Dengue and Hepatitis C Immunopathogenesis

Question 1

What does immunopathogenesis mean?

Answer 1

How a virus causes disease is driven by the immune system interactions

Question 2

What is tropism determined by?

Answer 2

• Cellular receptors for virus
• Other cellular factors required for replication
• Physical barriers
• Local temperature, pH, inactivating factors

Question 3

How does HCV evade immune clearance?

Answer 3

• Evasion of innate immune responses such as interferon induction/signalling
• Lack of sterilising antibody responses

Question 4

How does HCV evade antibody responses?

Answer 4

• Viral mimicry - virions mimic low-density lipoproteins to evade antibody targeting
• Viral hiding - cell-to-cell spread occurs via tight junctions to avoid exposure to antibodies
• Shifting antigens - shifting glycosylation patterns on envelope proteins means the immune system has to produce new antibodies for these new antigens

Question 5

Describe the interferon system

Answer 5

• One of the ways viruses can be detected by the cell is RIG-I which detects dsRNA
• RIG-I signals to MAVS
• Eventually activates a complex which activates the production of interferon
• Viral nucleic acids can be detected by TLRs present in the endosome or on the cell surface
• TLRs signal through TRIF and MYD88, culminating in the activation of IRF3 and IRF7 and the induction of interferon
• Interferon is excreted from the cell and recognised by IFN receptors
• Get activation and phosphorylation of Tyk2 and JAK1
• Leads to dimerisation and phosphorylation of STAT
• STAT is a transcription factor that forms a complex with IRF9 and activates interferon stimulated genes in the nucleus that protect the cell from viral infection

Question 6

How doe HCV evade the interferon system?

Answer 6

• NS3/4a viral protease cleaves RIG-I to block RIG-I-mediated interferon induction and cleaves MAVS to block RIG-I-mediated interferon induction - NS3 binds TBK1 to prevent binding with IRF3 and interferon induction
• NS4b blocks STING to prevent interferon induction
• NS5a binds MyD88 to block TLR signalling and interferon induction and binds a number of interferon stimulating genes to block antiviral effects
• Core protein directly interacts with STAT1, preventing STAT1 phosphorylation and activation
• Envelope protein binds the ISG PKR to block antiviral effects
• Replication in membranous compartments means its hidden from immune sensors

Question 7

How do cytotoxic immune cells drive hepatitis immunopathology?

Answer 7

• NK cells kill infected cells through perforins and granzymes which would normally help clear the infection but as hepatocytes are constantly replicating, new cells can be infected with HCV
• CD8+ cytotoxic T cells destroy infected cells through perforins and the destructions allows the virus to infect the newly generated cells - eventually the T cells will get exhaustion as they are seeing the same antigen repeatedly which reduces the effectiveness - HCV can also evade these cells by changing their epitope
• T reg cells regulate cell-mediated immune responses and attempt to dampen the T cell response to this chronic infection to prevent further liver damage

Question 8

How do cytotoxic T cell responses drive hepatitis immunopathology?

Answer 8

• CD8+ cytotoxic T cells kill infected hepatocytes and produce cytokines - prolonged exposure to viral antigen leads to increased expression of exhaustion markers, which reduces proliferation and degranulation and reduced cytotoxicity
• Regulatory T cells suppress CD8+ cytotoxic T cells to allo long-term persistence of HCV
• HCV has a high mutation rate that allows for epitope evasion and escape from cytotoxic killing
• Allows for viral persistence
• Leads to long-term liver damage - balanced by high proliferative capacity of liver
• Chronic inflammation leading to fibrosis and cirrhosis - eventually leads to end stage liver disease

Question 9

How do antibodies enhance dengue infection?

Answer 9

• Antibody-dependent enhancement of DENV infection contributes to haemorrhage
• Higher replication and more severe disease in secondary infection wit different serotype
• There is cross reactivity between the serotypes which increases infection
• Antibodies bind to the virus on the variable region and immune cells bind to the antibody on the Fc receptor, aiding the viral infection of immune cells

Question 10

How does dengue evade the interferon system?

Answer 10

• NS2B/3 viral protease cleaves STING to block interferon induction
• NS2A reduces STAT1 phosphorylation
• NS4A reduces STAT1 phosphorylation
• NS4B reduces STAT1 phosphorylation
• NS2A, NS4A and NS4B work synergistically
• NS5 binds and degrades STAT2
• Replication in membranous compartment hidden from immune sensors

Question 11

How does sequential infection with different dengue serotypes increase risk of haemorrhage?

Answer 11

• There is a window where the antibodies present will bind to the new serotype and increase uptake into macrophages
• This increases viral load and increases haemorrhage risk

Question 12

How does a cytokine storm lead to haemorrhage?

Answer 12

• Macrophages infected with dengue produce cytokines
• A positive feedback loop means immune cells produce more cytokines - cytokine storm
• Some cytokines cause vasodilation which can lead to haemorrhage