Passively acquired anti-tetanus and anti-Haemophilus antibodies and the response to Haemophilus influenzae type b-tetanus toxoid conjugate vaccine in infancy

Genetically detoxified tetanus toxin as a vaccine and conjugate carrier protein

Tetanus toxoid (TTxd), developed over 100 years ago, is a clinically effective, legacy vaccine against tetanus. Due to the extreme potency of native tetanus toxin, manufacturing and regulatory efforts often focus on TTxd production, standardization, and safety, rather than product modernization. Recently, a genetically detoxified, full-length tetanus toxin protein (8MTT) was reported as a tetanus vaccine alternative to TTxd (Przedpelski et al. mBio, 2020). Here we describe the production of 8MTT in Gor/Met^TM E. coli, a strain engineered to have an oxidative cytoplasm, allowing for the expression of soluble, disulfide-bonded proteins. The strain was also designed to efficiently cleave N-terminal methionine, the obligatory start amino acid for E. coli expressed proteins. 8MTT was purified as a soluble protein from the cytoplasm in a two-column protocol to > 99 % purity, yielding 0.5 g of purified 8MTT/liter of fermentation broth with low endotoxin contamination, and antigenic purity of 3500 Lf/mg protein nitrogen. Mouse immunizations showed 8MTT to be an immunogenic vaccine and effective as a carrier protein for peptide and polysaccharide conjugates. These studies validate 8MTT as commercially viable and, unlike the heterogenous tetanus toxoid, a uniform carrier protein for conjugate vaccines. The development of a recombinant, genetically detoxified toxin produced in E. coli aligns the tetanus vaccine with modern manufacturing, regulatory, standardization, and safety requirements.

Effect of Maternal Vaccination of PCV-10, PPV-23 or Placebo on the Immunogenicity of PCV-10 in HIV-exposed Uninfected Infants: A Randomized Clinical Trial

BACKGROUND

The effect of pneumococcal vaccination of mothers with HIV on infant responses to childhood vaccination has not been studied. We compared the immunogenicity of PCV-10 in HIV-exposed uninfected infants born to mothers who received PCV-10, PPV-23 or placebo during pregnancy.

METHODS

Antibody levels against seven serotypes were measured at birth, before the 1 st and 2 nd doses of PCV-10 and after the completion of the 2-dose regimen in 347 infants, including 112 born to mothers who received PPV-23, 112 PCV-10, and 119 placebo during pregnancy. Seroprotection was defined by antibody levels ≥0.35µg/ml.

RESULTS

At birth and 8 weeks of life, antibody levels were similar in infants born to PCV-10- or PPV-23-recipient mothers and higher than infants of placebo-recipient mothers. After the last dose of PCV-10, infants in the maternal PCV-10 group had significantly lower antibody levels against five serotypes compared to infants in the maternal PPV-23 group; against three serotypes compared to infants in the maternal placebo group; and did not have higher antibody levels against any serotype. The seroprotection rate against seven serotypes was 50% in infants in the maternal PCV-10 compared to 71% in each of the maternal PPV-23 and placebo groups (p<0.0001).

CONCLUSIONS

Administration of PCV-10 during pregnancy was associated with decreased antibody responses to PCV-10 and seroprotection rates in infants. Considering that PCV-10 and PPV-23 had similar immunogenicity in pregnant women with HIV and that administration of PPV-23 did not affect the immunogenicity of PCV-10 in infants, PPV-23 in pregnancy may be preferred over PCV-10.

Maternal immunisation: collaborating with mother nature

Maternal immunisation has the potential to substantially reduce morbidity and mortality from infectious diseases after birth. The success of tetanus, influenza, and pertussis immunisation during pregnancy has led to consideration of additional maternal immunisation strategies to prevent group B streptococcus and respiratory syncytial virus infections, among others. However, many gaps in knowledge regarding the immunobiology of maternal immunisation prevent the optimal design and application of this successful public health intervention. Therefore, we did an innovative landscape analysis to identify research priorities. Key topics were delineated through review of the published literature, consultation with vaccine developers and regulatory agencies, and a collaborative workshop that gathered experts across several maternal immunisation initiatives-group B streptococcus, respiratory syncytial virus, pertussis, and influenza. Finally, a global online survey prioritised the identified knowledge gaps on the basis of expert opinion about their importance and relevance. Here we present the results of this worldwide landscape analysis and discuss the identified research gaps.

