Amyloid-beta peptide (Abeta) seems to have a central role in the neuropathology of Alzheimer's disease (AD). Familial forms of the disease have been linked to mutations in the amyloid precursor protein (APP) and the presenilin genes. Disease-linked mutations in these genes result in increased production of the 42-amino-acid form of the peptide (Abeta42), which is the predominant form found in the amyloid plaques of Alzheimer's disease. The PDAPP transgenic mouse, which overexpresses mutant human APP (in which the amino acid at position 717 is phenylalanine instead of the normal valine), progressively develops many of the neuropathological hallmarks of Alzheimer's disease in an age- and brain-region-dependent manner. In the present study, transgenic animals were immunized with Abeta42, either before the onset of AD-type neuropathologies (at 6 weeks of age) or at an older age (11 months), when amyloid-beta deposition and several of the subsequent neuropathological changes were well established. We report that immunization of the young animals essentially prevented the development of beta-amyloid-plaque formation, neuritic dystrophy and astrogliosis. Treatment of the older animals also markedly reduced the extent and progression of these AD-like neuropathologies. Our results raise the possibility that immunization with amyloid-beta may be effective in preventing and treating Alzheimer's disease.

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.

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...

StlmrllsryTo study the processes of thymic development, we have established transgenic mice expressing an or/3 T cell antigen receptor (TCK) specific for cytochrome c associated with class II major histocompatibility complex (MHC) molecules. The transgenic TCK chains are expressed by most of the thymocytes in these mice, and these cells have been shown to efficiently mature in association with E L and Ab-encoded class II MHC molecules. This report describes a characterization of the negative selection of these transgenic thymocytes in vivo that is associated with the expression of A s molecules. Negative selection by A s molecules appears to result in the deletion of a late stage of CD4/CD8 double-positive thymocytes in that there is a virtual absence of transgenic TCK bearing CD4 single-positive thymocytes. This phenotype is accompanied by the appearance of CD4/CD8 double-negative thymocytes and peripheral T cells that are functionally antigen reactive. The process of negative selection has also been investigated using an in vitro culture system. Upon presentation of cytochrome c by Eb-expressing nonthymic antigen-presenting cells, there occurs an antigen dose-dependent deletion of the majority of CD4/CD8 double-positive thymocytes. In contrast, presentation of Staphylococcal enterotoxin A by E b in vitro results in minimal deletion of double-positive thymocytes. In addition, we use this in vitro model to examine the effects of cyclosporin A on negative selection. In contrast to its effects on mature T cells, and the findings of others in vivo, cyclosporin A does not inhibit antigen-induced deletion of double-positive thymocytes. Finally, a comparison of the antigen dose responses for thymocyte deletion and for peripheral T cell activation indicates that double-positive thymocyte recognition is more sensitive than mature T cells to antigen recognition. The establishment of immunological self-recognition and tolerance is known to occur primarily in the thymus, where thymocytes undergo a program of differentiation leading to mature functional T cells. Thymocytes are specifically selected based on the specificity of their ot/B TCR. Most of the information currently available relates to the results of thymic selection and not the selection process itself (1); the actual mechanism for selection has yet to be elucidated. T cells that survive thymic selection and are exported to the peripheral lymphoid organs are not overtly self-reactive, yet have the ability to recognize foreign antigenic peptides bound to MHC molecules that are also expressed in the thymus. The selection events that lead to a naive population of T cells specific for foreign peptides bound to self and not foreign alleles of the MHC molecules are termed positive selection; the deletion of T cells overtly reactive to self-MHC molecules, presumably containing self-peptides, is termed negative selection (2). The distinction between the recognition events that lead to positive selection versus negative selection is not understood.The developmental pathwa...

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