Herpesviridae

Herpesviridae is a large family of DNA viruses that cause diseases in animals, including humans.[1][2][3] The members of this family are also known as herpesviruses. The family name is derived from the Greek word herpein ("to creep"), referring to the latent, recurring infections typical of this group of viruses. Herpesviridae can cause latent or lytic infections.

At least five species of Herpesviridae – HSV-1 and HSV-2 (both of which can cause orolabial herpes and genital herpes), Varicella zoster virus (which causes chicken-pox and shingles), Epstein-Barr virus (which causes mononucleosis), and Cytomegalovirus – are extremely widespread among humans. More than 90% of adults have been infected with at least one of these, and a latent form of the virus remains in most people.[4][5][6] In total, there are 8 herpesvirus types that infect humans: herpes simplex viruses 1 and 2, varicella-zoster virus, EBV (Epstein-Barr virus), human cytomegalovirus, human herpesvirus 6, human herpesvirus 7, and Kaposi's sarcoma-associated herpesvirus.[7] There are more than 130 herpesviruses,[8] and some are from mammals, birds, fish, reptiles, amphibians, and molluscs.[7]

Viral structure

Herpesviruses all share a common structure—all herpesviruses are composed of relatively large double-stranded, linear DNA genomes encoding 100-200 genes encased within an icosahedral protein cage called the capsid which is itself wrapped in a protein layer called the tegument containing both viral proteins and viral mRNAs and a lipid bilayer membrane called the envelope. This whole particle is known as a virion.

Genus Structure Symmetry Capsid Genomic Arrangement Genomic Segmentation
IltovirusSpherical PleomorphicT=16EnvelopedLinearMonopartite
ProboscivirusSpherical PleomorphicT=16EnvelopedLinearMonopartite
CytomegalovirusSpherical PleomorphicT=16EnvelopedLinearMonopartite
MardivirusSpherical PleomorphicT=16EnvelopedLinearMonopartite
RhadinovirusSpherical PleomorphicT=16EnvelopedLinearMonopartite
MacavirusSpherical PleomorphicT=16EnvelopedLinearMonopartite
RoseolovirusSpherical PleomorphicT=16EnvelopedLinearMonopartite
SimplexvirusSpherical PleomorphicT=16EnvelopedLinearMonopartite
ScutavirusSpherical PleomorphicT=16EnvelopedLinearMonopartite
VaricellovirusSpherical PleomorphicT=16EnvelopedLinearMonopartite
PercavirusSpherical PleomorphicT=16EnvelopedLinearMonopartite
LymphocryptovirusSpherical PleomorphicT=16EnvelopedLinearMonopartite
MuromegalovirusSpherical PleomorphicT=16EnvelopedLinearMonopartite

Herpesvirus life cycle

All herpesviruses are nuclear-replicating—the viral DNA is transcribed to mRNA within the infected cell's nucleus.

Infection is initiated when a viral particle contacts a cell with specific types of receptor molecules on the cell surface. Following binding of viral envelope glycoproteins to cell membrane receptors, the virion is internalized and dismantled, allowing viral DNA to migrate to the cell nucleus. Within the nucleus, replication of viral DNA and transcription of viral genes occurs.

During symptomatic infection, infected cells transcribe lytic viral genes. In some host cells, a small number of viral genes termed latency associated transcript (LAT) accumulate instead. In this fashion the virus can persist in the cell (and thus the host) indefinitely. While primary infection is often accompanied by a self-limited period of clinical illness, long-term latency is symptom-free.

Reactivation of latent viruses has been implicated in a number of diseases (e.g. Shingles, Pityriasis Rosea). Following activation, transcription of viral genes transitions from latency-associated LAT to multiple lytic genes; these lead to enhanced replication and virus production. Often, lytic activation leads to cell death. Clinically, lytic activation is often accompanied by emergence of non-specific symptoms such as low grade fever, headache, sore throat, malaise, and rash as well as clinical signs such as swollen or tender lymph nodes and immunological findings such as reduced levels of natural killer cells.

