Carrier priming with CRM197 or diphtheria toxoid has a different impact on the immunogenicity of the respective glycoconjugates: Biophysical and immunochemical interpretation

Glycoconjugate vaccines play an enormous role in preventing infectious diseases. The main carrier proteins used in commercial conjugate vaccines are the non-toxic mutant of diphtheria toxin (CRM₁₉₇), diphtheria toxoid (DT) and tetanus toxoid (TT). Modern childhood routine vaccination schedules include the administration of several vaccines simultaneously or in close sequence, increasing the concern that the repeated exposure to conjugates based on these carrier proteins might interfere with the anti-polysaccharide response. Extending previous observations we show here that priming mice with CRM₁₉₇ or DT does not suppress the response to the carbohydrate moiety of CRM₁₉₇ meningococcal serogroup A (MenA) conjugates, while priming with DT can suppress the response to DT-MenA conjugates. To explain these findings we made use of biophysical and immunochemical techniques applied mainly to MenA conjugates. Differential scanning calorimetry and circular dichroism data revealed that the CRM₁₉₇ structure was altered by the chemical conjugation, while DT and the formaldehyde-treated form of CRM₁₉₇ were less impacted, depending on the degree of glycosylation. Investigating the binding and avidity properties of IgGs induced in mice by non-conjugated carriers, we found that CRM₁₉₇ induced low levels of anti-carrier antibodies, with decreased avidity for its MenA conjugates and poor binding to DT and respective MenA conjugates. In contrast, DT induced high antibody titers able to bind with comparable avidity both the protein and its conjugates but showing very low avidity for CRM₁₉₇ and related conjugates. The low intrinsic immunogenicity of CRM₁₉₇ as compared to DT, the structural modifications induced by glycoconjugation and detoxification processes, resulting in conformational changes in CRM₁₉₇ and DT epitopes with consequent alteration of the antibody recognition and avidity, might explain the different behavior of CRM₁₉₇ and DT in a carrier priming context.     

Comparison of CRM197, diphtheria toxoid and tetanus toxoid as protein carriers for meningococcal glycoconjugate vaccines

Glycoconjugate vaccines are among the most effective and safest vaccines ever developed. Diphtheria toxoid (DT), tetanus toxoid (TT) and CRM₁₉₇ have been mostly used as protein carriers in licensed vaccines. We evaluated the immunogenicity of serogroup A, C, W-135 and Y meningococcal oligosaccharides conjugated to CRM₁₉₇, DT and TT in naïve mice. The three carriers were equally efficient in inducing an immune response against the carbohydrate moiety in immunologically naïve mice. The effect of previous exposure to different dosages of the carrier protein on the anti-carbohydrate response was studied using serogroup A meningococcal (MenA) saccharide conjugates as a model. CRM₁₉₇ showed a strong propensity to positively prime the anti-carbohydrate response elicited by its conjugates or those with the antigenically related carrier DT. Conversely in any of the tested conditions TT priming did not result in enhancement of the anti-carbohydrate response elicited by the corresponding conjugates. Repeated exposure of mice to TT or to CRM₁₉₇ before immunization with the respective MenA conjugates resulted in a drastic suppression of the anti-carbohydrate response in the case of TT conjugate and only in a slight reduction in the case of CRM₁₉₇. The effect of carrier priming on the anti-MenA response of DT-based conjugates varied depending on their carbohydrate to protein ratio. These data may have implications for human vaccination since conjugate vaccines are widely used in individuals previously immunized with DT and TT carrier proteins.     

