Herpesviruses
Name comes from the Greek 'Herpein' - 'to creep' = chronic/latent/recurrent infections. Epidemiology of the common Herpesvirus infections puzzled clinicians for many years. In 1950, Burnet and Buddingh showed that HSV could become latent after a primary infection, becoming reactivated after later provocation. Weller (1954) isolated VZV from chicken pox and zoster, indicating the same causal agent. ~100 Herpesviruses have been isolated, at least one for most animal species which has been looked at. To date, there are 8 known human Herpesviruses.
The family is divided into 3 Sub-families:
Large (genomes up to 235kbp DNA) and complex viruses ~35 virion proteins.
All encode a variety of enzymes involved in nucleic acid metabolism, DNA synthesis and protein processing (protein kinase). The Herpesviruses are widely separated in terms of genomic sequence and proteins, but all are similar in terms of virion structure and genome organization:
| Size: |
180-200nm |
| Envelope: |
Present; associated glycoproteins. |
| Tegument: |
Protein-filled region between capsid and envelope. |
| Capsid: |
Icosahedral, 95-105nm diameter; 162 hexagonal capsomers. |
| Core: |
Toroidal (DNA around protein), ~75nm diameter. |
| Genome: |
Linear, d/s DNA, 130-230kbp |
| Replication: |
Nuclear. |
| Assembly: |
Nuclear. |
| Common Antigens: |
None! |
Structure:
The structure of the herpesvirus particle is very complex. The core consists of a toroidal shape with the large DNA genome would around a proteinaceous core. The complex capsid surrounds the core. Outside the capsid is the tegument, a protein-filled region which appears amorphous in electron micrographs. On the outside of the particle is the envelope, which contains numerous glycoproteins (see discussion of Herpes Simplex Virus, below). To view an electron micrograph of negatively-stained herpesvirus particles click here. N.B. All herpesviruses are almost indistinguishable in electron micrographs.
To view an image of the herpesvirus capsid click here.
Genome:
All herpesvirus genomes have a unique long (UL) and a unique short (US) region, bounded by inverted repeats. The repeats allow rearrangements of the unique regions and Herpesvirus genomes exist as a mixture of 4 isomers. Herpesvirus genomes also contain multiple repeated sequences and depending on the number of these, genome size of various isolates of a particular virus can vary by up to 10kbp.
The prototype member of the family is Herpes Simplex Virus (HSV): ~160kbp - complete sequence is known. Prior to this, the HSV genome had been mapped extensively by a large number of t.s. mutants. There are 2 antigenic types, HSV-1 and HSV-2 which share antigenic cross-reactivity but different neutralization patterns and tend to produce different clinical symptoms. Man is believed to be the natural host for HSV, but the virus is also capable of infecting various animals, including rodents (good animal models). Human infection is virtually universal - most adults are seropositive.
Replication:
The HSV envelope contains at least 9 glycoproteins. Most of these appear to be individually dispensable for infectivity. Interaction of glycoprotein(s) with cellular receptors (not known, but probably different receptors on different cell types - a characteristic of the larger, more complex DNA viruses, and probably common cell surface molecules rather than specific proteins) results directly in fusion of the envelope with the cell membrane. Endocytosis is not absolutely required, but may occur (alternative route for penetration).
Fusion deposits the capsid in the cytoplasm, where it migrates to the nucleus.
The core enters via a nuclear pore where the genome is circularized.
Transcription of the large, complex genome is sequentially regulated in a cascade fashion. ~50 mRNAs are produced by host cell RNA polymerase II.
Three distinct classes of mRNAs are made:
| Alpha - immediate early (IE) mRNAs |
5 trans-acting regulators of virus transcription |
| Beta - (delayed) Early mRNAs |
Further non-structural regulatory proteins & minor structural proteins |
| Gamma - Late mRNAs |
Major structural proteins |
Gene expression is co-ordinately regulated:
- If translation is blocked shortly after infection, IE mRNAs accumulate in the nucleus, but no other virus mRNAs are transcribed.
