Sensitivity for prion detection of SCEPA and mouse bioassay.

*The ratio between PrPSc-positive and total wells is shown for one representative out of eight independent experiments.†Infectious titers were calculated with the Spearman-Karber formula [55] and expressed as tissue culture infectious units (TCIU)/g brain for SCEPA and LD50 units/g brain for mouse bioassay, respectively.$Infectious titers were calculated using a GLM with binomial family complementary log-log link and expressed as mean log TCIU/g brain ± SE of 8 independent experiments for SCEPA and mean log LD50 units/g ± SE for two independent bioassays.‡Infectious titers were estimated for the combined two bioassays using a GLM regression with complementary log-log link function and expressed as log LD50 units/g brain.The sensitivity for prion detection of SCEPA and mouse bioassay was determined by endpoint titration using RML mouse brain homogenate I6200. Aliquots of I6200 (10% (w/v), 9.3 log LD50 units/g brain [7]) were serially diluted 1∶10 into uninfected CD1 brain homogenate (10% w/v) in a range between 10−4 and 10−10. For mouse bioassay, groups of six Tga20 mice were inoculated intracerebrally with 30 µl of 1% (w/v) RML homogenates and attack rates and scrapie incubation times (Inc. time) were determined. In parallel experiments brain homogenates were diluted 1∶1000 into OFCS and cell layers of highly prion susceptible N2aPK1-2 cells were infected with 300 µl aliquots. The input of prion infectivity for bioassay and SCEPA is expressed as mouse ic LD50 units. A 10−7 dilution of I6200 corresponds to 200 LD50 units/ml or 6 LD50 units per 30 µl inoculum for the mouse bioassay and 60 LD50 units per 300 µl per well for SCEPA, respectively. Infectious titers for SCEPA, expressed as TCIU/g brain represent mean values ± SE of 8 independent experiments. For mouse bioassay, two independent experiments are shown and titers are expressed as LD50/g brain ± SE.