IFNα protects lung epithelial cells from α-toxin-induced cell death and depletion of intracellular ATP. Human primary SAEC were pretreated with medium or IFNα (1000 U/ml) for 24 h and treated with α-toxin (0.1 μg/ml) for additional 24 h. (A): Representative photomicrographs of cells stained with calcein AM (green, live) and EthD-1 (red, dead); scale bar = 100 μM. (B): Percent dead cells (mean±SD of five independent experiments, each carried out in quadruplicate wells). (C): Relative levels of intracellular ATP (% remaining, relative to untreated cells at each time point) were measured in SAEC at the indicated time points after α-toxin (mean±SD of quadruplicate wells, the data are representative of five experiments). (D): Relative ATP levels in IFNα-pretreated A549 cells at 6 h after α-toxin (mean±SD of five independent experiments, each carried out in quadruplicate wells). (E) and (F): Relative ATP levels in medium- or IFNα-pretreated A549 cells after incubation with conditioned medium (diluted 1:100) from cultures of Hla+ or Hla- S. aureus. The data (mean±SD of quadruplicate wells) are representative of three independent experiments.