There are a lot of viruses out there – most of which we would all like to steer clear of! However, a subset of viruses are commonly used in the lab as tools and we all might benefit from getting to know them a little better. Here we will discuss the four most commonly used lab viruses: gamma-retrovirus, lentivirus, adenovirus, and adeno-associated virus.
Gamma-retrovirus
Retroviruses are single-stranded RNA viruses that use reverse transcriptase to synthesize complementary DNA copies of their genome upon infecting a host cell. This viral DNA (also called proviral DNA) is then inserted into the host's genome and is used to make more virus. The most studied (and used in the lab) type of gamma-retrovirus is Murine Leukemia virus (MLV). Naturally isolated gamma-retroviruses have been associated with cancer in mammals, which is thought to be due to their propensity to integrate into proto-oncogenes and disrupt their expression (Robinson, 1982). To add insult to injury, retroviruses generally have moderate to high immunogenicity, meaning they induce an immune response in the host.
So why use a gamma-retrovirus in the lab? Well, they are essentially evolutionarily optimized gene delivery devices. Their survival depends on their ability to deliver and permanently integrate a genetic cargo into a cell. These viruses have a cargo capacity of ~ 8 kb, ample room for most experimental needs, and can transduce (infect) a wide range of cell types with a high efficiency. However, gamma-retrovirus infection is restricted to dividing cells only, so is not suitable for infection of quiescent cells or infrequently cycling cells but is fine for experiments with immortalized cell line cultures.
Lentivirus
Lentivirus is a type of retrovirus and shares many common features with gamma-retrovirus including genome architecture, host integration, and its association with disease. The most well-studied lentivirus is HIV, which many lentiviral tools have been derived from. Lentiviruses have slightly larger packaging capacities than gamma-retrovirus, coming in around 9 kb. Unlike gamma, lentivirus can infect both dividing and non-dividing cells. Lenti particles enter the nucleus through nuclear pores and thus do not require breakdown of the nuclear envelope during cell division to enter the nucleus as gamma-retroviruses do.
Fun fact: Since lentivirus and gamma-retrovirus are so closely related, their required components are very similar. While plasmids containing virus-specific elements such as the LTRs or structural proteins are not interchangeable between the two systems, other, more general, viral components such as a heterologous envelope or a post-transcriptional regulatory element can be used for either type of retrovirus.
Adeno-associated virus
Adeno-associated virus (AAV) is a small, single-stranded DNA parvovirus. AAV can infect humans and some other mammals but is not currently known to cause any diseases. Recombinant AAV largely does not integrate, and the addition of a helper virus or plasmid allows it to replicate episomally, essentially remaining an extrachromosomal part of the host genome that can replicate and divide along with the endogenous DNA. AAV also has low immunogenicity, meaning it does not induce the same high immune response as retroviruses. This virus can also transduce dividing and non-dividing cells, making it conducive to targeting all cell types. Altogether, AAV has many excellent features for human gene therapy and is currently being used in clinical settings.
AAV is also a cool tool in the laboratory! It is ideal for whole animal administration, such as in mouse experiments. AAV also has multiple serotypes (12 naturally occurring and more synthetic capsids are routinely being developed) to choose from, which can target the virus to specific cell types of interest. This is a particularly nice feature as organ-specific injections, or the location of a specific cell type doesn’t always lend itself to direct application of virus. However, the cargo capacity of AAV is ~4.7 kb, almost half that of lenti and gamma-retro viruses. This is large enough for many applications, but in some cases multiple AAVs (co-infection), an alternative virus, or other strategies may need to be considered if size constraints are an issue.
Adenovirus
Adenovirus has a linear, double stranded DNA genome which is non-enveloped. It can infect humans, many mammals, and even the avian population. Some adenoviruses are known to cause respiratory disease across species, as well as cancer in hamsters, although no cancer-connection has been identified in humans to date. Over fifty serotypes of human-infecting adenovirus have been identified so far! Adenovirus generally has high immunogenicity, but some strains have been engineered to induce a lower inflammatory response within hosts.
Relative to the other viruses discussed in this article, adenovirus has the highest maximum titer, a plus for production and infection. This virus can infect dividing and non-dividing cells and doesn’t require genomic integration to express its cargo, in fact its viral life cycle does not include genomic integration. While the expression of adenovirus cargo is high, it’s generally not as long lasting as AAV (up to several weeks vs. AAV’s several years). But these viruses are roomy: some adenoviruses pack up to ~35 kb of space for cargo! Adenovirus is an effective option if you need high, transient expression of a genetic cargo or have a very large cargo.
Fast fact: All viruses discussed in this blog are generally broadly infective, with one exception. Blood cells are not transduced efficiently with adenovirus and AAV.
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| Table 1: Types of viruses and their features |
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Choosing a virus
Now that we’ve reviewed the common viruses used in biological research, you might have an idea which ones could be helpful to you. Interested in learning more about a specific virus? We have you covered with guides on lentivirus, retrovirus, adenovirus, and AAV. If you’re wondering more about what applications these viruses are good for, we have plenty of information on that too. May your titer always be high and your immunogenicity low!
References and Resources
References
Robinson HL. Retroviruses and cancer. Rev Infect Dis. 1982 Sep-Oct;4(5):1015-25. DOI: 10.1093/clinids/4.5.1015
Lee C.S., et al. Adenovirus-Mediated Gene Delivery: Potential Applications for Gene and Cell-Based Therapies in the New Era of Personalized Medicine. Genes Dis. 2017 Jun;4(2):43-63. DOI: 10.1016/j.gendis.2017.04.001
Maetzig T, Galla M, Baum C, Schambach A. Gammaretroviral vectors: biology, technology and application. Viruses. 2011 Jun;3(6):677-713. DOI: 10.3390/v3060677
Varanda C, Félix MDR, Campos MD, Materatski P. An Overview of the Application of Viruses to Biotechnology. Viruses. 2021 Oct 14;13(10):2073. DOI: 10.3390/v13102073
Resources on Addgene.org
- Adeno-associated Virus (AAV) Guide
- Addgene’s viral plasmids and resources
- Addgene’s viral services: AAV
Resources on the Addgene blog
Topics: Viral Vectors
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