The role of palatine tonsils as a prion entry site in classical and atypical experimental sheep scrapie

Atypical and classical scrapie-infected sheep brain tissue was monolaterally injected into the tonsils of lambs to investigate their role as a prion entry point. We firstly detected classical PrPSc within the inoculated tonsil and in the ipsilateral retropharyngeal lymph-node at 3 months post-inoculation (pi). At 7 months pi, PrPSc colonized other lymphoid tissues bilaterally, including ileal Peyer's patches. The earliest PrPSc deposition within the brain was ipsilaterally observed at 9 months pi in the substantia reticularis of the medulla oblongata. At 12 months pi, PrPSc deposition was present bilaterally in the nucleus parasympathicus nervi vagi as well as in the intermedio-lateral cell column of the thoracolumbar spinal cord. No PrPSc was detected in the lambs inoculated with atypical scrapie. These findings suggest that neuroinvasion may naturally occur from the tonsil after a widespread prion replication within the lymphoid tissues during classical scrapie only, thus mimicking the pathogenesis following oral ingestion.[...]