Infectivity titers of 22L scrapie from brains of Tg44 mice after 1 or 2 passages.

aZygosity of tg44 transgene in scrapie-infected mouse whose brain infectivity was titered.bNumber of passages in tg44 mice (1 = first; 2 = second). For passage 1, tg44+/− and tg44+/+ mice were inoculated intracerebrally with a 22L scrapie stock derived from C57BL/10 mice with a titer of 1.4×109 ID50per gram of brain tissue. For the second passage in tg44+/− the brain of the 286 dpi passage 1 tg44+/− mouse was used as the inoculum. The preliminary titration of the passage 1 mouse from 286 dpi was previously reported to be >2.4×108 ID50per brain [19].cDays post-inoculation when brain was harvested for titration.dBrain titer was determined by end-point dilution titration in C57BL/10 mice for tg44+/− or in tga20 mice for tg44+/+. Titer was expressed as ID50per gram of brain tissue.eThe titration of this brain is still in progress.