Therefore, we speculate that the BBB capacity to control cell entry into the CNS is impaired in MS patients, leading to the entry of T cells regardless whether they express PD-1 or not, but that PD-1-negative CD8 T cells will be favored for progressing into the inflamed parenchyma which abundantly expresses PD-L1. Here, PDCD1 is linked to myeloid sarcoma.