The Complement System explained

1. Complement System
Dr. Kanwal Deep Singh
Lyall
M.D. Microbiology

2. History
• Buchner’s observation (1889)
• Pfieffer’s phenomenon (1894)
• Bordet (1895)- heat labile substance
-alexine
• Paul Ehrlich- complement as it
complemented Abs function
• Jules Bordet & Gengou (1901)
• Pillemer (1954)

3. The term “complement (C) refers to a
system of factors which occur in the
normal serum & are activated
characteristically by antigen-
antibody interaction & subsequently
mediate a number of biologically
significant consequences

4. Components
• C1-C9
• C1 – C1q,C1r,C1s
• C3 highest conc. 1.2mg/ml)
• C2 in lowest conc. (0.015mg/ml)

5. Biosynthesis of complement
C component Site
C1 Intestinal
epithelium
C2, C4 Macrophages
C5, C8 Spleen
C3, C6, C9 Liver
C7 Not known

6. Pathways
• The Classical Pathway
• The Alternate Pathway
• The Lectin Pathway

7. Main effects
• Lysis
• Generation of mediators
• Opsonization

9. General properties
• Present in all mammalian sera
• Non-specific
• 5% of normal serum protein
• Not increased by immunization
• Heat labile
• Combines only with Ag-Ab complex
• Fixed only by IgM, IgG3, IgG1 & 2
• Binding site on Fc portion of Ig
• Not influenced by nature of Ag

10. Cont…
• Inactivation at 56 C X 30 mnts
• C components-proenzymes-cleaved to
form active enzymes
• Lectin & alt. – innate immunity

11. Nomenclature
• The components of the classical
pathway are called C1, C2 etc.
• Proteolytically cleaved components ,
the products are referred to as C2a +
C2b etc.
• Alternative Pathway specific
components are given letters as
names (factor B etc).

12. • The central molecule
• Marks microbes for destruction
• Generates C5 convertase
• Opsonizes
C3b

13. Complement Activation
General
• Hydrophobic surfaces
• Oxides
• Strong binding of C3(b) to nucleophilic
groups (-NH2, -OH)
• Higher absorption of C3 to crystalline
TiO2 than to amorphous
• Kallikrein directly activates C5
• Plasmin directly activates C5

14. Initiators of classical
pathway
• Antibodies IgM, IgG1, IgG2, IgG3
• Lectin via the mannan binding protein
(MBP) “Lectin Pathway”
• Hageman Factor (F XIIa)
• C-reactive protein (CRP)
• Rough surfaces

15. The Classical Pathway

18. • C1q binds Ag bound Ab
• C1r activates auto-catalytically & activates
the 2nd
C1r
• Both activate C1s

19. • C1S cleaves C4 & C2
• Cleaving C4 exposes the binding site for C2
• C4 binds the surface near C1 & C2 binds C4,
forming C3 convertase

21. • C3 convertase hydrolyzes many C3
molecules
• Some combine with C3 convertase to
form C5 convertase

22. • The C3b component of C5 convertase
binds C5, permitting C4b2a to cleave
C5

23. • C5b binds C6, initiating the formation of
the membrane-attack complex

26. The Alternate Pathway

27. Initiators of alternate
pathway
Pathogens & particles of
microbial origin
Many strains of Gr –ve bacteria
Lipopolysaccharides from Gr –ve
bacteria
Many strains of Gr +ve bacteria
Teichoic acid from Gr +ve
bacteria
Fungal & yeast cell walls
(zymosan)
Some virus & virus-infected cells
Some tumor cells
Parasites (trypanosomes)
Non-pathogens
Human IgG, IgA & IgE complexes
Rabbit & guinea pig IgG in
complexes
Cobra venom factor
Heterologous erythrocytes
(rabbit, mouse, chicken)
Anionic polymers (dextran
sulphate)
Pure carbohydrates
(agarose,inulin)

28. 1.C hydrolysis spontaneously; C3b
fragment attaches to foreign surface
2. Factor B binds C3b, exposes site
acted on by factor D. Cleavage generates
C3bBb, which has convertase activity
3. Binding of properdin stablizes convertase
4. Convertase generates C3b; some
Binds to C3 convertase . C5b binds
to antigenic surface

29. Lectin pathway (MBL -
MASP)

30. • Homologous to the classical pathway, but with the
opsonin, mannan-binding lectin (MBL), and
ficolins, instead of C1q.
• Activated by binding MBL to mannose residues
on the pathogen surface
• Activates the MBL-associated serine proteases,
MASP-1, and MASP-2 (very similar to C1r and
C1s, respectively),
• Can then split C4 into C4a and C4b and C2 into
C2a and C2b.
• C4b and C2a then bind together to form the C3-
convertase, as in the classical pathway.
• Ficolins are homologous to MBL and function via
MASP in a similar way.

