Viruses do replicate in cell-free systems

FreeCell

Proc Natl Acad Sci U S A. Feb 21, 2012; 109(8): E461.
) n# P) W! a  I! BPublished online Jan 30, 2012. doi:  10.1073/pnas.1118908109
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Viruses do replicate in cell-free systems

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Yervand E. Karapetyan1

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Klingeborn et al. (1) reported a systematic study of the infectivity of transmissible spongiform encephalopathy (TSE) infectious agent generated de novo in cell-free reactions. They convincingly demonstrated the generation of new TSE infectivity in reactions primed by infected brain homogenates. Absence of infectivity in control reactions with brain homogenates lacking prion protein (PrP) supports the requirement of host PrP for in vitro replication of the agent. Based on these findings, the authors assumed this new infectivity derives from a small proportion of newly generated misfolded PrP. They then concluded that “because viruses do not replicate in cell-free conditions, this result is against the concept that the prion infectious agent is a virus” (1).0 g( N, l& E6 g) U: H) |

' D" k9 ]. D( r! X( bThis statement and conclusion contradict the well-known fact that several viruses, including the mammalian poliovirus, can replicate in a cell-free system. Synthesis of polioviral RNA in cell-free conditions was discovered by Baltimore in 1964 (2). In 1991, Wimmer and colleagues (3) additionally demonstrated de novo formation of infectious poliovirus particles in uninfected HeLa cell lysates primed by poliovirus virion mRNA. Thus, the cell-free replication of TSE agent does not exclude a causative viral particle that requires host PrP for replication. Moreover, the destruction of virtually all forms of PrP without loss of infectious brain titer (4), as well as the environmental origin of many unique TSE strains, seriously undermines the protein-only prion hypothesis (5). Diverse strains strongly implicate an agent genome, and nucleic acids are present in all infectious preparations.+ ?% Z0 N8 }- P( Q" U$ Z4 M

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The author declares no conflict of interest.+ n+ L9 Z/ n  E, _& q' a; |+ `' m1 ^
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REFERENCES6 }, z- }7 k- n& }; _6 J0 h( d
1. Klingeborn M, Race B, Meade-White KD, Chesebro B. Lower specific infectivity of protease-resistant prion protein generated in cell-free reactions. Proc Natl Acad Sci USA. 2011;108:E1244–E1253. * A7 k) |" c  r3 a
2. Baltimore D. In vitro synthesis of viral RNA by the poliovirus RNA polymerase. Proc Natl Acad Sci USA. 1964;51:450–456.
8 s/ H7 f& ?: ?% Z2 W% D3. Molla A, Paul AV, Wimmer E. Cell-free, de novo synthesis of poliovirus. Science. 1991;254:1647–1651.
# a+ c( d) X9 A0 `5 a8 r4. Miyazawa K, Emmerling K, Manuelidis L. High CJD infectivity remains after prion protein is destroyed. J Cell Biochem. 2011;112:3630–3637.

5. Miyazawa K, Emmerling K, Manuelidis L. Replication and spread of CJD, kuru and scrapie agents in vivo and in cell culture. Virulence. 2011;2:188–199.

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