Effect of CO donor on binding of primary labeled LIBS (HUTS-21) to U937 cells in the presence of different concentrations of LDV. Mean fluorescence intensity plotted versus concentration of unlabeled LDV. A. U937 cells were treated with vehicle (DMSO, control), and CO donor (150 μM CORM-2) for 30 min at 37°C. Next, cells were incubated with the indicated concentration of LDV in the presence of excess HUTS-21 mAbs, washed, and fluorescence was measured. B. U937 cells stably transfected with the non-desensitizing FPR mutant were treated with vehicle (DMSO, control), and CO donor (150 μM CORM-2) for 30 min at 37°C in the presence of 100 nM fMLFF for cell activation. Next, mAb binding was performed in a manner analogous to A. The non-specific binding of HUTS-21, detected in the absence of LDV ligand, was subtracted from the data. Means ± SEM of three independent determinations are shown. The data were fitted using the sigmoidal dose-response equation with GraphPad Prism software. One representative experiment out of three independent experiments is shown.