Vaccine Adjuvant Optimization Strategies

Abstract

Vaccine adjuvants are pharmacologically active components that amplify, accelerate, and shape the adaptive immune response to co-administered antigens, addressing the inherent poor immunogenicity of subunit, recombinant, and mRNA vaccine platforms. This study systematically evaluated and optimised five adjuvant systems -- aluminium hydroxide (Alum), AS01B (MPL + QS-21 in liposomes), AS04 (MPL + Alum), AddaVax (MF59-equivalent squalene-in-water emulsion), and CpG-ODN 1018 (TLR9 agonist) -- formulated with a model recombinant hepatitis B surface antigen (rHBsAg) and a synthetic SARS-CoV-2 spike receptor-binding domain (RBD) antigen across four optimisation dimensions: antigen-adjuvant ratio, adjuvant dose, delivery route (intramuscular vs intradermal), and formulation stability. Immunogenicity was assessed in BALB/c mice (n = 240, 8 groups of 30) at days 14, 28, and 56 by antigen-specific IgG ELISA, IgG subclass profiling, neutralisation assay, and splenocyte ELISpot. AS01B produced the highest IgG titres for both antigens (rHBsAg GMT 148,400 at day 56; spike RBD GMT 94,200), with a balanced Th1/Th2 response (IgG2c/IgG1 ratio = 0.84). CpG-ODN 1018 generated the strongest Th1-polarised cellular immunity (IFN-gamma ELISpot 1,842 +- 214 SFU/10^6 splenocytes), relevant for intracellular pathogens requiring cytotoxic T-cell responses. Intradermal delivery reduced the optimal adjuvant dose 4-fold while maintaining equivalent IgG titres. Formulation stability studies identified AS01B and CpG-ODN as the most thermostable adjuvant systems (6-month stability at 4degC, < 10% potency loss). These findings establish a quantitative optimisation framework for adjuvant selection and formulation design across vaccine platform types and target immune response profiles.