Protective levels of serum antibodies stimulated in infants by two injections of Haemophilus influenzae type b capsular polysaccharide-tetanus toxoid conjugate

Revisiting the dimensions of universal vaccine with special focus on COVID-19: Efficacy versus methods of designing

Even though the use of SARS-CoV-2 vaccines during the COVID-19 pandemic showed unprecedented success in a short time, it also exposed a flaw in the current vaccine design strategy to offer broad protection against emerging variants of concern. However, developing broad-spectrum vaccines is still a challenge for immunologists. The development of universal vaccines against emerging pathogens and their variants appears to be a practical solution to mitigate the economic and physical effects of the pandemic on society. Very few reports are available to explain the basic concept of universal vaccine design and development. This review provides an overview of the innate and adaptive immune responses generated against vaccination and essential insight into immune mechanisms helpful in designing universal vaccines targeting influenza viruses and coronaviruses. In addition, the characteristics, safety, and factors affecting the efficacy of universal vaccines have been discussed. Furthermore, several advancements in methods worthy of designing universal vaccines are described, including chimeric immunogens, heterologous prime-boost vaccines, reverse vaccinology, structure-based antigen design, pan-reactive antibody vaccines, conserved neutralizing epitope-based vaccines, mosaic nanoparticle-based vaccines, etc. In addition to the several advantages, significant potential constraints, such as defocusing the immune response and subdominance, are also discussed.

Anti-glycan antibodies: roles in human disease

Carbohydrate-binding antibodies play diverse and critical roles in human health. Endogenous carbohydrate-binding antibodies that recognize bacterial, fungal, and other microbial carbohydrates prevent systemic infections and help maintain microbiome homeostasis. Anti-glycan antibodies can have both beneficial and detrimental effects. For example, alloantibodies to ABO blood group carbohydrates can help reduce the spread of some infectious diseases, but they also impose limitations for blood transfusions. Antibodies that recognize self-glycans can contribute to autoimmune diseases, such as Guillain-Barre syndrome. In addition to endogenous antibodies that arise through natural processes, a variety of vaccines induce anti-glycan antibodies as a primary mechanism of protection. Some examples of approved carbohydrate-based vaccines that have had a major impact on human health are against pneumococcus, Haemophilus influeanza type b, and Neisseria meningitidis. Monoclonal antibodies specifically targeting pathogen associated or tumor associated carbohydrate antigens (TACAs) are used clinically for both diagnostic and therapeutic purposes. This review aims to highlight some of the well-studied and critically important applications of anti-carbohydrate antibodies.

Glycoconjugate Vaccines for Prevention of Haemophilus influenzae Type b Diseases

This review summarizes the experience in laboratory- and industrial-scale syntheses of glycoconjugate vaccines used for prevention of infectious diseases caused by Haemophilus influenzae type b bacteria based on the linear capsular polysaccharide poly-3-β-D-ribosyl-(1→1)-D-ribitol-5-phosphate (PRP) or related synthetic oligosaccharide ligands. The methods for preparation of related oligosaccharide derivatives and results of the studies evaluating effect of their length on immunogenic properties of the conjugates with protein carriers are overviewed.

A guide to vaccinology: from basic principles to new developments

Immunization is a cornerstone of public health policy and is demonstrably highly cost-effective when used to protect child health. Although it could be argued that immunology has not thus far contributed much to vaccine development, in that most of the vaccines we use today were developed and tested empirically, it is clear that there are major challenges ahead to develop new vaccines for difficult-to-target pathogens, for which we urgently need a better understanding of protective immunity. Moreover, recognition of the huge potential and challenges for vaccines to control disease outbreaks and protect the older population, together with the availability of an array of new technologies, make it the perfect time for immunologists to be involved in designing the next generation of powerful immunogens. This Review provides an introductory overview of vaccines, immunization and related issues and thereby aims to inform a broad scientific audience about the underlying immunological concepts.

This Review, aimed at a broad scientific audience, provides an introductory guide to the history, development and immunological basis of vaccines, immunization and related issues to provide insight into the challenges facing immunologists who are designing the next generation of vaccines.

Antigen-specific immunoglobulin variable region sequencing measures humoral immune response to vaccination in the equine neonate

The value of prophylactic neonatal vaccination is challenged by the interference of passively transferred maternal antibodies and immune competence at birth. Taken our previous studies on equine B cell ontogeny, we hypothesized that the equine neonate generates a diverse immunoglobulin repertoire in response to vaccination, independently of circulating maternal antibodies. In this study, equine neonates were vaccinated with 3 doses of keyhole limpet hemocyanin (KLH) or equine influenza vaccine, and humoral immune responses were assessed using antigen-specific serum antibodies and B cell Ig variable region sequencing. An increase (p<0.0001) in serum KLH-specific IgG level was measured between days 21 and days 28, 35 and 42 in vaccinated foals from non-vaccinated mares. In vaccinated foals from vaccinated mares, serum KLH-specific IgG levels tended to increase at day 42 (p = 0.07). In contrast, serum influenza-specific IgG levels rapidly decreased (p≤0.05) in vaccinated foals from vaccinated mares within the study period. Nevertheless, IGHM and IGHG sequences were detected in KLH- and influenza- sorted B cells of vaccinated foals, independently of maternal vaccination status. Immunoglobulin nucleotide germline identity, IGHV gene usage and CDR length of antigen-specific IGHG sequences in B cells of vaccinated foals revealed a diverse immunoglobulin repertoire with isotype switching that was comparable between groups and to vaccinated mares. The low expression of CD27 memory marker in antigen-specific B cells, and of cytokines in peripheral blood mononuclear cells upon in vitro immunogen stimulation indicated limited lymphocyte population expansion in response to vaccine during the study period.

Targeting regulatory T cells to improve vaccine immunogenicity in early life

Human newborns and infants are bombarded with multiple pathogens on leaving the sterile intra-uterine environment, and yet have suboptimal innate immunity and limited immunological memory, thus leading to increased susceptibility to infections in early life. They are thus the target age group for a host of vaccines against common bacterial and viral pathogens. They are also the target group for many vaccines in development, including those against tuberculosis (TB), malaria, and HIV infection. However, neonatal and infant responses too many vaccines are suboptimal, and in the case of the polysaccharide vaccines, it has been necessary to develop the alternative conjugated formulations in order to induce immunity in early life. Immunoregulatory factors are an intrinsic component of natural immunity necessary to dampen or control immune responses, with the caveat that they may also decrease immunity to infections or lead to chronic infection. This review explores the key immunoregulatory factors at play in early life, with a particular emphasis on regulatory T cells (Tregs). It goes on to explore the role that Tregs play in limiting vaccine immunogenicity, and describes animal and human studies in which Tregs have been depleted in order to enhance vaccine responses. A deeper understanding of the role that Tregs play in limiting or controlling vaccine-induced immunity would provide strategies to improve vaccine immunogenicity in this critical age group. New adjuvants and drugs are being developed that can transiently suppress Treg function, and their use as part of human vaccination strategies against infections is becoming a real prospect for the future.

