- Email: [email protected]
For discussion: live attenuated vaccines for group B meningococcus
PERGAMON
Vaccine 17 (1999) 114±117
For discussion: live attenuated vaccines for group B meningococcus Christoph Tang a, *, Richard Moxon a, Myron M. Levine b a
University Department of Paediatrics, John Radclie Hospital, Headley Way, Oxford, OX3 9DU, UK b Center for Vaccine Development, 685, West Baltimore Street, Baltimore, MD 21021, USA Received 22 January 1998; received in revised form 6 April 1998; accepted 28 April 1998
Abstract Current attempts at preventing infections caused by group B Neisseria meningitidis are largely directed on generating immune responses to outer membrane proteins or the lipopolysaccharide of this organism. We suggest an alternative approach: the use of a live, attenuated strain of Neisseria meningitidis which could be delivered mucosally to elicit both local and systemic immune responses. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Meningococcus; Vaccine; attenuated; Group B
Invasive infections caused by group B Neisseria meningitidis are an important cause of morbidity and mortality in many countries, both industrialized and developing. During the past decade, group B meningococcal infections have emerged in epidemic form in regions where they had not been previously observed as an important problem, such as South America (Brazil, Argentina, Chile), the Caribbean (Cuba, the Dominican Republic), the Middle East (Israel) and Iceland [1±4]. Based on the excellent results with the use of puri®ed group A and C capsular polysaccharide (PS) vaccines in preventing invasive group A and C infections [5] in both endemic and epidemic situations, and the encouraging immunogenicity of group A and C conjugate vaccines in infants [6], the major challenge for the control of meningococcal disease is the development of vaccines to prevent group B infections. Regrettably, progress toward a group B vaccine has been slow and problematic for several reasons relating to speci®c characteristics of this organism and its interaction with human hosts. The critical dierence between group B and the other meningococcal serogroups is that the group B capsular PS, a homopolymer of a2-8 linked sialic acid residues, is not immunogenic in humans. It has been suggested that * Corresponding author. Tel.: +1-65-221072; fax: +1-865-220479; e-mail: [email protected] 0264-410X/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 4 - 4 1 0 X ( 9 8 ) 0 0 1 6 3 - 7
the human immune system is tolerant to the group B capsular PS because it shares epitopes with a human surface adhesion molecule, N-CAM 1, which is present on neural and other tissues [7]. Thus, many researchers have been reluctant to undertake strategies that generate immune responses against the group B capsule, lest they prove harmful [8]. Instead, most investigators studying group B meningococcal vaccines have sought to identify the non-capsular epitopes against which bactericidal antibodies are directed for inclusion in a parenteral vaccine. Much work has focused on the outer membrane proteins (OMPs) and the lipopolysaccharides (LPS) of this organism [9±11]. However, the problem with this approach is that there is considerable antigenic heterogeneity among both OMPs and LPS, and indeed the diversity of these molecules forms the basis for the serotype, sub-serotype, and immunotype classi®cations of group B meningococci [12]. This makes such vaccines largely strain-speci®c and requires that multivalent vaccines be developed to provide broad spectrum protection that would have global applicability. Moreover, many OMPs are composed of both hydrophilic and hydrophobic domains, and it is dicult to ensure that OMPs retain their native conformation (which may be necessary for their ability to induce protective immune responses) both during and after puri®cation as either recombinant proteins or a component of outer membrane vesicle (OMV) vaccines.