Immunization with Immune Complexes Modulates the Fine Specificity of Antibody Responses to a Flavivirus Antigen

The antibody response to proteins may be modulated by the presence of preexisting antigen-specific antibodies and the formation of immune complexes (ICs). Effects such as a general increase or decrease of the response as well as epitope-specific phenomena have been described. In this study, we investigated influences of IC immunization on the fine specificity of antibody responses in a structurally well-defined system, using the envelope (E) protein of tick-borne encephalitis (TBE) virus as an immunogen. TBE virus occurs in Europe and Asia and-together with the yellow fever, dengue, West Nile, and Japanese encephalitis viruses-represents one of the major human-pathogenic flaviviruses. Mice were immunized with a dimeric soluble form of E (sE) alone or in complex with monoclonal antibodies specific for each of the three domains of E, and the antibody response induced by these ICs was compared to that seen after immunization with sE alone. Immunoassays using recombinant domains and domain combinations of TBE virus sE as well as the distantly related West Nile virus sE allowed the dissection and quantification of antibody subsets present in postimmunization sera, thus generating fine-specificity patterns of the polyclonal responses. There were substantially different responses with two of the ICs, and the differences could be mechanistically related to (i) epitope shielding and (ii) antibody-mediated structural changes leading to dissociation of the sE dimer. The phenomena described may also be relevant for polyclonal responses upon secondary infections and/or booster immunizations and may affect antibody responses in an individual-specific way.

IMPORTANCE

Infections with flaviviruses such as yellow fever, dengue, Japanese encephalitis, West Nile, and tick-borne encephalitis (TBE) viruses pose substantial public health problems in different parts of the world. Antibodies to viral envelope protein E induced by natural infection or vaccination were shown to confer protection from disease. Such antibodies can target different epitopes in E protein, and the fine specificities of polyclonal responses can differ between individuals. We conducted a mouse immunization study with TBE E protein alone or complexed to monoclonal antibodies specific for each of the three protein domains. We demonstrated that phenomena such as epitope shielding and antibody-induced structural changes can profoundly influence the fine specificity of antibody responses to the same immunogen. The study thus provided important new information on the potential immunomodulatory role of preexisting antibodies in a flavivirus system that can be relevant for understanding individual-specific factors influencing antibody responses in sequential flavivirus infections and/or immunizations.

A cohort study of developmental polychlorinated biphenyl (PCB) exposure in relation to post-vaccination antibody response at 6-months of age

BACKGROUND

Extensive experimental data in animals indicate that exposure to polychlorinated biphenyls (PCBs) during pregnancy leads to changes in offspring immune function during the postnatal period. Whether developmental PCB exposure influences immunologic development in humans has received little study.

METHODS

The study population was 384 mother-infant pairs recruited from two districts of eastern Slovakia for whom prospectively collected maternal, cord, and 6-month infant blood specimens were available. Several PCB congeners were measured in maternal, cord, and 6-month infant sera by high-resolution gas chromatography with electron capture detection. Concentrations of IgG-specific anti-haemophilus influenzae type b, tetanus toxoid, and diphtheria toxoid were assayed in 6-month infant sera using ELISA methods. Multiple linear regression was used to estimate the relation between maternal, cord, and 6-month infant PCB concentrations and the antibody concentrations evaluated at 6-months of age.

RESULTS

Overall, there was little evidence of an association between infant antibody concentrations and PCB measures during the pre- and early postnatal period. In addition, our results did not show specificity in terms of associations limited to a particular developmental period (e.g. pre- vs. postnatal), a particular antibody, or a particular PCB congener.

CONCLUSIONS

At the PCB concentrations measured in this cohort, which are high relative to most human populations today, we did not detect an association between maternal or early postnatal PCB exposure and specific antibody responses at 6-months of age.

Relationship between the incidence of type 1 diabetes and maternal enterovirus antibodies: time trends and geographical variation

AIMS/HYPOTHESIS

We have previously observed an inverse correlation between the incidence of type 1 diabetes and enterovirus infections in the background population. The aim of this study was to analyse whether maternal enterovirus antibody status, which reflects both the frequency of enterovirus infections and the protection conferred by the mother on the offspring, also correlates with the incidence of type 1 diabetes.

METHODS

Maternal enterovirus antibodies were analysed from serum samples taken from pregnant women between 1983 and 2001 in Finland and Sweden using enzyme immunoassay and neutralisation assays. Comparable samples were also taken between 1999 and 2001 in countries with a lower incidence of diabetes (Estonia, Germany, Hungary, Israel, Lithuania, Russia).