Maternal vaccination: moving the science forward

BACKGROUND

Infections remain one of the leading causes of morbidity in pregnant women and newborns, with vaccine-preventable infections contributing significantly to the burden of disease. In the past decade, maternal vaccination has emerged as a promising public health strategy to prevent and combat maternal, fetal and neonatal infections. Despite a number of universally recommended maternal vaccines, the development and evaluation of safe and effective maternal vaccines and their wide acceptance are hampered by the lack of thorough understanding of the efficacy and safety in the pregnant women and the offspring.

METHODS

An outline was synthesized based on the current status and major gaps in the knowledge of maternal vaccination. A systematic literature search in PUBMED was undertaken using the key words in each section title of the outline to retrieve articles relevant to pregnancy. Articles cited were selected based on relevance and quality. On the basis of the reviewed information, a perspective on the future directions of maternal vaccination research was formulated.

RESULTS

Maternal vaccination can generate active immune protection in the mother and elicit systemic immunoglobulin G (IgG) and mucosal IgG, IgA and IgM responses to confer neonatal protection. The maternal immune system undergoes significant modulation during pregnancy, which influences responsiveness to vaccines. Significant gaps exist in our knowledge of the efficacy and safety of maternal vaccines, and no maternal vaccines against a large number of old and emerging pathogens are available. Public acceptance of maternal vaccination has been low.

CONCLUSIONS

To tackle the scientific challenges of maternal vaccination and to provide the public with informed vaccination choices, scientists and clinicians in different disciplines must work closely and have a mechanistic understanding of the systemic, reproductive and mammary mucosal immune responses to vaccines. The use of animal models should be coupled with human studies in an iterative manner for maternal vaccine experimentation, evaluation and optimization. Systems biology approaches should be adopted to improve the speed, accuracy and safety of maternal vaccine targeting.

Carrier priming or suppression: understanding carrier priming enhancement of anti-polysaccharide antibody response to conjugate vaccines

INTRODUCTION

With the availability of newer conjugate vaccines, immunization schedules have become increasingly complex due to the potential for unpredictable immunologic interference such as 'carrier priming' and 'carrier induced epitopic suppression'. Carrier priming refers to an augmented antibody response to a carbohydrate portion of a glycoconjugate vaccine in an individual previously primed with the carrier protein. This review aims to provide a critical evaluation of the available data on carrier priming (and suppression) and conceptualize ways by which this phenomenon can be utilized to strengthen vaccination schedules.

METHODS

We conducted this literature review by searching well-known databases to date to identify relevant studies, then extracted and synthesized the data on carrier priming of widely used conjugate polysaccharide vaccines, such as, pneumococcal conjugate vaccine (PCV), meningococcal conjugate vaccine (MenCV) and Haemophilus influenzae type b conjugate vaccines (HibV).

RESULTS

We found evidence of carrier priming with some conjugate vaccines, particularly HibV and PCV, in both animal and human models but controversy surrounds MenCV. This has implications for the immunogenicity of conjugate polysaccharide vaccines following the administration of tetanus-toxoid or diphtheria-toxoid containing vaccine (such as DTP).

CONCLUSION

Available evidence supports a promising role for carrier priming in terms of maximizing the immunogenicity of conjugate vaccines and enhancing immunization schedule by making it more efficient and cost effective.

Investigation of non-typeable Haemophilus influenzae outer membrane protein P6 as a new carrier for lipooligosaccharide conjugate vaccines

Non-typeable Haemophilus influenzae (NTHi) outer membrane protein P6 was used as a new protein carrier for NTHi detoxified lipooligosaccharide (dLOS) conjugates due to its conservation and potential to elicit bactericidal antibodies. P6 was covalently conjugated to dLOS of strain 9274 through adipic acid dihydrazide with different ratios of dLOS to P6, which resulted in two conjugate formulations with weight ratios of dLOS to P6 of 3.7 for dLOS-P6 (I) and 1.6 for dLOS-P6 (II). Binding activity of the conjugates was examined by an enzyme-linked immunosorbent assay with mouse monoclonal antibodies specific to LOS and P6 and a rabbit anti-P6 serum. The results showed that the conjugates bound not only to the LOS antibody but also to both P6 antibodies, suggesting that the conjugates retained epitopes of both LOS and P6 antigens. Animal studies revealed that dLOS-P6 (II) induced high levels of anti-LOS and anti-P6 IgGs in mice and rabbits. However, dLOS-P6 (I) induced lower levels of anti-LOS IgGs in mice and rabbits and anti-P6 IgGs in rabbits with no anti-P6 IgGs in mice. In addition, all rabbit, but not mouse, antisera elicited by the conjugates showed bactericidal activity against the homologous strain, and two of them elicited by each conjugate plus Ribi adjuvant showed cross-bactericidal activity against three of five major serotype stains. These data indicate that P6 could serve as an effective carrier for dLOS or other carbohydrate conjugates and that the ratio of carbohydrate to P6 might contribute to immune responses in vivo.

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.

Immunization of female mice with glycoconjugates protects their offspring against encapsulated bacteria

The immune system of the newborn is immature, and therefore it is difficult to induce protective immunity by vaccination in the neonatal period. Immunization of mothers during pregnancy against infections caused by encapsulated bacteria could thus be particularly attractive, as infants do not respond to polysaccharide (PS) antigens. Transmission of maternal vaccine-specific antibodies and protection of offspring against pneumococcal bacteremia and/or lung infection were studied in a neonatal murine model of pneumococcal immunization and infections. Adult female mice were immunized with native pneumococcal PS (PPS) of serotypes 1, 6B, and 19F or PPS conjugated to tetanus protein (Pnc-TT), and PPS-specific antibodies were measured in sera of mothers and their offspring. Effective transmission of maternal antibodies was observed, as PPS-specific immunoglobulin G levels in 3-week-old offspring of immunized mothers were 37 to 322% of maternal titers, and a significant correlation between maternal and offspring antibody levels was observed. The PPS-specific antibodies persisted for several weeks but slowly decreased over time. Offspring of Pnc-TT-immunized mothers were protected against pneumococcal infections with homologous serotypes, whereas PPS immunization of mothers did not protect their offspring, in agreement with the low titer of maternal PPS specific antibodies. When adult female mice were immunized with a meningococcal serogroup C conjugate vaccine (MenC-CRM), antibody response and transmission were similar to those observed for pneumococcal antibodies. Importantly, bactericidal activity was demonstrated in offspring of MenC-CRM-immunized mothers. These results demonstrate that this murine model of pneumococcal immunization and infections is suitable to study maternal immunization strategies for protection of offspring against encapsulated bacteria.

Antibody response to Haemophilus influenzae type b tetanus conjugate vaccine with two doses given at 3 and 5 months of age

BACKGROUND

In developed countries, the use of Hib conjugate vaccines has led to the near disappearance of invasive Hib disease, but costs have limited its use in developing countries. In order to identify more economical vaccination schedules, we carried out a trial to evaluate the immunogenicity of an alternative two-dose PRP-T regimen, based on a previous report in which carrier priming could be obtained with prior diphtheria-tetanus-pertussis (DTP) vaccination.

METHODS

Healthy infants were enrolled to receive the PRP-T given at 3 and 5 months of age, with DTP vaccination given at 2, 4 and 6 months of age. Serum specimens were obtained at 3, 6 and 15 months of age. IgG anti-Hib titer determination was performed using enzyme-linked immunosorbent assay to evaluate serologic response and its duration.

