Increased Dietary Manganese Impairs Neutrophil Extracellular Trap Formation Rendering Neutrophils Ineffective at Combating Staphylococcus aureus.

Dietary metals can modify the risk to infection. Previously, we demonstrated that heightened dietary manganese (Mn) during systemic Staphylococcus aureus infection increases S. aureus virulence. However, immune cells also operate in these same environments and the effect of dietary Mn on neutrophil function in vivo has not been assessed. This study reveals that increased concentrations of Mn impairs mitochondrial respiration and superoxide production in neutrophils responding to S. aureus. As a result, high Mn accelerates primary degranulation, while impairing suicidal neutrophil extracellular trap (NET) formation, which decreases bactericidal activity. In vivo, elevated dietary Mn accumulated extracellularly in the heart, indicating that excess Mn may be more bioavailable in the heart. Coinciding with this phenotype, neutrophil function in the heart was most impacted by a high Mn diet, as neutrophils produced lower levels of mitochondrial superoxide and underwent less suicidal NET formation. Consistent with an ineffective neutrophil response when mice are on a high Mn diet, S. aureus burdens were increased in the heart and mice were more susceptible to systemic infection. Therefore, elevated dietary Mn not only affects S. aureus but also renders neutrophils less capable of restricting staphylococcal infection.

Mitochondrial Calcium Uniporter Affects Neutrophil Bactericidal Activity during Staphylococcus aureus Infection.

Neutrophils simultaneously restrict Staphylococcus aureus dissemination and facilitate bactericidal activity during infection through the formation of neutrophil extracellular traps (NETs). Neutrophils that produce higher levels of mitochondrial superoxide undergo enhanced terminal NET formation (suicidal NETosis) in response to S. aureus; however, mechanisms regulating mitochondrial homeostasis upstream of neutrophil antibacterial processes are not fully resolved. Here, we demonstrate that mitochondrial calcium uptake 1 (MICU1)-deficient (MICU1-/-) neutrophils accumulate higher levels of calcium and iron within the mitochondria in a mitochondrial calcium uniporter (MCU)-dependent manner. Corresponding with increased ion flux through the MCU, mitochondrial superoxide production is elevated, thereby increasing the propensity for MICU1-/- neutrophils to undergo suicidal NETosis rather than primary degranulation in response to S. aureus. Increased NET formation augments macrophage killing of bacterial pathogens. Similarly, MICU1-/- neutrophils alone are not more antibacterial toward S. aureus, but rather, enhanced suicidal NETosis by MICU1-/- neutrophils facilitates increased bactericidal activity in the presence of macrophages. Similarly, mice with a deficiency in MICU1 restricted to cells expressing LysM exhibit lower bacterial burdens in the heart with increased survival during systemic S. aureus infection. Coinciding with the decrease in S. aureus burdens, MICU1-/- neutrophils in the heart produce higher levels of mitochondrial superoxide and undergo enhanced suicidal NETosis. These results demonstrate that ion flux by the MCU affects the antibacterial function of neutrophils during S. aureus infection.

Altered Mitochondrial Homeostasis during Systemic Lupus Erythematosus Impairs Neutrophil Extracellular Trap Formation Rendering Neutrophils Ineffective at Combating Staphylococcus aureus.

Inflammation involves a delicate balance between pathogen clearance and limiting host tissue damage, and perturbations in this equilibrium promote disease. Patients suffering from autoimmune diseases, such as systemic lupus erythematosus (SLE), have higher levels of serum S100A9 protein and increased risk for infection. S100A9 is highly abundant within neutrophils and modulates antimicrobial activity in response to bacterial pathogens. We reasoned that increased serum S100A9 in SLE patients reflects accumulation of S100A9 protein in neutrophils and may indicate altered neutrophil function. In this study, we demonstrate elevated S100A9 protein within neutrophils from SLE patients, and MRL/lpr mice associates with lower mitochondrial superoxide, decreased suicidal neutrophil extracellular trap formation, and increased susceptibility to Staphylococcus aureus infection. Furthermore, increasing mitochondrial superoxide production restored the antibacterial activity of MRL/lpr neutrophils in response to S. aureus These results demonstrate that accumulation of intracellular S100A9 associates with impaired mitochondrial homeostasis, thereby rendering SLE neutrophils inherently less bactericidal.

Neutrophil extracellular traps enhance macrophage killing of bacterial pathogens.

Neutrophils and macrophages are critical to the innate immune response, but cooperative mechanisms used by these cells to combat extracellular pathogens are not well understood. This study reveals that S100A9-deficient neutrophils produce higher levels of mitochondrial superoxide in response to Staphylococcus aureus and, as a result, form neutrophil extracellular traps (suicidal NETosis). Increased suicidal NETosis does not improve neutrophil killing of S. aureus in isolation but augments macrophage killing. NET formation enhances antibacterial activity by increasing phagocytosis by macrophages and by transferring neutrophil-specific antimicrobial peptides to them. Similar results were observed in response to other phylogenetically distinct bacterial pathogens including Streptococcus pneumoniae and Pseudomonas aeruginosa, implicating this as an immune defense mechanism that broadly enhances antibacterial activity. These results demonstrate that achieving maximal bactericidal activity through NET formation requires macrophages and that accelerated and more robust suicidal NETosis makes neutrophils adept at increasing antibacterial activity, especially when A9 deficient.

Clostridioides difficile Senses and Hijacks Host Heme for Incorporation into an Oxidative Stress Defense System.

Clostridioides difficile infection of the colon leads to severe inflammation and damage to the gastrointestinal epithelium due to the production of potent toxins. This inflammatory tissue damage causes the liberation of high concentrations of host heme at infection sites. Here, we identify the C. difficile heme-sensing membrane protein system (HsmRA) and show that this operon induces a protective response that repurposes heme to counteract antimicrobial oxidative stress responses. HsmR senses vertebrate heme, leading to increased expression of the hsmRA operon and subsequent deployment of HsmA to capture heme and reduce redox damage caused by inflammatory mediators of protection and antibiotic therapy. Strains with inactivated hsmR or hsmA have increased sensitivity to redox-active compounds and reduced colonization persistence in a murine model of relapse C. difficile infection. These results define a mechanism exploited by C. difficile to repurpose toxic heme within the inflamed gut as a shield against antimicrobial compounds.

Light pollution increases West Nile virus competence of a ubiquitous passerine reservoir species.

Among the many anthropogenic changes that impact humans and wildlife, one of the most pervasive but least understood is light pollution. Although detrimental physiological and behavioural effects resulting from exposure to light at night are widely appreciated, the impacts of light pollution on infectious disease risk have not been studied. Here, we demonstrate that artificial light at night (ALAN) extends the infectious-to-vector period of the house sparrow (Passer domesticus), an urban-dwelling avian reservoir host of West Nile virus (WNV). Sparrows exposed to ALAN maintained transmissible viral titres for 2 days longer than controls but did not experience greater WNV-induced mortality during this window. Transcriptionally, ALAN altered the expression of gene regulatory networks including key hubs (OASL, PLBD1 and TRAP1) and effector genes known to affect WNV dissemination (SOCS). Despite mounting anti-viral immune responses earlier, transcriptomic signatures indicated that ALAN-exposed individuals probably experienced pathogen-induced damage and immunopathology, potentially due to evasion of immune effectors. A simple mathematical modelling exercise indicated that ALAN-induced increases of host infectious-to-vector period could increase WNV outbreak potential by approximately 41%. ALAN probably affects other host and vector traits relevant to transmission, and additional research is needed to advise the management of zoonotic diseases in light-polluted areas.