Increased PD-L1 expression in breast and colon cancer stem cells.

Here we report the expression of programmed cell death ligand 1/2 (PD-L1/L2) in breast and colon cancer stem cells (CSCs). The stemness of these cells was confirmed by their surface markers. Using flow cytometry analysis we demonstrated that PD-L1 expression was higher in CSCs of both cancers compared to non-stem like cancer cells. Consistent with this, detection of cellular PD-L1 proteins by western blot assay also showed increased PD-L1 protein in CSCs. In contrast, only trace amounts of PD-L2 were detected in CSCs of both cancers. Our results suggest that breast and colon cancers may be sensitive to PD1/PD-L1 immunotherapy and thus warrant further investigations of CSC targeted PD1/PD-L1 therapy.