RESULTS

A clear decrease was observed in maternal enterovirus antibody levels over the past 20 years (p<0.0001). The frequency of enterovirus antibodies was higher in countries with a low or intermediate incidence of type 1 diabetes compared with high-incidence countries (p<0.0001).

CONCLUSIONS/INTERPRETATION

These findings are in line with our previous observations supporting the hypothesis that a low frequency of enterovirus infection in the background population increases the susceptibility of young children to the diabetogenic effect of enteroviruses.

Protective levels of polysaccharide-specific maternal antibodies may enhance the immune response elicited by pneumococcal conjugates in neonatal and infant mice

Maternal antibodies (MatAbs) may protect the offspring against infections but may also interfere with their immune responses to vaccination. We have previously shown that maternal immunization with pneumococcal polysaccharides (PPS) conjugated to tetanus protein (Pnc-TT) protected the offspring against infections caused by three important pediatric serotypes. To study the influence of MatAb on the immune response to Pnc-TT early in life, adult female mice were immunized twice with Pnc-TT of serotype 1 (Pnc1-TT), and their offspring received Pnc1-TT subcutaneously three times at 3-week intervals starting at 1 week (neonatal) or 3 weeks (infant) of age. High levels of PPS-1-specific MatAb (>3 log) in offspring of Pnc1-TT-immunized dams completely inhibited their anti-PPS-1 response elicited by Pnc1-TT. In contrast, low or moderate ( approximately 1 to 2 log) levels of MatAb did not interfere with and even enhanced the immune response of the offspring, and a booster response to a second Pnc1-TT dose was observed. Carrier-specific MatAbs had little effect on the response of offspring to the conjugate. All Pnc1-TT-immunized offspring were protected against pneumococcal bacteremia and had reduced lung infection. These results demonstrate that in the presence of MatAb, Pnc1-TT may elicit a protective PPS-1-specific antibody response and prime for PPS-1-specific memory in young offspring. Importantly, low or moderate levels of PPS-1-specific MatAb not only provided protection against pneumococcal infections but also enhanced the immune response elicited by Pnc1-TT in neonatal and infant mice. This murine model will be used to develop novel strategies combining maternal and neonatal immunization to protect against infections caused by encapsulated bacteria in early life.

Haemophilus influenzae type b conjugate vaccines

Haemophilus influenzae type b (Hib) is one of the leading causes of invasive bacterial infection in young children worldwide. During childhood, acquisition of antibody directed against the polysaccharide capsule of the organism, presumably as a result of asymptomatic carriage, confers protection and disease is much less common after the age of 4 years. Like other polysaccharides, the polyribosyl ribitol phosphate (PRP) of the Hib capsule is a T-independent antigen and not immunogenic when administered as a vaccine in infancy. Because the highest rates of disease occur in the first 2 years of life, efficacious Hib vaccines have been designed by covalently linking the PRP capsule to a carrier protein that recruits T-cell help for the polysaccharide immune response and induces anti-PRP antibody production even in the first 6 months of life. Introduction of Hib protein-polysaccharide conjugate vaccines into many industrialized countries over the past 15 years has resulted in the virtual elimination of invasive Hib disease. However, despite the success of the vaccine programme several factors may interfere with the effectiveness of the vaccine in the routine programme, as observed in the UK recently. Such factors may include interference with other concomitant vaccines, waning immunity in the absence of booster doses of vaccine, and reduced natural boosting as a result of decreased transmission of the organism. However, the burden of disease remains highest in resource-poor countries and urgent efforts are needed to provide the benefits of this vaccine for children living in regions where it cannot be used for economic and logistical reasons.

Humoral immune responses to a protective peptide-conjugate against measles after different prime-boost regimens

The current live-attenuated measles vaccine leaves many children unprotected until they reach the recommended age of vaccination. We have previously shown that the short peptide corresponding to the hemagglutinin noose epitope (HNE) of the measles virus (MV) hemagglutinin protein induced virus-neutralizing antibodies even in the presence of protective levels of anti-whole virus-specific antibodies. Here we investigate the immunogenicity of HNE peptide-conjugates of diphtheria or tetanus toxoid in mice after active and passive priming with antibodies against the peptide, toxoids and conjugates. Both conjugates induced high titers of peptide antibodies which crossreacted with the virus and protected against a lethal intracranial challenge with a rodent-adapted measles virus, even after active priming with homologous or heterologous toxoid or conjugate. Peptide-specific epitopic suppression was stronger after passive priming with carrier or conjugate antibodies, but diphtheria toxoid as a carrier was less susceptible to suppression than tetanus toxoid and suppression was overcome by an additional boost. Furthermore, prior immunization with peptide-conjugate did not interfere with the development of a complete response to a subsequent injection of MV, suggesting that the benefits of a follow-up vaccination with the current live-attenuated vaccine would not be lost. These results underline the potential of these peptide-based conjugates as vaccine candidates for use in early infancy to close the window of susceptibility before the live-attenuated vaccine can be administered.

