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Meningococcal Vaccines and Developments 1
1
Meningococcal Vaccines and Vaccine
Developments
Ian M. Feavers
1. Introduction
Despite rapid advances in the diagnosis of bacterial infections and the avail-
ability of effective antibiotics, meningococcal disease continues to represent a
substantial public health problem for most countries (1–4). Disease usually
develops rapidly, is notoriously difficult to distinguish from other febrile ill-
nesses, and generally has a high case-fatality rate. The death of an otherwise fit
and healthy individual can occur within a very short time from the first appear-
ance of symptoms, those who survive frequently suffer from permanent tissue
damage and neurological problems (4,5). Consequently, the development and
implementation of effective immunoprophylaxis is a sine qua nonfor the com-
prehensive control of meningococcal disease. From an historical perspective,
many meningococcal vaccines have been developed and evaluated in clinical
trials; unfortunately, no vaccine so far offers comprehensive protection. This
overview traces the development of the existing licensed vaccines and exam-
ines the prospects of vaccine candidates that are currently under development
or subject to clinical evaluation.
The challenges faced by the vaccine developer in designing meningococcal
vaccines that are safe, comprehensive, and efficacious in the age groups most
at risk of disease are a consequence of the complex biology of Neisseria
meningitidis. It is a Gram-negative, encapsulated organism that is naturally
competent for transformation with DNA. It only thrives in the human host and
is not known to colonize any other animal or environmental niches. Meningo-
coccal carriage is very much more common than disease (6)and, notwithstand-
ing the devastating impact of meningococcal disease, it may be more
From:Methods in Molecular Medicine, vol. 66: Meningococcal Vaccines: Methods and Protocols
Edited by: A. J. Pollard and M. C. J. Maiden © Humana Press Inc., Totowa, NJ
1
2 Feavers
appropriate to consider this bacterium as a commensal that rarely causes dis-
ease rather than as a strict pathogen. The meningococcus is, therefore, specifi-
cally adapted to the colonization of humans and has evolved a battery of
mechanisms that enable it to evade the human immune response.
Meningococcal meningitis and septicaemia are ostensibly childhood dis-
eases, with highest attack rates in infants (7). Carbohydrate antigens, such as
capsular polysaccharide or lipopolysaccharide (LPS), are poorly immunogenic
in the very young and frequently mimic host cell structures (8–10) posing
a dilemma for the vaccine developer: can immunity to a carbohydrate be
enhanced in infants and, if so, would such a vaccine elicit an autoimmune
response? Protein vaccine candidates present a different problem; they are gen-
erally better immunogens than carbohydrates, but the more immunogenic men-
ingococcal surface-protein antigens suffer from the disadvantage that they are
also antigenically highly variable (11,12). In this case, the vaccine developer is
faced with producing a vaccine that offers adequate cross-protection against
the majority of virulent meningococci circulating in the population.
Besides hiding behind a camouflage of poorly immunogenic and highly vari-
able cell-surface structures, meningococci utilize a variety of genetical mech-
anisms to facilitate their persistent colonization of humans. These simultaneously
provide them with the potential to circumvent anything less than comprehen-
sive immune protection. The mosaic structure of the genes and operons
that encode major cell surface structures provides evidence of the importance
of horizontal genetical exchange, mediated by transformation and recombina-
tion, in the generation of meningococcal antigenic diversity (13,14; see also
Chapter 24). It has profound implications for both the development and evalu-
ation of vaccine candidates, as well as for the implementation of vaccination
programs (15), as it provides a mechanism for the reassortment of antigen-
encoding genes among meningococcal clones and increases the prospect of
meningococci evading host immunity (16,17). In addition, the expression of
many antigen genes is tightly regulated so that critical antigens are not con-
tinuously expressed in vivo (18–22).
Like many other medically important bacteria, the meningococcus has his-
torically been characterized serologically on the basis of its surface antigens
(23–26). It can synthesize one of a number of polysaccharide capsules that
define the serogroup; pathogenic isolates invariably belong to one of five
serogroups, A, B, C, W135, or Y. Serogroups are further subdivided into sero-
types and serosubtypes on the basis the serological reactivity of major outer
membrane proteins (OMPs) and into immunotypes on the basis of differences
in LPS structure. Perhaps not surprisingly, the capsular antigens have been
critical in the development of the licensed vaccines. Arguably, if it had been
Meningococcal Vaccines and Developments 3
possible to produce a pentavalent vaccine based on the capsular polysaccha-
ride of the pathogenic serogroups that was safe and effective in infants, com-
prehensive control of meningococcal disease through routine immunization
would already be possible. However, the use of serogroup B capsule presents
particular problems, and as a result many of the other surface antigens are under
consideration as potential components of future vaccines (for review, seeref.27).
