Immunomodulatory Effects of Saponins

Saponins are amphipathic compounds consisting of one or more hydrophilic glycoside moieties attached to a lipophilic triterpene derivative. These naturally formed substances are used in many pharmaceutical applications.

Saponins are amphipathic compounds consisting of one or more hydrophilic glycoside moieties attached to a lipophilic triterpene derivative. These naturally formed substances are used in many pharmaceutical applications. The most prominent saponin-based adjuvant is Quil-A. Quil-A is a heterogeneous mixture of water-soluble saponins extracted from Quillaja saponaria, a tree native to South America. It has been shown that adding Quil-A to FMD and porcine reproductive and respiratory syndrome (PRRS) vaccines improves their immunogenicity compared to vaccine groups that did not receive an adjuvant. QS-21, the purified part of Q. saponaria, is currently used in multiple clinical trials for human cancer and infectious vaccines. Moreover, QS-21 is used as an adjuvant in a commercially available vaccine for feline leukemia.

Studies on the immune regulatory activities of Quil-A and QS-21 have shown that they can elicit cell-mediated immune responses by stimulating both Th1 and cytotoxic lymphocytes. Q. saponins can induce the production of IgG1 and IgG2a antibodies in mice. Great effort has been made to understand the interconnection between the structural components of saponin and their immunological functions. However, the sugar groups of saponins can bind to lectins in APCs. This promotes antigen phagocytosis and stimulates APCs to secrete cytokines responsible for activation of humoral and cell-mediated responses. It is assumed that the aldehyde group in the triterpene type of the saponin molecule interacts with T cell surface receptors and facilitates its co-stimulation. This information can be used in the development of synthetic analogs of naturally derived saponins and in the design of saponin molecules with preserved immune activity and reduced toxicity. For example, extract obtained from the leaves of Q. brasiliensis is shown to be less toxic.