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Background: CD8Foxp3 T lymphocytes have been detected in tumors. However, the distribution, phenotypic features, and regulation of these cells in gastric cancer remain unknown. Methods: The levels of CD8Foxp3 T lymphocytes in the peripheral blood, tumor-draining lymph nodes, non-tumor tissues, and tumor tissues of patients with gastric cancer were detected by flow cytometry. Foxp3 induction in CD8Foxp3 T cells was investigated in vitro. The suppressive function of CD8Foxp3 T lymphocytes was analyzed by their effect on CD4 T-cell proliferation and IFN-γ production. The percentages of CD8Foxp3 T lymphocytes were evaluated for the association with tumor stage. Results: The frequency of CD8Foxp3 T lymphocytes in tumor tissues was significantly higher than that in non-tumor tissues, and similar results were also observed in tumor-draining lymph nodes compared with peripheral blood. Most intratumoral CD8Foxp3 T lymphocytes were activated effector cells (CD45RACD27). TGF-β1 levels were positively correlated with the frequency of CD8Foxp3 T lymphocytes in tumor tissues, and in vitro TGF-β1 could induce the generation of CD8Foxp3 T lymphocytes in a dose-dependent manner. Furthermore, intratumoral CD8Foxp3 T lymphocytes suppressed the proliferation and IFN-γ production of CD4 T cells. Finally, intratumoral CD8Foxp3 T lymphocytes were significantly increased with tumor progression in terms of tumor-node-metastasis (TNM) stage. Conclusions: Our data have shown that increased intratumoral CD8Foxp3 T lymphocytes are associated with tumor stage and potentially influence CD4 T-cell functions, which may provide insights for developing novel immunotherapy protocols against gastric cancer. [ABSTRACT FROM AUTHOR]