PD-L1 is not detectable on brain endothelium of normal controls and MS patients. Micrographs showing brain sections stained for PD-L1 (green), caveolin-1 (red) and nucleus (blue) of one representative normal control (A-C) and two representative MS donors (E-G, I-K). In control subjects, PD-L1 immunoreactivity is not detectable (A). In contrast, PD-L1 is robustly expressed in MS lesions but it does not co-localize with the endothelial cell marker (E-G, I-K). Corresponding isotypes are shown in D, H and L. Scale bar: 25 μm.