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Objective: To establish a mouse model of breast cancer lung metastasis with miR-155 knockout (miR155-/-) mice, and to compare the difference of peripheral blood immune cell typing between miR155-/- mice and C57BL/6J wide-type (WT) mice. Methods: Bioinformatics analysis was used to explore the expression level of miR-155 in breast cancer tissues and peripheral serum, and its relationship with prognosis. Mouse model of lung metastasis of breast cancer was established by tail vein injection; peripheral blood was collected for flow cytometry, and the immune cell typing was analyzed; the lung tissues were collected for immunohisto-chemical detection to observe the tumor metastasis. Results: Percentage of T lymphocytes and monocytes in peripheral blood of miR155-/- mice was significantly decreased compared with WT mice (P<0.05), percentage of myeloid inhibitory cells (MDSCs) was increased significantly (P<0.05), in which the proportion of monocyte subsets (M-MDSC) was significantly decreased (P<0.05), while the proportion of granulocyte subsets (G-MDSC) was significantly increased (P<0.05). In lung metastasis model of breast cancer, percentage of T lymphocytes in peripheral blood of miR155-/- mice was significantly higher compared with WT mice, while percentage of NK cells was decreased significantly (P<0.05), percentage of neutrophil was significantly decreased (P<0.001), proportion of Th cells in T lymphocytes was significantly decreased (P<0.05), proportion of M-MDSCs was significantly decreased (P< 0.01), while proportion of G-MDSCs was significantly increased (P<0.01). Conclusion: Deletion of miR-155 gene leads to significant differences in peripheral immune cell typing, making mice more susceptible to lung metastasis of breast cancer. [ABSTRACT FROM AUTHOR]