QS-21 is one of the active fractions of the bark of Chilean tree, Quillaja saponaria, purified using a reverse-phase chromatography (RP-HPLC).QS denotes its source as Q. saponaria and the number 21 as the identity of the RP-HPLC peak. QS-21 is an acylated 3, 28-bisdesmodic triterpene glycosides or โsaponinโ with a molecular formula of C92O46H148 and molecular weight of 1990 Da. It is one of the most potent immunological adjuvants that has been widely used.
The adjuvant effect of saponins was first reported in 1925, when it was shown that the addition of bread crumbs, tapioca, saponin and โstarch oilโ to antigenic preparations greatly enhanced antibody responses to diphtheria or tetanus. In 1951, Espinet used a crude commercially available saponin preparation to increase the potency of foot-and-mouth disease vaccines. Further in 1974, Dalsgaard successfully isolated saponin Quil A from the cortex of the South-American Tree Quillaja saponaria Molina and found that Quil A stimulated both humoral and cellular immunity, as well as induced differential antibody isotypes. Since then, the Quil A has been commercialized and has gained widespread use in veterinary vaccines and pre-clinical studies. Additional studies showed its effects when co-formulated with aluminum salts, liposomes and oil-in-water emulsions, and with amphipathic proteins and lipids forming detergent/lipid/saponin complexs termed immune-stimulating complexes (ISCOMs). However, Quil A is still a heterogeneous product, consisting of up to 23 different saponin peaks detectable by HPLC and its toxicity precluded its use in human vaccines.
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