RESULTS

One-hundred and seventeen infants were enrolled. The geometric mean titer (GMT) of antibody to PRP was low in the pre-immunization samples (0.13 mg/mL), achieving high values after two doses of PRP-T (27.42 mg/mL), with all titers over 1 mg/mL; the GMT at 15 months was 5.45 mg/mL; 94.6% of infants had serologic responses after the two doses of vaccination, with average intervals of 27 and 22 days between DTP and PRP-T first-to-first and second-to-second administrations, respectively. However, these intervals were 11 and 3 days for infants who did not have serologic responses (P50.0013 and 0.0030, respectively).

CONCLUSIONS

These results indicate that two doses of PRP-T can induce high antibody titers using the proposed schedule; moreover, the GMT assessed at 15 months of age was also protective. The enhanced immune response observed in the study could be explained by the previous administration of the DTP vaccine, since the longer the interval between DTP and PRP-T, the better the response to Hib vaccine. The PRP-T vaccine given at 3 and 5 months of age may be an economical alternative to the current proposed schedule, which could make the introduction of Hib vaccination in developing countries more feasible, considering the relatively high cost of this vaccine.

Safety and antibody persistence following Haemophilus influenzae type b conjugate or pneumococcal polysaccharide vaccines given before pregnancy in women of childbearing age and their infants

BACKGROUND

Immunization of healthy women before pregnancy is a potential approach to providing increased levels of maternal antibody to newborns to protect them from infections occurring during the perinatal period and first months of life.

METHODS

Healthy nonpregnant Pima Indian women of childbearing age were randomized to receive one of two Haemophilus influenzae type b (Hib) conjugate vaccines [HbOC or Hib-meningococcal outer membrane protein complex (OMP)] or a 23-valent pneumococcal polysaccharide vaccine (PnPs). Infants received Hib-OMP vaccine at 2, 4 and 12 months of age. Vaccine safety and immunogenicity was evaluated in the women and their infants.

RESULTS

Anti-polyribose ribitol phosphate antibody titers were significantly higher in women in both Hib conjugate vaccine groups than in the pneumococcal vaccine group throughout the 37-month observation period. Antibody responses to HbOC vaccine were significantly higher than those to Hib-OMP. A subsequent booster dose of each Hib conjugate vaccine induced reactions and antibody responses similar to those of the first dose. Infants born to mothers immunized with Hib vaccines compared with PnPs had significantly higher polyribose ribitol phosphate-specific IgG antibody titers at birth and 2 months of age but lower antibody responses to Hib-OMP at 6 months and similar titers before and after boosting with Hib-OMP at 1 year of age. By contrast women immunized with PnPs did not have significantly elevated concentrations of pneumococcal-specific antibody at delivery, and their infants had pneumococcal antibody titers similar to those of infants born to mothers who did not receive pneumococcal vaccine before pregnancy.

CONCLUSION

Hib conjugate vaccine given to women before pregnancy significantly increased the proportion of infants who had protective Hib antibody levels at birth and 2 months of age.

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.

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.

Differential expression of Vibrio vulnificus capsular polysaccharide

Vibrio vulnificus is a human pathogen whose virulence has been associated with the expression of capsular polysaccharide (CPS). Multiple CPS types have been described; however, virulence does not appear to correlate with a particular CPS composition. Reversible-phase variation for opaque and translucent colony morphologies is characterized by changes in CPS expression, as suggested by electron microscopy of cells stained nonspecifically with ruthenium red. Isolates with opaque colony morphologies are virulent and appear to be more thickly encapsulated than naturally occurring translucent-phase variants, which have reduced, patchy, or absent CPS. Previously, we have shown that the virulence of translucent-phase variants was intermediate between opaque-phase variants and acapsular transposon mutants, suggesting a correlation between virulence and the amount of CPS expressed. In the present study, CPS expression of phase variants and genetically defined mutants of V. vulnificus M06-24/O was examined by using a CPS-specific monoclonal antibody with an enzyme-linked immunosorbent assay, flow cytometry, and immunoelectron microscopy. Semiquantitative analyses of CPS expression correlated well among these assays, confirming that the translucent-phase variant was intermediate in CPS expression and retained type I CPS-specific epitopes. Cell surface expression of CPS varied with the growth phase, increasing during logarithmic growth and declining in stationary culture. Significantly greater CPS expression (P = 0.026) was observed for cells grown at 30 degrees C than for those at 37 degrees C. These studies confirm that phase variation and virulence in V. vulnificus correlate with the amount of CPS expressed and demonstrate the fluidity of bacterial polysaccharide expression in response to environmental conditions.

Antibody response to accelerated Hib immunisation in preterm infants receiving dexamethasone for chronic lung disease

AIM

To study the effect of dexamethasone on the routine immunisation of preterm infants with chronic lung disease.

METHODS

Serum samples were obtained before and after immunisation from an unselected cohort of 59 preterm infants. Haemophilus influenzae antibodies were measured using an ELISA method and differences in the geometric mean values between the two groups of babies analysed.

RESULTS

Sixteen infants received no dexamethasone. Before and after immunisation antibody titres for those receiving no dexamethasone were 0.16 and 4.63 mcg IgG/ml. Corresponding values for those receiving dexamethasone were 0.10 and 0.51 mcg IgG/ml, respectively.

CONCLUSION

Dexamethasone used in the treatment of chronic lung disease seems to significantly affect the antibody response of preterm infants to immunisation against Haemophilus influenzae.

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.

Placental antibody transfer: influence of maternal HIV infection and placental malaria

AIM

To determine the influence of placental malaria, maternal HIV infection, and maternal hypergammaglobulinaemia on transplacental IgG antibody transfer.

METHODS

One hundred and eighty materno-neonatal pairs from a Malawian population were assessed. Cord and maternal serum samples were tested for total serum IgG antibody titres using nephelometry, and for specific IgG antibody titres to Streptococcus pneumoniae, measles, and tetanus toxoid antibodies using an enzyme linked immunosorbent assay (ELISA).

RESULTS

Multiple regression analyses showed that placental malaria was associated with a decrease in placental IgG antibody transfer to S pneumoniae and measles to 82% and 81%, respectively. Maternal HIV infection was associated with a reduction in IgG antibody transfer to S pneumoniae to 79%; raised maternal total serum IgG titres were correlated with S pneumoniae and measles IgG antibody transfer reduction to 86% and 87%, respectively. No effect was seen with tetanus toxoid antibody transfer.

CONCLUSION

The combined influence of placental malaria, maternal HIV infection, and maternal hypergammaglobulinaemia seems to be linked to the low transplacental antibody transfer observed in the Malawian population.

