1. Elements of the Immune System: Complement
S. Barbour

2. Hallmarks of Complement
Sequential Activation
Amplification
Regulation

3. Overview of Complement
Activation
(adaptive immunity)
Activation
(innate immunity)
Activation
(innate immunity)
Classical Pathway
Alternative Pathway
Lectin Pathway
Antibody binds to specific
antigen on pathogen surface
Mannose-binding protein
binds pathogen surface
Pathogen surface creates
environment conducive to
complement activation
a series of enzyme
(protease) reactions
Complement Activation
Formation of C3 and C5 convertases
Inflammatory response
Membrane Attack Pathway
Cytolysis of some pathogens
effecter function
effecter function
Activation of
B cells
Opsonization & phagocytosis
of some pathogens
Clearance of
immune complexes
effecter function
effecter function
effecter function

4. Complement Nomenclature
1. CX (X = 1-9); example: C5
2. Factor X (alternative pathway);
example: Factor B (B)
3. complement fragments (usually a and b);
example: C3 → C3a + C3b
Note: the “b” fragment remains surface associated;
“a” fragment is soluble
Note: “i” denotes “inactive” fragments that do not support complement activation. However, these fragments can have other biological functions. (eg. iC3b or C3bi)

5. Complement Activation

6. Activation of the Classical Pathway
IgG
(IgG1, IgG2, IgG3)
IgM C3
C3 convertase C3
C3 convertase Y Y Y Y
C3b Y Y C1
C3b C1
C3b Y
C3b
Antigen
(bacterial or yeast cell surface)
covalent bond
with surface
covalent bond
with surface
-Note: only antigen-antibodies complexes can activate the classical pathway! Antibody
alone cannot
-Classical pathway results in formation of a C3 convertase that generates C3b
-C3b forms a covalent bond with the surface of the pathogen and is part of C5 convertase

7. Activation of the Alternative Pathway (Sources of C3b)
Soluble
C3 convertase C3
C3 tickover
C3b C3
C3 convertase
C3b
C3b
C3b Y Y C1
covalent bond
with surface
covalent bond
with surface
“activating surface”
(bacterial or yeast cell surface)
-the alternative pathway is initiated by C3b binding to a bacterial or yeast cell surface
-C3b preferentially interacts with bacterial or yeast cells; host cells are spared
-this is a primitive distinction of self versus non-self

8. (bacterial or yeast cell surface)
Activation of the Alternative Pathway (C3 Convertase)
Soluble
C3 convertase C3
C3 tickover
C3b
C3b
C3b C3
C3 convertase
C3 convertase
covalent bond
with surface
C3b
“activating surface”
(bacterial or yeast cell surface)
covalent bond
with surface
-alternative pathway results in the formation of a C3 convertase that generates C3b
-C3b forms a covalent bond with the surface of the pathogen and is part of C5 convertase

9. Activation of the Lectin Pathway
C3b
mannan binding lectin
(MBL) C3
C3 convertase
C3b
mannose
sugars
MBL
covalent bond
with surface
“activating surface”
(bacterial or yeast cell surface)
-mannan binding lectin (MBL) recognizes mannose sugars on microbial cells
-host mannose is hidden and is not accessible to MBL
-lectin pathway results in the formation of a C3 convertase that generates C3b
-C3b forms a covalent bond with the surface of the pathogen and is part of C5 convertase

10. Regulation of Complement

11. Importance of C3
Activation of Classical, Lectin, and Alternative pathways (alternative pathway is constantly activated in serum!)
Fragments have biological activity
Opsonization / Phagocytosis
B cell activation
Inflammation
Most abundant complement protein in serum ( > 1 mg/ml)

12. P C3 Convertases C3 C3a + C3b Classical / Lectin Pathway
Alternative Pathway
receptor
subunit
receptor
subunit
C2b
C4b Bb
C3b
catalytic
subunit
catalytic
subunit P
covalent association
with pathogen surface
Covalent association ensures that C3 convertases remain localized to the
pathogens that activate complement!
Regulation: Dissociation of the subunits or proteolysis of receptor subunit by Factor I

13. Regulation of C3 Convertases
Dissociation
Proteolysis
(Factor I cofactor)
Protein
Classical/Lectin
Alternative
C4b
C3b
Factor H - +
C4bp
DAF
MCP
CR1
In general, regulatory proteins are expressed on host cells, but not
on pathogens. Therefore, host cells are spared from complement
attack.

14. Effecter Functions of Complement

15. The Membrane Attack Pathway
MAC
The MAC is especially important for the immune response against
Neisseria spp. Membrane proteins (CD59, HRF) prevent MAC
formation on host cells.

16. Opsonization / Phagocytosis
C3b or iC3b
opsonized bacteria
CR1, CR3, or CR4

17. Inflammation anaphylotoxins C3a, C4a---increased vascular permeability
C5a—chemoattraction
C3a, C4a----activation

18. Clearance of Immune Complexes
CR1

19. Activation of B Cells or C3dg -microbial cell expresses antigens
recognized by B cell receptor
-microbial cell is coated with C3d or
C3dg, recognized by CR2
-simultaneous binding to B cell
receptor and CR2 results in more
efficient B cell activation

20. Complement: Disease Correlations

21. Disease Correlations
-deficiency in classical pathway components is associated with accumulation of immune
complexes, autoimmune diseases.
-deficiency in terminal components (components of MAC) results in increased
susceptibility to Neisseria infections.

22. Disease Correlations
-C1 inhibitor (C1INH) blocks the activation of C1, the complement component that
interacts with antibody in the classical pathway. Deficiency results in uncontrolled
production of anaphylotoxins
-DAF, HRF, and CD59 are lipid (GPI) linked proteins. Deficiency in production of
lipid anchors results in loss of these regulatory proteins, complement activation on host
cells

23. Deficiencies in Complement Receptors
CR3, CR4
CR3, CR4 are integrins
Loss of these proteins compromises functions of phagocytes (LAD)
Result is increased susceptibility to infection
CR1
Most CR1 is expressed by erythrocytes
Loss of this protein is associated with accumulation of immune complexes
(similar to deficiencies in classical pathway components)

24. Overview of Complement
Activation
(adaptive immunity)
Activation
(innate immunity)
Activation
(innate immunity)
Classical Pathway
Alternative Pathway
Lectin Pathway
Antibody binds to specific
antigen on pathogen surface
Mannose-binding protein
binds pathogen surface
Pathogen surface creates
environment conducive to
complement activation
a series of enzyme
(protease) reactions
Complement Activation
Formation of C3 and C5 convertases
Inflammatory response
Membrane Attack Pathway
Cytolysis of some pathogens
effecter function
effecter function
Activation of
B cells
Opsonization & phagocytosis
of some pathogens
Clearance of
immune complexes
effecter function
effecter function
effecter function