Identification of QS-21 as an inflammasome-activating molecular component of saponin adjuvants

Robyn Marty-Roix, Gregory I. Vladimer, Kimberly Pouliot, Dan Weng, Rachel Buglione- Corbett†, Kim West, John D. MacMicking, Jonathan D. Chee, Shixia Wang, Shan Lu, Egil Lien,

Many immunostimulants act as vaccine adjuvants via activation of the innate immune system, although it is in many cases unclear which specific molecules contribute to the stimulatory activity. QS-21 is a defined, highly purified and soluble saponin adjuvant currently used in licensed and exploratory vaccines, including vaccines against malaria, cancer and HIV-1. However, little is known about the mechanisms of cellular activation induced by QS-21. We observed QS-21 to elicit caspase-1-dependent IL-1β and IL-18 release in antigen-presenting cells such as macrophages and dendritic cells when co-stimulated with the TLR4- agonist adjuvant monophosphoryl lipid A. Furthermore, our data suggests that the ASC- NLRP3 inflammasome is responsible for QS-21- induced IL-1β/IL-18 release. At higher concentrations, QS-21 induced macrophage and dendritic cell death in a caspase-1-, ASC- and NLRP3- independent manner while the presence of cholesterol rescued cell viability but did not alter IL-1 β release A nanoparticulate adjuvant that contains QS-21 as part of a heterogeneous mixture of saponins also induced IL-1β in a NLRP3-dependent manner. Interestingly, despite the role NLRP3 plays for cellular activation in vitro, NLRP3-deficient mice immunized with HIV-1 gp120 and QS-21 showed significantly higher levels of Th1 and Th2 antigen-specific T cell responses and increased IgG1 and IgG2c compared to wild type controls. Thus, we have identified QS-21 as a non-particulate single molecular saponin that activates the NLRP3 inflammasome, but this signaling pathway may contribute to decreased antigen-specific responses in vivo.