Aim To investigate the effect of theaflavin on oxidized low density lipoprotein(ox-LDL)-induced foam cell formation and oxidative stress in THP-1 macrophages and its mechanism.Methods THP-1 derived macro-phages were pretreated with 50 μmol/L theaflavin and(or)10 μmol/L nuclear factor erythroid 2-related factor 2(NRF2)inhibitor ML385,then 100 mg/L ox-LDL was added to the cells for 24 h to establish the foam cell model.The effect of theaflavin on THP-1 macrophages viability was evaluated by CCK-8 assay and LDH release.The expression of inflamma-tory cytokines interleukin-6(IL-6),interleukin-1 β(IL-1β),tumor necrosis factor-α(TNF-α)were analyzed by real-time quantitative polymerase chain reaction(RT-qPCR)and Western blot.The release of inflammatory cytokines were detected by enzyme linked immunosorbent assay(ELISA).Intracellular lipid accumulation was detected by Oil red O staining,and lipid absorption was observed by DiL-labeled oxidized low density lipoprotein(DiL-ox-LDL)staining.Re-active oxygen species(ROS)level was detected by DCFH-DA probe.The expression of lipid uptake,cholesterol efflux and oxidative stress-related proteins were detected by Western blot and RT-qPCR.Results Treatment with 100 mg/L ox-LDL significantly decreased cell viability and cholesterol efflux-related protein expressions,increased lipid uptake,ac-cumulation and lipid uptake-related protein expressions,and significantly promoted inflammation and ROS level,as well as the expressions of myeloperoxidase(MPO),NADPH oxidase 2(NOX2)in THP-1 macrophages(all P＜0.05).After pretreatment with theaflavin,cell viability was increased,intracellular lipid uptake,accumulation and lipid uptake-related protein expressions were significantly reduced,cholesterol efflux-related protein expressions were significantly increased,the expression and release of IL-6,IL-1β and TNF-α were significantly decreased,ROS level was significantly decreased,and the expression of MPO and NOX2 were decreased(all P＜0.05).Pretreatment with theaflavin effectively alleviated intracellular oxidative stress by altering the expression of NRF2,heme oxygenase-1(HO-1)and Kelch-like ECH-associated protein 1(KEAP1)in NRF2 signaling pathway,and enhanced the translocation of NRF2 into the nucleus.After pretreat-ment with ML385,the expression levels of NRF2,HO-1,KEAP1 and CD36 were significantly decreased.Conclusion Theaflavin can significantly inhibit ox-LDL-induced foam cell formation,inflammation,and oxidative stress through NRF2/HO-1 signaling pathway in THP-1 macrophages.