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Flashcards in Complement Deck (28):
1

The complement system is a group of more than 30 plasma and membrane proteins that play a critical role in host defense. When activated, complement components interact in a highly regulated fashion to generate products that:

Recruit inflammatory cells (promoting inflammation).

Opsonize microbial pathogens and immune complexes (facilitating antigen
clearance).

Kill microbial pathogens (via a lytic mechanism known as the membrane attack complex).

Generate an inflammatory response.

2

Describe complement activation:

Where does it take place?
What are the pathways?

Complement activation takes place on antigenic surfaces. However, the activation of
complement generates several soluble fragments that have important biologic activity.

There are three distinct pathways of activation of complement: the Classical, the Lectin, and the Alternative Pathways. See Figure 1, p 3.

Classical pathway (antigen: antibody complexes)
Lectin pathway (lectin binding to pathogen surfaces)
Alternative pathway (Pathogen surfaces) ... all lead to complement activation

3

Describe activation of classical pathway.

When is it initiated?
What component recognizes antigen-antibody complex?
What happens?

Classical pathway activation is initiated after immune complex formation.

Complement component C1 recognizes the antigen-antibody complex.

The binding of antibody to antigen induces a conformational change in the antibody constant region. This exposes a site on the Fc portion (of the antibody molecule) that can
be bound by the first complement component of the classical pathway, C1.

4

Describe the structure of C1.

When does activation of C1 occur?

C1 is a macromolecule that consists of C1q (comprised of 6 globular heads and extended
tails) in complex with C1r and C1s (the C1qrs complex). See Figure 3.

p 5

Activation of the C1qrs complex occurs when at least two of the C1q globular heads are simultaneously bound to antibody. See Figure 4. p 6

5

Describe the activation of the C1qrs complex. What must occur in order for activation to take place?

(How will IgG differ from IgM?)

Activation of the C1qrs complex occurs when at least two of the C1q globular heads are simultaneously bound to antibody. See Figure 4.

For this to occur, two Fc portions need to be in within close molecular proximity of each other on the antigenic surface. (C1q binds to at least 2 IgG molecules) p 6

In contrast to IgG, the pentameric nature of IgM allows a single molecule of antigen bound IgM to activate C1. See Figure 4.

6

What happens once C1q is bound to the antibody?

Once C1q is bound to antibody, C1r undergoes a conformational change and becomes enzymatically active.

C1r then cleaves C1s, which after cleavage is enzymatically active as well.

7

Describe the activation of the lectin pathway. How does it differ from the classical pathway?

Activation of the Lectin pathway is similar to the classical pathway.

Except that the Lectin pathway is initiated by a protein, Mannan Binding Lectin (MBL) or ficolins.

8

Describe MBL. What do they bind specifically? What about ficolins?

What proteases are MBL/ficolins associated with? How do these become activated?

MBL binds to mannose and certain other complex carbohydrates that are found on the surface of many microbial pathogens (like Candida albicans, a fungus with surface
mannose residues). See Figure 5.

Ficolins bind oligosaccharides.

MBL/ficolins are physically associated with two serine proteases, MASP-1 and MASP-2
(mannan binding lectin-associated serine protease-1) that are similar to C1r and C1s.

When MBL/ficolins bind to surfaces, MASP-1 and MASP-2 become activated.

9

Describe what happens once C1qrs is activated (or MBL/ficolines activated MASP-1 and MASP-2)?

Activated C1qrs, see Figure 6 (or separately MBL/ficolins activated MASP-1 and MASP-2 see Figure 7) cleave C4, and the C4b fragment becomes bound to a cell surface
(e.g. microorganism). Bound C4b fragment binds C2.

10

Activated C1qrs, see Figure 6 (or separately MBL/ficolins activated MASP-1 and MASP-2 see Figure 7) cleave C4, and the C4b fragment becomes bound to a cell surface
(e.g. microorganism). Bound C4b fragment binds C2.

What happens next?

Once bound to C4b, C2 is also cleaved by C1s, forming the C4b2a complex, which
remains bound to the cell surface. (For Lectin Pathway activation, MBL/ficolins can be
substituted for C1, substitute MASP-1 and MASP-2 for C1r and C1s in Figure 4).

