Autoimmunity in Alzheimer's disease: Increased levels of circulating IgGs binding Aβ and RAGE peptides

Plasma samples derived from 33 Alzheimer's disease (AD) and 42 control participants were subjected to several steps to purify specific anti-(amyloid)Aβ IgGs. Affinity-purified IgGs binding the peptide Aβ1-42, a neurotoxic sequence derived from the trans-membrane amyloid precursor protein, exhibited nearly four-fold higher titers in AD patients compared with their control non-AD cohort. Affinity-purified IgGs binding a fragment of the receptor for advanced glycation end products (RAGE) likewise were increased nearly three-fold in AD individuals. Aβ and RAGE IgG titers were negatively correlated with cognitive status, i.e. the more cognitively impaired individuals tended to exhibit higher IgG titers. Aβ IgG titers were negatively correlated with age in the control group, but not with the AD group. Levels of circulating Aβ̃- and RAGE-like proteins were not different between AD and control participants, nor was there a relationship between individual IgG titers and the respective Aβ- and RAGE-like proteins. Freshly prepared leukocyte preparations were subjected to flow cytometric analysis. AD individuals exhibited significantly increased populations of cells expressing binding sites for monoclonal antibodies directed against Aβ (5.5-fold), βAPP (3.5-fold), and RAGE (2.6-fold) relative to the control group. These findings confirm the presence of circulating IgGs specifically directed at proteins implicated in immunological processes linked to AD. The close relationship between titers for Aβ and RAGE IgGs suggests the possibility that the antibodies are being produced in response to a common mechanism or protein complex (with the respective epitopes) linked to the disease.