Lecture 5 - Innate immunity: Immediate response to infection Flashcards
Describe the steps of the inflammatory response:
Surface wound introduces bacteria which activates resident effector cells to secrete cytokines
Vasodilation and increased vascular permeability allow fluid, protein, and inflammatory cells to leave blood and enter tissue
Infected tissue becomes inflamed causing redness, heat, swelling, and pain
How do innate and adaptive immune responses differ?
innate response works at the local level whereas adaptive immune response has a greater systemic effect and forms memory
How do barriers prevent pathogens from crossing epithelia and colonizing tissue?
They act as mechanical barriers
They have chemical properties such as pH and antimicrobial enzymes that make them tough to move past
Microbiome compete with the pathogen reducing their virulence
What are some chemical barriers to entry on the skin?
Fatty acids and antimicrobial peptides
What is the complement system?
Soluble proteins constitutively made in the liver that are present in the blood lymph and extracellular fluids and are involved in the first wave of immunity.
What do complement proteins do?
One of the fastest responses of the body they mark pathogens for destruction.
Why is the bacterial capsule so important to their defence against the complement protein?
It prevents the attachment of complement proteins and therefore prevent phagocytosis
How does the complement protein mark the pathogen?
zymogens are circulating in blood (circulating inactive complement).
Complement cleavage is triggered by bacterial cell surface thus activating it
Infection triggers activation and cascade of enzymatic reactions
One fragment binds to the bacterium while the other attracts phagocyte in that direction
Complement fragments meet and this induces phagocytosis
What form do complement proteins circulate?
Zymogen form which means inactive form.
What does C3b do and what does C3a do?
C3a recruits phagocytes
C3b tags for phagocytosis (complement fixation)
What bond does inactive C3 have?
Thioester bond and this bond is stabilized within hydrophobic interior
What happens when C3 is cleaved?
C3b exposes thioester bond to hydrophilic environment
How does C3b work?
C3b exposes thioester bond to hydrophilic environment which reacts with hydroxyl and amino groups on pathogen’s surface.
What are the 3 pathways for complement activation?
Classical (antigen binding complement)
Alternative (pathogen surface creates environment for complement activation)
Lectin pathway (Mannose-binding lectin binds to pathogen surface)
All 3 pathways recruit C3 activation into C3a and C3b
Which of the 3 complement activation pathways are considered a part of both innate and adaptive immune responses?
The classical pathway (the antigen specific pathway)
At the start of infection which complement is activated?
Alternative pathway
Where is inactive C3 made?
In the liver
What is the first step of the C3 activation pathway?
In plasma close to microbial surface thioester bond of C3 spontaneously hydrolyses at low frequency iC3.
What is the pathway of Complement activation?
Inactive C3 is produced in liver
Inactive C3 is hydrolysed spontaneously near the surface of a microbe forming the product iC3
iC3 binds factor B
Serine protease factor D cleaves B and produces soluble C3 convertase aka iC3Bb which activates more C3 molecules by cleaving C3 into C3a and C3b
C3b fragments bind to pathogen surfaces
What can C3b that is bound to pathogen surfaces do?
It can bind to factor B to facilitate cleavage by factor D
What are the 2 convertases produced by the C3 complement system?
iC3Bb and the alternative C3 convertase
What does the alternative C3 convertase do?
It produces a feedback loop creating more C3a and C3b complexes which provide a rapid bacterial coating
What are the 2 broad categories of complement control proteins?
Plasma proteins that interact with C3b attached to microbial surfaces
Membrane proteins on human cells that prevent complement fixation on host cell surface
What measure is taken to ensure that complements are attached only to pathogens and not human cell surfaces?
Regulatory proteins
What does factor P do?
Stabilizes the alternative C3b convertase (C3bBb) on the surface of the pathogenic cell extending lifetime on cell surface
What is another name for factor P?
Properdin
How is the complement pathway inactivated?
Factor H comes in and alters the structure of C3b so that I can cleave it rendering it inactive
How is C3bBb prevented from degrading human cells?
On the cell membrane there are proteins known as decay-accelerating factor (DAF) and Membrane Cofactor Protein (MCP)
What is opsonisation?