γ3 Gene-Disrupted Mice Selectively Deficient in the Dominant IgG Subclass Made to Bacterial Polysaccharides Undergo Normal Isotype Switching After Immunization with Polysaccharide-Protein Conjugate Vaccines

Bacterial polysaccharides (PS) are T-independent type 2 Ags that elicit restricted Ab responses of IgM and IgG3 in mice and IgM and predominantly IgG2 in humans. Immunodeficiency in the dominant IgG subclass made to PS is associated with chronic sinus and pulmonary infections with PS-encapsulated bacteria. To elucidate the biologic role of the dominant IgG subclass in the immune response to PS and to make an animal model of human IgG subclass deficiency, we generated mice with a targeted disruption of the exon encoding the CH1 domain of the γ3 heavy-chain constant region gene. Homozygotes had no detectable serum IgG3, and their splenocytes did not produce IgG3 after LPS stimulation. IgG3−/− mice immunized with PS from Pseudomonas aeruginosa LPS O-side chain or Streptococcus pneumoniae type 19F capsule did not produce any IgG3 anti-PS Abs, in contrast to wild-type mice in which IgG3 was the major IgG subclass. Immunizing both wild-type and IgG3−/− mice with 19F PS-protein conjugate elicited IgG1 Abs. We conclude that IgG3−/− mice have a selective deficiency in the dominant murine IgG subclass made to T-independent type 2 Ags and may be a useful animal model of IgG subclass deficiency. In addition, we show that the anti-PS Ab class switching to IgG1 that occurs when mice are immunized with a PS-protein conjugate vaccine does not require sequential Ig expression or an intact, upstream γ3 heavy-chain gene.

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.

Economisation of vaccination against Haemophilus influenzae type b: a randomised trial of immunogenicity of fractional-dose and two-dose regimens

BACKGROUND

The cost of Haemophilus influenzae type b (Hib) conjugate vaccines has limited their use in non-industrialised countries. To identify more economical vaccination schedules, we carried out a randomised trial of the immunogenicity of alternative regimens to the standard three-dose series.

METHODS

627 Chilean infants were randomly allocated to one of four regimens with either Hib polysaccharide-tetanus toxoid conjugate vaccine (PRP-T) or Hib oligosaccharide-diphtheria mutant toxoid conjugate vaccine (PRP-CRM197), for a total of eight groups. All infants receive diphtheria-tetanus-pertussis (DTP) vaccine at ages 2, 4, and 6 months. The regimens included three full doses, three fractional doses consisting of one half or one third of the full dose, and a regimen of two full doses (at age 4 and 6 months). The primary outcome was the proportion of infants with serum anti-polyribosylribitol phosphate (PRP, the type b capsular polysaccharide) concentrations of 0.15 microg/mL or more at age 8 months.

FINDINGS

93% (95% CI 85-98) of infants vaccinated with three full doses of PRP-T or PRP-CRM197 (95% CI 84-98) achieved anti-PRP concentrations of 0.15 microg/mL or more at age 8 months, compared with 91% (83-96) to 100% (95-100) of infants immunised with any fractional-dose regimen. Of the infants vaccinated with two doses of PRP-T or PRP-CRM197, 99% (93-100) and 87% (77-93) developed anti-PRP concentrations of 0.15 microg/mL or more, respectively.

INTERPRETATION

91% (83-96) to 100% (95-100) of infants immunised with one-half or one-third of a full dose of Hib conjugate developed protective antibody concentrations. Carrier priming with DTP may make two-dose schedules an option in some places. These alternative regimens could bring the cost of Hib vaccines within reach of countries that currently cannot afford them.

Immunogenicity and tolerance of H. influenzae type b, tetanus toxoid conjugate vaccine given concurrently or in combination

The present prospective, open, controlled, randomised comparative trial was undertaken to evaluate the sero response and side effects of PRP-T Conjugate Vaccine (ACT-HIB) in infants and children aged 2 months and 16-24 months. Fifty four babies aged 2 months formed group A, 56 children aged 16-24 months formed group B. Groups A and B were further subdivided into two sub groups each destined to receive either PRP-T vaccine in association with DPT vaccine at different sites (I) or PRP-T and DPT both vaccines at the same site mixed in the same syringe (II). Group A received 3 doses at 2, 3 and 4 months of age and group B received one dose between 16-24 months. The Geometric mean titres of Anti PRP antibodies observed in primary immunisation schedule (A) and single dose vaccination schedule (B) were comparable and significantly higher to prevaccination titres. A serum anti PRP level of > 1.0 mcg/ml after immunisation is believed to correlate with long term protection. Ninety-six percent of infants in Group A and 98% in Group B achieved titres > 1.0 mcg/ml. The side effects were minimal, local and were comparable between the study and control groups, suggesting that PRP-T vaccine is highly immunogenic and well tolerated in Indian infants and children.

Immune responses of infants vaccinated with serotype 6B pneumococcal polysaccharide conjugated with tetanus toxoid

BACKGROUND

Streptococcus pneumoniae is a major cause of meningitis, bacteremia, pneumonia and otitis media. Pneumococcal polysaccharides are not immunogenic in infants, but improved immunogenicity of polysaccharide-protein conjugates has been demonstrated. Antibiotic-resistant pneumococci have increased the need for an effective vaccine.

OBJECTIVE

To study the safety and immunogenicity of a pneumococcal type 6B polysaccharidetetanus toxoid conjugate (Pn6B-TT) in infants and to assess the function of antibodies.

METHODS

Healthy infants were injected, Group A at 3, 4 and 6 months (n = 21) and Group B at 7 and 9 months (n = 19). Booster injection was given at 18 months. Antibodies were measured by enzyme-linked immunosorbent assay and radioimmunoassay, and functional activity was measured by opsonization of radiolabeled pneumococci. Nasopharyngeal cultures were obtained.

RESULTS

No significant adverse reactions were observed. Pn6B-IgG (enzyme-linked immunosorbent assay) increased to a geometric mean of 0.62 microgram/ml (P = 0.367, compared with prevaccination titers) in Group A at 7 months and 1.22 micrograms/ml (P < 0.001) in Group B at 10 months. Total Pn6B antibodies (radioimmunoassay) were 44 ng of antibody N/ml (P < 0.053) in Group A and 211 ng of antibody N/ml (P < 0.001) in Group B. A smaller increase in IgM and IgA anti-Pn6B was observed. Reinjection at 18 months elicited booster responses in total and IgG anti-Pn6B; 62% of those in Group A and 79% of those in Group B had > 300 ng of antibody N/ml. Opsonic activity, after initial and booster vaccinations, correlated with Pn6B-antibody titers. Three infants with nasopharyngeal cultures repeatedly positive for serogroup 6 had poor serum IgG responses.

CONCLUSION

Our results demonstrate that Pn6B-TT is safe, elicits functional antibodies and memory responses in infants.

Factors associated with superior antibody responses to a single dose of Haemophilus influenzae type b-tetanus toxoid conjugate vaccine administered to Chilean infants at 2 months of age

The anticapsular antibody response of Chilean infants to a single dose of Haemophilus influenzae type b capsular polysaccharide-tetanus toxoid conjugate, vaccine is substantially higher than that observed among infants of similar age from the USA. Comparison of selected demographic and environmental factors indicates that low maternal education and a greater number of persons in the home are significantly associated with the superior responder phenotype. High anticapsular antibody responses were associated with high antibody responses to tetanus toxoid, the carrier protein in this conjugate, but not to diphtheria toxoid. These data suggest that environmental factors may enhance the magnitude of the primary antibody response to PRP-T vaccine.

