1-3.10 Viral Vectors:

In recombinant molecular biology, virus particles has been modified to use as a carrier of nucleic acid into a cell, termed as viral vectors. Viral vectors are highly efficient in transferring target DNA/RNA segment to the host cells with high specificity. Wild type virus are modified by deleting the non-essential genes and incorporating exogenous nucleic acid segments to construct a viral vector. Viral vectors have wide application in gene therapy and targeted drug delivery systems. Main advantages of viral vectors are-high transfer efficiency and high cell specificity. Although, there are certain safety issues associated with viral vectors and careful handing is essential during the experimental procedure.

Commonly used viral vectors are- Adenovirus, retrovirus, lentivirus, adeno associated virus (AAV), herpes simplex virus (HSV) etc. The properties of different viral vectors are summarized in the table below.

Viral vector	Insert type	Insert size	Immunogenicity	Host genome integration
Adeno virus	DNA	2-8 kb	Very high	Non integrating
Retro virus	RNA	2-8 kb	Low	Integrating
Lentivirus	RNA	7-18 kb	Low	Integrating
Adeno associated virus (AAV)	DNA	4.5 kb	Low	Non integrating
Herpes simplex virus (HSV)	DNA	>30kb	Low	Non integrating

Table 1-3.10: Types of viral vectors

1-3.10.1 Simian Virus 40 (SV40):

Simian virus 40 (SV40) was the first mammalian expression vector whose genome size is 5.2kb. It is a DNA virus which can infect human as well other mammalian cell lines. SV40 may integrate into the host genome, permitting stable transmission of insert DNA to daughter cells. Recombinant SV40 vectors (rSV40) display some unique features:

•  SV40 is a well-known virus and nonreplicative vectors are easy-to-make

•  SV40 can be produced in high titers (10¹² IU/ml).

•  They can infect both resting and dividing cells.

•  Stable transgene expression can be achieved in a wide range of cell types.

The major disadvantage of SV40 vector is the low packaging capacity with insert size of <5kb.