QS-21 saponin (QS-21)

QS-21 is a purified saponin extract derived from the bark of the soap bark tree (Quillaja saponaria). It is a complex triterpene glycoside (saponin) composed of a quillaic acid core with attached sugar moieties and a unique acyl chain. QS-21 acts as a potent adjuvant in vaccines, markedly enhancing both humoral and cell-mediated immune responses. It triggers activation of antigen-presenting cells, facilitates antigen uptake and cross-presentation, and potently stimulates T lymphocytes via direct and indirect mechanisms, including inflammasome activation. QS-21 is included in several licensed vaccines (e.g., Shingrix, Mosquirix, Arexvy), usually formulated with other adjuvants (like monophosphoryl lipid A in AS01) to improve both efficacy and safety. Due to its instability and tendency to cause local and systemic side effects, QS-21 is typically delivered in liposomes or nanoparticles, which also direct its delivery to phagocytic immune cells and reduce reactogenicity and hemolysis. Its mechanism is distinct from classical receptor agonists or antagonists, acting as a non-specific enhancer of the adaptive immune response rather than a drug target per se.

Other names

Quillaja saponaria saponin QS-21QS21Quillaja saponin fraction 21Quillaja bark saponin

Acts as an immune adjuvant: stimulates antigen-presenting cells (APCs) and T-cell responses. Promotes Th1 and Th2 cytokine production. Activates NLRP3 inflammasome in APCs, leading to the release of IL-1β and IL-18. Facilitates antigen uptake, processing, cross-presentation to CD8+ T cells, enhances cytotoxic T lymphocyte responses. Stimulates direct T cell activation via CD2 receptor. May form Schiff base with T-cell surface receptors through its C4-aldehyde group, providing co-stimulatory signals.