<b><i>Francisella tularensis</i></b> Modulates a Distinct Subset of Regulatory Factors and Sustains Mitochondrial Integrity to Impair Human Neutrophil Apoptosis

Abstract

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.

Neutrophils and Bacterial Immune Evasion

“…F. tularensis is another intracellular bacterial pathogen that delays neutrophil apoptosis by affecting Bcl-2 family proteins. Namely, the pathogen impairs the processing and activation of caspase-8 and Bid, and significantly inhibits apoptosis by impairing translocation of Bax into the mitochondria [56, 57]. More recently, it has been suggested that F. tularensis- secreted bacterial lipoproteins play a role in apoptosis inhibition via a TLR-2 dependent route [58].…”

Bystander Cells Taking Action

“…The importance of microbial virulence factors is nicely illustrated in an article by Blaschke et al [25], in which they show that cytolysins of the important bacterial pathogen Streptococcus pyogenes induce the expression of cyclooxygenase 2, resulting in proinflammatory activities. The importance of the resolution of inflammation to avoid excessive tissue damage, but also from the microbial side to avoid eradication, illustrates the dichotomy of host responses [8,10,26,27,28]. We hope that you will find this issue of Journal of Innate Immunity interesting and thought provoking!…”

“…The same applies for the Chemistry Prize. Electron microscopy is an essential tool when studying innate immunity [6,7,8,9,10,11]. It will therefore be exciting to see how the new developments will advance the forthcoming research outcome.…”

With Complement

“…Francisella tularensis is an aerobic rod-shaped Gram-negative pathogen that can sustain mitochondrial integrity, thereby delaying apoptosis in neutrophils [9]. In this issue Ireland et al [10] show that phospholipids derived from F. tularensis act as immunomodulatory substances, executing anti-inflammatory activity.…”