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Clinical and Developmental Immunology
Volume 2008 (2008), Article ID 628963, 10 pages
Review Article

Neonatal and Infantile Immune Responses to Encapsulated Bacteria and Conjugate Vaccines

Department of Pediatric Infectious Diseases and Immunology, Wilhelmina Children's Hospital, University Medical Centre, Room KE4.133.1, P.O. Box 85090, 3508 AB Utrecht, The Netherlands

Received 22 March 2008; Revised 25 June 2008; Accepted 1 August 2008

Academic Editor: Michel Goldman

Copyright © 2008 Peter Klein Klouwenberg and Louis Bont. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Encapsulated bacteria are responsible for the majority of mortality among neonates and infants. The major components on the surface of these bacteria are polysaccharides which are important virulence factors. Immunity against these components protects against disease. However, most of the polysaccharides are thymus-independent (TI)-2 antigens which induce an inadequate immune response in neonates and infants. The mechanisms that are thought to play a role in the unresponsiveness of this age group to TI-2 stimuli will be discussed. The lack of immune response may be overcome by conjugating the polysaccharides to a carrier protein. This transforms bacterial polysaccharides from a TI-2 antigen into a thymus-dependent (TD) antigen, thereby inducing an immune response and immunological memory in neonates and infants. Such conjugated vaccines have been shown to be effective against the most common causes of invasive disease caused by encapsulated bacteria in neonates and children. These and several other approaches in current vaccine development will be discussed.

1. Introduction

Globally, more than 2.5 million infants die every year from bacteremia, respiratory, and diarrhoeal diseases [1]. A limited number of viral and bacterial pathogens are responsible for this burden of disease among neonates and infants. A study by the WHO has identified that the most pathogenic bacteria are encapsulated bacteria, such as Streptococcus pneumoniae, Streptococcus pyogenes, Staphylococcus aureus, Escherichia coli, Neisseriameningitidis, and Haemophilusinfluenza [2]. The capsule around these pathogens is formed by a polysaccharide coating. The immunologic advantage of this coating is evasion of phagocyte killing, as the coating blocks complement binding and opsonization. This can be overcome by C-reactive protein (CRP) binding [3] and the production of antibodies against the polysaccharide [4]. This immune response confers protection against disease. However, especially the young [5], but also the elderly [6], have a weak immunological response to these encapsulated bacteria due to the thymus independent (TI) nature of these bacteria.

The humoral immune response to antigens can be divided into thymus-dependent (TD) and TI-responses [7] (see Table 1). TD antigens consist of soluble proteins or peptides and associate with major histocompatibility complex (MHC) molecules at the surface of an antigen presenting cell (APC), thus allowing the APC to interact with CD4+ T cells. In contrast to TD antigens, TI antigens do not require T-cells to induce an immune response [8]. Therefore, TI antigens do not or poorly induce an immunological memory. The antibodies that are produced are primarily of the IgM isotype and in lesser quantities IgG2 [7]. The TI antigens are further divided into two categories based on their interaction with B cells: type 1 (TI-1) and type 2 (TI-2) antigens [79]. TI-1 antigens induce proliferation and differentiation of B lymphocytes and induce immune responses in adults, but also in neonates [5, 10]. TI-2 antigens on the other hand induce a limited immune response in children below two years of age, but older children and adults react to TI-2 antigens with the formation of sufficient antibody production by activated B-cells. TI-1 antigens include lipopolysaccharides, which is part of the Gram-negative bacteria cell wall [6]. Therefore, immune responses to Gram-negative bacteria are relatively sufficient in neonates and infants compared to encapsulated bacteria, but still below levels of those of adults [10, 11]. TI-2 antigens are bacterial polysaccharides from encapsulated bacteria such as most S. pneumonia serotypes, N.meningitidis, and H. influenza [12]. Infection with these bacteria results in a reduced immunological response in neonates and therefore they are at risk [5]. The polysaccharides of some other bacteria such as S.pneumoniae serotype 3, however, are not classified as TI-2 antigens.

Table 1: Characteristics of thymus-dependent and thymus-independent antigens

The global use of effective vaccines directed against encapsulated polysaccharide pathogens would reduce the morbidity and mortality among newborns and infants significantly [13]. In poor resource countries medical care strikingly decreases after the first year of life, necessitating the development of effective infant immunization programs [14]. The challenge for early life immunization is to induce sustained protection circumventing immaturity of the immune system to TI-2 antigens [15]. Neonatal and infantile antibody responses to vaccines are of short duration and decline rapidly within a few months [16]. This may be associated with a resurgence of vulnerability to infection, requiring the administration of repeated vaccinations already in the second year of life. A better understanding of the neonatal response to polysaccharide antigens may lead to the development of improved vaccines. This review will explore the neonatal immune response on polysaccharide antigens and on polysaccharide protein conjugates. Furthermore, the efficacy and drawbacks of the three main polysaccharide-conjugate vaccines will be discussed.

2. Neonatal Immune Responses to Polysaccharides

There is a marked limitation in neonatal and infantile antibody responses to most, but not all, bacterial capsular polysaccharides [5]. The mechanisms that are thought to account for the partial unresponsiveness of neonates to TI-2 stimuli will be reviewed in the next paragraphs.

2.1. B Cell Immaturity

Polysaccharide antigens localize preferentially to the marginal-zone (MZ) B cells, found only in the spleen. These B cells are present in low numbers at birth and the development is deficient in neonates [17]. MZ B cells with adult features appear after 2 years of life and coincide with the ability to induce an immune response to polysaccharides. Children under two years of age have a quantitative defect in IgG2 and IgG4 isotypes [18]. Although other isotypes reach adult levels by two to three years of age, IgG2 appears much later in development, and adult levels of this subclass are not reached until 5–10 years of age [19]. As the IgG2 isotype is considered as the most effective immunoglobulin against some polysaccharides [20], the susceptibility of neonates and infants might be due to the defect in immunoglobulin production. Furthermore, a dysfunctional spleen or splenectomy increases the risk of infection by encapsulated bacteria, such as S. pneumoniae and requires antibiotic prophylaxis [21] or vaccination [22].

The possibility that B cell immaturity in the MZ B cells might cause the reduced TI-2 antigen response was first observed in mice by Mosier et al. [23]. With the development of the murine immune system, B cells change from IgMhiIgDlo to IgMloIgDhi and the response to TI-2 antigens coincided with the appearance of IgD, which takes about one to two weeks in mice [24]. Furthermore, it was shown that mice with an X-linked immune deficiency (CBA/N mice) resulting in B cells that phenotypically resemble neonatal B-cells are unable to respond to TI-2 antigens [23]. The murine MZ expresses the specific intercellular adhesion molecule-grabbing nonintegrin receptor 1 (SIGNR1) which plays a relevant role in the immune response against encapsulated bacteria [25, 26], but its human homologue has not yet been found.