11

Once bound to C4b, C2 is also cleaved by C1s, forming the C4b2a complex, which
remains bound to the cell surface. (For Lectin Pathway activation, MBL/ficolins can be
substituted for C1, substitute MASP-1 and MASP-2 for C1r and C1s in Figure 4).

What is C4b2a? What is the its job and significance?

C4b2a is an active C3 convertase, capable of cleaving C3 into C3b and C3a.

This is a major point of amplification of the pathways, since one C3 convertase can cleave up to 1000 molecules of C3.

12

C4b2a is a C3 convertase, capable of cleaving C3 into C3b and C3a.

Describe C3b.

C3b covalently bound to the antigenic surface, acts as a powerful opsonin and enhances the uptake of antigenic particle by phagocytes.

13

What will MASP-2 do?

does same thing as C1s.
cleaves C4 to C4a and C4b

(C4b then binds C2)

MASP-2 cleaves C2 to C2a and C2b forming the C4b2a complex

14

Describe C4b2a3b. What does it do?

C4b2a3b complex, also called C5 convertase, cleaves C5 into C5a, which is a soluble
inflammatory mediator, and C5b, which is capable of complexing with additional complement components. The generation of C5b initiates the final phase of complement activation, which is the formation of the Membrane Attack Complex (MAC). See
Figure 8.

The MAC is identical for all pathways of complement activation. C3a and C5a remain
soluble and produce local inflammatory effects.

p9

15

What does activation of the alternative complement pathway depend on?

The alternative pathway depends upon the slow hydrolysis of C3, which spontaneously occurs in plasma.

16

The alternative pathway depends upon the slow hydrolysis of C3, which spontaneously occurs in plasma.

Describe the role of hydrolyzed C3.

Hydrolyzed C3 can bind Factor B, and the resulting C3 (H2O) Bb complex is a C3
convertase that generates additional molecules of C3b. See Figure 9.

p 9

17

Hydrolyzed C3 can bind Factor B, and the resulting C3 (H2O) Bb complex is a C3
convertase that generates additional molecules of C3b.

What happens next? Discuss up to formation of MAC complex.

C3b is rapidly degraded, unless stabilized by attachment to certain favorable pathogenic
surfaces.

Once attached, C3b binds Factor B, and Factor B is cleaved by Factor D, forming bound C3bBb.

When stabilized by Factor P (properdin), the C3bBb complex acts as a C3 convertase, analogous to C4b2a of the classical pathway.

When another molecule of C3b associates with C3bBb (forming C3bBbC3b), a C5
convertase is formed.

This C5 convertase is analogous to C4b2a3b of the classical pathway. From this point (the cleavage of C5), the alternative and classical pathways converge, leading to the formation of a MAC complex as in Figure 8.

18

Describe the biological consequence of complement activation:

Cell lysis and viral neutralization.

The MAC complex (C5b--> 9) creates a pore in the
cell membrane, and disrupts cell homeostasis, by cellular lysis (e.g. Gram-negative
bacteria like Neisseria spp.). Certain viruses with a membrane coat can also be lysed in this manner. See Figure 11.

p 11

19

Describe the biological consequence of complement activation:

Opsonization.

Phagocytic leukocytes, including neutrophils and macrophages, carry
receptors for C3b (CR1). When an antigenic particle is coated with C3b, C3b (also C4b)
assists in the adherence and ultimate ingestion of the particle by the phagocytic cell. The C5a fragment also enhances phagocytosis by stimulating phagocytic cells to ingest C3b coated antigens. See Figure 12.

p 11

Bacterium is coated with complement and IgG antibody
-When C3b binds to CR1 and antibody binds to Fc receptor, bacteria are phagocytosed

20

Describe the biological consequence of complement activation:

Clearance of Immune Complexes

The removal of antigen-antibody complexes from the circulation depends upon C3b. Via C3b, antigen-antibody complexes bind to
complement receptors on circulating red blood cells. As the RBCs pass through the
spleen and liver, the coated complexes are stripped off of the RBCs by resident
phagocytes. See Figure 13.

p 12

Complement receptor CR1 on erythrocytes binds the immune complexes via bound C3b.
In the spleen and liver, phagocytic cells remove the immune complexes from the erythrocyte surface.