Current state of pneumococcal vaccines

Streptococcus pneumoniae is a leading cause of bacterial pneumonia, meningitis, and acute otitis media in children and adults worldwide. According to World Health Organization estimates, at least 1 million children under 5 years of age die each year from pneumococcal pneumonia. The emergence of resistant strains necessitates the development of an effective vaccine with a large serotype coverage. The 11 most common serotypes cause 72-83% of all serious pneumococcal diseases worldwide. Currently marketed 23-valent pneumococcal polysaccharide vaccine provides large serotype coverage and offers a less expensive option. However, it is efficacious only in adults but not in infants. Conjugate vaccines offer a solution by generating immunological memory already at early age. A recently licensed 7-valent conjugate vaccine is immunogenic and efficacious in infants. Its serotype coverage might be sufficient in Europe and North America, but not in Africa, Asia and Oceania. A need exists to develop pneumococcal vaccines with lower cost and larger serotype coverage. Several 11-valent pneumococcal conjugate vaccines are being evaluated in phase I-III trials. This study reviews the current state of pneumococcal problem and pneumococcal vaccines in clinical use.

Immunogenicity of anti-Haemophilus influenzae type b CRM197 conjugate following mucosal vaccination with oligodeoxynucleotide containing immunostimulatory sequences as adjuvant

Most vaccines are delivered by injection. Mucosal vaccination would increase compliance and decrease the risk of spread of infectious diseases due to a reduction of mucosal colonization and of contaminated syringes. However, most vaccines are unable to induce immune responses when administered mucosally, and require the use of strong adjuvant or effective delivery systems. Synthetic oligodeoxynucleotides (ODN) containing CpG immunostimulatory sequences (ISS) have been shown to act as potent adjuvants of type-1 immune responses also when mucosally co-administered with protein or peptide vaccines. We have shown that ISS can increase the anti-polysaccharide polyribosyl ribitol phosphate (PRP) antibody titres and anti-diphtheria toxin neutralizing antibody, if used as adjuvant of anti-Haemophilus influenzae type b (Hib) PRP vaccine conjugated with cross-reacting material (CRM) of diphtheria toxin in mice. Here, we show that ISS have the potential to increase host local and systemic antibody response against both the PRP and the protein component of a conjugated vaccine when mucosally administered in mice. Mucosal administration of Hib-CRM vaccine induced anti-PRP and neutralizing anti-diphtheria toxin antibodies of all the IgG subclasses, with a predominance of type-1 immune response-associated IgG2a and IgG3. At odds with systemic administration, the mucosal delivery of Hib-CRM induced anti-PRP and anti-diphtheria toxin mucosal IgA. These data envisage the feasibility of a mucosal vaccination with an already licensed Hib-CRM vaccine to achieve both an anti-H. influenzae and -diphtheria effective protection.

Neonatal and early life vaccinology

Preclinical and human vaccine studies indicate that, although neonatal immunisation does not generally lead to rapid and strong antibody responses, it may result in an efficient immunological priming, which can serve as an excellent basis for future responses. The apparent impairment of CD4 and CD8 T-cell function in early life seems to result from suboptimal antigen-presenting cells-T cell interactions, which can be overcome by use of specific adjuvants or delivery systems. Although persistence of maternal antibodies may limit infant antibody responses, induction of T-cell responses largely remain unaffected by these passively transferred antibodies. Thus, neonatal priming and early boosting with vaccine formulations optimised for sufficient early life immunogenicity and maximal safety profiles, could allow better control of the huge infectious disease burden in early life.

