A novel manganese glycerophosphate vaccine gel elicits broad and durable immunity across an aged and pox virus model.

Subunit vaccines, composed of a protein antigen and an adjuvant, offer a safer and more versatile strategy than traditional live-attenuated vaccines, but limitations of conventional adjuvants like alum require improved design and delivery. Manganese (Mn) has emerged as a novel adjuvant that stimulates the cGAS-STING pathway, showing profound pre-clinical efficacy in vaccines against infectious diseases and cancer, but its potential dose-limiting toxicities require innovative delivery strategies. Herein, we report the development of gel derived from the generally recognized as safe (GRAS) material manganese glycerophosphate (MnGp). The gel displayed tunable controlled antigen release based on MnGp concentration that activated dendritic cells (DCs) in vitro, eliciting substantial production of type I interferons and upregulation of costimulatory markers. A single immunization of mice with ovalbumin (OVA) and 250 mg mL-1 MnGp gel generated the highest and most durable OVA-specific total IgG, IgG1, and IgG2c serum antibody titers. Subsequently, a prime-boost-boost immunization with 250 mg mL-1 MnGp gel elicited a long-lasting OVA-specific IgG, IgG1, and IgG2c sera antibody response and it was superior to MF59-mimic AddaVax and STING agonists 2,3-cGAMP. Splenocytes from mice immunized with MnGp secreted high levels of Th1-associated cytokines upon antigen recall and illustrated generation of memory CD4+ and CD8+ T cells. Immunizing MnGp in 18-month-old mice elicited superior IgG and IgG1 antibody titers compared to Addavax, in addition to specific T cell responses in spleen and the draining lymph node. Finally, the co-immunization of MnGp and B5R (a vaccinia virus protein) induced higher B5R-specific antibody titers than Addavax and achieved full protection against the challenge with vaccinia virus. Overall, these findings corroborate the potential for MnGp gels as a novel vaccine platform.