Coating of bacterium in C3b which facilitated phagocytosis (mostly macrophages)
What is the difference between neutrophils and macrophages?
Neutrophils are short lived and die after phagocytosis macrophages stay alive and continue moving around.
What are the first responding cells after pathogen invasion?
Macrophages (first wave of immune response involves complement activation so this is interesting for the big picture)
What are macrophages?
Long lived phagocytic cells that participate in innate and adaptive immunity
Where are macrophages typically found/
Connective tissue
GI tract
Respiratory tract
Alveoli of the lungs
Liver
What is another name for “mature forms of circulating monocytes that have left blood for permanent residence in tissue”?
Macrophages
How do phagocytes know which proteins have C3b?
C3b interacts with CR1 (Complement receptors) on phagocytes which triggers uptake of the bacterium.
What do the intracellular signals of CR1 do?
They facilitate the uptake of the pathogen and then facilitate the fusion of the phagosome with the lysosome that contain toxic molecules and degradative enzymes which ultimately lead to the bacterium being killed
Which complement proteins form hole in pathogenic cell membrane?
C5, C6, C7, C8, C9
How is a pore formed in the pathogenic cell membrane?
New convertase is produced from the alternative C3b2Bb convertase converting C5 into C5a and C5b.
C5b initiates assembly of terminal complement components to form membrane attack complex
By binding C6 C7 C8 successively the conformation of C7 and C8 change and are able to insert into the membrane. This causes membrane damage and polymerisation and insertion of C9. 16 C9 molecules start to follow and this results in a transmembrane channel
Channel disrupts bacterial outer membrane which kills the bacterium
How is Membrane Attack complex prevented on self cells?
CD59 binds to the C5b678 complex and prevents recruitment of C9 to form the pore.
Homologous Restriction Factor works in the same way
What are C3a and C5a classified as?
anaphylatoxins
What do anaphylatoxins do?
Increase inflammation at the site of the activation by binding to receptors on several cell types.
Is inflammation a major response to infection?
Yes
Why are C3a and C5a called anaphylatoxins?
Anaphylactic shock occurs when they are present in large numbers
What affect do anaphylatoxins have on blood vessels?
They increase blood vessel permeability which allows leakage of RBCs and WBCs into tissues
What triggers coagulation?
Damage to blood vessels
What is the purpose of clots?
Immobilizes microbes to prevent entry into the bloodstream and decreasing blood loss
How do platelets promote antimicrobial defence?
They release prostaglandins, hydrolytic enzymes, and growth factors to promote antimicrobial defence, wound healing and inflammation.
What are kinins?
Enzymatic cascade that is triggered by tissue damage
Increases supply of immune mediators to infection site
What do protease inhibitors do?
Prevent the proteases bacteria produce from damaging body proteins
How do alpha2 macroglobulins prevent damaging proteases from their actions?
They set a trap.
Alpha macroglobulins contain highly reactive thioester bond.
Microbial proteases are produced in host body which are attracted to bait sequence.
When protease cleaves the bait the alpha macroglobulin binds protease covalently through activation of thioester group.
How do antimicrobial peptides kill bacteria?
They destroy their membrane.
What are defensins?
major family of antimicrobial peptides of 35 - 40 amino acids
What amino acid are defensins rich in?
rich in arginine residues
How do defensins work?
They penetrate microbial membranes and disrupt integrity of membrane.
What pathogens can defensins destroy?
Can destroy bacteria fungi and enveloped viruses
How diverse are defensins?
Significant diversity within and between individuals
How does human beta 1 defensin work?
Electrostatic attraction to bilayer allows it to enter the bilayer and move the membrane around themselves thereby creating holes in pathogenic membranes.
What are paneth cells?
cells in microvilli valleys that produce defensins
How do defensins avoid destroying host cells?
mucous layer prevents them from touching intestinal cells.
What do pentraxins do?
Bridge molecules that connect host cells to pathogen to create cross linking signals for phagocytosis.
(i.e like antibodies)
What type of proteins are pentraxins?
Cyclin multimeric proteins circulating in blood and lymph