Priming with diphtheria-tetanus-pertussis vaccine enhances the response to the Haemophilus influenzae type b tetanus conjugate vaccine in infancy

Haemophilus influenzae type b (Hib) capsular polysaccharide (PS) conjugated to tetanus toxoid (PRP-T) was given at 4 and 6 months of age and anti-Hib PS antibody response to the first and second dose of PRP-T was compared in groups that received diphtheria-tetanus-pertussis (DTP) vaccine either simultaneously with PRP-T (34 infants) or separately at 3, 4 and 5 months of age (49 infants). The geometric mean anti-Hib PS antibody concentration after the first dose of PRP-T given at 4 months of age was eightfold higher if the infants primed with DTP at 3 months of age than if the first dose of DTP was given together with the first dose of PRP-T (0.81 microgram ml-1 vs 0.11 microgram ml-1). The positive influence of DTP priming was seen also after the second dose of PRP-T given at 6 months of age (7.55 micrograms ml-1 vs 3.45 micrograms ml-1).

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

OBJECTIVE

To study the influence of maternally inherited tetanus antitoxin (anti-TT) antibodies on the response to the Haemophilus influenzae type b (Hib) capsular polysaccharide (PS)-tetanus toxoid conjugate (PRP-T) vaccine.

DESIGN

One hundred thirty healthy infants received their first dose of PRP-T in the same syringe with diphtheria-tetanus-pertussis vaccine (DTP) at 1 to 2 months, and 66 of them received a second dose at 3 to 4 months of age.

RESULTS

Maternal anti-TT antibodies did not interfere with the anti-Hib PS response to the first PRP-T vaccination; the geometric mean concentration (GMC) of anti-Hib PS was 0.14 microgram/ml in those with the lowest preimmunization anti-TT (< 0.3 IU/ml, n = 15) and 0.13 microgram/ml in those with the highest anti-TT (> or = 3 IU/ml, n = 25). After the second dose of PRP-T there was a positive correlation (r = 0.37, P = 0.004) between the anti-Hib PS response and the preimmunization anti-TT; those with the lowest preimmunization anti-TT (< 0.3 IU/ml, n = 9) achieved GMC of anti-Hib PS of 1.22 micrograms/ml and those with anti-TT > or = IU/ml (n = 22) anti-Hib PS GMC of 2.67 micrograms/ml. High preimmunization anti-Hib PS antibodies did not interfere with the final antibody concentrations; the GMC of anti-Hib PS after the second dose of PRP-T was 1.60 micrograms/ml in those with a preimmunization titer > or = 1.0 microgram/ml (n = 12) and 1.57 micrograms/ml in those with a titer of < 1.0 microgram/ml (n = 53).

CONCLUSION

The data suggest that infants can be safely vaccinated with PRP-T even though they have received high concentrations of anti-TT from their mother.

Comparison of five different vaccination schedules with Haemophilus influenzae type b-tetanus toxoid conjugate vaccine

OBJECTIVE

To compare seroresponses to five different vaccination schedules for Haemophilus influenzae type b (Hib)-tetanus toxoid conjugate (PRP-T) vaccine in infants.

DESIGN

Four different two-dose schedules were compared, with doses given at 1 and 3 months, 2 and 4 months, 2 and 6 months, or 4 and 6 months of age. One group received three doses at 2, 4, and 6 months of age. The PRP-T vaccine was given in the same syringe with diphtheria-tetanus-pertussis (DTP) vaccine; inactivated polio vaccine (IPV) was given in a separate syringe. Anti-Hib polysaccharide antibodies were measured by radioimmunoassay in sera taken before each immunization, 1 month after the second dose, at 7 and at 12 to 24 months of age.

SUBJECTS

A total of 196 healthy infants were enrolled between November 1990 and November 1992.

RESULTS

After one dose of PRP-T there were no significant differences in geometric mean antibody concentrations (0.09 to 0.13 microgram/ml) or in fold responses among the schedules. The response to the second dose was significantly higher than the response to the first dose given at the same age. The geometric mean antibody concentration was lower in the group vaccinated at 1 and 3 months than in the groups vaccinated at 2 and 4 months, 2 and 6 months, or 4 and 6 months. The three-dose schedule resulted in a significantly higher final antibody concentration than the best two-dose schedule (p <0.001). In most children (64% to 93%), the antibody concentration remained at least 0.15 microgram/ml up to the age of 12 to 24 months.

CONCLUSIONS

The Hib conjugate vaccine, PRP-T, administered concomitantly with DTP vaccine and IPV, was immunogenic with schedules starting at 1 to 4 months of age. Two injections of PRP-T vaccine were immunogenic enough to maintain protection up to 12 to 18 months of age.

Serum, breast milk, and infant antibody after maternal immunisation with pneumococcal vaccine

Pneumococci are a leading cause of severe bacterial disease in infants and children world wide. A possible means of protecting infants in the first few months of life is immunisation of the mother during pregnancy. We prospectively assessed pneumococcal immunisation of pregnant women to determine the amount of pneumococcal antibody transmitted to the infants in serum and milk and the half-life of the passively acquired antibody. Healthy pregnant women in Dhaka, Bangladesh, were randomised to receive pneumococcal or meningococcal vaccine with routine prenatal tetanus immunisation at 30-34 weeks of gestation. Serum and breast milk specimens from the mothers and sera from infants were collected up to 22 weeks of age and assayed for specific serum IgG, IgG1, and IgG2 and for milk IgA antibodies to pneumococcal serotypes 6B and 19F. 55 mothers and 56 infants were followed from birth to five months. Women who received pneumococcal vaccine had geometric mean antibody increases of 2.6 and 3.4 to types 6B and 19F, respectively. The mean infant/maternal antibody ratios were 0.56 and 0.59 (range 0.11-1.46) for these serotypes. Infant cord antibody titres correlated with maternal titres. Infant/maternal IgG ratios correlated with the interval between immunisation and birth and were higher for specific IgG1 than for IgG2. Infants of pneumococcal vaccine recipients had geometric mean antibody concentrations of 6.8 and 7.5 micrograms/mL to serotypes 6B and 19F in cord blood; in cord blood and in all subsequent serum specimens the concentrations were 2-3 fold higher than in control infants. The median half-life of passive antibody was about 35 days; at five months of age 63-71% of infants of pneumococcal vaccine recipients had antibody concentrations greater than 0.15 micrograms/mL. Breast milk IgA antibodies for pneumococcal serotype 19F, but not for type 6B, were significantly higher in vaccine recipients up to five months after delivery. If maternal pneumococcal polysaccharide antibodies do not interfere with active immunisation of the infant with new glycoprotein conjugate pneumococcal vaccines, passive-active immunisation of infants can be a feasible strategy for developing regions.

Avidity of specific IgG antibodies elicited by immunisation against Haemophilus influenzae type b

AIM

To investigate the avidity of specific IgG polyribosyl ribitol phosphate (PRP) antibodies induced by three Haemophilus influenzae type b (Hib) conjugate vaccines: PRP-meningococcal outer membrane protein complex (PRP-OMP), PRP-non-toxic mutant diphtheria toxin, CRM197 (HbOC) and PRP-tetanus toxoid (PRP-T).

METHODS

One hundred and ten infants were immunised with one of the vaccines at two, three and four months of age. Blood samples were taken after each vaccination and serum stored at -20 degrees C. Serum samples collected after the third course (that is, at five months of age) were submitted to antibody avidity assessment, using a urea enzyme linked immunosorbent assay (ELISA).

