with a Genetically Disrupted Phage Integrase Gene Exhibits Attenuated Virulence and Induces Protective Immunity against Fatal Rickettsioses in Mice.

Although rickettsiae can cause life-threatening infections in humans worldwide, no licensed vaccine is currently available. To evaluate the suitability of live-attenuated vaccine candidates against rickettsioses, we generated a mutant RPATATE_0245::pLoxHimar (named 3A2) by insertion of a modified pLoxHimar transposon into the gene encoding a phage integrase protein. For visualization and selection, 3A2 expressed mCherry fluorescence and resistance to spectinomycin. Compared to the parent wild type (WT) , the virulence of 3A2 was significantly attenuated as demonstrated by significantly smaller size of plaque, failure to grow in human macrophage-like cells, rapid elimination of and ameliorated histopathological changes in tissues in intravenously infected mice. A single dose intradermal (i.d.) immunization of 3A2 conferred complete protection against both fatal and rickettsioses in mice, in association with a robust and durable rickettsiae-specific IgG antibody response. In summary, the disruption of RPATATE_0245 in resulted in a mutant with a significantly attenuated phenotype, potent immunogenicity and protective efficacy against two spotted fever group rickettsioses. Overall, this proof-of-concept study highlights the potential of mutants as a live-attenuated and multivalent vaccine platform in response to emergence of life-threatening spotted fever rickettsioses.