A recombinant Rickettsia conorii vaccine protects guinea pigs from experimental boutonneuse fever and Rocky Mountain spotted fever

Abstract

There are no vaccines against boutonneuse fever and Rocky Mountain spotted fever. Previous studies have identified a Rickettsia rickettsii surface protein as a vaccine candidate and shown that an antigenically related protein is present in R. conorii, which causes boutonneuse fever. The gene encoding the R. rickettsii protein has been cloned and expressed in Escherichia coli. We confirmed by 7.5% sodium dodecyl sulfate-polyacrylamide gel electrophoresis of rickettsial lysates followed by immunoblotting with a monoclonal antibody raised against the R. rickettsii protein that an analogous protein exists in R. conorii. Although these proteins were previously called 155-kilodalton (kDa) proteins, we found that their apparent molecular masses were 198 kDa for R. conorii Kenya tick typhus and 190 kDa for R. rickettsii R. Using the R. rickettsii gene probe, we cloned and expressed a 5.5-kilobase HindIII fragment from R. conorii Kenya tick typhus genomic DNA in E. coli JM107. The expressed recombinant product was recognized by a monospecific polyclonal rabbit antiserum prepared against the 198-kDa protein. Guinea pigs immunized with sonic lysates of the E. coli strain expressing the recombinant gene product developed antibodies recognizing R. conorii when tested by a microimmunofluorescence antibody assay. Upon immunoblotting of rickettsial lysates, those antisera specifically recognized the 198-kDa R. conorii protein and its 190-kDa analog in R. rickettsii. Guinea pigs immunized with sonic lysates of the recombinant E. coli expressing the 198-kDa protein were protected from experimental infections with the homologous R. conorii strain and partially protected from experimental infections with a strain of the heterologous species R. rickettsii. These findings show that the 198-kDa R. conorii protein is a candidate for a vaccine against boutonneuse fever.