Lecture 28– Herpes Viruses
Reading Assignments: (1) Text Chapters 41 and 43, pp. 408-411 (2) HANDOUT: Cunha, B. 1999. Patients 28, pp. 62-63 and 61, pp. 140-141. In: Infectious Disease Pearls, Hanley and Belfus, Inc., Philadelphia, PA, (3) Gilligan, P.H., M.L. Smiley, and D.S. Shapiro. 1997. Cases 4, 19, 35, and 59. In: Cases in Medical Microbiology and Infectious Diseases (2nd ed.), American Society for Microbiology, Washington, D.C (4) White, D.O., and F.J. Fenner. 1994. “Persistent Infections”, pp.147-156., In Medical Virology (4th ed.), Academic Press, New York.
I . INTRODUCTION
A. Herpesviridae - large, enveloped dougle stranded DNA viruses; morphologically alike; share a common mode of replication.
B. Herpesviruses - ubiquitous and cause infections ranging from painful skin ulcers to chickenpox to encephalitis.
C. An outstanding property - the ability to establish latent infections, to persist indefinitely in infected hosts, and to periodically become reactivated.
D. With all these viruses, immunocompromised patients, especially those with altered cellular immunity, have more frequent and severe infections, including severe disease from reactivation of the virus. (Cell mediated immunity is paramount in controlling herpes virus infections.
E. Effective antiviral drugs available to treat these infections.
II. CLASSIFICATION: Herpesviruses - 3 subfamilies based on their biologic properties.
A. Alphaherpesviruses are fast growing, cytolytic viruses that establish latent infections in neurons. Herpes simples virus type 1 and 2 (HSV-1 and HSV- 2) and Varicella- zoster virus (VSV) are members of this subfamily. These three viruses produce vesicular rashes both in their primary infections and in reactivation.
B. Betaherpesviruses include the slow growing Cytomegalovirus (CMV) - so named because it causes the cells it infects to become cytomegalic – become massively enlarged. These viruses become latent in secretory glands and kidneys. This subfamily also includes the newly identified human Herpesviruses 6 and 7. Human herpesvirus 6 causes a common childhood illness – sixth disease – roseola infantum. Human herpesvirus 7 – is closely related to human herpesvirus 6, but is not firmly associated with human disease.
C. Gammaherpesviruses, exemplified by Epstein Barr virus (EBV) infect and become latent in lymphoid cells. A new herpesvirus – also called Kaposi's sarcoma associated herpesvirus has been detected in over 90% of Kaposi's sarcomas and a rare type of B cell lymphoma from AIDS patients.
III. VIRAL STRUCTURE AND OVERVIEW OF VIRAL REPLICATION
A. Structure
1. Virion – large (150-200 nm in diameter), enveloped, spherical virus with an icosahedral capsid. The envelope is derived from the host cell membrane and it contains viral glycoprotein spikes. An amorphous structure called the tegument lies between the membrane and the nucleocapsid.
2. Genome- double-stranded, linear DNA. Herpesvirus genomes possess terminal and internal repeated sequences. There is little DNA homology between the herpesviruses except for HSV-1 and HSV-2 which share 50% sequence homology. Genome is large enough to code for 100 proteins of which 35 structural proteins are known. Several of the proteins synthesized are virus specified enzymes (DNA polymerase, thymidine kinase) necessary for the replication of new virions and others contribute to the pathogen's virulence in the host (see #7,below.
B. Replication Cycle – A Lytic Infection (HSV)
1. Virus attaches to the cell receptors (heparan sulfate moiety of cellular proteoglycans) via glycoprotein spikes.
2. Enters the cell after pH independent fusion with the cell membrane.
3. Tegument proteins are released, one of which shuts down cellular protein synthesis.
4. The nucleocapsid is transported along the cytoskeleton to a nuclear pore where viral DNA is released, enters the nucleus, and circularizes.
