Arsenic trioxide inhibits lung metastasis of mouse colon cancer via reducing the infiltration of regulatory T cells

The purpose of this study was to investigate the effects of arsenic trioxide (As2O3) on the infiltration of regulatory T cells (Tregs) in the local lung metastasis of mouse colon cancer in vivo and the regulation of Tregs in cytokine-induced killer cells (CIKs) in vitro. A high Tregs infiltration mouse colon cancer lung metastasis model was established by intravenous injection of CT26 murine colon carcinoma cells. Tumor-bearing mice were randomly divided into three groups: control group, low-dose As2O3 group, and high-dose As2O3 group. For in vitro studies, CIKs were treated with vehicle control or 0.1, 1, or 5 mu M As2O3. The level of Tregs was detected via flow cytometry, Foxp3 expression was assessed by immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR), the level of interferon gamma (IFN-gamma) was evaluated by enzyme-linked immunoassay (ELISA), and the cytotoxic activity of As2O3 treated CIKs was assessed through a lactate dehydrogenase (LDH) release assay. Obvious lung metastasis was observed 3 days after CT26 murine colon carcinoma cell injection. The numbers of Tregs in the lungs and spleens of tumor-bearing mice were significantly higher than those of the normal group (p < 0.01). As2O3 treatment increased the mouse weight as well as reduced the number of metastatic lung nodules and the lung/body weight ratio (p < 0.01). Moreover, As2O3 treatment significantly reduced the Tregs proportion and the Foxp3 messenger RNA (mRNA) levels in metastatic lung tissues (p < 0.01). In vitro, As2O3 significantly reduced the Tregs proportion and the Foxp3 mRNA levels (p < 0.01) and significantly increased the cytotoxic activity of CIKs and the IFN-gamma levels in the supernatant of cultured CIKs (p < 0.01). As2O3 might inhibit lung metastasis of colon cancer by reducing the local infiltration of Tregs and increase the cytotoxic activity of CIKs by suppressing Tregs.