Complement System

Complement System Study Notes

Define opsonization and cytolysis and the roles they play in body defense.

Complement Discovery

  • 1894: Discovery by German bacteriologist Richard Pfeiffer.

    • Demonstrated destruction of cholera organisms both in vitro and in vivo.

  • Contribution by Belgian bacteriologist Jules Bordet:

    • Observed bacteria lysed in fresh immune serum.

    • Bacteria agglutinated if the immune serum was heated; addition of fresh immune serum to agglutinated bacteria caused lysis.

  • Assumption based on findings:

    • Two fractions in serum:

      1. Heat “stable” fraction - identified as antibodies (Ab).

      2. Heat “labile” fraction - known as complement.

Complement: An Overview

  • Antibodies do not kill bacteria directly but initiate the activation of plasma proteins (complement).

  • Complement activity is a cascade of interactions involving over 20 circulating and membrane proteins.

  • Major roles:

    • Complement is crucial to innate immunity.

    • Kills invading organisms directly and enhances antibody lytic activity.

  • Final step in activation leads to cell lysis due to influx of fluid.

  • Complement helps recruit phagocytes to infection sites and enhances phagocytic activity.

Pathways of Complement Activation

  • Three pathways of complement activation:

    1. Classical pathway (antibody dependent)

    2. Alternative pathway (contact with foreign surfaces)

    3. Lectin pathway (mannose-binding lectin pathway)

  • All pathways converge at C3, which begins the common pathway, ultimately leading to the formation of the membrane attack complex (MAC) and resulting in cytolysis.

Components of the Complement System

  • Designated by letter “C” followed by a number (C1-C9), based on discovery order, not reaction order.

    • Components can be cleaved into active parts (C1-9 and parts a-e).

  • Activation is a cascade process:

    • Initial component activation leads to subsequent activations until completion.

  • Components produced mainly in the liver, macrophages, and epithelial cells (e.g., C1 in GI tract) circulate in inactive forms (Zymogens).

Fragment Designation

  • Fragments of activated components indicated with letters:

    • Example: C3 splits into C3a and C3b.

    • C3b for larger fragment (membrane binding) and C3a for smaller fragment.

  • Activation of a complex is denoted by a line over the symbol (e.g., C1qrs).

Complement Activation: Stages

  • Three distinct stages of complement activation:

    1. Recognition

    2. Activation

    3. Membrane Attack

Biological Consequences of Complement Activation

  • In addition to cell destruction, activation products exhibit:

    • Chemotaxis (attraction of immune cells)

    • Mediation of inflammation

    • Enhancement of phagocytosis

Cytolysis Event

  • All complement components must interact sequentially:

    • Multiple lesions form on target cell membranes leading to lysis and death.

Classical Pathway

  • Principal components: C1 to C9.

  • Activation sequence: C1, C4, C2, C3, C5, C6, C7, C8, C9.

  • C3 is the most abundant plasma protein in circulation.

  • Generally requires antigen/antibody complex to initiate (part of adaptive immunity).

  • Recognition unit: C1 complex (C1q, C1r, C1s). Triggered by antigen-antibody complexes on cell surfaces.

  • Binding of C1 to antibodies requires cross-linking at least two antibody molecules via C1q.

Activation Sequence

  • Binding of C1q activates:

    • C1r -> activates C1s

    • Forms activated C1qrs, cleaving C4 into C4a and C4b.

  • C4b binds to the membrane, while C2a binds with it and C2b is released.

  • Forming of C4bC2a, a C3 convertase that cleaves C3 into C3a and C3b.

  • Result: Activation of C5 convertase at C4bC2aC3b.

Biological Activities of Classical Pathway Products

  • C2b: Edema due to accumulation in fluid.

  • C3a and C4a: Have roles in degranulation of basophils and mast cells; enhance vascular permeability leading to smooth muscle contraction and anaphylaxis.

  • C3b: Functions as opsonin; promotes phagocytosis.

  • C4b: Also acts as an opsonin.

  • C5a: Anaphylatoxin related to basophil and mast cell activation.

Membrane Attack Complex (MAC)

  • C5 convertase cleaves C5 into C5a and C5b.

  • C5b complexes with C6 and C7, inserting into membranes.

  • This leads to a pore formation through which cellular contents leak, culminating in lysis.

Alternative Pathway

  • Activated by microbes prior to immune response: part of innate immunity.

  • Involves different proteins leading to C3 activation.

  • No involvement of C1, C4, or C2.

Activation of Effector Cells (Chemotaxis)

  • C5a and C5b67 complex serve as chemotactic signals, attracting leukocytes, especially neutrophils and macrophages, to sites of infection.

  • Important in mediating inflammatory responses.

Basic Comparison of Pathways

  1. Classical Pathway:

    • Antibody dependent

    • Produces anaphylatoxins

    • C3 convertase: C4bC2a

    • C5 convertase: C4bC2aC3b

  2. Alternative Pathway:

    • Contact dependent

    • Produces anaphylatoxins

    • C3 convertase: C3bBb

    • C5 convertase: C3bBbC3b

Complement Cascades and Activation Pathways

  • Classical Pathway: Involved antigen-antibody complexes; stabilizes through calcium ions with components C1, C4, C2.

  • Alternative Pathway: Initiated by microbial surfaces; relies on contact without antibody involvement.

  • Lectin Pathway: Involves mannose-binding lectins for complement activation.

Mediation of Inflammation

  • Essential in defending against infection and repairing tissue damage.

  • Functions:

    • Delivers phagocytic cells and plasma to infection sites.

    • Increases fluid flow to dilute toxins.

    • Enhances lymphatic flow for improved antigen exposure and immune response.

    • Anaphylatoxins (C3a, C4a, C5a) promote smooth muscle contraction, endothelial cell contraction, and release of histamine from mast cells.

Enhancement of Phagocytosis

  • If complement pathway halts before completing, targeted cells can still be coated with antibody/complement, leading to enhanced phagocytosis of the complex due to opsonization.

Regulation of the Complement System

  • C1 inhibitor: Inhibits activation of classical and lectin pathways.

  • Soluble inhibitors (e.g., C4BP, CR1, MCP, DAF) act on Factor I, inactivating complements C3b and C4b.

  • Factor H: Regulates alternative pathway by binding C3b and inhibiting convertase activity.

  • S protein: Binds to C5b-7 complex, preventing attachment to cell membranes and avoiding lysis.

Complement Component Deficiencies

  • Inherited deficiencies in any complement components can occur:

    • Lack of C1q, C1r, C1s, C4, or C2 can result in immune complex diseases, often fatal.

    • Defects in complement components of MAC lead to greater susceptibility to infections.

Additional Insights on Complement Activation

  • The classical pathway is reliant on specific antibodies like IgM (most common) and IgG1/G3.

  • Importance of calcium ions for stability during activation, methods including hemolysis to observe Ag/Ab reactions.

Intravascular vs. Extravascular Destruction

  • Extravascular: Antibody-mediated destruction occurs outside of blood vessels, typically in reticuloendothelial system.

  • Intravascular: Complement mechanisms result in cell destruction within blood vessels, relevant in cases like ABO incompatibility.

Complement and Macrophage Attachment Sites

  • Complement attaches at the CH2 domain on the heavy chain of IgG antibodies, facilitating macrophage engagements via Fc receptors.