Dose effect on detection of PrPSc in follicular dendritic cells of knock-in mice for rapid bioassay

We established a rapid bioassay method for transmissibility of human prions to knock-in mouse expressing human/mouse chimeric prion protein (Ki-ChM mouse). In Ki-ChM mice, we detected accumulation of abnormal isoform (PrPSc) of prion protein in follicular dendritic cells (FDCs) in lymphoid organ within 14 days following intraperitoneal administration of human prions. Intraperitoneal administration of the inoculum provides an advantage over intracerebral injection, considering only a limited volume of 20µl can be inoculated with the intracerebral route. To overcome this limitation and to confirm dose dependent effect on PrPSc accumulation with intraperitoneal injection, we made an attempt to inoculate brain homogenate of human sporadic CJD (sCJD) with large dose and repetitive injection of a volume up to ∼5000µl. Fifty micro liter of 10−2, 10−3, and 10−4 diluted sCJD were inoculated respective groups intraperitoneally. Five hundred µl of 10−3 and 10−4 dilution of sCJD inoculated with one shot were shown as equivalent infectivity to 50µl of 10−2 and 10−3 dilution of them respectively. Five hundred micro liter of 10−4 dilution of sCJD inoculated with once a day during 10 days were shown as an equivalent infectivity to 50µl of 10−2 dilution of it. These results indicate that the detection of PrPSc in FDCs has increased in proportion to the amount of prion inoculated. It is conceivable that the prion of a low density piles up the intake frequency and the pathogenicity rises as well as a high density prion.