2.2. CD21 and Complement

The immune response to polysaccharides is initiated when polysaccharides activate complement factor C3d via the alternative pathway [27]. The resulting polysaccharide-complement complex subsequently localizes in MZ B cells expressing CD21 (complement receptor 2) [27, 28]. Neonatal and infantile B cells have low expression of CD21 which explains the inadequate response to polysaccharides [17, 28]. Interestingly, the increase of CD21 that occurs during development coincides with the response to polysaccharides. Furthermore, neonates have relatively low levels of complement [29]. Therefore, in early age, CD21 cannot bind the polysaccharide-C3d complex sufficiently and antibodies are not produced. Conjugate vaccines, however, are complement independent and induce an antibody response in neonates and young children, which implies that MZ B cells are not needed for this response.

In a murine model, Breukels et al. [30] showed that polysaccharide-conjugates localize in the splenic MZ of neonatal mice without obvious relation to MZ B cells. Furthermore, it was shown in adult mice that the antibody response to polysaccharides is absent after cobra venom factor (CVF) treatment, which depletes complement.

2.3. T Lymphocyte and Cytokine Defects

In spite of their name, in mice TI-antigens need the assistance of unspecific T-cells and cytokines [31]. In neonatal mice these signals are absent or diminished and B cells activated by TI-antigens do therefore not differentiate. Neonatal accessory cells are deficient in interleukin (IL)-1 and tumor necrosis factor (TNF)-α, which are important for TI-2 responses [32, 33]. Supplementation of a mouse model with these cytokines enabled neonatal B cells to clonally expand and differentiate [6, 32]. The reduced IL-1 and -6 production in neonatal mice by macrophages seems to be related to a decreased level of several TLR’s (TLR2, TLR4) [34, 35]. Also the increased activity of p38 MAPK, a mitogen-activated protein kinase that is involved in the TLR intracellular signaling pathway, seems to play a pivotal role in the unique cytokine phenotype of the neonatal macrophages [35]. The identity of T cells involved in antibody responses to TI-2 antigens is still unknown. Kobrynski et al. [31] measured IgG antibody production after immunization with pneumococcal polysaccharides in mice with disruptions in selected genes of the T cell pathway. Nonclassical MHC class I-like CD1 molecules and a subset of MHC class I-dependent CD8+ cells were found to be essential for antibody responses to TI-2 polysaccharides.

2.4. Other Hypotheses

Several other explanations have been proposed and rejected. A specific B cell subset, B-1, is thought to be important for TI-2 responses since certain TI-2 specificities are found in this subset. However, neonates have concentrations of B1-cells comparable to adults and it seems unlikely that this is the cause of reduced TI-2 responses [6]. Another view was based on the increased susceptibility of immature B cells to tolerance induction [36]. Lastly, it was thought that TI-2 antigens cause a relative increase in suppressor T cells compared to amplifier T cells in a murine model, which reflects an imbalance between Th1 and Th2, in favor of Th1 cells. However, this is unsatisfactory since the current data suggest a Th1 deficiency in neonates [6].

3. Polysaccharide Conjugate Vaccines

The impaired neonatal and infantile immune response to polysaccharide vaccines can be circumvented by conjugating the polysaccharide to a protein carrier [3739], based on the old dogma that haptens when attached to a protein carrier can induce an immune response. The mechanism by which a polysaccharide-protein conjugate vaccine acts is depicted in Figure 1. Today, all current polysaccharide vaccines registered for children below age two are conjugated vaccines. Conjugates transform bacterial polysaccharides from a TI-2 antigen into a TD-antigen and thereby induce an immune response and immunological memory in neonates [40], however not to the extend as in adults [4]. Several factors which are related especially to immunizations might contribute to a suboptimal response to conjugate vaccines. Firstly, the route of immunization determines the recall response to polysaccharides in mice that have been vaccinated with a conjugate as neonates [41]. Secondly, the choice of type of protein conjugate is important and determines the amount of IgG antibody response [41]. Lastly, in neonatal mice there is a Th-2 skew which leads to a predominantly IgG1 response and impaired IgG2a antibody formation [4], the latter thought to be more protective against encapsulated bacteria [6].

Figure 1: Schematic diagram of mechanism of action of PS and PS-protein conjugate vaccines. (a) Polysaccharide vaccines stimulate marginal zone B-cells which will proliferate in loco and differentiate into short-living plasma cells, which are responsible for the rapid release of low-affinity antibodies and thus first-line defence against the pathogen; (b) conjugate vaccines stimulate B-blasts that will migrate into the germinal centre, where they proliferate, undergo somatic hypermutations and isotype class switch, and differentiate into either long-living plasma cells (that produce high-affinity antibodies) and memory B cells. Abbreviations: CD40L (CD40 ligand), GC (germinal centre), MZ (marginal zone), DC (dendritic cell), MZ DC (marginal zone dendritic cell).
3.1. Haemophilus Influenza B Conjugate Vaccine

Several Haemophilus influenza B (HiB) conjugate vaccines were introduced in the early 1990s. The reported efficacy against meningitis and epiglottitis ranged from 94% in children of 2-3 years of age to 99% for infants below 1 year of age [42]. In Brazil, the conjugate vaccine led to a decrease in HiB meningitis 2.39 to 0.06 cases per 100 000 population (98%) overall, and from 60.9 to 3.1 cases per 100 000 population (95%) in children <1 year of age, five years after the introduction of the vaccine [43]. Furthermore, the HiB conjugate vaccines reduced carriage of HiB [44] and probably led to lower transmission rates to children who lacked protective antibodies.

3.2. Neisseria Meningitidis Conjugate Vaccine

Purified polysaccharides from N. menigitidis serogroups A, C, W135, and Y are available vaccine products and elicit antibody responses with no memory function, with the exception of serogroup A polysaccharide which induces a marginal antibody response also in infants [45]. The serogroup C polysaccharide is not immunogenic in children below 2 years of age, and development of antibody titers is slow [46]. Conjugates have been developed using the same principles as for HiB. The type A and C conjugate vaccines are safe and well tolerated in infants and young children [47]. In Spain, the meningococcal C vaccine (MCC) was effective in 98% of infants vaccinated at two, four, and six months of age and 99% in those vaccinated after seven months of age [48]. However, the vaccine effectiveness fell after the first year, especially in those vaccinated as infants. In England, the estimated effectiveness was 66% in infants vaccinated at two, three, and four months of age and 83% in those vaccinated after seven months of age [16]. It fell to low levels after one year in those vaccinated in the first year of life. A vaccine against type B (MenB), which is a common cause of meningococcal disease, is presently unavailable [49, 50]. However, unpublished data provide a source of optimism to develop a safe and effective containing recombinant outer membrane surface proteins of MenB vaccine from the N. meningitidis strain NZ98/254. This investigational vaccine against MenB induced protective immune responses against 3 strains after the third dose (Miller et al. abstract 133, Annual Meeting of the European Society of Paediatric Infectious Diseases (ESPID, 2008)). Results from Phase 3 RCTs are expected during the coming years.

