Immunotherapy can attenuate amyloid neuropathology and improve cognitive function in transgenic models of Alzheimer's disease. However, the first clinical trial was halted when 6% of the Alzheimer's patients developed aseptic meningoencephalitis. Postmortem analysis of two cases with meningoencephalitis showed robust glial activation, T-cell infiltration and sporadic clearance of Aβ. Interestingly, transgenic mouse models of Alzheimer's disease failed as predictors of these adverse inflammatory events. However there are now several studies with amyloid precursor protein transgenic mice that have reported an increased risk of microhemorrhages at sites of cerebrovascular amyloid deposits and because approximately 80% of Alzheimer's patient's have cerebrovascular pathology, there is concern regarding clinical trials using passive administration of humanized anti-Aβ antibodies. Although many studies have now been published on immunotherapy in mouse models, the mechanism(s) of antibody-mediated clearance of β-amyloid from the brain, and the cause of the antibody-induced microhemorrhages remain unclear. In this review, we will discuss the most recent results from the first clinical trial, offer speculation on possible causes for the failure of the trial, review data on antibody-mediated clearance mechanisms, explore the role of complement and inflammation in the clearance of β-amyloid, and suggest novel strategies for avoiding problems in future clinical trials. The central hypothesis being proposed in this review is that anti-Aβ antibodies delivered directly to the CNS at the sites of amyloid deposits will be far more effective at clearing Aβ and safer than active or passive immunization strategies where the majority of the antibodies are in the periphery. © 2006 Elsevier Ltd. All rights reserved.
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