Lentivirus as Vaccine-vectors

The best characterized integrated viral vector originates from the retroviridae family, whose members are ideal for the genetic manipulation of mammalian cells due to their intrinsic ability to integrate into genomic DNA. At present, the use of simple retroviruses based on murine Moloney leukemia virus (MLV) is becoming overshadowed by more sophisticated lentiviral vector systems that have superior efficiency in modifying cells in vitro and in vivo compared with MLV-based vectors. Therefore, as a very effective viral vector, lentivirus plays an unparalleled role in gene therapy.

Introduction of Lentivirus

Lentiviruses are the members of the Retroviridae family and are characterized in that they use viral reverse transcriptase (RT) and integrase (IN) to stably insert viral genomic information into the host genome. Unlike other retroviruses, lentiviruses can replicate in non-dividing cells and cause slow-progressive diseases in their specific hosts. Lentiviruses from different species have different properties and pathogenicity in terms of their genomic structure and receptor binding. Since most lentiviral vectors are based on HIV-1, we mainly use HIV as an example to introduce the basic information of lentivirus including its genomic structure and viral proteins.

The three largest open reading frames encode three major structural proteins: Gag, Pol, and Env. The gag gene encodes a viral core protein. The pol gene encodes a set of enzymes required for viral replication. The env gene encodes the viral surface glycoprotein gp160. In addition to these major proteins, the viral genome also encodes regulatory proteins Tat and Rev, which activate viral transcription and control the splicing and nuclear exports of viral transcripts respectively. The other four genes encode the accessory proteins Vif, Vpr, Vpu, and Nef. The viral genome is flanked by the LTR (long terminal repeat) required for viral transcription, reverse transcription, and integration.

Fig.1 Schematic representation of the HIV-1 viral genome and HIV-1 virion structure. Above: The viral genome encodes three structural (i.e. gag, pol and env), regulatory (i.e. rev and tat) and accessory (i.e. vif, vpr, vpu, and nef) genes flanked by LTRs. The Gag precursor includes MA, capsid protein (CA), nucleocapsid protein (NC) and p6. The Gag-Pol precursor protein encodes three essential replication enzymes: RT, IN, and PR. The glycoprotein gp160 is cleaved by PR to produce viral SU and TM domains. Bottom: Two single-stranded viral RNAs, RT, IN, PR and CA as well as accessory proteins are surrounded by CA. Inner viral membrane and outer viral membranes are coated with MA and Env respectively. (Sakuma. 2012)

Production of Lentiviral Vectors

In general, lentivector particles are generated by the co-transfection of three or more plasmids in the human embryonic kidney (HEK) 293T cells. The vector system is a split genomic form in which the packaging and structural genes have been separated from portions of the virus that facilitate integration into the host genome. The env gene can be replaced by heterologous glycoproteins from a variety of viruses, including the most commonly used VSV-G. In the latest-generation vectors, the rev and tat genes are not included in the packaging plasmid but provided in a fourth plasmid. Three plasmids (or more) are transfected into cell lines (293T cells) to generate viral vectors, which are collected in the supernatant by infection or by direct injection for in vitro transgenic applications.

Fig.2 The production of lentiviral vector. Typically, three plasmids (or more) are transfected into cell lines (293T cells) to generate viral vectors, which are collected in the supernatant by infection (A) or by direct injection (B) for in vitro transgenic applications. Prom, virus or cell promoter; ZP, zona pellucida. (Park. 2007)

Advantages of Lentiviral Vectors

Lentiviruses have several advantages over other gene transfer vectors, including the following:

• Can be irreversibly integrated into the host genome to achieve sustained transgene expression;
• They are engineered to package relatively large payloads;
• Low immunogenicity due to the absence of all viral coding genes;
• They are extensively engineered to increase biosafety;
• Can be pseudotyped with numerous heterologous envelope glycoproteins.

Application of Lentiviral Vectors

• Genome-wide function studies of gene expression
• The generation of transgenic animals
• Cell engineering (genetic reprogramming to generate induced pluripotent stem cells)
• Gene silencing by combination with small interfering RNA- and microRNA-based technologies
• Recombinant protein production
• Clinical gene therapy

Lentivectors have arisen with the promise to become the substitutes of oncoretroviruses due to their improved performance and, possibly, enhanced biosafety. Creative Biolabs improvs vector performance and safety by modification of the packaging plasmid, transfer plasmid, promoters, and alteration of the viral envelope to meet the needs of any one of the customers. If you are interested in any of the lentiviral vectors, please feel free to contact us.

References

1. Sakuma T., et.al. (2012). Lentiviral vectors: basic to translational. Biochemical Journal, 443(3): 603-618.
2. Park F. (2007). Lentiviral vectors: are they the future of animal transgenesis? Physiological Genomics, 31(2): 159-173.

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