Evaluation of the non-toxic mutant of the diphtheria toxin K51E/E148K as carrier protein for meningococcal vaccines

Diphtheria toxin mutant CRM₁₉₇ is a common carrier protein for glycoconjugate vaccines, which has been proven an effective protein vector for, among others, meningococcal carbohydrates. The wide-range use of this protein in massive vaccine production requires constant increase of production yields and adaptability to an ever-growing market. Here we compare CRM₁₉₇ with the alternative diphtheria non-toxic variant DT-K51E/E148K, an inactive mutant that can be produced in the periplasm of Escherichia coli. Biophysical characterization of DT-K51E/E148K suggested high similarity with CRM₁₉₇, with main differences in their alpha-helical content, and a suitable purity for conjugation and vaccine preparation. Meningococcal serogroup A (MenA) glycoconjugates were synthesized using CRM₁₉₇ and DT-K51E/E148K as carrier proteins, obtaining the same conjugation yields and comparable biophysical profiles. Mice were then immunized with these CRM₁₉₇ and DT-K51E/E148K conjugates, and essentially identical immunogenic and protective effects were observed. Overall, our data indicate that DT-K51E/E148K is a readily produced protein that now allows the added flexibility of E. coli production in vaccine development and that can be effectively used as protein carrier for a meningococcal conjugate vaccine.     

Quality,immunogenicity and stability of meningococcal serogroup ACWY-CRM197, DT and TT glycoconjugate vaccines

A physicochemical and immunological study of the stability of three different meningococcal (Men) ACWY conjugate vaccines was performed to evaluate any patterns of serogroup oligo- or polysaccharide-specific or carrier protein-specific stability that would affect immunogenicity. Critical quality and stability-indicating characteristics were measured, with the study supporting the suitability of both HPLC-SEC and HPAEC-PAD methods to detect changes following inappropriate vaccine storage. All three final products, ACWY-CRM₁₉₇, -DT and -TT conjugate vaccines had expected quality indicator values and similar immunogenicity in a mouse model (anti-PS IgG and rSBA) when stored at +2–8 °C. When stored at ≥+37 °C, all conjugated carrier proteins and serogroup saccharides were affected. Direct correlations were observed between the depolymerization of the MenA saccharide as evidenced by a size-reduction in the MenA conjugates (CRM₁₉₇, DT and TT) and their immunogenicity. MenA was the most labile serogroup, followed by MenC; then MenW and Y, which were similar. At high temperatures, the conjugated carrier proteins were prone to unfolding and/or aggregation. The anti-MenC IgG responses of the multivalent conjugate vaccines in mice were equivalent to those observed in monovalent MenC conjugate vaccines, and were independent of the carrier protein. For any newly developing MenACWY saccharide-protein conjugate vaccines, a key recommendation would be to consider the lyophilization of final product to prevent deleterious degradation that would affect immunogenicity.     

Antigen processing of glycoconjugate vaccines; the polysaccharide portion of the pneumococcal CRM197 conjugate vaccine co-localizes with MHC II on the antigen processing cell surface

Pneumococcal (Pn) polysaccharides (PS) are T-independent (TI) antigens and do not induce immunological memory or antibodies in infants. Conjugation of PnPS to the carrier protein CRM₁₉₇ induces PS-specific antibody in infants, and memory similar to T-dependent (Td) antigens. Conjugates have improved immunogenicity via antigen processing and presentation of carrier protein with MHC II and recruitment of T cell help, but the fate of the PS attached to the carrier is unknown. To determine the location of the PS component of PnPS-CRM₁₉₇ in the APC, we separately labeled PS and protein and tracked their location. The PS of types 14-CRM₁₉₇ and 19F-CRM₁₉₇ was specifically labeled by Alexa Fluor^® 594 hydrazide (red). The CRM₁₉₇ was separately labeled red in a reaction that did not label PS. Labeled antigens were incubated with APC which were fixed, permeabilized and incubated with anti-MHC II antibody labeled green by Alexa Fluor^® 488, followed by confocal microscopy. Labeled CRM₁₉₇ was presented on APC surface and co-localized with MHC II (yellow). Labeled unconjugated 14 or 19F PS did not go to the APC surface, but PS labeled 14-CRM₁₉₇ and 19F-CRM₁₉₇ was internalized and co-localized with MHC II. Monoclonal antibody to type 14 PS bound to intracellular type 14 PS and PS-CRM₁₉₇. Brefeldin A and chloroquine blocked both CRM₁₉₇ and PS labeled 14-CRM₁₉₇ and 19F-CRM₁₉₇ from co-localizing with MHC II. These data suggest that the PS component of the CRM₁₉₇ glycoconjugate enters the endosome, travels with CRM₁₉₇ peptides to the APC surface and co-localizes with MHC II.     

