Innate immune response to microbial challenge Flashcards
What is the whole early immune system in vertebrates?
Innate immune system
What are the key concepts of the innate immune system?
Prevents infection
May eliminate infection with or without interaction with acquired immune system
Early response to infection (minutes/hours)
May drive adaptive immune responses
No memory
Recognises microbial conserved structures that aren’t found in host (PAMPs)
Doesn’t react with self
Targets products needed for microbial survival
Components encoded in germ line
What are the main components of the innate immune system?
Physical barriers- skin and mucociliary escalator
Cellular barrier- immune active (secretions)
Circulating effector leukocytes- monocytes/macrophages, neutrophils and NK cells
Circulating proteins- complements, collectins and pentraxins (CRP) and antimicrobial peptides (defensins
Commensal organisms- gut microbiome, cytokines and local enzymes
What are NK cells?
Specialised T lymphocytes
What are NK cells triggered by?
IL-12 and IL-15
What do NK cells detect?
Missing self- loss of MHC class I and kill infected and malignant cells
What do NK cells secrete?
Cytokines- mainly IFN-g (top of the cytokine cascade)
What do the cytokines released by NK cells do?
They activate macrophages
They activate receptors and signal via receptor protein tyrosine kinase.
What do mannose receptors on phagocytic cells bind to?
Mannose and fucose receptors on microbial glycoproteins and lipids
What other type of receptors are found on phagocytic cells?
Scavenger receptors including CD14
How do phagocytes cause opsonisation?
Complement (especially C3), antibody and ingestion into vesicles
What effect do macrophages have on mycobacterial infection?
Macrophages can disseminate mycobacterial infection
What do Toll-like receptors do?
They give specificity to the innate immune system, they recognise and discriminate specific microbial components
They activate the proinflammatory cascase
What do TLRs interact with?
Various adapter molecules such as CD14 and MyD88
They act intracellularly and extracellularly
What are the proinflammatory cytokines?
TNF-alpha
IL-1
These are at the top of inflammatory cytokine cascade- release of these leads to release of many
What regulatory cytokines are there?
IFN-g
IL-12
These activate adaptive immunity
What down-regulatory cytokines are there?
IL-10
TGF-b
What chemoattractants are there?
Chemokines
What is cytokine secretion like including regulation?
It is transcriptionally regulated and relatively transient
What are the 4 main properties of cytokines?
Pleiotropism- when a single gene affects a number of phenotypic traits within the same organism
Synergism
Antagonism
Redundancy- some don’t do anything
What is the major source of TNF-alpha?
Mononuclear phagocytes
When is TNF alpha secreted?
Early in infection
What does TNF-alpha do?
Mediates response to LPS (Gram-negative sepsis)
Causes cachexia and septic shock
Affects muscle function- depresses the heart
Drives metabolic disturbances
Decrease blood glucose
Emerging interaction between metabolism and immunity
What are the two main TNF-alpha receptors?
p75 TNF
p55 TNF
What do the death domains on some of the TNF receptors do?
They activate death signalling pathways involving caspases leading to cell death- it can cause necrosis or apoptosis
How does NF-kB exist?
It sits in the cytoplasm as a heterodimer of p65 + p50 bound to its inhibitor (IkB
What happens to NFkB when a pathogen triggers intracellular signalling via a receptor?
The IkB is phosphorylated, ubiquitinated and degraded
What does the degradation of IkB allow?
It allows the NFkB to pass from the cytoplasm to the nucleus where it binds to specific sites and leads to gene transcription
Where are NFkB binding sites found?
On nearly all pro-inflammatory genes
What is NFkB binding to its receptor like?
It will occur in a matter of minutes and for many cytokines and chemokines, it is essential for activity
What effect does p65 have by itself?
It can be activating
What effect does p50 and p52 have either by themselves or as bigger molecules?
Inhibitory
What is the classical IkB protein?
IkB-a
What often binds to NFkB to keep it in its active state?
IkB-b so instead of being inhibitory, it is maintaining NFkB as a more active molecule
How do transcription factors control inflammatory gene regulation?
There are transcription factor binding sites adjacent to the genes
What regulatory sites does mRNA have apart from transcription factor binding sites?
Mainly UA rich regions at the 3’ end- this provides stability to the mRNA
The more UA rich sequences you have, the longer the mRNA survives and hence more protein is produced from that mRNA sequence
Give a brief summary of what happens when interferon binds to IFN receptors?
This leads to phosphorylation of JAKs which then leads to activation of STAT1 which heterodimerises to activate the genes in the nucleus
What are interferon-g deficient patients very vulnerable to?
Mycobacterial infections and salmonella
What do chemokines do?
