Cerebral extracellular preamyloid deposits in Alzheimer's disease, Down syndrome and nondemented elderly individuals.

Using anti-SP28 (a polyclonal antibody to a 28 residue synthetic peptide homologous to the NH2-terminal region of Alzheimer amyloid beta-protein) to investigate the cerebral cortex of 8 Alzheimer and 6 Down patients and 13 nondemented individuals, we found that, besides senile plaques and congophilic vessels, extracellular deposits unrelated to degenerating neurites, tangle-bearing neurons or congophilic vessels were labelled. These deposits were observed in all Alzheimer and 4 Down patients, and in 5 nondemented individuals. The material accumulated in these deposits did not react with Congo red, thioflavine S or, on some occasions, silver salts and therefore might have been constituted by beta-protein and/or beta-protein precursors lacking the conformation of amyloid fibrils.