Development of a sandwich ELISA for evaluating soluble PD-L1 (CD274) in human sera of different ages as well as supernatants of PD-L1+ cell lines

► A novel sPD-L1 ELISA was developed with well specificity, precision and stability. ► PD-L1+ cell lines produce high levels of sPD-L1 protein. ► The concentration of sPD-L1 in human serum increases in an age-dependent manner. ► Matrix metalloproteinase inhibitor (MMPI) could suppress sPD-L1 production. ► The circulating sPD-L1 could specially bind to PD-1 receptor.