Weak anamnestic responses of inbred mice to Yersinia F1 genetic vaccine are overcome by boosting with F1 polypeptide while outbred mice remain nonresponsive.

The role of immunity to intracellular Ags in resistance to infection by Yersinia is not well established. The enteropathogenic bacteria Yersinia pseudotuberculosis and Yersinia enterocolitica actively translocate Ags to the cytosol of eukaryotic cells. Whereas Yersinia pestis does not always express the requisite cellular adhesins, results have varied as to whether similar cytosolic translocation of Ags occurs in vitro. We used a genetic vaccine to induce intracellular expression of the fraction 1 (F1) capsular protein of Y. pestis within host mammalian cells and examined the ensuing immune response. The F1 genetic vaccine stimulated only weak CTL responses in BALB/c mice. Substantial Ab responses to the F1 genetic vaccine were obtained in all inbred strains of mice tested, but Ab levels were less than those resulting from vaccination with the F1 polypeptide. In contrast, outbred mice did not respond to the F1 plasmid, suggesting that some inbred mouse strains may exhibit exaggerated responses to plasmid vaccines. A primary immunization with the F1 genetic vaccine followed by a boost with recombinant F1 polypeptide produced a vigorous Ab response from inbred mice that was equivalent to three injections of F1 polypeptide. We conclude that cytosolic expression of the F1 Ag efficiently primes immunity, while secondary exposure to the F1 polypeptide is required for optimal Ab induction.