One hallmark of Alzheimer disease is the accumulation of amyloid beta-peptide in the brain and its deposition as plaques. Mice transgenic for an amyloid beta precursor protein (APP) mini-gene driven by a platelet-derived (PD) growth factor promoter (PDAPP mice), which overexpress one of the disease-linked mutant forms of the human amyloid precursor protein, show many of the pathological features of Alzheimer disease, including extensive deposition of extracellular amyloid plaques, astrocytosis and neuritic dystrophy. Active immunization of PDAPP mice with human amyloid beta-peptide reduces plaque burden and its associated pathologies. Several hypotheses have been proposed regarding the mechanism of this response. Here we report that peripheral administration of antibodies against amyloid beta-peptide, was sufficient to reduce amyloid burden. Despite their relatively modest serum levels, the passively administered antibodies were able to enter the central nervous system, decorate plaques and induce clearance of preexisting amyloid. When examined in an ex vivo assay with sections of PDAPP or Alzheimer disease brain tissue, antibodies against amyloid beta-peptide triggered microglial cells to clear plaques through Fc receptor-mediated phagocytosis and subsequent peptide degradation. These results indicate that antibodies can cross the blood-brain barrier to act directly in the central nervous system and should be considered as a therapeutic approach for the treatment of Alzheimer disease and other neurological disorders.

Experimental autoimmune encephalomyelitis (EAE) is an inflammatory condition of the central nervous system with similarities to multiple sclerosis. In both diseases, circulating leukocytes penetrate the blood-brain barrier and damage myelin, resulting in impaired nerve conduction and paralysis. We sought to identify the adhesion receptors that mediate the attachment of circulating leukocytes to inflamed brain endothelium in EAE, because this interaction is the first step in leukocyte entry into the central nervous system. Using an in vitro adhesion assay on tissue sections, we found that lymphocytes and monocytes bound selectively to inflamed EAE brain vessels. Binding was inhibited by antibodies against the integrin molecule alpha 4 beta 1, but not by antibodies against numerous other adhesion receptors. When tested in vivo, anti-alpha 4 integrin effectively prevented the accumulation of leukocytes in the central nervous system and the development of EAE. Thus, therapies designed to interfere with alpha 4 beta 1 integrin may be useful in treating inflammatory diseases of the central nervous system, such as multiple sclerosis.

Abstract. Endothelial cells that make up brain capillaries and constitute the blood-brain barrier become different from peripheral endothelial cells in response to inductive factors found in the nervous system . We have established a cell culture model of the bloodbrain barrier by treating brain endothelial cells with a combination of astrocyte-conditioned medium and agents that elevate intracellular cAMP These cells form

Transgenic PDAPP mice, which express a disease-linked isoform of the human amyloid precursor protein, exhibit CNS pathology that is similar to Alzheimer's disease. In an age-dependent fashion, the mice develop plaques containing ␤-amyloid peptide (A␤) and exhibit neuronal dystrophy and synaptic loss. It has been shown in previous studies that pathology can be prevented and even reversed by immunization of the mice with the A␤ peptide. Similar protection could be achieved by passive administration of some but not all monoclonal antibodies against A␤. In the current studies we sought to define the optimal antibody response for reducing neuropathology. Immune sera with reactivity against different A␤ epitopes and monoclonal antibodies with different isotypes were examined for efficacy both ex vivo and in vivo. The studies showed that: (i) of the purified or elicited antibodies tested, only antibodies against the N-terminal regions of A␤ were able to invoke plaque clearance; (ii) plaque binding correlated with a clearance response and neuronal protection, whereas the ability of antibodies to capture soluble A␤ was not necessarily correlated with efficacy; (iii) the isotype of the antibody dramatically influenced the degree of plaque clearance and neuronal protection; (iv) high affinity of the antibody for Fc receptors on microglial cells seemed more important than high affinity for A␤ itself; and (v) complement activation was not required for plaque clearance. These results indicate that antibody Fc-mediated plaque clearance is a highly efficient and effective process for protection against neuropathology in an animal model of Alzheimer's disease.I mmunization of the transgenic PDAPP mice with ␤-amyloid peptide (A␤)-derived immunogens results in an antibody response that facilitates the clearance of plaques within the central nervous system (CNS) (1-4). Although a number of mechanisms are likely to operate in this clearance response (5, 6), our previous findings strongly indicate that antibodymediated, Fc-dependent phagocytosis by microglial cells and͞or macrophages is important to the process (7). Importantly, a T cell response was not required for amyloid plaque clearance. When peripherally administered, antibodies against A␤ entered the CNS of PDAPP transgenic mice, decorated amyloid plaques, and induced plaque clearance. Comparing different antibodies in an ex vivo assay with sections of PDAPP or Alzheimer's disease (AD) brain, there was a strong correlation between those that produced ex vivo efficacy and those that were efficacious in vivo. Fc receptors on microglial cells were found to be key for the clearance response in this assay. However, it has been reported that antibody efficacy can also be obtained in vivo by mechanisms that are independent of Fc interactions (8). Studies have indicated that an antibody directed against the midportion of A␤, which cannot recognize amyloid plaques, appears to bind to soluble A␤ and reduce plaque deposition (6). In addition, it has been reported recently that short-term...