C. Tang et al. / Vaccine 17 (1999) 114±117
Epidemiological data from both the USA and UK show that group B meningococcal disease is uncommon in the ®rst few months of life, but that agespeci®c attack rates rise to a peak between 6 months and 2 years of age. From then onwards, the incidence declines throughout childhood [13, 14]. Infants are thought to be protected from disease initially by transplacentally acquired maternal antibodies, but then become susceptible once these antibodies disappear from the circulation [13]. Subsequent protective immunity arises in most individuals without them suering a recognizable clinical infection, presumably developing from sub-clinical infection with Neisseria spp. or other bacteria expressing cross-reacting antigens [15, 16]. For instance, individuals exposed to non-groupable meningococci develop bactericidal antibodies against group A, B, C and Y organisms [13]. Most infections with meningococci are asymptomatic. Only in rare instances do individuals develop meningococcal disease. The situation is similar to polio in which most of those exposed to the virus remain well; for polio, the live attenuated vaccine has been extremely successful in reducing the incidence of infection with the real prospect of eradication in the next decade. It is not known whether individuals develop systemic meningococcal infection because they have inherent immune defects (and will never raise a protective response), or they are susceptible because they have not been exposed to antigens that elicit protective responses. Aside from those with defects in the complement cascade, the role of inherent immune function in determining the outcome of meningococcal infection has not been de®ned. However epidemiologic data (showing a reduction in the incidence of meningococcal infection from 2 years old and upwards) and the low incidence of second attacks of meningococcal disease (except in complement de®cient people) support the importance of acquired immunity in determining susceptibility to disease. An alternative approach to develop vaccines against group B meningococcal infections is based on the application of recombinant DNA techniques to attenuate group B strains and then use them as live, intranasal vaccines. This approach has the potential to overcome a number of the drawbacks of other approaches currently being pursued, and has the following theoretical advantages: 1. By identifying mutations that can reliably attenuate any group B strain irrespective of its OMP or LPS pro®le, it should be possible to prepare a multivalent vaccine consisting of several attenuated strains derived from wild-type isolates of epidemiological import. 2. In a live vaccine, the various major and minor surface antigens elaborated by each strain should
115
retain their native con®guration necessary to stimulate protective immune responses. 3. Antigens that are speci®cally expressed during growth in the host will be presented by immunization with live, attenuated strains but not by other methods such as OMVs. 4. A live, mucosally administered group B vaccine would have the potential to elicit mucosal immune responses (such as secretory IgA) that might interfere with colonization, in addition to systemic responses. 5. Responses to the individual components of parenteral combination vaccines (e.g. HIB and DTP) may be lower than when each antigen is given alone. Experience of Shigella [17±19], polio, and in¯uenza [20] vaccines suggests that responses to multivalent vaccines may be more reliable when given mucosally. The most notable examples of live, attenuated bacterial vaccines are the two safe, recently described Salmonella typhi strains which are both immunogenic following a single, oral dose [21, 22]; similar to N. meningitidis, wild-type S. typhi invades individuals through a mucosal surface, establishes a bacteraemic phase, and disseminates to deep organs. The feasibility of a live attenuated group B meningococcal vaccine is also supported by several precedents of other multivalent, mucosally administered, live vaccines. In the 1960s, several distinct serotypes of Shigella were responsible for causing endemic dysentery in Yugoslavia. Mel et al. derived streptomycin-dependent variant strains from the most common, epidemiologically important Shigella serotypes [17]. When administered as a multivalent oral vaccine, the live Shigella vaccine cocktail conferred signi®cant protection against dysentery caused by serotypes present in the vaccine [17±19]. Recently, a large ®eld trial of one or two doses of a trivalent cold-adapted attenuated in¯uenza A (H3N2 and H1N1) and B vaccine administered by the intranasal route to children 15 months to 6 years of age demonstrated remarkable protective ecacy (press release). The results of this trial demonstrate the practicality of immunizing young children by the intranasal route, and the ability to deliver multiple distinct antigens in combination. Earlier studies had shown that live, attenuated intranasal vaccines elicit prominent secretory IgA mucosal antibody responses, as well as serum antibodies [20]. There has also been encouraging data on mucosal immunization using OMV vaccines in human volunteers [23]. The major concern with using a live, attenuated strain as a group B vaccine is safety. In a proportion of those who manifest clinical disease, group B meningococcus can cause a fulminant bacteraemia with rapidly ensuing death. This mortality is believed to be
116
C. Tang et al. / Vaccine 17 (1999) 114±117