31. Lectin pathway
• Activated by bacterial lectins
• Mannan binding lectin (MBL)
• AKA mannose binding lectin(MBL)
• Binds to microbes bearing mannan
• Activates proteases associated with
MBL
• Cleaves C4 & C2 & activates classical
pathway
• Bypasses antibody activating step

34. Holes of approx. 100 in diameterǺ

36. Regulators and inhibitors

37. Protein Type of protein Pathway
affected
Immunological
function
C1 inhibitor (C1
inh)
Soluble Classical Inactivates
protease activity
of C1
C4b-binding
protein C4bBP)
Soluble Classical &
lectin
Blocks
formation of C3
convertase by
binding C4b:
cofactor for
cleavage of C4b
factor I
Factor H Soluble Alternative Blocks
formation of C3
convertase by
binding C3b;
cofactor for
cleavage of C3b
factor I
Complement
receptor type1
(CR1 Or CD35).
Membrane bound Classical,
alternate &
lectin
Blocks
formation of C3
convertase by

38. Decay-accelerating
factor (DAF or
CD55)
Membrane bound Classical, alternate
& lectin
Destabilizes C3 &
C5 convertase &
prevents formation
of MAC
Factor I Soluble Classical, alternate
& lectin
Serine protease:
cleaves C4b or C3b
S protein Soluble Terminal Binds soluble
C5b67 & prevents
its insertion into
cell membrane
HRF (homologous
restriction factor)
or MIRL
(membrane
inhibitor of
reactive lysis)or
CD59
Membrane bound Terminal Bind to C5b678 on
autologous cells,
blocking binding
of C9
Anaphylotoxin
inactivator
Soluble Effector Inactivates
anaphylotoxin
activity of
C3a,C4a,C5a
Protein Type of protein Pathway
affected
Immunological
function

39. 1.C1 inhibitor (C1inh) binds C1r2s2,
causing dissociation from C1q
2. Association of C4b & C2a is
blocked by binding C4b-binding protein
(C4bBP), complement receptor type I,
or membrane Cofactor protein (MCP)
3. Inhibitor-bound C4B is cleaved by factor I
4. In alternative pathway,CR1,MCP, or factor H
prevents binding of C3b & factor B
5.Inhibitor-bound C3b is cleaved by factor I
Regulation of the complement system
Before assembly of converatse activity

40. After assembly of
convertase
C3 co0nvertases are dissociated by
C4bBP, CR1, factor H & DAF

41. S protein prevents insertion of
C5b67 MAC components into membrane
Homologous restriction factor
(HRF) or membrane inhibitors
of reactive lysis (MIRL or
CD59), Preventing assembly of
poly-C9 & blocking formation
of MAC
Regulation of assembly of
MAC

42. Biological effects mediated by complement
products
Effect Complement product
mediating
Cell l lysis C5b-9, the MAC
Inflammatory response
• Degranulation of mast cells &
basophils
• Degaranulation of eosinophils
•Extravasation & chemotaxis of
leucocytes at inflammatory site
• Aggregation of platelets
• Inhibition of monocytes/ macrophage
migration & induction of their
spreading
• release of neutrophils from bone
marrown
• release of hydrolytic enzymes from
neutrophils
• increased expression of complement
receptors type 1 & 3 (CR1 & CR3) on
neutrophils
• C3a, C4a, C5a ( anaphylotxins)
•C3a, C5a
•C3a, C5a, C5b67
•C3a, C5a
•Bb
•C3C
•C5a
•C5a
Opsonization of particulate Ags,
increasing their phagocytes
C3b, C4b,IC3b

