Structure and adsorption properties of commercial calcium phosphate adjuvant

Calcium phosphate adjuvant is a commercially available vaccine adjuvant that potentiates the immune response to antigens. Although its name suggests that it is Ca3(PO4)2, X-ray diffraction, FTIR spectroscopy, thermal analysis and the Ca/P molar ratio identify commercial calcium phosphate adjuvant as non-stoichiometric hydroxyapatite, Ca10-x (HPO4)x (PO4)6-x (OH)2-x, where x varies from 0 to 2. The surface charge is pH-dependent (point of zero charge = 5.5). Consequently, commercial calcium phosphate adjuvant exhibits a negative surface charge at physiological pH and electrostatically adsorbs positively charged antigens. The presence of hydroxyls allows calcium phosphate adjuvant to adsorb phosphorylated antigens by ligand exchange with surface hydroxyls.