Objective To explore the myeloid-derived suppressor cell (MDSC) expression in the peripheral blood and lesions of 4NQO-induced tongue carcinoma in mouse. Methods The established 4NQO mouse model was used to analyze the distribution of MDSC and T cell subsets in the peripheral blood by flow cytometry. The relations of MDSC with T cell subsets and CD4+/CD8+changes were evaluated. The distribution of MDSC in the lesions of tongues was analyzed by immunohistochemistry, and the expression of arginase 1 (ARG-1) in tongue tissues was detected by real-time polymerase chain reaction. Results During tumor progression, a significant increase was observed in the frequency of MDSC in the peripheral blood of 4NQO treated mice (P+CD8+T cells but negatively correlated with the CD4+/CD8+ratio. Squamous cell carcinomas were extensively infiltrated with MDSC, whereas dysplastic area and normal tongue mucosa had only sparse MDSC infiltration. The majority of MDSCs were located in the stroma, particularly along the tumor invasive front. Moreover, 4NQO-treated mice showed significantly higher ARG-1 mRNA levels in the tumor site (P
