Objective To investigate the mechanism by which Scutellaria barbata D.Don water extract(SBW)inhibits lung metastasis of breast cancer by regulating the immune microenvironment.Methods A mouse model of lung metastasis was established using 4T1 breast cancer cells.Mice were divided into a control group(n=6)and an SBW treatment group(n=6).The effect of SBW on tumor growth was assessed by measuring the volume of the primary tumor,and the inhibitory effect on lung metastasis was evaluated by observing the number and area of metastatic nodules in lung tissue using H&E staining.Flow cytometry was used to analyze changes in the composition of immune cells in the tumor,peripheral blood,and lung tissue.Results Compared with the control group,the SBW treatment group inhibited the growth of the primary tumor(P<0.01)and reduced the number and area of lung metastatic nodules(P<0.01).Flow cytometry analysis showed that after SBW treatment,the numbers of CD86+macrophages(P<0.001)and polymorphonuclear myeloid-derived suppressor cells(PMN-MDSCs)(P<0.05)in the tumor tissue were increased.In lung tissue,the numbers of CD86+macrophages,natural killer T(NKT)cells,natural killer(NK)cells,and PMN-MDSCs were also elevated(P<0.05).Meanwhile,the numbers of regulatory T cells(Tregs)(P<0.05),CD206+macrophages(P<0.01),and monocytic myeloid-derived suppressor cells(M-MDSCs)(P<0.05)in both tumor and lung tissues were decreased.Conclusion SBW inhibits the breast cancer growth and lung metastasis by regulating the recruitment and distribution of immune cells in the tumor,peripheral blood,and lung tissue,thereby enhancing anti-tumor immune responses and reducing immune suppression.