Complement

Objectives

Discuss the role of complement in the immune system.

Discuss complement regarding its: Components Activation pathways Biological activities

Complement

System comprised of more than 25 glycoproteins Make up about 10% of total serum proteins

Components formed mainly in the liver C1 forms in intestinal epithelial cells Factor D forms in adipose tissues

Components form a cascade, with each step triggering (and often amplifying) the next step

Complement

In plasma – the components are in inactive form Once activated, each component splits

The smaller “a” fragment serves to stimulate the immune system

The larger “b” fragment further activates the cascade The exception to this is C2… C2a is the larger

molecule that promotes the cascade Activated components are written with a line over the

letter or number. For example: ____ C4b2a

3 Functions

1. Cell lysis – Cell swells and bursts

2. Opsonization - Neutrophils and macrophages have receptors for C4b or C3b, which promotes phagocytosis

3 Functions

3. Regulate immune and inflammatory response Immune adherence – enhanced response to an antigen.

Receptors for complement found on Red Blood Cells (RBCs), platelets, B lymphocytes, endothelial and epithelial cells

Anaphylatoxins - chemicals that increase vascular permeability, contract smooth muscle, and cause the release of histamine from basophils and mast cells

C3a, C4a, and C5a are anaphylatoxins Chemotaxins – signal leukocytes to migrate to an affected

area C5a is also a chemotaxin C5b67 promotes monocyte and neutrophil adherence

to blood vessel endothelium, and extravasation

3 Activation Pathways

Classical Alternative Lectin

Classical Pathway

Begins with antibody sensitization of antigen on cell

2 Fc pieces in close proximity IgM better than IgG (1 IgM vs.

800 IgG) IgG1 & IgG3 better at

activation than IgG2 or IgG4 C1 = Recognition unit

C1q C1r C1s

C1

YY

Recognition Unit

C1q combines with the Fc piece 6 globular heads attached to

collagen-like tails. C1r that acts as a protease on

C1s C1r and C1s intertwine with

the tails. 2 C1q heads must interact with

Fc pieces for complement to be activated.

YY

C4, C2, and C3 components participate Amplification of cascade

1 molecule of C1s activates approximately 30 C4 molecules.

C1

YY

C1s

C4

C2

C4a

C2b

C4b2a

Classical - Activation Unit

Activation Unit

C2 is active only if it binds to C4b before being cleaved by C1s This reaction is enhanced if C4b binds to the

antigen rather than being free in serum Once C4b2a is formed, antibody is no longer

necessary to ensure cell lysis

Activation Unit

C4b2a = C3 convertase One C4b2a complex converts about 200 C3

molecules Requires Mg+

C1

YY

C1s

C4

C2

C4a

C2b

C4b2a C3

C3a

C3b C4b2a3b

Activation Unit

C3 most abundant complement component C3 common to all pathways C3b on a cell enhances opsonization C3b combines with C4b2a to form C4b2a3b =

C5 convertase

Classical – MAC

Membrane Attack Complex = C5, C6, C7, C8, C9

C1

YY

C1s

C4

C2

C4a

C2b

C4b2a C3

C3a

C3b C4b2a3b C5

C5a

C5b

C9

C8

C7

C6

MAC

C5b binds to C6 and C7 in serum This complex will bind to any nearby cell membrane,

not just the cell that originally triggered the cascade. (The “innocent by-stander” effect)

May also form a micelle – free floating sphere- which has antiviral properties

C5b678 form a surface on the cell membrane for polymerization of C9

A pore forms in the cell, allowing an influx of water The cell swells and lyses

Hemolysin

An antibody that can activate complement, resulting in lysis of an RBC is termed a hemolysin. Y YC1

Alternative (Properdin) Pathway

Activated by bacteria, fungus, yeast, viruses, parasites, and tumor cells

Oldest pathway Relies on the natural splitting of C3 into C3a

and C3b (exact process unclear)

C3b

Factor B

C3bBb

Properdin

C3bBbP

C5a

C5

C5b

C9C8C7C6

Factor D

Alternative

C3b and Factor B need Mg+ to combine Factor D splits Factor B that has been bound

to C3b Properdin stabilizes the C3bBb complex C3bBbP is a C5 convertase Once C5b formed, cascade continues as in

the classical pathway

Lectin Pathway

Mannose is a sugar commonly found in bacterial cell walls

This pathway can be activated by bacteria, yeast, viruses and protozoa

MBL C4

C2

C4a

C2b

C4b2a C3

C3a

C3b C4b2a3b C5

C5a

C5b

C9C8C7C6

Lectin Pathway

MBL= Mannose Binding Lectin equivalent to C1q of classical pathway MBL also increases opsonization

MASP-1 and MASP-2 function similarly to C1r and C1s respectively MASP = Mannose Associated Serine

Proteases Once MBL activates the cascade, the

cascade continues as in the classical pathway

Inhibitors of Complement

Activated enzymes decay quickly. Half life of activated components ranges from

a fraction of a second to approximately 30 minutes.

If Ca+ and Mg+ are not present, the cascade can NOT be activated.

Carboxypeptidase N inactivates anaphylatoxins.

Heating serum at 56oC for 30 minutes destroys complement components.

Specific Inhibitors

C1-INH irreversibly binds to the active sites of C1r and C1s.

Factor I degrades C4b and C3b Membrane cofactor protein –cofactor for

Factor I C4-binding protein – prevents C4 and C2 from

joining (cofactor for Factor I)

Specific Inhibitors

Complement Receptor 1 (CR1) Found as a surface protein on most WBCs

and follicular dendritic cells Binds C3b allowing degradation by Factor I Increases clearance of C3b coated cells via

macrophages of liver and spleen Also increases the length of time antigen

remains near germinal centers of lymph nodes (May assist with B cell differentiation)

Specific Inhibitors

Sialic acid of cell membrane inactivates C3b bound to cells

Decay Accelerating Factor (DAF) – dissociates C2 from C3

S protein – prevents C5b67 complex from binding to cell

MIRL – Binds to C8 MIRL = Membrane Inhibitor of Reactive Lysis

Protectin – Prevents binding of C5b678 to cell, so no polymerization of C9 is allowed

Inhibitors - Alternative

DAF & Factor H – compete with Factor B for binding with C3b

C3b

Factor B

C3bBb

Properdin

C3bBbP

C5a

C5

C5b

C9C8C7C6

Complement Deficiencies

Complement deficiencies rare C2 deficiency most common (1 in 10,000) C3 deficiency most problematic as it

participates in all pathways Deficiencies in complement components can

lead to: Increased susceptibility to infection Accumulation of immune complexes Autoimmune disease

Pathologic Conditions

Complement is harmful if: Activated systemically Activated by tissue necrosis Allows build up of immune complexes

Immune complexes play a role in Goodpasture’s syndrome, SLE, MS, Guillain-Barre’ syndrome and other autoimmune diseases

Pathologic Conditions

Paroxysmal Nocturnal Hemoglobinuria (PNH) Abnormal DAF on RBCs Cells more susceptible to lysis

C1-INH deficiency – Hereditary angioedema C2b accumulates, but does not participate in

the complement cascade Increases vascular permeability

The End

Can you list the complement components in order of activation?