A cross-reacting material CRM197 conjugate vaccine induces diphtheria toxin neutralizing antibody response in children and adolescents infected or not with HIV

Anti-diphtheria antibody levels decrease with aging, and frequent booster vaccinations are required to maintain herd immunity. We analyzed the diphtheria toxin neutralizing antibody (DT-Nab) response induced by a conjugate vaccine (meningococcal C polysaccharide-CRM₁₉₇) in HIV-vertically infected (HI) children and adolescents and healthy controls (HC) with matched age. We report the association of DT-Nab with the bactericidal antibodies to serogroup C meningococcus (MenC). Before vaccination, 21 HI patients (50%) had no protection against diphtheria (≤0.01 IU/ml of antibody) and only 8 (19%) showed complete protection (≥0.1 IU/ml). About half of the HC (56%) had complete protection before immunization and 6 subjects (12%) had no protection against diphtheria. After one and two vaccine injections, 96% of HC and 64% of HI vaccinees, respectively, showed full protection against diphtheria. These data indicate that CRM₁₉₇ was able to induce primary and/or booster response in both groups of individuals.     

A clinical trial examining the effect of increased total CRM(197) carrier protein dose on the antibody response to Haemophilus influenzae type b CRM(197) conjugate vaccine

CRM(197) is a carrier protein in certain conjugate vaccines. When multiple conjugate vaccines with the same carrier protein are administered simultaneously, reduced response to vaccines and/or antigens related to the carrier protein may occur. This study examined responses of infants who, in addition to diphtheria toxoid/tetanus toxoid/acellular pertussis vaccine (DTaP) received either diphtheria CRM(197)-based Haemophilus influenzae type b conjugate vaccine (HbOC) or HbOC and a diphtheria CRM(197)-based combination 9-valent pneumococcal conjugate vaccine/meningococcal group C conjugate vaccine. Administration of conjugate vaccines with CRM(197) carrier protein load >50 microg did not reduce response to CRM(197) conjugate vaccines or immunogenicity to immunologically cross-reactive diphtheria toxoid.     

23F型肺炎球菌多糖与CRM197蛋白结合物制备及特性研究

目的采用改良胺化还原法,白喉无毒突变体CRM197蛋白作为载体,制备23F型肺炎球菌多糖蛋白结合物。方法在还原胺法的基础上,以1,6-己二酰肼（ADH）为分子臂,在碳二亚胺（EDAC）作用下衍化CRM197蛋白,衍化后蛋白再与多糖上的醛基反应,形成共轭结合;采用高效液相色谱技术（HPLC）、免疫扩散等方法分析结合物,并将获得的结合物免疫NIH小鼠,用ELISA方法检测多糖IgG水平。结果HPLC图谱和免疫扩散检测结果证明了结合物的获得;动物免疫产生了较多糖高的抗23F肺炎多糖抗体,并有免疫记忆发生。结论采用改良还原氨化法可以制备出动物免疫原性良好的23F型多糖蛋白结合物。     

Production and purification of an active CRM197 in Pichia pastoris and its immunological characterization using a Vi-typhoid antigen vaccine

