SCID mice are resistant to intraperitoneal infection with 10 and 10 intracerebral ID units of ME7 scrapie agent whereas they develop disease after intracerebral challenge. However, higher doses introduced, by intraperitoneal or subcutaneous routes, produce disease. Immunocompetent mice of the same strain (CB20) developed scrapie following either intracerebral or intraperitoneal infection. Bioassay of spleens from SCID mice infected with 10 dilutions of ME7 scrapie by intraperitoneal, intracerebral or abdominal subcutaneous injection showed traces or low levels of infectivity in spleen. However, subcutaneous injection beneath the skin of the neck failed to infect the spleen. CB20 bone marrow reconstitution of SCID mice resulted in the regeneration of a normal lymphoid architecture in the spleen. Spleens from these reconstituted mice, infected intracerebrally with a 10 dilution of ME7 contained high levels of infectivity. These results suggest that the ability to replicate scrapie agent in spleen or lymphoid tissue depends on the restoration of normal lymphoid structure and in particular the presence of differentiated follicular dendritic cells. The possibility that SCID mice can select minor strains of scrapie which are normally unrecognized in cloned ME7 is discussed.


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