What are the main effector functions in complements role in killing of microbes?
opsonisation; leukocyte activation and cell lysis
What complement factor is involved in the activation of leukocytes?
C5a
What are the functions of complement?
killing of microbes; briding innate and adaptive immunity; immune complex processing; removal of apoptotic cells
What are the functions of C5a?
potent anaphylatoxin; chemotaxis; endothelial activation; prothormbotic
What are the functions of C5b-9?
cell lysis; platelet activation; endothelial activation; prothrombotic
How is the classical pathway of complement activated?
by binding of C1q to Fc portions of IgG and IgM or recognises microbial surface directly
How is the lectin pathway of complement activated?
mannose binding lectin and ficolins interacting with bacterial carbohydrate residues
How is the alternative pathway of complement activated?
spontaneously
What is a zymogen?
inactive pro-enzyme which become activated after proteolytic cleavage
What associates with MBL and ficolins to trigger cleavage of complement?
MBL-associated serine proteases (MASPs)
What is C1 comprised of?
recognition protein C1q assocated with proteases C1s and C1s
What do all 3 complement pathways generate?
C3 convertase
What is C3 convertase?
a multisubunit protein with protease activity that cleaves complement component 3 (different types depending by pathway)
what is the main effector molecules of the complemetn system?
C3b
Where does C3 convertase cleave C3?
when it is bound covalently to the pathogen surface releasing C3a while leaving lots of C3b bound to the surface
What is the function of C3a?
binds to specific receptors and helps induce inflammation
What is C3b degraded into?
C3f and C3dg
How does covalent bond formation between C3b and the pathogen surface take palce?
due to highly reactive thioester bond hidden inside the folded C3 protein
What are the components of C3 convertase in the lectin and classical pathways?
C4b2a
What are the compoents of C3 convertase in the alternative pathway?
C3bBb
What is the difference in glycans between yeast and vetebrates?
yeast glycans terminate in mannose resudes rather than sialic acid residues
Where is MBL synthesised?
liver
What is a collectin?
protein that has an amino-terminal collagen-like domain and a carboxy-terminal C-type lectin domain
What is the difference in structure between ficolins and MBL?
ficolins have a fibrinogen-like domain rather than lectin domain
How many types of ficolin do humans have?
L-ficolin (2); M-ficolin (1) and H-ficolin (3)
Where are the ficolins produced?
L and M ficolins are produced by the liver; M-ficolin is procued by lung and blood cells
What happens when MBL bound to MASP 1,2 and 3 binds to a pathogen surface?
MASP-1 is conformationally changed and activates MASP-2 which can cleave C4 and C2
What happens to C4 once activated by MASP-2?
similarly to C3, C4b has a reactive thioester which binds to pathogen surface
What happens to C2 once activated by MASP-2?
C2a remains bound to C4b
What is the function of C4bC2a?
cleaves many molecules of C3 into C3b and C3a
Give examples of other collectins?
surfactant proteins A and D
What is the function of surfactnat proteins A and D?
coat surfaces of pathogens to opsonise
why do SP-A and SP-D not activate complement?
don’t associated iwth MASPs
What happens when the globular heads of C1q bind to ligand?
conformational change in C1r:C1s complex- activating C1r which then cleaves C1s
What does activated C1s do?
cleaves C4 into C4a and C4b which binds to pathogen surface, a molecule of C2 binds to C4b which is then cleaves by C1s to C2a
How does C1q bind to pathogen?
directly; via CRP and binds to the Fc regions of antibodies bound to antigen
What prevents C4b from diffusing from its site of activation to becoming attached to htealth host cells?
the thioester bond is rapidly hydrolysed and irreversibly inactviated
What are the 2 ways that the alternative pathway can be activated?
by the action of the lectin or classical pathways; spontaneous hydrolysis
How can the alternative pathway be activated by the lectin/classical patwhays?
C3b bound to the pathogen surface binds factor B whih is then cleaved by plasma protease factor D into Ba and Bb forming C3bBb`
What is fluid-phase C3 convertase?
