Protective immunity evoked by oral administration of attenuated aroA Salmonella typhimurium expressing cloned streptococcal M protein.

Attenuated strains of Salmonella have been used effectively as vaccines against typhoid fever. We have investigated the use of such strains to deliver cloned antiphagocytic virulence determinants of unrelated bacteria. The aroA strain of S. typhimurium SL3261 was transformed with a low-copy plasmid vector pMK207, which contains the cloned gene spm5 encoding streptococcal M protein, the major virulence factor of these organisms. The transformed SL3261 expressed type 5 M protein in the cytoplasmic fraction, and when fed orally to BALB/c mice, evoked both serum and salivary IgA, IgG, and IgM antibodies directed against type 5 M protein. The orally immunized mice were completely protected against both intranasal and intraperitoneal challenge infections with virulent S. typhimurium SL1344 or M5 streptococci. These studies provide evidence that an attenuated strain of Salmonella can be used effectively as a general vaccine vehicle to deliver antiphagocytic virulence determinants of unrelated bacteria.