3.3. Streptococcus Pneumoniae Conjugate Vaccine

Diseases caused by pneumococci include pneumonia, meningitis, otitis media, sinusitis, and bronchitis. Pneumococcal vaccines that are effective in infants have been welcomed as resistance against growing commonly used antibiotics [51]. An unconjugated 23-valent vaccine is registered for children over two years of age but is ineffective in younger children [52]. A 7-valent polysaccharide-protein conjugate vaccine (PCV-7) has been introduced for use in children below the age of 2 years. Serotypes included in PCV-7 cover 65–80% of serotypes that cause invasive pneumococcal infections [53]. Other conjugate vaccines with wider serotype coverage, including a 10-valent vaccine and a 13-valent vaccine, are in the late stage of development [54, 55]. A 94% decrease in vaccine type invasive pneumococcal disease in children with a coverage rate of just 68% was reported [56]. The number of all-cause pneumonia admission rates has declined by 39% for children younger than two years in the USA from 2000–2004 [57] and admissions due to pneumococcal meningitis were reduced by 66% in the same period [58]. A significant decrease was also seen in the unvaccinated groups as a result of herd immunity due to decreased transmission from vaccinated children to unvaccinated contacts [56]. The protective efficacy against acute otitis media however has been relatively modest. In a Finnish study, the efficacy against confirmed otitis media was 34% and the overall efficacy against otitis media regardless of cause was only 6% [59].

3.4. Other Conjugate Vaccines

Several other polysaccharide pathogens are under investigation. Group B streptococci are the major cause of meningitis and sepsis in neonates. In animal studies, group B streptococcal conjugate vaccines have shown to be able to induce protective antibodies [60]. Similar attempts have been made to develop immunogenic and safe vaccines against the Vi polysaccharide of S. typhi [61] and polysaccharides and LPS of E. coli [62] as well as S. aureus [63].

4. Drawbacks of Conjugate Vaccines

As described above, polysaccharide-conjugate vaccines are effective in children under the age of two years. However, conjugate vaccines are only available for H. influenza type B, some meningococcal subtypes, and recently for a limited number of subtypes of S. pneumoniae. Vaccine failure and rise of nonserotype bacteria have been noticed and require continuous attention [64]. There is evidence that coadministration with other vaccines may impair effectiveness of the vaccinations [65]. Finally, conjugate vaccines are expensive, which does not allow for global use of these vaccines [53].

4.1. Haemophilus Influenza B Conjugate Vaccine

A decade after its introduction in the UK, the first conjugate vaccine, HiB, became less effective and vaccine failures were seen [66]. Two explanations have been given for this effect. First, after early age immunization antibody levels are sufficient for protection but drop over the following years, sometimes to levels that are not considered protective. This was explained by a reduction in either the number or the quality of memory B cells induced by immunization or a loss of avidity in matured B cells prior to disease onset following defective priming [67]. The low titers observed in the UK may have been exacerbated by the loss of “natural boosting” associated with a reduction in carriage. Reboosting might be the solution [67]. The other important factor contributing to the reduced immune response to HiB-conjugate-vaccine is the current coadministration with other vaccines. Reduced antibody responses to HiB conjugates have been documented using acellular pertussis/HiB [65], DTaP-HiB [68], and MCC/HiB combinations [69], but not in other studies [70, 71]. The impaired immune responses were increased by accelerated immunization schedules such as that in the UK [72]. The precise immunological mechanism responsible for the excess of vaccine failures following the combination vaccine is not known.

4.2. Neisseria Meningitidis Conjugate Vaccine

The meningococcal serogroup B polysaccharide is poorly immunogenic in man [73]. The development of an effective vaccine against N. meningitidis serogroup B is complicated by the inability of this polysaccharide to induce a significant antibody response [73], even when conjugated to a carrier protein [74]. However, unpublished data show promising results (Miller et al. abstract 133, Annual Meeting of the European Society of Paediatric Infectious Diseases (ESPID, 2008)). In Spain and the UK, the MCC vaccine effectiveness fell after the first year, especially in those vaccinated as infants [16, 48]. These data suggest that the protection given by MCC vaccine may be age dependent and that children vaccinated at an older age may have greater and longer-lasting protection than those vaccinated as infants. This suggests that protection may be more reliant on circulating antibodies at the time of exposure than on the ability to mount a booster response [75]. A recent study found that the immune response and length of protection were dependent on the formation of a large germinal center one month after primary immunization with the MCC vaccine [76]. One third of infants in this study produced a very low number of memory B cells after the initial immunization and did not maintain protective antibody levels by one year of age. In these children, the germinal center was underdeveloped. Understanding of the factors that determine the production of these germinal centers could lead to improved conjugate vaccines. Until then, booster doses of MCC may be required in order to extend the duration of protection offered by the vaccine.

4.3. Streptococcus Pneumoniae Conjugate Vaccine

A major drawback of pneumococcal conjugate vaccines is that the serotypes included in PCV-7 cover only 65–80% of serotypes that cause invasive pneumococcal infections [53]. Efforts to include more subtypes in a conjugate vaccine prove to be very complicated and costly. Furthermore, it has been shown that combination of PCV-7 with other vaccines can lead to reduced immune responses. The response to hepatitis B vaccine was nonsignificantly reduced with concomitant administration with PCV-7 [77]. Another problem is that the PCV-7 vaccine replaces disease by nonvaccine serotypes especially 19A [78] and 16F [59]. A recent study in Alaska, where routine vaccination in children has started in 1999–2000, showed an increase in invasive pneumococcal disease rate caused by nonvaccine serotypes of 140% compared with the prevaccine period [64]. In the first three years after introduction of the PCV-7 vaccine, there was a 96% decrease in heptavalent vaccine serotype disease. This led to a decrease in overall invasive pneumococcal disease of 67% in Alaskan children younger than 2 years (from 403.2 per 100 000 in 1995–2000 to 134.3 per 100 000 per year in 2001–2003) but to an 82% increase in invasive disease in the following years to 244.6/100 000. Serotype 19A accounted for 28% of invasive pneumococcal disease among Alaska children younger than 2 years during 2004–2006. There was no significant increase in disease due to nonvaccine serotypes in nonnative Alaskan children younger than 2 years [64]. This emphasizes the importance of continuing surveillance and development of expanded valency vaccines. The question remains whether this serotype shift leads to increased morbidity and mortality rates as these nonvaccine types are typically less pathogenic. Other limitations are a modest effect on nasopharyngeal colonization [79], cost (US 32 000–166 000 per life-year saved) [53], and difficulties in production that have led to shortages.

5. New Developments

The currently registered conjugated polysaccharide vaccines have been developed based upon the principle that CD4 T cell recruitment is necessary for the activation of the infant B cell immune response [8]. The underlying thought was to promote a transformation of the neonatal immune response from a TI one to a TD one by conjugating the polysaccharides to immunogenic carrier proteins. The neonatal immune response to TD-antigens has been shown to be better than the response to TI-2 antigens but did not reach levels seen in healthy adults [4]. One of the most important factors determining the neonatal immune response to conjugate vaccines seems to the type of carrier protein used in the conjugate [4]. The carrier determines the level of induction of specific T cells and therefore the levels of polysaccharide antibodies and hence the protective effect gained by administration of the conjugate vaccine. The optimal carrier might be different for different pneumococcal serotypes [80]. In one study pneumococcal polysaccharide conjugated to a diphtheria carrier was more efficient in inducing a mucosal response, while tetanus conjugate resulted in improved systemic responses [81]. Another study showed that a tetanus conjugate resulted in a better serotype 4 response, while a diphtheria conjugate evoked a better response to types 3, 9 V, and 14 [82].

