Complement Cascade Flashcards
Complement system
Collection of circulating and cell membrane proteins
complements the humoral branch of innate and adaptive immunity
coordinated enzymatic cascade –> seqeuntial proteolytic cleavage of C’ proteins (zymogens)
C’ functions
triggers and amplifies inflammation rxns
attraction of phagocytes by chemotaxis
clearance of immune complexes
cellular activation
Direct microbial killing
Development of humoral responses
what are the components activated by cleavage into peptide fragments
smaller fragment “a” - away
- anaphylatoxins
- diffuse AWAY from site
- helps initiate localized inflammatory response
larger fragment “b”
- active complement component
- BINDS to target near site of activation
what does a bar over a symbol mean in the nomenclature of C’ system
it means the complex (such as C4b2b3b) is activated and has enzymatic activity
Complement system activation differs in the stimulus that triggers the cascade but all merge into a common pathway leading to the formation of what?
lethal membrane attack complex (MAC)
What are the three different C’ activation pathways?
Classical
Alternative
Lectin
What activates the classical pathway
antibody binding to antigen
antibody defines the target antigen
this is part of humoral arm of adaptive immunity
Alternative pathway
Independent of antibody
- not necessary for the host to have had prior exposure (so innate)
- constant trickle (surveillance)
- don’t have exquisite specificity that you have to have in classical pathway
Lectin Pathway
activated when a carbohydrate-binding plasma protein, mannose-binding lectin (MBL), binds to terminal mannose residues on the surface glycoproteins of microbes.
component of innate immunity
What are the initiation steps of Classical pathway
Antibodies bind antigens
C1 component
What is C1 and what does it bind to?
C1 is a hexamer (6 globular domains) associated to two C1s and C1r molecules
binds to antigen/antibody complex
binds 1 IgM
binds 2 IgG
***Classical pathway
what does C1qrs cleave?
C4
C2
generates an amplification b/c it cleaves and activates many C4 and C2 molecules
C4b2b
aka C3 convertase
cleaves many molecules of C3
responsible for the distinction b/w self and non-self b/c C3b deposits deposits rapidly on non-self surfaces which leads to opsonization
self surfaces limit the deposition of C3b
C4b2b3b
aka C5 Convertase
this brings in C5 and cleaves it to C5b and C5a
initiates the final series of events leading to MAC formation
C5b67 complex
Inserts itself into the membrane and forms a small pore
eventually leads to slow lysis of target cells
association of C5b67 can occur in fluid phase as well as on membrane… so that means this complex can diffuse and insert itself into other near by membranes
C8
associated with C5b67
C8 recruits 10-16 copies of C9
C9 forms hole in membrane and leads to lysis of cell
MAC
C5b67 with C8 and C9
Alternative pathway
Protects from pathogens in the absence of antibody
slow. less efficient
initiation of alternative pathway?
spontaneous conversion of C3 to C3b in serum
if there is a microbe in the area then C3b will deposit itself onto the membrane surface of the microbe
How does C3 identify self from non self
Self membranes
- high levels of sialic acid
- rapidly inactivates bound C3b molecules on host cells
Non self membranes
- bacterial cell walls, yeast walls and viral envelopes
- low levels of sialic acid
- bound C3b will remain active longer
what does factor B bind to
binds to deposited C3b on microbe surface
alternative pathway
cleaved by Factor D
Complex C3bBb
aka C3 convertase
cleaves more C3 into C3b
alternative pathway
what does factor D do…
cleaves factor B into Bb and Ba
alternative pathway
Alternative C5 convertase?
C3bBb3bP
formed when a second C3b binds to the C3BbP complex (which is the C3 convertase stabilized by a P)
Lectin pathway
innate immune response (antibody independent)
another surveillance method
plasma contains Collectin Family
- Mannose-binding lectin (MBL) or ficolins
- MBL- associated serine proteases (MASPS)
what is MBL
used in lectin pathway
Mannose binding lectin attaches to microbes
structurally similar to C1 in classical pathway and acts to activates C4
what is MASP in the lectin pathway equivalent to in the classical pathway
C1qrs complex
so it needs to be activated and then cleaves C4 as well as C2
Activation of lectin pathway
MBL binds mannose residues on bacterial surface polysaccharides
MBL/MASP complex cleaves C4 and C2 components
steps following are similar to classical pathway in that it leads to formation of C3 convertase
C’ regulation
required to prevent:
- consumption of components (C shit) through unregulated amplification
- protect host and host self cells
regulation occurs during activation, amplification, and membrane attack
Two main effector functions of regulation
binding with complement molecules to cause dissociation
proteolytic digestion to decrease half life
first control of convertases is…..
