Viral vectors work like a "nanosyringe" to deliver nucleic acid to a target. They are often more efficient than other transfection methods, are useful for whole organism studies, have a relatively low toxicity, and are a likely route for human gene transfer.
All viral vectors require a host for replication. The production of a viral vector is typically separated from the ability of the viral vector to infect cells. While viral vectors are not typically considered infectious agents, they do maintain their ability to "infect" cells. Viral vectors just don't replicate (although there are some replicating viral vectors in use) under experimental conditions. An HIV-based lentiviral vector no longer possesses the ability to infect an individual with HIV, but it does maintain the ability to enter a cell and express genetic information. This is why viral vectors are useful, but also require caution.
If a viral vector can transduce a human cell line on a plate, it can also transduce YOUR cells if accidentally exposed.
There are more than 49 immunologically distinct types of adenovirus that can cause infection. Recombinant adenoviruses used for biomedical research are typically based on Adenovirus 5. These are linear, non-enveloped, icosahedral, double-stranded DNA viruses of approximately 36kb with a lytic infection cycle. Virus packaged via transfection of HEK293 cells are capable of transfecting human cells. Deletion of E1 renders the virus replication incompetent. Deletion of E3 allows for larger inserts. Because recovery of E1 is the only recombination event required to create a replication competent virus, all adenoviral vectors are 1st generation.
Adenovirus is a pathogen of respiratory, gastrointestinal mucosa, and mucous membranes. Symptoms of respiratory illness resulting from adenovirus infection can range from asymptomatic disease, common cold, pneumonia, croup, and bronchitis. Additional clinical symptoms include conjunctivitis (pink eye), cystitis, gastroenteritis (stomach flu), tonsillitis, rash-associated illness, and rare cases of severe disease (especially in immune compromised individuals). Adenoviral vectors DO NOT have to be replication competent to cause corneal and conjunctival damage.
Wold WS, Toth K. Adenovirus vectors for gene therapy, vaccination and cancer gene therapy.Curr Gene Ther. 2013;13(6):421-433. doi:10.2174/1566523213666131125095046
https://www.canada.ca/en/public-health/services/laboratory-biosafety-biosecurity/pathogen-safety-data-sheets-risk-assessment/adenovirus-types1-2-3-4-5-7-pathogen-safety-data-sheet.html
Baculoviruses are lytic DNA viruses that are primarily pathogenic for insects. The nucleocapsids of Baculoviruses are rod-shaped and enveloped, with circular genomes of double-stranded DNA, ranging in size from 80-180kbp. Baculoviruses produce two distinct types of virions: occlusionderived virions (ODV), embedded in large protein crystals called occlusion bodies, and budded virions (BV). ODV are responsible for horizontal transmission between insects, whereas BV help spread infection from cell to cell. There have been more than 500 baculovirus isolates identified based on hosts of origin. Apart from their utility as gene expression vectors, they are also useful as biological pesticides. The two most common isolates used for gene expression are Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) and Bombyx mori (silkworm) nuclear polyhedrosis virus (BmNPV).
Non-genetically modified, wild-type baculoviruses are typically not capable of replicating in vertebrate cells, and therefore do not pose much risk to laboratory personnel. Baculoviruses for gene expression which utilize polyhedrin or p10 promoters will only transfect insect cells. Baculoviruses that have been engineered with mammalian specific promoters do achieve expression of foreign genes in mammalian cell lines and primary cell cultures.
Ono C, Okamoto T, Abe T, Matsuura Y. Baculovirus as a Tool for Gene Delivery and Gene Therapy.Viruses. 2018;10(9):510. Published 2018 Sep 19. doi:10.3390/v10090510
Epstein-Barr virus (EBV) is a ubiquitous B-lymphotrophic herpesvirus. EBV causes the common childhood disease mononucleosis. It is an icosahedral, lipid enveloped, double-stranded DNA virus sized 120-150 nm in diameter. EBV has been found in the tumor cells of a heterogeneous group of malagnancies (i.e. Burkitt?s lymphoma, lymphomas associated with immunosuppression, other non-Hodgkin?s lymphomas, Hodgkin?s Disease, nasopharyngeal carcinoma, gastric adenocarcinoma, lymphoepithelioma-like carcinomas, and immunodeficiency-related leiomyosarcoma). 80-90% of adults worldwide are infected with EBV. Most wild-type EBV infections are asymptomatic and acquired during childhood, with symptoms indistinguishable from other childhood acute viral syndromes.