C3(H2O)Bb
What stabilises the alternative pathway C3 convertases?
properdin (factor P)
Where is properdin synthesised?
neutrohpils
What is significant about hte alternative pathway?
amplification loop to increase C3b production rapidly
What do ficolins bind to?
acetylated sugar residues
What does a H-ficolin deficiency lead to?
necrotising entercolitis
What does mutations in MASP3 or CL-K1 lead to?
3MC syndrome- developmental abnormalities and mental retardation
What is the active enzyme of the C3/C5 convertases?
C2a
What is DAF/CD55?
decay-accelerating factor - complement-regulatory protein
How does DAF work?
competes with factor B for binding to C3b and can displace Bb from a convertase
What is the function of factor I in complement?
cleaves C3b to iC3b an inactive derivative
What is required for factor I to work?
cofactors- membrane cofactor of proteolysis (MCP/CD46)
Name a protein that has similar actions of DAF and MCP (inhibits convertase formation and inc. breakdown of C3b)?
CR1/CD35- cell-surface complement receptor type 1
What are teh function of factor H?
competes with factor B; acts as cofactor for factor I
What is significant about factor D?
only activating protease of hte complement system to circulate as an active enzyme rather than zymogen
How is C5 convertase generated in the classical and lectin pathways?
binding of C3b to C4b2a
How is C5 convertase generated in the alternative pathwat?
binding of C3b to C3bBb to form C3b2Bb
How does C5 convertase cleave C5?
C5 has to bind to C3b on C5 convertase and can then be cleaved by C2a or Bb
What complement receptor does C3b bind to?
CR1
What is required to activate phagocytosis when C3b binds to CR1?
C5a
What else does C5a bind to aside from CR1?
C5a receptor- a GPCR
What receptors bind forms of C3b which have been cleaved by factor I but which remain attached to pathogen surface?
CR2; CR3 and CRIg
What does CR2 recognise?
C3dg
How is C3dg produced?
C3b is cleaved releasing C3f to form iC3b; which is then cleaved to release C3c leaving C3dg
What cells have CR2?
B cells - amplifies antibody response
What is the function of binding of iC3b to CR3?
sitmulates phagocytosis
Give eamples of circulating regulators of complement?
C1 inhibitor; C4-binding protein and factor H
What is the role of CD59 as a regulator?
prevents assembly of MAC
When does factor H preferentially bind factor C3b?
when it is boudn to vertebrate cells
What is complementopathy?
pathology arising from abnormal complement activation
What diseases can result from overactivation o the complement system?
paroxysmal nocturnal haematuria; atypical HUS; C3 glomerulopathy; macular degeneration; IgA nephroptahy
How does a pore in thel ipid bilayer membrane caause pathogen death?
destroying the proton gradient
which part of the MAC binds to the lipid bilayer?
C7 amphiphilic copmlex
What is the function of C9n?
polymerizes C5b678 to form a membraen-spanning channel, lysing the cell
What part of the MAC inserts into the lipid bilyaer?
C8a-y
what does a deficiency in C5-C9 result in?
vulnerability to Neisseria species
What is C1 inhibitor?
a plasma serine protease inhibitor or serpin
What is the function of C1 inhibitor?
binds to the active enzymes C1r:C1s and causes them to dissociate from C1q and also limits the spontaneous activation of C1 in plasma
What complement levels are low in C1 inhibitor defieicny ?
C4 (being used up a lot)
What does C1INH inhibit in the coagulation system?
factor XI and thrombin
What does C1INH inhibit in the bradykinin system?
kallikrein ( inhibits bradykinin)
What proteins are affected in PNH?
decay-acclerating factor (inactivates membrane C3 convertase); CD59 (prevents MAC assembly)
What treatment can be give to patietns with PNH?
eculizumab- monoclonal ab against C5
What are the risks and benefits of eculizumba?
reduces blood transfusions but increases risk of meningitis (C5 important against neisseria)
What happens in atypical haemolytic uraemic sydnrome?
C5 mediated damage to the renal endothelium leading to glomerular thrombosis
What drug is highly effective in atypical HUS?
eculizumab