Modeling the neurovascular niche: implications for recovery from CNS injury.

While survival from stroke, traumatic brain and spinal cord injuries, neurodegenerative diseases and hypoxia has improved over the past several years, treatments are limited and impacts of these injuries and diseases to patients, families and society can be devastating. Recovery from these injuries is variable and involves in part an orchestrated angiogenesis and neurogenesis in the neurogenic zones (neurovascular niches) of the CNS. In this focused review the roles of HIF-1alpha mediated responses to hypoxia in CNS neurovascular niches is discussed. Using in vivo and in vitro murine models of sublethal hypoxia we mimicked the variable responses observed in the human population and correlated differences in baseline and hypoxia-induced induction of HIF-1alpha and several downstream signaling components including BDNF, VEGF, SDF-1, TrkB, Nrp-1, CXCR4 and NO with differences in survival as well as endothelial cell and neural stem cell survival and proliferation, providing insight into this important and timely problem and suggesting that optimization of expression levels of some or all of these signaling components may have the potential of maximizing recovery following CNS injury.

Astrocyte-derived VEGF mediates survival and tube stabilization of hypoxic brain microvascular endothelial cells in vitro.

Chronic sublethal hypoxia has been associated with changes in neurovascular behavior, mediated, in part, by induction of vascular endothelial growth factor-A (VEGF-A(165)). In this report we demonstrate that RBE4 cells (derived from rodent cerebral microvasculature), when cultured in three-dimensional collagen gels: (1) Are induced to undergo increased tube formation in response to VEGF-A(165) in a dose-dependent manner; (2) undergo apoptosis under mild hypoxic conditions; (3) are rescued from the effects of hypoxia by the addition of exogenous VEGF-A(165) in a dose-dependent and inhibitable manner or by co-culture with primary newborn rat astrocytes, which are induced to express increased amounts of VEGF-A in hypoxic conditions. Further, we demonstrate that: (4) The observed astrocyte-produced, VEGF-mediated protection from apoptosis (survival) is inhibitable with soluble recombinant VEGF receptor-1 (sFlt), and is associated with a robust induction of MAPK tyrosine phosphorylation. These findings illustrate the importance of VEGF in the process of neurovascular survival in response to injury in developing brain and provide insight into the signaling pathways involved.

pp60(c-src) modulates microvascular endothelial phenotype and in vitro angiogenesis.

The tyrosine kinase c-src associates with the platelet-derived growth factor (PDGF) receptor. Overexpression of wild-type c-src, a kinase-negative c-src mutant, and v-src in microvascular endothelial cells modulated the mitogenic effect of PDGF, suggesting that c-src kinase activity inhibits PDGF signals. Analyses of cell morphology in two-dimensional culture revealed changes in cell shape and size induced by the overexpression of c-src proteins. Investigations in three-dimensional culture unveiled a modulatory role of c-src during in vitro angiogenesis. Overexpression of c-src resulted in an increased diameter of tube-like structures, and the number of branching segments was decreased. Expression of the kinase-negative c-src mutant resulted in abortive tube formation consisting of disconnected multicellular fragments. These results indicate that the c-src tyrosine kinase exerts regulatory effects on endothelial proliferation, size, and cytoskeletal organization in two-dimensional culture and on the formation of a differentiated multicellular network in three-dimensional culture.

Hyperglycemia-induced vasculopathy in the murine conceptus is mediated via reductions of VEGF-A expression and VEGF receptor activation.

Major congenital malformations, including those affecting the cardiovascular system, remain the leading cause of mortality and morbidity in infants of diabetic mothers. Interestingly, targeted mutations of several genes (including VEGF and VEGF receptors) and many teratogenic agents (including excess D-glucose) that give rise to embryonic lethal phenotypes during organogenesis are associated with a failure in the formation and/or maintenance of a functional vitelline circulation. Given the similarities in the pathology of the abnormal vitelline circulation in many of these conditions, we hypothesized that the hyperglycemic insult present in diabetes could cause the resultant abnormalities in the vitelline circulation by affecting VEGF/VEGF receptor signaling pathway(s). In this study we report that hyperglycemic insult results in reduced levels of VEGF-A in the conceptus, which in turn, leads to abnormal VEGF receptor signaling, ultimately resulting in embryonic (vitelline) vasculopathy. These findings and our observation that addition of exogenous rVEGF-A(165) within a defined concentration range blunts the hyperglycemia-induced vasculopathy in the conceptus support the concept that VEGF levels can be modulated by glucose levels. In addition, these findings may ultimately lead to novel therapeutic approaches for the treatment of selected congenital cardiovascular abnormalities associated with diabetes.

