Immunization of mice by oral colonization with live recombinant commensal streptococci

To test the use of recombinant streptococci as live vaccine vectors, colonization/immunization experiments were performed with Streptococcus gordonii expressing heterologous cell-surface antigens. Three isogenic strains of S. gordonii were used: a wild-type, a recombinant expressing the M6 protein of Streptococcus pyogenes, and a recombinant expressing the E7 protein of human papillomavirus type 16 as a fusion with the M6 protein. A single dose of live bacteria was used to inoculate outbred mice, and it was found that: (i) mice were stably colonized by a single intranasal/oral inoculum of S. gordonii; (ii) recombinant strains were equally effective as wild-type in colonizing mice; (iii) two months after the inoculum, oral/pharyngeal swabs of 83.3% of animals were still positive for isolation of S. gordonii; (iv) recombinant S. gordonii isolated from colonized mice were always positive for expression of the heterologous antigens; (v) live bacteria induced a systemic immune response, since sera of mice colonized with recombinant S. gordonii contained IgG specific for the heterologous cell-surface antigens; (vi) this immune response depended upon the effective colonization by live bacteria, since killed bacteria did not induce such a response.