Matrix metalloproteinases in the normal human central nervous system, microglial nodules, and multiple sclerosis lesions

Matrix metalloproteinases (MMPs) comprise a family of proteolytic enzymes that are involved in remodeling of the extracellular matrix (ECM) of many tissues. They have been implicated in degradation of vascular basement membranes thereby facilitating leukocyte migration into inflammatory sites. To determine the cellular localization and levels of MMPs in the normal human central nervous system (CNS), multiple sclerosis (MS) lesions, and other conditions, cryostat sections of CNS samples were immunostained with antisera to MMP-1, -2, -3 and -9. In control white matter the principal cells that express the MMPs were perivascular and parenchymal microglia. Cellular MMP expression was also found in sporadic microglial nodules in MS white matter. Most CNS microvessel endothelial cells expressed MMP-3 and -9 but not MMP-1 or 2. The majority of macrophages in active MS and necrotic lesions were MMP-1-, -2-, -3-, and -9-positive whereas chronic MS lesions had fewer MMP-positive macrophages. Small numbers of astrocytes were MMP-2-, -3- and -9-positive in acute and chronic MS lesions. These data suggest that microglia-derived MMPs may mediate turnover of the CNS ECM under normal conditions and in microglial nodules. In sites of CNS tissue injury there is complex and dynamic regulation of MMP expression by different cell populations. In MS lesions MMP-mediated proteolysis may contribute to breakdown of the blood-brain barrier and leukocyte migration into the CNS, in situ immune activation, demyelination, metabolism of bioactive peptides, and the formation of an ECM that does not promote remyelination or axonal repair.