Pecam-1 is a modulator of stat family member phosphorylation and localization: lessons from a transgenic mouse.

PECAM-1 (CD31) is a member of the immunoglobin (Ig) superfamily of cell adhesion molecules whose expression is restricted to hematopoietic and vascular cells. PECAM-1 can recruit adapter and signaling molecules via its immunoreceptor tyrosine activation motif (ITAM), suggesting that PECAM-1 plays a role in signal transduction pathways. To study the involvement of PECAM-1 in signaling cascades in vivo, we used the major histocompatibility (MHC) I gene promoter to target ectopic PECAM-1 expression in transgenic mice. We noted an attenuation of mammary gland development at early stages of virgin ductal branching morphogenesis. STAT5a, a modulator of milk protein gene expression during lactation, was localized to the nuclei of ductal epithelial cells of 6-week-old virgin PECAM-1 transgenics, but not in control mice. This correlated with decreases in ductal epithelial cell proliferation and induction of p21, an inhibitor of cell cycle progression. Using in vitro model systems we demonstrated PECAM-1/STAT5a association and found that residue Y701 in PECAM-1's cytoplasmic tail is important for PECAM-1/STAT5 association and that PECAM-1 modulates increases in STAT5a tyrosine phosphorylation levels. We suggest that by serving as a scaffolding, PECAM-1 can bring substrates (STAT5a) and enzymes (a kinase) into close proximity, thereby modulating phosphorylation levels of selected proteins, as previously noted for beta-catenin.

Focal adhesion kinase activates Stat1 in integrin-mediated cell migration and adhesion.

Recent studies suggest that focal adhesion kinase (FAK) is important for cell migration. We now suggest a mechanism by which FAK activates the signal transducer and activator of transcription (STAT) pathway, regulating cell adhesion and migration. In particular, we observe that FAK is capable of activating Stat1, but not Stat3. Co-immunoprecipitation and in vitro binding assays demonstrate that Stat1 is transiently and directly associated with FAK during cell adhesion, and Stat1 is activated in this process. FAK with a C-terminal deletion (FAKDeltaC14) completely abolishes this interaction, indicating this association is dependent on the C-terminal domain of FAK, which is required for FAK localization at focal contacts. Moreover, Stat1 activation during cell adhesion is diminished in FAK-deficient cells, correlating with decreased migration in these cells. Finally, we show that depletion of Stat1 results in an enhancement of cell adhesion and a decrease in cell migration. Thus, our results have demonstrated, for the first time, a critical signaling pathway from integrin/FAK to Stat1 that reduces cell adhesion and promotes cell migration.

PECAM-1 shedding during apoptosis generates a membrane-anchored truncated molecule with unique signaling characteristics.

Shedding of cell surface molecules, including growth factor receptors, provides a mechanism by which cells regulate signal transduction events. Here we show that platelet-endothelial cell adhesion molecule (PECAM)-1 is shed from the endothelial cell surface during apoptosis and accumulates in the culture medium as a approximately 100 kDa soluble protein. The cleavage mediating the shedding is matrix metalloproteinase (MMP) dependent, as GM6001, a broad-spectrum MMP inhibitor, inhibits PECAM-1 accumulation in the culture medium in a dose-responsive manner. In addition to the 100 kDa soluble fragment, PECAM-1 cleavage generates the formation of a truncated (Tr.) approximately 28 kDa molecule, composed of the transmembrane and the cytoplasmic PECAM-1 domains. Transfections of the full-length (Fl) and the Tr. PECAM-1 gene constructs into endothelial and nonendothelial cells were performed. We found 1) significantly more gamma-catenin and SHP-2 bound to the truncated than to the full-length PECAM-1; 2) stable expression of the truncated PECAM-1 in SW480 colon carcinoma cells resulted in a dramatic decrease in cell proliferation, whereas expression of comparable levels of the full-length PECAM-1 had no effect; 3) the decrease observed in cell proliferation is due, in part, to an increase in programmed cell death (apoptosis) and correlated with continuous caspase 8 cleavage and p38/JNK phosphorylation. These results support the intimate involvement of PECAM-1 in signal transduction cascades and also suggest that caspase substrates (e.g., PECAM-1) may possess distinct and unique functions on cleavage.

