Summary Advanced Immunology Janeway test 1) Innate immunity, complement & phagocytosis

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)