CD274 and breast cancer: To reveal whether breast cancer cell lines expressing mPD-L1 can produce soluble PD-L1, PD-L1 expression on breast cancer cells was analyzed by flow cytometry and the level of sPD-L1 in the supernatant of breast cancer cells was determined with an ELISA kit (PDCD1LG1 ELISA kit, USCN Life Science, Wuhan, China).