The addition of adjuvants to conjugate vaccines could potentially reduce the number of doses needed to establish protective immunity and thereby provides protective immunity within a shorter time period and at a reduced cost. Adjuvants also lead to more consistent induction of responses to various polysaccharide serotypes [83]. Toll-like receptor (TLR) ligands have been considered as vaccine adjuvants [84, 85], such as CpG containing oligodeoxynucleotides [86]. Peptide p458 is a peptide derived from the human or mouse 60-kDa heat shock protein (hsp60) and stimulates TLR4 [84]. Conjugated to pneumococcal polysaccharide type 4 it could induce protection in mice against a supralethal S. pneumoniae challenge. Protection was associated with polysaccharide type 4-specific IgG antibodies in most but not in all the mice, a T cell response to the p458 carrier and long-term memory. Vaccines composed of p458 conjugated to the polysaccharides of Salmonella [87], or meningococcus B and C [88] were also immunogenic in mice, even when injected without an added adjuvant. Other TLR agonists that stimulate TLR8, such as R-848 (TLR7/8), the imidazoquinoline congeners 3M-003 (TLR7/8) and 3M-002 (TLR8), as well as single-stranded viral RNAs (TLR8), also induce a strong immune response in neonates and infants by stimulating p38 MAPK phosphorylation [85]. Furthermore, LT-K63, a nontoxic mutant of E. coli heat-labile enterotoxin [89], when administered concomitantly with a conjugated pneumococcal polysaccharide serotype 1, enhanced IgG responses in infant mice compared to conjugated polysaccharide alone [89]. A second dose of conjugated pneumococcal polysaccharide resulted in very high IgG responses and significantly improved protection against lethal pneumococcal infections in this animal model. Similar results were obtained with an MCC vaccine [90].

Furthermore, the route of administration is also shown to be an important determinant in eliciting protective immunity in neonates [4, 89]. Intranasal immunization with conjugated polysaccharides [89, 91] seemed to be effective both in infantile and neonatal mice. A single intranasal dose of conjugate vaccine elicited a sufficient high IgG response to protect neonatal mice against pneumococcal infections, whereas subcutaneous administration required two doses to induce complete protection. The increased efficiency could be explained by the additional induction of a salivary IgA response after intranasal administration. However, antibody responses and protective efficacy remained significantly lower than in adult mice [89]. One of the main reasons for this seems to be the lack of effective adjuvants [92].

Another strategy in early stage of development is the use of surface proteins. An example is the Pneumococcal surface protein A (PspA) that is a cell-wall-associated surface protein [93]. It is known to play a major role in the pneumococcal virulence; it binds human lactoferrin and interferes with complement deposition on the bacterial surface. It is thought that it might result in better immune responses in infants and neonates. The antibody response to PspA has been studied in children [93, 94]. The pneumococcal surface antigen protein A (PsaA) is currently explored as a vaccine candidate. It is structurally conserved [95] and plays a role in adherence to host mucosae [96]. Until now, however, it has not been used as vaccine antigen in humans.

Another potential vaccine strategy is the development of peptides that mimic polysaccharide antigens [97]. The main advantage of using peptides over polysaccharides is that peptides induce a TD antigen response as they are processed by APC’s and presented to T cells. A drawback of the use of peptides in vaccines is their poor chemical stability and subsequently lower immunogenicity in vivo. DNA-based vaccines are another potential approach as they are more stable but were initially not considered a viable option for pathogens coated with polysaccharides since carbohydrate antigens are secondary gene products [97]. However, it was recently shown that a DNAvaccine could induce an IgG2a isotype response against a polysaccharide antigen [98]. Other possible advantages of DNA-vaccines are the relatively straightforward and cheaper production techniques compared to conjugate vaccines.

6. Conclusion

Neonatal immune responses to polysaccharide pathogens are very weak. Therefore, neonates and young children are at risk for invasive infections with S. pneumococcus, N. meningitidis, and H. influenza. An important percentage of deaths among neonates are caused by these bacteria. The efficacy of the currently used conjugate vaccines is already very high in the population most at risk, but worldwide utilization of these vaccines is hampered by high production costs. Knowledge about neonatal immunological responses to polysaccharide antigens may open the way for the application of newly designed conjugated vaccines or vaccines based on other principles in this patient group. Currently, several strategies are being explored to get insight into the mechanisms underlying the limitations of infant responses and to thereby improve neonatal vaccination efficiency.


