Introduction. The activation of the adaptive immune system requires the engagement of co-stimulatory pathways in addition to primary antigen receptor signaling, and adjuvants play a central role in this process. Surprisingly, many bacterial polysaccharide vaccines, including typhoid Vi polysaccharide and quadrivalent meningococcal conjugate (MCV4) vaccines, do not incorporate adjuvants, and induce highly variable antibody responses. We found that Toll-Like Receptor 4 ligands present in typhoid vaccines account for the immunogenicity of typhoid vaccines. Based on this rationale, we developed a non-toxic TLR4 ligand, a monophosphoryl lipid A-based adjuvant formulation called Turbo.
Methods. The adjuvanticity of Turbo was tested with WHO-prequalified or FDA-approved typhoid and MCV4 vaccines, as well as a haptenated antigen system (NP-CGG) in several inbred and outbred strains of mice of all ages. The methodology employed included transgenic mouse systems, challenge models, serum bactericidal assays, ELISA, ELISpot, flow cytometry, and immunohistochemistry.
Results: Turbo promoted antibody responses across all ages and eliminated or minimized the booster requirement. In contrast to the commonly used adjuvant alum, Turbo promoted a superior germinal center response accompanied by affinity maturation and class switching to all four IgG isotypes. The magnitude of these IgG responses was sustained for more than one year in mice, which is concurrent with long-lived plasma cells present in bone marrow. Unlike alum, Turbo adjuvanticity is not dependent on canonical or non-canonical inflammasome activation or pyroptosis. Turbo upregulated the expression of the co-stimulatory molecules CD86 and CD40 on B cells, and Turbo adjuvanticity is dependent primarily on the TLR4-MyD88 axis and is lost in mice deficient in CD86 or CD40.
Conclusion: Turbo efficiently engages the required co-stimulatory pathways and promotes long-lasting antibody responses across all ages. Therefore, Turbo can be incorporated into a wide range of mono and multivalent polysaccharide vaccines to enhance immunogenicity and maximize efficacy.