1.1. Historical Perspective
Historically, attempts to prevent meningococcal disease by immuno-
prophylaxis seem to have been inspired by successes in the prevention of other
important diseases through vaccination. Following the use of killed whole-
cell vaccines for the prevention of typhoid at the turn of the last century (28),
numerous studies explored the potential of immunization with heat-killed men-
ingococcal cells to prevent disease (29). Many of the clinical trials that were
conducted with whole cell formulations were poorly controlled and the effi-
cacy of these preparations was at best questionable. This, together with the
unacceptable reactogenicity caused by their high endotoxin content, ultimately
resulted in the abandonment of the killed whole-cell vaccine approach.
In the 1930s, the successful prevention of diphtheria and tetanus by immu-
nization with toxoids prompted the search for a meningococcal toxin in cell-
free culture supernatants. Kuhns et al. evaluated the vaccine potential of culture
filtrates in studies that provided limited evidence for the efficacy of this approach
(30,31). Because the culture supernatants would have been contaminated with
capsular polysaccharide, endotoxin, and OMPs, it is impossible to attribute the
protection observed to a particular antigen. These preliminary observations do
not appear to have been pursued further. In common with research on vaccines
against other infectious diseases at that time, perhaps the optimism surround-
ing the introduction of antibiotics suppressed interest in meningococcal vac-
cine development.
During the early 1940s, the association of meningococcal disease with the
increase in the recruitment of Allied Forces rekindled interest in vaccination to
control disease outbreaks. Once again it was a vaccine against another patho-
gen that was to provide the inspiration for subsequent developments. Promis-
ing results with a multivalent pneumococcal polysaccharide vaccine indicated
that capsular polysaccharides may be able to elicit protective immune responses
(32). The clinical evaluations of early preparations of meningococcal serogroup
A and C polysaccharides were far from encouraging, probably because the
capsular material was degraded to low molecular-weight oligosaccharides by
the purification methods employed at the time. However, during the 1960s the
development of an innovative purification procedure permitted the production
4 Feavers
of highly purified, high molecular-weight meningococcal capsular polysaccha-
rides (33). Polysaccharides produced in this way have proved to be safe and
immunogenic in adults and older children (34–36). They form the basis of the
currently licensed meningococcal polysaccharide vaccine formulations.
Unfortunately, polysaccharides are usually T-cell independent antigens.
Consequently, they are poorly immunogenic in the very young, they fail to
stimulate a good anamnestic response, and they often elicit low-avidity anti-
body responses. Meningococcal capsular polysaccharides are no exception
(37); the currently licensed polysaccharide vaccines are not indicated for chil-
dren under 2 yr of age and the vaccines are not used in long-term immunization
programs. Recently, the successful introduction of the Hibvaccine into a num-
ber of national immunization programs (38) has been followed by the rapid
development of meningococcal glycoconjugate vaccines (39–41). These con-
sist of partially hydrolyzed, size-fractionated oligosaccharides chemically con-
jugated to either tetanus or diphtheria toxoids as carrier proteins. In clinical
studies they have proved to be safe, immunogenic, and to give a good anam-
nestic response regardless of the age of the vaccinee (42–49). The first such
vaccine was licensed in the UK at the end of 1999 and has since been licensed
for use in a number of other European countries.
Assuming that such glycoconjugate vaccines prove to be effective in infant
immunization schedules, the development of safe and effective vaccines that
offer protection against serogroup B disease remains a major challenge. Today
serogroup B organisms are responsible for most meningococcal disease in
developed countries (7). However, attempts to develop vaccines based upon
serogroup B polysaccharide have proved unsuccessful (9). Purified B polysac-
charide, a polymer of (cid:95) 2-8 linked sialic acid, has failed to elicit a significant
increase in antibody responses in clinical trials. The lack of response in man
may be explained by immunological tolerance to similar sialic-acid structures
on human cells and raises the question of whether a serogroup B polysaccha-
ride vaccine that overcame tolerance would be acceptable in terms of its safety.