largely mediated by the induction of a cytokine cascade initiated by LPS on the bacterial surface [24], and may be seen predominantly in genetically predisposed individuals [25, 26]. Examination of the virulence of the vaccine strain in both rodent and primate models of infection must be undertaken prior to clinical testing to determine whether the vaccine strain inherently retains the potential for causing disease. It may be necessary that an eective vaccine strain retains sucient `vigour' in the host for the induction of protective immunity. This paradigm has been established in attenuated S. typhi vaccines. This can only be determined in clinical trials for meningococcal vaccines, but the starting point for must be candidate strains that are unable to sustain bacteraemia in mammalian hosts. A further consideration for live attenuated vaccines is that Neisseria spp. can take up exogenous DNA which can then integrate into the host chromosome. This mechanism is believed to be responsible for the `mosaic' structure found in some meningococcal genes [27], with separate fragments of individual genes derived from dierent organisms, and could provide a means by which an attenuated strain reverts to wildtype. Therefore, the possibility of reversion must be minimized by engineering into a vaccine strain not just one, but several independently attenuating mutations, as well as by introducing speci®c deletions in the genes encoding the DNA uptake or recombination machinery. It will be also helpful to include in the vaccine strain a selectable marker so that it can be readily distinguished from wild-type bacteria. An example of the use of such a marker is the mercury resistance gene introduced into the live, attenuated cholera vaccine strain, CVD 103-HgR, which has been granted licences in countries in Europe, Asia, and America [28]. It could be argued that non-pathogenic Neisseria spp. are better options as live vaccines for meningococcus. The problem is that the antigenic pro®le of commensal bacteria is poorly understood, and the crossreactive epitopes that induce protection against meningococcal infection have not been de®ned. It would therefore be preferable to modify genetically bacteria that have already been characterized in some detail, and are known to express antigens that can induce bactericidal responses. The outcome of infection of an individual with group B meningococcus depends on the interaction of several factors that include the pathogenic potential of the infecting strain [29], the infecting dose, the presence of other infections, and the speci®c and innate cellular or humoral immunity of the host. It should be borne in mind that group B meningococcus is part of the normal nasopharyngeal ¯ora and only rarely causes disease [30]. The majority of individuals develop protective immunity without experiencing overt clinical disease. Viewed in this way, the use of a live, attenu-
ated intranasal vaccine against group B meningococci seeks merely to mimic and extend the natural acquisition of immunity, by providing more comprehensive protection than aorded by sporadic exposure of individuals to meningococci in the community. We believe this approach represents a novel strategy for preventing group B meningococcal disease.
Acknowledgement We are grateful to Dr Wendell Zollinger for his helpful comments during the preparation of this manuscript.
References [1] Scholten RJ, Poolman JT, Valkenburg HA, Bijlmer HA, Dankert J, Caugant DA. Phenotypic and genotypic changes in a new clone complex of Neisseria meningitidis causing disease in The Netherlands, 1958±1990. Journal of Infectious Diseases 1994;169(3):673±6. [2] Cruz C, Pavez G, Aguilar E et al. Serotype-speci®c outbreak of group B meningococcal disease in Iquique, Chile. Epidemiology and Infection 1990;105(1):119±26. [3] Fischer M, Perkins BA. Neisseria meningitidis serogroup B: emergence of the ET-5 complex. Seminars in Pediatric Infectious Diseases 1997;8(1):50±6. [4] Sacchi CT, Pessoa LL, Ramos SR et al. Ongoing group B Neisseria meningitidis epidemic in Sao Paulo, Brazil, due to increased prevalence of a single clone of the ET-5 complex. Journal of Clinical Microbiology 1996;30(7):1734±8. [5] Anderson EL, Bowers T, Mink CM et al. Safety and immunogenicity of meningococcal A and C polysaccharide conjugate vaccine in adults. Infection and Immunity 1994;62(8):3391±5. [6] Lieberman JM, Chiu SS, Wong VK et al. Safety and immunogenicity of a serogroups A/C Neisseria meningitidis oligosaccharide±protein conjugate vaccine in young children. A randomized controlled trial. JAMA 1996;275(19):1499±503. [7] Finne J, Bitter-Suermann D, Goridis C, Finne U. An IgG monoclonal antibody to group B meningococci cross-reacts with developmentally regulated polysialic acid units of glycoproteins in neural and extraneural tissues. Journal of Immunology 1987;138(12):4402±7. [8] Finne J, Leinonen M, Makela PH. Antigenic similarities between brain components and bacteria causing meningitis. Implications for vaccine development and pathogenesis. Lancet 1983;ii:355±7. [9] Diaz Romero J, Outschoorn IM. Current status of meningococcal group B vaccine candidates: capsular or noncapsular?. Clinical Microbiology Review 1994;7(4):559±75. [10] Herbert MA, Heath PT, Mayon-White RT. Meningococcal vaccines for the United Kingdom. Communications in Disease Report CDR Review 1995;5(9):R130±R135. [11] Zollinger, W., New and Improved Vaccines Against Meningococcal Vaccines, 2nd edn. Marcel Dekker, New York, 1997. [12] Frasch CE, Zollinger WD, Poolman JT. Serotype antigens of Neisseria meningitidis and a proposed scheme for designation of serotypes. Reviews in Infectious Diseases 1985;7(4):504±10.