CRM197 is a commonly used glycoconjugate carrier that improves the immunogenicity of vaccines, particularly in infants. Despite the advantages of this diphtheria toxoid mutant, low yields, production in inclusion bodies, and the requirement for specific growth conditions have limited the breadth of successful recombinant protein expression platforms available for its expression. We evaluated Pichia pastoris as a production host, using the methanol inducible AOX1 promoter and a modified α-mating factor signal peptide for secretion into the supernatant. Final purified yields >100 mg L⁻¹ culture were achieved when produced in a bioreactor, which is equivalent to the productivity obtained from bioprocesses using the native Corynebacterium diphtheriae host. Recombinant CRM197 was purified to ≥95% homogeneity and showed the expected endonuclease activity. Furthermore, mice immunized with a Salmonella enterica serovar Typhi capsular Vi antigen conjugated to our recombinant CRM197 showed greater than 5-fold increase in immune response. Overall, the results demonstrate that Pichia pastoris is a suitable expression host for the production of high quality CRM197 for vaccine applications.     

Glycoconjugate vaccines and immune interference: A review

Bacterial polysaccharide–protein conjugate vaccines (Haemophilus influenzae type b [Hib], pneumococcal and meningococcal conjugates) have revolutionized pediatric vaccination strategies. The widely used carrier proteins are tetanus toxoid (TT), diphtheria toxoid (DT) and diphtheria toxoid variant CRM197 protein, DT conjugates being in general less immunogenic. Multivalent conjugates using TT were found to be at risk for reduced polysaccharide responses, whilst multivalent CRM197 conjugates are at lower risk for this, but may be at higher risk of inducing bystander interference, particularly affecting Hib and hepatitis B immune responses. Novel carriers avoiding these issues could enable the further development of pediatric schedules and combinations.     

Vi-CRM 197 as a new conjugate vaccine against Salmonella Typhi

An efficacious, low cost vaccine against typhoid fever, especially for young children, would make a major impact on disease burden in developing countries. The virulence capsular polysaccharide of Salmonella Typhi (Vi) coupled to recombinant mutant Pseudomonas aeruginosa exoprotein A (Vi-rEPA) has been shown to be highly efficacious. We investigated the use of carrier proteins included in infant vaccines, standardized the conjugation process and developed key assays required for routine lot release at production scale. Vi from a BSL1 organism, Citrobacter freundii, strain WR7011, was used as an alternative to Vi from S. Typhi. We showed that Vi conjugated to CRM₁₉₇, a non-toxic mutant of diphtheria toxin, widely used in commercial vaccines, was produced at high yield. Vi-CRM₁₉₇ proved immunogenic in animal studies, even without adjuvant. Thus, Vi-CRM₁₉₇ appears to be a suitable candidate for the development of a commercially viable, effective typhoid vaccine for developing countries.     

A mucosal vaccine against diphtheria: formulation of cross reacting material (CRM(197)) of diphtheria toxin with chitosan enhances local and systemic antibody and Th2 responses following nasal delivery

The development of new generation vaccines against diphtheria is dependent on the identification of antigens and routes of immunization that are capable of stimulating immune responses similar to, or greater than, those obtained with the parenterally-delivered toxoid vaccine, while reducing the adverse effects that have been associated with the traditional vaccine. In this study, we examined the cellular and humoral immune responses in mice generated after both parenteral and mucosal immunizations with cross-reacting material (CRM(197)) of diphtheria toxin. We found that both native and mildly formaldehyde-treated CRM(197) and conventional diphtheria toxoid (DT) induced mixed Th1/Th2 responses and similar levels of anti-DT serum IgG following parenteral immunization. In contrast, CRM(197) preparations were poorly immunogenic when administered intranasally in solution. However, formulation of the antigens with chitosan significantly enhanced their immunogenicity, inducing high levels of antigen-specific IgG, secretory IgA, toxin-neutralizing antibodies and T cell responses, predominately of Th2 subtype. Furthermore, intranasal immunization with CRM(197) and chitosan induced protective antibodies against the toxin in a guinea pig passive challenge model. We also found that priming parenterally with DT in alum and boosting intranasally with CRM(197) was a very effective method of immunization in mice, capable of inducing high levels of anti-DT IgG and neutralizing antibodies in the serum and secretory IgA in the respiratory tract. Our findings suggest that boosting intranasally with CRM(197) antigen may be very effective in adolescents or adults who have previously been parenterally immunized with a conventional diphtheria toxoid vaccine.     

