Surface expression of a protective recombinant pertussis toxin S1 subunit fragment in Streptococcus gordonii

In this study, the expression of the Bordetella pertussis S1 subunit was te sted in Streptococcus gordonii, a commensal oral bacterium which has the po tential to be a live oral vaccine vehicle. The DNA fragment encoding the N- terminal 179 amino acids of the S1 subunit was ligated into the middle part of spaP, the surface protein antigen PI gene originating from Streptococcu s mutans, The resulting construct, carried on the Escherichia coli-Streptoc occus shuttle vector pDL276, was introduced into S. gordonii DL-1 by natura l transformation. One of the transformants (RJMIII) produced a 187-kDa prot ein (the predicted size of the SpaP-S1 fusion protein) which was recognized by both the anti-pertussis toxin (anti-PT) and anti-SpaP antibodies, sugge sting that an in-frame fusion had been made. Results from immunogold-electr on microscopic studies and cellular fractionation studies showed that the f usion protein was surface localized and was mainly associated with the cell wall of RJMIII, indicating that SpaP was able to direct the fusion protein to the cell surface. A rabbit antiserum raised against heat-killed S. gord onii RJMIII recognized the native S1 subunit of PT in Western blotting and showed a weak neutralization titer to PT by the Chinese hamster ovary cell- clustering assay. BALB/c mice immunized with the heat-killed S. gordonii RJ MIII were protected from the toxic effect of PT in the leukocytosis-promoti ng and histamine sensitization assays. In conclusion, a fragment of the S1 subunit of PT was successfully surface expressed in S. gordonii; the recomb inant SI fragment was found to be immunogenic and could induce protection a gainst the toxic effect of PT in mice.