Antitoxic immunity in experimental cholera: protection, and serum and local antibody responses in rabbits after enteral and parenteral immunization.

The protective effect of enternal and parenteral immunization with cholera toxin antigen against experimental cholera in rabbits was studied by using the small-bowel loop technique. Subcutaneous injection of crude toxin as well as purified toxin or toxoids gave rise to significant protection against toxin challenge. The enhanced resistance to toxin was found to correspond to a many-fold higher magnitude of protection against challenge with live vibrios. In the primary response the protection increased during the first month. Booster immunization gave rise to a further increased immunity which, however, declined rapidly. Multiple oral or repeated intraintestinal antigen administrations also induced protective antitoxic immunity although of less magnitude than that obtained by parenteral immunization. Enteral and, to a lesser extent, parenteral immunization gave rise to increased antitoxic antibody titers and immunoglobulin levels in intestinal washings and mucosa scraping. Immunoglobulin G (IgG) and IgG antitoxins predominated, but after enteral immunization total IgA and specific IgA antibodies occasionally reached levels similar to those for IgG. In serum, significantly increased antibody levels (IgG) were only recorded after parenteral immunization. Both the primary binding and the neutralizing antitoxin titers showed a stayistically significant correlation with the degree of protection against toxin challenge; however, for the neutralizing antibodies this correlation was not without exceptions. No relation to protection was found for intestinal antibodies. The results of the present study indicate that enternal as well as parenteral immunization with toxin antigen can give rise to effective cholera immunity. After enternal immunization, the protection appears to be medicated by locally synthesized antibodies. After parenteral vaccination both serum-derived and locally produced antibodies seem to be effective.