Fewer PrPc myeloid-based cells in sheep with the prion-resistant genotype

Small-ruminant lentivirus enhances PrPSc accumulation in cultured sheep microglial cells

Sheep scrapie is the prototypical transmissible spongiform encephalopathy (prion disease), which has a fundamental pathogenesis involving conversion of normal cellular prion protein (PrP(C) [C superscript stands for cellular]) to disease-associated prion protein (PrP(Sc) [Sc superscript stands for sheep scrapie]). Sheep microglial cell cultures, derived from a prnp 136VV/171QQ near-term fetal brain, were developed to study sheep scrapie in the natural host and to investigate potential cofactors in the prion conversion process. Two culture systems, a primary cell culture and a cell line transformed with the large T antigen of simian virus 40, were developed, and both were identified as microglial in origin as indicated by expression of several microglial phenotype markers. Following exposure to PrP(Sc), sheep microglial cells demonstrated relatively low levels (transformed cell line) to high levels (primary cell line) of PrP(Sc) accumulation over time. The accumulated PrP(Sc) demonstrated protease resistance, an inferred beta-sheet conformation (as determined by a commercial enzyme-linked immunosorbent assay), specific inhibition by anti-PrP antibodies, and was transmissible in a dose-dependent manner. Primary microglia coinfected with a small-ruminant lentivirus (caprine arthritis encephalitis virus-Cork strain) and PrP(Sc) demonstrated an approximately twofold increase in PrP(Sc) accumulation compared to that of primary microglia infected with PrP(Sc) alone. The results demonstrate the in vitro utility of PrP(Sc)-permissive sheep microglial cells in investigating the biology of natural prion diseases and show that small-ruminant lentiviruses enhance prion conversion in cultured sheep microglia.

Effects of nutrition and genotype on prion protein (PrPC) gene expression in the fetal and maternal sheep placenta

For placental transmission of scrapie to occur, the normal cellular prion protein (PrPC) must be converted to an abnormal infectious form known as PrPSc. PrPC genotype influences susceptibility to contracting scrapie, but we still do not understand whether genotype or expression levels of PrPC are important in transmission of scrapie. Some evidence exists that nutrition affects expression levels of PrPC. Thus, we evaluated the effects of genotype and nutrition on PrPC mRNA and protein expression in adolescent ewes fed at control (100% of National Research Council [NRC] requirements) or restricted (60% of NRC) levels of diet intake during two periods of pregnancy (days 50-90 and days 90-130)]. Gravid uteri (n=50) from singleton pregnancies were collected at day 130, and placentomes were either separated into caruncular (CAR; maternal) or cotyledonary (COT; fetal) placenta and snap-frozen for PrPC mRNA expression or perfusion fixed for PrPC protein expression. PrPC genotypes were determined (codons 136 and 171) using SNP assay. There were no genotype effects on PrPC mRNA expression in CAR or on PrPC protein expression in either CAR or COT, but PrPC mRNA expression in COT was greater (P<0.02) when codon 136 was homozygous for alanine. Some PrPC protein-positive cells were found in the epithelium of CAR, but most were found in trophoblast binucleate and mononucleate cells of COT. In CAR, from days 90 to 130, PrPC protein abundance was greater (P=0.003) in diet-restricted ewes than in control ewes, but was less uniformly distributed (P<0.007). Additionally, in COT, from days 90 to 130, PrPC protein was less uniformly distributed (P<0.01) in diet-restricted ewes. The localized increase in PrPC protein expression, found in ewes diet-restricted late in pregnancy, may suggest a protective role for PrPC in placental biology. Further study is needed to evaluate whether nutrition, PrPC genotype, and PrPC expression levels influence placental transmission of scrapie.

Immunological differences between susceptible and resistant sheep during the preclinical phase of scrapie infection

In order to investigate the relationship between the immune response to scrapie infection and genetic susceptibility to the disease in sheep, immune cell subsets and prion protein (PrP) expression were determined in susceptible and resistant Suffolk sheep in the preclinical phase of infection. At 6 months of age, 12 ARQ/ARQ (susceptible) and nine ARR/ARR (resistant) scrapie-free Suffolk lambs were challenged subcutaneously with scrapie inoculum. Prefemoral lymphadenectomies were carried out at 14 and 180 days post-inoculation (p.i.) and serial bleeds were collected at monthly intervals for up to 1 year p.i. An indirect double-labelling procedure was carried out on peripheral blood mononuclear cells (PBMCs) and lymph node cell preparations and analysed using flow cytometry. Prior to scrapie challenge, significantly more PrP+cells were detected in PBMCs from the susceptible sheep. Furthermore, following challenge, significantly more CD8+andγΔ+T cells were detected in the PBMCs of the resistant sheep. However, at both 14 and 180 days p.i, CD21+cell expression was significantly higher in the lymph node preparations of the susceptible sheep. In contrast, more CD4+cells were detected in the lymph nodes of the resistant sheep at both time points. It was concluded that significant differences in immune cell subsets and PrP expression occur between ARQ/ARQ and ARR/ARR Suffolk sheep in the preclinical phase of infection.