Herpesviridae Infections-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). . N.B. All herpesviruses are almost indistinguishable in electron micrographs.

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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):

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 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.

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Varicella Zoster Virus (VZV):

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.

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Cytomegalovirus (CMV):

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.