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Entry of virus particle is accomplished by the fusion of viral surface glycoprotein with the host cell receptor. The viral RNA genome is converted into DNA by the enzyme reverse transcriptase. The DNA after combining with the viral proteins then migrates to nucleus and integrates with the host cell chromosome with the help of enzyme integrase. The transcription of the viral mRNA starts from LTR using host cell transcription factors. The new virus particles are formed after assembling of two copies of ssRNA along with the essential enzyme inside core. The mature virus particles are released by budding from the host cell membrane.
Retroviral vectors are based on replication deficient retroviruses. The vectors are mainly derived from Rous sarcoma virus, avian leukosis virus, and murine leukemia virus. The retroviral vectors are made by replacing the viral proteins with the gene of interest driven by a tissue specific promoter. The vector also contains the long terminal repeat (LTR) which is an essential packaging signal. In addition, the vector also contains essential enzyme such as reverse transcriptase and integrase. Vector RNA production is achieved by either the LTR or promoters present upstream to the transgene. The packaging of the vector is completed by the incorporation of viral structural proteins in trans from packaging cell lines. Alternatively, the vector can also be produced by the transfection of plasmid expressing the structural proteins. The latter method is less time consuming as it avoids the use of packaging cell line. Viruses are recovered from the supernatants of actively growing producer cells.
Major issue of using retroviral vectors is the possibility of the defective genome to be recombined with the host cell chromosome. The concern leads to the development of self inactivating retroviral vectors by using tissue specific promoter for transgene transcription. In self inactivating vectors, the transcription of the transgene is carried out by using an internal promoter instead of LTR. Apart for gene delivery, retroviral vectors are extensively used for many other applications. Retroviruses have ability to integrate with the host cell chromosome; the property is explored to express the protein of interest in a cell. Long term expression of a protein in a suitable cell is a very useful way to make a stable cell line. The replication depends on actively dividing cells making these an important tool to manipulate stem cells and tumor cells. On the other hand lentiviral vectors are used for gene delivery in many organs including brain, eye, liver, muscles, and hematopoietic cells.
The transduction of retroviral vector is limited because of its ability to have limited cellular tropism. The cellular tropism of the retrovirus is broadened by the incorporation of envelope from related or unrelated viruses, making them pseudotype virus. Incorporation of vesicular stomatitis virus glycoproteins into the retrovirus virion or the envelope of murine leukemia virus allows the broad host range to the transducing vectors. Pseudotyping of retrovirus by the use of lyssavirus glycoprotein makes it transducible to the brain while incorporation of surface protein of ebola virus helps in transducing airway epithelium.