Transmissions of variant Creutzfeldt-Jakob disease from brain and lymphoreticular tissue show uniform and conserved bovine spongiform encephalopathy-related phenotypic properties on primary and secondary passage in wild-type mice.

Prion strains are defined by their biological properties after transmission to wild-type mice, specifically by their incubation periods and patterns of vacuolar pathology ('lesion profiles'). Preliminary results from transmissions of variant Creutzfeldt-Jakob disease (vCJD) to wild-type mice provided the first compelling evidence for the close similarity of the vCJD agent to the agent causing bovine spongiform encephalopathy (BSE). Complete results from this investigation, including the transmission characteristics of vCJD from brain and peripheral tissues of 10 cases (after primary transmission and subsequent mouse-to-mouse passage), have now been analysed. All 10 vCJD sources resulted in consistent incubation periods and lesion profiles, suggesting that all 10 patients were infected with the same strain of agent. Incubation periods suggested that infectious titres may be subject to regional variation within the brain. Comparison of incubation periods and lesion profiles from transmission of brain and peripheral tissues showed no evidence of tissue-specific modification in the biological properties of the agent. Analysis of the protease-resistant prion protein (PrP(res)) by Western blotting from primary and subsequent passages in mice showed a glycosylation pattern closely resembling that of vCJD in humans, the so-called BSE 'glycoform signature'. Minor variations in PrP(res) fragment size were evident between mouse strains carrying different alleles of the gene encoding PrP both in primary transmissions and on further passages of vCJD brain. Overall, the results closely resembled those of previously reported transmissions of BSE in the same mouse strains, consistent with BSE being the origin of all of these vCJD cases.

Thermostability of mouse-passaged BSE and scrapie is independent of host PrP genotype: implications for the nature of the causal agents.

Five experimentally maintained strains of scrapie and BSE agents have been passaged in two PrP genotypes of mice. Brain macerates were autoclaved at 126 degrees C and the levels of surviving infectivity were measured by titration. There was a large difference in the survival properties of transmissible spongiform encephalopathy (TSE) infectivity between TSE strains. PrP genotype had little effect. Phenotypic properties of the TSE strains were not affected with the exception that with one strain (ME7), incubation periods of the heated sample were longer than the controls given equivalent doses. It is concluded that PrP is probably not responsible for differences in thermostability between strains. More likely, a host-independent molecule which differs in covalent structure between strains accounts for these properties.

Follicular dendritic cell dedifferentiation by treatment with an inhibitor of the lymphotoxin pathway dramatically reduces scrapie susceptibility.

Transmissible spongiform encephalopathies (TSEs) may be acquired peripherally, in which case infectivity usually accumulates in lymphoid tissues before dissemination to the nervous system. Studies of mouse scrapie models have shown that mature follicular dendritic cells (FDCs), expressing the host prion protein (PrP(c)), are critical for replication of infection in lymphoid tissues and subsequent neuroinvasion. Since FDCs require lymphotoxin signals from B lymphocytes to maintain their differentiated state, blockade of this stimulation with a lymphotoxin beta receptor-immunoglobulin fusion protein (LT beta R-Ig) leads to their temporary dedifferentiation. Here, a single treatment with LT beta R-Ig before intraperitoneal scrapie inoculation blocked the early accumulation of infectivity and disease-specific PrP (PrP(Sc)) within the spleen and substantially reduced disease susceptibility. These effects coincided with an absence of FDCs in the spleen for ca. 28 days after treatment. Although the period of FDC dedifferentiation was extended to at least 49 days by consecutive LT beta R-Ig treatments, this had little added protective benefit after injection with a moderate dose of scrapie. We also demonstrate that mature FDCs are critical for the transmission of scrapie from the gastrointestinal tract. Treatment with LT beta R-Ig before oral scrapie inoculation blocked PrP(Sc) accumulation in Peyer's patches and mesenteric lymph nodes and prevented neuroinvasion. However, treatment 14 days after oral inoculation did not affect survival time or susceptibility, suggesting that infectivity may have already spread to the peripheral nervous system. Although manipulation of FDCs may offer a potential approach for early intervention in peripherally acquired TSEs, these data suggest that the duration of the treatment window may vary widely depending on the route of exposure.

Strain characterization of natural sheep scrapie and comparison with BSE.

Scrapie was transmitted to mice from ten sheep, collected in the UK between 1985 and 1994. As in previous natural scrapie transmissions, the results varied between scrapie sources in terms of the incidence of disease, incubation periods and neuropathology in challenged mice. This contrasted with the uniformity seen in transmissions of BSE to mice. The scrapie and BSE isolates were characterized further by serial passage in mice. Different TSE strains were isolated from each source according to the Sinc or PrP genotype of the mouse used for passage. The same two mouse-passaged strains, 301C and 301V, were isolated from each of three BSE sources. Despite the variation seen in the primary transmissions of scrapie, relatively few mouse-passaged scrapie strains were isolated and these were distinct from the BSE-derived strains. The ME7 scrapie strain, which has often been isolated from independent sheep sources in the past, was identified in isolates from four of the sheep. However, a new distinct strain, 221C, was derived from a further four scrapie sheep. These results suggest that there is agent strain variation in natural scrapie in sheep and that the spectrum of strains present may have changed over the last 20 years. The tested sample is too small to come to any conclusions about whether the BSE strain is present in sheep, but the study provides a framework for further more extensive studies.