The problem of host cell nonpermissiveness to retrovirus infection is characterized and illustrated on several retroviral models, including the role of viral receptors, cell fusion, and endogenous retroviral genomes as modifiers of the outcome of retroviral infection. Special attention is paid to different barriers against the infection of mammalian cells with avian leukosis/sarcoma viruses (ALV/ASV). Even when avian retroviruses become integrated in mammalian cells, several blocks at the level of provirus expression, processing of viral RNAs, and posttranslational modification prevent virus production in such virogenic cells. The significance of these blocks and new strategies making it possible to overcome some of them are discussed in relation to the development of ALV/ASV-based vectors suitable for gene therapy in mammals.

Institute of Molecular Genetics Academy of Sciences of the Czech Republic Department of Cellular Viral Genetics Flemingovo n 2 Prague 6 166 37 Czech Republic

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Molecular events accompanying rous sarcoma virus rescue from rodent cells and the role of viral gene complementation

CpG island protects Rous sarcoma virus-derived vectors integrated into nonpermissive cells from DNA methylation and transcriptional suppression