Virus latency

Virus latency (or viral latency) is the ability of a pathogenic virus to lie dormant within a cell, denoted as the lysogenic part of the viral life cycle. A latent viral infection is a type of persistent viral infection which is distinguished from a chronic viral infection. A latent infection is a phase in certain viruses' life cycles in which after initial infection, virus production ceases. However, the virus genome is not fully eradicated. The result of this is that the virus can reactivate and begin producing large amounts of viral progeny without the host being infected by new outside virus, denoted as the lytic part of the viral life cycle stays within the host indefinitely [N.J. Dimmock et al. "Introduction to Modern Virology, 6th edition." Blackwell Publishing, 2007.] .

Virus latency is not to be confused with clinical latency during the incubation period when a virus is not dormant.

Mechanisms

Episomal latency

Episomal latency refers to the use of genetic episomes during latency. In this type, viral genes are floating in the cytoplasm or nucleus as distinct objects, both as linear or lariat structures. Episomal latency is more vulnerable to marauding ribozymes or host foreign gene degradation than provirus latency.

One example is Herpes Virus family, "Herpesviridae", all of which establish latent infection. Herpes virus include Chicken-pox virus and Herpes simplex viruses (HSV-1, HSV-2), all of which establish episomal latency in neurons and leave linear genetic material floating in the cytoplasm.cite journal
title=Epigenotypes of latent herpesvirus genomes
journal=Current topics in microbiology and immunology
date=2006
author=Minarovits J
volume=310
pages=61–80
pmid=16909907] The "Gammaherpesvirinae" subfamily is associated with episomal latency established in cells of the immune system, such as B-cells in the case of Epstein-Barr Virus.cite journal
title=Influence of EBV on the peripheral blood memory B cell compartment
journal=Journal of immunology (Baltimore, Md. : 1950)
date=2007 Sep 1
author = Souza TA, Stollar BD, Sullivan JL, Luzuriaga K, Thorley-Lawson DA
volume=179
issue=5
pages=3153–60
pmid=17709530]

Advantages of episomal latency include the fact that the virus may not need to enter the nucleus, and hence may avoid ND10 domains from activating interferon via that pathway.

Disadvantages include more exposure to cellular defenses, leading to possible degradation of viral gene via cellular enzymes. [Burton EA, Fink DJ, Glorioso JC. "Gene delivery using herpes simplex virus vectors." DNA Cell Biol. 2002 Dec;21(12):915-36. Review.] .

Proviral latency

Proviral latency begin with the virus genome integrates into the host genome, effectively become a provirus. This requires that the viral gene get into the nucleus and insert itself into the host genome, the family of which exemplifies this behavior being the Retroviruses. For example, the Retrovirus, HIV, enters the nucleus and inserts its gene between Long terminal repeats using integrase and remains within the hosts own gene [Bagasra O. "A unified concept of HIV latency." Expert Opin Biol Ther. 2006 Nov;6(11):1135-49. Review.] .

Advantages include automatic host cell division results in replication of the viruses gene, and the fact that it is near impossible to remove an integrated provirus from an infected cell without killing the cell [Marcello A. "Latency: the hidden HIV-1 challenge." Retrovirology. 2006 Jan 16;3(1):7] .

Disadvantages include the need to enter the nucleus (and the need for packaging proteins that will allow for that) and increased difficulty in maintaining the latency.

Maintaining latency

Both proviral and episomal latency may require maintenance for continued infection and fidelity of viral genes. Latency is generally maintained by viral genes expressed primarily during latency. Expression of these "latency-associated" genes may function to keep the viral genome from being digested by cellular ribozymes or being found out by the immune system. Certain viral gene products (RNA transcripts and proteins) may also inhibit apoptosis or induce cell growth and division to allow more copies of the infected cell to be produced [Divito S, Cherpes TL, Hendricks RL. "A triple entente: virus, neurons, and CD8+ T cells maintain HSV-1 latency." Immunol Res. 2006;36(1-3):119-26. Review.] .

An example of such a gene product is the "Latency Associated Transcripts (LAT)" in Herpes simplex virus, which interfere with apoptosis by downregulating a number of host factors, including MHC and inhibiting the apoptotic pathway [Carpenter D, Hsiang C, Brown DJ, Jin L, Osorio N, Benmohamed L, Jones C, Wechsler SL. "Stable cell lines expressing high levels of the herpes simplex virus type 1 LAT are refractory to caspase 3 activation and DNA laddering following cold shock induced apoptosis." Virology. 2007 Dec 5;369(1):12-8. Epub 2007 Aug 28.] .

A certain type of latency could be ascribed to the endogenous retroviruses. These viruses have incorporated into the human genome in the distant past, and are now passed through reproduction. Generally these types of viruses have become highly evolved, and have lost the expression of many gene products. [Buzdin A. "Human-specific endogenous retroviruses." ScientificWorldJournal. 2007 Nov 26;7:1848-68.] Some of the proteins expressed by these viruses have co-evolved with host cells to play important roles in normal processes. [Hayashida K, Omagari K, Masuda JI, Kohno S. "An integrase of endogenous retrovirus is involved in maternal mitochondrial DNA inheritance of the human mammal." Biochem Biophys Res Commun. 2007 Dec 3] .

Ramifications

While viral latency exhibits no active viral shedding nor causes any pathologies or symptoms, the virus is still able to reactivate via external activators (i.e. sunlight, stress) to cause an acute infection. In the case of Herpes simplex virus, which generally infects an individual for life, a serotype of the virus reactivates occasionally to cause cold sores. The sores are quickly resolved by the immune system, however may be a minor annoyance from time to time. In the case of varicella zoster virus, after an initial acute infection (chickenpox) the virus lies dormant until reactivated as herpes zoster.

More serious ramifications of a latent infection could be the possibility of transforming the cell, and forcing the cell into uncontrolled cell division. This is a result of the random insertion of the viral genome into the hosts own gene and suppression of host cellular growth factors for the benefit of the virus. A famous event of this actually happening with gene therapy through the use of retroviral vectors is the Necker Hospital in Paris, where 8 young boys received treatment for a genetic disorder, after which 4 developed leukemia [http://www.esgct.org/upload/4th_CaseofLeukemial.pdf] .

This is also seen with infections of the human papilloma virus in which persistent infection may lead to cervical cancer as a result of cellular transformation [Wang XG, Revskaya E, Bryan RA, Strickler HD, Burk RD, Casadevall A, Dadachova E. "Treating cancer as an infectious disease-viral antigens as novel targets for treatment and potential prevention of tumors of viral etiology." PLoS ONE. 2007 Oct 31;2(10):e1114.] [Molho-Pessach V, Lotem M. "Viral carcinogenesis in skin cancer." Curr Probl Dermatol. 2007;35:39-51. Review.] [Carrillo-Infante C, Abbadessa G, Bagella L, Giordano A. "Viral infections as a cause of cancer (review)." Int J Oncol. 2007 Jun;30(6):1521-8. Review.] .

References