RESULTS

All three conjugate vaccines elicited IgG PRP antibodies of high median avidity. The resultant antibody populations were heterogeneous with regard to avidity, which in turn was independent of PRP antibody concentration.

CONCLUSIONS

With the recent findings of a correlation between bactericidal killing and affinity, our data highlight the need for a protective level to be expressed qualitatively as well as quantitatively.

Laboratory and preliminary clinical characterization of Vi capsular polysaccharide-protein conjugate vaccines

To improve its immunogenicity for children and adults and to make it suitable for routine immunization of infants against typhoid fever, the capsular polysaccharide of Salmonella typhi (Vi) was bound to the B subunit of the heat-labile toxin (LT-B) of Escherichia coli or the recombinant exoprotein A (rEPA) of Pseudomonas aeruginosa. The conjugates elicited higher levels of antibodies (micrograms per milliliter of serum) in mice and in guinea pigs than did Vi and, unlike Vi alone, elicited booster antibody responses in both species. In adult volunteers, Vi-LT-B and Vi-rEPA, respectively, elicited higher levels of antibodies than Vi alone after the first injection (4.74 versus 1.77 and 4.91 versus 1.77; P < 0.005) and 26 weeks later (2.32 and 2.69 versus 0.54; P < 0.04); a second injection of the conjugates did not elicit a booster response of Vi antibodies. None of the 51 vaccinees had fever or significant local reactions. Vi-rEPA elicited slightly higher levels of Vi antibodies than did Vi-LT-B at all intervals after injection, but these differences were not significant. Each conjugate elicited antibodies to its carrier protein. The antibody responses elicited in adults by Vi bound to LT-B and rEPA are similar to those of other polysaccharide-protein conjugates. These conjugates promise to be an improved Vi vaccine. Studies of Vi conjugates with adults and infants in areas where typhoid is endemic are planned.

Heavy-chain isotype patterns of human antibody-secreting cells induced by Haemophilus influenzae type b conjugate vaccines in relation to age and preimmunity

The influence of preexisting immunity on the heavy-chain isotypes of circulating antibody-secreting cells (AbSC) induced by vaccination with Haemophilus influenzae type b (Hib) capsular polysaccharide (HibCP) coupled to tetanus toxoid (TT) or diphtheria toxoid (DT) and by vaccination with TT or DT alone in 51 healthy adults and 9 infants was studied. In adults, the isotypes of TT and DT AbSC were dominated by immunoglobulin G1 (IgG1) followed by IgG4 and IgA1. HibCP AbSC were dominated by the isotype IgA1 followed by (in decreasing order) IgG2, IgA2, IgM, and IgG1. The isotype distributions of TT and DT AbSC were independent of whether the toxoids were coupled to HibCP, and the isotypes of HibCP AbSC were not influenced by the nature of the carrier (TT or DT). Furthermore, the isotype distributions were unaffected by recent immunization with components of the conjugates, although this reduced the numbers of AbSC. The heavy-chain gene usage of HibCP AbSC in adults differed clearly from that in infants, which was restricted largely to the genes mu, gamma 1, and alpha 1, all lying upstream in the heavy-chain constant-region gene locus, while the usage in adults also, to different extents, involved the downstream genes gamma 2 and alpha 2. The ratio between the numbers of HibCP AbSC using heavy-chain genes from the downstream duplication unit (gamma 2, gamma 4, and alpha 2) and those using genes from the upstream duplication unit (gamma 3, gamma 1, and alpha 1) correlated with the preimmunization level of natural HibCP antibodies (r = 0.59; P = 0.00002). A possible role of natural exposure for Hib or cross-reactive bacteria on the mucosal surfaces in the shaping of the isotype response to HibCP conjugate vaccines is discussed.

Haemophilus type B conjugate vaccine: postimmunization kinetics of IgM and IgG antibody responses in ten vaccinated children

A new Haemophilus type b conjugate vaccine coupling capsular polysaccharide of Haemophilus influenzae b to tetanus toxoid is available in France and other countries. We have studied the kinetics of the immune response in ten children aged 17 to 50 months during the 4 weeks after immunization with one dose of Haemophilus type b conjugate vaccine. Eight serum samples were collected from each child at day 0 (D0), D2, D4, D7, D10, D14, D21 and D28. An ELISA method has been used to discriminate between IgM and IgG classes of anti-polyribosylribitol phosphate antibodies. A high level of IgM appeared at D7 and persisted until D28. The increase in IgG was regular and progressive from D7.

Qualitative and quantitative analyses of the antibody response elicited by Haemophilus influenzae type b capsular polysaccharide-tetanus toxoid conjugates in adults with IgG subclass deficiencies and frequent infections

Twenty-one IgG subclass-deficient adult patients with repeated infections of the respiratory tract, were immunized with Haemophilus influenzae type b capsular polysaccharide (HibCP) covalently bound to tetanus toxoid (TT). Specific immunoglobulin and IgG subclasses to HibCP and TT were quantified; the biological activities of HibCP antibodies were also investigated. Most patients showed an antibody response similar to that observed in healthy adults, and the bactericidal activity related to the post-immunization levels of HibCP antibodies. No relation was found between immunoglobulin isotype deficiency, the clinical symptoms and the IgG subclass responsiveness, and no relation was observed between HibCP and TT antibody responses. Our data indicate that some, but not all, patients with recurrent infections and IgG subclass deficiency have an abnormal serum antibody response to polysaccharide and protein epitopes of Hib-TT conjugate vaccine. Analysis of the antibody response after vaccination with HibCP-TT conjugate vaccine did not seem to predict the clinical course of such patients.

The immunogenicity and safety of Haemophilus influenzae type b-tetanus toxoid conjugate vaccine in Gambian infants

In developing countries, Haemophilus influenzae type b is a major cause of meningitis and pneumonia in the 1st year of life. The safety and immunogenicity of a Haemophilus influenzae type b polysaccharide-tetanus toxoid conjugate vaccine (PRP-T) were evaluated in two studies of Gambian infants. In the first study, 131 infants were recruited and randomized into three groups to receive PRP-T at 1 and 3 months (group A), PRP-T at 2 and 4 months (group B) or no PRP-T (group C). The liquid form of PRP-T was used. The geometric mean titre (GMT) of anti-PRP antibody 1 month after the second dose was 0.26 microgram/ml in group A and 0.41 microgram/ml in group B. In the second study, 66 infants were given PRP-T in the lyophilized form at 2, 3 and 4 months of age. The GMT 1 month after the first dose was 0.09 microgram/ml, after the second 0.74 microgram/ml and after the third 2.32 micrograms/ml. After the third dose, 72% of children had antibody levels greater than 1.0 microgram/ml and 93% greater than 0.15 microgram/ml. No serious side-effects were observed and the rate of adverse reactions was consistent with the concurrent administration of diphtheria-tetanus-pertussis (DPT) vaccine.

