Replication-competent hybrids between murine leukemia virus and foamy virus

Molecular biology of foamy viruses

One of the most fascinating areas in retrovirology is the study of foamy viruses (FVs), because these viruses appear to do everything that is common to all other retroviruses differently. FVs have found a completely new way to propagate their genome. And they do this extremely successfully because most of wild non-human primates, felines, bovines, equines, and small ruminants are likely to be non-pathogenically infected. The success of FVs can also be viewed from a different angle, since they replicate very conservatively and do not need to shape their genotypic and phenotypic makeup every now and then. The elucidation of the underlying basic mechanisms of the FV replication strategy is the topic of this review.

Adaptive evolution of a tagged chimeric gammaretrovirus: identification of novel cis-acting elements that modulate splicing

Retroviruses are well known for their ability to incorporate envelope (Env) proteins from other retroviral strains and genera, and even from other virus families. This characteristic has been widely exploited for the generation of replication-defective retroviral vectors, including those derived from murine leukemia virus (MLV), bearing heterologous Env proteins. We investigated the possibility of "genetically pseudotyping" replication-competent MLV by replacing the native env gene in a full-length viral genome with that of another gammaretrovirus. Earlier, we developed replication-competent versions of MLV that stably transmit and express transgenes inserted into the 3' untranslated region of the viral genome. In one such tagged MLV expressing green fluorescent protein, we replaced the native env sequence with that of gibbon ape leukemia virus (GALV). Although the GALV Env protein is commonly used to make high-titer pseudotypes of MLV vectors, we found that the env replacement greatly attenuated viral replication. However, extended cultivation of cells exposed to the chimeric virus resulted in selection of mutants exhibiting rapid replication kinetics and different variants arose in different infections. Two of these variants had acquired mutations at or adjacent to the splice acceptor site, and three others had acquired dual mutations within the long terminal repeat. Analysis of the levels of unspliced and spliced viral RNA produced by the parental and adapted viruses showed that the mutations gained by each of these variants functioned to reverse an imbalance in splicing caused by the env gene substitution. Our results reveal the presence of previously unknown cis-acting sequences in MLV that modulate splicing of the viral transcript and demonstrate that tagging of the retroviral genome with an easily assayed transgene can be combined with in vitro evolution as an approach to efficiently generating and screening for replicating mutants of replication-impaired recombinant viruses.

Genetic engineering of onco/lentivirus hybrids results in formation of infectious but not of replication-competent viruses

To achieve specific gene transfer into human CD4(+) cells, murine leukaemia virus (MLV)-based pseudotype vector particles were generated employing Env variants derived from human or simian immunodeficiency virus (HIV-1 or SIVagm). Here, we describe the generation of full-length onco/lentivirus hybrid genomes comprising components of MLV and HIV-1 or SIVagm, respectively, to assess the possibility of replication-competent hybrid virus formation. The env reading frame of an infectious molecular clone of MLV was replaced with the analogous coding regions of HIV-1 or SIVagm encompassing the env gene and accessory genes. Resulting MLV/HIV-1 or MLV/SIVagm hybrid genomes were transfected into 293T cells. Expression of viral proteins and budding of retroviral particles was shown by specific immunostaining and electron microscopy. The viral particles mediated CD4- and co-receptor-specific infection of human cells as demonstrated by PCR and immunostaining in the respective target cells. However, no productive infection resulting in the generation of infectious virus was detected in these cells. Thus, these onco/lentivirus hybrids, although able to initiate single-round infection, were not replication competent. Thus, MLV-based pseudotype vectors carrying Env variants of HIV-1 or SIVagm are not prone to form replication-competent retroviruses, suggesting a favourable safety profile for MLV-based CD4-specific pseudotype vectors.