Background Tumor cells can reduce the number of dendritic cells (DCs) in the tumor environment and cause DC dysfunction through autocrine or paracrine pathways.We sought to measure cyclooxygenase-2 (COX-2) expression in bombesin-inhibited DCs treated with theanine in vitro and to explore the protection and activation effects of theanine on DCs.Methods Enzyme-linked immunosorbent assay (ELISA),reverse transcription-polymerase chain reaction (RT-PCR),and West blotting were used to analyze the effects of theanine on COX-2 expression and intedeukin (IL)-12/IL-10 secretion of bombesin-treated DCs.Results DCs acquired an impaired phenotype as a result of bombesin treatment.Theanine increased the expression of mature DC surface molecules.The number of cell apoptosis with the treatment of bombesin and theanine significantly decreased,accounting for 15.9％,compared with 26.1％ of cell apoptosis with bombesin.COX-2 expression in bombesintreated DCs was inhibited by theanine in a dose-dependent manner.Theanine promoted DC secretion of IL-12.IL-12 levels reached (137.4±4.9) pg/ml with theanine at 200 μmol/L.However,theanine inhibited the secretion of IL-10 in a dose-dependent manner.IL-10 levels were only (58.4±6.9) pg/ml with theanine at 200 μmol/L.Conclusion Theanine inhibits the transcription and translation of COX-2 and regulates the balance of IL-10/IL-12 secretion in bombesin-inhibited DCs,leading to the recovery of a state of activation in DCs.