10. Dengue virus immunity and vaccination 1 Flashcards
What is the global burden of dengue?
- It is the most abundant and rapidly spreading arboviral infection.
- Estimated 390 million infections with 96 million clinically apparent.
- 500,000 cases of dengue hemorrhagic fever and dengue shock syndrome. These are more severe forms of dengue.
- Causes 20,000 deaths annually.
- Mostly exists in tropical and sub-tropical areas but it is spreading.
What type of virus is dengue virus?
- An orthoflavivirus
- ssRNA virus
What are examples of orthoflaviviruses?
- Dengue virus
- Zika
- Yellow fever
- West Nile Virus
How is dengue virus transmitted?
- By the Aedes mosquitoes.
- Aedes aegypti is the main vector
- Aedes Albopictus is the secondary vector
How many dengue serotypes are there?
- 4 infectious serotypes
- They cocirculate.
- This provides a challenge for vaccine design.
How is Dengue fever treated?
- There are no specific therapies.
- Only general pain killers and fluids can be given
What vaccines are available for dengue virus?
- 2 partially protective vaccines
- Dengvaxia by Sanofi-Pasteur
- Qdengue by Takeda
- Dengvaxia has some serious problems.
Are cases of dengue fever increasing or decreasing?
Increasing
Where is dengue spreading to geographically?
- Places it shouldn’t and hasn’t been.
- Coastal regions of Europe like Italy and Spain.
- South America
Why is dengue spreading out of tropical regions?
- The dengue vector Aedes Albopictus is now endemic in Europe due to rising temperatures.
- Travellers return from dengue endemic regions with the infection and can pass the infection to the vectors.
- This then causes local transmission and spread of the virus in Europe and other places.
- The winters are still too cold for the vector so infection drop but climate change increases this risk.
- South and Central America is mostly effected by this.
What are the 4 clinical stages of Dengue Virus?
- Asymptomatic
- Self-limiting dengue fever
- Severe dengue: Dengue hemorrhagic fever and dengue shock syndrome
What is the normal course of dengue infection?
- mosquito bite
- Around 5 days of incubation
- Then viremia starts to develop alongside fever.
- After 5 days is the biggest risk of developing severe dengue.
- around 10% of cases become severe dengue
When does severe dengue manifest?
- Around day 5 of infection
- Symptoms start to appear
- This is later in infection, so the viral load is decreasing, but the immune response is increasing.
What are the forms of severe dengue?
- Dengue hemorrhagic fever
- Dengue shock syndrome
- These occur in the minority of cases, but the effects can be debilitating and last a long time.
What is non-severe dengue?
- A normal dengue infection.
- symptoms include vomiting, rashes and aches.
- Some symptoms act as warning signs for severe dengue.
- This includes abdominal pain, fluid accumulation or liver enlargement.
What are the symptoms of severe dengue?
- Severe plasma leakage due to increased vascular permeability.
- This leads to hypervolemic shock, severe bleeding and organ impairment.
- This is caused by a massive immune mediated cytokine storm.
- It is not clear what part of immunity mediates severe dengue
What are the host risk factors for severe dengue?
- Secondary dengue infection
- Age - very young or old
- Co morbidities like hypertension and diabetes.
- Obesity
- Genetic polymorphisms, SNPs.
How does secondary dengue infection increase the risk for severe dengue?
- There are 4 different stereotypes of dengue circulating
- Infection with a different serotype at the same time or later. (heterologous infection)
- This risk of severe dengue increases for the 2nd exposure massively.
- After two exposures, the risk decreases again, suggesting immunity develops to protect from subsequent infections.
How does SNPs increase the risk for severe dengue?
- Some SNPs in genes important with T cell responses and cytokines can increase the risk of severe dengue.
- MICB is the main one. It was found through a GWAS study.
- Also PLCE1, TNFa, IL-10, HLA class 1 and 2
What is heterologous dengue infection?
When someone is infected with a different serotype of dengue then they were previously infected with.
What are the key features of the adaptive immune response?
- Consists of T and B lymphocytes and their secreted products.
- It is a sophisticated antigen-specific defence system.
- Immunological memory
How do T cell recognise antigens?
- MHC/HLA molecules.
- MHC1 presents cytosolic proteins via the TAP pathway to CD8+ T cells.
- MHC2 presents extracellular proteins and activate CD4+ T cells.
How do T cells recognise viral antigens?
- Viruses are intracellular pathogens so their proteins are found in the cytosol.
- This means they are leaded via the TAP pathway into MHC1.
- These are then presented to CD8+ T cells to activate them.
What are the 3 phases of a primary T cell response?
- Expansion
- Contraction
- Memory
What happens in the contraction phase of the T-cell response?