5. Viral gene expression is tightly regulated, with three classes of mRNA's, alpha, beta, and gamma, being transcribed in an ordered sequence by cellular RNA polymerase II.
a. The second tegument protein travels to the nucleus and turns on the transcription of the alpha mRNA's. The alpha mRNA's travel to the cytoplasm, are translated, and return back to the nucleus, and turn on the transcription of the beta mRNA's.
b. The beta mRNA's travel to the cytoplasm, are translated, and return to the nucleus. The beta proteins are enzymes required to increase the pool of nucleotides (thymidine kinase, ribonucleotide reductase), and to replicate the viral DNA (viral DNA polymerase (along with primase-helicase, topoisomerase, single-strand and double-strand DNA-binding proteins).
c. Viral DNA is replicated by a rolling circle mechanism, which results in unit-length concatemers of DNA.
d. Meanwhile, certain beta proteins induce the transcription of the gamma mRNA's which are the viral structural proteins. These travel to the cytoplasm, are translated, and return to the nucleus.
e. The icosahedral capsids self assemble in the nucleus. Unit length viral DNA, cleaved from newly synthesized DNA is packaged to produce nucleocapsids.
6. The virus buds through the nuclear membrane.
7. Enveloped virions accumulate in endoplasmic reticulum and the mature virions are released by exocytosis. The length of the replication cycle varies from 18h (HSV) to over 70h (CMV).
IV. ANTIVIRAL DRUGS: 2 viral enzymes– thymidine kinase and viral DNA polymerase - are the tarets of the antiviral drugs used against the herpes viruses.
A. Nucleoside Analogs:
1. Acyclovir, Valacylovir, and Famciclovir – used for HSV –1, HSV-2 and VZV infections but ineffective against CMV
2. Gancylovir – a modified acyclovir molecule with activity against CMV
Mechanism of action of these two drugs?
How does HSV become resistant to acyclovir?
B. Foscarnet- an organic analog of inorganic pyrophosphate used to treat cytomegalovirus retinitis and herpes simplex infections that are resistant to the nucleoside analogs, listed above.
Mechanism of action of this drug?
V. EFFECT OF LYTIC INFECTIONS ON HOST CELLS
A. Herpes simplex viruses causes cytocidal infections of epithelial cells of the oral mucosa and genital tract; cell death results from several mechanisms. Morphologic changes (cytopathic effects) are seen prior to cell lysis. (Review Lecture 11- How Viral Infections Affect Individual Cells)
1. Host protein and DNA synthesis are shut down
2. Cell apoptosis is induced
(Note- viruses can encode for genes that both induce and block apoptosis- facilitating completion of the infections cycle and the production of new virions or lysis of cells and promoting viral spread.)
3. Cytopathic effects seen microscopically include:
a. ballooning of infected cells
b. production of intranuclear inclusion bodies
c. margination and pulverization of chromatin
d. the formation of multinucleated giant cells
(Cell fusion provides an efficient method of cell-to cell spread of viruses, even in the presence of neutralizing antibody.)
4. The characteristic pathology produced is the result of the damage to infected cells together with the host's inflammatory response.
B. All herpesviruses cause latent infections (see below)
C. Some herpesviruses can cause malignant cell transformation (EBV) (B cell lymphomas)
VI. LATENCY- THE HALLMARK OF INFECTIONS WITH HERPESVIRUSES (see reading Assignment (4) and Paradigm in the Text, p. 385. Review Lecture 11 - How Viral Infections Affect the Host - Patterns of Clinical Illness:
A. Acute Infections (rhinoviruses, influenza viruses)
B. Persistent Infections:
1. Persistent infection with shedding (called chronic infections in Reading Assignment (4) – Hepatitis B virus
2. Persistent slow infection following acute infection (called acute infection with rare complication in Reading Assignment (4)- SSPE
3. Persistent slow infection without an acute stage- kuru/scrapie
C. Latent Infections (acute infections followed by latency and reactivation) (Latency can be classified as a form of persistence)
STEP BY STEP: LATENCY AND REACTIVATION
1. Virus infects epithelial cells and causes a productive infection with the production of many progeny virions (primary infection).
