COMPLEMENT SYSTEMFe A. Bartolome, MD, FPASMAPDepartment of MicrobiologyOur Lady of Fatima University

• Consists of approx. 20 proteins that are present in normal human serum synthesized mainly by liver

• Heat-labile inactivated by heating serum at 560C for 30 minutes

• Able to augment the effects of other components of the immune system

• Important component of innate host defenses

• Three main effects:

1. Lysis of cells (bacteria, allografts, tumor cells)

2. Generation of mediators of inflammation

3. Opsonization – enhancement of phagocytosis

• Sequential activation of complement components occurs via one of three pathways:1. Classic pathway2. Lectin pathway3. Alternative or Properdin pathway

• Lectin and alternative pathways are more important the first time we are infected by microorganisms because antibody required to activate the classic pathway is not yet present

• Part of acquired or adaptive immunity

• Activated by Ag-Ab complexes

• Immunoglobulins involved: IgM and IgG (except IgG4)

• Involves activation of C1 Composed of C1q, C1r, and C1s

binds to Fc portion of IgG and IgM Requires calcium for activation

Classic Pathway

• Other activators include:

1. Viruses – Murine and Retroviruses2. Bacteria – Mycoplasma3. Polyanions, especially bound to

cationsa. PO4

3- - DNA, lipid A, cardiolipin

b. SO42- - dextran, heparin,

chondroitin4. Arrays of terminal mannan groups

Classic Pathway

Classic PathwayComponents of the Classical Pathway

Native component Active component(s) Function(s)

C1(q,r,s)

C1q Binds to antibody that has bound antigen, activates C1r.

C1r Cleaves C1s to activate protease function.

C1s Cleaves C2 and C4.

C2C2a Unknown.

C2b Active enzyme of classical pathway; cleaves C3 and C5.

C3

C3a Mediates inflammation; anaphylatoxin.

C3b

Binds C5 for cleavage by C2b.Binds cell surfaces for opsonization and activation of alternate pathway.

C4

C4a Mediates inflammation.

C4bBinds C2 for cleavage by C1s. Binds cell surfaces for opsonization.

Classic Pathway

Components of the Membrane-Attack ComplexNative component

Active component(s) Function(s)

C5

C5aMediates inflammation; anaphylatoxin, chemotaxin.

C5bInitiates assembly of the membrane-attack complex (MAC).

C6 C6 Binds C5b, forms acceptor for C7.

C7 C7Binds C5b6, inserts into membrane, forms acceptor for C8.

C8 C8 Binds C5b67, initiates C9 polymerization.

C9 C9nPolymerizes around C5b678 to form channel that causes cell lysis.

Classic Pathway

Alternative Pathway

• Also known as the Properdin Pathway

• Part of innate immunity

• Bypasses C1, C4, and C2

• Does not require an antigen-binding protein

• Does not wait for antibody to be formed for activation

• Acts synergistically with the classical pathway

Alternative Pathway

• Usually activated by products of micro-organisms like endotoxin

• Other activators include:1. Complexes containing IgA2. Some virus-infected cells (e.g. EBV)3. Many gram negative and gram

positive organisms4. Parasites – Trypanosomes,

Leishmania5. Dextran SO4

6. Erythrocytes7. Carbohydrates (agarose)

Alternative Pathway

Components of the Alternate Pathway

Native component Active component(s) Function(s)

C3

C3a Mediates inflammation; anaphylatoxin.

C3b

Binds cell surfaces for opsonization and activation of alternate pathway.

Factor B

BBinds membrane bound C3b. Cleaved by Factor D.

Ba Unknown.

BbCleaved form stabilized by P produces C3 convertase.

Factor D D Cleaves Factor B when bound to C3b.

Properdin PBinds and stabilizes membrane bound C3bBb.

Alternative Pathway

Lectin Pathway

• Also known as the MBL Pathway

• Activated by binding of mannose-binding lectin (or mannose-binding protein) to surface of microbes bearing mannan (polymer of the sugar mannose) in a calcium dependent manner

• Binding causes activation of MASP (MBP- associated serine proteases) cleave C2 and C4

Lectin Pathway

• All three pathways lead to production of C3b central molecule of complement cascade

• Presence of C3b on surface of a microbe marks it as foreign and targets it for destruction

• C3b with two important functions:1. Combines with other complement

components to generate C5 convertase

2. Opsonizes bacteria

Biologic Effects:

1. Opsonization • C3b & C1q; enhance phagocytosis

2. Chemotaxis • C5a and C5,6,7 complex attract

neutrophils• C5a – enhance adhesiveness of

neutrophils to the endothelium

3. Anaphylatoxin (C3a, C4a, C5a)• Cause degranulation of mast cells• Bind directly to smooth muscles of

bronchioles bronchospasm

Biologic Effects:

4. Cytolysis (MAC)• Disrupt the membrane & the entry

of water and electrolytes into the cell

5. Enhancement of antibody production• Binding of C3b to its receptors on

the surface of activated B cells enhanced antibody production

Regulation of Complement System

1. C1 inhibitor (C1-INH)• Important regulator of classic

pathway• A serine protease inhibitor (serpin)• Irreversibly binds to and

inactivates C1r and C1s, as well as MASP in lectin pathway

2. Factor H• Regulate alternative pathway• Reduce amount of C5 convertase

available• With both cofactor activity for the

factor I- mediated C3b cleavage, and decay accelerating activity against C3bBb (C3 convertase)

Regulation of Complement System

3. Properdin • Protects C3b and stabilizes C3

convertase

4. Factor I• Cleaves cell-bound or fluid phase

C3b and C4b inactivates C3b and C4b

5. Decay accelerating factor (DAF)• Glycoprotein on surface of human

cells• Prevents assembly of C3bBb or

accelerates disassembly of preformed convertase no formation of MAC

• Acts on both classical and alternative

Regulation of Complement System

6. C4b-binding protein (C4BP)• Inhibits the action of C4b in

classical pathway• Splits C4 convertase and is a

cofactor for factor I7. Complement Receptor 1 (CR-1)• Co-factor for factor I, together with

CD468. Protectin (CD59) and Vitronectin (S

protein)• Inhibits formation of MAC by

binding C5b678• Present on “self” cells to prevent

complement from damaging them

Clinical Aspects

1. Deficiency of C5-C8 & Mannan-binding lectin• Predispose to severe Neisseria

bacteremia2. Deficiency of C3• Severe, recurrent pyogenic sinus &

resp. tract infections3. Deficiency of C1 esterase inhibitor• Angioedema inc. capillary

permeability and edema4. Deficiency of DAF• Increased complement-mediated

hemolysis paroxysmal nocturnal hemoglobinuria

Clinical Aspects

5. Transfusion mismatches• Activation of complement

generate large amounts of anaphylatoxins & MAC red cell hemolysis

6. Autoimmune diseases• Immune complexes bind

complement low complement levels + activate inflammation tissue damage

7. Severe liver disease• Deficient complement proteins

predispose to infection with pyogenic bacteria

Clinical Aspects

8. Factor I deficiency

• Low levels of C3 in plasma due to unregulated activation of alternative pathway recurrent bacterial infections in children

• Mutations in factor I gene implicated in development of Hemolytic Uremic Syndrome