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Complement System (CS)
Part II
Lukas Menges
24.5.2016
Molecular Immunology
CONTENTS
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2
Most basic facts about the CS (repetition)
Control of recognition
Modulation of increasing
Complement receptors (CRs)
Small fragments C5a and C3a
Regulation of membrane attacking
24.5.2017 – LUKAS MENGES – MOLECULAR IMMUNOLOGY
Most basic facts about the
complement system (repetition)
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Antibodies only detect antigens. The complement system is one system which
complements the antibody function
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3 modes of action:
- complement fixation (binding and lysis of the cell membrane)
- opsonisation (stimulation of phagocytosis)
- enticement of phagocytes by triggering of inflammatory reaction
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CS consists of an cascade of approx. 20 zymogens (mostly serine proteases)
activating each other
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24.5.2017 – LUKAS MENGES – MOLECULAR IMMUNOLOGY
Prevention of autoimmunity
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Complement activation should take
place on pathogens, not on normal
„harmless“ cells. How it is ensured?
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All activated zymogens bind nearby,
otherwise they are rapidly inactivated
by hydrolysis
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Complement regulatory proteins
protect endogenous cells from
complement activation on their
surfaces
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Prevention of autoimmunity
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Complement activation is regulated by
proteins that serve to protect cells from
accidental damage
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These proteins act on different stages of
the complement cascade, dissociating
complexes or catalyzing the enzymatic
degradation of covalently bound
complement proteins
Regulation of
recognition
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Recognition unit
The recognition unit C1 consits of C1q, C1r and
C1s and binds to the Fc terms of IgM and IgG.
Binding is only possible if IgM and IgG have
bound to antigens.
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C1q is composed of 18 polypeptide chains
(A6B6C6) forming 6 triple bundles (“bunch of
tulips”)
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At least two C1q subunits must be linked to
immunoglobulins to activate C1 („lower limit“)
Prevention that one unspecific antibody
activates CS
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Two IgGs or one IgM activate(s) C1. Why?!
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24.5.2017 – LUKAS MENGES – MOLECULAR IMMUNOLOGY
EM-picture
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Stage at which complement activity is regulated
C1q binding to antibodies: C1r
and C1s are activated
C1 inhibitor (C1NH)
dissociates C1r and C1s from
the active C1 complex
C1r and C1s cannot propagate the complement cascade
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Regulation of increasing
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Important role of C3
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Formation of C3 convertase: All pathways of complement
activation converge.
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C3 most abundant complement protein in plasma (1.2 mg/ml)
Approx. 100 proteins in plasma, abundance: 60-80 mg/ml
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C3 is cleaved into C3b and C3a. C3b binds to molecules on the
pathogen surface
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C3b forms a covalently bonded coat that can signal the ultimate
destruction of the pathogen by phagocytes (macrophages,…).
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Complement receptors (CRs)
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CRs bind pathogens opsonized with
complement components
C3b is an opsonin
Receptor
Specificity
Functions
Cell types
CR1 (CD35)
C3b, iC4b
Promotes C3b and
C4b decay.
Stimulates
phagocytosis
(requires C5a)
Erythrocyte
transport of
immune complexes
Erythrocytes,
macrophages,
monocytes,
polymorphonuclear
leucocytes, B cells,
FDC (Dendritic cell)
CR2 (CD21)
C3d, iC3b, EpsteinBarr virus
Part of B-cell coreceptor
B cells, FDC
CR3 (Mac-1)
iC3b
Stimulates
phagocytosis
Macrophages,
monocytes,
polymorphonuclear
leucocytes, FDC
CR4
iC3b
Stimulates
phagocytosis
Macrophages,
monocytes,
polymorphonuclear
leucocytes,
dendritic cells
CRIg
C3b, iC3b
Phagocytosis of
circulating
pathogens
Tissue-resident
macrophages
Hepatitic sinusoid
macrophages
C5a
C5a
Binding of C5a
activates G protein
Endothelial cells,
Mast cells,
phagocytes
C3a
C3a
Binding of C3a
activates G protein
Endothelial cells,
Mast cells,
phagocytes
C3b receptor is called CR1, specific to C3b
CR1 promotes C3b decay. Stimulates
phagocytosis (requires C5a)
+ C5a
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24.5.2017 – LUKAS MENGES – MOLECULAR IMMUNOLOGY
Stage at which complement activity is regulated
C4b2a is the active C3
convertase, cleaving C3 into
C3a and C3b
DAF, C4BP and CR1 displace
C2a from the C4b2a complex,
C4b bound by C4BP, MCP or
CR1 is cleaved by a soluble
protease I to inactive forms
C4d and C4c
Destruction of the C3 convertase
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C3b is another target of regulation
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1. Competition. Decay-accelerating factor DAF,
membrane-attached protein, competes with Bb for
binding to C3b on the cell surface - preventing
formation of the C3bBb C3 convertase
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2. Cleaving. The plasma protease Factor I
cleaves C3b to inactive iC3b. Needs cofactors
(membrane cofactor of proteolysis = MCP or
CD46)
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Further C3b binding proteins: Factor H, CR1
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Factor I deficiency
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Genetic disorder
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Symptoms: Repeated bacterial infections
Uncontrolled complement activation
Complement proteins rapidly become depleted
because they are mostly active on “wrong” cells
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A closer look at the pathway…increase
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A closer look at the pathway
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Generation of the C5 convertases
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C5b and C5a propagate the complement cascade
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Classical pathway: C4b2a3b
Alternative pathway: C3b2Bb
From different pathways, but same function
C5 is cleaved into C5a and C5b. What is the small
fragment?