Maternal immunization to prevent infectious diseases in the neonate or infant

The approach of providing passive protection to young infants by immunizing pregnant women can bypass the problems of immunological immaturity in the neonate, avoid or delay active immunization of the infant in the first year of life, and prevent transmission of an infection from the mother to the neonate. Optimal vaccines for this approach should induce high immunoglobulin G antibody titers that quickly reach their maximum level after immunization and persist at protective levels for several years, thus providing passive protection in subsequent pregnancies. Specific applications of this approach include the worldwide practice of maternal immunization with tetanus toxoid vaccine and ongoing studies of maternal immunization to prevent Haemophilus influenzae type b, group B streptococcal, pneumococcal, meningococcal, and human immunodeficiency virus infection in the infant. Addressing the cultural, sociological, and legal aspects of maternal immunization will be required to ensure the success of this approach.

Opposite effects of actively and passively acquired immunity to the carrier on responses of human infants to a Haemophilus influenzae type b conjugate vaccine

Vaccination of infants with Haemophilus influenzae type b (Hib) capsular polysaccharide (HibCP) coupled to carrier proteins has proven protective against invasive Hib diseases in several trials. However, insufficient immunogenicity has been noted in certain populations. Therefore, studies analyzing factors influencing the antibody response to conjugate vaccines are needed. In this study, the response to HibCP coupled to tetanus toxoid (TT) was examined in relation to (i) priming with or coadministration of the carrier protein and (ii) the levels of passively acquired maternal TT antibodies. One hundred forty-four infants were vaccinated with HibCP-TT at 5 and 6 months. They were randomized into three groups that received TT as part of a diphtheria-tetanus-polio vaccine at either 6 and 7 months (group A), 5 and 6 months (group B), or 4 and 5 months (group C). Maternally acquired TT antibodies inhibited the anti-HibCP response to the first HibCP-TT dose in groups A and B (r = -0.5 and -0.4, respectively; P < 0.005). In these groups, infants with prevaccination anti-TT levels above the median failed to reach the defined long-term protective level of HibCP antibodies (1 microgram/ml) more often than infants with low prevaccination levels after the first (P = 0.0001) and the second (P = 0.01) doses of HibCP-TT. In contrast, active priming with TT at 4 months resulted in a threefold-higher median level of anti-HibCP (group C; 1.34 micrograms/ml) than in the unprimed group (group A; 0.40 microgram/ml) after the first dose of HibCP-TT (P = 0.01). Coadministration of TT had no enhancing effect (group B; 0.58 microgram/ml). No significant differences between the median anti-HibCP levels were seen after the second HibCP-TT dose (6.72, 9.63, and 11.44 micrograms/ml in groups A, B, and C, respectively; P = 0.25).

Non-epitope-specific suppression of the antibody response to Haemophilus influenzae type b conjugate vaccines by preimmunization with vaccine components

Recently, conjugate vaccines containing Haemophilus influenzae type b capsular polysaccharide (HibCP) coupled to protein carriers were introduced for use in infants and certain adult risk groups. Similar conjugate vaccines against other capsulated bacteria are currently under development for both children and adults. Despite its potential importance, the possible influence of preexisting immunity to the components of such conjugates on the vaccination response in humans has been addressed by few studies. To study this issue, we randomized 82 healthy adult volunteers into six groups and vaccinated them twice, with a 4-week interval between immunizations. Four groups received tetanus toxoid (TT) or diphtheria toxoid (DT) and then HibCP coupled to TT (HibCP-TT) or DT (HibCP-DT). Two groups received HibCP-TT followed by HibCP-DT or vice versa. The total antibody levels to HibCP, TT, and DT and the anti-HibCP immunoglobulin G1 (IgG1) and IgG2 levels were measured before and 4 weeks after the immunizations. For some of the vaccinees, the number of circulating antibody-secreting cells was evaluated 7 days after immunization. Surprisingly, preimmunization with the relevant carrier protein reduced the subsequent increase in the total HibCP antibody levels (P < 0.05), affecting the IgG1 and the IgG2 subclasses equally. Also, the responses to the carrier portions of the conjugates were suppressed, as demonstrated by the lack of significant increases in the antibody levels (P > or = 0.29) and, for HibCP-TT, by reduced numbers of anticarrier antibody-secreting cells (P = 0.009). Similar non-epitope-specific suppression was seen in the groups receiving both conjugates. Thus, preimmunization with one conjugate reduced the subsequent response to the carrier portion of the other conjugate (HibCP-DT and then HibCP-TT, P = 0.00002; HibCP-TT and then HibCP-DT, P = 0.06) as well as to HibCP itself. Possible mechanisms behind this non-epitope-specific suppression and its relevance for vaccine development are discussed.

Characterization of the serum antibody response induced by Haemophilus influenzae type b tetanus protein-conjugate vaccine in infants receiving a DTP-combined vaccine from 2 months of age

The serum antibody response induced by Haemophilus influenzae type b capsular polysaccharide (CPS)-tetanus protein conjugate vaccine combined to DTP vaccine was characterized in infants receiving three injections from 2 months of age. Sixty-five per cent and 94% of infants had anti-CPS antibody levels > or = 1 micrograms ml-1 after the second and third dose, respectively. The antibody response was mostly made up of IgG with a marked IgG1 predominance. Significant rises in bactericidal and in complement-mediated opsonic activities were observed after immunization. These data clearly show that this vaccine can be successfully administered in one syringe together with DTP vaccine during the regular infant immunization programme.

Haemophilus influenzae type B conjugate vaccines

OBJECTIVE

To review the epidemiology of Haemophilus influenzae type b (Hib) disease, the first Hib vaccine and its limitations, the characteristics and clinical efficacy of the newer conjugate vaccines, and the current recommendations for administration of Hib vaccines.

DATA SOURCES

Pertinent literature was identified via a MEDLINE search. Additionally, references cited in published articles were used as data sources.

STUDY SELECTION

Studies describing the epidemiology of Hib disease and the efficacy and/or immunogenicity of the Hib vaccines are reviewed.

DATA SYNTHESIS

Serious invasive disease secondary to Hib infection causes significant morbidity and mortality in children between the ages of three months and five years. The original Hib vaccine was found to be ineffective in stimulating an adequate immune response in children younger than two years of age. The new Hib conjugate vaccines provide superior efficacy and immunogenicity compared with the original unconjugated vaccine. They stimulate an immune response that is distinctly different from that elicited by the original vaccine. Two vaccine products are currently licensed for use in children as young as two months of age, thus conferring immunity to those children at highest risk for Hib disease.

CONCLUSIONS

The new Hib conjugate vaccines provide excellent efficacy and, when used as recommended, may significantly reduce the incidence of invasive Hib disease and its sequelae.

Synthetic trimer and tetramer of 3-beta-D-ribose-(1-1)-D-ribitol-5-phosphate conjugated to protein induce antibody responses to Haemophilus influenzae type b capsular polysaccharide in mice and monkeys

Synthetic oligosaccharides derived from the capsular polysaccharide (PRP) of Haemophilus influenzae type b were conjugated to carrier proteins via a thioether linkage. Conjugates were made of trimeric and tetrameric ribose-ribitol-phosphate and tetanus toxoid or diphtheria toxin. All conjugates elicited anti-PRP antibody responses with an increasing immunoglobulin G/immunoglobulin M ratio in adult mice and monkeys. Trimer conjugates elicited lower anti-PRP antibody responses compared with tetramer conjugates. Adult monkeys responded equally well to the tetrameric oligosaccharide-tetanus toxoid conjugate as to the oligosaccharide-CRM197 conjugate (HbOC), which elicits protective levels of serum antibodies in human infants after two or three injections.

