LAUSR

Abstract Most FDA‐approved adjuvants for infectious agents boost humoral but not cellular immunity, and have poorly‐understood mechanisms. Stimulator of interferon genes (STING, also known as MITA, MPYS, or ERIS) is an exciting adjuvant target due to its role in cyclic dinucleotide (CDN)‐driven anti‐viral immunity; however, a major hindrance is STING's cytosolic localization which requires intracellular delivery of its agonists. As a result, STING agonists administered in a soluble form have elicited suboptimal immune responses. Delivery of STING agonists via particle platforms has proven a more successful strategy, but the opportunity for improved formulations and bioactivity remains. In this study we evaluated the adjuvant activity of the potent STING agonist, CDN 3′3′‐cGAMP (cGAMP), encapsulated in acid‐sensitive acetalated dextran (Ace‐DEX) polymeric microparticles (MPs) which passively target antigen‐presenting cells for intracellular release. This formulation was superior to all particle delivery systems evaluated and maintained its bioactivity following a sterilizing dose of gamma irradiation. Compared to soluble cGAMP, the Ace‐DEX cGAMP MPs enhanced type‐I interferon responses nearly 1000‐fold in vitro and 50‐fold in vivo, caused up to a 104‐fold boost in antibody titers, increased Th1‐associated responses, and expanded germinal center B cells and memory T cells. Furthermore, the encapsulated cGAMP elicited no observable toxicity in animals and achieved protective immunity against a lethal influenza challenge seven months post‐immunization when using CDN adjuvant doses up to 100‐fold lower than previous reports. For these reasons, Ace‐DEX MP‐encapsulated cGAMP represents a potent vaccine adjuvant of humoral and cellular immunity. Graphical abstract Figure. No Caption available.