  1. World Health Organization, “Maternal Health and Safe Motherhood Programme,” MSM96.7, 1996.
  2. The WHO Young Infants Study Group, “Conclusions from the WHO multicenter study of serious infections in young infants,” Pediatric Infectious Disease Journal, vol. 18, no. 10, supplement, pp. S32–S34, 1999. View at Publisher · View at Google Scholar
  3. D. Thomas-Rudolph, T. W. Du Clos, C. M. Snapper, and C. Mold, “C-reactive protein enhances immunity to Streptococcus pneumoniae by targeting uptake to FcγR on dendritic cells,” The Journal of Immunology, vol. 178, no. 11, pp. 7283–7291, 2007. View at Google Scholar
  4. H. Jakobsen, S. Hannesdottir, S. P. Bjarnarson et al., “Early life T Cell responses to pneumucoccal conjugates increase with age and determine the polysaccharide-specific antibody response and protective efficacy,” European Journal of Immunology, vol. 36, no. 2, pp. 287–295, 2006. View at Publisher · View at Google Scholar · View at PubMed
  5. G. T. Rijkers, E. A. M. Sanders, M. A. Breukels, and B. J. M. Zegers, “Infant B cell responses to polysaccharide determinants,” Vaccine, vol. 16, no. 14-15, pp. 1396–1400, 1998. View at Publisher · View at Google Scholar
  6. S. Bondada, H.-J. Wu, D. A. Robertson, and R. L. Chelvarajan, “Accessory cell defect in unresponsiveness of neonates and aged to polysaccharide vaccines,” Vaccine, vol. 19, no. 4-5, pp. 557–565, 2000. View at Publisher · View at Google Scholar
  7. D. E. Mosier, J. J. Mond, and E. A. Goldings, “The ontogeny of thymic independent antibody responses in vitro in normal mice and mice with an X-linked B cell defect,” The Journal of Immunology, vol. 119, no. 6, pp. 1874–1878, 1977. View at Google Scholar
  8. J. J. Mond, A. Lees, and C. M. Snapper, “T cell-independent antigens type 2,” Annual Review of Immunology, vol. 13, no. 1, pp. 655–692, 1995. View at Publisher · View at Google Scholar · View at PubMed
  9. J. J. Mond, Q. Vos, A. Lees, and C. M. Snapper, “T cell independent antigens,” Current Opinion in Immunology, vol. 7, no. 3, pp. 349–354, 1995. View at Publisher · View at Google Scholar
  10. S. R. Yan, D. M. Byers, and R. Bortolussi, “Role of protein tyrosine kinase p53/56lyn in diminished lipopolysaccharide priming of formylmethionylleucyl- phenylalanine-induced superoxide production in human newborn neutrophils,” Infection and Immunity, vol. 72, no. 11, pp. 6455–6462, 2004. View at Publisher · View at Google Scholar · View at PubMed
  11. O. Levy, R. B. Sisson, J. Kenyon, E. Eichenwald, A. B. Macone, and D. Goldmann, “Enhancement of neonatal innate defense: effects of adding an N-terminal recombinant fragment of bactericidal/permeability-increasing protein on growth and tumor necrosis factor-inducing activity of gram-negative bacteria tested in neonatal cord blood ex vivo,” Infection and Immunity, vol. 68, no. 9, pp. 5120–5125, 2000. View at Publisher · View at Google Scholar
  12. G. T. Rijkers, L. A. M. Sanders, and B. J. M. Zegers, “Anti-capsular polysaccharide antibody deficiency states,” Immunodeficiency, vol. 5, no. 1, pp. 1–21, 1993. View at Google Scholar
  13. P. F. Wright and P. F. Wright, “Infectious diseases in early life in industrialized countries,” Vaccine, vol. 16, no. 14-15, pp. 1355–1359, 1998. View at Publisher · View at Google Scholar
  14. J. Kovarik and C.-A. Siegrist, “Optimization of vaccine responses in early life: the role of delivery systems and immunomodulators,” Immunology & Cell Biology, vol. 76, no. 3, pp. 222–236, 1998. View at Publisher · View at Google Scholar
  15. M. Pihlgren, M. Friedli, C. Tougne, A.-F. Rochat, P.-H. Lambert, and C.-A. Siegrist, “Reduced ability of neonatal and early-life bone marrow stromal cells to support plasmablast survival,” The Journal of Immunology, vol. 176, no. 1, pp. 165–172, 2006. View at Google Scholar
  16. C. L. Trotter, N. J. Andrews, E. B. Kaczmarski, E. Miller, and M. E. Ramsay, “Effectiveness of meningococcal serogroup C conjugate vaccine 4 years after introduction,” The Lancet, vol. 364, no. 9431, pp. 365–367, 2004. View at Publisher · View at Google Scholar · View at PubMed
  17. W. Timens, A. Boes, T. Rozeboom-Uiterwijk, and S. Poppema, “Immaturity of the human splenic marginal zone in infancy. Possible contribution to the deficient infant immune response,” The Journal of Immunology, vol. 143, no. 10, pp. 3200–3206, 1989. View at Google Scholar
  18. B. J. M. Zegers, M. Van Der Giessen, E. E. Reerink-Brongers, and J. W. Stoop, “The serum IgG subclass levels in healthy infants of 13–62 weeks of age,” Clinica Chimica Acta, vol. 101, no. 2-3, pp. 265–269, 1980. View at Publisher · View at Google Scholar
  19. N. Kutukculer, N. E. Karaca, O. Demircioglu, and G. Aksu, “Increases in serum immunoglobulins to age-related normal levels in children with IgA and/or IgG subclass deficiency,” Pediatric Allergy and Immunology, vol. 18, no. 2, pp. 167–173, 2007. View at Publisher · View at Google Scholar · View at PubMed
  20. D. J. Barrett and E. M. Ayoub, “IgG2 subclass restriction of antibody to pneumococcal polysaccharides,” Clinical and Experimental Immunology, vol. 63, no. 1, pp. 127–134, 1986. View at Google Scholar
  21. E. Castagnola and F. Fioredda, “Prevention of life-threatening infections due to encapsulated bacteria in children with hyposplenia or asplenia: a brief review of current recommendations for practical purposes,” European Journal of Haematology, vol. 71, no. 5, pp. 319–326, 2003. View at Publisher · View at Google Scholar
  22. T. V. Adamkiewicz, B. J. Silk, J. Howgate et al., “Effectiveness of the 7-valent pneumococcal conjugate vaccine in children with sickle cell disease in the first decade of life,” Pediatrics, vol. 121, no. 3, pp. 562–569, 2008. View at Publisher · View at Google Scholar · View at PubMed
  23. D. E. Mosier, I. M. Zitron, J. J. Mond, A. Ahmed, I. Scher, and W. E. Paul, “Surface immunoglobulin D as a functional receptor for a subclass of B lymphocytes,” Immunological Reviews, vol. 37, no. 1, pp. 89–104, 1977. View at Google Scholar
  24. P. Balogh, Y. Aydar, J. G. Tew, and A. K. Szakal, “Ontogeny of the follicular dendritic cell phenotype and function in the postnatal murine spleen,” Cellular Immunology, vol. 214, no. 1, pp. 45–53, 2001. View at Publisher · View at Google Scholar · View at PubMed
  25. A. Lanoue, M. R. Clatworthy, P. Smith et al., “SIGN-R1 contributes to protection against lethal pneumococcal infection in mice,” Journal of Experimental Medicine, vol. 200, no. 11, pp. 1383–1393, 2004. View at Publisher · View at Google Scholar · View at PubMed
  26. E. A. Koppel, M. Litjens, V. C. van den Berg, Y. van Kooyk, and T. B. H. Geijtenbeek, “Interaction of SIGNR1 expressed by marginal zone macrophages with marginal zone B cells is essential to early IgM responses against Streptococcus pneumoniae,” Molecular Immunology, vol. 45, no. 10, pp. 2881–2887, 2008. View at Publisher · View at Google Scholar · View at PubMed
