IMSE 311: Complement Pathways

Complement System

Heat labile series of plasma proteins, many of which are enzymes or proteases and has “housekeeping” roles

• Lysis of foreign cells (Main function)

• Recognizes cellular debris (includes infected host cells)

• Opsonize and tag invaders for clearance

• Proinflammatory (Diapedesis)

  • Increase vascular permeability

  • Recruit phagocytes

• Directs the adaptive immune system to site of infection (Chemotaxis)

Liver

Organ that is the major source of complement proteins

Classical Pathway

Antigen-antibody complexes that involves 9 proteins

• Recognition unit: C1 complex + Calcium (maintain structure)

  • C1q: binds to antibody molecules

  • C1r: autoactivation; cleaves C1s to activate it

  • C1s: cleaves C4 and C2 into “a” and “b” fragments

• Activation unit:

  • C4: C4b binds to C2a in the presence of Mg ions

  • C2: cleaves C3 once bound to C4b

  • C3: Cleaves C5 once bound to C4b2a

• Membrane Attack Complex (MAC): produces a pore in the target membrane of 70-100 Å (Angstrom)

  • C5: C5b attaches to cell membrane

  • C6: binds to C5b

  • C7: binds to C5b6

  • C8: binds to C5b67

  • C9: binds to C5b678

C3

Most abundant complement protein with a plasma concentration of 1-1.5 mg/mL

• Convergence point of all 3 complement pathways

Cleavage of C3 into C3b

Most significant step in complement activation

C4b2a

Also known as “C3 convertase” of the classical and lectin pathway, with a half-life between 15 secs - 3 mins

Classical Pathway Triggers

1. IgM (most effective; multiple binding sites)

2. IgG3

3. IgG1

4. IgG2

5. CRP

6. Several viruses

7. Mycoplasma

8. Some protozoa

9. Some gram (-) bacteria (Escherichia coli)

C4b2a3b

Also known as “C5 convertase” of the classical and lectin pathway

Carboxy-Terminal End

Hydrophobic part of C9 that anchors the MAC w/n the target membrane

False

True or False: C9 is required for MAC to create a pore for cell lysis

Alternative Pathway

“Properdin System”, relies on spontaneous hydrolysis of C3 into C3b-like molecule or C3H2O and involves 6 proteins + 2 factor proteins

• Recognition unit: none

• Activation unit:

  • C3: Cleaved by hydrolysis into C3H2O to start activation of pathway

    • C3b: binds to Factor B

    • Factor B: Analogous to C2 in classical pathway

    • Factor D: Cleaves Factor B

• Membrane Attack Complex (MAC): produces a pore in the target membrane of 70-100 Å (Angstrom)

  • C5: C5b attaches to cell membrane

  • C6: binds to C5b

  • C7: binds to C5b6

  • C8: binds to C5b67

  • C9: binds to C5b678

C3bBb

Also known as “C3 convertase” of the alternative pathway; with a half-life of 90 secs

C3bBbPC3b

Also known as “C5 convertase” of the alternative pathway

Properdin

A plasma glycoprotein and the only known positive regulator of the complement system

• amplifies and stabilizes C3bBb by binding to it

Alternative Pathway Triggers

1. Bacterial cell wall w/ lipopolysaccharide

2. Fungal cell wall

3. Yeast

4. Viruses

5. Virally-infected cells

6. Tumor cell lines

7. Some parasites (especially trypanosome)

Lectin Pathway

Activated by direct recognition of surface moieties found on pathogens and plays an important role as a defense mechanism in infancy (during the interval between loss of maternal Ab and acquisition of full-fledged Ab response to pathogens) and involves 8 proteins + lectins

• Recognition unit:

  • Lectins, Ficolins, & Collectins: analogous to C1q in classical pathway

  • Mannose-binding Lectin (MBL): requires Calcium to bind to sugars (PAMPs) found on microbial surfaces

  • MBL-associated Serine Proteases (MASPs): analogous to C1r and C1s in classical pathway

• Activation unit:

  • C4: C4b binds to C2a in the presence of Mg ions

  • C2: cleaves C3 once bound to C4b

  • C3: Cleaves C5 once bound to C4b2a

• Membrane Attack Complex (MAC): produces a pore in the target membrane of 70-100 Å (Angstrom)

  • C5: C5b attaches to cell membrane

  • C6: binds to C5b

  • C7: binds to C5b6

  • C8: binds to C5b67

  • C9: binds to C5b678

Short Half-Life

Serves as a mechanism of control (“system control”) for complement complex enzymes, keeping the reaction localized

C1 Inhibitor (C1-INH)

Inhibits activation at recognition phases of both classical and lectin pathways by binding to C1r, C1s and MASP-2

• C4-binding Protein (C4BP)

• Cell-bound Receptors

  • Complement Receptor Type 1 (CR1)

  • Membrane Cofactor Protein (MCP or CD46)

  • Decay-accelerating Factor (DAF or CD55)

Inhibits formation of C3 convertase (inactivates C3b and C4b) together with Factor I (for degradation)

Factor H

Principal soluble regulator of alternative pathway

• binds to C3b to inhibit factor B

S Protein (Vitronectin)

Inhibits C5b67 complex binding on cell surface preventing polymerization of C9

• note: binding of C8 and C9 still proceeds but polymerization of C9 doesn’t occur

Membrane Inhibitor of Reactive Lysis (MIRL or CD59)

Present on cell membranes (RBCs, endothelial, epithelial, & etc.) to block the formation of membrane attack complex (MAC)

Complement Receptor 1 (CR1 or CD35)

Large polymorphic glycoprotein found mainly on peripheral blood cells that binds to C3b and C4b, but has the greatest affinity for C3b

Complement Receptor 2 (CR2 or CD21)

Present only on mature B-cells and is lost when conversion to plasma cells occurs

• binds complement-coated antigen (antigen opsonized by complement molecules) and cross-links it to membrane immunoglobulin to activate B cells

Complement Receptor 3 (CR3 or CD11b/CD18)

Found on monocytes, macrophages, neutrophils, and natural killer (NK) cells (basically phagocytic cells)

• specifically binds particles opsonized with iC3b (C3b degradation product) in calcium-dependent manner

Complement Receptor 4 (CR4 or CD11c/CD18)

Similar function to CR3 but also found in dendritic cells, and activated T- or B-cells,

C5a

An anaphylatoxin (proinflammatory) that also serves as a chemotaxin

Deficiency

• C1 complex (C1q, C1r, C1s): Lupus-like syndrome; recurrent infections

• C2: Lupus-like syndrome; recurrent infections; atherosclerosis

• C3: Severe recurrent infections; glomerulonephritis

• C4: Lupus-like syndrome

• C5 - C8: Neisseria infections

• C9: no known disease association

• C1-INH: Hereditary angioedema

• DAF and MIRL: Paroxysmal nocturnal hemoglobinuria

• Factor H or I: Recurrent pyogenic infections

• MBL: Pneumococcal diseases; sepsis; Neisseria infections

• Properdin: Neisseria infections

• MASP-2: Pneumococcal diseases