Mucosal unresponsiveness to aflatoxin B1 is not broken by cholera toxin.

Rabbits immunized via chronically isolated ileal loops with aflatoxin B1 (AFB) conjugated to porcine thyroglobulin (TG) mixed with the mucosal adjuvant cholera toxin (CT) produced very small mucosal antibody responses to AFB. Strong mucosal and systemic antibody responses to CT and TG were generated by this immunization protocol, suggesting that the observed unresponsiveness was specific to AFB. Parenteral immunization with AFB-TG produced strong serum IgG anti-AFB responses, indicating that the conjugate preparation was immunogenic and that the rabbits possess the requisite systemic B and T cell repertoires to recognize and respond to AFB. This mucosal unresponsiveness was distinct from oral tolerance, as animals immunized mucosally with AFB-TG mixed with CT produced vigorous serum IgG anti-AFB responses upon subsequent parenteral immunization with AFB-TG. In vitro mitogen stimulation of lymphocytes isolated from Peyer's patches and mesenteric lymph nodes of unimmunized rabbits revealed the presence of AFB-specific B cells at levels comparable with these found in the spleen. These observations indicate that unresponsiveness to AFB is hapten-specific, restricted to the mucosa, and refractory to the adjuvancy of CI.