Synergistic protective effect in rabbits of immunization with Vibrio cholerae lipopolysaccharide and toxin/toxoid

Subcutaneous immunization of rabbits with a combination of Vibrio cholerae lipopolysaccharide (LPS) and enterotoxin induced a more than 100-fold-higher degree of protection against intestinal challenge with live cholera vibrios than did vaccination with either of the two antigens alone. Such a synergistic effect was also obtained by immunization with a combination of LPS and choleragenoid. The immunization with LPS and toxin (or toxoid) in combination did not enhance the reistance to toxin challenge above that induced by the toxin component alone. This, together with data from titrations of anti-LPS and antitoxin antibodies in serum and in intestinal washings, contradicts enhanced immune responses due to adjuvant action of the two antigens as the explanation for the synergistic effect of the combined vaccines. A more likely explanation would be that the antibacterial and antitoxic immune responses, without being increased in themselves, function synergistically by interfering with two separate events in cholera pathogensis.