Complement System Notes

Republic Polytechnic A231 Immunology - Learning Package 04: Complement System

Case Study: Mike's Recurrent Staphylococcal Infection

  • Mike, a 25-year-old, experiences recurrent Staphylococcal infections.

  • His complement protein levels are:

    • C1r: 87 mg/L (Normal: 34-100 mg/L)

    • C1q: 250 mg/L (Normal: 70-300 mg/L)

    • C1s: 75 mg/L (Normal: 30-80 mg/L)

    • C2: 21 mg/L (Normal: 15-30 mg/L)

    • C3: 22 mg/L (Normal: 900-1800 mg/L) - Significantly low

    • C4: 587 mg/L (Normal: 350-600 mg/L)

  • Antibody levels are normal.

  • The question is: How do complement components relate to Mike's condition?

Staphylococcal Infection

  • Caused by Staphylococcus aureus.

  • Treatment: Antibiotics (e.g., penicillin).

Immune System Response to Recurrent Infection

  • Antibody effector functions:

    • Neutralization

    • Opsonization

    • Agglutination of microbe

    • Precipitation of soluble antigen

    • ADCC (Antibody-Dependent Cell-mediated Cytotoxicity)

Significance of Abnormal Complement Protein Levels

  • Above-normal levels may indicate infection or inflammation.

  • Below-normal levels may indicate disorders affecting the immune system's effectiveness.

Nomenclature of Complement Proteins

  • Classical Pathway:

    • Proteins are designated by "C" followed by a number (C1-C9) based on order of discovery.

    • Smaller cleaved fragment: "a" suffix (e.g., C3a).

    • Larger cleaved fragment: "b" suffix (e.g., C3b).

    • Exceptions:

      • C1: Complex of C1q, C1r, and C1s.

      • C2: Larger fragment initially C2a, now often C2b; C4b2b is the C3 convertase.

  • Alternative Pathway:

    • Proteins are called factors (e.g., Factor B, Factor D).

Complement System Overview

  • Activation pathways.

  • Central role of C3.

  • Effector functions linked to C3a.

Classical Pathway Example

  • Objective: Observe how complement proteins work together to eliminate pathogens.

Three Complement Activation Pathways

  • Classical Pathway:

    • C1q interacts with the pathogen surface or antibodies bound to the surface.

    • Involves C1q, C1r, C1s, C4, and C2.

  • Lectin Pathway:

    • Mannose-binding lectin (MBL) and ficolins recognize and bind carbohydrates on the pathogen surface.

    • Involves MBL/ficolin, MASP-2, C4, and C2.

  • Alternative Pathway:

    • C3 undergoes spontaneous hydrolysis to C3(H2O) to initiate C3 convertase deposition on microbial surfaces.

    • Involves factor B, factor D, and properdin.

  • All pathways converge to generate a C3 convertase that cleaves C3 into C3a and C3b.

    • C3→C3a+C3bC3 \rightarrow C3a + C3b

    • C3b binds to the microbial surface.

Effector Functions of Complement

  • C3 convertase cleaves C3 into C3a and C3b.

    • C3→C3a+C3bC3 \rightarrow C3a + C3b

  • C3a and C5a:

    • Recruit phagocytic cells to the infection site.

    • Promote inflammation.

  • C3b:

    • Opsonization: Phagocytes with C3b receptors engulf and destroy pathogens.

  • Completion of the complement cascade:

    • Formation of the membrane-attack complex (MAC).

    • Disrupts the cell membrane, causing cell lysis.

Antibody Efficiency in Complement Activation

  • IgM and IgG can activate the complement system.

  • C1q recognizes and binds to the Fc region of IgG and IgM after they bind to pathogen surface antigens.

Classical Pathway (Antibody-Dependent)

  • C1q is the sensor for surface-bound IgM or IgG.

  • C1q links adaptive immunity to complement effector function.

  • C1 activates C4 and C2 to form the classical C3 convertase: C4b2a.

    • C1+C4+C2→C4b2aC1 + C4 + C2 \rightarrow C4b2a

  • The C3 convertase cleaves C3 into C3a and C3b.

