resultant high prevalence of multiple Salmonella serovars in many large dairy farms and feedlots. Thus, it is imperative to develop livestock vaccines that are capable of eliciting potent states of cross-protective immunity against a diversity of serovars of a given species. Immunization with modified live Salmonella enterica serovar Typhimurium vaccine strains, that lack the DNA adenine methylase (Dam), confers cross-protective immunity in murine and avian models of typhoid fever as well as in a bovine model of salmonellosis. Here we examined whether a dam mutant Typhimurium vaccine (serogroup B) has the capacity to elicit cross-protection against a virulent challenge with an emerging, clinically relevant, and multi-drug resistant strain of serovar Newport (serogroup C2-C3) that has been associated with clinical disease in recent salmonellosis outbreaks in calves. Vaccinated animals challenged with Newport exhibited a significant attenuation of clinical disease (improved attitude scores, increased daily weight gains and reduced fever and diarrhea) and a concomitant reduction in Newport fecal shedding and colonization of mesenteric lymph nodes and lungs compared to non-vaccinated control animals. The capacity to elicit cross-protective immunity in calves suggests that dam mutant vaccines have potential application toward the prevention and control of Salmonella infection in commercial livestock production systems wherein livestock are exposed to a diversity of Salmonella serovars.     

Epidemiology of meningococcal disease

In the UK, serogroup A strains disappeared 50 years ago, but in the 1990s, numbers of cases rose again to a 50-year high. Following the very successful introduction of conjugated meningitis C vaccines, effective meningitis B vaccines are now the highest priority.