C. Tang et al. / Vaccine 17 (1999) 114±117 [13] Goldschneider I, Gotschlich EC, Artenstein MS. Human immunity to the meningococcus.I. The role of humoral antibodies. Journal of Experimental Medicine 1969;129(6):1307±26. [14] Jones DM, Kaczmarski EB. Meningococcal infections in England and Wales: 1994. Communications in Disease Report CDR Review 1995;5(9):R125±R130. [15] Gold R, Goldschneider I, Lepow ML, Draper TF, Randolph M. Carriage of Neisseria meningitidis and Neisseria lactamica in infants and children. Journal of Infectious Diseases 1978;137(2):112±21. [16] Reller LB, MacGregor RR, Beaty HN. Bactericidal antibody after colonization with Neisseria meningitidis. Journal of Infectious Diseases 1973;127(1):56±62. [17] Mel D, Gangarosa EJ, Radovanovic ML, Arsic BL, Litvinjenko S. Studies on vaccination against bacillary dysentery. 6. Protection of children by oral immunization with streptomycindependent Shigella strains. Bulletin of the World Health Organisation 1971;45(4):457±64. [18] Mel DM, Arsic BL, Nikolic BD, Radovanic ML. Studies on vaccination against bacillary dysentery. 4. Oral immunization with live monotypic and combined vaccines. Bulletin of the World Health Organisation 1968;39(3):375±80. [19] Mel DM, Arsic BL, Radovanovic ML, Litvinjenko SA. Live oral Shigella vaccine: vaccination schedule and the eect of booster dose. Acta Microbiologica Academy of Science Hungary 1974;21(1/2):109±14. [20] Belshe RB, Mendelman PM, Treanor J, King J, Gruber WC, Piedra P, Bernstein DI, Hayden FG, Kotlo K, Zangwill K, Lacuzio D, Wol M. The ecacy of live attenuated, coldadapted, trivalent, intranasal in¯uenza virus vaccine in children. N Engl J Med 1998;338(20):1405±12. [21] Tacket CO, Sztein MB, Losonsky GA et al. Safety of live oral Salmonella typhi vaccine strains with deletions in htrA and aroC
[22]
[23]
[24] [25]
[26] [27]
[28] [29] [30]
117
aroD and immune response in humans. Infection and Immunity 1997;65(2):452±6. Hohmann EL, Oletta CA, Killeen KP, Miller SI. phoP/phoQdeleted Salmonella typhi (Ty800) is a safe and immunogenic single-dose typhoid fever vaccine in volunteers. Journal of Infectious Diseases 1996;173(6):1408±14. Haneberg, B., Dalseg, R., Haugen, I.L., et al., Prospects for a nasal vaccine against group B meningococcal disease. Tenth International Pathogenic Neisseria Conference, Baltimore, USA. 1996, Abstr. 47, pp. 152±153. Bone RC. Gram-negative sepsis: a dilemma of modern medicine. Clinical Microbiology Review 1993;6(1):57±68. Nadel S, Newport MJ, Booy R, Levin M. Variation in the tumor necrosis factor-alpha gene promoter region may be associated with death from meningococcal disease. Journal of Infectious Diseases 1996;174(4):878±80. Westendorp RG, Langermans JA, Huizinga TW et al. Genetic in¯uence on cytokine production and fatal meningococcal disease. Lancet 1997;i:170±3. Feil E, Zhou J, Maynard Smith J, Spratt BG. A comparison of the nucleotide sequences of the adk and recA genes of pathogenic and commensal Neisseria species: evidence for extensive interspecies recombination within adk. Journal of Molecular Evolution 1996;43(6):631±40. Levine MM, Kaper JB, Herrington DA et al. Safety, immunogenicity and ecacy in man of recombinant live oral cholera vaccines. Lancet 1988;ii:467±70. Moxon ER, Rainey PB, Nowak MA, Lenski RE. Adaptive evolution of highly mutable loci in pathogenic bacteria. Current Biology 1994;4(1):24±33. Cartwright KA, Stuart JM, Jones DM, Noah ND. The Stonehouse survey: nasopharyngeal carriage of meningococci and Neisseria lactamica. Epidemiology and Infection 1987;99(3):591±601.
