PD-L2 is expressed on brain endothelial cells of normal controls, but down-regulated in MS lesions. Micrographs showing brain sections stained for PD-L2 (green), caveolin-1 (red) and nucleus (blue) of one representative normal control (A-C, M) and two representative MS donors (E-G, I-K, N). PD-L2 immunoreactivity is detected on all blood vessels visualized by caveolin-1 positive labeling in control subjects (C, M). However, in MS lesions, while PD-L2 reactivity is detected on blood vessels (E-G, N), a subset of blood vessels visualized by caveolin-1 labeling does not express detectable PD-L2 (I-K, N). Corresponding isotypes are shown in D, H and L. Scale bar: 25 μm (A-L), 400 μm (M, N). White arrows indicate examples of caveolin-1+ PD-L2- blood vessels, whereas orange arrows indicate examples of caveolin-1+ PD-L2+. O. Quantification of the percentage of blood vessels expressing PD-L2 in sections from normal controls (n = 3) and MS patients (n = 7). At least 6 blood vessels were counted for each section. Student's t-test: *** P < 0.001.