Studies on prion replication in spleen

Some of the early events following scrapie infection take place in the lymphoreticular system (LRS) and result in significant replication of prions in lymphoid organs. The identity of the cells in the LRS that produce prions and their role in neuroinvasion are still unknown. We find that in the spleen of scrapie-infected mice, prions are associated with T and B cells and to a somewhat lesser degree with the stroma, which contains the follicular dendritic cells (FDC's); curiously, no infectivity was found in lymphocytes from blood of the same mice. Thus, splenic lymphocytes either replicate prions or acquire them from another source. Studies on PrP knockout mice with ectopic expression of PrP restricted to only B or T lymphocytes suggest that neither of these by themselves are competent for prion replication. To determine whether B and T cells are able to pick up prions from other sources, irradiated wild-type mice were reconstituted with PrP-deficient lymphohaematopoietic stem cells. Following intraperitoneal inoculation of these mice, no infectivity was found on splenic lymphocytes whereas the stroma (comprising the radiation-resistant, PrP-expressing FDC's) contained prions. These results imply that splenic lymphocytes can acquire prions, possibly from FDC's, but only if they express PrP.

Ectopic expression of prion protein (PrP) in T lymphocytes or hepatocytes of PrP knockout mice is insufficient to sustain prion replication

The cellular form of the Prion protein (PrPC) is necessary for prion replication in mice. To determine whether it is also sufficient, we expressed PrP under the control of various cell- or tissue-specific regulatory elements in PrP knockout mice. The interferon regulatory factor-1 promoter/Emu enhancer led to high PrP levels in the spleen and low PrP levels in the brain. Following i.p. scrapie inoculation, high prion titers were found in the spleen but not in the brain at 2 weeks and 6 months, showing that the lymphoreticular system by itself is competent to replicate prions. PrP expression directed by the Lck promoter resulted in high PrP levels on T lymphocytes only but, surprisingly, did not allow prion replication in the thymus, spleen, or brain following i.p. inoculation. A third transgenic line, which expressed PrP in the liver under the control of the albumin promoter/enhancer-albeit at low levels-also failed to replicate prions. These results show that expression of PrP alone is not sufficient to sustain prion replication and suggest that additional components are needed.