Medical Virology Lecture 19: Gammaherpesviruses

Herpesviruses II - Gammaherpesviruses

• Lecture 19, Medical Virology, Level H
• Professor Andrew Davidson, E46 Biomedical Sciences Building, [email protected]
• Date: 19/11/24

Overview

• Epstein Barr virus (EBV) introduction and impact
• EBV primary and persistent infection
• EBV - cancer associations and potential mechanisms involved
• EBV and multiple sclerosis
• EBV vaccines
• Kaposi's sarcoma virus (KSHV)

Intended Learning Outcomes

• Describe the uniting features of human Gammaherpesviruses.
• Describe the epidemiology of EBV and recognize the diseases associated with EBV infection.
• Understand how EBV causes primary and persistent infections.
• Understand the EBV gene expression patterns in different stages of latency and their relationship to EBV-induced cancers.
• Describe the evidence for the association of EBV infection and multiple sclerosis.
• Describe the epidemiology and diseases caused by KSHV infection.

Subfamily Gammaherpesvirinae

• Gammaherpesviruses have a very restricted host range.
• In vitro, all members infect lymphoblastoid cells and some cause infection in epithelial or fibroblastic cells.
• Typically target B or T lymphocytes.
• Four genera: Lymphocryptovirus, Rhadinovirus, Percavirus, and Macavirus.
  • Lymphocryptovirus: mainly infects humans and non-human primates, includes human gammaherpesvirus 4 = Epstein Barr Virus
  • Rhadinovirus: mainly infects human and non-human primates, includes human gammaherpesvirus 8 = Kaposi's Sarcoma Virus
• dsDNA genome
  • EBV ~ 170 kbp, ~ 100 protein-coding genes, also non-coding RNAs and miRNAs (>46)
  • KSHV ~ 165 kbp, at least 80 protein-coding genes, also non-coding RNAs and miRNAs
• In both cases, linear dsDNA in particles but found as a closed circular dsDNA in the nucleus of persistently infected cells ("episome")

Epstein Barr Virus

• Causes most common persistent asymptomatic virus infection in humans.
• Ubiquitous, infects 90-95% of the human population as a lifelong persistent infection.
• Mostly causes benign infections.
• First human tumor virus to be discovered (1964).
• Associated with a range of diseases
  • Infectious mononucleosis (glandular fever)
  • Human cancers (200,000/year; 1.8% of all cancer deaths)
  • Multiple sclerosis
• Primarily found in the body as a latent infection of B cells.
• dsDNA genome completely sequenced in 1984.

Epidemiology

• Transmission by saliva.
• Globally, most people are infected by the age of 5-8.
• EBV infection at a young age is typically asymptomatic.
• In resource-rich environments, EBV infection is frequently delayed until adolescence or early adulthood.
• Prevalence of infectious mononucleosis depends on the age of initial EBV infection in different countries.
• In many cases, EBV-associated cancers are most prevalent in specific geographical areas.

EBV Associated Diseases

• Typically causes a subclinical persistent infection
• Primary Infection: Infectious mononucleosis (IM) ("kissing disease", glandular fever), Chronic IM, Fatal IM
• Fatal lymphoproliferative (B-cell) disorders in various immunosuppressive conditions (e.g., after organ transplant/AIDS): Burkitt’s lymphoma (BL), Hodgkin's and non-Hodgkin's lymphomas, Nasopharyngeal carcinoma (NPC), Gastric carcinomas, Natural killer (NK)/T cell lymphomas, Multiple sclerosis

EBV: Structure

• Envelope proteins
• Outer Tegument
• Inner Tegument
• Major capsid protein
• Triplex
• Icosahedron
• Nucleocapsid
• Portal vertex

EBV: dsDNA genome

• Linear form virion
• Circular form episome

EBV: Primary and persistent infection

• (detailed diagrams showing the process)
• Transmission: Oropharynx → epithelial cells → infected naive B cells → lymphoid tissue→ mucosal lymphoid tissue →dissemination→blood→latency→infection of epithelial (other) cells

EBV: Infection of primary target cells

-Attachment protein gp350/220 and glycoproteins gB, gp42, and gH/gL complex are essential for EBV entry into both epithelial and B cells, targeting for neutralizing antibodies.

Infectious mononucleosis (IM) - Glandular fever

• As age increases to young adulthood, primary EBV infection more often causes IM.
• Linked to host genetics.
• High replication of EBV in B cells in periphery and the oral cavity.
• Up to 50% of resting memory B cells may be infected in the acute phase of IM.
• Characterized by a rapid expansion in NK cells and CD8+ T cells.
• Strong inflammatory response and emergence of atypical lymphocytes.
• High fever, severe sore throat, swollen glands, extreme tiredness, ongoing tonsillitis.
• Recovery typically after 2-3 weeks, but extreme tiredness can last for months (infectious for up to 7 weeks before symptoms).

EBV: Latency

• In latently infected cells, the genome is maintained as a transcriptionally silent genome.
• The genome is circularised; if the cell divides, the virus genome is replicated and each daughter cell gets a copy.
• This is achieved by a virus protein called EBNA1, which binds to the virus genome and interacts with dividing chromatin.
• EBNA1 is among a group of at least nine EBNA-encoded proteins produced during various stages of latency, but it is the only EBV protein expressed in all types of latency.

