Lecture 9 - Innate immunity - molecules

Question 1

Why is inflammation important?

Answer 1

A protective response involving host cells, blood vessels, and proteins.

A process occurring as a result of infection of damage (wounding) of a tissue aiming to clear the infection and/or repair tissue damage

A prerequisite for a successful immune response

Also a potentially harmful process: Components of inflammation that are capable of
destroying microbes can also injury bystander normal tissue (if the response is too strong or too long).

Question 2

What is the complement system?

Answer 2

The Complement is a system of soluble pattern-recognition receptors and effector molecules that detect and destroy microbes

Three main groups of receptors initiate the response:

C1q (classical pathway)
Interacts with an antibody coated pathogen surface

Mannose-binding lectin (MBL) and ficolins (lectin pathway)
Recognise carbohydrates on pathogen surfaces

C3 (alternative pathway)
It undergoes spontaneous hydrolysis leading to deposition of C3 convertase on pathogen surface

Question 3

What is the complement activation cascade?

Answer 3

Circulating proteins react with each other to fight infection

A number of complement proteins are inactive precursor proteases (zymogens) that are themselves activated by proteolytic cleavage.

At sites of infection, the complement system is activated through a triggered-enzyme cascade:
An active complement enzyme generated by cleavage of its zymogen precursor cleaves its substrate (another complement zymogen) to its active enzymatic form.

The activation of a small number of complement proteins at the start of the pathway is hugely amplified resulting in the rapid generation of a disproportionately large complement response.

Strong regulatory mechanisms to prevent uncontrolled complement activation

Question 4

What are the functions of the complement?

Answer 4

The complement helps antibodies and phagocytes clear microbes from an organism in three ways:

Opsonisation
enhancing phagocytosis

Chemotaxis
some complement proteins act as chemoattractants to recruit more phagocytes to the site of complement activation

Pathogen lysis
complement components damage certain bacteria by creating pores in the bacterial membrane.

Question 5

How do innate immune cells recognise pathogens?

Answer 5

broad Pathogen Associated Molecular Patterns (PAMPs)
are recognised by
a small number of Pattern Recognition Receptors (PRRs)

Question 6

What are PAMPs?

Answer 6

Found on (nearly) all members of a pathogen class
e.g. Lipopolysaccharide (LPS) on the surface of Gram negative bacteria
or double-stranded RNA in dsRNA viruses

Essential for microbial survival

Question 7

What are PRRs?

Answer 7

Just a few proteins can be used to sense millions of pathogens!

All cells express some PRRs
Dendritic cells and macrophages express all PRRs

Question 8

What are the 4 families of PRRs?

Answer 8

Toll-like receptors (TLR)

Nucleotide-binding oligomerization domain (NOD)-like receptors (NLR)

C-type lectin receptors (CLR)

RIG-1 like receptors (RLR)

Question 9

What are Toll-like receptors?

Answer 9

Localised at the plasma membrane or in the membranes of endosomes

Broad range of specificities

Question 10

What are NOD-like receptors (NLR)?

Answer 10

Cytoplasmic proteins

Form multiprotein complexes known as inflammasomes

Question 11

What are C-type lectin receptors (CLR)?

Answer 11

Localised at the plasma membrane

Broad range of specificities

Recognise glycans from the wall of fungi or some bacteria

Question 12

What are RIG-1 like receptors (RLR)?

Answer 12

Cytoplasmic proteins

Sense viral RNA

Signal through mitochondrial adaptor proteins

Question 13

Explain how TLR activation depends on where it is

Answer 13

TLR4 is the only TLR localised on the membrane and endosome

MyD88 and TRIF are important adaptors

IRF pathway leads to production of type I interferons

NF-kB and AP1 pathways lead to production of inflammatory regulators

Question 14

Describe NF-kB induction of cytokines and chemokines

Answer 14

NF-kB activation downstream of PRRs leads to induction of transcription of:

Chemokines that attract other adaptive and innate immune cells

Cytokines that promote inflammation (e.g. IL1b, TNFa, IL6, IL8)
many of them are induced very rapidly – early

Question 15

Describe how IRF induces transcription of IFNb , IFNa, and ISGs

Answer 15

Following PRR activation IRF3 and IRF7 induce transcription of IFNb and IFNa (type I interferons)

Type I IFNs is translated and secreted and binds the IFNAR (Interferon A Receptor) – positive feedback loop

This induces the second wave of interferon stimulated genes (ISGs) through the action of other transcription factors of the IRF and STAT families

Question 16

Describe Type I interferon function in immune responses

Answer 16

Inhibition of pathogen (virus, parasite) replication through inhibition of protein translation in the infected cell

Induction of cell-death (apoptosis) of infected and neighbouring cells

Activation of other innate immune cells (e.g. NK cells)

Activation of the antigen presentation machinery (particularly in dendritic cells and macrophages) -> initiation of adaptive immunity

Question 17

What are the two key pathways in innate immune activation?

Answer 17

The type I interferon pathway: driven by activation of the IRF family transcription factors leading to production of IFNb and other interferon stimulated genes (ISGs).

The NF-kB pathway: driven by activation of the NF-kB transcription factor leading to production of cytokines (e.g. IL1b, TNFa, IL6).

Transcription of several inflammatory cytokines and chemokines as well as surface receptors can be activated downstream of either of these pathways.

Question 18

Describe how the innate immune gene expression programme induces its own negative regulators

Answer 18

anti-inflammatory cytokines – e.g. IL10

phosphatases that stop the signalling cascade – e.g. dephosphorylation of p38 by Dual Specificity Phosphatases (DUSPs)

intracellular factors that inhibit inflammatory transcription factors – e.g. IkB prevents NFkB from going to the nucleus

microRNAs that suppress translation of inflammatory mRNAs – e.g. suppression of TRAF6 and IRAK1 by miR-146a

These genes are usually induced at the later stages of the innate immune response

Prolonged immune response can lead to tissue damage and disease caused by the immune system rather than the pathogen

e.g. non-cytopathic viruses (example: hepatitis viruses) that establish chronic infection leading to chronic inflammation