Manganese-Coordinated Rhein Hydrogel as a Vaccine Adjuvant for Synergistically Enhanced Humoral and Cellular Immunity

Manganese ions (Mn2+), which act as agonists of the cGAS-STING pathway, hold considerable promise as vaccine adjuvants. However, free Mn2+ is rapidly cleared after injection, making it difficult to sustain local immune stimulation. In this study, we exploited the coordination between Mn2+ and rhein, a naturally occurring anthraquinone small molecule, to construct a novel injectable Rhe@Mn hydrogel through one-step self-assembly. Within the hydrogel, densely packed supramolecular nanofibers intertwine to form a three-dimensional network, and energy-dispersive X-ray spectroscopy (EDS) confirmed the uniform distribution of Mn along the nanofibers. Rheological testing showed that the Rhe@Mn hydrogel had favorable viscoelasticity and structural stability. In vitro studies, the Rhe@Mn hydrogel retained the STING-agonist activity of Mn2+ and effectively promoted antigen uptake by DCs and their maturation. In vivo, the hydrogel formed an antigen depot at the injection site, enabling sustained antigen release for up to 10 days. It also efficiently recruited immune cells, including DCs and macrophages, thereby establishing a local immune microenvironment enriched in antigen-presenting cells. After a single immunization, the Rhe@Mn hydrogel vaccine increased the abundance of germinal-center B (GC B) cells in lymph nodes by approximately 3.7-fold, significantly enhanced antigen-specific humoral and cellular immune responses, and established durable immune memory. This study provides a facile, functionally integrated platform for adjuvant-antigen co-delivery in subunit vaccines.