Mechanism of immunopotentiation by aluminum-containing adjuvants elucidated by the relationship between antigen retention at the inoculation site and the immune response.

The relationship between depot formation and immunopotentiation was studied by comparing the retention of antigen at the inoculation site with antibody production in rats. A model (111)In-labeled alpha casein (IDCAS) antigen was formulated into four vaccines: IDCAS adsorbed onto either aluminum hydroxide adjuvant (AH) or aluminum phosphate adjuvant (AP); non-adsorbed IDCAS with phosphate-treated AP (PTAP); and IDCAS solution. Gamma scintigraphy showed the order of retention following subcutaneous administration to be: AH adsorbed>AP adsorbed>non-adsorbed with PTAP=solution. The antibody titers followed the order: non-adsorbed with PTAP=AP adsorbed>AH adsorbed>>solution. The presence of an aluminum-containing adjuvant was essential for immunopotentiation, but retention of the antigen at the inoculation site was not required.