Immunohistochemical detection of prion protein in lymphoid tissues of sheep with natural scrapie

The scrapie-associated form of the prion protein (PrP(Sc)) accumulates in the brain and lymphoid tissues of sheep with scrapie. In order to assess whether detecting PrP(Sc) in lymphoid tissue could he used as a diagnostic test for scrapie, we studied the localization and distribution of PrP(Sc) in various lymphoid tissues collected at necropsy from 55 sheep with clinical scrapie. Samples collected from the spleen, palatine tonsil, ileum, and five different lymph nodes were immunohistochemically stained for PrP(Sc), PrP(Sc) was found to be deposited in a reticular pattern in the center of both primary and secondary lymphoid follicles. In addition, granules of PrP(Sc) were seen in the cytoplasm in macrophages associated with the lymphoid follicles. In 54 (98%) of the 55 scrapie-affected sheep, PrP(Sc) was detected in the spleen, retropharyngeal lymph node, mesenteric lymph node, and the palatine tonsil. However, only in the palatine tonsils was PrP(Sc) present in a consistently high percentage of the lymphoid follicles. PrP was not detected in any of the lymphoid tissues of 12 sheep that had no neurohistopathological signs of a scrapie infection. We conclude that the tonsils are the best-suited lymphoid tissue to be biopsied for the detection of PrP(Sc) in the diagnosis of clinical scrapie in living sheep.