Humoral and cellular immune responses in the murine respiratory tract following oral immunization with cholera toxin or Escherichia coli heat-labile enterotoxin

Cholera toxin (CT) and Escherichia coli heat-labile enterotoxin (LT) are the strongest mucosal immunogens identified to date and are also good adjuvants when given orally together in combination with unrelated antigens. We used these potent immunogens to monitor local and systemic immune responses following oral immunization of BALB/c mice, and compared their action on the following: (a) immunoglobulin production rates (IgG, IgM and IgA) in mucosal inductive (Peyer's patches-PPs), effector (intestinal lamina propria-LP, respiratory tract) and systemic (spleen) sites; (b) analysis of systemic antigen-specific antibodies (IgG subclasses, IgA and IgE); (c) time monitoring of fecal anti-CT and anti-LT antibodies, and (d) in vivo relevance of interleukin-6 (IL-6) to mucosal responses. Both mucosal immunogens elicited specific antibody responses (IgA, IgG) not only in the gastrointestinal tract (PP's and intestinal LP), but also in the respiratory tract and spleens of orally immunized mice. These mucosal responses were accompained by elevated secretion of IL-6 in all investigated tissues, indicating involvement of this cytokine in B-cell maturation processes. Furthermore, oral immunization with CT and LT induced elevated serum titers of IgG1 followed by IgG2a, IgG2b, IgG3 and IgA, while high antigen-specific IgA and IgG1 responses were found in fecal extracts. These findings illustrate the action of orally administered CT and LT, respectively, on several humoral and cellular immune responses not only at the gastrointestinal tract, the application site, but also in distant mucosal effector sites such as the respiratory tract. These data suggest the potential use of these mucosal adjuvants in oral immunization strategies to improve the local immune response in remote mucosal tissues, in accordance with the concept of a common mucosa-associated immune system.