In vivo conversion of cellular prion protein to pathogenic isoforms, as monitored by conformation-specific antibodies

The central event in prion disease is thought to be conformational conversi on of the cellular isoform of prion protein (PrPC) to the insoluble isoform PrPSc. We generated polyclonal and monoclonal antibodies by immunizing PrP C-null mice with native PrPC. All seven monoclonal antibodies (mAbs) immuno precipitated PrPC, but they immunoprecipitated PrPSc weakly or not at all, thereby indicating preferential reactivities to PrPC in solution. Immunopre cipitation using these mAbs revealed a marked loss of PrPC in brains at the terminal stage of illness. Histoblot analyses using these polyclonal antib odies in combination of pretreatment of blots dissociated PrPC and PrPSc in situ and consistently demonstrated the decrease of PrPC at regions where P rPSc accumulated. interestingly, same mAbs showed immunohistochemical react ivities to abnormal isoforms. One group of mAbs showed reactivity to materi als that accumulated in astrocytes, while the other group did so to amorpho us plaques in neuropil. Epitope mapping indicated that single mAbs have rea ctivities to multiple epitopes, thus implying dual specificities. This sugg ests the importance of octarepeats as a part of PrPC-specific conformation. Our observations support the notion that loss of function of PrPC may part ly underlie the pathogenesis of prion diseases. The conversion of PrPC to P rPSc may involve multiple steps at different sites.