Viral vectors: a look back and ahead on gene transfer technology.

No matter what their origin, strain and family, viruses have evolved exquisite strategies to reach and penetrate specific target cells where they hijack the cellular machinery to express viral genes and produce progeny particles. The ability to deliver and express genetic information to cells is the basis for exploiting viruses as "Trojan horses" to genetically modify the natural cell target or, upon manipulation of the viral receptor to retarget the virus, to genetically engineer different cell types. This process, known as transduction, is accomplished using viral vectors derived from parental wild type viruses whose viral genes, essential for replication and virulence, have been replaced with the heterologous gene(s) required for cell manipulation. Rearrangement of the viral genome to impede replication or generation of infectious virions but maintaining the ability to deliver nucleic acids has been the object of intense research since the early 1980s. Technological advances and the ever-growing knowledge of molecular virology and virus-host cell relationships have constantly improved the safety profile of viral vectors that are now used in vitro and in vivo to study cellular gene function, correct genetic defects (gene therapy), express therapeutic proteins, vaccinate against infectious agents and tumors, produce experimental animal models, and for other purposes. This review illustrates the strategies used to generate some of the most used viral vectors, and their advantages, limitations and principal applications.