Ultrastructural evidence that ependymal cells are infected in experimental scrapie

During the last stage of infection in the experimental scrapie-infected hamster model, light microscopy reveals typical immunostaining of PrP^sc in the subependymal region and at the apical ependymal cell borders. Whereas the subependymal immuno-staining is known to originate from extracellular amyloid filaments and residual membranes of astrocytes as constituents of plaque-like structures, the ultrastructural correlate of the supraependymal PrP^sc staining remains uncertain. To decipher this apical PrP^sc immunopositivity and subsequently the ependymocyte–scrapie agent interaction, we employed highly sensitive immuno-electron microscopy for detecting PrP^sc in 263K scrapie-infected hamster brains. The results revealed the supraependymal PrP^sc signal to be correlated not only with extracellular accumulation of amyloid filaments, but also with three distinct ependymal cell structures: (1) morphologically intact or altered microvilli associated with filaments, (2) the ependymal cell cytoplasm in proximity of apical cell membrane, and (3) intracytoplasmic organelles such as endosomes and lysosomal-like structures. These findings suggest a strong ependymotrope feature of the scrapie agent and recapitulate several aspects of the cell–prion interaction leading to the formation and production of PrP^sc amyloid filaments. Our data demonstrate that in addition to neurons and astrocytes, ependymocytes constitute a new cellular target for the scrapie agent. In contrast, the absence of PrP^sc labeling in choroid plexus and brain vascular endothelial cells indicates that these cells are not susceptible to the infection and may inhibit passage of the infectious agent across the blood–brain barrier.