Hypoxia, reactive oxygen species and glucose metabolism in the regulation of the innate immune system
Item statusRestricted Access
Embargo end date01/07/2020
Willson, Joseph Alexander
Introduction - Neutrophils are the most abundant innate immune cells, essential for clearing pathogens from injured or infected sites and directing the ensuing inflammatory response. Sites of inflammation tend to be hypoxic due to rapid tissue volume expansion, loss of vascular integrity and oxygen consumption by infiltrating immune cells. Neutrophils are well suited to the hypoxic niche and have metabolic adaptations that enable them to exist in hypoxia, including a reliance on anaerobic respiration, despite possessing a mitochondrial network. The role of neutrophil mitochondria is unclear, and relatively few studies have examined what role the mitochondria play in regulating neutrophil apoptosis and function, and how hypoxia modulates anaerobic and aerobic neutrophil metabolism. Methods - Neutrophils isolated from healthy volunteers were cultured in normoxia (21% O2) and hypoxia (1% O2) in vitro and neutrophil function and metabolic flux interrogated ex vivo. Functional consequences of hypoxic response pathway manipulation were investigated in infection models carried out in mice with myeloid specific PHD3 knockout. Results – Neutrophil mitochondrial membrane potential and reactive oxygen species production are enhanced in hypoxia through a mechanism independent of oxidative phosphorylation. Neutrophil mitochondrial reactive oxygen species are capable of stabilising HIF-1α and enhancing hypoxic survival of neutrophils. Manipulation of the hypoxic response pathway and neutrophil metabolism through knockout of PHD3 enhances control of bacterial infection through changes in neutrophil metabolic flux. Conclusion - We describe a signalling role for the neutrophil mitochondria in hypoxic conditions that is independent of oxidative phosphorylation activity. Neutrophil anaerobic metabolism is also linked closely to antioxidant production through the pentose phosphate pathway that compensates for mitochondrial oxidant signalling from the mitochondria. Regulation of glycolysis is therefore required for both reactive oxygen signalling and for managing oxidative stress in hypoxic conditions. These pathways are also regulated by the activity of PHD3, in a manner capable of modulating neutrophil bactericidal function.