Inhibition of TG2 activity decreases superoxide production in differentiating NB4 cells. NB4 cells were treated with 1 μM ATRA or 1 μM ATRA plus 15 μM MDC for 5 days. (A) Ratio of NBT-positive cells following treatment with MDC. At the indicated days, cell smears were fixed and analyzed for nitroblue tetrazolium reduction (at least 300 cells were scored for each experimental condition). (B-C) Chemiluminescence reaction of stimulated cells. (B) In NB4 cells (10⁶), extracellular and intracellular NADPH-oxidase activity was induced by 50 nM PMA and measured in the presence of isoluminol (0.5 mM) or luminol (0.5 mM), respectively. (C) NB4 cells (10⁶) were stimulated by opsonized S aureus and C albicans. Extracellular and intracellular NADPH-oxidase response was determined in the presence of isoluminol (0.5 mM) or luminol (0.5 mM), respectively. In contrast to luminol-amplified chemiluminescence reactions, which measure reactive oxygen species (ROSs) both in the extracellular and the intracellular compartments, isoluminol clearly detects the intracellular oxidase activity only.²⁸  Results are the mean ± SD of 3 experiments.