  27. M.-J.P. Llopis, G. Harms, M. J. Hardonk, and W. Timens, “Human immune response to pneumococcal polysaccharides: complement-mediated localization preferentially on CD21-positive splenic marginal zone B cells and follicular dendritic cells,” Journal of Allergy and Clinical Immunology, vol. 97, no. 4, pp. 1015–1024, 1996. View at Publisher · View at Google Scholar
  28. A. W. Griffioen, S. W. Franklin, B. J. M. Zegers, and G. T. Rijkers, “Expression and functional characteristics of the complement receptor type 2 on adult and neonatal B lymphocytes,” Clinical Immunology and Immunopathology, vol. 69, no. 1, pp. 1–8, 1993. View at Publisher · View at Google Scholar
  29. C. A. Davis, E. H. Vallota, and J. Forristal, “Serum complement levels in infancy: age related changes,” Pediatric Research, vol. 13, no. 9, pp. 1043–1046, 1979. View at Google Scholar
  30. M. A. Breukels, A. Zandvoort, G. T. Rijkers et al., “Complement dependency of splenic localization of pneumococcal polysaccharide and conjugate vaccines,” Scandinavian Journal of Immunology, vol. 61, no. 4, pp. 322–328, 2005. View at Publisher · View at Google Scholar · View at PubMed
  31. L. J. Kobrynski, A. O. Sousa, A. J. Nahmias, and F. K. Lee, “Cutting edge: antibody production to pneumococcal polysaccharides requires CD1 molecules and CD8+ cells,” The Journal of Immunology, vol. 174, no. 4, pp. 1787–1790, 2005. View at Google Scholar
  32. R. L. Chelvarajan, N. L. Gilbert, and S. Bondada, “Neonatal murine B lymphocytes respond to polysaccharide antigens in the presence of IL-1 and IL-6,” The Journal of Immunology, vol. 161, no. 7, pp. 3315–3324, 1998. View at Google Scholar
  33. D. F. Angelone, M. R. Wessels, M. Coughlin et al., “Innate immunity of the human newborn is polarized toward a high ratio of IL-6/TNF-α production in vitro and in vivo,” Pediatric Research, vol. 60, no. 2, pp. 205–209, 2006. View at Publisher · View at Google Scholar · View at PubMed
  34. R. L. Chelvarajan, S. M. Collins, I. E. Doubinskaia et al., “Defective macrophage function in neonates and its impact on unresponsiveness of neonates to polysaccharide antigens,” Journal of Leukocyte Biology, vol. 75, no. 6, pp. 982–994, 2004. View at Publisher · View at Google Scholar · View at PubMed
  35. L. Chelvarajan, D. Popa, Y. Liu, T. V. Getchell, A. J. Stromberg, and S. Bondada, “Molecular mechanisms underlying anti-inflammatory phenotype of neonatal splenic macrophages,” Journal of Leukocyte Biology, vol. 82, no. 2, pp. 403–416, 2007. View at Publisher · View at Google Scholar · View at PubMed
  36. G. J. Nossal, “Cellular mechanisms of immunologic tolerance,” Annual Review of Immunology, vol. 1, no. 1, pp. 33–62, 1983. View at Publisher · View at Google Scholar · View at PubMed
  37. S. Kurikka, H. Käyhty, L. Saarinen, P.-R. Ronnberg, J. Eskola, and P. H. Makela, “Immunologic priming by one dose of Haemophilus influenzae type b conjugate vaccine in infancy,” The Journal of Infectious Diseases, vol. 172, no. 5, pp. 1268–1272, 1995. View at Google Scholar
  38. J. Eskola and H. Käyhty, “Early immunization with conjugate vaccines,” Vaccine, vol. 16, no. 14-15, pp. 1433–1438, 1998. View at Publisher · View at Google Scholar
  39. J. Eskola, J. Ward, R. Dagan, D. Goldblatt, F. Zepp, and C.-A. Siegrist, “Combined vaccination of Haemophilus influenzae type b conjugate and diphtheria-tetanus-pertussis containing acellular pertussis,” The Lancet, vol. 354, no. 9195, pp. 2063–2068, 1999. View at Publisher · View at Google Scholar
  40. B. Adkins, C. Leclerc, and S. Marshall-Clarke, “Neonatal adaptive immunity comes of age,” Nature Reviews Immunology, vol. 4, no. 7, pp. 553–564, 2004. View at Publisher · View at Google Scholar · View at PubMed
  41. S. P. Bjarnarson, H. Jakobsen, G. Del Giudice, E. Trannoy, C.-A. Siegrist, and I. Jonsdottir, “The advantage of mucosal immunization for polysaccharide-specific memory responses in early life,” European Journal of Immunology, vol. 35, no. 4, pp. 1037–1045, 2005. View at Publisher · View at Google Scholar · View at PubMed
  42. R. Booy, P. T. Heath, P. E. M. Slack, N. Begg, and E. R. Moxon, “Vaccine failures after primary immunisation with Haemophilus influenzae type-b conjugate vaccine without booster,” The Lancet, vol. 349, no. 9060, pp. 1197–1202, 1997. View at Publisher · View at Google Scholar
  43. G. S. Ribeiro, J. B. T. Lima, J. N. Reis et al., “Haemophilus influenzae meningitis 5 years after introduction of the Haemophilus influenzae type b conjugate vaccine in Brazil,” Vaccine, vol. 25, no. 22, pp. 4420–4428, 2007. View at Publisher · View at Google Scholar · View at PubMed
  44. M. L. Barbour, R. T. Mayon-White, C. Coles, D. W. M. Crook, and E. R. Moxon, “The impact of conjugate vaccine on carriage of Haemophilus influenzae type b,” The Journal of Infectious Diseases, vol. 171, no. 1, pp. 93–98, 1995. View at Google Scholar
  45. P. H. Mäkelä, H. Peltola, H. Käyhty et al., “Polysaccharide vaccines of group A Neisseria meningtitidis and Haemophilus influenzae type b: a field trial in Finland,” The Journal of Infectious Diseases, vol. 136, pp. S43–S50, 1977. View at Google Scholar
  46. H. Peltola, “Meningococcal vaccines. Current status and future possibilities,” Drugs, vol. 55, no. 3, pp. 347–366, 1998. View at Google Scholar
  47. L. O. Conterno, C. R. Silva Filho, J. U. Rüggeberg, and P. T. Heath, “Conjugate vaccines for preventing meningococcal C meningitis and septicaemia,” Cochrane Database of Systematic Reviews, no. 3, Article ID CD001834, 2006. View at Publisher · View at Google Scholar · View at PubMed
  48. A. Larrauri, R. Cano, M. García, and S. de Mateo, “Impact and effectiveness of meningococcal C conjugate vaccine following its introduction in Spain,” Vaccine, vol. 23, no. 32, pp. 4097–4100, 2005. View at Publisher · View at Google Scholar · View at PubMed
  49. J. Findlow, S. Taylor, A. Aase et al., “Comparison and correlation of Neisseria meningitidis serogroup B immunologic assay results and human antibody responses following three doses of the Norwegian meningococcal outer membrane vesicle vaccine MenBvac,” Infection and Immunity, vol. 74, no. 8, pp. 4557–4565, 2006. View at Publisher · View at Google Scholar · View at PubMed
  50. H. Nókleby, P. Aavitsland, J. O'Hallahan, B. Feiring, S. Tilman, and P. Oster, “Safety review: two outer membrane vesicle (OMV) vaccines against systemic Neisseria meningitidis serogroup B disease,” Vaccine, vol. 25, no. 16, pp. 3080–3084, 2007. View at Publisher · View at Google Scholar · View at PubMed
  51. M.-C. Demachy, V. Vernet-Garnier, J. Cottin et al., “Antimicrobial resistance data on 16,756 Streptococcus pneumoniae isolates in 1999: a pan-regional multicenter surveillance study in France,” Microbial Drug Resistance, vol. 11, no. 4, pp. 323–329, 2005. View at Publisher · View at Google Scholar · View at PubMed
