Mechanism of adsorption of three recombinant Streptococcus pneumoniae (Sp) vaccine antigens by an aluminum adjuvant.

The adsorption of three Streptococcus pneumoniae (Sp) vaccine antigens by aluminum-containing adjuvants was studied. The antigens showed high binding affinity isotherms with aluminum hydroxide adjuvant described by the Langmuir equation but virtually no binding to aluminum phosphate adjuvant. The effects of ionic strength and ethylene glycol were evaluated to determine whether electrostatic or hydrophobic interactions were responsible for the observed binding to aluminum hydroxide, but no significant change in the adsorptive capacity was observed at either high ionic strength nor high concentrations of ethylene glycol for any of the antigens. This indicates that neither electrostatic nor hydrophobic interactions appear to be responsible for the observed binding, which means that ligand exchange may be the primary mechanism for this interaction. Further studies to evaluate the ability to elute a Sp antigen from aluminum hydroxide using fibrinogen (adsorptive coefficient 2.2 mL/microg) as a competitive protein resulted in evidence that Sp antigen follows the trend that proteins with higher adsorptive coefficients are able to displace those with lower adsorptive coefficients. It was also noted that the Sp antigens and alpha-lactalbumin (adsorptive coefficient 1.8 mL/microg) have similar adsorptive coefficients indicative of high affinity binding isotherms but do not contain phosphate, which has previously been used to explain ligand exchange for such proteins as alpha-casein and hepatitis B surface antigen (HBsAg). Further investigations using alpha-lactalbumin as a model protein may elucidate the binding interaction between the antigens in this study and aluminum adjuvants.