2. As disease progresses, the viruses penetrate into deeper layers and come in contact with nerve terminals.
3. The virus travels by axonal transport to sensory nerve cell ganglia.
4. There is limited replication in these sensory nerve cells which is followed by:
a. Latent infections of the surviving cells. The latent genome can persist for the life of the host. What happens to the viral genome during latency? (For some viruses like HIV the viral genome is integrated into the host's DNA, but for herpesviruses, the genome survives as a free plasmid (episome) in the cytoplasm or nucleus of the host cell. Sensory neurons latently infected with herpes simplex virus harbor 10-100 copies of the viral genome as nonintegrated, circular extended concatemers.
b. Further spread of the virus to the CNS- causing meningitis and encephalitis
5. Following reactivation, the virus exits latency and there is a limited burst of replication in sensory nerve cells.
6. Newly formed virions proceed by axonal transport back to susceptible epithelial cells causing recurrent vesicular lesions at or near the site of the primary infection.
HOW IS LATENCY MAINTAINED?
During latency, viral gene expresssion is wholly or partially repressed, but is able to be fully expressed again upon reactivation. How is this possible? Although latency in herpesviruses is not fully understood, study the paradigm (Text p. 385) and be able to describe what is known about the mechanisms used by HSV, EBV, and CMV to maintain latency.
VII. WHAT MAKES A VIRAL PATHOGEN HOT? VIRULENCE FACTORS OF HSV?
Almost half of the gene products of herpes simplex are not essential for virus replication in tissue culture cells, but they are important for replication and spread within the host. Beyond, the basic gene products needed for replication, what other gene products does this virus express that correlate with virulence in vivo?
A. Virulence is correlated with the capacity of the virus to replicate in epithelial cells and spread to the nervous system, infecting the peripheral nervous system and sometimes the brain.
HSV has viral gene products that:
1. maintain nucleotide pools needed for replication - thymidine kinase/ ribonucleotide reductase
2. interfere with neuronal apoptosis
3. destablilize cellular mRNAs –help the virus shut off host protein synthesis An observation- the more severe the primary infection, the greater the frequency and duration of recurrences. This may be related to the initial viral load (i.e. a high initial infectious dose)
B. Virulence is correlated with the capacity of the virus to avoid nonspecific and specific host defenses. (Review Lecture 10)
1. Several viral gene products expressed during lytic infections interfere with the host's ability to recognize and destroy virus-infected cells.
a. One viral glycoprotein spike binds to C3b-inactivating it and interfering with the alternative complement pathway.
b. Two other glycoprotein spikes together form a heterodimer that bind the Fc portion of IgG- interfering with opsonization and ADCC.
c. One gene product interferes with the transporter protein used for antigen presentation, inhibiting MHC-class I-mediated antigen presentation in infected epithelial cells- interfering with HSV- specific TC cell recognition.
2. The virus can hide latently inside of cells – evading the specific immune response for long periods of time.
See Reading Assignment (4) (p. 153) for three reasons the nondividing neurons give herpes simplex viruses a survival advantage:
1.
2.
3.
VIII. HERPES SIMPLEX VIRUS DISEASE:
A. Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) can be distinquished by their antigenicity and in general the location of the lesions they produce. Lesions caused by HSV-1 are generally above the waist, and those of HSV-2 are below the waist. HSV-2 has a greater potential to cause viremia and associated influenza-like symptoms.
B. Epidemiology
1. Transmission in vesicle fluid, saliva, and vaginal secretions – MMMM (the “mixing and matching of mucous membranes”). (The viruses are not hardy and they require direct inoculation areas where they can replicate)
a. HSV-1- usually transmitted in saliva (kissing, sharing of glasses, etc.)
b. HSV-2 transmitted by
1. sexual contact (HSV-2 can infect genitalia, anorectal tissues, or the orpharynx)
2. autoinoculation
3. from an infected mother infecting her infant at birth (during passage through the vagina) . Neonates may also become infected by a primary HSV-2 infection during pregancy (HSV-2 ascends to the uterus) Either way, these infections are devastating to the infants. Disseminated disease in neonates has a 50% mortality rate and results in multiple sequelae among the survivors. See attached case study- Introduction and Resolution of Clinical Case.
c. Both types can cause oral and genital lesions.
2. Transmission occurs most often when active lesions are present but asymptomatic shedding can occur and is important in transmission.
3. Occurrence
a. HSV-1 Common infections with 80% of people in the U.S. infected. Most primary infections occur in childhood as evidenced by early appearance of antibody.
b. HSV-2 occurs later in life and correlates with increased sexual activity. Antibody appears at the time of sexual activity.