Function of EBV latent proteins

• (detailed table showing specific genes and their actions)
• Some gene products promote cell growth, while others inhibit it.

Function of EBV latent non-coding RNAs

• BART miRNAs
• BHRF1 miRNAs
• EBV-encoded RNA 1 (EBER1) and EBER2

Patterns of latent cycle gene expression in EBV

• Latency III
• Latency II
• Latency I

Latency patterns and cancer

• (table linking types of cancer with latency patterns)

Burkitt Lymphoma (BL) and EBV

• Aggressive non-Hodgkin B-cell lymphoma (1-5%)
• Associated with EBV, HIV, and chromosomal translocations resulting in c-myc oncogene overexpression.
• Endemic, sporadic, and immunodeficiency types of BL.
• Endemic BL: coincident with malaria in equatorial Africa, Brazil, New Guinea, etc.
• Investigation led to the discovery of EBV (Burkitt, Epstein, Barr, Achong)
• Endemic BL is nearly 100% EBV+.
• Tumours are at a state of EBV latency.
• Sporadic BL: 89% EBV+, accounts for 40% of pediatric lymphomas.
• EBV proteins prevent apoptosis of cells with c-myc overexpression.
• Treatment: intensive chemotherapy and hospitalization (60-90% long-term survival).
• EBV was discovered in 1964.

Burkitt lymphoma (BL) and EBV discovery

• Epstein had previously worked on Rous Sarcoma Virus, a retrovirus that causes sarcoma in chickens

Nasopharyngeal carcinoma (NPC)

• Epithelial cell tumor associated with EBV latency.
• EBV is almost always present in malignant epithelial cells.
• Predominantly found in SE Asia and southern China.
• Major health issue (2018: 129,000 new cases and 73,000 deaths).
• Co-factors: environment (diet, smoking, etc.), genetic predisposition (HLA type), EBV subtypes.

Nasopharyngeal carcinoma (NPC) Mechanism

• Mechanism of cellular transformation is not clear.
• Infection of epithelial cells in the laboratory does not immortalize cells or give rise to long-term persistent infection.
• Cells lacking cyclin-dependent kinase inhibitors or overexpressing cyclin D1 can be persistently infected.
• Therefore, cells may already contain specific mutations before EBV transformation.

Nasopharyngeal carcinoma (NPC) - additional details

• EBv genomes in cancer are clonal.
• EBV infection detected in high-grade pre-invasive lesions in NPC but not in low-grade disease.
• Suggests that EBV infection must precede clonal selection of cells.

Association of EBV lytic replication with clinical manifestations

• (diagram displaying the relationship between EBV lytic replication and different clinical manifestations)

EBV and Multiple Sclerosis

• Multiple sclerosis (MS): most common chronic inflammatory and neurodegenerative disease of the central nervous system (2.8 million people worldwide).
• Cause of MS is complex and multifactorial: genetic susceptibility, environmental factors, lack of sun/vitamin D, smoking, obesity, etc.
• EBV is a prime candidate and has been studied as a risk factor for MS in many epidemiological studies.
• Difficult to establish causality – long period between disease initiation and clinical disease expression.
• 95% of people infected with EBV do not develop MS.

EBV and MS: Evidence

• Epidemiological evidence
• Immunological evidence
• Virological evidence

EBV Latest Evidence

• Epidemiological evidence
• Mechanism

EBV Vaccines

• Aiming to prevent/reduce the severity of infection with EBV.
• Reduce the incidence of infectious mononucleosis and EBV-associated malignancies and autoimmune diseases.
• Vaccines in development for 40 years: 36 preclinical and 4 clinical trials.
• Spurred on by recent findings linking EBV and MS.
• Vaccine development hindered by undefined correlates of immune protection, lack of an appropriate animal model, lack of knowledge regarding ideal EBV antigens for vaccination, and lack of knowledge regarding ideal vaccine delivery platform.
• Number of new vaccines in phase I clinical trials (safety/immune response): Moderna mRNA vaccine (targets four glycoprotein antigens), NIH gp350-Ferritin nanoparticle vaccine (targets gp350).

Kaposi's Sarcoma Virus (KSHV)

• One of the most recently discovered human tumor viruses (1994).
• Causative agent of all forms of Kaposi's sarcoma (KS) in combination with impaired host immunity.
• KSHV can infect several different cell types, including endothelial, B cells, epithelial cells, dendritic cells, monocytes, and fibroblasts.
• Latency can be established in B cells and endothelial cells.
• KS tumors are of endothelial lineage—highly aggressive in immunosuppressed/AIDS patients.

KSHV Epidemiology

• (detailed information on different forms of KS, clinical presentation, risk factors, and progression)

KSHV Seroprevalence

• (map showing global distribution of KSHV seroprevalence)

KSHV: Diseases

• Kaposi's sarcoma
• Primary effusion lymphoma (PEL)
• Multicentric Castleman disease (MCD)
• Treatment of KSHV related diseases

Extra Reading

• (list of relevant research papers and articles)