Isolation and characterization of new human carrier peptides from two important vaccine immunogens

In the preparation of glycoconjugate vaccines in clinical practice, two highly immunogenic carrier proteins, CRM₁₉₇ and tetanus toxoid (TT), are predominantly conjugated with the capsular polysaccharides (CPSs) of bacterial pathogens. In addition, TT has long been used as an effective vaccine to prevent tetanus. While these carrier proteins play an important role in immunogenicity and vaccine design alike, their defined human major histocompatibility complex class II (MHCII) T cell epitopes are inadequately characterized. In this current work, we use mass spectrometry to identify the peptides from these carrier proteins that are naturally processed and presented by human B cells via MHCII pathway. The MHCII-presented peptides are screened for their T cell stimulation using primary CD4+ T cells from four healthy adult donors. These combined methods reveal a subset of eleven CD4+ T cell epitopes that proliferate and stimulate human T cells with diverse MHCII allelic repertoire. Six of these peptides stand out as potential immunodominant epitopes by responding in three or more donors. Additionally, we provide evidence of these natural epitopes eliciting more significant T cell responses in donors than previously published TT peptides selected from T cell epitope screening. This study serves toward understanding carrier protein immune responses and thus enables the use of these peptides in developing novel knowledge-based vaccines to combat persisting problems in glycoconjugate vaccine design.     

Effect on meningococcal serogroup W immunogenicity when Tdap was administered prior,concurrent or subsequent to the quadrivalent (ACWY) meningococcal CRM197-conjugate vaccine in adult Hajj pilgrims: A randomised controlled trial

Immune responses to the capsular polysaccharide administered in the polysaccharide-protein conjugate vaccines can be either improved or suppressed by the pre-existence of immunity to the carrier protein. Receiving multiple vaccinations is essential for travellers such as Hajj pilgrims, and the use of conjugated vaccines is recommended.We studied the immune response to meningococcal serogroup W upon prior, concurrent and sequential administration of a quadrivalent meningococcal conjugate vaccine (MCV4) conjugated to CRM₁₉₇ (coadministered with 13 valent pneumococcal vaccine conjugate CRM₁₉₇ [PCV13]), and tetanus-diphtheria-acellular pertussis (Tdap) vaccine in Australian adults before attending the Hajj pilgrimage in 2014.Participants were randomly assigned, by computer-generated numbers, to three study arms by 1:1:1 ratio. Group A received Tdap followed by MCV4-CRM₁₉₇ (+PCV13) 3–4 weeks later. Group B received all three vaccines in a single visit. Group C received MCV4-CRM₁₉₇ (+PCV13) followed by Tdap 3–4 weeks later. Blood samples obtained prior to and 3–4 weeks after immunisation with MCV4-CRM₁₉₇ were tested for meningococcal serogroup W-specific serum bactericidal antibody responses using baby rabbit complement (rSBA).One hundred and seven participants aged between 18 and 64 (median 40) years completed the study. No significant difference in meningococcal serogroup W rSBA geometric mean titre (GMT) was observed between the study arms post vaccination with MCV-CRM₁₉₇ but Group A tended to have a slightly lower GMT (A = 404, B = 984 and C = 1235, p = 0.15). No statistical difference was noticed between the groups in proportions of subjects achieving a ≥4-fold rise in rSBA titres or achieving rSBA titre ≥8 post vaccination.In conclusion, receipt of MCV4-CRM₁₉₇ vaccine prior, concurrent or subsequent to Tdap has similar immunologic response, and hence concurrent administration is both immunogenic and practical. However, further investigation into whether carrier induced suppression is a public health issue is suggested.Clinical trial registration: ANZCTR no. ACTRN12613000536763.     