- Synthesis of early gene products turns off IE products and initiates genome replication.
- Some of the late structural proteins (gamma-1) are produced independently of genome replication, others (gamma-2) are only produced after replication.
Both IE and E proteins are required for genome replication. A virus-encoded DNA-dependent DNA polymerase and DNA-binding protein are involved in replication, together with a number of enzymes (e.g. thymidine kinase) which alter cellular biochemistry. In addition, cellular proteins are required for genome replication, therefore HSV replication occurs in the nucleus (c.f. Poxviruses).
Viral DNA replication is the target for a number of successful anti-Herpesvirus drugs (e.g. acyclovir, gancyclovir, etc). The pattern of replication is complex, involving at least 3 potential origins of replication, and resulting in the formation of high molecular weight DNA concatemers.
Virus particles (core plus capsid) assemble in the nucleus - genomic concatemers are cleaved and packaged into pre-assembled capsids.
The envelope is acquired from the inner lamella of the nuclear membrane, and particles accumulate in the space within the inner and outer lamellae. How these particles are transported to the cell surface is not clear and may or may not involve the golgi apparatus. Mutations in certain envelope glycoproteins interfere with cytoplasmic transport. Any remaining virus particles are released when the cell lyses (~24h after infection).
HSV infection appears to be a 'wasteful' process, only ~25% of viral DNA/protein produced is incorporated into virions. The rest accumulates in the cell, which eventually dies. This process produces characteristic nuclear inclusion bodies.
Pathogenesis:
Herpes simplex (HSV/ HHV-1/2):
Primary infection occurs through a break in the mucus membranes of the mouth or throat, via the eye or genitals or directly via minor abrasions in the skin. because of the universal distribution of the virus, most individuals are infected by 1-2 years of age; initial infection is usually asymptomatic, although there may be minor local vesicular lesions. Local multiplication ensues, followed by viraemia and systemic infection. There then follows life-long latent infection with periodic reactivation.
During primary infection, the virus enters peripheral sensory nerves and migrates along axons to sensory nerve ganglia in the CNS - allows virus to escape immune response! During latent infection of nerve cells, viral DNA is maintained as an episome (not integrated) with limited expression of specific virus genes required for the maintenance of latency - true latency.
The delicate balance of latency may be upset by various disturbances, physical (injury, U.V, hormones, etc) or psychological (stress, emotional upset - perhaps affecting immune system/hormonal balance).
Reactivation of latent virus leads to recurrent disease - virus travels back down sensory nerves to surface of body and replicates, causing tissue damage:
- HSV-1: Primarily associated with oral (cold sores) and ocular lesions
- HSV-2: Primarily associated with genital and anal lesions
Although painful, most recurrent infections resolve spontaneously, usually to reoccur later. More serious are herpetic keratitis (ulceration of cornea due to repeated infection which can lead to blindness) and encephalitis (very rare but often fatal). Incidence of genital herpes has increased sharply during the last few decades - sexual promiscuity and oral contraceptives.
No vaccines currently licensed but a number under development - particularly for HSV-2 - a good candidate for post-exposure vaccination.
HSV-1 is under active development as a vector for gene therapy.
Is HSV infection associated with Alzheimer's disease?
Alzheimer's disease affects 10% of people aged over 65 and 20% of those aged over 75. In the UK, this amounts to about 750,000 cases, and 4 million in the USA. A large proportion of elderly people have HSV in their brains, irrespective of whether they have Alzheimer's disease. A particular version (allele) the human ApoE4 gene is known to be a risk factor for the development of Alzheimer's disease, but by no means all those who carry this allele get Alzheimer's. The same ApoE4 allele is a strong risk factor for cold sores caused by HSV infection. Could HSV infection and the ApoE4 allele combine to play a role in Alzheimer's disease?
Itzhaki et al. "Herpes simplex virus 1 in brain & risk of Alzheimer's disease" Lancet 349: 241-244, 1997.
