Abstract: Objective To investigate whether triple negative breast cancer (TNBC) cells can regulate the activation, proliferation and apoptosis of co-cultured CD4⁺T cells through the PD-1/PD-L1 signaling pathway, so as to provide theoretical evidence for the immunotherapy of TNBC patients. Methods Peripheral blood CD4⁺T cells were isolated from healthy volunteers and co-cultured with cancer cell line MDA-MB-231. The following groups were used: a blank control group, where CD4⁺T cells were cultured alone; a co-culture group, where CD4⁺T cells were co-cultured with MDA-MB-231 cells; a PD-L1 co-culture group, where CD4⁺T cells were cultured with the presence of MDA-MB-231+PD-L1 monoclonal antibody; and a isotype control co-culture group, where CD4⁺T cells were cultured with the presence of MDA-MB-231 isotype control. Then, MTT assay and flow cytometry were used to detect the proliferation and apoptosis of CD4⁺T cells. Fluorescent quantitative PCR was performed to detect the expression of Treg related markers in CD4⁺T cells. ELISA was performed to measure the contents of interleukin (IL)-2, IL-10 and transforming growth factor (TGF)-β1 in the supernatant of each group. Western blot was used to detect the expression of Bax and Bcl-2 in CD4⁺T cells. Results Compared with the blank control group, the co-culture group showed remarkable decreases in the proliferation rate of CD4⁺T cells, increases in the apoptosis rate, decreases in the levels of forkhead box P3 (Foxp3), CD25, cytotoxic T lymphocyte associated antigen-4 (CTLA-4), glucocorticoid-induced tumor necrosis factor receptor (GITR) mRNA and Bcl-2 protein in CD4⁺T cells, increases in the levels of CD127 mRNA and Bax protein (all P<0.01); increases in the contents of TGF-β1 and IL-10 in the supernatant and decreases in the contents of IL-2 (all P<0.01). Compared with the co-culture group, the above indexes was reversed in the PD-L1 co-culture group (all P<0.01). Conclusions Triple negative breast cancer cells can inhibit the activation and proliferation of co-cultured CD4⁺T and promote its apoptosis through the PD-1/PD-L1 signaling pathway.