Much available evidence points to a pathological isoform of the prion protein PrP being the infectious agent that causes transmissible spongiform encephalopathies, but the mechanisms controlling the neurotropism of prions are still unclear. We have previously shown that mice that do not express PrP (Prnp o/o mice) are resistant to infection by prions 1, 2, and that if a Prnp+/+ neurograft is introduced into such animals and these are infected intracerebrally with scrapie, the graft but not the surrounding tissue shows scrapie pathology 3. Here we show that PrP-expressing neurografts in Prnp o/o mice do not develop scrapie histopathology after intraperitoneal or intravenous inoculation with scrapie prions. Prion titres were undetectable in spleens of inoculated Prnp o/o mice, but were restored to wild-type levels upon reconstitution of the host lymphohaemopoietic system with PrP-expressing cells. Surprisingly, however, ip or iv inoculation failed to produce scrapie pathology in the neurografts of 27 out of 28 reconstituted animals, in contrast to intracerebral inoculation. We conclude that transfer of infectivity from the spleen to the central nervous system is crucially dependent on the expression of PrP in a tissue compartment that cannot be reconstituted by bone marrow transfer. Thus the requirement for the normal isoform of PrP in peripheral tissues represents a bottleneck for the spread of prions from peripheral sites to the central nervous system.

We placed neurografts derived from wild-type mice or transgenic mice overexpressing PrP (tga20; ref. 4) into the brains of Prnp o/o mice and delivered scrapie prions (6.5 log LD 50 units, where LD 50 is the half-maximal lethal dose) by intraperitoneal (ip) or intravenous (iv) injection. Neither clinical disease nor the histopathological changes characteristic of spongiform encephalopathy developed during the observation period, up to 401 days after inoculation. Histoblots 5 revealed no protease-resistant PrP Sc in Prnp o/o mice carrying tga20 (n= 17) or wild-type (n= 2) neurografts. In contrast, the same amount of infectious agent produced scrapie when administered ip to 3/3 wild-type mice (incubation time, 209±0 days) or 5/5 tga20 mice (incubation time, 85±2 days)(Table 1).