Cell migration in the immune system: the evolving inter-related roles of adhesion molecules and proteinases.

Leukocyte extravasation into perivascular tissue during inflammation and lymphocyte homing to lymphoid organs involve transient adhesion to the vessel endothelium, followed by transmigration through the endothelial cell (EC) layer and establishment of residency at the tissue site for a period of time. In these processes, leukocytes undergo multiple attachments to, and detachments from, the vessel-lining endothelial cells, prior to transendothelial cell migration. Transmigrating leukocytes must traverse a subendothelial basement membrane en route to perivascular tissues and utilize enzymes known as matrix metalloproteinases to make selective clips in the extracellular matrix components of the basement membrane. This review will focus on the evidence for a link between adhesion of leukocytes to endothelial cells, the induction of matrix metalloproteinases mediated by engagement of adhesion receptors on leukocytes, and the ability to utilize these matrix metalloproteinases to facilitate leukocyte invasion of tissues. Leukocytes with invasive phenotypes express high levels of MMPs, and expression of MMPs enhances the migratory and invasive properties of these cells. Furthermore, MMPs may be used by lymphocytes to proteolytically cleave molecules such as adhesion receptors and membrane bound cytokines, increasing their efficiency in the immune response. Engagement of leukocyte adhesion receptors may modulate adhesive (modulation of integrin affinities and expression), synthetic (proteinase induction and activation), and surface organization (clustering of proteolytic complexes) behaviors of invasive leukocytes. Elucidation of these pathways will lead to better understanding of controlling mechanisms in order to develop rational therapeutic approaches in the areas of inflammation and autoimmunity.

Matrix metalloproteinase activity is required for activity-induced angiogenesis in rat skeletal muscle.

Proteolysis of the capillary basement membrane is a hallmark of inflammation-mediated angiogenesis, but it is undetermined whether proteolysis plays a critical role in the process of activity-induced angiogenesis. Matrix metalloproteinases (MMPs) constitute the major class of proteases responsible for degradation of basement membrane proteins. We observed significant elevations of mRNA and protein levels of both MMP-2 and membrane type 1 (MT1)-MMP (2.9 +/- 0.7- and 1.5 +/- 0.1-fold above control, respectively) after 3 days of chronic electrical stimulation of rat skeletal muscle. Inhibition of MMP activity via the inhibitor GM-6001 prevented the growth of new capillaries as assessed by the capillary-to-fiber ratio (1.34 +/- 0.08 in GM-6001-treated muscles compared with 1.69 +/- 0.03 in control 7-day-stimulated muscles). This inhibition correlated with a significant reduction in the number of capillaries with observable breaks in the basement membrane, as assessed by electron microscopy (0.27 +/- 0.27% in GM-6001-treated muscles compared with 3.72 +/- 0.65% in control stimulated muscles). Proliferation of capillary-associated cells was significantly elevated by 2 days and remained elevated throughout 14 days of stimulation. Capillary-associated cell proliferation during muscle stimulation was not affected by MMP inhibition (80.3 +/- 9.3 nuclei in control and 63.5 +/- 8.5 nuclei in GM-6001-treated animals). We conclude that MMP proteolysis of capillary basement membrane proteins is a critical component of physiological angiogenesis, and we postulate that capillary-associated proliferation precedes and occurs independently of endothelial cell sprout formation.

Distinct roles for matrix metalloproteinase-2 and alpha4 integrin in autoimmune T cell extravasation and residency in brain parenchyma during experimental autoimmune encephalomyelitis.

Expression of alpha4 integrin by auto-reactive T cells is critical for their ability to induce EAE, an autoimmune disease of the central nervous system in mice, used as a model to study human multiple sclerosis. Having previously identified one role for alpha4 integrin in adhesion-mediated induction of matrix metalloproteinase-2 (MMP-2), an enzyme that degrades the subendothelial basement membrane matrix, we investigated independent roles for MMP-2 and alpha4 integrin during EAE. The data suggest that expression of alpha4 integrin by auto-reactive T cells is important not only in mediating MMP-2 induction to facilitate entry into the CNS, but also plays a role in maintaining residency within the CNS.

