Rational design of next-generation vaccine adjuvants: From molecular mechanisms to hybrid delivery platforms

This review provides a comprehensive analysis of the molecular innovations driving vaccine adjuvant research, moving from empirical formulations to rational designs. After outlining clinical benchmarks, we systematically categorize and evaluate emerging experimental adjuvant classes, including cytokine-based agonists, advanced particulate delivery systems, emulsion-based platforms, and nucleotide-based strategies. Notably, we focus on natural product-derived adjuvants, highlighting how their structure-activity relationships (SAR) drive potent immunomodulatory activities. We further dissect the molecular mechanisms underpinning these adjuvants, detailing how they target specific pattern recognition receptors (PRRs) to orchestrate dendritic cell maturation, antigen cross-presentation, and Th1/Th2 polarization. Crucially, we discuss the rational selection of adjuvants based on specific clinical targets and antigen modalities. Finally, we propose a forward-looking perspective on developing multi-modal strategies that synergize molecular immunostimulants with synthetic delivery vectors.