Small blood vessels are composed of two different but interdependent cellular compartments, endothelial cells (ECs) and pericytes [10, 11]. Pericytes are perivascular stromal cells that are located on the abluminal vessel wall of brain capillaries and regulate vascular tone and morphology, in a similar way as vascular smooth muscle cells do in big blood vessels [10, 12]. Pericytes may have stem cell properties and represent an immunological defense in the brain [13, 14]. Indeed, they have phagocytic activity and express numerous macrophage markers, supporting an ability to acquire functional properties of macrophages [9, 15].