This presentation is organized with the help of other presentations, text book of immunology and some internet resources for better understanding of students.

1. THE COMPLEMENT
SYSTEM
Organized by Tooba Kanwal (with the
help of text book and other resources)

2. Complement:
kills cells by making pores on cell membranes
But how?
Molecular mechanisms of complement-mediated killing and
diseases associated with the failure of these pathways are
discussed in this presentation

4. OBJECTIVES
To understand:
1. That complement plays an important role
as an effector of innate and adaptive
immune responses.
2. The mechanisms of activation and function
of the complement system.
3. That complement is able to distinguish between
self (host cells) and non-self (pathogens).
4. That deficiency of complement and/or
complement regulatory proteins can
result in disease.

5. • Complement was discovered by Jules Bordet as a heat-labile component of
normal plasma that augments the opsonization of bacteria by antibodies and
allows antibodies to kill some bacteria. This activity was said to ‘complement’ the
antibacterial activity of antibody.
• The complement system is made up of a large number of distinct plasma
proteins that react with one another to opsonize pathogens and induce a series of
inflammatory responses that help to fight infection.
• Several complement proteins are proteases that are themselves activated by
proteolytic cleavage. Such enzymes are called zymogens and were first found in
the gut.
• The digestive enzyme pepsin, for example, is stored inside cells and secreted as
an inactive precursor enzyme, pepsinogen, which is only cleaved to pepsin in the
acid environment of the stomach. The advantage to the host of not being auto-
digested is obvious.
• In the case of the complement system, the precursor zymogens are widely
distributed throughout body fluids and tissues without adverse effect. At sites of
infection, however, they are activated locally and trigger a series of potent
inflammatory events.
• The complement system activates through a triggered-enzyme cascade.
Background and Introduction

6. Complement is a system of proteases that mostly
exists in plasma but some components are found
on the surface of cells.
These proteases circulate as inactive precursor
proteases (or zymogens) that can be activated by
proteolytic cleavage.
Activation of the complement system is triggering
of a cascade of activated proteases. An activated
protease generated by cleavage of its zymogen
precursor cleaves its substrate, the next protease
precursor.
Complement is a Protease Cascade

7. A cascade of proteases can be an exponential
biological response amplifier
Protease cascades as biological amplifiers
10,000 molecules
100 molecules
1 molecule

8. FUNCTIONS OF COMPLEMENT SYSTEM:
There are three ways in which the Complement System
protects against pathogens,
1. Opsonization and pathogen clearance: First, it generates large
numbers of activated complement proteins that bind covalently to pathogens,
opsonizing them for engulfment by phagocytes bearing receptors for complement.
2. Inflammatory cell stimulation: Second, the small fragments of some
complement proteins act as chemoattractants to recruit more phagocytes to the
site of complement activation, and also to activate these phagocytes.
3. Direct lysis of pathogens: Third, the terminal complement components
damage certain bacteria by creating pores in the bacterial membrane.

9. Three major biological activities of the Complement system
OPSONIZATION
LYSI
S
CELL ACTIVATION

10. Nomenclature of Complement Peptides
• All components of the classical complement pathway and the membrane-attack
complex are designated by the letter C followed by a number.
• The native components have a simple number designation, for example, C1 and
C2, but unfortunately, the components were numbered in the order of their
discovery rather than the sequence of reactions.
• The reaction sequence is C1, C4, C2, C3, C5, C6, C7, C8, and C9.
• The products of the cleavage reactions are designated by adding lower-case
letters, the larger fragment being designated b and the smaller fragment being
designated a
e.g. C4 is cleaved to C4b, the large fragment of C4 that binds covalently to
the surface of the pathogen, and C4a, a small fragment with weak pro-
inflammatory properties
• The components of the alternative pathway instead of being numbered, are
designated by different capital letters, for example factor B and factor D.
• As with the classical pathway, their cleavage products are designated by the
addition of lower-case a and b: thus, the large fragment of B is called Bb and the
small fragment Ba.

11. Substances that cause Complement Activation
There are many substances that can activate complement cascade. These include
★ In Classical Pathway :
• Protein A: It’s an integral part of Staphylococcus aureus, which has an affinity
for the Fc portion of IgG, IgM & IgA. A single molecule of protein A can bind
two IgG molecules simultaneously leading to the formation of a complex
resembling antigen-antibody complex.
• C-reactive protein: is found in the serum of patients suffering from rheumatic
fever due to Streptococcus pyogenes or inflammatory diseases. This protein can
bind to the C1 component of complement system to activate it.
• Lipopolysaccharide also activates the complement system through classical
pathway. It is found in the cell wall of Gram negative bacteria and binds to the
C1 protein.
★ In Alternate Pathway :
The substances called activators activate the Alternate Pathway of the complement
system. these include Polyanions and certain bacterial and viral substances.
Polyanions are negatively charged including DNA, polyinosinic acid, dextran
substrate etc. Viruses also act as activators of the alternate pathway.