Type IV collagen modulates angiogenesis and neovessel survival in the rat aorta model.

Type IV collagen is a major basement membrane component that has been implicated in the regulation of angiogenesis. The purpose of this study was to evaluate the effect of type IV collagen on the angiogenic response of native endothelial cells in three-dimensional vascular organ culture. Rings of rat aorta were cultured under serum-free conditions in gels of type I collagen with or without type IV collagen. In the absence of type IV collagen, aortic rings generated neovessels, which proliferated until day 9 and gradually regressed during the second and third weeks of culture. Type IV collagen promoted neovessel elongation and survival in a dose-dependent manner. Microvascular length increased by 43, 57, and 119% over control values in cultures treated with 3, 30, and 300 microg/ml type IV collagen, respectively. When used at high concentrations (300 microg/ml) type IV collagen stabilized the neovascular outgrowths and prevented vascular regression. Type IV collagen also promoted the formation of neovessels, but significant stimulatory effects were observed only at an intermediate concentration (30 microg/ml) and were no longer significant at the high concentration (300 microg/ml). The observation that type IV collagen has dose-dependent effects on vascular elongation, proliferation, and stabilization, supports the concept that the developing basement membrane of neovessels acts as a solid-phase regulator of angiogenesis, whose function varies depending on the concentration of its molecular components.

PECAM-1 (CD31) expression modulates bleeding time in vivo.

PECAM-1 is a 130-kd member of the Ig superfamily present on endothelial cells, platelets, polymorphonuclear leukocytes, monocytes, and lymphocytes. Its expression begins early in development and persists through adulthood. PECAM-1 functions as an adhesion and signaling molecule between adjacent endothelial cells and between endothelial cells and circulating blood elements. Antibodies directed against PECAM-1 have been shown to affect angiogenesis, endothelial cell migration, and polymorphonuclear leukocyte transmigration. Furthermore, its dimerization is associated with the modulation of integrin affinity. Antibody inhibition studies suggest that PECAM-1 plays a role in modulating thrombosis; however, recent in vitro aggregation studies performed on platelets harvested from PECAM-1-deficient mice revealed no abnormalities. In this report we demonstrate prolonged in vivo bleeding times in PECAM-1-deficient mice. This abnormality was not corrected when wild-type hematopoietic precursors were engrafted into marrow-ablated PECAM-1-deficient mice. Furthermore, normal bleeding times were observed when marrow-ablated wild-type mice were engrafted with hematopoietic precursors harvested from PECAM-1-deficient mice. These studies are consistent with a role for PECAM-1 in modulating thrombosis in the vasculature, which is potentially mediated by endothelial cell PECAM-1 expression.

Platelet-endothelial cell adhesion molecule-1 (CD31), a scaffolding molecule for selected catenin family members whose binding is mediated by different tyrosine and serine/threonine phosphorylation.

Platelet-endothelial cell adhesion molecule (PECAM)-1 is a 130-kDa glycoprotein commonly used as an endothelium-specific marker. Evidence to date suggests that PECAM-1 is more than just an endothelial cell marker but is intimately involved in signal transduction pathways. This is mediated in part by phosphorylation of specific tyrosine residues within the ITAM domain of PECAM-1 and by recruitment of adapter and signaling molecules. Recently we demonstrated that PECAM-1/beta-catenin association functions to regulate beta-catenin localization and, moreover, to modulate beta-catenin tyrosine phosphorylation levels. Here we show that: 1) not only beta-catenin, but also gamma-catenin is associated with PECAM-1 in vitro and in vivo; 2) PKC enzyme directly phosphorylates purified PECAM-1; 3) PKC-derived PECAM-1 serine/threonine phosphorylation inversely correlates with gamma-catenin association; 4) PECAM-1 recruits gamma-catenin to cell-cell junctions in transfected SW480 cells; and 5) gamma-catenin may recruit PECAM-1 into an insoluble cytoskeletal fraction. These data further support the concept that PECAM-1 functions as a binder and modulator of catenins and provides a molecular mechanism for previously reported PECAM-1/cytoskeleton interactions.

Neuronal VEGF expression correlates with angiogenesis in postnatal developing rat brain.

