Gene Transfection of Animal Target-organs

At present, the most mainstream virus vector tool has three kinds:Adeno virus, Lentiviral, Adeno-associated virus(AAV).Compared with the limitations of genetically engineered mice, virus carriers have the following advantages：
i)Viral vectors directly carry out acute gene interventions in adult animals, avoiding the problem of developmental decompensation；
ii)Compared with genetically engineered mice, the cycle and maintenance costs of the package are much smaller;
iii)Viral vector-mediated gene intervention can be done after animal modeling, as a gene therapy intervention, closer to the clinical, rather than just completing the study of gene function.

1)Lentivirus vectors: Lentiviral genes are widely used in cell gene transfection to establish a gene-stabilized cell line because of their own genomic integration and long-time expression.But in the animal living (in vivo) level, the lentiviral and adeno-associated viruses are significantly inferior in comparison to the virus—
a.Restriction of lentiviral Titer: Gene transfection in animals has a high demand for the total amount of active viral vectors injected and the volume of injection,Injection volume is often neglected but in fact the most limiting factor, such as the mouse ventricle injection, the volume of injections can not exceed 2ul.The same amount of viral injections, the higher the viral titer, the smaller the volume of injections, and the slow virus due to the titer limit (108~109tu/ml), the amount of virus active virus particles contained in the unit volume is much less than the adenovirus and AAV, so in the animal body (in vivo) gene Transfection application, Lentiviral Transfection is far less effective than adenovirus and AAV.

b. Potential for tumor-causing: lentiviral integration into the genome of host cells forms a stable and theoretically long-time expression. Although the slow virus is generally considered to be relatively safe in practical experiments, this random insertion will theoretically still have the potential risk of inoculating animal-induced tumors.The NIH security level for Lentiviral is rk2+ level, which is the highest risk level in three viruses (adenovirus is RK2 level, AAV is RK1 level).

2)Adenovirus vector: adenovirus vector is characterized by high titer (1011PFU/ML), large insertion fragment, broad spectrum of infection, strong instantaneous expression ability, short time for gene expression.Adenovirus is the first viral vector used in clinical trials of gene therapy, and it is widely used in animal gene transfection.Adenovirus vectors are prominent and distinct in animal gene transfection experiments:
a.Adenovirus can begin to express strongly in the 72 hours of injection intervention, thus having a significant advantage in the acute in vivo gene transfection adenovirus, which is particularly short in the intervention window;
b.The expression of adenovirus vectors in vivo is 2-4 weeks, four weeks after expression of gene expression attenuation is obvious, if the need to extend the expression time, the first injection 2 weeks after the two injections.
c.Adenovirus has a higher immunogenicity, through systemic administration is easier to activate the immune system to produce the corresponding antibodies, affecting the gene transfection effect in vivo, so the adenovirus vector is more of the target organs of the local injection (liver exception). The most common method of adenovirus vector administration is in situ multi-point injection, such as skeletal muscle, myocardium, tumor tissue, etc,generally choose 3-5 injection points, each point around 10ul injection volume.

3)Adeno-associated virus (AAV): Gene transfection artifact in animals
Adeno-associated virus, namely AAV, is the best and safest animal gene transfection tool, compared with other viral vectors such as slow virus, AAV has high titer, very low immunogenicity, long expression time, no pathogenic and pathological toxicity, viral particles of small diffusion range, multi-serum type and has the advantages of relative tissue affinity and so on. AAV has a variety of serotypes, each serum type of target organ affinity is different, which determines that the AAV and other viral vectors are more organ-selective carrier.