Imaging matrix metalloproteinase activity in multiple sclerosis as a specific marker of leukocyte penetration of the blood-brain barrier.

The enzymes gelatinase A/matrix metalloproteinase-2 (MMP-2) and gelatinase B/MMP-9 are essential for induction of neuroinflammatory symptoms in experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis (MS); in the absence of these enzymes, the disease does not develop. We therefore investigated the cellular sources and relative contributions of MMP-2 and MMP-9 to disease at early stages of EAE induction. We demonstrated that MMP-9 from an immune cell source is required in EAE for initial infiltration of leukocytes into the central nervous system and that MMP-9 activity is a reliable marker of leukocyte penetration of the blood-brain barrier. We then developed a molecular imaging method to visualize MMP activity in the brain using fluorescent- and radioactive-labeled MMP inhibitors (MMPis) in EAE animals and used the radioactive MMP ligand for positron emission tomography (PET) imaging of MMP activity in patients with MS. In contrast to traditional T1-gadolinium contrast-enhanced MRI, MMPi-PET enabled tracking of MMP activity as a unique feature of early lesions and ongoing leukocyte infiltration. MMPi-PET therefore allows monitoring of the early steps of MS development and provides a sensitive, noninvasive means of following lesion formation and resolution in murine EAE and human MS.