Vaccine 17 (1999) 114±117
For discussion: live attenuated vaccines for group B meningococcus Christoph Tang a, *, Richard Moxon a, Myron M. Levine b a
University Department of Paediatrics, John Radclie Hospital, Headley Way, Oxford, OX3 9DU, UK b Center for Vaccine Development, 685, West Baltimore Street, Baltimore, MD 21021, USA Received 22 January 1998; received in revised form 6 April 1998; accepted 28 April 1998
Abstract Current attempts at preventing infections caused by group B Neisseria meningitidis are largely directed on generating immune responses to outer membrane proteins or the lipopolysaccharide of this organism. We suggest an alternative approach: the use of a live, attenuated strain of Neisseria meningitidis which could be delivered mucosally to elicit both local and systemic immune responses. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Meningococcus; Vaccine; attenuated; Group B
Invasive infections caused by group B Neisseria meningitidis are an important cause of morbidity and mortality in many countries, both industrialized and developing. During the past decade, group B meningococcal infections have emerged in epidemic form in regions where they had not been previously observed as an important problem, such as South America (Brazil, Argentina, Chile), the Caribbean (Cuba, the Dominican Republic), the Middle East (Israel) and Iceland [1±4]. Based on the excellent results with the use of puri®ed group A and C capsular polysaccharide (PS) vaccines in preventing invasive group A and C infections [5] in both endemic and epidemic situations, and the encouraging immunogenicity of group A and C conjugate vaccines in infants [6], the major challenge for the control of meningococcal disease is the development of vaccines to prevent group B infections. Regrettably, progress toward a group B vaccine has been slow and problematic for several reasons relating to speci®c characteristics of this organism and its interaction with human hosts. The critical dierence between group B and the other meningococcal serogroups is that the group B capsular PS, a homopolymer of a2-8 linked sialic acid residues, is not immunogenic in humans. It has been suggested that * Corresponding author. Tel.: +1-65-221072; fax: +1-865-220479; e-mail: [email protected] 0264-410X/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 4 - 4 1 0 X ( 9 8 ) 0 0 1 6 3 - 7
the human immune system is tolerant to the group B capsular PS because it shares epitopes with a human surface adhesion molecule, N-CAM 1, which is present on neural and other tissues [7]. Thus, many researchers have been reluctant to undertake strategies that generate immune responses against the group B capsule, lest they prove harmful [8]. Instead, most investigators studying group B meningococcal vaccines have sought to identify the non-capsular epitopes against which bactericidal antibodies are directed for inclusion in a parenteral vaccine. Much work has focused on the outer membrane proteins (OMPs) and the lipopolysaccharides (LPS) of this organism [9±11]. However, the problem with this approach is that there is considerable antigenic heterogeneity among both OMPs and LPS, and indeed the diversity of these molecules forms the basis for the serotype, sub-serotype, and immunotype classi®cations of group B meningococci [12]. This makes such vaccines largely strain-speci®c and requires that multivalent vaccines be developed to provide broad spectrum protection that would have global applicability. Moreover, many OMPs are composed of both hydrophilic and hydrophobic domains, and it is dicult to ensure that OMPs retain their native conformation (which may be necessary for their ability to induce protective immune responses) both during and after puri®cation as either recombinant proteins or a component of outer membrane vesicle (OMV) vaccines.
