Effects of hyperbaric oxygen treatment on antimicrobial function and apoptosis of differentiated HL-60 (neutrophil-like) cells

Neutrophil apoptosis is important in the resolution of inflammation in chronic wounds. Hyperbaric oxygen (HBO) therapy, an intermittent inhalation of 100% oxygen at greater than atmospheric pressure, appears to be an effective treatment for chronic wounds. The aim was to use HL-60 cells differentiated using all-trans retinoic acid (ATRA) (neutrophil-like cells) to test the hypothesis that an HBO-induced increase in antimicrobial activity might lead to an increase in apoptosis, thereby contributing to neutrophil clearance from chronic wounds. ATRA differentiated HL-60 cells, an in vitro neutrophil model, were used to test the effects of normoxia, hypoxia (5% O2), hyperoxia (95% O2), hyperbaric normoxia (pressure) (8.8% O2 at 2.4 ATA) and HBO (97.9% O2 at 2.4 ATA) on antimicrobial function [NBT staining, superoxide and H2O2 production, and phagocytosis activity] and apoptosis (caspase 3/7 activity and morphological changes observed using SYBR Safe staining). A single 90 min HBO exposure caused an increase in the respiratory burst activity of neutrophil-like cells post exposure. Phagocytosis of Staphylococcus aureus was also increased. HBO pre-treatment had a pro-apoptotic effect, increasing caspase 3/7 activity and causing morphological changes associated with apoptosis. The potential detrimental effect of enhanced antimicrobial activity induced by HBO may be offset by enhanced apoptosis. Both hyperoxia and pressure alone seemed to contribute to the HBO-induced increases in antimicrobial activity and apoptosis, although there was no consistent pattern. These data contribute to explaining the effectiveness of HBO in the treatment of chronic wounds.