2. Vaccines
2.1. Polysaccharide Vaccines
The currently licensed polysaccharide vaccines include two formulations—
a bivalent A and C vaccine and a tetravalent formulation containing A, C,
W135, and Y polysaccharides—that are produced by a number of European
and North American companies. The high molecular size polysaccharides used
in these vaccines are produced by essentially the same method as first described
by Gotschlich et al. (33). All four polysaccharide components have been shown
to be immunogenic in adults and older children (34,50,51), although it has only
been possible to demonstrate protective efficacy against infection with
Meningococcal Vaccines and Developments 5
serogroup A and C organisms because of the low incidence of W135 and Y
disease. In early protective efficacy trials in US military recruits, monovalent
serogroup C vaccines were demonstrated to have an efficacy in the region of
90% (35). Similar levels of protection were observed when serogroup A vac-
cines were studied in Africa and Finland (36).
Serum bactericidal antibodies play a crucial role in the protection of the host
against meningococcal disease. The evidence for this includes an association
between the lack of serogroup specific bactericidal antibodies and occurrence
of disease among military recruits (52) and the susceptibility of individuals,
who congenitally lack complement components in the membrane-attack com-
plex, to repeated meningococcal infections (53). Although there has been con-
siderable debate over the way in which the assay should be performed, the
serum bactericidal-antibody titer provides an important immunological surro-
gate for protection, without which the subsequent development of
glycoconjugate vaccines would have been severely hampered.
The size and duration of the immune response is age-dependent, reflecting
the fact that meningococcal polysaccharides, like other carbohydrate antigens,
are T-independent antigens, and suggests that B-cell maturation is critical for
an effective immune response (37,54,55). The serogroup C response was not
effective in children under 2 yr of age and the licensed vaccines are con-
sequently not indicated for use below this age. Serogroup A polysaccharide
appears to be more immunogenic than C polysaccharide in young children but
neither is capable of inducing long-term immunological memory. The polysac-
charide vaccines are therefore generally not used in routine immunization pro-
grams due to the lack of protection that they offer in infancy and the relatively
short-lived immune response that they elicit. Nevertheless, they are frequently
offered to individuals who are at particular risk of infection including: military
recruits, undergraduate students, patients with immunodeficiencies, and trav-
elers to the so-called “meningitis belt” countries and the Haj pilgrimage
(27,56). They are also used together with chemotherapy to control localized
outbreaks of serogroup C disease in schools and colleges in industrialized coun-
tries (57). In the meningitis belt, polysaccharide vaccine has proved effective
at controlling the spread of serogroup A epidemics (58,59) and recently the
World Health Organization (WHO) has established a stock of vaccine that can
be dispatched to sub-Saharan Africa at short notice whenever a sudden increase
in disease rate indicates the potential onset of an epidemic.
2.2. Glycoconjugate Vaccines
The success of the Hib glycoconjugate vaccine has highlighted the advan-
tages of converting polysaccharides into T-dependent antigens by chemical
conjugation to protein-carrier molecules (38,60,61) and has led to the clinical
6 Feavers
development of similar vaccines based on the meningococcal serogroup A and
C capsular polysaccharides (41,62). Size-fractionated oligosaccharides derived
from purified capsular polysaccharides conjugated to either the nontoxic, cross-
reacting mutant of diphtheria toxin, CRM197, or tetanus toxoid have been
evaluated for their safety and immunogenicity in clinical trials. The depoly-
merization, activation, and conjugation of meningococcal serogroup C polysac-
charide to tetanus toxoid is detailed in Chapter 4.
Miller and Farrington, in Chapter 6 of this volume, review the rationale
behind the conduct of clinical trials and the particular problems encountered
in the evaluation of meningococcal vaccines. Generally, meningococcal-
conjugate vaccines have been well-tolerated; both local and systemic reactions
have been relatively mild and similar to those expected for unconjugated
polysaccharide vaccines. They have proved to be highly immunogenic over a
wide age range, including very young infants (42–45,47–49). Studies in which
infants have received three doses of vaccine at 2, 3, and 4 mo have shown that
serogroup C- CRM197 conjugates induce high levels of high-avidity, anti-C
polysaccharide antibodies that are bactericidal. Richmond et al. also demon-
strated that the immune response of infants primed with the conjugate vaccine
was boosted by the administration of serogroup C polysaccharide, confirming
that the vaccine induces immunological memory (49). These data indicate the
successful induction of a T-cell dependent antibody response by serogroup
C-CRM197 conjugate vaccines. Other clinical studies have shown that
serogroup C conjugates in which tetanus toxoid has been used as the carrier
protein or the C polysaccharide is O-deacetylated to be similarly immunogenic
and well-tolerated (46).
