The Great Pretender

NEWS & VIEWS

the possibility of reversion to the virulent alphaviruses, usually contain a gene encod- uisite immune responses for protection. The phenotype during production. However, ing the desired antigen in place of the viral ‘borrowed’ technologies—such as ballistic reversion to the virulent phenotype in the structural or coat protein genes. Such vec- injection of nucleic acids or vaccines con- vaccinated recipient still remains a concern. tors are known as replicons because they taining adjuvants like CpG motifs or In addition, the authors state that RNA- retain the genes encoding the viral repli- monophosphoryl lipid A—may provide the mediated genetic immunization avoids case, allowing for amplification of vector critical boost in potency. Ultimately, a suc- safety concerns (such as chromosomal inte- RNA and high expression levels of antigen cessful ‘marriage’ is likely to require a strate- gration) that are a problem with plasmid in transduced cells. Delivery of these repli- gic combination of various technologies, DNA-based immunization. A PCR-based cons is accomplished using several modal- such as a DNA version of a replicon encod- assay was performed prior to initiating DNA ities, including in vitro transcribed RNA, ing an antigen delivered with an adjuvant, vaccine clinical trials to address the issue plasmid DNA, or recombinant vector par- to conquer the diseases that have resisted of potential DNA vaccine integration into ticles in which the replicons can be pack- vaccine strategies to date. the host chromosome. This analysis failed aged by a variety of efficient methods (see 1. Mandl, C.W. et al. In vitro synthesized infectious to reveal any evidence of integration, and Fig.). In particular, the recombinant vector RNA as an attenuated live vaccine in a flavivirus even if integration did occur (a worst case particles show great promise for stimulat- model. Nature Med. 4, 1438–1440 (1998). scenario), it would be three orders of mag- ing broad antigen-specific immune 2. Donnely, J.J. et al. DNA vaccine. Annu. Rev. Immunol. 15, 617–648 (1997). 9 nitude less than the spontaneous mutation responses in animal models . These parti- 3. Wang, R. et al. Induction of antigen-specific cyto- rate in the host chromosome5. Nevertheless, cles retain the efficiency of infection and toxic T lymphocytes in humans by a malaria DNA vaccine. Science 282, 476–480 (1998). using RNA eliminates a theoretical concern. tropism properties of the wild-type virus, 4. Calarota, S. et al. Cellular cytotoxic response in- As TBE virus is cytolytic, all cells trans- are propagation incompetent, and gener- duced by DNA vaccination in HIV-1-infected pa- duced with the ‘infectious’ in vitro tran- ate class I-restricted cytotoxic T lympho- tients. Lancet 351, 1320–1325 (1998). 5. Nichols, W. et al. Potential DNA vaccine integration scribed RNA are ultimately killed as a result cytes as a result of replicon-based expres- into host cell genome. Ann. N.Y. Acad. Sci. 772, of productive infection. More importantly, sion of antigen. Thus, replicon particles 30–39 (1995). however, a significant potential limitation may indeed provide the efficacy charac- 6. Agapov, E. et al. Noncytopathic Sindbis virus RNA vectors for heterologous gene expression. Proc. of this approach is that the manufacture teristic of live virus vaccines, yet avoid Natl. Acad. Sci. USA 95, 12989–12994 (1998). of in vitro transcribed RNA is rather ineffi- safety issues related to reversion to a path- 7. Driver, D. et al. Plasmid DNA-based alphavirus expression vectors for nucleic acid immunization. Curr. cient, and RNA itself is extremely labile. For ogenic virus in vaccinated individuals. Res. Mol. Therapeut. 1, 510–517 (1998). example, in this study, efficacy was There are now several publications in a vari- 8. Ward, G. et al. Plasmid DNA encoding replicating observed with ballistic approaches but not ety of model systems that demonstrate the foot-and-mouth disease virus genomes induces an-

http://medicine.nature.com tiviral immune responses in swine. J. Virol. 71, • with in vitro transcribed RNA administered efficacy of nucleic acid- or particle-based 7442–7447 (1997). in solution. Furthermore, the fidelity of in vectors derived from alphaviruses9–11. 9. Pushko, P. et al. Replicon-helper systems from atten- vitro transcription is far less than that of The challenges that lie ahead in the quest uated Venezuelan equine encephalitis virus: Expression of heterologous genes in vitro and immu- DNA replication and, in fact, has been to develop effective vaccines against the nization against heterologous pathogens in vivo. exploited to select for positive-stranded most serious virus infections are daunting Virology 239, 389–401 (1997). 10. Hariharan, M. et al. DNA immunization against her- RNA viruses with phenotypes that are dif- but may be conquered given the technolo- pes simplex virus: Enhanced efficacy using a Sindbis ferent from those of their wild-type coun- gies under development. The ‘old’ live virus virus-based vector. J. Virol. 72, 950–958 (1998). terparts6. Thus, whereas commercial-scale vaccines are successful, but may not be fea- 11. Berglund, P. et al. Enhancing immune responses using suicidal DNA vaccines. Nat. Biotechnol. 16, manufacture of plasmid DNA-based vac- sible, safe, or sufficiently efficacious to pro- 562–565 (1998). 1998 Nature America Inc. cines would be expected to circumvent tect against viruses such as HIV or hepati- © some of the quality control issues associated tis C. The ‘new’ nucleic acid vaccines show Vaccines and Gene Therapy Research with production of attenuated virus vac- promise, but in their current form may be Chiron Technologies cines, it remains to be seen whether the of insufficient potency to induce the req- Emeryville, California 94608, USA same is true for in vitro transcribed RNA. It has been demonstrated that the infectious cycle of positive-stranded RNA The great pretender viruses can be launched directly from Identifying a new tumor suppressor gene is simply the end of the beginning as far as genome length cDNA copies precisely understanding its function goes. Take the PTEN gene, for example, which is mutated in positioned within RNA polymerase II glioblastoma, breast and prostate cancer, and in Cowden disease. Shortly after its dis- expression cassettes, when these are trans- covery last year, Tonks and colleagues (Cold Spring Harbor Laboratory, New York) 7 fected into permissive cells . Indeed, reported that PTEN is a dual-specificity phosphatase—that is, it removes phosphate induction of antiviral immune responses groups from serine, threonine and tyrosine residues of target proteins. This all made sense in mice vaccinated with plasmid DNA because tyrosine phosphatases are thought to act as tumor suppressors by removing encoding infectious genome RNA has been phosphates from (and thus deactivating) growth-promoting kinases. observed for foot-and-mouth disease virus, Then, earlier this year, another group showed that PTEN removes a specific phosphate 8 a positive-stranded RNA picornavirus . from a lipid, phosphatidylinositol-3,4,5-trisphosphate (PIP3), which (when all three phos- A rapidly emerging area of vaccine phates are in place) activates downstream kinases. Now, reporting in PNAS (95, research is the development of viral vector- 13513–13518; 1998), Tonks and co-workers describe a missense mutation in PTEN that based delivery platforms derived from pos- wipes out the lipid phosphatase activity yet leaves the protein phosphatase function itive-stranded RNA viruses for the expres- intact. In other words, PTEN acts in a completely unexpected way to switch off cell sion of heterologous antigens. Vectors growth signals by dephosphorylating phospholipids. derived from positive-stranded RNA Alison Mitchell, Nature viruses, such as picornaviruses and

1358 NATURE MEDICINE • VOLUME 4 • NUMBER 12 • DECEMBER 1998