A series of dilutions were prepared from the remaining bacteria. Bacteria were cultured on Luria broth agar plates without antibiotic at 37° overnight. Colonies were counted the next day. The phagocytosis assay was performed as described previously.19,20 In brief, FITC-conjugated killed S. aureus (Invitrogen,
Darmstadt, Germany) was used for assay. The bacteria were opsonized before the assay. For this purpose, bacteria SRT1720 in vivo were incubated with 5% serum (from the same donor from whom neutrophils were isolated) for 25 min at 37°. Non-infected neutrophils were pre-stimulated with PAR2-cAP 10−4 m, PAR2-cRP 10−4 m and/or IFN-γ 100 ng/ml for 2 hr at 37° and 5% CO2. Neutrophils and opsonized bacteria were co-incubated at 1 : 20 ratio (neutrophils : S. aureus). During co-incubation of bacteria and neutrophils, PAR2-cAP 10−4 m, PAR2-cRP 10−4 m and/or IFN-γ 100 ng/ml were applied in the concentrations indicated above. Co-incubation took place in assay medium on a shaker for 30 min at 37°. The phagocytosis assay was stopped Ferroptosis targets by the addition
of ice-cold PBS containing 0·5 mm EDTA (500 μl PBS to 1 ml of sample medium). Samples were then centrifuged at 169 g and neutrophil pellets were resuspended in ice-cold PBS containing 0·9% FCS and 2 mm EDTA. Trypan blue quench, which helps to discriminate adherent and ingested bacteria, was performed as described previously.21 The efficacy of phagocytosis was estimated using flow cytometry (FACS analysis). Measurements were performed for the next 15 min and all samples were kept on ice during measurements. At least 30 000 cells were analysed with the FACScalibur
and cell quest pro Software (Becton Dickinson, Heidelberg, Germany). Bacteria. The S. aureus (SH1000) was kindly provided by Dr C. Eiff22 and S. aureus was grown for 18 hr in Mueller–Hinton bouillon at 37°. Bacterial Oxalosuccinic acid density was measured spectrophotometrically at 540 nm, after two PBS washings. The number of bacterial cells was calculated using a previously determined standard curve (based on the counts of colony-forming units). Finally, the concentration of bacteria in PBS was adjusted to 5 × 108 cells/ml. For the purpose of the quantitative analysis of phagocytosis by flow cytometry, S. aureus was incubated in PBS containing 0·1% FITC (Sigma Aldrich, Munich, Germany) for 1 hr at 37°. After being labelled, bacteria were washed three times before incubation with pre-treated leucocytes. Assay. During pre-treatment, human monocytes or neutrophils (1 × 106 cells) were cultured in medium either without stimuli (‘control’) or containing the following stimuli: 100 ng/ml LPS; 1 × 10−4 m PAR2-cAP, 10 ng/ml or 100 ng/ml of IFN-γ. Monocytes or neutrophils were pre-treated for 2 hr at 37° and subsequently co-incubated with live FITC-labelled S. aureus at a ratio of 1 : 10 (cells : S. aureus) for 30 min at 37°.