Because IgA appears to play an important role in controlling microbial adherence and in limiting antigen absorption into the body, its regulation is of great consequence, but is poorly understood. In previous years of this grant we have discovered that the feeding of most protein antigens does not result in IgA responses in the intestine, that this is due at least in part to active inhibition by suppressor T cells, and that the same suppressor Tcell appears to mediate oral tolerance as well. The one exception to this rule is cholera toxin (CT) which is a very potent immunogen in the intestine and which does not cause oral tolerance. CT is able also to alter the response to another protein antigen when both are fed together, stimulating an intestinal IgA response and abrogating oral tolerance to the other antigen. We have postulated that these effects may be due to CT's ability to inhibit suppressor cells in GALT, and in fact CT and its B subunit are potent inhibitors of T cell activation in vitro. However, the mechanisms of CT's properties and potency in the intestine remain unclear and form the focus of this renewal. The specific aims of this proposal include first, and evaluation of the mucosal immunogenicity of CT subunit derivatives, including A subunit conjugated to various lectins and B subunit coupled to various lectin and non-lectin protein antigens; this should provide new insights in to the importance of these subunits in the holotoxin's mucosal immunogenicity and adjuvanticity. Second, the hypothesis that the inhibitory properties of CT and its B subunit are important in their mucosal immunogenicity will be examined by further defining the cellular basis of this inhibition and by testing lectins with similar inhibitory properties for mucosal immunogenicity. Third, the epitopes important in antigen presentation to CT-specific T cells will be defined. Once these are defined, epitope- specific T cell clones will be developed and used to examine which cell types in GALT, eg, macrophages, dendritic cells, B cells, or enterocytes, are important in presenting CT to T cells. Fourth, CT-specific T cell clones from GALT will be compared to similar clones from peripheral lymphoid tissue for the production of lymphokines or immunoglobulin binding factors that specifically help IgA responses. Altogether these studies should yield important new insights into the induction and regulation of secretory IgA responses to CT, which may allow the future manipulation of mucosal IgA responses to other types of protein antigens.