Circulating TNF and mitochondrial DNA are major determinants of neutrophil phenotype in the advanced-age, frail elderly

• Blood neutrophils appear basally activated and are immature in the advanced-age, frail elderly.
• CD11b and HLA-DR expression is related to the levels of circulating TNF and mtDNA in humans.
• Loss of TNF in mice abrogates age-related increases in neutrophil CD11b in vivo.
• mtDNA is able to induce neutrophil HLA-DR in a dose-dependent fashion ex vivo.

Tumor necrosis factor (TNF), a potent inflammatory cytokine, and mitochondrial DNA (mtDNA), a product of inflammation-induced tissue damage, increase with age (“inflammaging”) and many chronic diseases. Peripheral blood neutrophils, a critical component of innate immunity, have also been shown to be altered with age, and are exceptionally sensitive to external stimuli. Herein, we describe that the phenotype of neutrophils from the advanced-age, frail elderly (ELD) is determined by levels of circulating TNF and mtDNA.Neutrophils from ELD donors are morphologically immature, and have higher levels of intracellular reactive oxygen species (ROS) and expression of the activation markers CD11b and HLA-DR. The frequency of CD11b++ neutrophils correlated with plasma TNF, and recombinant TNF elevated neutrophil CD11b ex vivo and in vivo. Furthermore, neutrophils from aged TNF-deficient mice expressed CD11b similar to young counterparts. The frequency of HLA-DR+ neutrophils, on the other hand, positively correlated with circulating mtDNA, which increased neutrophil HLA-DR expression in a dose-dependent manner ex vivo. Cell-surface TLR-9 expression, however, was unaltered on neutrophils from ELD donors.In summary, we provide novel evidence that products of age-related inflammation modulate neutrophil phenotype in vivo. Given this, anti-inflammatory therapies may prove beneficial in improving neutrophil functionality in the elderly.