Development of invasive non-typhoidal Salmonella conjugate vaccines and their evaluation in a trivalent formulation with typhoid conjugate vaccine

Invasive nontyphoidal Salmonella (iNTS) infections, primarily caused by Salmonella enterica serovars Typhimurium (S. Typhimurium) and Enteritidis (S. Enteritidis), represent a significant public health concern, particularly in sub-Saharan Africa, where multidrug-resistant (MDR) strains are increasingly prevalent. Despite the substantial disease burden, no vaccines are currently licensed for iNTS. This study aimed to develop an iNTS conjugate vaccine by conjugating O-specific polysaccharide (OSP) antigens to carrier proteins using chemical conjugation, a proven method known for its efficiency and scalability in licensed glycoconjugate vaccines. Various carrier proteins and chemical conjugation processes were evaluated to optimize the iNTS OSP conjugate vaccine candidates. Through this optimization, diphtheria toxoid (DT) was identified as the carrier protein that significantly enhances the anti-OSP immunogenicity of the iNTS conjugates. Key properties, such as the molecular weight and OSP:DT ratio in the iNTS OSP conjugate were found to be controllable by adjusting the ratios of CDAP conjugate reagent and DT to iNTS OSP. Optimal conjugation process parameters were identified by evaluating the relationship between these property and immunogenicity through tests in mice. The optimized iNTS conjugates for S. Typhimurium and S. Enteritidis were further developed into a bivalent formulation. This formulation was selected based on a dose-dependent immunogenicity study and included alum as an adjuvant to enhance immune response. Ultimately, a trivalent drug product formulation was developed by combining the bivalent iNTS conjugate vaccine with a typhoid conjugate vaccine.

Our findings demonstrated that the iNTS OSP-DT conjugates, at the optimal conjugation ratios, induced robust immune responses with high anti-OSP IgG titers for both iNTS serovars, comparable to or exceeding those of other formulations. The inclusion of alum further enhanced immunogenicity across all formulations. Notably, the trivalent vaccine formulation showed promising results, maintaining robust immunogenic responses against all iNTS OSP antigens and the Vi polysaccharide antigen of Salmonella Typhi, without compromising the immunogenicity of any individual antigens.

This study suggests that a bivalent iNTS vaccine combined with a typhoid conjugate vaccine could provide broad protection against both iNTS infections and typhoid fever, addressing a critical unmet need in regions with limited resources.