Use of a quartz crystal microbalance platform to study protein adsorption on aluminum hydroxide vaccine adjuvants:...

Aluminum hydroxide (AH) salts are widely used as vaccine adjuvants and controlling antigen-AH interactions is a key challenge in vaccine formulation. In a previous work, we have developed a quartz crystal microbalance (QCM) platform, based on stable AH-coated sensors, to explore the mechanisms of model antigen adsorption. The QCM study of bovine serum albumin (BSA) adsorption at different pH and ionic strength (I) values showed that protein adsorption on AH adjuvant at physiological pH cannot be explained mainly by electrostatic interactions, in contrast with previous reports. Here, we exploit further the developed QCM platform to investigate the role of phosphate-hydroxyl ligand exchanges in the adsorption mechanism of BSA, human serum albumin (HSA) and ovalbumin (OVA) on two commercial AH adjuvants. BSA adsorption decreased on immobilized AH particles previously treated with KH₂PO₄, highlighting the role of exchangeable sites on AH particles in the adsorption process. BSA and OVA were dephosphorylated by treatment with an acid phosphatase to decrease their phosphate content by about 80% and 25%, respectively. Compared to native BSA, adsorption of dephosphorylated BSA decreased significantly on one AH adjuvant at pH 7. Adsorption of dephosphorylated OVA was comparable to the one of native OVA. Further QCM assays showed that phospho-amino acids (PO₄-serine and PO₄-threonine) displaced previously adsorbed BSA and OVA from AH particles in conditions that were depending on the protein and the AH. Taken together, these observations suggest that phosphate-hydroxyl ligand exchange is an important adsorption mechanism of proteins on AH. These results moreover confirm that the developed AH-coated QCM sensors offer a new platform for the study of antigen adsorption, to the benefit of vaccine formulation.