␣ 4 ␤ 1 integrin (VLA-4) appears to be unique among the leukocyte integrins in that it can initiate the adhesion of circulating lymphocytes without cellular activation. It is not known how lymphocytes or other cell types maintain constitutive levels of ␣ 4 ␤ 1 integrin activity. The current report describes a monoclonal antibody, 15/7, that recognizes a high affinity or ligand-occupied conformation of ␤ 1 integrin. Studies with 15/7 revealed that ␣ 4 ␤ 1 integrin-dependent adhesion of leukocytic cell lines is mediated by a population of low affinity receptors that is conformationally responsive to ligand; the 15/7 epitope could be induced by nanomolar concentrations of soluble VCAM-1 or by micromolar concentrations of a peptide derived from the type III connecting segment domain of fibronectin (as ligands for ␣ 4 ␤ 1 integrin). The same receptors were also responsive to adhesion activating reagents, such as Mn 2؉ , activating anti-␤ 1 integrin antibodies, and phorbol myristate acetate, which induced the 15/7 epitope directly and/or decreased the concentration of ligand required for epitope induction. In addition to the responsive receptor pool, cells expressed a second population of ␣ 4 ␤ 1 integrin that was conformationally restrained, failing to respond to ligand or to any of the activating reagents. The relative size of the responsive and inactive receptor pools, as well as the affinity of the responsive receptors, represented a stable phenotype of different cell types and played important roles in defining the cells' adhesive capacity and ligand specificity. Similar receptor populations were measured on lymphocyte subsets in whole blood. These studies provide insight into how cells maintain different constitutive levels of ␣ 4 ␤ 1 integrin activity, and how the activity of ␤ 1 integrin can be modulated by activators of cell adhesion.Integrins are heterodimeric adhesion molecules that contribute to the specificity of cellular interactions through the recognition of numerous matrix and cell-associated ligands (1). Importantly, the ligand binding activity of integrins can be modulated rapidly, allowing cells to specify the timing and location of integrin-mediated adhesive interactions. On circulating leukocytes, for example, the ␤ 2 integrins LFA-1 and Mac-1 are thought to be activated by site-specific factors (cytokines or other adhesive interactions) during transient cell interactions with the vascular wall; the receptors then establish firm adhesive contacts and mediate leukocyte extravasation (2, 3). Likewise, platelets are stimulated at sites of vascular injury allowing ␣II b ␤ 3 integrin to bind to fibrinogen and initiate thrombosis (4).The regulation of ␣ 4 integrin activity on circulating immune cells appears to be different from that of the other leukocyte integrins described above. ␣ 4 ␤ 1 and ␣ 4 ␤ 7 integrin can mediate leukocyte adhesion to their endothelial ligands VCAM-1 (5) and MAdCAM-1 (6), respectively, without cellular activation, and can do so even in the presence of the shear forces encount...