4 - Innate 2: Complement Flashcards
What is complement
Group of proteins arranged in a biochemical cascade that aid in elimination of pathogens through mediating inflammation, opsonization, and direct lytic attack
Complement is a major component of ____ and ______ immune responses
Primitive and advanced
Is complement used by innate or adaptive immune responses?
Both
Complement consists of
16 different serum proteins and glycoproteins comprising 10% of total serum protein
What % of total serum protein is complement
Aroun 10%
Where is complement produced
Mostly in the liver
3 initiating complement activation pathways:
- Classical
- Alternative
- Lectin
List 3 complement activation pathways & what immune system they correlate to
- Classical - adaptive immune system
- Alternative - innate immune system
- Lectin - innate immune system
The classical pathway is initiated by
IgG or IgM (needs antigen presenting cell)
The alternative pathway is initiated by
Microbial surface vicinity (** and can also amplify classical cascade**)
Lectin pathway is initiated by
Mannose-binding lectin
Which pathway is the most ancient
Lectin
Do all 3 pathways result in the same outcome
Yes
How does complement kill invaders?
Pathogen presents foreign surface —> proteins recognize foreign stuff—> initiates common terminal pathway —> complement forms a big pore in pathogen which kills it
Opsonization
The coating of a particle with host protein
The classical activation pathway is activated by
Antigen-antibody complex (specifically IgG1, IgG3, and IgM)
In the classical pathway, immobilized antibody is recognized by
The C1 complex through C1q units
Classical pathway - once the immobilized antibody is recognized by the C1 complex through C1q units, what happens?
C1q binds to the complex, then undergoes a conformational change to activate C1r which starts cleaving subunit C1s. C1r is the first enzyme in the cascade
The classical activation pathway initiation is dependent on
Ca2+
Classical pathway - once C1 protease is active, what does it do next?
Cleaves C4 and C2
Classical pathway - once C4 and C2 are cleaved by active C1, what happens
Cleaved components C4b and C2a form classical C3 convertase C4b2a which is associated with the activating surface
Classical pathway - once classical C3 convertase C4b2a is formed, what happens next?
C4b2a cleaves C3 into C3a and C3b
Classical pathway - once C3a and C3b have been cleaved, what happens
Both C4b and C3b covalently bind to the activating surface via thiolester linkage
Classical pathway - a proportion of C3b will complex with
C4b2a to form the C5 converting complex C4b2a3b
Classical pathway - which molecules are “sticky”?
C4b2a and C4b2a3b - both stay physically attached to pathogen
Lectin pathway initiation
Antibody independent initiation (not specific). Mannose-binding lectin (MBL) behaves similarly to C1q. Binds to sugary residues common on pathogen surfaces.
MBL binds to
Mannose, fucose, and N-acetyl glucosamine (NAG) that are commonly present in cell walls of microbes.
Lectin pathway - MBL binding induces
Conformational change that induces autocatalysis of MBL-associated serine proteases (MASPs) that subsequently cleave C4 and C2.
Lectin pathway - Once the MASPs cleave C4 and C2, what happens
Classical pathway!
What is different between Classical and Lectin activated pathways?
Initiation event
Alternative activation pathway - relies on
A low-level serum hydrolysis of C3
Alternative Activation pathway - C3 in serum is
Continually hydrolyzed to C3(H20)
Alternative pathway - C3(H20) comes into contact with Factor B and then
Turns in to C3(H20)B which then recruits factor D
Alternative Pathway - once C3(H20)B is formed, what happens?
It recruits Factor D, and Ba is cleaved off. This creates C3(H20)Bb, which is also called Fluid-phase C3 convertase
Alternative Activation Pathway - hydrolysed C3 in the presence of an “activator surface” binds
serum factors that form the C3 convertase C3bBB
Alternative pathway - once C3bBb has been formed, what does it do?
C3bBb hydrolyses C3 into C3a and C3b, which establishes an amplification loop in the absence of regulators
Alternative pathway - Once C3b has been cleaved, what does it do?
C3b complexes with C3bBb to form the C5 converting complex C3bBb3b (which is an alternative C5 convertase).
Alternative Activation pathway - how does the body prevent activation on body cells?
Host cells possess surface carbohydrates and regulators that inactivate C3b, so they have non-activating surfaces
Is the alternative pathway inherent to the classical pathway?
Yes, as it provides an amplification loop for C3 deposition
Alternative pathway - which molecules are sticky
C3b - will stick to any surface nearby
What is the terminal membrane attack pathway?
Classical, Lectin, and Alternative C’ pathways end with the formation of a C5 convertase complex (C4b2a3b & C3bBb3b)
Terminal Membrane Attack Pathway - After formation of C5 convertase complex (C4b2a3b & C3bBb3b), what happens?
Cleavage of C5 to C5b initiates pathway involving C6, C7, C8, and multiple copies of C9 to form a rigid pore (Membrane Attack Complex (MAC) within the lipid bilayer of the target
Terminal Membrane Attack Pathway - once the rigid pore/Membrane Attack Complex has been formed, what happens?
