DNA vaccination for the priming of neutralizing antibodies against non-immunogenic STa enterotoxin from enterotoxigenic Escherichia coli

In order to test the use of DNA vaccination for its capacity to induce antibodies against the non-immunogenic heat-stable enterotoxin STa from Escherichia coli, BALB/c mice were immunized with plasmid DNA encoding hybrid proteins made by the insertion of wild type STa or insertion of the Cys6Ala, Cys17Ala and Cys6Ala-Cys17Ala STa mutants at positions 195 or 216 of the TEM-1 β-lactamase. No STa specific antibodies could be detected after three plasmid injections, but a subsequent boost with native STa peptide was capable of inducing low levels of neutralizing antibodies, as tested in the suckling mouse assay. Highest STa specific responses were found in mice primed with the double mutated STa inserted in position 195. This plasmid induced highest T-cell responses to the TEM-1 protein, indicating that priming of helper T-cell responses to the carrier protein was essential. Mixed IgG1/IgG2a isotypes also reflected this T helper 1 type priming. Moreover, insertion into loop A of the TEM-1 carrier may be more suitable than insertion into loop B, because of reduced competition between carrier and hapten B cell responses.