Significance of antigen adsorption by aluminum-containing adjuvants for immunopotentiation

Ilia Z Romero Mendez, Purdue University


The degree of antigen adsorption by aluminum-containing adjuvants is considered an important issue during vaccine formulation. Regulatory agencies, i.e. the FDA, require that a high percentage of the antigen is adsorbed by the aluminum-containing adjuvant in a vaccine formulation. This regulation is based on the depot mechanism, which stipulates that the antigen is slowly released from the adjuvant surface following administration. Thus, the immune system is constantly stimulated inducing an enhanced response. However, the degree of antigen adsorption in the vaccine formulation might change when administered. It was of interest to study systems in which a model antigen was not adsorbed to an aluminum-containing adjuvant in the vaccine formulation and upon administration. Three different model vaccines were developed in which the antigen was not adsorbed by an aluminum-containing adjuvant. In the first system aluminum phosphate adjuvant was treated with a phosphate solution to minimize its ability to adsorb antigens through ligand exchange mechanism. Because of the pre-treatment adsorption of dephosphorylated alpha casein was prevented in the vaccine formulation as well as when exposed to interstitial fluid. Aluminum phosphate adjuvant and ovalbumin were used as a second system. Ovalbumin was treated with potato acid phosphatase to reduce phosphorylation from 1.8 to 0.14 mol PO4/mol ovalbumin. In vitro adsorption in the vaccine formulation was prevented as well as when exposed to interstitial fluid. The third system consisted of lysozyme and aluminum phosphate adjuvant. In order to prevent lysozyme adsorption, aluminum phosphate adjuvant was pre-treated with fibrinogen, a protein present in interstitial fluid known to bind strongly to aluminum phosphate adjuvant. The immune response was evaluated in terms of antibody production. It was found that all three systems produced antibody titers statistically higher than vaccines composed of a solution of the antigen and comparable to vaccines with adsorbed antigens. The adjuvant effect observed when the antigen was not adsorbed is believed to be related to the structure of aluminum-containing adjuvants. The primary adjuvant particles do not exist as separate entities but as aggregates. It is thought that the antigen is trapped inside these aggregates thereby, retaining the antigen at the site of injection.




Hem, Purdue University.

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