The MAC in the lipid bilayer of the target allows influx of solutes and electrolytes leading to osmotic swelling and sometimes lysis.
- a hole is punched in the surface of the pathogen which kills the pathogen
Terminal Membrane Attack Pathway - in which order do C6, C7, C8, and C9 bind?
Numerical - have to go C6-C9 and each protein goes farther into the membrane than the protein before it (ie C9 goes deepest)
Outcomes of complement activation:
- Opsonization of particles with proteins (C3b)
- Chemotaxis and activation
- Increased vascular permeability
- Target osmodysregulation and lysis
- Enhances adaptive immunity
- Immune complex clearance
Outcomes of complement - Opsonization
C3b deposition enhances binding and uptake of microbes via CR1 and CR3 in professional phagocytes - enhanced clearance
Outcomes of complement - Chemotaxis & activation
PMNs and Macrophages migrate along C3a and C5a gradients to site of complement activation
outcomes of complement activation - increased vascular permeability
C3a and C5a triggers degranulation of Mast cells
Mast cells release vasoactive amines that increase vascular permeability and local blood flow
Outcomes of complement - enhances adaptive immunity
Adjuvant effect in priming humoral immune response
Outcomes of complement - immune complex clearance
Disaggregates immune complexes and aids erythrocyte-mediated transport and clearance
Outcome: Direct target Lysis
Complement product: MAC
Action:
Osmodysregulation and lysis of target cells
Outcome: Tissue Inflammation
Complement products: C3a & C5a
Action
Activation of mast cell degranulation leading to release of vasoactive amines (histamine and serotonin)
Outcome:Endothelial Activation
Complement product: C3a & C5a
Increased expression of adhesion molecules
Outcome: Chemotaxis
Complement product: C3a & C5a
Action:
Promotes migration of neutrophils, eosinophils, and macrophages towards the site of complement activation
Outcome: Leukocyte Activation
Complement product: C5a (C3a & C4a)
Action:
Upregulation of adhesion moleculwes, phagocytes receptors, and antimicrobial effectors by neutrophils and monocytes
Outcome: Opsonization
Complement product: C3b & iC3b
Action:
Enhancement of particle phagocytosis by macrophages and neutrophils
Outcome: Promotion of humoral responses
Complement product: C3dg
Action:
Enhanced B cell activation. Retention of antigen complexes in B cell follicles
Outcome: Immune complex clearance
Complement product: C3b (and iC3b)
Action:
Blocks growth and facilitates dissociation of immune complexes. Immobilization and clearance of immune complexes through interaction with CR1 on erythrocytes
How are the complement pathways regulated?
The classical, lectin, alternative, and terminal attack pathways can all be stopped or downregulated at key points by soluble or membrane-associated complement control proteins
Regulation of complement activation prevents
The complement innate defense system from acting on inappropriate targets, and also from acting in perpetuity
Regulation of complement activation - alternative activation pathway works by:
Host membranes are rich in sialic acid residues (not present on pathogen surfaces) that promote binding of C3b to factor H.
Factor H, along with Factor I, inactivates and degrades inappropriately bound C3
On host surfaces, spontaneously bound C3b is
Destroyed as fast as it is deposited
SLIDE 19 !!! PICK 3 AND MEMORIZE THEM !!!!! HE SAID IN LECTURE!
Good luck & don’t f it up
Can you be deficient in complement
Yes - genetic
Deficiencies in components of the classical pathway have what effect
Little or no increased risk of infection. This suggests alternative and lectin pathways are sufficient for controlling infection
Deficiencies in classical pathway components are associated with increased risk of
SLE (systematic Lupus Like) autoimmune disease thought to be due to impaired clearance of immune complexes by monocyte macrophage system
A factor H deficiency results in
Unregulated activation of alternative pathway
In pigs, factor H deficiency leads to
Type 2 membrane proliferation glomerulonephritis due to deposition of C3 components in glomerulus of kidney
What are acute phase proteins
Proteins that increase or decrease in plasma concentrations in response to inflammation
APP are mostly synthesized in
Liver in response to cytokines (particularly IL-6)
APP actions
Maintains homeostasis of tissue
Reduces tissue damage associated with inflammatory process
Aids in propagating inflammatory rxn
Regulation of immune response
Protection against infection
Aids in tissue repair
Positive APP (increase when animal’s infected)
C-reactive protein (CRP)
Serum Amyloid A (SAA)
Haptoglobin
Alpha-1-acid glycoprotein (AGP)
Ceruloplasmin
Fibrinogen
Negative APP (goes down when animal infected)
Albumin
Transferrin
What is the major APP in primates, dogs, and pigs
C-reactive protein (CRP)
CRP binds to
Phosphocholine, polysaccharides, and glycolipids commonly found on surface of bacteria, parasites, and damaged cells
CRP also binds to
For subsets on neutrophils and macrophages - increased clearance by phagocytosis
What pathway does CRP induce
Classical pathway - binds C1q then induces cytokines and has both pro and anti-inflammatory actions
CRP inhibits
Chemotaxis and modulation of neutrophil function
