Mitochondria in human neutrophils mediate killing of Staphylococcus aureus

Abstract

Background Neutrophils play a role in innate immunity and are critical for clearance of Staphylococcus aureus . Current understanding of neutrophil bactericidal effects is that NADPH oxidase produces reactive oxygen species (ROS), mediating bacterial killing. Neutrophils also contain numerous mitochondria; since these organelles lack oxidative metabolism, their function is unclear. We hypothesize that mitochondria in human neutrophils contribute to the bactericidal capacity of S. aureus . Methods and Findings: Using human neutrophils isolated from healthy volunteers (n = 13; 7 females, 6 males), we show that mitochondria are critical in the immune response to S. aureus . Using live-cell and fixed confocal, and transmission electron microscopy, we show mitochondrial tagging of bacteria prior to ingestion and surrounding of phagocytosed bacteria immediately upon engulfment. Further, we demonstrate that mitochondria are ejected from intact neutrophils and engage bacteria during vital NETosis . Inhibition of the mitochondrial electron transport chain at Complex III, but not Complex I, attenuates S. aureus killing by 50 ± 7%, comparable to the NADPH oxidase inhibitor apocynin. Similarly, mitochondrial ROS scavenging using MitoTEMPO attenuates bacterial killing 112 ± 60% versus vehicle control. Antimycin A treatment also reduces mitochondrial ROS production by 50 ± 12% and NETosis by 53 ± 5%. Conclusions We identify a previously unrecognized role for mitochondria in human neutrophils in the killing of S. aureus . Inhibition of electron transport chain Complex III significantly impairs antimicrobial activity. This is the first demonstration that vital NETosis, an early event in the antimicrobial response, occurring within 5 min of bacterial exposure, depends on the function of mitochondrial Complex III. Mitochondria join NADPH oxidase as bactericidal ROS generators that mediate the bactericidal activities of human neutrophils.

Unveiling Leukocyte Extracellular Traps in Inflammatory Responses of the Central Nervous System

“…In conclusion, these data indicate that the vital release of genetic material from neutrophils is always dependent on ROS originating from either NADPH (42) or mitochondria (44,50). These extracellular DNA fibers have been proposed to boost autoimmune responses in SLE (43) or to exert anti-microbial functions (48).…”

“…According to this model, neutrophils survive as anucleated cytoplasts while still retaining their ability to crawl or exert phagocyte functions (48). Mitochondrial DNA is a negligible component of these early NETs induced by S. Aureus (47), but mitochondrial complex III and mtROS contribute to their formation (50).…”

Roles of mitochondria in neutrophils

“…A recent study found that inhibition of complex III (cytochrome c reductase) with antimycin A can significantly reduce neutrophil superoxide O 2 – production ( 17 ). MitoTEMPO can accumulate in mitochondria and reduce superoxide.…”

“…Pre-incubation of neutrophils with MitoTEMPO can significantly attenuate neutrophils’ ability to kill Staphylococcus aureus. This effect is not enhanced by co-incubation with MitoTEMPO and antimycin A, suggesting that electron transport complex III is critical for mROS production and required for the bactericidal function of neutrophils ( 17 ). Rice et al.…”

Neutrophils: Musketeers against immunotherapy

“…Neutrophils are the most profuse leukocytes, representing 50%–70% of all the circulating leukocytes in human, and are regarded as body’s first responders to injury, infections, and inflammation ( 1 , 2 ). In response to infection-associated signals, neutrophils initiate multiple effector functions such as the production of neutrophil extracellular traps (NETs), generation of reactive oxygen species (ROS), and production of antibacterial peptides to eradicate pathogens ( 3 , 4 ). The functional importance of neutrophils had been overlooked previously on the basis of their reported short life span; however, recent studies suggesting that they can survive in the circulation from 19 h to 5 days have ensured renewed attention toward the role of neutrophils under varying biological conditions ( 5 ).…”