TABLE 1.
Endpoint titration by the intracerebral route of a reference scrapie isolate in transgenic mice that express the ovine PrP and in ARQ/VRQ sheepa
| Dilution of the 4% (wt/vol) brain homogenate | Brain material per animal (g) | Transmission inb: |
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|---|---|---|---|---|---|---|---|---|---|
| tg338 mice |
tg338 × PrPKo mice |
tg338 × tgShpXI mice |
ARQ/VRQ sheep |
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| No. of infection-positive mice/total no. of mice | Incubation period (days) (mean ± SD) | No. of infection-positive mice/total no. of mice | Incubation period (days) (mean ± SD) | No. of infection-positive mice/total no. of mice | Incubation period (days) (mean ± SD) | No. of infection-positive sheep/total no. of sheep | Incubation period (days) (mean ± SD) | ||
| 10−3 | 8 × 10−7 | 6/6 | 94 ± 6 | 6/6 | 103 ± 4 | 6/6 | 111 ± 9 | 6/6 | 212 ± 16 |
| 10−4 | 8 × 10−8 | 4/6 | 117 ± 12 | 4/6 | 128 ± 12 | 2/6 | 124, 154 | 3/6 | 242, 263, 275 |
| 10−5 | 8 × 10−9 | 0/6 | >250 | 0/6 | >250 | 0/6 | >250 | 0/6 | >650 |
| 10−6 | 8 × 10−10 | 0/6 | >250 | 0/6 | >250 | 0/6 | >250 | 0/6 | >650 |
| 10−7 | 8 × 10−11 | 0/6 | >250 | 0/6 | >250 | 0/6 | >250 | 0/6 | >650 |
Successive 1/10 dilutions of a 4% tissue homogenate prepared using the brainstem from 70 ARQ/VRQ scrapie affected sheep (Langlade flock) were inoculated into groups of tg338 mice (n = 6), tg338 × PrPKo, tg338 × tgShpXI and ARQ/VRQ sheep (n = 6). In camparison with ARQ/VRQ sheep, (i) tg338 mice express about 8-fold, (ii) Tg338 × PrPKo mice express about 4-fold, and (iii) tg338 × tgShpXI mice express about 8-fold the ovine PrPC in their brain (see Fig. 1). Animals were euthanized when they showed clinical signs of infection or after 250 days postinoculation (mice) or 650 days postinoculation (sheep). Animals were considered infected when PrPres deposition was detected in the brain by Western blot analysis using the Sha31 monoclonal antibody which recognizes amino acids 145 to 152 (YEDRYYRE) of the sheep PrP. Infectious titers were estimated by the Spearman-Karber method (7), and they are reported as the number of ID50 per gram of brain tissue.
The most likely value and the lower and upper values of the 95% confidence intervals (in parentheses) of the infectious titers (ID50 per gram of tissue) are as follows: for tg338 mice, 107.26 (106.88 to 107.65); for tg338 × PrPKo mice, 107.25 (106.88 to 107.65); for tg338 × tgShpXI mice, 106.93 (106.55 to 107.32); and for ARQ/VRQ sheep, 107.1 (106.69 to 107.51).