43. Clinical syndrome associated with
genetic deficiencies components
Group Deficiency Syndrome
1 C1 inhibitor Hereditary
angionecrotic edema
2 Early components of
classical pathway
C1,C2,,C4
SLE & other collagen
vascular diseases
3 C3 & its regulatory
protein C3b activator
Sever recurrent
pyogenic infections
4 C5 & C8 Bacteremia, mainly
GNDC, toxoplasmosis
5 C9 No particular disease

47. C3 is the key component of
Complement
• An abundant serum protein (1.2mg/ml) C
• Contains an unusual internal thiolester bond.
• In native C3 this bond is stable
• Can become highly reactive as a result of
conformational changes in the C3 protein structure.
• Generally the activation of C3 comes about as a
result of proteolytic cleavage of the C3 molecule into
2 biologically active fragments.

49. THE BIOLOGICAL EFFECTS OF
COMPLEMENT
• 1. Opsonisation. The C3b & C4b-coat foreign organisms either by
the AP or those already bound by antibody. Opsonisation of
particles greatly enhances their phagocytosis by means of binding
to specific complement receptors (see below).
• 2. Inflammation. The C5a and, less potently, the C4a and C3a
fragments are important inflammatory activators inducing
vascular permeability, recruitment and activation of phagocytes.
• 3. Lysis. C5b binds and recruits C6 and C7 to the target surface.
C7 and subsequently C8 change conformation to expose
hydrophobic domains which insert in the lipid bilayer. The C5b678
complex catalyses the polymerisation of the final component C9
which forms a transmembrane pore of ~ 10nm diameter causing
lysis of the cell. This macromolecular assembly is known as the
Membrane Attack Complex (MAC).
• 4. Immune complex clearance. Complement has a very important
role in solubilising and causing removal from the circulation of
immune complexes. It does this by the binding of C4b and C3b,
covalently bound to the immune complex, to CR1 complement
receptors on red blood cells which transport the complexes to the

51. Lectin pathway (MBL -
MASP)• Homologous to the classical pathway, but with the
opsonin, mannose-binding lectin (MBL), and
ficolins, instead of C1q.
• Activated by binding MBL to mannose residues
on the pathogen surface
• Activates the MBL-associated serine proteases,
MASP-1, and MASP-2 (very similar to C1r and
C1s, respectively),
• Can then split C4 into C4a and C4b and C2 into
C2a and C2b.
• C4b and C2a then bind together to form the C3-
convertase, as in the classical pathway.
• Ficolins are homologous to MBL and function via
MASP in a similar way.

52. • E. THE COMPLEMENT SYSTEM
• 2. The Lectin Pathway
• The lectin pathway is mediated bycirculating proteins called mannan-
binding lectin (MBL) - also known as mannan-binding protein or
MBP. MBL is a soluble pattern-recognition receptor that binds to mannose-
rich glycans - short carbohydrate chains with the sugar mannose or fructose
as the terminal sugar. Mannose-rich glycans are common in microbial
glycoproteins and glycolipids but rare in those of humans MBL is synthesized
by the liver and released into the bloodstream as part of the acute phase response
that will be discussed later in Unit 4. The MBP is equivalent to C1q in the
classical complement pathway.
• Activation of the lectin pathway begins when mannan-binding lectin
(MBL) binds to the mannose groups of microbial carbohydrates.
Two more lectin pathway proteins called MASP1 and MASP2 (equivalent to
C1r and C1s of the classical pathway) now bind to the MBL (see Fig. 1). This
forms an enzyme similar to C1 of the classical complement pathway that is able
to cleave C4 and C2 to form C4bC2a, the C3 convertase capable of
enzymatically splitting hundreds of molecules of C3 into C3a and C3b.

57. These proteolytic reactions are mediated by a C1r/C1s-like serine protease termed MASP
(MBL-associated serine protease), which is bound to MBL.
In humans, three types of MASP (MASP-1, MASP-2 and MASP-3) and sMAP (small MBL-
associated protein, or called MAp19) form complexes (MBL-MASP) with MBL.
MASPs and C1s share domain structures . I
n serum, MASPs are present as proenzymes consisting of a single polypeptide chain.
Upon cleavage of a peptide bond, MASPs are converted to their activated forms consisting
of two polypetides linked by a disulfide bridge.
Activated MASP-1 cleaves C3 and C2, while activated MASP-2 cleaves C4 and C2. The
function of MASP-3 is not known. sMAP is a truncated protein