Unsaturated Squalene Content in Emulsion Vaccine Adjuvants Plays a Crucial Role in ROS-Mediated Antigen Uptake and Cellular Immunity

Emulsion-based adjuvants have been demonstrated to be an effective tool in increasing vaccine efficacy. Here, we aimed to launch a mechanistic study on how emulsion adjuvants interact with immune cells and to elucidate the roles of the core oil in vaccine immunogenicity. Our results showed that treatment of dendritic cells (DCs) and splenocytes with a squalene-based emulsion (referred as SqE) induced reactive oxidative species (ROS) production and resulted in an increase in apoptotic and necrotic cells in a concentration- and time-dependent manner. Furthermore, DCs cocultured with cellular debris of SqE-pretreated splenocytes resulted in a higher level of ovalbumin (OVA) antigen uptake by DCs than those cocultured with untreated splenocytes. Interestingly, the potency was rather attenuated when splenocytes were pretreated with N-acetyl-cysteine, an antioxidant. Notably, SqE possesses a high impact on eliciting ROS-mediated antigen uptake compared with a squalane-based emulsion (SqA). Concordantly, immunogenicity studies have shown that SqE is better able than SqA to activate antigen-presenting cells, and to enhance antigen-specific T-cell immunity. Taken together, our results show that unsaturated squalene oil cored within emulsions plays a crucial role in ROS-mediated antigen uptake and cellular immunity, providing a basis for the design and development of vaccine adjuvant.