Genus Host Details Tissue Tropism Entry Details Release Details Replication Site Assembly Site Transmission
IltovirusBirds: galliform: psittacineNoneCell receptor endocytosisBuddingNucleusNucleusOral-fecal; aerosol
ProboscivirusElephantsNoneGlycoproteinsBuddingNucleusNucleusContact
CytomegalovirusHumans; monkeysEpithelial mucosaGlycoproteinsBuddingNucleusNucleusUrine; saliva
MardivirusChickens; turkeys; quailNoneCell receptor endocytosisBuddingNucleusNucleusAerosol
RhadinovirusHumans; mammalsB-lymphocytesGlycoproteinsBuddingNucleusNucleusSex; saliva
MacavirusMammalsB-lymphocytesGlycoproteinsBuddingNucleusNucleusSex; saliva
RoseolovirusHumansT-cells; B-cells; NK-cell; monocytes; macrophages; epithelialGlycoproteinsBuddingNucleusNucleusRespiratory contact
SimplexvirusHumans; mammalsEpithelial mucosaCell receptor endocytosisBuddingNucleusNucleusSaliva
ScutavirusSea turtlesNoneCell receptor endocytosisBuddingNucleusNucleusAerosol
VaricellovirusMammalsEpithelial mucosaGlycoproteinsBuddingNucleusNucleusAerosol
PercavirusMammalsB-lymphocytesGlycoproteinsBuddingNucleusNucleusSex; saliva
LymphocryptovirusHumans; mammalsB-lymphocytesGlycoproteinsBuddingNucleusNucleusSaliva
MuromegalovirusRodentsSalivary glandsGlycoproteinsBuddingNucleusNucleusContact

Taxonomy

Group: dsDNA

[9]

The herpesvirus was first isolated from the blue wildebeest in 1960 by veterinary scientist Walter Plowright.[10] The genus Herpesvirus was established in 1971 in the first report of the International Committee on Taxonomy of Viruses (ICTV). This genus consisted of 23 viruses and 4 groups of viruses. In the second ICTV report in 1976 this genus was elevated to family level - the Herpetoviridae. Because of possible confusion with viruses derived from reptiles this name was changed in the third report in 1979 to Herpesviridae. In this report the family Herpesviridae was divided into 3 subfamilies (Alphaherpesvirinae, Betaherpesvirinae and Gammaherpesvirinae) and 5 unnamed genera: 21 viruses were listed. In 2009 the family Herpesviridae was elevated to the order Herpesvirales. This elevation was necessitated by the discovery that the herpes viruses of fish and molluscs were only distantly related to those of birds and mammals. Two new families were created - the family Alloherpesviridae which incorporates bony fish and frog viruses and the family Malacoherpesviridae which contains those of molluscs.

This order currently has 3 families, 3 subfamilies plus 1 unassigned, 17 genera, 90 species and plus 48 as yet unassigned viruses.[11]

Virus naming system

The system of naming herpes viruses was originated in 1973 and has been elaborated considerably since. The recommended naming system specified that each herpes virus should be named after the taxon (family or subfamily) to which its primary natural host belongs. The subfamily name is used for viruses from members of the family Bovidae or from primates (the virus name ending in –ine, e.g. bovine) and the host family name for other viruses (ending in –id, e.g. equid). Human herpes viruses have been treated as an exception (human rather than hominid). Following the host-derived term, the word herpes virus is added, followed by an Arabic numeral (1,2,3,...). These last two additions bear no implied meaning about taxonomic or biological properties of the virus.

Some exceptions to this system exist. A number of viruses' names (e.g. Epstein–Barr virus) are so widely used that it is impractical to attempt to insist on their replacement. This has led to a dual nomenclature in the literature for some herpes viruses. All herpes viruses described since this system was adopted have been named in accordance with it.

Evolution

The three mammalian subfamilies - Alpha, Beta and Gamma - arose approximately 180 million years ago to 220 million years ago.[12] The major sublineages within these subfamilies were probably generated before the mammalian radiation of 80 million years ago to 60 million years ago. Speciations within sublineages took place in the last 80 million years probably with a major component of cospeciation with host lineages.

All the currently known bird and reptile species are alphaherpesviruses. Although the branching order of the herpes viruses has not yet been resolved, because herpes viruses and their hosts tend to coevolve this is suggestive that the alphaviruses may have been the earliest branch.

The date of evolution of the iltovirus genus has been estimated to be 200 million years ago while those of the mardivirus and simplex genera have been estimated to be between 150 million years ago and 100 million years ago.[13]

Immune system evasions

Herpesviruses are known for their ability to establish lifelong infections. One way this is possible is through immune evasion. Herpesviruses have many different ways of evading the immune system. One such way is by encoding a protein mimicking human interleukin 10 (hIL-10) and another is by downregulation of the major histocompatibility complex II (MHC II) in infected cells.