24.5.2017 – LUKAS MENGES – MOLECULAR IMMUNOLOGY
Stage at which complement activity is regulated
The C5 convertases cleave C5
into C5a and C5b
CR1 and H displace C3b from
the convertase. CR1 and H act
as cofactors in the cleavage of
C3b by I
Destruction of the C5 convertase
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A closer look at the pathway…C3a and C5a
Small fragments
with a big effect
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The small complement fragments C5a and C3a
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C5a and C3a produce local inflammatory
responses by acting directly on local blood
vessels, stimulating an increase in blood flow,
increased vascular permeability and increased
binding of phagocytes to endothelial cells.
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C5a also activates mast cells to release mediators,
such as histamine that contributes to the
inflammatory response
enticement of phagocytes by triggering of
inflammatory reaction
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C5a is more active
than C3a, which is
more active than C4a
Higher
permeability
of blood
vessels
Infiltrates
get into
the tissue
Regulation of membrane-attacking
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Membrane attack complex (MAC)
C5b is the cleavage product from the C5convertase and binds to C6 and C7
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The C5b/C6/C7-complex (amphiphilic) binds to
C8 and to (up to 18) C9-molecules forming a
tube in the membrane
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The MAC is a pore with a diameter of 3-10 nm
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One single MAC is sufficient for killing a cell
Cell loses electrolytes but no macromolecules
(interior of the cell is hypertonic). Thereby the
cell takes water and bursts („osmotic lysis“ or
„colloidal osmotic lysis“)
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EM-picture
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Stage at which complement activity is regulated
The terminal components of
complements form a
membrane pore: The
membrane attack complex
CD59 prevents final assembly
of the membrane-attack
complex at the C8 to C9 stage
The terminal complement proteins
cannot polymerize to form a pore
in membranes that kill pathogens
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Knowledge about CS helps us to develop immunotherapies
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The initiating Cells of the diffuse
large B Cell Lymphoma (DLBCL) are
white blood cells (B-lymphocytes)
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The B-lymphocyte-antigen CD20 is a
glycosylated phosphoprotein, which
is expressed on the membrane
surface of B-cells.
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CD20 is a target of monoclonal
antibodies due to cure B-celllymphoma and leukemia with the
aid of CS
https://www.youtube.com/watch?v=sINGPr6buvw
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Take home messages
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Complement activation must be regulated to
protect cells from accidental damage
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Regulation by “lower limitation”, regulating
proteins and rapid inactivation
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C3b receptor called: CR1 (effects phagocytosis)
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The membrane attacking complex (MAC) forms
a pore in the membrane that kill pathogens
The small complement fragments C5a and C3a
produce local inflammatory responses
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References
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Janeway, 2009, pp. 59-71
Lecture: Biochemistry III, part 15, WS2015, Michael Hollmann
Voeth & Voeth, 1995, figs 34.41
Alberts et al., 1994, fig. 23.27, 23.28, 23.29
https://www.youtube.com/watch?v=sINGPr6buvw, access 20.5.2017
24.5.2017 – LUKAS MENGES – MOLECULAR IMMUNOLOGY
Thank you for attention!
Any questions left?