DNCB shows hormetic effects in THP-1 cells: low-dose enhancement of metabolic activity

2,4-Dinitrochlorobenzene (DNCB) is a well-characterized skin sensitizer that has been widely used in immunological and toxicological research and, historically, in clinical immunotherapy. Although it is a well-investigated chemical, this is the first study focusing on the dose–response behavior at low-level concentrations. The aim was to reveal potential hormetic effects due to its known Nrf2 inducting activity. Therefore, THP-1 cells were treated with low doses of DNCB and two endpoints were evaluated for hormetic responses: metabolic activity using a resazurin-based assay and immune activation by measuring CD86 and CD54 expression using flow cytometry. The results showed a significant hormetic effect on the metabolic endpoint at the lower cell density for both analyzed time points, and a hormetic tendency at the higher cell density. Metabolic activity increased to approximately 125% of the control at 0.05 µM DNCB. For the immunological endpoint a slight decrease in CD86 and CD54 surface marker expression was observed, up to −16% and up to −12% compared to control at 0.5 µM DNCB. These findings highlight the importance of including low-dose concentrations when characterizing chemical dose-response relationships and evaluating toxicological risk.