Aliquots of exosomes (10 μg protein) isolated from AML patients’ plasma or leukemia cell lines were suspended in 50 μL PBS and plated in wells of a 96-well flat-bottom microtiter plate. The assay was carried out by combining the exosomes with 50 μL of the DPP4 substrate, Gly-Pro-aminoluciferin, and the buffer optimized for this assay (Promega, Madison, WI, USA). Plates were incubated at 37 °C for 60 min, and surface DPP4 enzyme activity was measured as luminescence signals using the Promega GloMax multidetection system. Diprotin A, a DPP4 inhibitor, was used for blocking DPP4 activity [24 (link)]. Titration experiments were performed using different doses of Diprotin A (0.1–0.6 mmol) to identify the optimal inhibitory dose of Diprotin A for the blocking experiments. Aliquots of exosomes (10 μg protein) were co-incubated with Diprotin A (0.2 mmol) for 30 min prior to the addition of DPP4.