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 Ca-3(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), wherex varies from 0 to 2. The sur lace 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 hydroxylss allows calcium phosphate adjuvant to adsorb phosphorylated antigens by ligand exchange with surface hydroxyls. (C) 2004 Elsevier Ltd. All rights reserved.