Immunogenicity of liposome-associated oral cholera vaccine prepared from combined Vibrio cholerae antigens

Liposomes were prepared from bovine brain sphingomyelin and cholesterol. They were reinforced by incorporation of osmium tetroxide to prevent their immediate degradation inside the host. Combined Vibrio cholerae antigens (lipopolysaccharide, crude cell-bound hemagglutinin and procholeragenoid) were orally administered to experimental rats either as free or liposome-associated. A total of 70 experimental rats was utilized in experiments comparing the immune responses of rats to liposome-associated vaccine, free vaccine, liposomes, or placebo, and to vaccines where the lipid or antigen levels were reduced. Immediately after feeding with sodium bicarbonate to lower the gastric acidity, they were fed either cholera vaccines or placebo. Results from serum ELISA revealed that the liposomes localized the immune response to the intestinal mucosa. They displayed an adjuvant property in terms of evoking a higher immune response to V. cholerae antigens, as measured by the appearance of specific antibody-producing cells in the intestinal mucosa, than when the antigens were fed alone. The adjuvanticity was found to be lipid dose dependent. Liposomes prepared with high lipid content enhanced immunogenicity of the admixture antigens to a greater degree.