Potentiality of gelatin microsphere as immunological adjuvant

This paper describes a new attempt to enhance the production of antibody by delivery of an antigen to phagocytic antigen-presenting cells (e.g. macrophages) using gelatin microspheres. A model protein antigen, human gamma globulin (HGG), was incorporated into microspheres composed of gelatin which have an opsonic ability for macrophage phagocytosis. Subcutaneous injection of the microspheres induced the production of HGG-specific IgG antibody in the mouse serum to a great extent compared with that of HGG in soluble form or in Freund's incomplete adjuvant (FIA) form. There was an optimal concentration of cross-linking agent (glutaraldehyde) for the highest production of antibody. When gelatin microspheres were cross-linked at lower concentrations of glutaraldehyde, they were more extensively swollen in an aqueous solution, leading to an increase in the size of hydrated microspheres because of their lower cross-linking densities. The increased size of microspheres caused a decrease in their macrophage phagocytosis, whereas the release rate of HGG from the microspheres increased as the concentration of cross-linking agent became low. The balance of the two factors, the microsphere susceptibility to macrophage phagocytosis and the rate of HGG release, seemed to affect the efficacy of gelatin microspheres to enhance the antibody production. In addition, incorporation of HGG into gelatin microspheres enhanced the delayed-type hypersensitivity reaction. Moreover, the microspheres developed a strong secondary response in comparison with FIA. The gelatin microspheres induced a minimal inflammatory response around the injection site in contrast to FIA. These findings demonstrate that the gelatin microsphere is promising as an adjuvant to enhance both humoral and cellular immune responses to antigen.