Coinfecting Prion Strains Compete for a Limiting Cellular Resource

Prion strain interference can influence the emergence of a dominant strain from a mixture; however, the mechanisms underlying prion strain interference are poorly understood. In our model of strain interference, inoculation of the sciatic nerve with the drowsy (DY) strain of the transmissible mink encephalopathy (TME) agent prior to superinfection with the hyper (HY) strain of TME can completely block HY TME from causing disease. We show here that the deposition of PrP^Sc, in the absence of neuronal loss or spongiform change, in the central nervous system corresponds with the ability of DY TME to block HY TME infection. This suggests that DY TME agent-induced damage is not responsible for strain interference but rather prions compete for a cellular resource. We show that protein misfolding cyclic amplification (PMCA) of DY and HY TME maintains the strain-specific properties of PrP^Sc and replicates infectious agent and that DY TME can interfere, or completely block, the emergence of HY TME. DY PrP^Sc does not convert all of the available PrP^C to PrP^Sc in PMCA, suggesting the mechanism of prion strain interference is due to the sequestering of PrP^C and/or other cellular components required for prion conversion. The emergence of HY TME in PMCA was controlled by the initial ratio of the TME agents. A higher ratio of DY to HY TME agent is required for complete blockage of HY TME in PMCA compared to several previous in vivo studies, suggesting that HY TME persists in animals coinfected with the two strains. This was confirmed by PMCA detection of HY PrP^Sc in animals where DY TME had completely blocked HY TME from causing disease.Prions are infectious agents of animals, including humans, which are comprised of PrP^Sc, a misfolded isoform of the noninfectious host encoded protein PrP^C (17, 24, 50, 63). Prion diseases of humans are unique neurodegenerative disorders in that they can have either a sporadic, familial, or infectious etiology. Prions cause disease in economically important domestic and wild animal species such as bovine spongiform encephalopathy in cattle and chronic wasting disease in wild and captive cervids (20, 62). Prion diseases can be zoonotic as illustrated by the transmission of bovine spongiform encephalopathy to humans that resulted in the emergence of variant Creutzfeldt-Jacob disease (14, 19, 22, 23, 46, 61, 68). Prion diseases are inevitably fatal and there are currently no effective treatments (21).Prion strains are defined by a characteristic set of features that breed true upon experimental passage (33, 34). Strain-specific differences have been identified in incubation period, clinical signs, agent distribution, overdominance, host range, neuropathology, and biochemical properties of PrP^Sc (5, 10, 11, 13, 28, 34, 42, 44). Strain-specific conformations of PrP^Sc are hypothesized to encode prion strain diversity; however, it is not understood how these differences result in the distinct strain properties (11, 19, 40, 47, 59, 66).Prion strain interference may be involved in the emergence of a dominant strain from a mixture as could occur during prion adaptation to a new host species or during prion evolution (4, 36, 43, 48, 56). In the natural prion diseases, there are examples where an individual host may be infected with more than one prion strain (15, 25, 55, 57, 58). Experimentally, coinfection or superinfection of prion strains can result in interference where a blocking, long incubation period strain extends the incubation period or completely blocks a superinfecting, short incubation period strain from causing disease (26, 27). Prion interference has been described in experimental studies of mice and hamsters infected with a wide variety of prion strains and routes of inoculation, suggesting it may be a common property of prion disease (3, 27, 52, 53, 60).It has been proposed that prion strains compete for a shared “replication site”; however, mechanistic details are not known, and it is unclear whether the blocking strain destroys or occupies the replication sites required for the superinfecting strain (28). The transport to and relative onset of replication of interfering strains in a common population of neurons is an important factor that can determine which strain will emerge (8). In the present study, we sought to determine whether the blocking strain disables transport and spread of the superinfecting strain or whether prion interference is due to competition for a cellular resource.