  52. G. B. Lesinski and M. A. Westerink, “Vaccines against polysaccharide antigens,” Current Drug Targets—Infectious Disorders, vol. 1, no. 3, pp. 325–334, 2001. View at Google Scholar
  53. “Pneumococcal conjugate vaccine for childhood immunization—WHO position paper,” Weekly Epidemiological Record, vol. 82, no. 12, pp. 93–104, 2007.
  54. “Study Evaluating 13-Valent Pneumococcal Conjugate Vaccine Catch-Up Regimens in Older Infants and Children,” April 2008, http://www.clinicaltrials.gov/.
  55. “Safety and Immunogenicity Study of a Booster Dose of GSK Biologicals' 10-Valent Pneumococcal Conjugate Vaccine,” April 2008, http://www.clinicaltrials.gov/.
  56. S. Black, H. Shinefield, B. Fireman et al., “Efficacy, safety and immunogenicity of heptavalent pneumococcal conjugate vaccine in children,” Pediatric Infectious Disease Journal, vol. 19, no. 3, pp. 187–195, 2000. View at Publisher · View at Google Scholar
  57. C. G. Grijalva, J. P. Nuorti, P. G. Arbogast, S. W. Martin, K. M. Edwards, and M. R. Griffin, “Decline in pneumonia admissions after routine childhood immunisation with pneumococcal conjugate vaccine in the USA: a time-series analysis,” The Lancet, vol. 369, no. 9568, pp. 1179–1186, 2007. View at Publisher · View at Google Scholar · View at PubMed
  58. C. J. Tsai, M. R. Griffin, J. P. Nuorti, and C. G. Grijalva, “Changing epidemiology of pneumococcal meningitis after the introduction of pneumococcal conjugate vaccine in the United States,” Clinical Infectious Diseases, vol. 46, no. 11, pp. 1664–1672, 2008. View at Publisher · View at Google Scholar · View at PubMed
  59. J. Eskola, T. Kilpi, A. Palmu et al., “Efficacy of a pneumococcal conjugate vaccine against acute otitis media,” The New England Journal of Medicine, vol. 344, no. 6, pp. 403–409, 2001. View at Publisher · View at Google Scholar · View at PubMed
  60. F. Michon, C. Uitz, A. Sarkar et al., “Group B streptococcal type II and III conjugate vaccines: physicochemical properties that influence immunogenicity,” Clinical and Vaccine Immunology, vol. 13, no. 8, pp. 936–943, 2006. View at Publisher · View at Google Scholar · View at PubMed
  61. F. Y. C. Lin, V. A. Ho, H. B. Khiem et al., “The efficacy of a Salmonella typhi Vi conjugate vaccine in two-to-five-year-old children,” The New England Journal of Medicine, vol. 344, no. 17, pp. 1263–1269, 2001. View at Publisher · View at Google Scholar
  62. S. J. Cryz, Jr., J. O. Que, A. S. Cross, and E. Fürer, “Synthesis and characterization of a polyvalent Escherichia coli O-polysaccharide-toxin A conjugate vaccine,” Vaccine, vol. 13, no. 5, pp. 449–453, 1995. View at Publisher · View at Google Scholar
  63. T. Tollersrud, L. Zernichow, S. R. Andersen, K. Kenny, and A. Lund, “Staphylococcus aureus capsular polysaccharide type 5 conjugate and whole cell vaccines stimulate antibody responses in cattle,” Vaccine, vol. 19, no. 28-29, pp. 3896–3903, 2001. View at Publisher · View at Google Scholar
  64. R. J. Singleton, T. W. Hennessy, L. R. Bulkow et al., “Invasive pneumococcal disease caused by nonvaccine serotypes among Alaska Native children with high levels of 7-valent pneumococcal conjugate vaccine coverage,” The Journal of the American Medical Association, vol. 297, no. 16, pp. 1784–1792, 2007. View at Publisher · View at Google Scholar · View at PubMed
  65. J. Eskola, H. Peltola, and A. K. Takala, “Efficacy of Haemophilus influenzae type b polysaccharide-diphtheria toxoid conjugate vaccine in infancy,” The New England Journal of Medicine, vol. 317, no. 12, pp. 717–722, 1987. View at Google Scholar
  66. D. F. Kelly, E. R. Moxon, and A. J. Pollard, “Haemophilus influenzae type b conjugate vaccines,” Immunology, vol. 113, no. 2, pp. 163–174, 2004. View at Publisher · View at Google Scholar · View at PubMed
  67. C. L. Yeh, D. F. Kelly, L.-M. Yu et al., “Haemophilus influenzae type b vaccine failure in children is associated with inadequate production of high-quality antibody,” Clinical Infectious Diseases, vol. 46, no. 2, pp. 186–192, 2008. View at Publisher · View at Google Scholar · View at PubMed
  68. J. McVernon, N. Andrews, M. P. E. Slack, and M. E. Ramsay, “Risk of vaccine failure after Haemophilus influenzae type b (Hib) combination vaccines with acellular pertussis,” The Lancet, vol. 361, no. 9368, pp. 1521–1523, 2003. View at Publisher · View at Google Scholar
  69. N. R. E. Kitchin, J. Southern, R. Morris et al., “Evaluation of a diphtheria-tetanus-acellular pertussis-inactivated poliovirus-Haemophilus influenzae type b vaccine given concurrently with meningococcal group C conjugate vaccine at 2, 3 and 4 months of age,” Archives of Disease in Childhood, vol. 92, no. 1, pp. 11–16, 2007. View at Publisher · View at Google Scholar · View at PubMed
  70. H.-J. Schmitt, G. Maechler, P. Habermehl et al., “Immunogenicity, reactogenicity, and immune memory after primary vaccination with a novel Haemophilus influenzae-Neisseria meningitidis serogroup C conjugate vaccine,” Clinical and Vaccine Immunology, vol. 14, no. 4, pp. 426–434, 2007. View at Publisher · View at Google Scholar · View at PubMed
  71. S. Gatchalian, E. Palestroque, I. De Vleeschauwer et al., “The development of a new heptavalent diphtheria-tetanus-whole cell pertussis-hepatitis B-Haemophilus influenzae type b-Neisseria meningitidis serogroups A and C vaccine: a randomized dose-ranging trial of the conjugate vaccine components,” International Journal of Infectious Diseases, vol. 12, no. 3, pp. 278–288, 2008. View at Publisher · View at Google Scholar · View at PubMed
  72. E. Vidor, A. Hoffenbach, and M. A. Fletcher, “Haemophilus influenzae type b vaccine: reconstitution of lyophilised PRP-T vaccine with a pertussis-containing paediatric combination vaccine, or a change in the primary series immunisation schedule, may modify the serum anti-PRP antibody responses,” Current Medical Research and Opinion, vol. 17, no. 3, pp. 197–209, 2001. View at Publisher · View at Google Scholar
  73. F. A. Wyle, M. S. Artenstein, B. L. Brandt et al., “Immunologic response of man to group B meningococcal polysaccharide vaccines,” The Journal of Infectious Diseases, vol. 126, no. 5, pp. 514–521, 1972. View at Google Scholar
  74. S. J. N. Devi, W. D. Zollinger, P. J. Snoy et al., “Preclinical evaluation of group B Neisseria meningitidis and Escherichia coli K92 capsular polysaccharide-protein conjugate vaccines in juvenile rhesus monkeys,” Infection and Immunity, vol. 65, no. 3, pp. 1045–1052, 1997. View at Google Scholar
  75. C. Auckland, S. Gray, R. Borrow et al., “Clinical and immunologic risk factors for meningococcal C conjugate vaccine failure in the United Kingdom,” The Journal of Infectious Diseases, vol. 194, no. 12, pp. 1745–1752, 2006. View at Publisher · View at Google Scholar · View at PubMed