cmvIL-10

Research conducted on cytomegalovirus (CMV) indicates that the viral human IL-10 homolog, cmvIL-10, is important in inhibiting pro-inflammatory cytokine synthesis. The cmvIL-10 protein has 27% identity with hIL-10 and only one conserved residue out of the nine amino acids that make up the functional site for cytokine synthesis inhibition on hIL-10. There is, however, much similarity in the functions of hIL-10 and cmvIL-10. Both have been shown to down regulate IFN-γ, IL-1α, GM-CSF, IL-6 and TNF-α, which are all pro-inflammatory cytokines. They have also been shown to play a role in downregulating MHC I and MHC II and up regulating HLA-G (non-classical MHC I). These two events allow for immune evasion by suppressing the cell-mediated immune response and natural killer cell response, respectively. The similarities between hIL-10 and cmvIL-10 may be explained by the fact that hIL-10 and cmvIL-10 both use the same cell surface receptor, the hIL-10 receptor. One difference in the function of hIL-10 and cmvIL-10 is that hIL-10 causes human peripheral blood mononuclear cells (PBMC) to both increase and decrease in proliferation whereas cmvIL-10 only causes a decrease in proliferation of PBMCs. This indicates that cmvIL-10 may lack the stimulatory effects that hIL-10 has on these cells.[14]

It was found that cmvIL-10 functions through phosphorylation of the Stat3 protein. It was originally thought that this phosphorylation was a result of the JAK-STAT pathway. However, despite evidence that JAK does indeed phosphorylate Stat3, its inhibition has no significant influence on cytokine synthesis inhibition. Another protein, PI3K, was also found to phosphorylate Stat3. PI3K inhibition, unlike JAK inhibition, did have a significant impact on cytokine synthesis. The difference between PI3K and JAK in Stat3 phosphorylation is that PI3K phosphorylates Stat3 on the S727 residue whereas JAK phosphorylates Stat3 on the Y705 residue. This difference in phosphorylation positions seems to be the key factor in Stat3 activation leading to inhibition of pro-inflammatory cytokine synthesis. In fact, when a PI3K inhibitor is added to cells, the cytokine synthesis levels are significantly restored. The fact that cytokine levels are not completely restored indicates there is another pathway activated by cmvIL-10 that is inhibiting cytokine system nthesis. The proposed mechanism is that cmvIL-10 activates PI3K which in turn activates PKB (Akt). PKB may then activate mTOR, which may target Stat3 for phosphorylation on the S727 residue.[15]

MHC downregulation

Another one of the many ways in which herpes viruses evade the immune system is by down regulation of MHC I and MHC II. This is observed in almost every human herpesvirus. Down regulation of MHC I and MHC II can come about by many different mechanisms, most causing the MHC to be absent from the cell surface. As discussed above, one way is by a viral chemokine homolog such as IL-10. Another mechanism to down regulate MHCs is to encode viral proteins that detain the newly formed MHC in the endoplasmic reticulum (ER). The MHC cannot reach the cell surface and therefore cannot activate the T cell response. The MHCs can also be targeted for destruction in the proteasome or lysosome. The ER protein TAP also plays a role in MHC down regulation. Viral proteins inhibit TAP preventing the MHC from picking up a viral antigen peptide. This prevents proper folding of the MHC and therefore the MHC does not reach the cell surface.[16]

It is important to note that HLA-G is often up regulated in addition to downregulation of MHC I and MHC II. This prevents the natural killer cell response.

Human herpesvirus types

There are eight distinct viruses in this family known to cause disease in humans.[17][18][19]

Human Herpesvirus (HHV) classification[1][18]
Name Synonym Subfamily Primary Target Cell Pathophysiology Site of Latency Means of Spread
HHV‑1 Herpes simplex virus-1 (HSV-1) α (Alpha) Mucoepithelial Oral and/or genital herpes (predominantly orofacial), as well as other herpes simplex infections Neuron Close contact (oral or sexually transmitted infection)
HHV-2 Herpes simplex virus-2 (HSV-2) α Mucoepithelial Oral and/or genital herpes (predominantly genital), as well as other herpes simplex infections Neuron Close contact (oral or sexually transmitted disease)
HHV-3 Varicella zoster virus (VZV) α Mucoepithelial Chickenpox and shingles Neuron Respiratory and close contact (including sexually transmitted disease)
HHV-4 Epstein-Barr virus (EBV), lymphocryptovirus γ (Gamma) B cells and epithelial cells Infectious mononucleosis, Burkitt's lymphoma, CNS lymphoma in AIDS patients,
post-transplant lymphoproliferative syndrome (PTLD), nasopharyngeal carcinoma, HIV-associated hairy leukoplakia 		B cell Close contact, transfusions, tissue transplant, and congenital
HHV-5 Cytomegalovirus (CMV) β (Beta) Monocyte, lymphocyte, and epithelial cells Infectious mononucleosis-like syndrome,[20] retinitis Monocyte, lymphocyte, and ? Saliva, urine, breast milk
HHV-6A and 6B Roseolovirus, Herpes lymphotropic virus β T cells and ? Sixth disease (roseola infantum or exanthem subitum) T cells and ? Respiratory and close contact?
HHV-7 β T cells and ? (roseola infantum or exanthem subitum) T cells and ? ?
HHV-8 Kaposi's sarcoma-associated herpesvirus
(KSHV), a type of rhadinovirus 		γ Lymphocyte and other cells Kaposi's sarcoma, primary effusion lymphoma, some types of multicentric Castleman's disease B cell Close contact (sexual), saliva?