The adjuvant effect of synthetic oligodeoxynucleotide containing CpG motif converts the anti-Haemophilus influenzae type b glycoconjugates into efficient anti-polysaccharide and anti-carrier polyvalent vaccines

Synthetic oligodeoxynucleotides containing CpG immunostimulatory sequences (ISS) have been shown to act as potent adjuvants of type 1 immune responses when co-administered with protein or peptide vaccines. We have recently shown that ISS can increase the anti-polysaccharide (CHO) and anti-tetanus toxoid (TT) or anti-diphtheria (CRM) toxoid antibody levels if used as adjuvant of anti-Haemophilus influenzae type b (Hib) CHO vaccine conjugated with TT or CRM. The analysis of anti-TT and anti-CRM IgG subclasses showed a significant increase in IgG2a, IgG2b and/or IgG3 in the presence of ISS. Anti-TT and anti-CRM antibodies were shown to neutralize the activity of both the tetanus and diphtheria toxin in vivo or in vitro tests respectively. These data show that ISS have the potential to increase host antibody response against both the CHO and the protein component of a conjugated vaccine, and encourage the investigation to identify strategies of vaccination with schedules aimed at the valuation of protein carriers as protective immunogens.

Vaccination in the neonatal period and early infancy

Immune maturation is responsible for a progressive increase in antibody responses that can be elicited during the first year of life, such that neonatal immunization may currently not be expected to induce strong antibody responses. In contrast, B and T cell priming can be induced very early in life, without interference of maternal immunity. Strong IL-5 and IL-13 responses in young mice, and limited IL-12 and IFN-gamma release capacity by early life APC and T cells both in young mice and infants, could contribute to the severity of infections with intracellular pathogens in early life. It calls for evaluation of novel delivery systems, adjuvants and/or prime-boost immunization strategies capable to meet the challenge of both strong neonatal immunogenicity and acceptable reactogenicity. The extent to which early life murine immunization models may be useful for preclinical evaluation of infant responses is outlined in this review.

Natural Haemophilus influenzae type b capsular polysaccharide antibodies in 412 infants and children from West Africa (Burkina-Faso) and France: a cross-sectional serosurvey

Prior to possible introduction of large-scale vaccination programmes, an estimation and comparison of naturally acquired immunity against Haemophilus influenzae type b (Hib) was carried out in two populations of age-stratified infants and children (from birth to 14 years old) in Burkina-Faso (West Africa) (n = 206) and France (n = 206). Hib capsular polysaccharide antibodies were detected by an ELISA method. The difference in the percentages of minimum protective levels for the two populations were not significant (0.15 microg/ml) for newborns (0-1 month) but became significant as early as 2 to 3 months of age (p < 0.01) when lower levels were found among infants from Burkina-Faso. Subsequently, the percentages in both countries remained low until 11 months of age and showed no significant differences. For children between 12 and 35 months, the results > or = 0.15 microg/ml were significantly higher in France (p < or = 0.05). From 36 months, the percentage of minimum seropositivity increased in Burkina-Faso, so that the difference was no longer significant. In each country, the percentage of children with the minimum protective level varied significantly (p < or = 0.05) according to age (0-47 months). None of the children from Burkina-Faso or France had antibody levels > 1.0 microg/ml before one year of age. Thereafter, only 9.51% of French children in the 12- to 17-month age stratum and 19.2% over 4 years of age had antibody levels > 1.0 microg/ml. There were no non-detectable results for children over 4 years of age, and the means for natural detectable Hib CP antibodies were > 0.15 microg/ml for both populations. Hib invasive infections depend on climate, socioeconomic status and ethnic and genetic factors. In Burkina-Faso, the large number of infants and children under 4 years of age susceptible to Hib infections suggests that large scale vaccination programmes are needed soon after birth. However, it would first be necessary to evaluate such factors as the frequency of Hib diseases in this population.

Transplacental transmission of serotype-specific pneumococcal antibodies in a Brazilian population

