Combinations of alum and synthetic toll-like receptor agonists as adjuvants for CoVID-19 vaccines

Abstract

Abstract Aluminum-based adjuvants (“alum”) are the most widely-used adjuvants for human vaccines, with an extensive history of safety and efficacy. Alum is known to elicit high antibody titers by driving a Th2-based immune response, which may not be optimal for many viral infections, such as SARS-CoV-2. Therefore, combining the strong safety and efficacy attributes of alum with a Th1-polarizing adjuvant could improve immunity against viral antigens. Here we test the combination of alum with synthetic TLR4- and TLR7/8 -ligands in vaccines against SARS-CoV-2. The combination of alum and TLR agonists resulted in efficient adjuvantation of both humoral and cell-mediated immunity. Alum adsorption studies showed low association between the small, positively-charged receptor binding domain (RBD) antigen and aluminum hydroxide (Alhydrogel) and high adsorption to negatively charged aluminum phosphate (Adju-phos). However, Adju-phos and Alhydrogel both enhanced immunity to the SARS-CoV-2 RBD antigen, suggesting antigen adsorption may not be required for the immune enhancing effects of alum. These data demonstrated that adjuvants combining alum with a TLR agonist result in an improved anti-viral immune response. However, the ability of alum to adsorb to an antigen may not predict its ability to effectively adjuvant.