Background::Despite improvements in disease diagnosis, treatment, and prognosis, breast cancer is still a leading cause of cancer death for women. Compelling evidence suggests that targeting cancer stem cells (CSCs) have a crucial impact on overcoming the current shortcomings of chemotherapy and radiotherapy. In the present study, we aimed to study the effects of T cells and a critical anti-tumor cytokine, interferon-gamma (IFN-γ), on breast cancer stem cells.Methods::BALB/c mice and BALB/c nude mice were subcutaneously injected with 4T1 tumor cells. Tumor growth and pulmonary metastasis were assessed. ALDEFLOURn TM assays were performed to identify aldehyde dehydrogenasen bright (ALDHn br) tumor cells. ALDHn br cells as well as T cells from tumor-bearing BALB/c mice were analyzed using flow cytometry. The effects of CD8n + T cells on ALDHn br tumor cells were assessed n in vitro and n in vivo. The expression profiles of ALDHn br and ALDHn dim 4T1 tumor cells were determined. The levels of plasma IFN-γ were measured by enzyme-linked immunosorbent assay, and their associations with the percentages of ALDHn br tumor cells were evaluated. The effects of IFN-γ on ALDH expression and the malignancy of 4T1 tumor cells were analyzedn in vitro.n Results::There were fewer metastatic nodules in tumor-bearing BALB/c mice than those in tumor-bearing BALB/c nude mice (25.40 n vs. 54.67, n P < 0.050). CD8 n + T cells decreased the percentages of ALDHn br 4T1 tumor cells n in vitro (control n vs. effector to target ratio of 1:1, 10.15% n vs. 5.76%, n P < 0.050) and n in vivo (control n vs. CD8n + T cell depletion, 10.15% n vs. 21.75%, n P < 0.001). The functions of upregulated genes in ALDH n br 4T1 tumor cells were enriched in the pathway of response to IFN-γ. The levels of plasma IFN-γ decreased gradually in tumor-bearing BALB/c mice, while the percentages of ALDHn br tumor cells in primary tumors increased. IFN-γ at a concentration of 26.68 ng/mL decreased the percentages of ALDHn br 4T1 tumor cells (22.88% n vs. 9.88%, n P < 0.050) and the protein levels of aldehyde dehydrogenase 1 family member A1 in 4T1 tumor cells (0.86 n vs. 0.49, n P < 0.050) and inhibited the abilities of sphere formation (sphere diameter <200 μm, 159.50 n vs. 72.0; ≥200 μm, 127.0 n vs. 59.0; both n P < 0.050) and invasion (89.67 n vs. 67.67, n P < 0.001) of 4T1 tumor cells.n Conclusion::CD8n + T cells and IFN-γ decreased CSC numbers in a 4T1 mouse model of breast cancer. The application of IFN-γ may be a potential strategy for reducing CSCs in breast cancer.n “,”Background::Despite improvements in disease diagnosis, treatment, and prognosis, breast cancer is still a leading cause of cancer death for women. Compelling evidence suggests that targeting cancer stem cells (CSCs) have a crucial impact on overcoming the current shortcomings of chemotherapy and radiotherapy. In the present study, we aimed to study the effects of T cells and a critical anti-tumor cytokine, interferon-gamma (IFN-γ), on breast cancer stem cells.Methods::BALB/c mice and BALB/c nude mice were subcutaneously injected with 4T1 tumor cells. Tumor growth and pulmonary metastasis were assessed. ALDEFLOURn TM assays were performed to identify aldehyde dehydrogenasen bright (ALDHn br) tumor cells. ALDHn br cells as well as T cells from tumor-bearing BALB/c mice were analyzed using flow cytometry. The effects of CD8n + T cells on ALDHn br tumor cells were assessed n in vitro and n in vivo. The expression profiles of ALDHn br and ALDHn dim 4T1 tumor cells were determined. The levels of plasma IFN-γ were measured by enzyme-linked immunosorbent assay, and their associations with the percentages of ALDHn br tumor cells were evaluated. The effects of IFN-γ on ALDH expression and the malignancy of 4T1 tumor cells were analyzedn in vitro.n Results::There were fewer metastatic nodules in tumor-bearing BALB/c mice than those in tumor-bearing BALB/c nude mice (25.40 n vs. 54.67, n P < 0.050). CD8 n + T cells decreased the percentages of ALDHn br 4T1 tumor cells n in vitro (control n vs. effector to target ratio of 1:1, 10.15% n vs. 5.76%, n P < 0.050) and n in vivo (control n vs. CD8n + T cell depletion, 10.15% n vs. 21.75%, n P < 0.001). The functions of upregulated genes in ALDH n br 4T1 tumor cells were enriched in the pathway of response to IFN-γ. The levels of plasma IFN-γ decreased gradually in tumor-bearing BALB/c mice, while the percentages of ALDHn br tumor cells in primary tumors increased. IFN-γ at a concentration of 26.68 ng/mL decreased the percentages of ALDHn br 4T1 tumor cells (22.88% n vs. 9.88%, n P < 0.050) and the protein levels of aldehyde dehydrogenase 1 family member A1 in 4T1 tumor cells (0.86 n vs. 0.49, n P < 0.050) and inhibited the abilities of sphere formation (sphere diameter <200 μm, 159.50 n vs. 72.0; ≥200 μm, 127.0 n vs. 59.0; both n P < 0.050) and invasion (89.67 n vs. 67.67, n P < 0.001) of 4T1 tumor cells.n Conclusion::CD8n + T cells and IFN-γ decreased CSC numbers in a 4T1 mouse model of breast cancer. The application of IFN-γ may be a potential strategy for reducing CSCs in breast cancer.n