Innate immunity, complement & phagocytosis – Ilse
Jongerius
2-5
Lectin pathway: Initiated by
mannose binding lectin (MBL)
and ficolins that recognise
carbohydrate structures on
microbial surfaces. MBL-
associated serine proteases
(MASPs) associate with MBL
and ficolins and trigger the rest
of the pathway.
Classical pathway: Initiated by
C1 (consists of recognition
protein C1q and proteases C1r
and C1s). C1q recognises
microbial surface or
antibodies.
Alternative pathway:
Spontaneous hydrolysis and
activation of C3 (bind directly
to microbial surfaces).
Any pathway C3 convertase is activated cleaves C3 C3b (main effector) & C3a (small peptide
binds receptors induce inflammation).
C3b binds covalently to surface acts as opsonin phagocytes can destroy microbe.
C3b can also bind to C3 convertase C5 convertase is formed cleaves C5 into C5a and C5b.
C5b helps form the membrane attack complex (MAC).
C3b: Thioester bond can react with hydroxyl or amino group on microbial surface. no
bond hydrolyse thioester inhibition alternative pathway
Key activated complement components are rapidly inactivated unless they bind to a
pathogen (otherwise potentially dangerous).
Regulatory proteins protect host cells by preventing activation on their surfaces.
o Unless they undergo apoptosis (helps limiting risk autoimmune response).
2-6
Microorganisms have PAMPs (pathogen associated molecular patterns).
Gram positive: Lipoteichoic acids.
Gram negative: Lipopolysaccharide.
Mannose binding lectin (MBL) is a receptor that circulates in blood and other fluids and triggers the
lectin pathway and is synthesised in the liver. Amino terminus: collagen-like domain, carboxy
terminus: lectin domain (protein called collectins). In the blood it consists of 2-6 trimers (main form
, trimers and tetramers). 1 MBL has low affinity (for mannose, fucose, GlcNAc) but multimers have a
high total binding strength (avidity) for repetitive carbohydrate structures.
Production increased during infection (acute-phase response).
Ficolins (more abundant dan MBL): Have a fibrinogen-like domain and a collagen-like domain.
Fibrinogen-like domain gives the specificity for oligosaccharides with acetylated sugars (not
mannose). We have 3 forms L & H (synthesised by liver) and M (lung and blood).
MBL in plasma forms complexes with MBL-associated serine proteases MASP-1, MASP-2 & MASP-3.
MBL binds with a pathogen surface conformational change MASP-1 it cleaves and activates
MASP-2 it cleaves C4 and C2.
Ficolins also form complex with MASP-1 and MASP-2 etc.
When C4 is cleaved (also thioester bond) releases C4a C4b has reactive thioester binds to
microbial surface binds C2. C2 is cleaved by MASP-2 producing C2a binds to C4b to form C4b2a
(=C3 convertase in lectin pathway). This cleaves many C3 into C3a and C3b.
Defect in MBL or MASP-2: Respiratory infections by common bacteria in early childhood.
SP-A and SP-D (surfactant proteins): Collectin proteins, coat surface of pathogens for
phagocytosis. No complement (no association with MASPs).
2-7
Classical pathway uses C1 as pathogen sensor. C1q is the pathogen sensor and C1r and C1s are serine
proteases. C1q is a hexamer of trimers. Amino terminal: globular domain, Carboxy terminal:
collagen-like domain. C1r and C1s are closely related to MASP-2.
C1q recognises a pathogen conformational change in C1r, C1s complex activation of
autocatalytic enzymatic activity in C1r cleaves C1s to active serine protease. C1s cleaves C4 to
produce C4b (leading to C4b2a again).
C1q can bind to the surface of pathogens in different ways:
Binding directly to surface components (proteins cell wall, lipoteichoic acid gram positive).
Binding to C-reactive protein (acute phase protein, binds to phosphocholine residues on the
bacterial surface)
Jongerius
2-5
Lectin pathway: Initiated by
mannose binding lectin (MBL)
and ficolins that recognise
carbohydrate structures on
microbial surfaces. MBL-
associated serine proteases
(MASPs) associate with MBL
and ficolins and trigger the rest
of the pathway.
Classical pathway: Initiated by
C1 (consists of recognition
protein C1q and proteases C1r
and C1s). C1q recognises
microbial surface or
antibodies.
Alternative pathway:
Spontaneous hydrolysis and
activation of C3 (bind directly
to microbial surfaces).
Any pathway C3 convertase is activated cleaves C3 C3b (main effector) & C3a (small peptide
binds receptors induce inflammation).
C3b binds covalently to surface acts as opsonin phagocytes can destroy microbe.
C3b can also bind to C3 convertase C5 convertase is formed cleaves C5 into C5a and C5b.
C5b helps form the membrane attack complex (MAC).
C3b: Thioester bond can react with hydroxyl or amino group on microbial surface. no
bond hydrolyse thioester inhibition alternative pathway
Key activated complement components are rapidly inactivated unless they bind to a
pathogen (otherwise potentially dangerous).
Regulatory proteins protect host cells by preventing activation on their surfaces.
o Unless they undergo apoptosis (helps limiting risk autoimmune response).
2-6
Microorganisms have PAMPs (pathogen associated molecular patterns).
Gram positive: Lipoteichoic acids.
Gram negative: Lipopolysaccharide.
Mannose binding lectin (MBL) is a receptor that circulates in blood and other fluids and triggers the
lectin pathway and is synthesised in the liver. Amino terminus: collagen-like domain, carboxy
terminus: lectin domain (protein called collectins). In the blood it consists of 2-6 trimers (main form
, trimers and tetramers). 1 MBL has low affinity (for mannose, fucose, GlcNAc) but multimers have a
high total binding strength (avidity) for repetitive carbohydrate structures.
Production increased during infection (acute-phase response).
Ficolins (more abundant dan MBL): Have a fibrinogen-like domain and a collagen-like domain.
Fibrinogen-like domain gives the specificity for oligosaccharides with acetylated sugars (not
mannose). We have 3 forms L & H (synthesised by liver) and M (lung and blood).
MBL in plasma forms complexes with MBL-associated serine proteases MASP-1, MASP-2 & MASP-3.
MBL binds with a pathogen surface conformational change MASP-1 it cleaves and activates
MASP-2 it cleaves C4 and C2.
Ficolins also form complex with MASP-1 and MASP-2 etc.
When C4 is cleaved (also thioester bond) releases C4a C4b has reactive thioester binds to
microbial surface binds C2. C2 is cleaved by MASP-2 producing C2a binds to C4b to form C4b2a
(=C3 convertase in lectin pathway). This cleaves many C3 into C3a and C3b.
Defect in MBL or MASP-2: Respiratory infections by common bacteria in early childhood.
SP-A and SP-D (surfactant proteins): Collectin proteins, coat surface of pathogens for
phagocytosis. No complement (no association with MASPs).
2-7
Classical pathway uses C1 as pathogen sensor. C1q is the pathogen sensor and C1r and C1s are serine
proteases. C1q is a hexamer of trimers. Amino terminal: globular domain, Carboxy terminal:
collagen-like domain. C1r and C1s are closely related to MASP-2.
C1q recognises a pathogen conformational change in C1r, C1s complex activation of
autocatalytic enzymatic activity in C1r cleaves C1s to active serine protease. C1s cleaves C4 to
produce C4b (leading to C4b2a again).
C1q can bind to the surface of pathogens in different ways:
Binding directly to surface components (proteins cell wall, lipoteichoic acid gram positive).
Binding to C-reactive protein (acute phase protein, binds to phosphocholine residues on the
bacterial surface)