C. Tang et al. / Vaccine 17 (1999) 114±117
Epidemiological data from both the USA and UK show that group B meningococcal disease is uncommon in the ®rst few months of life, but that agespeci®c attack rates rise to a peak between 6 months and 2 years of age. From then onwards, the incidence declines throughout childhood [13, 14]. Infants are thought to be protected from disease initially by transplacentally acquired maternal antibodies, but then become susceptible once these antibodies disappear from the circulation [13]. Subsequent protective immunity arises in most individuals without them suering a recognizable clinical infection, presumably developing from sub-clinical infection with Neisseria spp. or other bacteria expressing cross-reacting antigens [15, 16]. For instance, individuals exposed to non-groupable meningococci develop bactericidal antibodies against group A, B, C and Y organisms [13]. Most infections with meningococci are asymptomatic. Only in rare instances do individuals develop meningococcal disease. The situation is similar to polio in which most of those exposed to the virus remain well; for polio, the live attenuated vaccine has been extremely successful in reducing the incidence of infection with the real prospect of eradication in the next decade. It is not known whether individuals develop systemic meningococcal infection because they have inherent immune defects (and will never raise a protective response), or they are susceptible because they have not been exposed to antigens that elicit protective responses. Aside from those with defects in the complement cascade, the role of inherent immune function in determining the outcome of meningococcal infection has not been de®ned. However epidemiologic data (showing a reduction in the incidence of meningococcal infection from 2 years old and upwards) and the low incidence of second attacks of meningococcal disease (except in complement de®cient people) support the importance of acquired immunity in determining susceptibility to disease. An alternative approach to develop vaccines against group B meningococcal infections is based on the application of recombinant DNA techniques to attenuate group B strains and then use them as live, intranasal vaccines. This approach has the potential to overcome a number of the drawbacks of other approaches currently being pursued, and has the following theoretical advantages: 1. By identifying mutations that can reliably attenuate any group B strain irrespective of its OMP or LPS pro®le, it should be possible to prepare a multivalent vaccine consisting of several attenuated strains derived from wild-type isolates of epidemiological import. 2. In a live vaccine, the various major and minor surface antigens elaborated by each strain should
115
retain their native con®guration necessary to stimulate protective immune responses. 3. Antigens that are speci®cally expressed during growth in the host will be presented by immunization with live, attenuated strains but not by other methods such as OMVs. 4. A live, mucosally administered group B vaccine would have the potential to elicit mucosal immune responses (such as secretory IgA) that might interfere with colonization, in addition to systemic responses. 5. Responses to the individual components of parenteral combination vaccines (e.g. HIB and DTP) may be lower than when each antigen is given alone. Experience of Shigella [17±19], polio, and in¯uenza [20] vaccines suggests that responses to multivalent vaccines may be more reliable when given mucosally. The most notable examples of live, attenuated bacterial vaccines are the two safe, recently described Salmonella typhi strains which are both immunogenic following a single, oral dose [21, 22]; similar to N. meningitidis, wild-type S. typhi invades individuals through a mucosal surface, establishes a bacteraemic phase, and disseminates to deep organs. The feasibility of a live attenuated group B meningococcal vaccine is also supported by several precedents of other multivalent, mucosally administered, live vaccines. In the 1960s, several distinct serotypes of Shigella were responsible for causing endemic dysentery in Yugoslavia. Mel et al. derived streptomycin-dependent variant strains from the most common, epidemiologically important Shigella serotypes [17]. When administered as a multivalent oral vaccine, the live Shigella vaccine cocktail conferred signi®cant protection against dysentery