Lin et al. "Alzheimer's disease, herpes virus in brain apolipoprotein E4 and herpes labialis" Alzheimer's Reports 1: 173-178, 1998.
Top
Varicella Zoster Virus (VZV / HHV-3):
Gives rise to 2 distinct clinical syndromes:
- Varicella (Chicken pox) - (nothing to do with chickens or pox!). Infection normally occurs in childhood (~90% - more?), via respiratory tract or conjunctiva. After multiplication at the inoculation sites, virus spreads to bloodstream and reticuloendothelial system. Secondary multiplication involves skin and mucosa, producing vesicles filled with very high titres of infectious virus! Complications are rare, but may include CNS infection.
- Zoster (Shingles) - After primary infection, virus persists in sensory ganglia of CNS. It is not clear if this is a latent or a persistent infection, but 'reactivation' after many years leads to infection and tissue damage to dermatosome served by infected ganglia - most serious when cranial nerves are involved, affecting face/head - can lead to blindness.
Therapy - acyclovir.
Complete sequence of VZV genome is known, ~125kbp. Infects a variety of human and animal cell types in vitro.
Top
Cytomegalovirus (CMV / HHV-5):
- The largest of the Herpesviruses, genome ~240kbp. The kinetics of CMV infection are 'slow' - 7-14 days c.f. 24-48h for HSV. As with some other Herpesviruses, certain parts of the CMV genome have considerable homology with cellular DNA, implying that the virus has acquired cellular genes during evolution. The complete nucleotide sequence is known and expression has been studied in detail. Upstream of the IE genes, there is a promoter/enhancer region which has been characterized in detail and is remarkable for its strength - is often used for heterologous expression of recombinant genes. This is the first region to be transcribed after infection and initiates replication.
- CMV infection is common; 60% of the UK population have experienced infection by the age of 40. Most infections are asymptomatic. Apart from during pregnancy and newborn infants exposed in utero, active (as opposed to latent) CMV infection only occurs in people with immune defects, specifically T-cell defects, e.g. AIDS patients and immunosuppressed transplant patients.
- Transmission is believed to be by oral/respiratory route. Infection produces enlargement of cells and nuclear inclusion bodies in a wide range of tissues - systemic infection.
- In spite of the widespread distribution, CMV-related illness is rare and occurs only in two groups:
|
- Immunocompromised: Evidence that the host immune response (particularly cell-mediated) plays a role in latency comes from the evidence of what occurs on immunosuppression. Latent virus is reactivated and AIDS/transplant patients experience frequent and severe infections with the potential for involvement of many possible organs.
- Foetal Infections: Particularly a problem when primary infection of the mother occurs, resulting in congenital abnormalities in a proportion of cases.
Online CMV Experiment:
Mr Campbell's Kidney |
Therapy - Gancyclovir.
Top
Epstein-Barr Virus (EBV / HHV-4):
Dual cell tropism for human B-lymphocytes (generally non-productive infection) and epithelial cells (productive infection). There is no suitable animal host, but replication/latency has been studied extensively in transformed human cell lines. Widespread worldwide, >90% infection in most human populations (lifelong!). The usual outcome of infection is polyclonal B-cell activation and benign proliferation, which may be sub-clinical or produce infectious mononucleosis (glandular fever). Uniquely among Herpesviruses, there is a well-established relationship between EBV and oncogenesis - Burkitts lymphoma and nasopharyngeal carcinoma. Complete nucleotide sequence (~172kbp) is known, contains many internal repeats. Therapy/Vaccine - none.