https://www.canada.ca/en/public-health/services/laboratory-biosafety-biosecurity/pathogen-safety-data-sheets-risk-assessment/epstein-barr-virus.html
Kazuyuki Kiyosue, Yoshihiro Miwa, Epstein-Barr virus-derived vector suitable for long-term expression in neurons, Heliyon, Volume 6, Issue 3, 2020, e03504, ISSN 2405-8440, https://doi.org/10.1016/j.heliyon.2020.e03504.
Herpes Simple Virus (Types I and II) are icosahedral, lipid enveloped, double-stranded linear DNA viruses approximately 110-200nm in diameter. HSV types I and II can be differentiated immunologically. HSV-I is herpes gingivostomatitis; whereas HSV-II is herpes genitalis, or genital herpes. HSV-derived vectors are unique in that the vectors have a wide host range and cell tropism in dividing and non-dividing cells, and are able to infect almost every cell type in most vertebrates. HSV has a dual life cycle ? a lytic growth cycle in epithelial cells and latent infection of neuronal cells. This latency in neuronal cells leads to persistent, long-term expression.
https://www.canada.ca/en/public-health/services/laboratory-biosafety-biosecurity/pathogen-safety-data-sheets-risk-assessment/herpes-simplex-virus.html
Burton EA, Fink DJ, Glorioso JC. Gene delivery using herpes simplex virus vectors.DNA Cell Biol. 2002;21(12):915-936. doi:10.1089/104454902762053864
The Poxviridae family is divided into two subfamilies: Chordopoxviridae, with a vertebrate host range, and Entomopoxviridae, with an insect host range. Chordopoxviridae is further broken down into eight genera: Avipoxvirus, Capripoxvirus, Leporipoxvirus, Molluscipoxvirus, Orthopoxvirus, Parapoxvirus, Suipoxvirus, and Yatapoxvirus.
Poxviruses are enveloped, with a double-stranded DNA genome with hairpin loops at each end and a lytic infection cycle. Poxviruses do not integrate into the hosts? genome because they remain in the cytoplasm and utilize virally encoded polymerases to carry out replication and transcription. Members of the Orthopoxvirus genus have both narrow and broad host range. Variola, the agent of smallpox, only infects humans. The absence of other host species has made the eradication of smallpox possible. On the other hand, Vaccinia virus has a very broad host range. Vaccinia is used as a live vaccine for protection against smallpox. Vaccinia?s large genome (approximately 190kb) allows for the stable insertion of DNA as large as 25kb.
Unlike many viral vectors utilized, vaccinia is a replication competent vector. Vaccinia virus presents varying levels of health risk to laboratory personnel, depending on the strain utilized. Highly attenuated strains are typically unable to replicate or replicate poorly in human cells. Non-highly attenuated strains can replicate in human cells and pose a health risk. The classical symptom of poxvirus infection is a vesicular or pustular lesion on the skin at the inoculation site. Vaccinia can cause severe disease in people with active skin disorders (i.e. eczema, psoriasis), pregnant women, and immune compromised individuals.
https://www.canada.ca/en/public-health/services/laboratory-biosafety-biosecurity/pathogen-safety-data-sheets-risk-assessment/vaccinia-virus.html
GuoZS,LuB,GuoZ, et alVaccinia virus-mediated cancer immunotherapy: cancer vaccines and oncolyticsJournal for ImmunoTherapy of Cancer2019;7:6.doi:10.1186/s40425-018-0495-7
Murine Leukemia Virus (MLV) is an enveloped, icosahedral, single-stranded virus with a linear RNA genome, approximately 100nm in diameter. MLV integrates into the host genome and is present in infected cells as a DNA provirus. Cell division is required for infection.