12. There are 3 pathways of complement activation

13. Overview of the Main Components and Effector Actions of
ComplementJaneway 9.32

14. Overview of the main components and effector actions of complement
The early events of all three pathways of complement activation involve a series of cleavage reactions that
culminate in the formation of an enzymatic activity called a C3 convertase, which cleaves complement component
C3 into C3b and C3a. The production of the C3 convertase is the point at which the three pathways converge and
the main effector functions of complement are generated. C3b binds covalently to the bacterial cell membrane and
opsonizes the bacteria, enabling phagocytes to internalize them. C3a is a peptide mediator of local inflammation.
C5a and C5b are generated by cleavage of C5b by a C5 convertase formed by C3b bound to the C3 convertase (not
shown in this simplified diagram). C5a is also a powerful peptide mediator of inflammation. C5b triggers the late
events in which the terminal components of complement assemble into a membrane-attack complex that can
damage the membrane of certain pathogens. C4a is generated by the cleavage of C4 during the early events of the
classical pathway, and not by the action of C3 convertase, hence the *; it is also a peptide mediator of inflam-mation
but its effects are relatively weak. Similarly, C4b, the large cleavage fragment of C4 (not shown), is a weak opsonin.
Although the classical complement activation pathway was first discovered as an antibody-triggered pathway, it is
now known that C1q can activate this pathway by binding directly to pathogen surfaces, as well as paralleling the
MB-lectin activation pathway by binding to antibody that is itself bound to the pathogen surface. In the MB-lectin
pathway, MASP stands for mannan-binding lectin-associated serine protease.

15. The classical pathway of complement
activation

17. The Classical Activation Pathway
The large fragments function by binding to membranes
(esp. pathogen membranes)
The small fragments act as soluble
mediators
C
1
C4
Ab
C4
a
C4b
C
3
C3a
C
5
C5a
C5b C6 C7 C8 C9
C
6
C
7
C
8
C
9
MAC
Membrane
Classical Activation Pathway
C2
C2a
C2b C3b

18. Janeway 9.34
Structure of C1
● C1q is composed of six identical subunits with globular heads and long collagen like tails.
● the tails combine to two molecules each of C1r and C1s, forming the C1 complex C1q: C1r2
:C1s2
● the heads can bind to the constant regions of Ig molecules or directly to the pathogen surface,
causing a conformational change in C1r, which then cleaves and activates the C1s zymogen.

19. Recognition unit :
● Binding of C1 complex via C1q globular domains to antibody bound to the
pathogen is the first event in the Classical pathway of complement activation in
the presence of calcium.
● Activation occurs only when C1q cross links to the two or more CH3 domains of
a single pentameric IgM or CH2 domains of two different IgG molecules.
● The first major enzyme of the pathway is called C1 esterase which acts on C4.
● In humans a serum protease inhibitor, C1 inactivator (C1INA) controls the
function of C1.
The Activation Unit :
● The activation is initiated by C1 in the presence of Mg.
● C1 acts as enzyme and cleaves C4 into its fragments.
● C4b forms a covalent bond with sugar residue on a cell surface glycoprotein
adjacent to the Ag-Ab complex.
● C2 is the third component to be activated by C1s by being cleaved.
● C2b has a hydrophobic region that binds to the C4b forming a C4b2b complex
known as C3 convertase.
● C3 convertase cleaves C3 into its fragments.
● C3b can bind to the pathogen surface for opsonization or to the C3 convertase to
form C5 convertase to cleave C5.
Early events of the Classical Pathway
of Complement Activation-1

20. The Membrane Attack Unit:
• C5 convertase cleaves C5 into its fragments C5a and C5b.
• C5a acts as anaphylotoxin and participates in the inflammatory
process.
• C5b attaches to a new site on the cell membrane and serves as a site
for the assembly of a trimolecular complex, C5b678 to which is then
attached C9.
• C8 inserts itself into cell membrane.
• These processes do not involve any enzyme.
• 12 to 15 C9 molecules binds to the complex which helps in the
cytolysis of cell.