X-linked lymphoproliferative syndrome (XLP) is a rare condition usually seen in males where on initial infection with EBV (usually around the age of 2-3) results in a hyperimmune response, sometimes causing a fatal form of glandular fever and sometimes cancer of the lymph nodes. Death by the age of 40 is inevitable. XLP is an inherited defect due to a faulty gene on the X chromosome. Females have two X chromosomes, so if one copy is faulty, the other can usually compensate. But males have just one X chromosome and one Y. There is no backup X, hence the predominance of the disease in males. The gene responsible (SH2D1A or SAP) causes a failure in the communication between the cells of the immune system. A protein known as SLAM ("signalling lymphocyte activation molecule") stimulates activity and proliferation of cells in the immune system. SLAM is found on the surface of both B and T cells, effectively coupling them together. When this happens, it signals changes inside the cells that cause them to become active and to develop and proliferate. SAP ("SLAM-associated protein") is produced by T cells, and is an inhibitor of SLAM. This is the gene which is defective in XLP. EBV first infects the throat but shortly afterwards invades the B cells, and triggers them to multiply, partly by increasing the number of SLAM proteins on the surface. Without functional SAP protein, the body is unable to control the B-cell proliferation triggered by infection. T cells recognize the EBV-infected B cells as foreign and instigate a massive inflammatory response. (J.Sayos et al. Nature 395: 462-469, 1998).
Top
Human Herpesvirus 6 (HHV-6):
(a.k.a. "HBLV"). Isolated in 1986 in lymphocytes of patients with lymphoreticular disorders - tropism for CD4+ lymphocytes. Genome ~160kbp. HHV-6 is now recognised as being a universal human infection. Discovery of the virus solved a longstanding mystery - primary infection in childhood causes "roseola infantum" a.k.a. "fourth disease", a common childhood rash whose cause was previously unknown. Ab titres are highest in children and decline with age. Consequences of childhood infection appear to be mild. Primary infections of adults are rare but have more severe consequences - mononucleosis/hepatitis. HHV-6 infection is a problem in immunocompromised patients.
Human Herpesvirus 6 by Stephen Dewhurst, David Skrincosky and Nanette van Loon, Expert Reviews in Molecular Medicine.
Top
Human Herpesvirus 7 (HHV-7):
First isolated from human CD4+ cells in 1990. Genome ~170kbp, organization similar to but distinct from HHV-6. The complete genome sequence of HHV-7 has been determined and this shows a high degree of conservation of genetic content and encoded amino acid sequences to HHV-6. However, there is only limited antigenic cross-reactivity between the two viruses. At present, there is no clear evidence for the direct involvement of HHV-7 in any human disease, but HHV-7 might be a co-factor in HHV-6-related syndromes ???
Human Herpesvirus 7 by Stephen Dewhurst, David Skrincosky and Nanette van Loon, Expert Reviews in Molecular Medicine.
Top
Human Herpesvirus 8 (HHV-8):
(a.k.a. KSHV). Sequences of an unique herpesvirus were recently identified in 100% of amplifiable samples from AIDS patients with Kaposi's sarcoma (KS) and 15% of non-KS tissue samples from AIDS patients.
There is a strong correlation (>95%) with KS in HIV+ and HIV- patients. The virus can be isolated from PBMC as well as KS tumour cells; appears to have a less ubiquitous world distribution than other HHVs???
Recent evidence suggests that one of the genes of HHV-8, vGPCR (viral G-protein coupled receptor) acts as a vascular switch, turning on synthesis of a powerful angoigenic agent, vascular endothelial growth factor (VEGF), which is responsible for the development of KS. However, HHV-8 also contains a considerable number of other 'pirated' cellular genes in an 'oncogenic cluster' within the virus genome which may also be involved in the development of malignancy (See Boshoff, C. Nature 391: 24-25, 1998), e.g. the K1 gene (Nature Med. 4:435 1998).
In addition to KS, this virus may also cause other tumours such as B-cell lymphomas (±EBV as 'helper'). HHV-8 resembles EBV in that:
- It has a tropism for epithelial and B-cells.
- It is kept under immunological control and only presents a problem during immunosuppression.
HHV-8 & AIDS-Related Neoplasms
Medscape Article: Kaposi's Sarcoma & HHV-8: Implications for Therapy
Top
© AJC 1998