    • C3→C3a+C3bC3 \rightarrow C3a + C3b

  • C3b covalently attaches to the pathogen surface.

Lectin Pathway (MBL/Ficolin-Dependent)

  • Lectins (MBL/Ficolin) can bind to specific carbohydrates on pathogen surfaces.

  • C4 and C2 are activated as in the classical pathway.

  • C3 convertase formed: C4b2a.

    • C4+C2→C4b2aC4 + C2 \rightarrow C4b2a

Alternative Pathway (C3 Spontaneous Activation)

  • C3 spontaneously splits into C3a + C3b.

    • C3→C3a+C3bC3 \rightarrow C3a + C3b

  • C3b links to the pathogen surface.

  • Requires factors B, D, and P.

  • Surface C3b forms alternative C3 convertase: C3bBb.

    • C3b+FactorB→C3bBbC3b + FactorB \rightarrow C3bBb

  • C3b formed by the classical or lectin pathway can initiate the alternative pathway.

Comparison: Classical vs. Alternative Pathway

  • All three pathways lead to C3 cleavage.

Effector Functions After C3 Cleavage

  • C3 convertase cleaves C3:

    • C3→C3a+C3bC3 \rightarrow C3a + C3b

  • C3a: In tissue fluid.

  • C3b: Surface-bound and critical for function.

Opsonization

  • C3b-coated bacteria are efficiently cleared by macrophages via complement receptors.

  • Phagocytosed antigens activate specific B and T cells.

Lysis of Microbe

  • C3b joins C3 convertase to form C5 convertase.

    • C3b+C3 convertase →C5 convertaseC3b + C3 \text{ convertase } \rightarrow C5 \text{ convertase}

  • C5 convertase cleaves C5:

    • C5→C5a+C5bC5 \rightarrow C5a + C5b

  • C5a: In tissue fluid.

  • C5b initiates the membrane attack complex (MAC).

Inflammation

  • Anaphylatoxins: C3a, C4a, and C5a.

  • Activate local blood vessels, inducing inflammation and attracting immune cells, antibodies, and more complement proteins to the infection site.

  • Activate mast cells and neutrophils to produce more inflammatory mediators.

  • C5a is the most potent chemoattractant for neutrophils.

Membrane Attack Complex (MAC)

  • C5b required for MAC formation.

  • MAC allows leakage of ions and water, causing cell lysis.

Summary of Complement Pathways

  • Three pathways.

  • Formation of C3 convertase.

  • Formation of C5 convertase.

  • Formation of MAC.

  • Cause inflammation.

Summary of Key Components

Step in pathway

Alternative

Lectin

Classical

Relationship

Initiating serine protease

D

MASP

C1s

Homologous (C1s and MASP)

Covalent binding to the cell surface

C3b

C4b C3b

C3b

Homologous

C3 convertase

C3bBb

C4b2a

C4b2a

Homologous

C5 convertase

C3bBb3b

C4b2a3b

C4b2a3b

Homologous

Opsonization

Identical

Lysis of Microbe

Identical

Local inflammation

Identical

Importance of C3

  • Critical in the eradication of bacteria.

Mike's Condition Explained

  • Deficiency in C3 complement makes Mike susceptible to bacterial infections.

Complement System Functions

  • Inflammation.

  • Opsonization leading to phagocytosis.

  • Cell lysis through formation of the membrane attack complex.

  • Generation of mediators involved in inflammation.

Learning Objectives

  • Explain how antibodies function with complement proteins to eradicate bacteria.

  • Describe the three pathways of the complement system (classical, alternative, lectin).

  • Identify the complement proteins involved in the three pathways and the effector functions.

  • Understand the roles of basic components of the complement cascade.

  • Appreciate the importance of the complement system and discuss the consequences of complement deficiency.

Key Words & Phrases

  • Complement pathways.

  • Complement proteins: C1 – C9.

  • Role of each complement within different pathways.

  • Sequence of events.

  • Complement substrates and enzymatic cleavage products.

  • Consequence of alteration/defect/loss of each complement protein.

  • Effect on the complement cascade.

  • Effect on outcome (i.e., opsonisation, MAC formation).