Pathogenesis of Prion Diseases I

Peripheral exposure is the most common route for transmission of prion diseases. To understand the spread of peripherally administered prions to the brain, we studied the role of the complement system in this process termed neuroinvasion. It has been shown that neuroinvasion is facilitated by both soluble complement factors (C1q, Bf/C2 and C3) and the complement receptors CD21/CD35. Since mouse prions target follicular dendritic cells (FDCs) in the lymphoid tissues, it has been suggested that prions might bind to complement factors and thereby transport to FDCs via complement receptors. In the first project, we have
analyzed mice devoid of CD19, a co-receptor of CD21/CD35. After intraperitoneal challenge, mice developed the disease significantly sooner than wild-type mice. Morphological studies
of the spleens revealed that the distance between FDCs and the adjacent nerve fibers is markedly reduced. This suggests that the closer contact between FDCs and splenic nerve fibers might result in accelerating of the neuroinvasion. In a second project, we have
generated bone marrow chimaeras expressing CD21/CD35 either on lymphoid cells or FDCs, because we have recently shown that the deficiencies of CD21/CD35 on both cell type delayed prion neuroinvasion in mice. Surprisingly, expression of CD21/CD35 on either cell type is sufficient to restore the delayed neuroinvasion of prions, but only if the cellular prion protein (PrPC) and CD21/CD35 are co-expressed on lymphoid cells. In a third project, we challenged mice expressing an inhibitor of the complement system (sCrry) restricted to the
nervous system. In strong contrast, prion pathogenesis in the nervous system appears to be complement-independent. Since C1q null mice are resistant to peripheral exposure with a low prion dose, we analyzed in a fourth project the spleens of these mice for the expression of PrPC, which is required for prion propagation. Surprisingly, we found that PrPC expression within the splenic FDC network is up-regulated in wild-type mice, but not in C1q null mice upon antigen stimulation. A novel PrP-expressing compartment was found on capsule and trabeculae in the spleen. In a fifth project, we determined whether orally administered prions can reach the brain in mice expressing hamster PrPC only on neurons. Indeed, mice are still susceptible either after oral or peripheral challenge. However, immunodeficiencies of mice render them completely resistant to oral infection, while they are still susceptible to
intraperitoneal infection. In a sixth project, we performed a therapeutic approach to block prion neuroinvasion in mice by using STI571 (Gleevec), which clears prion-infected cells from PrPSc without targeting PrPC. We found that PrPSc levels in the spleen were markedly
reduced, when the drug was administered at the point of maximal levels of prion infectivity in the spleen. Treatment provided a week after peripheral challenge for a month delayed appearance of PrPSc in the brain and prolonged the onset of clinical disease. However, once
prions had already reached the brain, no therapeutic effect on the incubation time was found.