Development of a bivalent conjugate vaccine candidate against rotaviral diarrhea and tuberculosis using polysaccharide from Mycobacterium tuberculosis conjugated to ΔVP8* protein from rotavirus

Conjugation of carbohydrate antigens with a carrier protein is a clinically proven strategy to overcome the poor immunogenicity of bacterial polysaccharide. In addition to its primary role, which is to help generate a T cell-mediate long-lasting immune response directed against the carbohydrate antigen, the carrier protein in a glycoconjugate vaccine can also play an important role as a protective antigen. Among carrier proteins currently used in licensed conjugate vaccines, non-typeable Haemophilus influenzae protein D has been used as an antigenically active carrier protein. Our previous studies also indicate that some carrier proteins provide B cell epitopes, along with T cell helper epitopes.Herein we investigated the dual role of truncated rotavirus spike protein ΔVP8* as a carrier and a protective antigen. Capsular polysaccharide lipoarabinomannan (LAM), purified from Mycobacterium tuberculosis (M.tb), was chemically conjugated with ΔVP8*. Mouse immunization experiments showed that the resultant conjugates elicited strong and specific immune responses against the polysaccharide antigen, and the responses were comparable to those induced by Diphtheria toxoid (DT)-based conjugates. The conjugate vaccine induced enhanced antibody titers and functional antibodies against ΔVP8* when compared to immunization with the unconjugated ΔVP8*. Thus, these results indicate that ΔVP8* can be a relevant carrier protein for glycoconjugate vaccine and the glycoconjugates consisting of ΔVP8* with LAM are effective bivalent vaccine candidates against rotavirus and tuberculosis.     

The B-cell response to a primary and booster course of MenACWY-CRM197 vaccine administered at 2, 4 and 12 months of age

A quadrivalent meningococcal vaccine conjugated to CRM₁₉₇ (MenACWY-CRM₁₉₇) is immunogenic in young infants. We assessed the memory B-cell and antibody responses after a primary and booster course of MenACWY-CRM₁₉₇ in children. At 5 months of age, following primary immunisation, serogroup-specific memory B-cells were detectable in fewer than 25% of children, although protective antibody titres (hSBA ≥ 4) were detectable in 69% of children against serogroup A and more than 95% against the other serogroups. At 12 months, before booster immunisation the percentages with hSBA ≥ 4 were 5% for serogroup A, and between 44 and 70% for the other serogroups. One month after booster immunisation with MenACWY-CRM₁₉₇ over 50% of children had detectable memory B-cells, and 91% had hSBA ≥ 4 against serogroup A and more than 99% against the other serogroups. These data show that few antigen-specific anticapsular memory B-cells can be detected after two-doses priming with MenACWY-CRM₁₉₇. For MenC and CRM₁₉₇, the antigens with the highest number of B-cells at 5 months, there was a definite (p ≤ 0.02) but weak correlation with antibody persistence at 12 months. Although previous studies suggest that measuring memory B-cell responses after priming immunisations in infancy can be used to predict antibody persistence and memory responses, this may not be suitable for all antigens in young children.     

Overview of the development and current use of CRM197 conjugate vaccines for pediatric use

Glycoconjugate vaccines have been proven safe and effective against various diseases in children. Although these vaccines have a history of effectiveness, there are still many unanswered questions to be addressed, including conjugate interference when multiple vaccines are administered at one time, expansion of serotype coverage, effectiveness in special populations, and issues relating to conjugate vaccine use in the developing world. This paper focuses on the use of CRM₁₉₇ as a carrier protein, contrasting it to other carrier proteins used in single-antigen pediatric vaccines as well as identifying areas for future study.     

