Bacterial lipoproteins and other factors released byFrancisella tularensismodulate human neutrophil lifespan: Effects of aTLR1SNP on apoptosis inhibition

Abstract

Francisella tularensis infects several cell types including neutrophils, and aberrant neutrophil accumulation contributes to tissue destruction during tularaemia. We demonstrated previously that F. tularensis strains Schu S4 and live vaccine strain markedly delay human neutrophil apoptosis and thereby prolong cell lifespan, but the bacterial factors that mediate this aspect of virulence are undefined. Herein, we demonstrate that bacterial conditioned medium (CM) can delay apoptosis in the absence of direct infection. Biochemical analyses show that CM contained F. tularensis surface factors as well as outer membrane components. Our previous studies excluded roles for lipopolysaccharide and capsule in apoptosis inhibition, and current studies of [14C] acetate‐labelled bacteria argue against a role for other bacterial lipids in this process. At the same time, studies of isogenic mutants indicate that TolC and virulence factors whose expression requires FevR or MglA were also dispensable, demonstrating that apoptosis inhibition does not require Type I or Type VI secretion. Instead, we identified bacterial lipoproteins (BLPs) as active factors in CM. Additional studies of isolated BLPs demonstrated dose‐dependent neutrophil apoptosis inhibition via a TLR2‐dependent mechanism that is significantly influenced by a common polymorphism, rs5743618, in human TLR1. These data provide fundamental new insight into pathogen manipulation of neutrophil lifespan and BLP function.

Temporal Manipulation of Mitochondrial Function by Virulent Francisella tularensis To Limit Inflammation and Control Cell Death

“…tularensis is likely variable depending on the cell type or tissue environment. Our results are consistent with reports in which SchuS4 infection delayed apoptosis in human neutrophils (30,31). However, we found capsule inhibited apoptosis in BMM, which was not evident with human neutrophils (30).…”

“…Our results are consistent with reports in which SchuS4 infection delayed apoptosis in human neutrophils (30,31). However, we found capsule inhibited apoptosis in BMM, which was not evident with human neutrophils (30). Dissimilarity in metabolic programming between neutrophils and BMM, as well as the use of immortalized cell lines which are more glycolytic, could account for observations of apoptosis in SchuS4 infection (30,32,33).…”

“…However, we found capsule inhibited apoptosis in BMM, which was not evident with human neutrophils (30). Dissimilarity in metabolic programming between neutrophils and BMM, as well as the use of immortalized cell lines which are more glycolytic, could account for observations of apoptosis in SchuS4 infection (30,32,33). The presence or absence of apoptosis with SchuS4 infection may also be influenced by the tissue environment in vivo.…”

Neutrophils and Bacterial Immune Evasion

“…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]. …”

Production of Neutrophil Extracellular Traps Contributes to the Pathogenesis of Francisella tularemia

“…Previous studies have shown that Ft parasitizes inside the neutrophil and suppresses the neutrophil's antimicrobial defense by impairing respiratory burst, thus allowing the Ft to escape from phagosomal killing (15)(16)(17)(18).Ft-mediated suppression of neutrophil respiratory burst occurs via disruption of NADPH oxidase enzyme complex that aids in bacterial survival in these immune cells (15,32). Simultaneously, Ft also enhances neutrophil life-span by inhibiting their apoptotic signaling pathway in vitro (18,33,34).…”