Immune Response to Intracellular Pathogens 2

Question 1

Epstein Barr Virus (EBV)

Answer 1

A herpes virus with enveloped dsDNA

• Part of gammaherpesvirus family along with the oncogenic HHV-8 (Kaposi’s Sarcoma)
• Latent or persistant infection following primary infection (reactivation most likely during periods of immunosuppression)
• Infection more serious in immunocompromised patients
• Latency –> lytic during stress

Question 2

EBV discovery

Answer 2

Discovered in 1964

• Lymphocryptovirus with a DNA genome
• Icosahedral core surrounded by a protein tegument and enveloped in lipids, proteins and external glycoprotein spikes

Question 3

EBV associated diseases

Answer 3

Infectious Mononucleosis (a.k.a. Glandular Fever)
- very common in teens

Cancer

• Burkitt’s Lymphoma
• Nasopharyngeal carcinoma

AIDs patients

• Oral leukoplakia
• Chronic interstitial pneumonitis

Question 4

EBV genome

Answer 4

184kb of linear dsDNA producing >85 proteins
- during latency, very few proteins are actually expressed

2 EBV subtypes (EBV-1 and EBV-2)
- only 1% difference in DNA

Question 5

EBV pathogenesis

Answer 5

Ubiquitous infecting 95% of all humans

• if infected in early childhood, generally asymptomatic
• if infected later, infectious mononucleosis (IM) occurs (a Lymphoproliferative disorder [LD] resulting in the expansion of CD8 cells)
• transmission through salivary contact

Latent infection (resting memory B cells, capable of immortalising)

Have several different expression programmes for different stages of life cycle

Question 6

EBV primary infection

Answer 6

Following oral transmission from kissing/inhaling cough droplets, EBV replicates in a permissive cell type like the throat epithelial cells (also targeting B cells?)

• Very high levels of lytic replication occur to produce more virions
• Shedding of high titre virus into throat occurs
• Then infects nearby B cells

Question 7

EBV infecting B cells

Answer 7

Turns them into Lymphoblasts or a Lymphoblastoid Cell Line (LCL)

• they travel to the extra-follicular areas of the tonsils and appear in very high numbers
• 1-10% of all B cells infected during primary infection)

Question 8

Innate immune response to primary EBV infection

Answer 8

Phagocytes (DCs) identify invading pathogens/materials using their germline encoded receptors:

• enters phagocyte’s endocytic pathway and activates TLRs
• Activates phagocyte’s secretion of cytokines to promote an intracellular Th1 response

NK cells identify infected cells (loss of MHC) and kill by injecting toxic cargo
- Also secrete cytokines to activate T cells and enhance phagocytic killing abilities

Question 9

Displaying viral peptides/antigens

Answer 9

In ER, fragments of viral protein bind to MHCI: secretory pathway and expressed on cell surface for T cells to detect

Phagocytic cells (DCs) do this presentation and migrate to secondary lymphoid organ to activate CD8 cells
- some lytic antigen specific; some latent antigen specific

Question 10

CD8 recognising antigens on infected cells

Answer 10

Interaction between T cell Receptor (TCR) and cell’s MHCI:

• All nucleated cells express MHCI so CD8 can kill any nucleated cells
• MHC-antigen complex recognised and bound to
• TCR-MHC binding has low affinity (10^-5M) so a CD8 coreceptor binds to stabilise MHCI (increasing affinity 10-fold)

Question 11

Mass epithelial lining destruction explains pathology of infection

Answer 11

Up to 40% of all CD8 cells are focussing on EBV
- releasing cytotoxic perforin and granzymes

Only 0.1-5% of CD8 cells can respond to latent antigens

Question 12

Immune system state after initial infection

CD8 and CD4 fates/roles

Answer 12

Once foreign antigen is removed, expression of Bcl2 survival proteins decreases: CD8 cells die by apoptosis

CD4 T helper cells, in low number, maintain a memory response against EBV antigens
- predominantly, Th1 is polarised to secret IFNg

Question 13

EBV’s generation of a reservoir of latency

Answer 13

Upon infecting B cells, a Lymphoblastoid Cell Line (LCL) develops:

• EBV alters B cell signalling (using latency transcription programmes) to differentiate them into memory B cells which can last longer than naive B cells
• EBV produce proteins like LMP2a that mimic B cell receptor activators, tricking it to differentiate and immortalise
• Mimic protein LMP1 activates B cell’s CD40 receptor)

Question 14

EBV latent infection (reactivating LCLs)

Answer 14

Certain stresses can reactivate LDLs by altering the gene expression profile:
- LCLs can produce virions and the cycle repeats (virions infect other B cells etc)

However, the immediate adaptive immune response occurs making the re-infection short lived
- enough time to be transmitted, not enough for significant pathology)

Question 15

EBV in Cancer

Answer 15

EBV is associated with 1% of all human cancers. E.g. Burkitt’s Lymphoma (BL):

• Not caused by virus!
• BL is an aggressive cancer appearing in extranodule masses

Question 16

EBV’s involvement in Burkitt’s Lymphoma

Answer 16

1) EBV infection leads to B cell differentiation (led by viral oncogenes)
2) Rarely, a translocational mutation occurs in the cell’s genome
3) The promoter is translocated upstream: overexpressing the oncogene Myc
4) Normal pro-apoptotic/anti-cancer regulators (e.g. BIM) are repressed by the virus so cell proliferation occurs

Question 17

Lymphomas of Immunocompromised

Answer 17

Primary disorders including X-linked Lymphoproliferative disease (XLP)
- 75% of males carrying mutations in XLP gene die within a month of EBV infection

Death occurs due to massive invasion of bone marrow and liver by CD8s, macrophage, LCLs…
- XLP gene normally encodes SAP protein that acts as a ‘break’ for the immune response (negatively regulating T and B cell signalling to limit response)