Coxiella burnetii Inhibits Neutrophil Apoptosis by Exploiting Survival Pathways and Antiapoptotic Protein Mcl-1.

ABSTRACT

Our previous study demonstrated that neutrophils play an important role in host defense against Coxiella burnetii infection in mice. In this study, avirulent strain C. burnetii Nine Mile phase II (NMII) was used to examine if C. burnetii can modulate mouse bone marrow-derived neutrophil apoptosis. The results indicated that NMII can inhibit neutrophil apoptosis. Western blotting demonstrated that caspase-3 cleavage was decreased in NMII-infected neutrophils, while phosphorylated mitogen-activated protein kinase (MAPK) p38 and extracellular signal-regulated kinase 1 (Erk1) were increased. Additionally, p38, Erk1/2, phosphoinositide 3-kinase (PI3K), or NF-κB inhibitors reduced the ability of NMII to inhibit neutrophil apoptosis. These results suggest that NMII-mediated inhibition of neutrophil apoptosis depends on its ability to activate neutrophil MAPK pathways. Antiapoptotic protein myeloid cell leukemia-1 (Mcl-1) was significantly increased in NMII-infected neutrophils, and an Mcl-1 inhibitor significantly reduced the ability of NMII to inhibit neutrophil apoptosis. Mcl-1 protein stability was enhanced by phosphorylation at Thr-163 by Erk, and the protein levels were regulated by p38, Erk, PI3K, and NF-κB. Furthermore, the observation that a type IV secretion system (T4SS)-deficient dotA mutant showed a significantly reduced ability to inhibit neutrophil apoptosis compared to wild-type (WT) NMII suggests that T4SS-secreted factors may be involved in NMII-induced inhibition of neutrophil apoptosis. Collectively, these results demonstrate that NMII inhibits neutrophil apoptosis through inhibition of caspase-3 cleavage and activation of MAPK survival pathways with subsequent expression and stabilization of antiapoptotic protein Mcl-1, a process that may partially require the T4SS.