- Around 5-10% of T cell from the main response survive and become memory cells.
- IL-7Ra (CD127) is important for this persistence without antigen stimulation.
- This allows the T cells to respond to omostatic cytokines, which allows persistence.
When does the T cell response peak?
- After the peak of viral infection
- After this peak most T cells die.
How many naive T cells are there for any given antigen?
- Around 1 in 100,000 T cells.
- This is very low frequency
How many memory T cells are there for any given antigen?
- Memory T cell precursors are much more common than naive T cell precursors.
- They also have a lower activation threshold
- This is an advantage of immune memory.
- The frequency of memory cells depends on the pathogen.
What are the main characteristics of T memory cells?
- They clonally expand from naive precursors.
- They have a lower activation threshold then naive cells.
- They mount secondary responses and confer immediate protection. This response is bigger then a naive response.
- Memory cells persist for a lifetime in the absence of antigen.
- Heterogeneous for effector functions and migratory capacities. There are lots of different subsets.
What does the immune response to primary dengue infection look like?
- Bite from mosquito and infection.
- 5 day incubation that leads to viremia
- Day 0 is when symptoms appear.
- Peak of viremia is around day 2/3.
- Peak of T cell response is around day 8/9. About 6 days later.
- Antibodies also develop in patients.
- IgM response starts around day 1.
- IgG response appears around day 5/6.
What does the immune response to secondary dengue infection look like?
- Bite from mosquito and infection.
- Viremia peaks higher and earlier than in primary infection. Around Day 0
- Viremia is shorter due to pre-existing antibodies that can clear the infection.
- Risk point for developing severe dengue is around day 5.
- The immune response is increasing as viremia decreases.
- Vascular permeability increases and peaks when the risk is highest for severe dengue.
What is severe dengue associated with?
- A higher dengue virus load early in infection.
- This is independent of dengue serotype or host immune status.
What is the structure of the dengue virus proteome?
- It is an ssRNA virus that is encoded by 1 polyprotein.
- There are 3 structural proteins. The capsid, membrane and envelope proteins.
- There are 7 non-structural proteins: NS1, NS2a, NS2b, NS3, NS4a, NS4b, NS5.
- The non-structural proteins encode enzymes important for viral lifecycle and replication.
What is the structure of the dengue virus particle?
- Mature dengue has a smooth surface made by repetitive E protein pattern.
- Immature dengue has a spikey surface made by the prM precursor peptide.
- This immature structure prevents premature membrane fusion.
- The cleavage of prM depends on pH and is done by host or viral proteases.
How were dengue T cell epitopes determined?
- Using ELISpot assays.
- These are used to detect and measure immune cells that produce antibodies or cytokines.
- It is good as it is high throughput.
- The limitation is that you don’t know which type of T cell is causing the response.
What dengue epitopes were recognised by T cells?
- All dengue proteins were recognised to some degree.
- The T cell response to non-structural proteins was bigger especially to NS3 and NS5.
- Structural proteins still also trigger an immune response.
Does dengue cause a CD4 or CD8 T cell response?
- Using flow cytometry and intracellular cytokine staining you can characterise the T cell response.
- The majority of the response is a CD8+ response to non structural proteins.
- NS3 and NS5 produce the largest immune response.
- There is still a CD4+ T cell response that mainly targets the Capsid and envelope proteins.
What regions in the dengue virus polyprotein trigger the immune response?
- The T cell response can be mapped to the dengue proteome and map how frequently a peptide region is recognised.
- 50% of the T cell responses to dengue target 25 antigenic regions across NS3 and NS5.
- This shows the T cell response is broad but also targeted.
- This is important to consider for vaccine design.
How does dengue activate the immune response?
- The virus enters through the skin.
- The dengue antigens are picked up by Langerhans cells which present antigens to T cells.
- The T cells recognise the antigen presented by the Langerhans cell which also imprint skin-homing signals so the T cells go back to the skin.
- This is an evolutionary response to increase the chance of antigen re-encounter.
- This homing signal is CLA and all dengue specific T cells express CLA.
- Later in infection they start to express CXCR6 which homes to the liver and other tissues.
- This aids dengue spreading through the blood to the liver, and T cells migrate to these secondary sites.
What are Langerhans cells?
Skin resident dendritic cells
What other immune cell is involved in dengue infection?
- Mast cells
- They degranulate to limit infection
- They can have a role in vascular permeability changes in severe dengue.
- They also produce cytokines to recruit T cells.
How do we know adaptive immunity has a role in dengue immunopathology?
The fact that heterologous dengue infection is a big risk factor for severe dengue.
What are the 2 ways pre-existing antibodies can act in secondary dengue infection?
