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Hydroethidine (HE) and 2',7'-dichlorofluorescin (DCFH) were used for the flow cytometric measurement of reactive oxygen metabolites in leukocytes. Hydroethidine and DCFH were both rapidly oxidized in a cell-free cuvette assay to ethidium bromide (EB) and 2',7'-dichlorofluorescein (DCF) by H2O2 and peroxidase, but not by H2O2 alone, while only HE was oxidized by KO2, a source of O2-. Quiescent lymphocytes, monocytes, and neutrophils spontaneously oxidized HE to EB, while DCFH was only oxidized to a low degree. Neutrophils increased 6.9-fold in EB red fluorescence and 12.5-fold in DCF green fluorescence during the respiratory burst induced by phorbol 12-myristate 13-acetate or 6.1-fold and 4.7-fold, respectively, during the respiratory burst induced by Escherichia coli bacteria. The HE or DCFH oxidation during the respiratory burst, unlike the spontaneous HE oxidation, was not inhibitable by 10 mM NaNe indicating a non-mitochondrial source of cellular oxidants during the respiratory burst such as NADPH oxidase, which produces O2-. The oxidation of DCFH, but not of HE, was decreased in stimulated neutrophils, which were simultaneously loaded with HE and DCFH. Intracellular DCFH oxidation induced by incubation of resting neutrophils with extracellular H2O2 was not influenced by the presence of HE. This indicates that HE is oxidized at an earlier step in the reactive oxygen metabolism of neutrophils than DCFH, i.e., by early oxygen metabolites like O2-, while DCFH is oxidized in part by H2O2 and phagosomal peroxidases. The differential oxidation of HE and DCFH during simultaneous cellular staining permits the analysis of up to three functionally different neutrophil populations in septic patients. This is of interest for the determination of disease-related alterations of oxygen metabolism in quiescent and stimulated leukocytes.
Rothe et al. (Tue,) studied this question.