When exposed to chronic sublethal hypoxia the developing brain responds with increases in permeability and angiogenesis. Vascular endothelial growth factor (VEGF) may mediate this response. Here, we present data on the localization of VEGF in the rat brain cortex during postnatal development and its correlation to vascularization. We reared newborn rats under normoxic conditions and in hypoxic chambers (FiO(2) 9.5%), removed them at postnatal days (P) 3, 8, 13, 24, and 33 and prepared the cortical brain tissue for immunohistochemistry, in situ hybridization (ISH), Western blot analyses and vessel density counting. When compared to age-matched controls, hypoxic-reared animals displayed a significant increase in platelet endothelial cell adhesion molecule 1 (PECAM-1) protein levels, cerebral microvascular lumen diameter and number and density of vessels (number of capillaries per area). In control animals, ISH and immunohistochemistry revealed that localization of VEGF is restricted almost exclusively to cortical neurons at early stages of development. As the vascular bed begins to stabilize, predominant VEGF expression switches to maturing glial cells which invest vessels while neuronal expression is reduced to a basal level. In hypoxic animals, early localization of VEGF is also restricted to cortical neurons, however, during later developmental stages, glial cells express elevated levels of VEGF protein and high neuronal expression also persists. Thus chronic sublethal hypoxia disrupts the temporal-spatial expression of VEGF, which correlates with continuing hypoxia-driven angiogenesis.

Extracellular matrix-driven matrix metalloproteinase production in endothelial cells: implications for angiogenesis.

The process of new blood vessel growth, angiogenesis, involves orchestrated alterations in endothelial cell interactions with adjacent cells and with components of the underlying basement membrane matrix. The activity of matrix metalloproteinases (MMPs), proteases that can cleave basement membrane and interstitial matrix molecules, has been shown to be necessary for angiogenesis as it occurs in several different in vivo and in vitro models. This review discusses the potential roles of two particular MMPs, MMP-2 and MT1-MMP, in angiogenesis, with emphasis on current understanding of how endothelial cell-extracellular matrix interactions may regulate the production of these MMPs via matrix-induced signaling leading to transcriptional activation and subsequent formation of active multiprotease complexes on the cell surface.

New paradigms of signaling in the vasculature: ephrins and metalloproteases.

As our understanding of the control of vasculogenesis and angiogenesis continues to grow, we will be confronted with an increasing number of interacting and intersecting receptor-mediated signaling pathways. If we are to be successful in developing new and novel effective therapeutic reagents that can function as stimulators or inhibitors of these critically important processes, we will have to develop a sophisticated, full understanding of the complex interactions associated with ephrin-based and metalloprotease-based signaling pathways.

PECAM-1 (CD31) functions as a reservoir for and a modulator of tyrosine-phosphorylated beta-catenin.

Catenins function as regulators of cellular signaling events in addition to their previously documented roles in adherens junction formation and function. Evidence to date suggests that beta and gamma catenins can act as signaling molecules, bind transcriptional factors and translocate to the nucleus. Beta- and gamma-catenin are also major substrates for protein tyrosine kinases, and tyrosine phosphorylation of junctional proteins is correlated with decreased adhesiveness. One way in which catenin functions are modulated is by dynamic incorporation into junctional complexes which controls, in part, the cytoplasmic levels of catenins. Here we show that: (1) vascular endothelial growth factor (VEGF) induces beta-catenin tyrosine phosphorylation in a time-, and dose-dependent manner and that VEGF receptors co-localize to areas of endothelial cell-cell contact in vitro and in vivo. (2) Platelet-endothelial cell adhesion molecule (PECAM)-1 can function as a reservoir for, and modulator of, tyrosine phosphorylated beta-catenin. (3) PECAM-1 can prevent beta-catenin nuclear translocation in transfected SW480 colon carcinoma cells. We suggest that PECAM-1 may play a role in modulating beta-catenin tyrosine phosphorylation levels, localization and signaling and by doing so, functions as an important modulator of the endothelium.

Egr-1 mediates extracellular matrix-driven transcription of membrane type 1 matrix metalloproteinase in endothelium.

