Cloned Gene of Rickettsia rickettsii Surface Antigen: Candidate Vaccine for Rocky Mountain Spotted Fever

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Abstract
Two major protective antigens of Rickettsia rickettsii have been previously described. In this study, we cloned the gene encoding one of these antigens into Escherichia coli and tested the effectiveness of the recombinant-made product as a vaccine for Rocky Mountain spotted fever. A clone bank of R strain R. rickettsii DNA was made in E. coli K-12 by using the plasmid vector pBR322. Transformants were screened for their ability to make rickettsial antigens by reactivity with rabbit antibodies to R. rickettsii. One of the transformants, EM24(pGAM21), made a product reactive with two monoclonal antibodies that recognize a 155-kilodalton protein of R. rickettsii. One of the monoclonal antibodies was a member of a class of antibodies that react to heat-sensitive epitopes and protect mice injected with a potentially lethal dose of viable R. rickettsii. The cloned product contained this protective heat-sensitive epitope. In order to obtain enhanced expression, the gene was subcloned downstream of the lactose promoter on the plasmid vector pUC8. A sonic lysate of E. coli harboring the pUC8 subclone was used successfully as a vaccine to protect mice injected with a lethal dose of the viable R. rickettsii.