The polarity of CXCL12 expression in the active and inactive MS plaques changes from abluminal toward the luminal surface of the BBB vascular endothelium, which is considered to be a critical element for the trafficking of CXCR4-positive leukocytes into the perivascular and then into CNS parenchyma space [34]. Here, CXCR4 is linked to myeloid sarcoma.