From toxin to adjuvant: basic mechanisms for the control of mucosal IgA immunity and tolerance

We provide compelling evidence that delivery of Ag in the absence of ADP-ribosylation can promote tolerance, whereas ADP-ribosyltransferase activity induces IgA immunity and prevents tolerance. By linking Ag to the ADP-ribosylating enzyme, cholera toxin subunit A1 (CTA1), we could show that the combination of targeting to antigen-presenting cells (APC) and enzymatic activity is a highly effective means of controlling the induction of tolerance or immunity. Firstly, we demonstrated that cholera toxin (CT), although potentially binding to all nucleated cells, in fact, bound preferentially to dendritic cells (DC) in vivo. Following injection of CT-conjugated Ag, we found that DC in the marginal zone (MZ) of the spleen accumulated Ag, a process that was GM1-ganglioside receptor dependent. Contrary to CTB, which also delivered Ag to the MZ DC, CT matured and activated co-stimulatory functions in the targeted DC and greatly augmented immune responses to Ag. Secondly, when Ag was incorporated into the CTA1-DD fusion protein, which equals the CT in adjuvant function but lacks GM1-ganglioside-binding ability, we greatly augmented specific responses to Ag. The DD-bound Ag was distinctly targeted to B cells and probably also to follicular dendritic cells (FDC) in vivo. Thus, in both constructs Ag was targeted to APC and associated with an ADP-ribosylating enzyme, which resulted in greatly enhanced immunogenicity. When the enzymatic activity was absent, as in CT B-subunit (CTB) or in the inactive CTA1R7K-DD mutant, Ag largely failed to stimulate an active immune response. Rather, this type of Ag exposure resulted in Ag-specific tolerance, especially when mucosal delivery of Ag was attempted. Therefore, targeting to APC in the absence or presence of the CTA1-enzyme appears to be an effective means to control tolerance and active protective IgA immunity.