The highest incidence of severe pneumococcal infections in children occurs in the first 6 months of life; however, immunization of infants with the existing polysaccharide vaccines is ineffective. We wished to determine the prevalence of immunoglobulin G (IgG) pneumococcal antibodies in unimmunized Brazilian mothers and their transplacental transmission to term and preterm infants. Total IgG, IgG1 and -2 subclass levels, and IgG antibodies against Streptococcus pneumoniae serotypes 1, 3, 6B, 9V, and 14 were determined in 15 pairs of mothers and term newborns (gestational age, >/=37 weeks) and in 18 pairs of mothers and preterm newborns (gestational age, 32 to 36 weeks). Serotype-specific anti-pneumococcal antibodies were detected by a recently standardized enzyme-linked immunosorbent assay calibrated with the 89-SF reference serum. Varying percentages of the mothers had antibody concentrations below arbitrarily defined protective levels: 33% for serotype 1, 67% for serotype 3, 30% for serotype 6B, 52% for serotype 9V, and 22% for serotype 14. In term newborns, IgG1 concentrations were slightly higher than maternal concentrations; in preterm newborns, the concentrations were much lower. Concentrations of IgG2 in term and preterm infants were significantly lower than in the mothers. Transplacental transmission of antibodies to serotypes 3 and 14 was clearly different from that of antibodies to serotypes 1, 6B, and 9V. Concentrations of IgG antibodies against serotypes 3 and 14 were similar to or higher than those of the mothers; against serotypes 1, 6B, and 9V they ranged from 77 to 83% of maternal concentrations in term newborns and also in preterm infants, although transplacental transmission of antibodies was proportionally lower for each specific serotype in preterm than in term infants. These data are relevant for developing strategies to protect infants against pneumococcal infections in the first months of life. Our findings and a review of existing information stress the importance of understanding the relationships among pneumococcal immunization, IgG subclass antibodies to individual serotypes, transplacental transport, half-life, and antibody function and their protective values against infection.

Effect of transplacentally acquired tetanus antibodies on the antibody responses to Haemophilus influenzae type b-tetanus toxoid conjugate and tetanus toxoid vaccines in Filipino infants

BACKGROUND

Pregnant women in developing countries are vaccinated with tetanus toxoid (TT) to prevent neonatal tetanus. In populations in which the maternal TT-vaccination program is efficiently implemented, responses of the infant to TT and TT-conjugated vaccines such as Haemophilus influenzae type b (Hib) capsular polysaccaride (PS) TT-conjugate (Hib-TT) vaccine may be depressed.

OBJECTIVES

To study the influence of transplacentally acquired anti-TT antibodies on responses to TT vaccination and to Hib-TT vaccine.

METHODS

One hundred ninety-four healthy Filipino infants received three doses of a Hib conjugate (either Hib-TT, PRP-OMP or HbOC) with diphtheria-tetanus-pertussis vaccine (DTP) given simultaneously but in a separate syringe at the age of 6, 10 and 14 weeks (primary series). In addition 54 of the study children received a booster dose of Hib-TT at 9 months simultaneously with the measles vaccine.

RESULTS

Transplacentally acquired anti-TT did not interfere with the anti-Hib PS antibody (anti-Hib PS) response to any of the conjugates. The transplacentally acquired anti-TT was not significantly associated with the concentration of anti-Hib PS either before or after the booster dose of Hib-TT. High concentrations (> or =1 IU/ml) of transplacentally acquired anti-TT inhibited the infants' anti-TT responses.

CONCLUSIONS

High concentration of transplacentally acquired anti-TT did not depress anti-Hib PS responses to the Hib-TT vaccine. On the other hand the high anti-TT concentrations somewhat depressed the anti-TT responses of the infants. However, the anti-TT concentrations attained were in the protective range in all study children after either the primary series (DTP + Hib-TT) or the booster dose of Hib-TT.

Early immunization with conjugate vaccines

Conjugate vaccines have been used during the neonatal period or in early infancy in order to test their safety and immunologic behaviour. Adverse events even in neonates and premature babies have been mild and spontaneously resolving. Anticapsular antibody concentrations after first doses of Hib conjugates in early infancy are generally low, but analysis of responses clearly demonstrates active antibody production. Concentrations after the whole primary series are at or close to the level achieved in routine vaccination programmes. Noteworthy, immunologic memory seems to be induced already by first doses during newborn period. These data suggest that protection against invasive Hib infections may be achieved early in life.

Determinants of infant responses to vaccines in presence of maternal antibodies

Presence of maternally-derived antibodies at time of immunization is known to often interfere with active infant immunization, although with variable degrees of clinical significance. In order to progressively decipher the rules that form the basis for these inhibitory effects on infant vaccine responses, two antigens (measles, tetanus) and various antigen presentation systems were evaluated in murine early life immunization models either in absence or presence of maternal antibodies. Both conventional (proteins, conjugate vaccines) and new (live viral vectors, DNA plasmids) antigen presentation systems were found to be similarly susceptible to the inhibitory influence of maternal antibodies. Factors emerging as crucial determinants of maternal antibody-mediated effects on responses to both live and non-live vaccines include (i) the level of maternal antibodies present at immunization, (ii) the use of distinct vaccines in mothers and pups and (iii) their distinct influence on B cell and T cell vaccine responses.