Experimental Verification of a Traceback Phenomenon in Prion Infection

The clinicopathological phenotypes of sporadic Creutzfeldt-Jakob disease (sCJD) correlate with the allelo-types (M or V) of the polymorphic codon 129 of the human prion protein (PrP) gene and the electrophoretic mobility patterns of abnormal prion protein (PrPSc). Transmission of sCJD prions to mice expressing human PrP with a heterologous genotype (referred to as cross-sequence transmission) results in prolonged incubation periods. We previously reported that cross-sequence transmission can generate a new prion strain with unique transmissibility, designated a traceback phenomenon. To verify experimentally the traceback of sCJD-VV2 prions, we inoculated sCJD-VV2 prions into mice expressing human PrP with the 129M/M genotype. These 129M/M mice showed altered neuropathology and a novel PrPSc type after a long incubation period. We then passaged the brain homogenate from the 129M/M mouse inoculated with sCJD-VV2 prions into other 129M/M or 129V/V mice. Despite cross-sequence transmission, 129V/V mice were highly susceptible to these prions compared to the 129M/M mice. The neuropathology and PrPSc type of the 129V/V mice inoculated with the 129M/M mouse-passaged sCJD-VV2 prions were identical to those of the 129V/V mice inoculated with sCJD-VV2 prions. Moreover, we generated for the first time a type 2 PrPSc-specific antibody in addition to type 1 PrPSc-specific antibody and discovered that drastic changes in the PrPSc subpopulation underlie the traceback phenomenon. Here, we report the first direct evidence of the traceback in prion infection.