Sites of prion protein accumulation in scrapie‐infected mouse spleen revealed by immuno‐electron microscopy

Abstract

Prion protein (PrP) from the brains of animals with transmissible spongiform encephalopathies is partially protease resistant (PrP res ) compared with fully sensitive PrP (PrP sen ) from uninfected brains. In most experimental models, PrP res is a reliable indicator of infectivity. Light microscopic studies have suggested that both PrP sen and disease-speci®c accumulations of PrP are associated with follicular dendritic cells (FDCs). Using immunogold electron microscopy, this study has demonstrated disease-speci®c accumulation of PrP in the spleens of C57 BL mice, 70 days after intracerebral infection with the ME7 strain of scrapie and at the terminal stage of disease at 170 days. At both stages, tingible body macrophages contained PrP within lysosomes and PrP was also detected at the plasmalemma of FDCs. In the light zone of follicles of terminally diseased mice, all FDC dendrites were arranged in the form of highly reactive or hyperplastic labyrinthine glomerular complexes, within which PrP was consistently seen between FDC processes in association with abundant electron dense material, interpreted as antigen±anti-body complexes. Within some glomeruli, ®brillar forms of PrP consistent with amyloid were seen. At 70 days after challenge, large or hyperplastic labyrinthine complexes were rare and invariably labelled for PrP. However, sparse PrP labelling was also seen on simple FDC processes at this stage. The ubiquitous accumulation of extracellular PrP in complex glomerular dendrites of FDCs in spleens from terminally affected mice, contrasted with simple FDC pro®les, sparse PrP and limited electron dense deposits in all but a few FDCs of 70-day post-infected mice. This suggests that FDCs continually release PrP from the cell surface, where it is associated with trapped antigen±antibody complexes and dendritic extension. It is likely that tingible body macrophages acquire PrP following phagocytosis of PrP within iccosomes or from the extracellular space around FDC dendrites. These studies would not support an intracellular phase of PrP accumulation in FDCs but show that PrP is produced in excess by scrapie-infected cells from where it is released into the extracellular space. We suggest that PrP sen is involved in dendritic extension or in the process of antibody±antigen trapping, perhaps as part of the binding mechanism for antigen±antibody complexes.

Follicular Dendritic Cell Dedifferentiation by Treatment with an Inhibitor of the Lymphotoxin Pathway Dramatically Reduces Scrapie Susceptibility

“…Within the lymphoid tissues of patients with vCJD (24), most sheep with natural scrapie (56), or after experimental peripheral inoculation of rodents with scrapie (6,28,37,39,44), early PrP Sc accumulation takes place in the germinal centre on follicular dendritic cells (FDCs). Studies of mouse scrapie models have shown that mature FDCs are critical for replication in lymphoid tissues and, in their absence, neuroinvasion after peripheral challenge is significantly impaired (6, 17, 31, 37-39, 46, 51).…”

Orally Administered Prion Protein Is Incorporated by M Cells and Spreads into Lymphoid Tissues with Macrophages in Prion Protein Knockout Mice

“…Macrophages containing PrP Sc have been detected in ovine Peyer's patches, 46 other secondary lymph nodes of variant CJD patients, 47 and the spleen of mice. 48 In the spleen, PrP-positive cells were entirely CD14-positive macrophages (Figure 8). Jeffrey et al 48 revealed that such macrophages contained PrP Sc within lysosomes.…”

“…48 In the spleen, PrP-positive cells were entirely CD14-positive macrophages (Figure 8). Jeffrey et al 48 revealed that such macrophages contained PrP Sc within lysosomes. Additionally, the depletion of spleen macrophages by administrating clodronate to mice shows that macrophages partly control scrapie infection in peripheral tissues by sequestration of the scrapie inoculum and impairment of the replication of scrapie infectious agents in the spleen during the early stages of the disease.…”

Les prions exploitent les communications neuro-immunitaires

“…Ainsi, le long de l'artériole centrale et de ses branches, on trouve des fibres sécrétant le neuropeptide-Y et des fibres à Met-enképhaline (peptide opioïde dérivé du clivage d'un précurseur, la pro-enképhaline A). Dans la pulpe blanche et la pulpe rouge on peut observer des fibres à cholécystokinine-8 3 . Enfin, le long de fins vaisseaux dans la pulpe blanche on trouve des fibres à neurotensine 4 .…”

“…La détection de PrP Sc à la surface des cellules folliculaires dendritiques (FDC), qui sont les cellules résidentes des centres germinatifs (CG), laissait présager un rôle important de ces cellules dans la rétention du pathogène [36] (➜). En effet, il a été démontré qu'après une infection périphérique, les FDC matures accumulent la PrP Sc [2,3]. La maturation des FDC est un point crucial dans le phénomène de lymphoinvasion et elle requiert la présence de lymphocytes B, porteurs de lymphotoxine /.…”