Matrix metalloproteinase activity is instrumental in processes of cellular invasion. The interstitial invasion of endothelial cells during angiogenesis is accompanied by up-regulation of several matrix metalloproteinases, including membrane type 1 matrix metalloproteinase (MT1-MMP). In this study, we show that endothelial cells stimulated to undergo angiogenesis by a three-dimensional extracellular matrix environment increase production of the transcription factor Egr-1. Increased binding of Egr-1 to the MT1-MMP promoter correlates with enhanced transcriptional activity, whereas mutations in the Egr-1 binding site abrogate the increased transcription of MT1-MMP in the stimulated cells. These data identify Egr-1-mediated transcription of MT1-MMP as a mechanism by which endothelial cells can initiate an invasive phenotype in response to an alteration in extracellular matrix environment, thus functionally associating MT1-MMP with a growing number of proteins known to be up-regulated by Egr-1 in response to tissue injury or mechanical stress.

Hyperglycemia-induced vasculopathy in the murine vitelline vasculature: correlation with PECAM-1/CD31 tyrosine phosphorylation state.

Maternal diabetes mellitus is associated with an increased incidence of congenital abnormalities as well as embryonic and perinatal lethality. In particular, a wide range of cardiovascular abnormalities have been noted in children of diabetic mothers and in the offspring of diabetic animals. The vascular system is the first organ system to develop in the embryo and is critical for normal organogenesis. The organization of mesodermal cells into endothelial and hematopoietic cells and into a complex vascular system is, in part, mediated by a series of specific cell-cell, cell-extracellular matrix, and cell-factor interactions. PECAM-1 expression has been observed during the earliest stages of vasculogenesis, and changes in PECAM-1 tyrosine phosphorylation have been associated with endothelial cell migration, vasculogenesis, and angiogenesis both in vitro and in vivo. In this report we demonstrate that exposure to hyperglycemia during gastrulation causes yolk sac and embryonic vasculopathy in cultured murine conceptuses and in the conceptuses of streptozotocin-induced diabetic pregnant mice. In addition, we correlate the presence of yolk sac and embryonic vasculopathy with the failure of PECAM-1 tyrosine dephosphorylation during the formation of blood islands/vessels from clusters of extra-embryonic and embryonic angioblasts in the murine conceptus using both in vitro and in vivo models. The importance of these findings in the development of vasculopathy in the offspring of diabetic mothers and the potential effects and benefits of glucose regulation during the periods of vasculogenesis/angiogenesis in embryonic development are discussed.

The interrelationship of alpha4 integrin and matrix metalloproteinase-2 in the pathogenesis of experimental autoimmune encephalomyelitis.

Previous studies have suggested that surface expression of alpha4 integrin by autoreactive T-cell clones is necessary for the clones to induce experimental autoimmune encephalomyelitis (EAE), a mouse model for human multiple sclerosis. To provide direct evidence for this phenomenon, we have transfected alpha4 integrin into C19alpha4-LO, a myelin basic protein-reactive T-cell clone that does not express alpha4 integrin and does not induce EAE when adoptively transferred into a susceptible mouse strain. Transfection of alpha4 integrin converted this clone to an alpha4 integrin-expressing clone that induced EAE. We then examined potential mechanisms by which alpha4 integrin may facilitate the disease process. C19 T-cell clones adhered equally to a monolayer of microvascular endothelial cells, regardless of level of alpha4 integrin expression. However, in contrast to T-cell clones that do not express alpha4 integrin, T-cell clones that express alpha4 integrin (endogenously or by transfection) transmigrated through an endothelial cell layer and subendothelial matrix at an enhanced rate and adhered to recombinant vascular cell adhesion molecule-1 (rVCAM-1) and the CS1 fragment of fibronectin, and after adhesion to these ligands, a matrix-degrading metalloproteinase (MMP-2) was induced and activated. The clones were also shown to constitutively express the membrane-type matrix metalloproteinase (MT1-MMP), an enzyme that activates MMP-2. GM6001 and UK-221,316, inhibitors of metalloproteinases, reduced alpha4 integrin-mediated transmigration and EAE induction by C19 T-cell clones. In addition, we studied a second EAE-inducing T-cell clone, MM4, which constitutively expresses alpha4 integrin and MMP-2. Engagement of alpha4 integrin on the MM4 clone up-regulated the expression and activation of MMP-2, without changing the expression of MT1-MMP. MMP inhibitors also reduced transmigration of and EAE induction by the MM4 T-cell clone. These studies demonstrate directly that expression of alpha4 integrin by autoreactive T-cell clones is required for adoptive transfer of EAE in this model. We also define a role for alpha4 integrin in the disease process in mediating the induction and coordinate activation of a matrix metalloproteinase (MMP-2), which facilitates T-cell transmigration.