The host range of MLV is dependent on the specificity of the viral envelope. The ecotropic env gene produces particles that infect only rodent cells. Amphotropic env gene allows infection of both murine and non-murine cells, including human. VSV-G envelope allows infection in a wide range of mammalian and non-mammalian cells.
Recent data suggests a pathogenic mechanism by which chronic productive retroviral infection allows insertional mutagenesis leading to cell transformation and tumor formation. The nature of the transgene or additional introduced genetic element(s) may pose additional risk. The provirus integrates randomly into the genome which can lead to inactivation of genes for protein expression. The 5? and 3? LTRs have promoter functions that can deregulate the expression of genes.
Armin Blesch, Lentiviral and MLV based retroviral vectors for ex vivo and in vivo gene transfer, Methods, Volume 33, Issue 2, 2004, Pages 164-172, ISSN 1046-2023, https://doi.org/10.1016/j.ymeth.2003.11.005
The genus of the family Retroviridae consists of non-oncogenic retroviruses that produce multi-organ diseases characterized by long incubation periods and persistent infection. There are five (5) serotypes recognized, based upon the mammalian hosts with which they are associated: Bovine, Equine, Feline, Ovine/Caprine, and Primate.
Most lentiviral vectors in use today are HIV-derived vectors. The cis- and trans- acting factors of the lentiviruses are often on separate plasmid vectors, with packaging being provided in trans. The vector constructs contain the viral cis elements, packaging sequences, the Rev response element (RRE), and a transgene. Lentiviral vectors can transfect dividing and non-dividing cells. Replacing the HIV envelope glycoprotein with VSV-G allows a broad host-range for the vectors, allows the viral particles to be concentrated via centrifugation, and alters the mode of transmission.
Lentiviruses are transmitted via direct exposure to infected bodily fluids, sexual contact, and sharing unclean needles. Lentiviruses persist lifelong ? being both a function of their ability to integrate into the host chromosome and ability to evade host immunity. Lentiviruses replicate, mutate, and undergo selection by host immune responses. The clinical manifestations of infection include non-specific symptoms such as lymphadenopathy, anorexia, chronic diarrhea, weight loss, fever, and fatigue. The use of lentiviruses also present the risk of insertional mutagenesis, potentially leading to cancer. The nature of the transgene may pose additional risk.
Milone, M.C., O?Doherty, U. Clinical use of lentiviral vectors.Leukemia 32,1529?1541 (2018). https://doi.org/10.1038/s41375-018-0106-0
Most viral vectors used today are disabled such that replication competent viruses are not readily formed by any biological process that might occur in normal hosts. The Department of Biological Safety encourages the use of such vectors in all relevant applications. In particularly sensitive applications, demonstrating that the viral stock used has no apparent contamination with replication competent vectors is essential. The issue is not whether replication competent virus (RCV) is present, but how much replication competent virus is present. Of course, assays for replication competence will never be perfect or absolute, so the Institutional Biosafety Committee (IBC) asks that one use a current procedure of demonstrated sensitivity and specificity. Even more rigorous testing may be required in some instances, such as a vector bearing a pathogenic gene, in human gene transfer, or in any materials that could be released to the environment.
Wilson CA, Ng TH, Miller AE. Evaluation of recommendations for replication-competent retrovirus testing associated with use of retroviral vectors.Hum Gene Ther. 1997;8(7):869-874. doi:10.1089/hum.1997.8.7-869
Uchida E, Sato K, Iwata A, et al. An improved method for detection of replication-competent retrovirus in retrovirus vector products.Biologicals. 2004;32(3):139-146. doi:10.1016/j.biologicals.2004.08.002
U.S. Food and Drug Administration. Center for Biologics Evaluation and Research. Testing of Retroviral Vector-Based Human Gene Therapy Products for Replication Competent Retrovirus During Product Manufacture and Patient Follow-up. January 2020. Accessed February 28, 2022. https://www.fda.gov/media/113790/download