21. Janeway
9.36

22. Janeway
9.35
Structure of IgM in solution
Structure of IgM bound to
bacterial surface

23. Pathogen
C4
C4a
C4b
C4b
THIOESTER-MEDIATED BINDING OF C4b
TO THE SURFACE OF A PATHOGEN
Thioester mediated binding of C4b

25. Deficiency of C1, C4, C2 Components
C1, C4, C2 deficiency leads to a failure to clear
immune complexes. This leads to immune
complex disease [Systemic Lupus
Erythematosus (SLE), Glomerulonephritis].

26. The Alternate Pathway of
Complement Activation

27. Hydrolysis of C3 causes initiation of the alternative
pathway of complement
• The alternate pathway is so named because it was discovered as a second, or
‘alternative,’ pathway for complement activation after the classical pathway
had been defined.
• This pathway can proceed on many microbial surfaces in the absence of
specific antibody, and it leads to the generation of a distinct C3 convertase
designated C3b,Bb
• In contrast to the classical and MB-lectin pathways of complement activation,
the alternative pathway does not depend on a pathogen-binding protein for its
initiation; instead it is initiated through the spontaneous hydrolysis of C3.
• Several mechanisms ensure that the alternative pathway will only proceed on
the surface of a pathogen.

29. Ro
itt
4.
8
Spontaneous Hydrolysis of C3 and formation of
a C3 convertase

30. Alternate pathway C3 Convertase depositing C3b
On the Pathogen Surface
Factor P: stabilizes C3bBb
on the pathogen surface
P

31. Amplification Loop
C3b B
C3bB
D
Ba
C3bBb
C3
C3a
(C3 convertase)
C3
C3
One C3 convertase molecule cleaves many C3 molecules.
Properdi
nstabilization
C3b interacts with factor B, and more C3 convertase is
generated.
Amplification Loop

32. The Alternate Pathway of Complement Activation
Early Events-1
First event: This pathway is initiated by spontaneous
hydrolysis of the thio-ester bond in C3 to form “activated”
C3i [or C3(H2O)]. C3 is abundant in plasma and C3b is
generated at a significant rate by such a spontaneous
cleavage.
Second event: C3i binds to a plasma protein, factor B, to
give rise to C3iB [or C3(H2O)B].
Third event: Factor B bound to C3i (in C3iB), is a substrate
for a plasma protease, factor D. Factor D cleaves B to give
rise to C3iBb and Ba (small mediator of inflammation).
C3iBb is a fluid phase C3 convertase.

33. Fourth event: C3iBb cleaves additional molecules of
C3 to C3b and C3a. Some of the C3b binds the
pathogen surface.
Fifth Event is an amplification loop:
(a) Pathogen-bound C3b binds factorB to give rise to
C3bB.
(b) Factor D cleaves B bound to C3b to give
rise to C3bBb, a pathogen-bound C3 convertase.
This starts a loop in which additional C3b makes
more C3 convertase, leading to amplified
formation of C3b and the C3 convertase C3bBb.
The Alternate Pathway of Complement Activation
Early Events-2

34. C3 deficiency:
C3 deficiency, whether genetic, or due to incessant
consumption, leads to susceptibility to infections
with pyogenic bacteria and Neisseria spp.
Sometimes also leads to immune complex disease.

35. Late events in Complement activation

36. a
a
a
a
a
a

37. Roitt 4.13
panel 1

38. Janeway
9.49 part
2

39. Late events in Complement activation
Formation of C5 convertase: C3b, in addition to
opsonizing pathogens, complexes with C3
convertases (the classical and the alternate) to give
rise to C5 convertase.
Cleavage of C5: C5 is cleaved by C5 convertase to
release C5b and C5a.
Formation of membrane attack complex (MAC):
C5b anchors the MAC by first forming “C5bC6C7C8”
complex. CD8 part of this complex induces
polymerization of 10-16 molecules of C9 into pore
forming structure called MAC.

40. Regulation:
• It is clear that a pathway leading to such potent inflammatory and destructive
effects, and which, moreover, has a series of built-in amplification steps, is
potentially dangerous and must be subject to tight regulation.
• The key activated complement components are rapidly inactivated unless
they bind to the pathogen surface on which their activation was initiated.
• There are also several points in the pathway at which regulatory proteins act
on complement components to prevent the inadvertent activation of
complement on host cell surfaces, hence protecting them from accidental
damage

41. Classical pathway regulation

42. Alternative pathway regulation

43. Anaphylotoxis
C5a and C3a are potent anaphylotoxins
produced by the pathogen-triggered
complement cascade and play a role in
the recruitment of polymorphonuclear
leucocytes for the phagocytosis of C3b or
C4b opsonized pathogens via CR1.
Ro
itt
12
.1
9

44. Actions of anaphylotoxins