21

The soluble fragments that are produced during complement activation
play several roles in inflammation. Describe chemotaxis and vascular changes.

Chemotaxis. C5a is an important chemoattractant for neutrophils, eosinophils,
basophils, and monocytes. The development of a C5a gradient at sites of
complement activation assists in the recruitment of leukocytes to the area of
antigenic challenge.

Vascular Changes. The fragments C3a, C4a, and C5a are capable of binding to
specific receptors on mast cells and basophils, triggering granule release by these
cells. The release of histamine leads to vascular changes, including increased
vascular permeability. Because of this property, C3a, C4a and C5a are called
anaphylatoxins.

(increased permeability allows increased fluid leakage from blood vessels and extravasation of immunoglobin and complement molecules)
-migration of macrophages, polymorphonuclear leukocytes (PMNs) and lymphocytes is increased. Microbial activity of macrophages and PMNs is also increased

p 13

22

A series of proteins serve to protect host cells from accidental damage by acting
at various different stages of complement activation and dissociating complexes
or catalyzing enzymatic degradation of covalently bound complement proteins.
If not regulated, what could happen?

activated complement can cause excessive inflammation and tissue damage. See Figures 15.

Regulation by regulatory proteins.

23

Describe the regulatory proteins and function:

Which factors serve to...
Inhibit C1
Inhibit C3 convertase
Inhibit C5 convertase

Inhibition of C1.
C1 inhibitor (C1INH) – inhibits C1 by dissociating C1r and C1s from the C1 complex.

Inhibit C3 convertase.
Decay-accelerating factor (DAF)
C4-binding protein (C4BP)
Complement receptor 1 (CR1)
Membrane cofactor protein (MCP)
Factor I (I)

Inhibit C5 convertase.
Factor I (I)
Factor H (H)
Complement receptor 1 (CR1)

see p 14

24

Deficiencies of complement components are very rare.
How are they handled if they do occur in the classical pathway?

Defects in the early components of the classical pathway do not lead to overwhelming
infection, as the MBL and alternative pathways can bypass this defect.

25

Describe deficiency in classical pathway, lectin pathway and alternative pathway.

C1
C2
C4
MBL
MASP1
MASP2
C2
C4
Factor D
Factor P
C3
C5
C6
C7
C8
C9

Figure 16 p 15

C1
C2
C4-Classical- deficiency leads to immune-complex disease

MBL
MASP1
MASP2
C2
C4- Lectin- deficiency of MBL leads to bacterial infections, mainly in childhood

Factor D
Factor P- Alternative pathway- deficiency leads to infection with pyogenic bacteria and Neisseria spp. but no immune-complex disease

C3- deficiency leads to infection with pyogenic bacteria and Neisseria spp. Sometimes immune-complex disease

C5-C9- deficiency leads to infection with Niesseria spp. only

26

Describe the following complement deficiencies and associated diseases:

Hereditary Angioneurotic Edema- (What type of deficiency and what are the clinical implications?)

Paroxysmal Nocturnal Hemoglobinurea- (What deficiency and what clinical implications?)

What will a deficiency of early components of complement result in?

Hereditary Angioneurotic Edema – C1 INH deficiency – failure to regulate C1
resulting in fluid accumulation, epiglottal swelling.

Paroxysmal Nocturnal Hemoglobinurea –CD59 failure to function – lack of
complement regulation leads to RBC lysis.

A deficiency of the early components of complement results in poor clearance of
immune complexes resulting in increased immune complex disease.

27

Ten important items to know:

C1
MBL
MASP
C3 comprised of C3a and C3b
C3 convertases: C4b2a : C3bBb
C5 convertases: C4b2a3b : C3bBbC3b
MAC
C1 INH
CR1
CD59

....

28

A deficiency of complement component C9 results in susceptibility to:
A. Pyogenic bacteria.
B. Neisseria spp. only.
C. Immune complex disease.
D. Reduced immune complex clearance.
E. No phenotype.

B