decay acceleration- components of complement under go spontaneous inactivation as they diffuse away from the site of activation
rapid hydrolysis of C3b
also happens to C4b
Factor H
prevents the formation of C5 convertase
Factor H/C3b complex is also a target for Factor I
binds to C3b only if the C3b has been deposited on the surface of a host cell
Protected site concept of C3b
Deposition of C3b on a microorganism
C3b is then protected from Factor H–> activates the alternative pathway
C3b on a host cell is a target for factor H and subsequently for factor I
S protein (Vitronectin)
binds to fluid phase form of C5b67
prevents binding to membranes
DOES NOT prevent association with C8 and the C9 proteins so still building a MAC just not poking holes in anything
C1 inh
binds Clr and Cls in classical
removes MASP from MBL complex in lectin path
prevents these from becoming active so helps in early regulation of C’
Factor I
C3b inactivator
cleaves C4b or C3b
requires cofactors such as:
C4-bp, MCP or CR1
prevents formation of C3 or C5 convertases
C4-bp
prevents C4b from associating with C2b
also causes C4b to dissociate from C3 convertase
cofactor for Factor I
CR1
binds to C3b molecules on the surface of cells
allows cleavage by factor I
mechanism for distinguishing between self and non-self b/c pathogens don’t have self markers
MCP (CD46)
co-factor for factor I
binds to either C4b or C3b
MCP is found on “self” membranes
“self” verses “non-self” recognition
DAF
Promotes dissociation of the C3 convertase
C2b from C4b in classical
Removes Bb from C3b in alternative pathway
CD59
Blocks C9 binding to C5bC678 complex on the cell surface
Functions of complement
Opsonization and phagocytosis/ waste management
Osmotic lysis of microbes (via MAC)
Stimulation of inflammatory reactions (due to anaphylatoxins the “a” C’ particles)
Opsonization
WShower of C4b and C3b in the area of activation
- coats antigens/microbes
- these C4b or C3b bind to one of the complement receptors (CR1-CR4) on phagocytic cells
- leads to enhanced phagocytosis
ALSO
Anaphylatoxins (C5a in particular) increased the number of complement receptors on the cell surface which greatly facilitates their phagocytosis of C3b-coated antigens
Opsonins in body
C3b (main)
C4b
C5a anaphylatoxin
increases the number of complement receptors on the cell surface which greatly facilitates their phagocytosis of C3b-coated antigens
Waste management (steps?)
Removal of immune complexes from circulation
1) Immune complexes form in circulation (Antibody/antigen)
2) IC is opsonized with C3b/C4b
classical or alternative
complement binds to C’ receptors of the Fc portion of the antibody (C3b)
3) IC binds to RBC C’ receptor I
4) RBC’s then travel to liver and spleen where they offload opsonized antibody C’ complex to phagocytes in liver or spleen
very large complex in capillaries (immune complex disease)
Antibodies bind…
bind extracellular antigens
Cell Lysis
done with the MAC
can do this to:
microorganisms
erythrocytes (just takes 1 MAC)
nucleated cells (requires multiple MAC’s)
what is responsible for the inflammatory response of complement?
Anaphylatoxins -the small diffuseable “a” fragments
Functions of Anaphylatoxins
Bind to receptors on mast cells and blood basophils–> degranulation/histamine release
induce smooth muscle contraction
-increase vascular permeability
induce monocytes and neutrophils to adhere to vascular endothelial cells (by upregulating adhesion molecules on endothelial cells)
CHEMOTACTIC
C5a is most potent in mediating these proceses (C3a and C5b67 can do this to)
C2a
Prokinin
cleaved by plasmin to yield kinin
which results in edema
C3 deficiency
life-threatening problems
severe recurrent infections soon after birth
central role of C3b plays in opsonin of infectious particles
Deficiencies in Factor H and Factor I mimic C3 why?
b/c unregulated C3b generation completely exhausts C3 from the serum
MAC deficiencies (C5-C9)
Generally healthy
except increase infection by Neisseria gonorrhea and N. meningitidis
deficiencies in early C’ activation
C1, C2, C4
C2 deficiency most common
pt’s have high degree of systemic lupus erythematosis (SLE)
can’t efficiently clear circulating immune complexes
IC’s deposit in blood vessel walls and tissues
C1inh deficiency
C1 is not regulated properly –> so C4 and C2 levels are low
physical trauma and emotional stress and precipitating factors
HANE (hereditary angioneurotic edema)
(swelling of face, GI tract, extremities)
treat with androgens (transcriptional expression problem)
DAF deficiency
causes Paroxysmal nocturnal hemoglobinuria (PHN)
-bright red urine in the morning
spontaneous episodes of RBC lysis
RBC’s, leukocytes, and platelets have increased sensitivity to C’ lysis (MORE MACs)
Treat with Erythropoietin
DAF normally promotes dissociation of C2b from C4b and Bb from C4b
what are the levels of C’ stuff when you have a DAF deficiency??
what is causing edema in the HANE
c2a or other anaphyatoxins
Viruses and C’ pathway
can be used to enhance infectivity
Epstein-Barr virus
uses CR2 as a receptor for attachment
Measles virus
uses MCP (CD46) as a receptor
West nile virus
C3b coats viral particle
gains entry into cells via the CR3 receptor
classical pathway deficiencies result in…
tissue inflammation
why?
Deficiencies of MBL are associated with…
infections in infants
Alternative pathway and C3 deficiencies are associated with…
bacterial infections
Terminal pathway deficiencies …
predispose to Gram-negative bacterial infections
C1 inhibitor deficiency leads to …
hereditary angioedema
Deficiencies in alternative pathway regulators produce …
a secondary loss of C3