Three serogroup C conjugate vaccines have been licensed in the UK to date.
Given the low incidence of disease caused by serogroup C organisms, it was
impractical to conduct controlled protective efficacy studies and the license
was granted on the basis that: 1) the conjugate was more immunogenic than the
existing licensed polysaccharide vaccine, particularly in the very young; 2) it
induced a good anamnestic response; and 3) the success of glycoconjugate
vaccine technology in reducing disease had been established with the Hibvac-
cine. Careful monitoring of serogroup C disease throughout the phased intro-
duction of the vaccine into national immunization schedules should provide
some assessment of the effectiveness of these vaccines.* Provided that there is
sufficient vaccine coverage, the introduction of serogroup C conjugate vaccine
*Recent estimates based on surveillance during the first 9 mo following the introduction of
the serogroup C conjugate in England indicate that the short-term efficacy of the vaccine was
97% (95% CI 77–99) for teenagers and 92% (65–98) for toddlers (Ramsay, Andrews, Kaczmarski
and Miller, 2001, Lancet357, 195, 196).
Meningococcal Vaccines and Developments 7
can reasonably be expected to parallel the previous success of the Hibvaccine,
eventually leading to the eradication of serogroup C disease. Although draw-
ing such parallels has been expeditious in the development of the new vaccines
this optimism is, however, tempered by the knowledge that certain aspects of
meningococcal disease and invasive Haemophilus influenzae type b disease
are quite different (15).
Type b organisms account for almost all septicaemic isolates of H. influ-
enzae, whereas several different meningococcal serogroups cause invasive
infections. In addition, there is little evidence that virulent isolates of non-
type b H. influenzaearise through the genetical exchange of capsular polysaccha-
ride loci (63), whereas there is extensive evidence that virulent meningococci
frequently exchange antigen genes, including those encoding their capsular
polysaccharides (17,64,65). The licensed serogroup C conjugate vaccines
offer no cross-protective immunity to the non-serogroup C meningococci that
are responsible for most of the meningococcal disease in industrialized coun-
tries, and that may arise as consequence of capsular switching. With the wide-
spread use of monovalent serogroup C conjugate vaccines, the associated
increase in the level of serogroup C specific salivary antibody together with
the induction of immunological memory in the vaccinated population is likely
to serve to reduce nasopharyngeal carriage, thereby increasing herd immunity
(66). This would represent a important shift in the immunological selection
acting on meningococci circulating in the vaccinated population and could
ultimately result in an increase in disease caused by the other pathogenic
serogroups. Further development of meningococcal glycoconjugate compo-
nents will inevitably lead to the availability of more comprehensive formula-
tions comprising combinations of serogroup A, C, W135, and Y conjugates,
but the development of an effective vaccine offering protection against disease
caused by serogroup B organisms clearly remains the decisive obstacle in the
elimination of meningococcal disease.
The poor immunogenicity of vaccine candidates consisting of native
serogroup B polysaccharide conjugated to carrier proteins has been attributed
to immunological tolerance associated with the presence of sialylated glyco-
peptides in human and animal tissues (10). During embryonic and neonatal
development, the neural cell adhesion molecule (N-CAM), which is widely
distributed in human tissue, has long polysialic acid chains that are recognized
by anti-serogroup B antibodies (67). A number of studies have shown that the
sialylation of N-CAM modulates cell-cell interactions during organogenesis
and has led to concern that pregnancy or fetal development may be adversely
affected by high levels of high avidity cross-reacting antibodies produced in
response to a serogroup B conjugate vaccine. Jennings et al. postulated that
chemical modification of the polysaccharide might overcome immunological
8 Feavers
tolerance and induce a safe and protective immune response (68). A modified
B polysaccharide, in which the N-acetyl groups at position C-5 of the sialic
acid residues are replaced with N-propionyl groups, conjugated to tetanus tox-
oid proved to be immunogenic in mice. More recently, N-propionylated
serogroup B polysaccharide conjugated to a recombinant meningococcal outer-
membrane protein (rPorB) has been shown to be highly immunogenic in non
human primates (69). Importantly, no adverse reactions to the trial vaccine
were observed in these studies, providing grounds for optimism, although the
absence of an autoimmune response and the overall safety of such a vaccine
remain to be substantiated by clinical trials, and it will inevitably take many
years to establish its long-term safety. The preparation and characteristics of
N-propionylated serogroup B polysaccharide conjugated to tetanus toxoid are
described in Chapter 5.