Herpesviruses of other animals

In addition to the herpesviruses considered endemic in humans, some viruses associated primarily with animals may infect humans. These are zoonotic infections:

Zoonotic Herpesviruses
Species Type Synonym Subfamily Human Pathophysiology
Macaque monkey CeHV-1 Cercopithecine herpesvirus-1, (monkey B virus) α Very unusual, with only approximately 25 human cases reported.[21] Untreated infection is often deadly; sixteen of the 25 cases resulted in fatal encephalomyelitis. At least four cases resulted in survival with severe neurologic impairment.[21][22] Symptom awareness and early treatment are important for laboratory workers facing exposure.[23]
Mouse MuHV‑4 Murid herpesvirus 68 (MHV-68) γ Zoonotic infection found in 4.5% of general population and more common in laboratory workers handling infected mice.[24] ELISA tests show factor-of-four (x4) false positive results, due to antibody cross-reaction with other Herpes viruses.[24]

Animal herpesviruses

In animal virology, the most important herpesviruses belong to the subfamily Alphaherpesvirinae. Research on pseudorabies virus (PrV), the causative agent of Aujeszky's disease in pigs, has pioneered animal disease control with genetically modified vaccines. PrV is now extensively studied as a model for basic processes during lytic herpesvirus infection, and for unraveling molecular mechanisms of herpesvirus neurotropism, whereas bovine herpesvirus 1, the causative agent of bovine infectious rhinotracheitis and pustular vulvovaginitis, is analyzed to elucidate molecular mechanisms of latency. The avian infectious laryngotracheitis virus is phylogenetically distant from these two viruses and serves to underline similarity and diversity within the Alphaherpesvirinae.[2][3]

Reptilian alphaherpesviruses[25]

Research

Research is currently ongoing into a variety of side-effect or co-conditions related to the herpesviruses. These include:

References

  1. 1 2 Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. ISBN 0-8385-8529-9.
  2. 1 2 Mettenleiter; et al. (2008). "Molecular Biology of Animal Herpesviruses". Animal Viruses: Molecular Biology. Caister Academic Press. ISBN 1-904455-22-0. http://www.horizonpress.com/avir.
  3. 1 2 Sandri-Goldin RM (editor). (2006). Alpha Herpesviruses: Molecular and Cellular Biology. Caister Academic Press. ISBN 978-1-904455-09-7. .
  4. ↑ Chayavichitsilp P, Buckwalter JV, Krakowski AC, Friedlander SF (April 2009). "Herpes simplex". Pediatr Rev 30 (4): 119–29; quiz 130. doi:10.1542/pir.30-4-119. PMID 19339385.
  5. ↑ In the United States, as many as 95% of adults between 35 and 40 years of age have been infected. National Center for Infectious Diseases
  6. ↑ Staras SA, Dollard SC, Radford KW, Flanders WD, Pass RF, Cannon MJ (November 2006). "Seroprevalence of cytomegalovirus infection in the United States, 1988–1994". Clin. Infect. Dis. 43 (9): 1143–51. doi:10.1086/508173. PMID 17029132. Retrieved 2009-12-04.
  7. 1 2 John Carter, Venetia Saunders. Virology, Principles and Applications. John Wiley & Sons. ISBN 978-0-470-02386-0.
  8. ↑ Jay C. Brown, William W. Newcomb (August 1, 2011). "Herpesvirus Capsid Assembly: Insights from Structural Analysis". Current Opinion in Virology 1 (2): 142–149. doi:10.1016/j.coviro.2011.06.003. PMC 3171831. PMID 21927635.
  9. ↑ ICTV. "Virus Taxonomy: 2014 Release". Retrieved 15 June 2015.
  10. ↑ O.A., Ryder; Byrd, M.L. (1984). One Medicine: A Tribute to Kurt Benirschke, Director Center for Reproduction of Endangered Species Zoological Society of San Diego and Professor of Pathology and Reproductive Medicine University of California San Diego from his Students and Colleagues. Berlin, Heidelberg: Springer. pp. 296–308. ISBN 978-3-642-61749-2.
  11. ↑ Davison, A.J. (2010). "Herpesvirus systematics". Veterinary Microbiology (Elsevier) 143 (1): 52–69. doi:10.1016/j.vetmic.2010.02.014. PMC 2995426. PMID 20346601.
  12. ↑ McGeoch DJ, Cook S, Dolan A, Jamieson FE, Telford EA (1995) Molecular phylogeny and evolutionary timescale for the family of mammalian herpesviruses" J Mol Biol 247(3) 443-458
  13. ↑ McGeoch DJ, Rixon FJ, Davison AJ (2006) Topics in herpesvirus genomics and evolution. Virus Res117:90–104
  14. ↑ Spencer, Juliet; et al. (Feb 2002). "Potent Immunosuppressive Activities of Cytomegalovirus- Encoded Interleukin-10". Journal of Virology 76 (3): 1285–1292. doi:10.1128/JVI.76.3.1285-1292.2002. PMC 135865. PMID 11773404.
  15. ↑ Spencer, Juliet (2007). "The Cytomegalovirus Homolog of Interleukin-10 Requires Phosphatidylinositol 3-Kinase Activity for Inhibition of Cytokine Synthesis in Monocytes". Journal of Virology 81 (4): 2083–2086. doi:10.1128/JVI.01655-06. PMC 1797587. PMID 17121792.
  16. ↑ Lin, Aifen; Huihui Xu; Weihua Yan (April 2007). "Modulation of HLA Expression in Human Cytomegalovirus Immune Evasion". Cellular and Molecular Immunology 4 (2): 91–98. PMID 17484802.
  17. ↑ Adams, MJ; Carstens EB (Jul 2012). "Ratification vote on taxonomic proposals to the International Committee on Taxonomy of Viruses (2012)". Arch Virol. 157 (7): 1411–22. doi:10.1007/s00705-012-1299-6. PMID 22481600.
  18. 1 2 Whitley RJ (1996). Herpesviruses. in: Baron's Medical Microbiology (Baron S et al., eds.) (4th ed.). Univ of Texas Medical Branch. ISBN 0-9631172-1-1.
  19. ↑ Murray PR; Rosenthal KS; Pfaller MA (2005). Medical Microbiology (5th ed.). Elsevier Mosby. ISBN 978-0-323-03303-9.
  20. ↑ Bottieau E, Clerinx J, Van den Enden E, Van Esbroeck M, Colebunders R, Van Gompel A, Van den Ende J (2006). "Infectious mononucleosis-like syndromes in febrile travelers returning from the tropics". J Travel Med 13 (4): 191–7. doi:10.1111/j.1708-8305.2006.00049.x. PMID 16884400.
  21. 1 2 Weigler BJ (February 1992). "Biology of B virus in macaque and human hosts: a review". Clinical Infectious Diseases 14 (2): 555–67. doi:10.1093/clinids/14.2.555. PMID 1313312.
  22. ↑ Huff J, Barry P (2003). "B-Virus (Cercopithecine herpesvirus 1) Infection in Humans and Macaques: Potential for Zoonotic Disease". Emerg Infect Dis 9 (2): 246–50. doi:10.3201/eid0902.020272. PMC 2901951. PMID 12603998.
  23. ↑ Herpes-B Fact Sheet
  24. 1 2 Hricova M, Mistrikova J (2007). "Murine gammaherpesvirus 68 serum antibodies in general human population". Acta virologica 51 (4): 283–7. PMID 18197737.
  25. ↑ Origgi FC, Tecilla M, Pilo P, Aloisio F, Otten P, et al (2015) A genomic approach to unravel host-pathogen interaction in Chelonians: The example of testudinid herpesvirus 3. PLoS ONE 10(8): e0134897
  26. ↑ Fenner, Frank J.; Gibbs, E. Paul J.; Murphy, Frederick A.; Rott, Rudolph; Studdert, Michael J.; White, David O. (1993). Veterinary Virology (2nd ed.). Academic Press, Inc. ISBN 0-12-253056-X.
  27. ↑ Estep, R. D.; Hansen, S. G.; Rogers, K. S.; Axthelm, M. K.; Wong, S. W. (2012). "Genomic Characterization of Japanese Macaque Rhadinovirus, a Novel Herpesvirus Isolated from a Nonhuman Primate with a Spontaneous Inflammatory Demyelinating Disease". Journal of Virology 87 (1): 512–523. doi:10.1128/JVI.02194-12. PMC 3536378. PMID 23097433.

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