Distinct Cell Death Programs in Monocytes Regulate Innate Responses Following Challenge with Common Causes of Invasive Bacterial Disease

Department of Infection and Immunity, Medical School, University of Sheffield, Sheffield, United Kingdom.
The Journal of Immunology (Impact Factor: 4.92). 09/2010; 185(5):2968-79. DOI: 10.4049/jimmunol.1000805
Source: PubMed


Peripheral blood monocytes represent the rapid response component of mononuclear phagocyte host defense, generating vigorous but finite antibacterial responses. We investigated the fate of highly purified primary human monocytes following phagocytosis of different bacteria. Exposure to high bacterial loads resulted in rapid loss of cell viability and decreased functional competence. Cell death typically involved classical apoptosis. Exposure to high numbers of Escherichia coli and Klebsiella pneumoniae induced nonapoptotic death with loss of cell membrane integrity, marked disruption of phagolysosomes, and caspase-1 activation, while a subset of cells also released caspase-1-regulated extracellular traps. Classical apoptosis increased if extracellular bacterial replication was reduced and decreased if intracellular ATP levels were reduced during these infections. Both classical apoptosis and the alternative forms of cell death allowed monocytes, whose functional competence was exhausted, to downregulate reactive oxygen species and proinflammatory cytokine responses. In contrast, sustained stimulation of glycolytic metabolism and mitochondrial oxidative phosphorylation, with associated hypoxia inducible factor-1alpha upregulation, maintained intracellular ATP levels and prolonged monocyte functional longevity, as assessed by maintenance of phagocytosis, reactive oxygen species production, and proinflammatory cytokine generation. Monocyte innate responses to bacteria are short-lived and are limited by an intrinsic program of apoptosis, a response that is subverted by overwhelming infection with E. coli and K. pneumoniae or bacterial stimulation of cell metabolism. In this regard, the fate of monocytes following bacterial challenge more closely resembles neutrophils than macrophages.

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    • "One hundred microlitres were streaked onto selective plates (LK plus 5 μg ml À1 of tetracycline (Sigma) and 5 μg ml À1 of citrate (Sigma)), incubated at 37 °C for 16 h and then resubcultured the next day. E. coli strain C29, group 2 capsular serotype K54, was grown in BHI medium to mid-log phase (Webster et al., 2010), S. pneumoniae D39 was grown as previously described (Dockrell et al., 2003). S. aureus and S. pneumoniae were stored in frozen aliquots at À80 °C and thawed before infection. "
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    • "IL-6, another cytokine of the pro-inflammatory pathway, could contribute as well to bystander-apoptosis, since IL-6 may lead to NF-kappaB driven production of reactive oxygen species, resulting in activation of the intrinsic apoptotic pathway [28], [32]. In contrast to TNF-α we found IL-6 neither to induce apoptosis in the concentration range used, nor to enhance PICD after infection with E. coli (Fig. 4B). "
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