Effective protective immunity to Yersinia pestis infection conferred by DNA vaccine coding for derivatives of the F1 capsular antigen

Three plasmids expressing derivatives of the Yersinia pestis capsular F1 antigen were evaluated for their potential as DNA vaccines. These included plasmids expressing the full-length F1, F1 devoid of its putative signal peptide (deF1), and F1 fused to the signal-bearing E3 polypeptide of Semliki Forest virus (E3/F1). Expression of these derivatives in transfected HEK293 cells revealed that deF1 is expressed in the cytosol, E3/F1 is targeted to the secretory cisternae, and the nonmodified F1 is rapidly eliminated from the cell. Intramuscular vaccination of mice with these plasmids revealed that the vector expressing deF1 was the most effective in eliciting anti-F1 antibodies. This response was not limited to specific mouse strains or to the mode of DNA administration, though gene gun-mediated vaccination was by far more effective than intramuscular needle injection. Vaccination of mice with deF1 DNA conferred protection against subcutaneous infection with the virulent Y. pestis Kimberley53 strain, even at challenge amounts as high as 4,000 50% lethal doses. Antibodies appear to play a major role in mediating this protection, as demonstrated by passive transfer of anti-deF1 DNA antiserum. Taken together, these observations indicate that a tailored genetic vaccine based on a bacterial protein can be used to confer protection against plague in mice without resorting to regimens involving the use of purified proteins.