Francisella tularensis Modulates a Distinct Subset of Regulatory Factors and Sustains Mitochondrial Integrity to Impair Human Neutrophil Apoptosis

Tularemia is a disease characterized by profound neutrophil accumulation and tissue destruction. The causative organism, Francisella tularensis, is a facultative intracellular bacterium that replicates in neutrophil cytosol, inhibits caspase activation and profoundly prolongs cell lifespan. Here, we identify unique features of this infection and provide fundamental insight into the mechanisms of apoptosis inhibition. Mitochondria are critical regulators of neutrophil apoptosis. We demonstrate that F. tularensis significantly inhibits Bax translocation and Bid processing during 24-48 h of infection, and in this manner sustains mitochondrial integrity. Downstream of mitochondria, X-linked inhibitor of apoptosis protein (XIAP) and proliferating cell nuclear antigen (PCNA) inhibit caspase-9 and caspase-3 by direct binding. Notably, we find that PCNA disappeared rapidly and selectively from infected cells, thereby demonstrating that it is not essential for neutrophil survival, whereas upregulation of calpastatin correlated with diminished calpain activity and reduced XIAP degradation. In addition, R-roscovitine is a cyclin-dependent kinase inhibitor developed for the treatment of cancer; it also induces neutrophil apoptosis and can promote the resolution of several infectious and inflammatory disorders. We confirm the ability of R-roscovitine to induce neutrophil apoptosis, but also demonstrate that its efficacy is significantly impaired by F. tularensis. Collectively, our findings advance the understanding of neutrophil apoptosis and its capacity to be manipulated by pathogenic bacteria.