- They can be cross-reactive and strongly neutralising and efficient at clearing dengue virus.
- Not specific enough to neutralise dengue so non-neutralising antibodies.
How do non neutralising antibodies cause antibody dependent enhancement of dengue infection?
- Non-neutralising antibodies don’t coat the viral particle properly but can still bind to dengue.
- The Fc part of the antibody binds to FcR and then the dengue virus can be internalised into phagocytes and replicate.
- This leads to higher viral loads.
- This is Fc receptor mediated viral entry and causes ADE.
What are the 3 types of antibodies that cause antibody-dependent enhancement (ADE)?
- Cross-reactive low affinity antibodies.
- Low antibody concentrations.
- Antibodies targeting specific epitopes.
What is the main antibody epitope involved in ADE?
- Anti prM antibodies.
- prM is the membrane precursor protein for dengue.
- These particles are not infectious but if antibodies bind the immature virus prM they can then enter cells through Fc-FcR binding.
- This makes the immature virus particles infectious so they can replicate.
What are the 2 types of ADE?
- Extrinsic ADE
- Intrinsic ADE
What is Extrinsic ADE in dengue?
The increase viral entry into cells due to Fc-FcR binding and non-neutralising antibodies.
What is intrinsic ADE in dengue?
- This is the modulation of the immune response once the antibody-virus complex is internalised.
- This is the blocking of the type 1 interferon response.
- This creates a more favourable environment for viral replication.
How does intrinsic ADE modulate the immune response to dengue?
- The antibody virus complex binds to activation Fc receptors
- This includes FcyR1, FcyR2a, FcyR3.
- These should activate the anti-viral response but they don’t.
- It causes a drop in Type 1 IFN responses and leads to ADE.
- There is decreased type 1 interferon signalling and production of interferon stimulated genes.
How does dengue binding to LILRB1 inhibit the FcyR induced anti-viral response?
- When the virus is not properly coated by antibodies some viral receptors are left open.
- These bind to LILR-B1 which triggers Syk dephosphorylation.
- This prevents signalling for type 1 interferon production and interferon stimulated genes.
- This increases viral replication inside cells and triggers ADE.
Do pre-existing antibodies increase the risk of severe dengue or ADE?
- A paediatric cohort was followed for 12 years and blood samples were collected every time they were sick.
- If these kids had antibody levels within a specific window, that put them at risk of severe dengue.
- The risk decreases against in really high antibody levels.
What else can increase the risk of severe dengue?
increased levels of afucosylated IgG
What is afucosylated IgG?
- 94% of circulating IgG is fucosylated.
- Fucosylation is a post translational modification.
Why does afucosylated IgG increase the risk of severe dengue?
- It has an increase affinity for FcyR3a/b.
- We don’t know why but patients with severe dengue have more afucosylated IgG of specific dengue antibodies or total antibodies.
What is antigenic sin in dengue infection?
- In primary infection, T cells develop to the specific dengue serotype.
- On secondary infection with another serotype, T memory cells can be activated due to the lower activation threshold and higher frequency.
- Dengue memory cells for other serotypes can recognise, clear and lyse infected cells in secondary infection.
- OR these T memory cells can be sub-optimally activated as the TCR has lower affinity for the dengue antigens.
- This virus is not cleared, and the cells are not lysed. This can cause cytokine storms and plasma leakage.
What are the differing roles of T cells during dengue infection?
- T cell could cause immunopathology and poor viral clearance and cytokine storms.
- They can also cause immune protection and viral clearance.
- This depends on the situation and T cell affinity.
What are T cell response like in severe dengue?
- T cell responses to all the dengue proteins is bigger in severe dengue patients.
- Particularly NS3.
How does T cell function change in primary and secondary dengue infections?
- CD107a shows degranulation capacity hasn’t changed.
- But in secondary infection patients the T cells how more cytokine production and less cytotoxic functions.
- Dengue could cause impairments of cytotoxic functions.
- It could also skew the T cell response towards cytokine production.
How does MICB SNPs increase the risk for severe dengue?
- MICB is a stress ligand expressed during dengue infection and other stress.
- SNPs in MICB increase the risk of developing severe dengue.
- MICB binds to NKG2D which activated CD8 and NK cells and is important for cytotoxic functions.
- Dengue could be compromising these cytotoxic functions.
What is antigenic sin?
- When the epitope of an infection is similar to the epitope of a previous similar infection, the immune system relies on the memory of the previous infection rather than mounting a new primary response.
- This can result in an inadequate immune response to the infection as the immune system is not adapting to the new infection.
- This is antigenic sin
- This can apply to vaccines where the vaccine can act as this first infection.
- This can cause antibody dependent enhancement of disease