Recruit leukocytes to areas of infection and inflammation- important in angiogenesis as well
What are the major two groups for the majority of chemokines?
CC- two adjacent cysteine residues
CXC- two cysteine residues that are separated by another amino acid
What cysteine molecules are responsible for the classification of chemokines?
There are four cysteine residues in the chemokine molecules but it is the proximal two that give them their classification as CC or CXC
What is lymphotactin?
Small family of chemokines that have a single cysteine residue
What is fractalkine?
CX3C chemokine
What do CC do?
They classically recruit mononuclear cells (monocytes and lymphocytes)
Give some examples of CCs?
MCP-1 (CCL2)
RANTES (CCL5)
What do CXCs tend to do?
They tend to be neutrophil chemoattractants:
IL-8- neutrophil attractant
IP-10 (CXCL10)- T cell attractant
Describe what leukocyte recruitment is like?
They will roll along endothelial wall. If the endothelium is activated it will bind to selectin molecules (loose binding). It will then bind more tightly via integrins (tight binding). They then transmigrate through the cells into tissues
What is Il-8 (CXCL8) released by?
Neutrophils and monocytes
What is IL-8 associated with?
It is present in animal models of sepsis- raised plasma conc of IL-8 is associated with poor prognosis in sepsis.
In what direction does chemotaxis occur?
Generally down concentration gradients (measured using Boyden chambers)
How does haplotaxis work?
There are chemokines attached to tissues but at different concentrations and cells can migrate down that tissue-bound gradient
What are anchored chemokines and what are they used for?
Potential adhesion molecules- a chemokine is bound to a cell or GAG and it sits there acting like an adhesion molecule for the cell that it attracts
What is the chemokine receptor?
G protein coupled receptor
What mimics chemokine?
Pathogens. Herpes virus express ECRF3- mimic of IL8Rbeta
Why would viruses want to express the chemokine receptors?
The virus would want more chemokine around it so that it’s able to spread from one cell to the next. Also by expressing chemokine receptors on the cell surface it acts as a sponge that soaks up the chemokines and it stops the leukocytes being recruited to the site of infection
What are the pathways of activation for the complement pathway?
Alternative- activated by microbe
Classical- activated by antibody
Lectin- activated by lectins e.g. Mannan-binding lectin
What are the early effects of complement activation?
Inflammation (C3a) and opsonisation and phagocytosis (C3b)
What are the late effects of complement activation?
Inflammation (C5a)
Lysis of microbes (Membrane attack complex- MAC)
What can the tadpole tail do?
It produces an enzyme (collagenase) so it breaks down collagen
What do you need to cause inflammatory tissue damage?
Enzymatic digestion
What is collagen broken down by?
Matrix metalloproteinases (MMPs)
What are matrix metalloproteinases?
Family of zinc containing proteases
How are matrix metalloproteinases classified?
By substrate specificity: Gelatinises Collagenases Stromelysins MT-MMPs
What do matrix metalloproteinases play a key role in?
They are key in tissue remodelling and embryonic development, they also have important immunological functions for leukocyte migration- it breaks down tissue to carve a pathway for leukocytes to migrate. Activate/deactivate a range of cytokines and chemokines
What happens when there is excessive MMP activity?
It is destructive- rheumatoid arthritis and emphysema
How are matrix metalloproteinases involved in tuberculosis?
They are involved in leukocyte recruitment to sites of infection, they drive tissue destruction and are required for spread of infection- pulmonary cavitation creates an immune-privileged site. They act in networks- granulomas are formed in TB
What types of collagen are broken down by MMP-1?
Types 1-3
What collagen is broken down by MMP-9?
Type 4
Which MMP is critical for causing tissue destruction in TB?
MMP-1
What is cavitation?
A key feature of human TB and that results from an up regulation of protease activity leading to lung extracellular matrix destruction
What are defensins?
They are found in phagocytic and endothelial cells. At least 9 different human defensin genes. Microbicidal at micro molar conc- stored in granules. They target bacteria, viruses and fungi. They form pores in the cell membranes.
What are cathelicidins?
They have a variable peptide structure with conserved N-terminal cathelin domain. Found in phagocytic and epithelial cells. They often undergo proteolytic cleavage- stored as pro-forms. Release is vit D dependent
What are the mechanisms of avoiding innate immunity?
Antigenic variation- neisseria and salmonella
Inhibition of complement activation- mycobacterium tuberculosis
Resistant to phagocytosis- pneumococcus and yersinia
Scavenging of reactive oxygen intermediates (ROIs)- catalase-positive staphylococcus aureus
Inhibit phagolysosome formation- mycobacterium tuberculosis
Escape from phagosome into cytoplasm- listeria monocytogenes
