Scrapie‐associated fibrils (SAF) are unique structures characteristic of the group of unconventional slow infections which includes scrapie and Creutzfeldt‐Jakob disease. A major component of hamster fibrils has been described as a protease‐resistant glycoprotein with an apparent mol. wt of 27,000‐30,000 (PrP27‐30). However, we report here that if fibrils are prepared by procedures designed to minimise proteolysis the PrP proteins co‐purifying with hamster SAF have mol. wts of 33,000‐35,000 (PrP33‐35) and 26,000‐29,000 (PrP26‐29). We find a Lys‐Lys‐Arg‐Pro‐Lys sequence at the amino terminus of these SAF proteins, that is absent from PrP27‐30, and which has recently been predicted to be the N‐terminal sequence of the native PrP protein of uninfected brain. The major SAF protein (PrP33‐35) and its normal brain homologue are shown to have the same apparent mol. wt and ionic charge distribution by two‐dimensional gel analysis, silver staining and immunoblotting. These results support our view that PrP33‐35 and the normal brain PrP protein may have the same covalent structure, and that the PrP protein is recruited into these amyloid‐like SAF or into association with a non‐protein component of SAF by an irreversible event initiated directly or indirectly by scrapie infection.

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Bovine spongiform encephalopathy, variant Creutzfeldt-Jakob disease (vCJD) and possibly also sheep scrapie are orally acquired transmissible spongiform encephalopathies (TSEs). TSE agents usually replicate in lymphoid tissues before they spread into the central nervous system. In mouse TSE models PrP c -expressing follicular dendritic cells (FDCs) resident in lymphoid germinal centres are essential for replication, and in their absence neuroinvasion is impaired. Disease-associated forms of PrP (PrP Sc ), a biochemical marker for TSE infection, also accumulate on FDCs in the lymphoid tissues of patients with vCJD and sheep with natural scrapie. TSE transport mechanisms between gut lumen and germinal centres are unknown. Migratory bone marrow-derived dendritic cells (DCs), entering the intestinal wall from blood, sample antigens from the gut lumen and carry them to mesenteric lymph nodes. Here we show that DCs acquire PrP Sc in vitro, and transport intestinally administered PrP Sc directly into lymphoid tissues in vivo. These studies suggest that DCs are a cellular bridge between the gut lumen and the lymphoid TSE replicative machinery.The transmissible spongiform encephalopathies (TSEs), or ' prion ' diseases, are neurodegenerative disorders which include Creutzfeldt-Jakob disease (CJD) and kuru in humans, bovine spongiform encephalopathy (BSE), transmissible mink encephalopathy, chronic wasting disease (CWD) in mule deer and elk, and scrapie in sheep and goats. Replication of the infectious TSE agent depends critically on the host prion protein (PrP c ), which accumulates as an abnormal, detergentinsoluble, relatively proteinase-resistant isoform, PrP Sc , in diseased tissues (Bolton et al., 1982 ; Bueler et al., 1992). The

In most peripheral infections of rodents and sheep with scrapie, infectivity is found first in lymphoid tissues and later in the central nervous system (CNS). Cells within the germinal centers (GCs) of the spleen and lymph nodes are important sites of extraneural replication, from which infection is likely to spread to the CNS along peripheral nerves. Here, using immunodeficient mice, we investigate the identity of the cells in the spleen that are important for disease propagation. Despite possessing functional T and B lymphocytes, tumor necrosis factor alpha-deficient (TNF-α−/−) mice lack GCs and follicular dendritic cell (FDC) networks in lymphoid tissues. In contrast, lymphoid tissues of interleukin-6-deficient (IL-6−/−) mice possess FDC networks but have impaired GCs. When the CNSs of TNF-α−/−, IL-6−/−, and wild-type mice were directly challenged with the ME7 scrapie strain, 100% of the mice were susceptible, developing disease after closely similar incubation periods. However, when challenged peripherally (intraperitoneally), most TNF-α−/− mice failed to develop scrapie up to 503 days postinjection. All wild-type and IL-6−/− mice succumbed to disease approximately 300 days after the peripheral challenge. High levels of scrapie infection and the disease-specific isomer of the prion protein, PrPSc, were detectable in spleens from challenged wild-type and IL-6−/− mice but not from TNF-α−/−mice. Histopathological analysis of spleen tissue demonstrated heavy PrP accumulations in direct association with FDCs in challenged wild-type and IL-6−/− mice. No PrPScaccumulation was detected in spleens from TNF-α−/−mice. We conclude that, for the ME7 scrapie strain, mature FDCs are critical for replication in lymphoid tissues and that in their absence, neuroinvasion following peripheral challenge is impaired.