caused by serotypes present in the vaccine [17±19]. Recently, a large ®eld trial of one or two doses of a trivalent cold-adapted attenuated in¯uenza A (H3N2 and H1N1) and B vaccine administered by the intranasal route to children 15 months to 6 years of age demonstrated remarkable protective ecacy (press release). The results of this trial demonstrate the practicality of immunizing young children by the intranasal route, and the ability to deliver multiple distinct antigens in combination. Earlier studies had shown that live, attenuated intranasal vaccines elicit prominent secretory IgA mucosal antibody responses, as well as serum antibodies [20]. There has also been encouraging data on mucosal immunization using OMV vaccines in human volunteers [23]. The major concern with using a live, attenuated strain as a group B vaccine is safety. In a proportion of those who manifest clinical disease, group B meningococcus can cause a fulminant bacteraemia with rapidly ensuing death. This mortality is believed to be
116
C. Tang et al. / Vaccine 17 (1999) 114±117
largely mediated by the induction of a cytokine cascade initiated by LPS on the bacterial surface [24], and may be seen predominantly in genetically predisposed individuals [25, 26]. Examination of the virulence of the vaccine strain in both rodent and primate models of infection must be undertaken prior to clinical testing to determine whether the vaccine strain inherently retains the potential for causing disease. It may be necessary that an eective vaccine strain retains sucient `vigour' in the host for the induction of protective immunity. This paradigm has been established in attenuated S. typhi vaccines. This can only be determined in clinical trials for meningococcal vaccines, but the starting point for must be candidate strains that are unable to sustain bacteraemia in mammalian hosts. A further consideration for live attenuated vaccines is that Neisseria spp. can take up exogenous DNA which can then integrate into the host chromosome. This mechanism is believed to be responsible for the `mosaic' structure found in some meningococcal genes [27], with separate fragments of individual genes derived from dierent organisms, and could provide a means by which an attenuated strain reverts to wildtype. Therefore, the possibility of reversion must be minimized by engineering into a vaccine strain not just one, but several independently attenuating mutations, as well as by introducing speci®c deletions in the genes encoding the DNA uptake or recombination machinery. It will be also helpful to include in the vaccine strain a selectable marker so that it can be readily distinguished from wild-type bacteria. An example of the use of such a marker is the mercury resistance gene introduced into the live, attenuated cholera vaccine strain, CVD 103-HgR, which has been granted licences in countries in Europe, Asia, and America [28]. It could be argued that non-pathogenic Neisseria spp. are better options as live vaccines for meningococcus. The problem is that the antigenic pro®le of commensal bacteria is poorly understood, and the crossreactive epitopes that induce protection against meningococcal infection have not been de®ned. It would therefore be preferable to modify genetically bacteria that have already been characterized in some detail, and are known to express antigens that can induce bactericidal responses. The outcome of infection of an individual with group B meningococcus depends on the interaction of several factors that include the pathogenic potential of the infecting strain [29], the infecting dose, the presence of other infections, and the speci®c and innate cellular or humoral immunity of the host. It should be borne in mind that group B meningococcus is part of the normal nasopharyngeal ¯ora and only rarely causes disease [30]. The majority of individuals develop protective immunity without experiencing overt clinical disease. Viewed in this way, the use of a live, attenu-
ated intranasal vaccine against group B meningococci seeks merely to mimic and extend the natural acquisition of immunity, by providing more comprehensive protection than aorded by sporadic exposure of individuals to meningococci in the community. We believe this approach represents a novel strategy for preventing group B meningococcal disease.
Acknowledgement We are grateful to Dr Wendell Zollinger for his helpful comments during the preparation of this manuscript.
