Matching preclinical and clinical evidences for a vaccine against Shigella

In the present thesis, insights into the global epidemiology of Shigella, together with evaluations of the humoral and cellular immunogenicity and cross-reactive potential of the quadrivalent GMMA‐based vaccine altSonflex1-2-3, currently in phase II development, were investigated to address the challenge of shigellosis. A review of Shigella serotypes distribution revealed its high burden among children under five in low- and middle-income countries, complicated by extensive O-antigen variability and rising antimicrobial resistance. This analysis underscored the urgent need for innovative vaccine strategies targeting the predominant S. sonnei and S. flexneri serotypes in these regions. Preclinical studies in mice, rats, and rabbits were conducted to examine the humoral immune responses elicited by altSonflex1-2-3 against its target strains and provide translational insights. Vaccination induced robust O-antigen-specific IgG responses and functional bactericidal activity, with distinct dose-response patterns revealing interspecies differences. Notably, the immune profiles in rats closely mirrored those of human adults, suggesting high predictive value for clinical outcomes. To further evaluate vaccine-induced functionality and potential broad protection against strains not targeted by the vaccine, a high-throughput luminescence-based serum bactericidal assay was characterized for five heterologous S. flexneri serotypes. This assay demonstrated excellent repeatability, precision, and specificity with human sera and effectively assessed both homologous and heterologous antibody responses. Subsequent serological studies using preclinical animal sera and clinical sera from European adults enrolled in a Phase I trial, confirmed that altSonflex1-2-3 was able to generate cross-reactive antibodies capable of binding and killing non-vaccine S. flexneri strains. These findings were supported by phylogenomic analyses, which verified that the heterologous strains represent circulating isolates, and by antigenic cartography, which provided a clear visualization of cross-reactivity. For the first time in the present thesis, the O-antigen-specific cellular response elicited by altSonflex1-2-3 was investigated. Multiparametric flow cytometric analyses revealed that the vaccine stimulated both immediate and long-lasting immunity through rapid induction of germinal centers, activation of T follicular helper cells, and generation of durable memory B cells and long-lived plasma cells, alongside a predominantly Th1-polarized CD4⁺ T cell response, with activation sustained up to 22 weeks post-boost. Collectively, these integrated findings establish altSonflex1-2-3 as a promising candidate for broadly protective Shigella vaccination, laying a strong foundation for its continued clinical development and future global impact.