  76. G. Blanchard Rohner, M. D. Snape, D. F. Kelly et al., “The magnitude of the antibody and memory B cell responses during priming with a protein-polysaccharide conjugate vaccine in human infants is associated with the persistence of antibody and the intensity of booster response,” The Journal of Immunology, vol. 180, no. 4, pp. 2165–2173, 2008. View at Google Scholar
  77. I. Tichmann-Schumann, P. Soemantri, U. Behre et al., “Immunogenicity and reactogenicity of four doses of diphtheria-tetanus-three-component acellular pertussis-hepatitis B-inactivated polio virus-Haemophilus influenzae type b vaccine coadministered with 7-valent pneumococcal conjugate vaccine,” Pediatric Infectious Disease Journal, vol. 24, no. 1, pp. 70–77, 2005. View at Publisher · View at Google Scholar
  78. M. H. Kyaw, R. Lynfield, W. Schaffner et al., “Effect of introduction of the pneumococcal conjugate vaccine on drug-resistant Streptococcus pneumoniae,” The New England Journal of Medicine, vol. 354, no. 14, pp. 1455–1463, 2006. View at Publisher · View at Google Scholar · View at PubMed
  79. R. Veenhoven, D. Bogaert, C. Uiterwaal et al., “Effect of conjugate pneumococcal vaccine followed by polysaccharide pneumococcal vaccine on recurrent acute otitis media: a randomised study,” The Lancet, vol. 361, no. 9376, pp. 2189–2195, 2003. View at Publisher · View at Google Scholar · View at PubMed
  80. J. Bernatoniene and A. Finn, “Advances in pneumococcal vaccines advantages for infants and children,” Drugs, vol. 65, no. 2, pp. 229–255, 2005. View at Publisher · View at Google Scholar
  81. T. Nieminen, J. Eskola, and H. Käyhty, “Pneumococcal conjugate vaccination in adults: circulating antibody secreting cell response and humoral antibody responses in saliva and in serum,” Vaccine, vol. 16, no. 6, pp. 630–636, 1998. View at Publisher · View at Google Scholar
  82. A. Nurkka, H. Ahman, M. Korkeila, V. Jantti, H. Käyhty, and J. Eskola, “Serum and salivary anti-capsular antibodies in infants and children immunized with the heptavalent pneumococcal conjugate vaccine,” Pediatric Infectious Disease Journal, vol. 20, no. 1, pp. 25–33, 2001. View at Publisher · View at Google Scholar
  83. A. Marchant and M. Newport, “Prevention of infectious disease by neonatal and early infantile immunization: prospects for the new millennium,” Current Opinion in Infectious Diseases, vol. 13, no. 3, pp. 241–246, 2000. View at Google Scholar
  84. N. Cohen, M. Stolarsky-Bennun, H. Amir-Kroll et al., “Pneumococcal capsular polysaccharide is immunogenic when present on the surface of macrophages and dendritic cells: TLR4 signaling induced by a conjugate vaccine or by lipopolysaccharide is conducive,” The Journal of Immunology, vol. 180, no. 4, pp. 2409–2418, 2008. View at Google Scholar
  85. O. Levy, E. E. Suter, R. L. Miller, and M. R. Wessels, “Unique efficacy of Toll-like receptor 8 agonists in activating human neonatal antigen-presenting cells,” Blood, vol. 108, no. 4, pp. 1284–1290, 2006. View at Publisher · View at Google Scholar · View at PubMed
  86. R. S. Chu, T. Mccool, N. S. Greenspan, J. R. Schreiber, and C. V. Harding, “CpG oligodeoxynucleotides act as adjuvants for pneumococcal polysaccharide-protein conjugate vaccines and enhance antipolysaccharide immunoglobulin G2a (IgG2a) and IgG3 antibodies,” Infection and Immunity, vol. 68, no. 3, pp. 1450–1456, 2000. View at Publisher · View at Google Scholar
  87. S. Konen-Waisman, M. Fridkin, and I. R. Cohen, “Self and foreign 60-kilodalton heat shock protein T cell epitope peptides serve as immunogenic carriers for a T cell-independent sugar antigen,” The Journal of Immunology, vol. 154, no. 11, pp. 5977–5985, 1995. View at Google Scholar
  88. H. Amir-Kroll, L. Riveron, M. E. Sarmiento, G. Sierra, A. Acosta, and I. R. Cohen, “A conjugate vaccine composed of a heat shock protein 60 T-cell epitope peptide (p458) and Neisseria meningitidis type B capsular polysaccharide,” Vaccine, vol. 24, no. 42-43, pp. 6555–6563, 2006. View at Publisher · View at Google Scholar · View at PubMed
  89. H. Jakobsen, S. Bjarnarson, G. Del Giudice, M. Moreau, C.-A. Siegrist, and I. Jonsdottir, “Intranasal immunization with pneumococcal conjugate vaccines with LT-K63, a nontoxic mutant of heat-labile enterotoxin, as adjuvant rapidly induces protective immunity against lethal pneumococcal infections in neonatal mice,” Infection and Immunity, vol. 70, no. 3, pp. 1443–1452, 2002. View at Publisher · View at Google Scholar
  90. B. C. Baudner, J. C. Verhoef, M. M. Giuliani et al., “Protective immune responses to meningococcal C conjugate vaccine after intranasal immunization of mice with the LTK63 mutant plus chitosan or trimethyl chitosan chloride as novel delivery platform,” Journal of Drug Targeting, vol. 13, no. 8-9, pp. 489–498, 2005. View at Publisher · View at Google Scholar · View at PubMed
  91. A. Sabirov and D. W. Metzger, “Intranasal vaccination of neonatal mice with polysaccharide conjugate vaccine for protection against pneumococcal otitis media,” Vaccine, vol. 24, no. 27-28, pp. 5584–5592, 2006. View at Publisher · View at Google Scholar · View at PubMed
  92. M. T. De Magistris, “Mucosal delivery of vaccine antigens and its advantages in pediatrics,” Advanced Drug Delivery Reviews, vol. 58, no. 1, pp. 52–67, 2006. View at Publisher · View at Google Scholar · View at PubMed
  93. S. Rapola, V. Jantti, R. Haikala et al., “Natural development of antibodies to pneumococcal surface protein A, pneumococcal surface adhesin A, and pneumolysin in relation to pneumococcal carriage and acute otitis media,” The Journal of Infectious Diseases, vol. 182, no. 4, pp. 1146–1152, 2000. View at Publisher · View at Google Scholar
  94. A. Virolainen, W. Russell, M. J. Crain, S. Rapola, H. Käyhty, and D. E. Briles, “Human antibodies to pneumococcal surface protein A in health and disease,” Pediatric Infectious Disease Journal, vol. 19, no. 2, pp. 134–138, 2000. View at Publisher · View at Google Scholar
  95. K. E. Morrison, D. Lake, J. Crook et al., “Confirmation of psaA in all 90 serotypes of Streptococcus pneumoniae by PCR and potential of this assay for identification and diagnosis,” Journal of Clinical Microbiology, vol. 38, no. 1, pp. 434–437, 2000. View at Google Scholar
  96. M. C. Lawrence, P. A. Pilling, V. C. Epa, A. M. Berry, A. D. Ogunniyi, and J. C. Paton, “The crystal structure of pneumococcal surface antigen PsaA reveals a metal-binding site and a novel structure for a putative ABC-type binding protein,” Structure, vol. 6, no. 12, pp. 1553–1561, 1998. View at Publisher · View at Google Scholar
  97. A. Weintraub, “Immunology of bacterial polysaccharide antigens,” Carbohydrate Research, vol. 338, no. 23, pp. 2539–2547, 2003. View at Publisher · View at Google Scholar
  98. T. Kieber-Emmons, B. Monzavi-Karbassi, B. Wang, P. Luo, and D. B. Weiner, “Cutting edge: DNA immunization with minigenes of carbohydrate mimotopes induce functional anti-carbohydrate antibody response,” The Journal of Immunology, vol. 165, no. 2, pp. 623–627, 2000. View at Google Scholar