References [1] Scholten RJ, Poolman JT, Valkenburg HA, Bijlmer HA, Dankert J, Caugant DA. Phenotypic and genotypic changes in a new clone complex of Neisseria meningitidis causing disease in The Netherlands, 1958±1990. Journal of Infectious Diseases 1994;169(3):673±6. [2] Cruz C, Pavez G, Aguilar E et al. Serotype-speci®c outbreak of group B meningococcal disease in Iquique, Chile. Epidemiology and Infection 1990;105(1):119±26. [3] Fischer M, Perkins BA. Neisseria meningitidis serogroup B: emergence of the ET-5 complex. Seminars in Pediatric Infectious Diseases 1997;8(1):50±6. [4] Sacchi CT, Pessoa LL, Ramos SR et al. Ongoing group B Neisseria meningitidis epidemic in Sao Paulo, Brazil, due to increased prevalence of a single clone of the ET-5 complex. Journal of Clinical Microbiology 1996;30(7):1734±8. [5] Anderson EL, Bowers T, Mink CM et al. Safety and immunogenicity of meningococcal A and C polysaccharide conjugate vaccine in adults. Infection and Immunity 1994;62(8):3391±5. [6] Lieberman JM, Chiu SS, Wong VK et al. Safety and immunogenicity of a serogroups A/C Neisseria meningitidis oligosaccharide±protein conjugate vaccine in young children. A randomized controlled trial. JAMA 1996;275(19):1499±503. [7] Finne J, Bitter-Suermann D, Goridis C, Finne U. An IgG monoclonal antibody to group B meningococci cross-reacts with developmentally regulated polysialic acid units of glycoproteins in neural and extraneural tissues. Journal of Immunology 1987;138(12):4402±7. [8] Finne J, Leinonen M, Makela PH. Antigenic similarities between brain components and bacteria causing meningitis. Implications for vaccine development and pathogenesis. Lancet 1983;ii:355±7. [9] Diaz Romero J, Outschoorn IM. Current status of meningococcal group B vaccine candidates: capsular or noncapsular?. Clinical Microbiology Review 1994;7(4):559±75. [10] Herbert MA, Heath PT, Mayon-White RT. Meningococcal vaccines for the United Kingdom. Communications in Disease Report CDR Review 1995;5(9):R130±R135. [11] Zollinger, W., New and Improved Vaccines Against Meningococcal Vaccines, 2nd edn. Marcel Dekker, New York, 1997. [12] Frasch CE, Zollinger WD, Poolman JT. Serotype antigens of Neisseria meningitidis and a proposed scheme for designation of serotypes. Reviews in Infectious Diseases 1985;7(4):504±10.
C. Tang et al. / Vaccine 17 (1999) 114±117 [13] Goldschneider I, Gotschlich EC, Artenstein MS. Human immunity to the meningococcus.I. The role of humoral antibodies. Journal of Experimental Medicine 1969;129(6):1307±26. [14] Jones DM, Kaczmarski EB. Meningococcal infections in England and Wales: 1994. Communications in Disease Report CDR Review 1995;5(9):R125±R130. [15] Gold R, Goldschneider I, Lepow ML, Draper TF, Randolph M. Carriage of Neisseria meningitidis and Neisseria lactamica in infants and children. Journal of Infectious Diseases 1978;137(2):112±21. [16] Reller LB, MacGregor RR, Beaty HN. Bactericidal antibody after colonization with Neisseria meningitidis. Journal of Infectious Diseases 1973;127(1):56±62. [17] Mel D, Gangarosa EJ, Radovanovic ML, Arsic BL, Litvinjenko S. Studies on vaccination against bacillary dysentery. 6. Protection of children by oral immunization with streptomycindependent Shigella strains. Bulletin of the World Health Organisation 1971;45(4):457±64. [18] Mel DM, Arsic BL, Nikolic BD, Radovanic ML. Studies on vaccination against bacillary dysentery. 4. Oral immunization with live monotypic and combined vaccines. Bulletin of the World Health Organisation 1968;39(3):375±80. [19] Mel DM, Arsic BL, Radovanovic ML, Litvinjenko SA. Live oral Shigella vaccine: vaccination schedule and the eect of booster dose. Acta Microbiologica Academy of Science Hungary 1974;21(1/2):109±14. [20] Belshe RB, Mendelman PM, Treanor J, King J, Gruber WC, Piedra P, Bernstein DI, Hayden FG, Kotlo K, Zangwill K, Lacuzio D, Wol M. The ecacy of live attenuated, coldadapted, trivalent, intranasal in¯uenza virus vaccine in children. N Engl J Med 1998;338(20):1405±12. [21] Tacket CO, Sztein MB, Losonsky GA et al. Safety of live oral Salmonella typhi vaccine strains with deletions in htrA and aroC
[22]
[23]
[24] [25]
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