Activation of complement and contact system in Alzheimer's disease

beta -Amyloid protein (betaA) has been implicated in the pathogenesis of Al zheimer's disease (AD) because of its neurotoxicity and ability to trigger a local inflammatory response. Although assembly of betaA in particular agg regates seems to be crucial event in AD pathogenesis, soluble, non-fibrilla r betaA may also be involved. Non-fibrillar beta A1-42, and truncated pepti de 1-28, induced dose-dependent activation of C4 sparing C3. The mechanism of C4 activation was not dependent on Clq, because non-fibrillar betaA can still activate C4 in plasma genetically deficient in Clq. A Clq independent mechanism of complement classical pathway activation could be via the acti vation of contact/kinin system. The possible involvement of contact system in AD is suggested by the finding that this system is massively activated i n CSF of AD patients. The mechanism of activation of contact system could b e the result of an anionic interaction of residues within the region 1-11 o f beta A1-42 with factor XII, and of kallikrein generation. Concomitant inc ubation of a small cationic. peptide (lysine(4)) with betaA abrogated its a bility to trigger the cleavage of high molecular weight kininogen. In vivo, prevention of contact system activation beside the reduction of kallikrein generation, can also decrease the activation of complement system and the release of interteukin-6, both factors being considered to play an importan t role in the inflammatory reactions in AD brain. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.