Comparison of carrier proteins to conjugate malaria transmission blocking vaccine antigens,Pfs25 and Pfs230

Malaria transmission blocking vaccines (TBV) target the sexual stage of the parasite and have been pursued as a stand-alone vaccine or for combination with pre-erythrocytic or blood stage vaccines. Our efforts to develop TBV focus primarily on two antigens, Pfs25 and Pfs230. Chemical conjugation of these poorly immunogenic antigens to carrier proteins enhances their immunogenicity, and conjugates of these antigens to Exoprotein A (EPA) are currently under evaluation in clinical trials. Nonetheless, more potent carriers may augment the immunogenicity of these antigens for a more efficacious vaccine; here, we evaluate a series of proteins to identify such a carrier. Pfs25 and Pfs230 were chemically conjugated to 4 different carriers [tetanus toxoid (TT), a recombinant fragment of tetanus toxin heavy chain (rTThc), recombinant CRM₁₉₇ produced in Pseudomonas fluorescens (CRM197) or in E. coli (EcoCRM®)] and compared to EPA conjugates in mouse immunogenicity studies. Conjugates of each antigen formulated in Alhydrogel® elicited similar antibody titers but showed differences in functional activity. At a 0.5 µg dose, Pfs230 conjugated to TT, CRM197 and EcoCRM® showed significantly higher functional activity compared to EPA. When formulated with the more potent adjuvant GLA-LSQ, all 4 alternate conjugates induced higher antibody titers as well as increased functional activity compared to the EPA conjugate. IgG subclass analysis of Pfs230 conjugates showed no carrier-dependent differences in the IgG profile. While Alhydrogel® formulations induced a Th2 dominant immune response, GLA-LSQ formulations induced a mixed Th1/Th2 response.     

Comparison of serum bactericidal and antibody titers induced by two Haemophilus influenzae type b conjugate vaccines: A phase III randomized double-blind study

Haemophilus influenzae type b (Hib) conjugate vaccines have drastically reduced disease incidence worldwide. Protection against Hib infection has relied on the serum bactericidal activity (SBA) of antibodies to the Hib capsular polysaccharide (polyribosylribitol phosphate). However, licensure usually relies on measuring induction of antibodies to PRP as a surrogate for SBA. In a phase III clinical trial we compared a PRP-conjugate vaccine using the nontoxic diphtheria toxin mutant, CRM₁₉₇, as carrier protein with the licensed tetanus toxoid conjugate when administered subcutaneously as a three dose primary series in Japanese infants. As an addition to the phase III study, we have now evaluated SBA and show PRP-CRM₁₉₇ induces higher levels of SBA than PRP-T four weeks after the primary series, with a statistically significant correlation with anti-PRP titers. This data confirms the superior immunogenicity of PRP-CRM₁₉₇ compared with PRP-T assessed as SBA following a three-dose primary series by subcutaneous administration.Clinical trial registry: Registered on ClinicalTrials.gov (NCT01379846).     

Impact of conjugation chemistry on the immunogenicity of S. Typhimurium conjugate vaccines

Salmonella Typhimurium is major cause of invasive nontyphoidal Salmonella disease in Africa. Conjugation of S. Typhimurium O-antigen to an appropriate carrier protein constitutes a possible strategy for the development of a vaccine against this disease, for which no vaccines are currently available. The conjugation chemistry used is one of the parameters that can affect the immunogenicity of glycoconjugate vaccines. Herein different glycoconjugates were synthesized to investigate the impact of this variable on the immunogenicity of S. Typhimurium conjugate vaccines in mice, all with CRM₁₉₇ as carrier protein. Random derivatization along the O-antigen chain was compared with site-directed activation of the terminal KDO sugar residue of the core oligosaccharide. In particular, two different random approaches were used, based on the oxidation of the polysaccharide, which differently impact the structure and conformation of the O-antigen chain. For the selective conjugation methods, linkers of two different lengths were compared.