Immunogenicity and safety of PRP-T conjugate vaccine given according to the British accelerated immunisation schedule

The immunogenicity and safety of a new Haemophilus influenzae type b conjugate vaccine, PRP-T, was studied in 107 infants from the Oxford district. The vaccine was given concurrently with diphtheria, pertussis, tetanus, and polio vaccines at 2, 3, and 4 months of age. Symptoms after immunisation were recorded by a parent. Sera were obtained before the first immunisation and at 5 months of age and the antibodies were measured by both radioimmunoassay and enzyme linked immunosorbent assay (ELISA). No serious adverse reactions were observed and there was no increase in the incidence of expected minor side effects. By radioimmunoassay, the geometric mean titre of serum anticapsular antibody increased from 0.09 micrograms/ml before immunisation to 5.01 micrograms/ml after three immunisations. Ninety eight per cent of children had antibody concentrations consistent with protection (greater than or equal to 0.15 micrograms/ml). IgG antibody concentrations measured by ELISA correlated well with total antibody concentrations measured by radioimmunoassay (r = 0.864). These results provide encouragement that routine immunisation against H influenzae type b at 2, 3, and 4 months of age, could prevent most cases of disease in children in the UK.

Determination of neutralizing antibodies and specific immunoglobulin isotype levels in infants after vaccination against diphtheria

Serum samples were obtained from 44 infants vaccinated against diphtheria at the ages of 3, 5 and 12 months with an aluminium-adsorbed diphtheria-tetanus toxoid vaccine containing 15 Lf units of diphtheria toxoid. Toxin-neutralizing antibodies (antitoxin) were measured by the Vero cell assay and IgG, IgM and IgA antibodies against diphtheria toxoid by enzyme-linked immunosorbent assay. A neutralizing antibody titer of 10 corresponded to 0.01 IU/ml, the level considered necessary for short-term protection. Geometric mean neutralizing antibody titers at 3, 5, 6, 12, 13 and 30 months were 28, 21, 173, 61, 1076 and 61. All children had titers of greater than or equal to 10 (greater than or equal to 0.01 IU/ml) between 6 and 30 months of age. At 30 months only 48% had titers of greater than or equal to 100 (greater than or equal to 0.01 IU/ml), the level considered necessary for long-term protection. Geometric mean IgG antibody levels were 13, 36, 216, 64, 649 and 57. IgG antibodies significantly correlated with neutralizing titers and predicted neutralizing antibodies above or below 10 and 100 with an accuracy of 96 and 82%, respectively. IgG antibodies could not, however, be used to predict individual neutralizing antibody titers with great accuracy. IgM antibodies were only detected after the third vaccination. IgA antibodies were not detected in any serum sample from ten infants tested. In conclusion, the Swedish vaccination schedule results in protective antibody levels in infants until at least 30 months of age. The decline of the antibody titers indicates a need for further studies to establish the duration of protection.

Immunogenicity of Haemophilus influenzae type b polysaccharide-tetanus protein conjugate vaccine in children with sickle hemoglobinopathy or malignancies, and after systemic Haemophilus influenzae type b infection

To determine the immunogenicity of Haemophilus influenzae type b polysaccharide-tetanus protein conjugate vaccine in specific populations at risk, we administered vaccine to children with sickle cell anemia (n = 19; mean age, 18.3 months, malignancies (n = 18; mean age, 43.1 months), or a recent history of systemic H. influenzae type b infection (n = 17; mean age, 11.9 months). After one dose of polyribosylribitol phosphate-tetanus toxoid conjugate vaccine the geometric mean titers for polyribosylribitol phosphate antibody were 4.8 micrograms/ml (14/19 greater than 1 microgram/ml), 1.4 micrograms/ml (9/18 greater than 1 microgram/ml), and 5.6 micrograms/ml (15/17 greater than 1 microgram/ml) in these three groups, respectively. Children with sickle cell anemia or a recent history of systemic H. influenzae type b infection had polyribosylribitol phosphate antibody levels comparable to those of normal children of similar age after one or two doses of polyribosylribitol phosphate-tetanus toxoid conjugate vaccine. We conclude that this vaccine is immunogenic in children with underlying conditions associated with an increased risk of H. influenzae type b infection.

Relation between enzyme-linked immunosorbent assay and radioimmunoassay for detection of antibodies to the capsular polysaccharide of Haemophilus influenzae type b

The measurement of antibodies to the capsular polysaccharide (PRP) of Haemophilus influenzae type b (Hib) is important because vaccines inducing such antibodies are now available. We developed and evaluated an enzyme-linked immunosorbent assay (ELISA) for detection of these antibodies based on direct coating of the plates with tyraminated PRP. The assay fulfilled the requirements for parallel line assays; it was sensitive, specific, and reproducible with a coefficient of variation between days of 19%. Results from the ELISA were compared with results from radioimmunoassay and a correlation coefficient of 0.93 was found. Results obtained by the two methods were proportional and the relation was independent of the antibody level. The relation between them was also unaffected by the contribution of different antibody isotypes, indicating that these were measured to the same extent by both methods. ELISA employing direct coating of the plates with tyraminated PRP represents a useful alternative for detection of antibodies when studying immunogenicity of Hib vaccines.

Comparative trial in infants of four conjugate Haemophilus influenzae type b vaccines

We performed a double-blind, randomized trial to compare the immunogenicity and reactogenicity of four conjugate Haemophilus influenzae type b vaccines given to infants 2, 4, and 6 months of age. Adverse reactions attributable to the vaccines were few and minor. The rates of systemic reactions did not differ among the various vaccines and were similar to those seen among children receiving conventional diphtheria-tetanus-pertussis vaccine. However, the four conjugate H. influenzae type b vaccines differed markedly in ability to stimulate antibody production. Mean antibody levels after three injections of polyribosylribitol phosphate conjugated with mutant diphtheria protein (PRP-CRM) or polyribosylribitol phosphate conjugated with tetanus toxoid (PRP-T) were 3.08 micrograms/ml and 3.64 micrograms/ml, respectively, significantly higher than those after the use of polyribosylribitol phosphate conjugated with outer-membrane protein of Neisseria meningitidis (PRP-OMP) (1.14 micrograms/ml) or polyribosylribitol phosphate conjugated with diphtheria toxoid (PRP-D) (0.28 microgram/ml). Only PRP-OMP produced a clinically pertinent elevation in antibody level after two injections (0.84 microgram/ml); the third injection of PRP-OMP produced a modest but statistically significant further elevation in mean antibody level (1.14 micrograms/ml). Only 29% of infants receiving PRP-D had antibody levels of 1 micrograms/ml, compared with 55%, 75%, and 83% of those receiving PRP-OMP, PRP-CRM, and PRP-T, respectively. We conclude that all four vaccines are safe and that all but PRP-D appear appropriate for use in a primary immunization series during infancy. The unique serologic response to PRP-OMP offers both advantages and disadvantages in comparison with PRP-CRM and PRP-T.