Infected splenic dendritic cells are sufficient for prion transmission to the CNS in mouse scrapie

Abstract

Transmissible spongiform encephalopathies display long incubation periods at the beginning of which the titer of infectious agents (prions) increases in peripheral lymphoid organs. This "replication" leads to a progressive invasion of the CNS. Follicular dendritic cells appear to support prion replication in lymphoid follicles. However, the subsequent steps of neuroinvasion remain obscure. CD11c(+) dendritic cells, an unrelated cell type, are candidate vectors for prion propagation. We found a high infectivity titer in splenic dendritic cells from prion-infected mice, suggesting that dendritic cells carry infection. To test this hypothesis, we injected RAG-1(0/0) mice intravenously with live spleen cell subsets from scrapie-infected donors. Injection of infected dendritic cells induced scrapie without accumulation of prions in the spleen. These results suggest that CD11c(+) dendritic cells can propagate prions from the periphery to the CNS in the absence of any additional lymphoid element.

Figure 1

Western blot detection of PrP^Sc (PrP-res) in brain extracts from four mice inoculated with scrapie-infected spleen cell fractions. Each lane was loaded with 0.075 brain equivalents, with (+) and without (–) previous treatment by proteinase K as described in the text. Extracts were prepared from brains of RAG-1^0/0 that were sacrificed at terminal disease or more than 400 days after inoculation with the following 139A-infected spleen cell preparations: (lane group A) Total mononuclear cells, (lane group B) B cell–enriched cells, (lane group C) killed B cells, and (lane group D) dendritic cells (CD11c⁺). Note that the extraction procedure does not recover normal PrP^C because of its detergent solubility, which explains the absence of signal in lane C(–). Extracts in lane group A, lane group B, and lane group D display high amounts of PrP that are resistant to proteinase K, with a shift in apparent sizes of the PrP glycoforms that is characteristic of PrP^Sc.

Figure 2

Absence of lymphoid reconstitution in mice injected with infected CD11c⁺ cells. (a–d) Hematoxylin and eosin staining of paraformaldehyde-fixed sections at the same magnification. ×25. A normal lymphoid architecture of the spleen is found in RAG1^0/0 mice that had been injected with live total spleen cells (a) and the B cell–enriched fraction (b), but not in RAG1^0/0 mice injected with killed spleen cells (c) or the live CD11c⁺ cell–enriched fraction (d). (e and f) Higher magnification (×1,000) of lymphoid follicles in spleens from mice injected with total spleen cells (e) reveals typical aspect including immunoblasts and centroblast, which is clearly different from that of nodules found in spleens from CD11C⁺ cell–injected mice (f); the latter are made of smaller cells, essentially with a nonlymphoid morphology and likely belonging to other hematopoietic lineages. (g and h) Frozen sections from spleens of RAG1^0/0 mouse recipients injected with total spleen cells (g; ×400) or the CD11c⁺ cell–enriched fraction (h; ×400) were triple stained with anti-CD3-FITC (green), anti-CD19-phycoerythrin (red), and anti-CD11c-Cychrome5 (blue). The absence of lymphoid structure in RAG1^0/0 mice injected with CD11c⁺ cells is confirmed on h, which is representative of the entire spleen and shows no staining for B (CD19) and T (CD3) cells. (i and j) Immunohistochemical evidence of PrP^Sc (see Methods) accumulation in spleens from RAG1^0/0 mice infected with total spleen cells (i; ×200) but not in those from mice infected with the CD11c⁺ cell–enriched fraction (j; ×200).