Alzheimer's disease (AD) is characterized by extracellular amyloid-b (Ab) deposition, which activates microglia, induces neuroinflammation and drives neurodegeneration. Recent evidence indicates that soluble pre-fibrillar Ab species, rather than insoluble fibrils, are the most toxic forms of Ab. Preventing soluble Ab formation represents, therefore, a major goal in AD. We investigated whether microvesicles (MVs) released extracellularly by reactive microglia may contribute to AD degeneration. We found that production of myeloid MVs, likely of microglial origin, is strikingly high in AD patients and in subjects with mild cognitive impairment and that AD MVs are toxic for cultured neurons. The mechanism responsible for MV neurotoxicity was defined in vitro using MVs produced by primary microglia. We demonstrated that neurotoxicity of MVs results from (i) the capability of MV lipids to promote formation of soluble Ab species from extracellular insoluble aggregates and (ii) from the presence of neurotoxic Ab forms trafficked to MVs after Ab internalization into microglia. MV neurotoxicity was neutralized by the Ab-interacting protein PrP and anti-Ab antibodies, which prevented binding to neurons of neurotoxic soluble Ab species. This study identifies microglia-derived MVs as a novel mechanism by which microglia participate in AD degeneration, and suggest new therapeutic strategies for the treatment of the disease. Cell Death and Differentiation (2014) 21, 582-593; doi:10.1038/cdd.2013.180; published online 13 December 2013Alzheimer's disease (AD) is the major cause of dementia in humans. Neuronal loss and cognitive decline occurring in AD patients are traditionally linked to the accumulation in the brain of extracellular plaques consisting of short amyloid-b (Ab) peptides of 39-42 amino acids, generated by amyloidogenic cleavage of the amyloid precursor protein. 1 Among Ab peptides, Ab 1-42 and pyroglutamate-modified Ab very rapidly aggregate and initiate the complex multistep process that leads to mature fibrils and plaque. 2,3 Although association of amyloid plaques with AD has long been assumed, Ab load does not correlate with neuronal loss 4,5 and high plaque burden does not necessarily lead to dementia in humans. 6,7 Accordingly, recent evidence clearly showed that the amyloid load reaches a plateau early after the onset of clinical symptoms in AD patients 8 and does not substantially increase in size during clinical progression. 9 These observations agree with the current view that small, soluble pre-fibrillar Ab species, rather than plaques formed by insoluble Ab fibrils, are the most toxic forms of Ab. 10 These cause synaptic dysfunction and spine loss, and correlate most closely with the severity of human AD. 5,8,11 Recent biochemical studies indicated that natural sphingolipids and gangliosides, whose metabolism has been shown to be altered in AD patients, 12 destabilize and rapidly resolubilize long Ab fibrils to neurotoxic species. 13 These studies also showed that phospholipids stabilize toxic oligomers...