AbstractLymph node metastasis is the cornerstone of survival of gastric cancer patients. The increased populations of regulatory T cells in peripheral blood of patients reflect compromised host immunity. Tumor might use these regulatory T cells in regional lymph nodes to suppress host immunity against tumor growth. In this study, the murine model of benzo[a]pyrene induced forestomach carcinoma was used to elucidate the difference of regulatory T cells distributed in different lymphoid tissues and its enhancement on tumour growth before and after depletion of regulatory T cells by PC61 mAb. The proportions of regulatory T cells in total CD4+ T cells is significantly increased in peri-stomach regional lymph nodes than those in other lymphoid tissues in benzo[a]pyrene induced forestomach carcinoma mice. These regulatory T cells in regional lymph nodes expressed higher level of Foxp3 transcript and protein. They were also enriched in CD62L- cell subset and expressed both CCR1 and CCR5 chemokine receptors. After depletion, the tumour mass reduced significantly with decrease of the proportion of regulatory T cells in the regional lymph nodes, massive infiltrating cells and apoptosis of the tumour cells were observed. These results demonstrate that the accumulation of Treg cells in the regional lymph nodes suppress host local immunity during growth of tumours.