2.3. Protein Vaccines
Concern over the safety of vaccines based on the serogroup B capsular
polysaccharide has focused attention on alternative cell-surface antigens as
vaccine candidates (Table 1). The most advanced of these, in terms of their
clinical development, consist of meningococcal outer-membrane vesicles
(OMVs)(70–72)or purified outer-membrane proteins (OMPs) (73). Grown in
broth culture, N. meningitidisproduces substantial quantity of outer-membrane
blebs, containing the same complement of OMPs as the organism itself (74).
These vesicles can be readily purified from detergent treated meningococcal
cultures to form the basis of vaccine formulations (Chapters 6 and 7). Unfortu-
nately, such vaccines suffer from significant drawbacks: 1) the most immuno-
genic antigens they contain are also the most variable, suggesting that OMV
vaccines may not offer comprehensive protection against all meningococci; 2) their
protective efficacy in young infants, the group most at risk of meningococcal dis-
ease, has not been demonstrated; and 3) protection appears to be short-lived. It has
been suggested that mucosally administered OMV formulations may overcome
some of these shortcomings and to explore this possibility immunogenicity studies
have been performed in human volunteers (see Chapter 16) (75).
Efficacy trials have been conducted with both OMV and purified OMP for-
mulations. In response to an outbreak of disease in Cuba in the late 1980s, the
Finlay Institute produced an OMV vaccine, based on this B(cid:1)4(cid:1)P1.19,15 (ET-5
complex) isolate, that also contained serogroup C capsular polysaccharide.
Case controlled studies using the Cuban vaccine in Brazil revealed that protec-
tive efficacy was age-dependent; an efficacy of greater than 70% was recorded
for children older than four years, while in younger children no efficacy was
demonstrated(76). Similarly, an increase in meningococcal disease in Norway
caused by a B(cid:1)15(cid:1)P1.7,16 isolate belonging to the ET5 complex prompted the
Meningococcal Vaccines and Developments 9
Table 1
Summary of Protein-Vaccine Candidates That Might Offer Protection
Against Serogroup B Disease
Vaccine candidate Stage of development Reference
Outer membrane vesicle:
Finlay Institute Licensed in some Central (70)
and Southern American countries
NIPH Completed efficacy (phase III)
studies in teenagers (71)
RIVM Immunogenicity (phase II) (72,85)
studies in various age groups
Purified outer membrane Efficacy studies (73)
proteins
Transferrin binding protein B Preliminary clinical studies (87)
(TbpB) in adult volunteers
Neisseria surface protein Preclinical research (113)
(NspA)
Transferrin binding protein A Preclinical research (114)
(TbpA)
FrpB Preclinical research (115)
Recombinant PorA Preclinical research (116)
Peptides from PorA Preclinical research (117)
TspA Preclinical research (118)
development of an OMV vaccine, the protective efficacy of which proved to
be 57% in a double-blind, placebo-controlled trial conducted in secondary-
school pupils (71). A serotype-specific outbreak of serogroup B meningococ-
cal disease in Iquique, Chile during the 1980s lead to the evaluation of a vaccine
consisting of purified meningococcal OMPs noncovalently complexed to
serogroup C polysaccharide in a randomized, controlled trial. The vaccine effi-
cacy was 70% in the volunteers aged from 5–21 yr, but was not protective in
children aged between 1 and 4 yr (73). In all three studies, which used two dose
schedules, there was evidence of better protection early after immunization,
indicating that protection is short-lived and leading to suggestions that a third
dose of vaccine may improve protective efficacy (27). Each of these vaccines
was based on a specific meningococcal isolate. Given the antigenic diversity
ofN. meningitidisisolates, this raises concerns that they cannot be relied upon
to offer cross-protection against all virulent meningococci; fears that have been
substantiated by immunogenicity studies showing that the ability of OMV vac-
cines to elicit cross-protective bactericidal antibodies is limited (77).
