## 3 Results
### HO-1 and IL-8 Knockdown Modify Jurkat and THP-1 Cell Oxidant Sensitivity  
A previous study compared the baseline expression of genes relevant for redox regulation and antioxidant detoxification between Jurkat and THP-1 cells [38]. Catalase (CAT), heme oxygenase 1 (HMOX-1), and interleukin (IL)-8 gene expression were found to be markedly elevated in THP-1. Therefore, the role of the three corresponding proteins in the different responses to oxidative stress of Jurkat and THP-1 cells was investigated by siRNA-mediated transient protein knockdown (Figure 4a). Knockdown efficiency was validated 24 h after transfection in Jurkat (Figure 4b) and THP-1 cells (Figure 4c) by Western blotting. The day after siRNA-mediated protein knockdown, the cells were exposed to H\({}_{2}\)O\({}_{2}\) or HOCI at the respective IC20 concentrations, and the metabolic activity of the cells was assessed. For comparison and excluding cytotoxic effects of the transfection process, metabolic activity was normalized to the untreated controls of the respective transfection condition. Since IC20 oxidant concentrations were used for treatment, the metabolic activity of the different controls was at approximately 80%. In Jurkat cells (Figure 4d), knockdown of catalase caused no visible additive effect with both H\({}_{2}\)O\({}_{2}\) and HOCI treatment. The knockdown of HO-1 showed no effect regarding H\({}_{2}\)O\({}_{2}\) exposure but an increased cytotoxic effect with HOCI treatment. Knockdown of IL-8 resulted in slightly increased cytotoxicity after H\({}_{2}\)O\({}_{2}\) treatment and an even more pronounced effect with HOCI. In contrast, there were elevated cytotoxic effects for all three knockdown targets in THP-1 cells (Figure 4e). Catalase knockdown had only modest effects. For knockdown of HO-1 and IL-8, the cytotoxic effect was more potent than for catalase and, in contrast to Jurkat cells, more pronounced for H\({}_{2}\)O\({}_{2}\) than for HOCI treatment.  
Figure 3: **Caspase 3/7 activation and glutathione levels.** (**a**) Representative microscopy overlay images of brightfield and green fluorescence (caspase 3/7); (**b**,**c**) fold-change increase of caspase activation in untreated, H\({}_{2}\)O\({}_{2}\)-treated (IC20), and HOCI-treated (IC20) Jurkat (**b**) and THP-1 (**c**) cells followed by time-lapse fluorescence microscopy up to 12 h; (**d**–**g**) total glutathione (**d**), reduced GSH (**e**), GSSG (**f**), and GSH/GSSG (**g**) ratio in Jurkat and THP-1 cells that had remained untreated or were exposed to H\({}_{2}\)O\({}_{2}\) (IC20) or HOCI (IC20). Results are representative of three experiments.  
Figure 4: **siRNA mediated knockdown of catalase, HO-1, and IL-8.** (**a**) Workflow of the siRNA electroporation followed by Western blotting and cell line testing; (**b,c**) representative Western blots in Jurkat (**b**) and THP-1 (**c**) cells, with white arrows indicating protein knockdown of catalase, HO-1, or IL-8; (**d,e**) normalized metabolic activity of Jurkat (**d**) and THP-1 (**e**) cells following exposure to H\({}_{2}\)O\({}_{2}\) (IC20) or HOCI (IC20) after being subjected to mock or siRNA-mediated transfection. NT = non-transfected; luc = luciferase.