The Coxiella burnetii QpH1 plasmid is a virulence factor for colonizing bone marrow-derived murine macrophages

Abstract

Coxiella burnetii carries a large conserved plasmid or plasmid-like chromosomally integrated sequence of unknown function. Here we report the curing of QpH1 plasmid from C. burnetii Nine Mile phase II, the characterization of QpH1-deficient C. burnetii in in vitro and in vivo infection models, and the characterization of plasmid biology. A shuttle vector pQGK, which is composed of the CBUA0036-0039a region (predicted for QpH1 maintenance), an E. coli plasmid ori, eGFP and kanamycin resistance genes was constructed. The pQGK vector can be stably transformed into Nine Mile II and maintained at a similar low copy like QpH1. Importantly, transformation with pQGK cured the endogenous QpH1 due to plasmid incompatibility. Compared to a Nine Mile II transformant of a RSF1010-based vector, the pQGK transformant shows an identical one-step growth curve in axenic media, a similar growth curve in Buffalo green monkey kidney cells, an evident growth defect in macrophage-like THP-1 cells, and dramatically reduced ability of colonizing bone marrow-derived murine macrophages. In the SCID mouse infection model, the pQGK transformants caused a lesser extent of splenomegaly. Moreover, the plasmid biology was investigated by mutagenesis. We found CBUA0037-0039 are essential for plasmid maintenance, and CBUA0037-0038 account for plasmid compatibility. Taken together, our data suggest that QpH1 encodes factor(s) essential for colonizing murine macrophages, and to a lesser extent for colonizing human macrophages. This study highlights a critical role of QpH1 for C. burnetii persistence in rodents, and expands the toolkit for genetic studies in C. burnetii. Author summaryIt is postulated that C. burnetii recently evolved from an inherited symbiont of ticks by the acquisition of novel virulence factors. All C. burnetii isolates carry a large plasmid or have a chromosomally integrated plasmid-like sequence. The plasmid is a candidate virulence factor that contributes to C. burnetii evolution. Here we describe the construction of novel shuttle vectors that allow to make plasmid-deficient C. burnetii mutants. With this plasmid-curing approach, we characterized the role of the QpH1 plasmid in in vitro and in vivo C. burnetii infection models. We found that the plasmid plays a critical role for C. burnetii growth in macrophages, especially in murine macrophages, but not in axenic media and BGMK cells. Our work highlights an essential role of the plasmid for the acquisition of colonizing capability in rodents by C. burnetii. This study represents a major step toward unravelling the mystery of the C. burnetii cryptic plasmids.