Mucosal tolerance to haptens is not broken by cholera toxin

Date of Completion

January 1997


Health Sciences, Toxicology|Health Sciences, Immunology




In preliminary experiments, mucosal unresponsiveness to the dietary carcinogen aflatoxin B$\sb1$ (AFB) was observed in rabbits immunized mucosally with AFB conjugated to porcine thyroglobulin (TG) mixed with cholera toxin (CT). In the first part of this study, the hypothesis was tested that unresponsiveness was AFB specific and restricted to the mucosal immune system. Unresponsiveness was specific to AFB, as strong mucosal and systemic antibody responses to CT and TG were generated by this immunization protocol. Parenteral immunization with AFB-TG produced strong serum IgG anti-AFB responses, indicating that the conjugate preparation was systemically immunogenic. Mucosal unresponsiveness was distinct from oral tolerance, as animals immunized mucosally with AFB-TG plus CT produced vigorous serum IgG anti-AFB responses following parenteral immunization with AFB-TG. In vitro mitogen stimulation of mucosal lymphocytes isolated from unimmunized rabbits revealed the presence of AFB-specific B cells at levels comparable to those found among splenic lymphocytes. These observations indicated that unresponsiveness to AFB is hapten-specific, restricted to the mucosa, and not broken by CT. In the second phase of the study, the ability of an orally administered antigen to induce mucosal tolerance was studied. The hypotheses tested were that (i) a low dose of antigen given orally may induce mucosal tolerance without systemic "oral" tolerance, (ii) an antigen dose that induces oral tolerance would also induce mucosal tolerance, and (iii) CT would not break an established tolerance to an orally administered antigen. Mice were given either a high (5.0 mg) or low (0.05 mg) oral dose of the contact sensitizing agent dinitrochlorobenzene (DNCB) prior to mucosal or systemic immunization. The low dose induced mucosal tolerance, evidenced by reduced antigen-specific fecal antibody responses following oral immunization, but not oral tolerance, whereas the high dose induced oral and mucosal tolerance. CT-induced systemic antibody responses were also suppressed by pretreatment with either dose of DNCB. These results suggest that the mucosal arm of the immune system may be more sensitive to the induction of tolerance to orally administered antigens than the systemic arm, and that CT may not be an effective adjuvant for antigens to which the mucosal immune system has previously been exposed. ^