4. Special “at risk” groups:
a. Neonates (see above)
b. Immunocompromised
c. Physicians, nurses, dentists, etc. in contact with oral and genital secretions.
C. Pathogenesis and Immunity – similar for HSV-1 and 2. THE DANCE BETWEEN THE PATHOGEN AND THE HOST
1. Two patterns of disease:
a. In the immunocompetent host- infections of the mucous membranes/ latency/ reactivation (with less severe disease); rare spread to the CNS via peripheral nerves- aseptic meningitis and encephalitis
b. In the immunocompromised host- disseminated, life threatening infections
(1) Age – most susceptible premature infants
(2) Patients with leukemia, AIDS- prolonged and severe disease.
Text: 2/3 of all bone marrow transplant patients have recurrence of disease within one month of transplant.
2. Pathogenesis of Primary Infections of the Mucous Membranes:
a. Viruses require a break in the skin for entry but can easily enter the mucosal epithelial cells.
b. A lytic viral infection occurs producing destruction of the epithelial cells. During primary (acute) infections a variety of cytopathic effects are seen (see #5, above). The visible evidence of infection is the classic fluid filled vesicle on an erythematous base. There is an inflammatory reponse characterized first by neutrophils and then by mononuclear cells.
c. The spreading viruses quickly enter innervating neurons in the area and these cells are damaged by viral replication or by the host's inflammatory response. The resulting symptoms include, itching, tingling, burning, and pain.
d. Viruses then travel up the neurons and become latent in sensory ganglion cells. In general, HSV-1 becomes latent in trigeminal ganglia and HSV-2 become latent in lumbar and sacral ganglia.
e. The virus can be reactivated at a later date by a variety of inducers in including stress, trauma, hormonal changes, sunlight, immunosuppression, etc. At this time, they travel back down the neruon and replicate again in the skin or mucous membranes causing lesions. Recurrent infections are generally less severe, more localized, and of shorter duration than the primary infectons. Why?
f. Immunity
a. During primary infection, interferon and natural killer cells limit the progression of the infection.
b. Antibody directed against envelope glycoproteins can neutralize extracellular viruses and help limit their spread. Antibody can also participate in ADCC. Viruses can escape antibody neutralization and clearance by:
(1) Direct cell-to cell spread- limiting their time spent outside the cell.
(2) Latent infections of the neurons
(3) Binding the antibody “upside down” with Fc receptors and/or inhibiting C3b (see #7 above)
c. Cell mediated immunity is essential for controlling and resolving HSV infections. Without functional cell mediated immunity, the virus may disseminate to vital organs and the brain.
D. Clinical Presentations (Clinical Features)
The classical presentation of HSV-1 and 2 is the lesion – a clear vesicle on an erythematous base – “the dewdrop on a rose petal”- which then progresses to pustular lesions, ulcers, and crusted lesions.
HSV-1
1. Oral Herpes – Herpetic Gingivostomatitis – an infection of the oral mucosa characterized by redness of oral tissues, formation of multiple vesicles, painful ulcers and fever. Occurs primarily in children characterized by fever, irritability and vesicular mouth lesions. Primary disease is more severe and lasts longer than recurrences. Lesions heal spontaneously in 2-3 weeks. Many children have asymptomatic primary disease and produce neutralizing antibody.
2. Herpes labialis (fever blisters or cold sores) – a milder, recurrent form of infection characterized by crops of vesicles, usually at the mucocutanous junction of the lips or nose. Reoccurs frequently at the same site usually with less severe symptoms.
3. Herpetic whitlow – pustuar lesion of the skin of the finger or the hand. Occurs in medical personnel as a result of contact with patient's lesions.
4. Eczema herpeticum – a disease in children wih active exzema. The underlying skin disease promotes the spread of the infection along the skin and potentially to other organs.
5. Keratoconjunctivitis – characterized by corneal ulcers and lesions of the conjunctival epithelium. Recurrences can lead to scarring, corneal damage, and blindness.
6. Encephalitis – most common cause of encephalitis. Lesions are generally limited to the temporal lobe. Viral pathology and immunopathology lead to destruction of the temporal lobe leading to seizures, permanent neurological sequelae or death. HSV-1 encephalitis has a high mortality rate.
