Epstein Barr Virus in Health & Disease

General:

EBV is a double stranded DNA virus or the Herpesviridae family
- It is also known as human herpesvirus 4 and is a gammaherpesvirus
- Genome is about 172kbp
Primarily causes infectious mononucleosis as a primary infection
It is the most efficient transforming agent known to man
It typically becomes latent in memory B cells and can reactivate to cause lymphoid and epithelial malignancies (e.g. Kaposi Sarcoma)
Typically presents in young adults and adolescents as it is primarily transmitted via saliva
- Extremely common - it is predicted that 95% of adults are latently infected with EBV
Two EBV types in circulation:
- EBV-1 most prevalent in developed world
- EBV-2 most prevalent in developing world
- Type 1 is the more efficient transforming agent

Mechanism of Action (Primary Infection):

Once transmitted, EBV targets the epithelial cells and the B cells within the tonsils. Incubation period following B cell infection is 4-6 weeks.
- Infection in epithelial cells is lytic in nature
- Replicates in B cells, and the virus is shed repeatedly into saliva secretions
- This is typically asymptomatic
- Interestingly, the virus produced in B Cells is more efficient in infecting epithelial cells and vice versa
Innate immune response via NK cells and adaptive immune response via humoral IgM and IgG as well as EBNA1 Ab response
This brings about an immune response, as B cells present the viral antigen to CD8+ Cytotoxic T cells in lymphatic tissues, the liver and the spleen. This, in turn, activates the B cells to produce viral capsid IgM (signifying acute infection) before producing viral capsid IgG later in the infection.
- Following resolution of the infection, nuclear antigen IgG (EBNA) is produced by B cells signalling that the acute infection has been resolved (although the virus will still exist latently)
- It is important to note that while it signifies acute infection, following EBV infection viral capsid IgM is high in the blood for up to 120 days and thus may in fact be a marker of a recent infection
In the blood, monocytes recruit cytokines as a response to the infection. This can cause the individual to feel symptoms such as fever and malaise.
B cells infected with EBV produce 'Heterophile antibodies' which can be used as a marker for confirming an EBV infection

Replication:

Replicates in the nucleus of the cell where it is episomal (not integrated into the host genome)

Symptoms of EBV Infection:

Tonsilitis
Pharyngitis
Fever
Headache
Malaise
Cervical Lymphadenopathy
Hepatosplenomegaly
Leukoplakia (white-grey patches generally on the tongue)

Diagnosis:

Diagnosis of EBV infection is mostly clinical
Full Blood Count showing increased and atypical lymphocytes in the blood
Presence of the Heterophile antibodies found by conducting a Monospot test
Testing for EBV specific antibodies (VCA-IgM, VCA-IgG, EBNA IgG)
Real Time PCR of blood/CSF for EBV DNA
Throat swab should be done if there is concern of a bacterial infection (EBV is commonly misdiagnosed as Strep Throat)

Treatment:

Generally symptomatic supportive treatment
- Fluids
- Nutrition
- Rest
- Paracetamol and Ibuprofen
- In some cases corticosteroids are prescribed to bring down the swelling of tonsils to stop them blocking the airway
Interesting point:
- Sometimes, if EBV infection is misdiagnosed as tonsilitis then Ampicillin/Amoxicillin will be prescribed. In the EBV infected patient, this will likely cause a macular rash. Discontinue the treatment to suppress rash. Mechanism is unknown.

Mechanism of Action (Latent & Secondary Infection):

In healthy individuals, between 1 and 100 B cells per million are EBV infected
When memory B cells divide, the EBNA1 protein is expressed in order to establish and maintain the altered state that cells take when infected with EBV. It is the only EBV protein found in all EBV-related malignancies.
EBV reactivation is highly associated with Lymphoma:
- Burkitt Lymphoma
  - Most common between age 5-10
  - Non-Hodgkin Lymphoma, cancer of the centroblasts (activated B cells) of the germinal centres of the lymph nodes and spleen
  - Dysregulation of the c-myc gene (gene 8q24) by one of three translocations - most commonly t(8;14)(q24;q32)
  - Cure rate is 90% in developed countries
- Hodgkin Lymphoma (50% of cases are EBV related)
  - Most common in early adulthood (20s)
  - Derived form the pre-apoptotic germinal centre B cells (normal cells of this type would be destroyed but some slip through)
  - Cancer of the lymphocytes within the lymph nodes.
  - Diagnosis via biopsy, which would show multi-nucleated Reed-Sternberg cells on histological analysis
  - Treatment via chemotherapy
- Post-Transplant Lymphoproliferative Disease (B-LPD)
It is also highly associated with Carcinoma:
- Nasopharyngeal Carcinoma
  - Most prevalent in certain areas including SE Asia, Northern Africa and the Arctic (inuit populations)
  - EBV is present in the epithelial cells not the lymphocytes
  - Latency Pattern II
- EBV Positive Gastric Carcinoma (approx 10% of all GCs)
Latent Antigen Expression:
- EBNA-1 is the only protein expressed by every latent EBV infected B cell. It maintains the viral episome. Transcriptional activation of EBV latent genes by EBNA-1 is essential for EBV transformation of primary B-cells
- EBNA-2 activates both viral and cellular genes, the main one being c-myc
- Latent Membrane Protein 1 (LMP1) produced by latent B cells infected with EBV acts similar to CD40 upregulating the NF-kB and JAK/STAT signalling pathway leading to an exaggerated immune response. It is a classical oncogene (ha the potential to cause cancer)
- LMP2 prevents the lytic infection of EBV infected B cells thus ensuring its survival

Latency Expression Patterns:

EBV latently exists in either B cells or epithelial cells, but the mechanism by which it does so can occur in three different ways. These are called latency patterns, and show which viral proteins and RNA are expressed from the episome during latency. Below shows each pattern and the malignant condition it is associated with:
- Latency I (Burkitt Lymphoma)
  - Proteins: EBNA-1
  - RNAs: EBV-encoded small RNAs (EBERs)
- Latency II (Hodgkins Lymphoma)
  - Proteins: EBNA-1, LMP1, LMP2
  - RNAs: EBV-encoded small RNAs (EBERs)
- Latency III (Post-Transplant Lymphoma)
  - Proteins: EBNA-1, EBNA-2, EBNA-3a, EBNA-3b, EBNA-3c, EBNA-LP, LMP1, LMP2
  - RNAs: EBV-encoded small RNAs (EBERs)

Endemic Burkitt Lymphoma:

Less common in developed world, more common in Africa
Malaria infection reduces resistance to EBV, thus allowing it to take hold
Typically affects the jaw causing distortion of the facial bones
Latency Type I (only EBNA-1 expressed upon replication

Diagnosis of EBV:

EBER RNAs are expressed in all latently infected cells at very high levels. This is an excellent target for in situ hybridisation assays
Excellent antibodies reactive with LMP1 available and work well on formalin-fixed paraffin-embedded sections. However these are not expressed by all EBV-associated Lymphomas

Immune Response to EBV:

During IM there is massive expansion of CD8-positive T-cells directed against EBV
Up to 44% of CD8-positive T-cell have been shown to be reactive with a single epitope during IM
EBV-associated lymphomas occur in individuals who are immunosuppressed (look into papers)

	Criado por Matthew Coulson mais de 4 anos atrás

Honours Degree Microbiology (Medical Virology) Notas sobre Epstein Barr Virus in Health & Disease, criado por Matthew Coulson em 16-03-2020.