Objective To investigate the effects of menaquinone-4(MK4)on the activation and function of CD8+T cells.Methods An in vitro culture system for primary mouse CD8+T cells was established by isolating these cells from the spleen using CD8a T cell isolation kit.The isolated CD8+T cells were then incubated and activated in a 96-well plate coated with anti-CD3/CD28 antibodies.The impact of MK4 on the activation and cytokine secretion of CD8+T cells was explored by quantifying the expression levels of CD8+T cell activation receptors and cytokines using flow cytometry.Additionally,the concentrations of these cytokines in the culture supernatant were measured by enzyme-linked immunosorbent assay(ELISA).The influence of MK4 on the anti-tumor function of CD8+T cells was evaluated by co-culturing colorectal cancer MC38 cells of mice with CD8+T cells at different ratios,and the effect of MK4 was assessed by detecting tumor cell apoptosis.Results High-purity primary CD8+T cells of mice(97.5％)were isolated using the magnetic bead,which could be activated by pre-coated CD3/CD28 antibodies and showed proliferation.Compared with the control group,the MK4-treated group exhibited increased expressions of CD25,CD69 and CD44 on CD8+T cells,as well as higher production and secretion levels of interleukin-2(IL-2),interferon-γ(IFN-γ),tumor necrosis factor-α(TNF-α)and granzyme B.In addition,CD8+T cells in the MK4-treated group induced a higher percentage of tumor cell apoptosis(36.7％)compared with the control group(29.1％).Conclusion MK4 can enhance the activation of CD8+T cells and promote anti-tumor functions.