7. Disseminated infections –immunocompromised patients with depressed T cell function.
HSV-2
1. Genital herpes – painful vesicular lesions of the male and female genitals and anal area. Lesions more severe and protracted in primary disease than in recurrence. Asymptomatic infections occur in both men (in the prostate and urethra) and women (in the cervix) and can be source of infection to others. Many infections are asymptomatic – many people have antibody to HSV-2 but have no history of disease. (See Text, Case Study- “Mr. H”, p. 380 and Reading Assignment (2) Patient 28)
2. Neonatal herpes – My be acquired in utero but usually is contracted during passage of the infant through the vaginal canal when the mother is shedding herpesviruses at the time of delivery. This is a usually a fatal disease, because without a developed cell-mediated immune system, the virus disseminates to the liver, lung and other organs, as well as the CNS. (See attached case study)
3. Aseptic meningitis – usually a mild, self-limited disease with few sequelae. (See Reading Assignment (3) Case 4)
E. Laboratory Diagnosis
1. Rapid Diagnosis
a. From skin lesions (Tzanck smear) – scrape the base of the lesion, stain, and look for characteristic CPE – multinucleated giant cells and Cowdry Type A intranuclear inclusions. Not definitive.
b. Assay of tissue biopsy, smear, or vesicular fluid for HSV antigen by direct fluorescent antibody technique, ELISA and DNA probe analysis. Rapid, definitive diagnosis. (FA of herpes brain biopsies)
2. Definitive Diagnosis – isolation of the virus in cell culture followed by identification with direct FA or assaying for virus specific glyoproteins by ELISA..
3. Serology – not useful – many adults have circulating Ab
F. Treatment and Control
1. Acyclovir – treatment for encephalitis and systemic disease. It shortens the duration of lesions and reduces the extent of shedding in genital herpes. No drug treatment will prevent recurrences; drugs have no effect on the latent state.
2. Prevention – avoiding contact with vesicular lesions or ulcers. Cesarean section is recommended for women at term who have genital lesions and positive viral cultures.
IX. VARICELLA ZOSTER VIRUS
A. Varicella (chickenpox) is the primary disease – herpes zoster or shingles is the recurrent form.
B. Transmission and Epidemiology
1. Humans are the natural host
2. Transmitted by respiratory droplets
3. Highly contagious; over 90% of people have antibody by age 10
C. Pathogenesis and Immunity- a typical invasive viral infection (Review Lectures 5 and 6)
1. VZV infects the upper respiratory tract, replicates on the mucosa and then spreads via the bloodstream and the lymphatics to the cells of the RES system. A secondary viremia that occurs after 11-13 days spreads the virus throughout the body and to the skin. The virus causes a vesiculopustular rash on the skin; fever and systemic symptoms occur with the rash.
2. After the host has recovered, the virus becomes latent in the dorsal root ganglia. Later in life, at times of lowered cell immunity or local trauma, the virus is activated and causes the vesicular skin lesions and nerve pain of zoster. (Severe nerve pain usually precedes the chickenpox-like lesions.) The rash is usually limited to a dermatome. A chronic pain syndrome can persist for months to years (occurs in 30% of patients). The frequency of zoster increases with advancing age, probably because of waning immunity.
3. Immunity is life long; a person gets varicella once, but zoster can occur despite this immunity.
D. Treatment and Prevention
1. No antiviral therapy is necessary for chickenpox or zoster in normal hosts. Acyclovir is used for systemic disease in immunocompromised individuals.
2. A vaccine containing live, attenuated VZV (Varivax) was approved by the FDA in 1995. Very effective for preventing varicella, but zoster can occur in those previously infected because the vaccine does not eliminate the latent state.
X. CYTOMEGALOVIRUS (CMV)
A. The most common viral cause of congenital defects. An important pathogen of immunocompromised patients causing pneumonia, retinitis, and a mononucleosis syndrome. The most common infection following organ transplants! (See Reading Assignment (3) Case 19)
XI. Epstein Barr Virus (EBV)
A. Causes infectious mononucleosis. Associated with Burkitt's lymphoma, other B cell lymphomas, and nasopharyngeal carcinoma. (See Reading Assignments (2) Patient 61 and Reading Assignment (3) Case 59) .