Innate Immunity and Viral Immune Evasion

Question 1

How do TLRs detect viruses?

Answer 1

TLRs are able to recognise viruses that are found in the endosome and when the viruses are broken down with proteases, they release some of their nucleic acid. These can be recognised by the TLRs including TLR3, TLR8, TLR7 and TLR9.

Question 2

What are the adaptor molecules that can interact with the TLRs?

Answer 2

The Myd88 molecule can interact with TLR8, TLR7 and TLR9 while TRIF interacts with TLR3.

Question 3

What happens when the TLRs interact with their adaptor molecules?

Answer 3

Both Myd88 and TRIF lead to the activation of transcription factors such as NFkB, IRF7 and IRF3. These transcription factors normally reside within the cytosol but when they become activated, they are going to move into the nucleus to stimulate transcription.

Question 4

What molecules do NFkB, IRF3 and IRF7 produce?

Answer 4

NFkB produce pro-inflammatory cytokines such as IL-6 and TNF-a while IRF3 leads to the transcription of type I interferons (IFN-a/b).

Question 5

Which molecules are able to recognise RNA?

Answer 5

There are two cytosolic RNA helicases that are specifically there to recognise viral RNA, particularly triphosphate RNA that is not capped. These are RIG-1 and MDA5 and these are always present in the cell.

Question 6

What are RIG-1 and MDA5 good at recognising?

Answer 6

RIG-1 is good at recognising - strand RNA viruses but can also recognise + stand viruses. MDA5 is much better at recognising the + sense RNA viruses as dsRNA intermediate complexes are formed.

Question 7

What do RIG-1 and MDA5 interact with?

Answer 7

They interact with the adaptor molecule MAVS on the mitochondrial membrane and this interaction is facilitated via the CARD (caspase activation and recruitment domain). The activation of MAVS induces the dimerisation and phosphorylation of IRF3 and IRF7 as well as NFkB. This results in the production of type I interferons as well as pro-inflammatory molecules.

Question 8

What is able to recognise cytosolic DNA?

Answer 8

Cytosolic DNA is foreign to our cells and acts as a danger signal for virus infection. We have many DNA receptors in the cytoplasm to monitor and detect foreign DNA. cGAS (cyclic-GMP-AMP synthase) is a host protein that recognises foreign DNA in the cytoplasm.

Question 9

What does cGAS interact with?

Answer 9

cGAS generates cGAMP that binds to STING in the ER and this in turn, activates TBK1, NFkB and IRF3 to produce IFNs and cytokines. cGAMP can also be secreted from infected cells into a neighbouring cell and that induces an anti-viral response that can have a bystander effect.

Question 10

Can mitochondrial DNA also be sensed?

Answer 10

DNA that can be sensed is also mitochondrial as many viruses will infect the mitochondria and will leak mitochondrial DNA into the cytoplasm, which can be detected by the host cell.

Question 11

What happens when type I interferons bind to their cytokine receptors?

Answer 11

Type I interferons (a/b) are able to bind to their receptors and the interaction phosphorylates Jak1 that then phosphorylates the receptor itself. This leads to the recruitment of Stat1 and Stat2 transcription factors. These heterodimerise, become phosphorylated and move from the cytoplasm into the nucleus. They bind to the ISRE (interferon stimulated response element) that goes on to produce a whole range of interferon gene products.

Question 12

What happens when the type II interferons bind to their cytokine receptors?

Answer 12

You still get the phosphorylation of Jak1 and Jak2 and the phosphorylation of the receptor. However, instead of STAT1 and STAT2, you get a homodimer of STAT1. This then undergoes phosphorylation and translocation to the nucleus that binds to GAS (gamma activation site).

Question 13

What are your interferon stimulated gene products?

Answer 13

MxA protein, OAS and RNaseL and protein kinase R (PKR).

Question 14

What does MxA do?

Answer 14

This recognises viral capsids. It sequesters the nucleocapsid in bundles and is used widely as a marker of infection.

Question 15

What does OAS and RNaseL do?

Answer 15

This induces the OAS monomer to form a tetramer. The OAS tetramer makes a nucleotide that induces the inactive RNaseL monomer to dimerise making it active. The active RNase L then degrades the viral RNA. This RNase is always present in cells but in very small amounts (inactive).

Question 16

What does protein kinase R do?

Answer 16

The inactive PKR monomer contains a dsRNA binding domain. Upon binding dsRNA, PKR is phorylated and dimerises making it active. Seeing as it is a kinase, it is able to phosphorylate the host cell translation factor (Elf2a), stopping its activity and this stops all host cell translation as well as the viral protein translation.
- It is a shut down, lockout mechanism to halt viral replication.

Question 17

How do viruses countermeasure this?

Answer 17

Many of the viruses actually target the TLRs and completely halt the sensing of the viral nucleic acid in the cells, thereby limiting the downstream effects. Other viruses can interfere with RNA recognition, helicase activation, MAVS signaling and can prevent the activation of transcription factors. The same can be done for the JAK-STAT pathway and almost every virus has devised a way to prevent PKR activation and activity.

Question 18

How does the flavivirus, West Nile virus inhibit IFN signaling?

Answer 18

This virus prevents phosphorylation through redistributing cholesterol from the plasma membrane. You have lipid rafts that arise at the plasma membrane and they influence the signaling molecules via the interferon receptor. It can be rescued through providing exogenous addition of cholesterol.

Question 19

Which viruses prevent nuclear transport?

Answer 19

The SARS CoV, measles, mumps and most likely Norovirus. If the transcription factors can’t get into the nucleus, they can’t produce the interferon stimulated genes.