Induction of virus-neutralizing antibodies by bacteria expressing the C3 poliovirus epitope in the periplasm. The route of immunization influences the isotypic distribution and the biologic activity of the antipoliovirus antibodies.

Two viral epitopes (C3 neutralization epitope from poliovirus type 1 and the 132-145 peptide from the PreS2 region from hepatitis B virus) have been expressed in the Escherichia coli periplasm as protein fusion with the maltose binding protein (MalE protein). Immunization of mice with live bacteria expressing the foreign viral epitopes in their periplasm elicited high antibody titers against the viral peptide as well as against the corresponding virus. This demonstrates for the first time in the case of defined epitopes that, when live bacteria are used as immunogens, presentation at the cell surface is not a prerequisite to obtain an antibody response. On the other hand, the induction of antiviral antibody responses by these recombinant bacteria depended dramatically on the route of immunization: a response was induced by live bacteria through the i.v. route but not through the s.c. route. However, when bacteria were heat killed or when the MalE hybrid protein was released under a soluble form from the cell, a response was induced even upon s.c. immunization. From these results, we suggest that in order to induce high levels of antibodies by the s.c. route, a major parameter for bacterial Ag would be their capacity to be released into a soluble form before the interaction of the bacteria with the APC. This would permit the presentation by B cells rather than by phagocytic cells. Finally, we demonstrate that the route of immunization influences the isotypic distribution and the neutralizing activity of the antipoliovirus antibodies. Such results may have major implications for the development of bacterial vaccines based on fusion proteins.