Antitoxic immunity in experimental cholera: observations with purified antigens and the rat foot edema model.

The recently introduced choleragen-induced rat foot edema model has been employed as a bioassay for evaluating the immunogenicity of three purified preparations containing cholera exo-enterotoxin antigen, choleragen, choleragenoid, and Formalin-treated choleragen (formagen). The results indicated that choleragen evoked antitoxic immunity. Both the degree of resistance to challenge and the serum antibody levels of immunized animals were found to be related to the immunizing dose. Responses to the natural toxoid, choleragenoid, were erratic: some animals responded well and some failed to respond with either serum antibody or resistance to challenge. On the other hand, the artificially prepared toxoid, formagen, was found to be superior to the parent toxin in immunogenicity. Resistance to the choleragen-induced rat foot edema could be transferred passively by means of antibody-containing serum from previously immunized animals. Each of the antigens induced a state of hypersensitivity manifested by an immediate edematous response to challenge with either choleragen or choleragenoid. This condition, which was also passively transferable, suggests that untoward reactions should be anticipated in people receiving multiple doses of immunogens containing the cholera exo-enterotoxin antigen. Some of these observations were repeated, in a preliminary fashion, in an apparently equally suitable mouse foot edema model.