Glycoconjugation is a well-established technology for vaccine development: linkage of the polysaccharide (PS) antigen to an appropriate carrier protein overcomes the limitations of PS T-independent antigens, making them effective in infants and providing immunological memory. Glycoconjugate vaccines have been successful in reducing the burden of different diseases globally. However, many pathogens still require a vaccine, and many of them display a variety of glycans on their surface that have been proposed as key antigens for the development of high-valency glycoconjugate vaccines. CDAP chemistry represents a generic conjugation strategy that is easily applied to PS with different structures. This chemistry utilizes common groups to a large range of PS and proteins, e.g., hydroxyl groups on the PS and amino groups on the protein. Here, new fast analytical tools to study CDAP reaction have been developed, and reaction conditions for PS activation and conjugation have been extensively investigated. Mathematical models have been built to identify reaction conditions to generate conjugates with wanted characteristics and successfully applied to a large number of bacterial PSs from different pathogens, e.g., Klebsiella pneumoniae, Salmonella Paratyphi A, Salmonella Enteritidis, Salmonella Typhimurium, Shighella sonnei and Shigella flexneri. Furthermore, using Salmonella Paratyphi A O-antigen and CRM₁₉₇ as models, a design of experiment approach has been used to study the impact of conjugation conditions and conjugate features on immunogenicity in rabbits. The approach used can be rapidly extended to other PSs and accelerate the development of high-valency glycoconjugate vaccines.

Background: Typhoid and paratyphoid fevers represent a global health burden, especially in Southern Asia, exacerbated by the increase in antimicrobial resistance. While vaccines against Salmonella Typhi have been successfully introduced, a vaccine against S. Paratyphi A is not available, yet. Efforts to develop an effective vaccine targeting both Salmonella serovars are currently ongoing. GVGH is developing a bivalent vaccine constituted by the Vi-CRM₁₉₇ typhoid conjugate vaccine (TCV), and the Salmonella Paratyphi A O-antigen (O:2), also conjugated to the CRM₁₉₇ carrier protein (O:2-CRM₁₉₇). In this work we have characterized a panel of S. Paratyphi A clinical isolates from endemic regions, differing in terms of their O:2 structural features. Methods: Rabbits were immunized with the S. Paratyphi A component of the vaccine candidate and the resulting sera were tested for their ability to bind and kill the isolates using flow cytometry and luminescence-based serum bactericidal assay (L-SBA). Results: The O:2-CRM₁₉₇ glycoconjugate induced a functional immune response in rabbits, effectively binding and killing a diverse panel of clinical isolates. The sera demonstrated bactericidal activity independent of the O:2 structural variations, including differences in O-acetylation and glucosylation levels. Additionally, the study found that the O:2-CRM₁₉₇ conjugate’s adsorption to Alhydrogel did not significantly impact its immunogenicity or bactericidal efficacy. Conclusions: The O:2-CRM₁₉₇ component of the bivalent vaccine candidate shows promise in providing broad protection against S. Paratyphi A isolates, regardless of their O-antigen structural variations. The ongoing clinical studies on human sera are expected to confirm these results.