A genetically engineered non-toxic vaccine adjuvant that combines B cell targeting with immunomodulation by cholera toxin A1 subunit

Cholera toxin (CT) is an exceptionally potent adjuvant but, unfortunately, also very toxic. Here we present a powerful new approach to separate toxicity from adjuvanticity by constructing a fusion protein that combines the enzymatically active cholera toxin A1 subunit (CTA1) with targeting to B cells. The CTA1 was genetically linked at its C-terminal end to two Ig-binding domains, DD, of staphylococcal protein A and produced in Escherichia coli. The highly purified, monomeric CTA1-DD fusion protein, with a molecular mass of 37 kDa, was found to exhibit strong ADP-ribosyltransferase activity and bound, via the DD moiety, to both Fc and Fab fragments and to all IgG subclasses--IgE, IgA, and IgM. After i.v. injection of the fusion protein, FACS analysis revealed binding of CTA1-DD to splenic IgM+ B cells, but not CD3+ T cells, indicating cell-specific targeting in vivo. Strikingly, we found that the adjuvant ability of CTA1-DD to enhance systemic IgG as well as mucosal IgA responses to the unrelated Ags, OVA, or keyhole limpet hemocyanin, administered i.v or intranasally, was comparable to that of intact CT. In addition, the enhancing effect on specific IgG1, IgG2a, and IgG2b responses mimicked that of CT and suggested involvement of both Th1 and Th2 CD4+ T cell activity. The CTA1-DD, as well as CT, up-regulated expression of the CD80 and CD86 molecules on the targeted B cells, indicating that enhanced T cell costimulation may be responsible for the adjuvant effect. Contrary to CT, however, CTA1-DD was completely nontoxic. Thus, the CTA1-DD adjuvant should find general applicability in systemic and mucosal vaccines, and the strategy used may also be explored for other regimens requiring targeted immunomodulation.