Complement System Lecture Notes

Complement System

Discovery of Complement System

The complement system was discovered by Buchner in 1893 and further elucidated by Bordet in 1895, who was awarded the Nobel Prize in 1919. The discovery stemmed from the observation that antisera lost their ability to kill cholera bacteria upon heating to 56°C. This ability was restored by adding unheated, non-immune serum. Bordet deduced that antibodies are heat-stable, but another heat-labile serum component is necessary for the antibody's action to destroy microbes, terming this component 'complement'.

Overview of the Complement System

The complement system comprises over 20 individual components, making it a very complex system. These components are primarily inactive precursor molecules (e.g., C1, C2, C3). Proteins are produced in the liver and present at low levels in normal serum. These precursors can be converted into active enzymes or powerful effector molecules when the cascade mechanism is triggered.

The complement system is a component of the innate immune system but can be activated by the adaptive immune response.

Enzyme-Amplifying Cascade Principle

The complement system operates via an enzyme-amplifying cascade:

Pro-enzyme 1 ===> activated enzyme 1 + cleavage fragment (CF)
Pro-enzyme 2 ===> activated enzyme 2 + CF
Pro-enzyme 3 ===> activated enzyme 3 + CF
etc.

In this cascade, many of the generated complement molecules act as enzymes to propagate the process and possess powerful biological effector roles.

Activation Pathways

The complement system can be activated through three different pathways:

  1. Classical Pathway: Involves antibody binding to an antigen. This pathway depends on the adaptive immune response.

  2. Alternative Pathway: Does not involve antibodies and is triggered by direct contact with certain microbes. It is considered part of the innate immune response.

  3. Lectin Pathway: Does not involve antibodies but is triggered by mannose-binding lectin (MBL) sticking to mannose on the surface of certain microbes. It is considered part of the innate immune response.

All three pathways converge at the point where complement component C3 is cleaved.

Classical Pathway: Detailed Mechanism

C1 Activation

The classical pathway is initiated by the activation of the C1 complex. The C1 complex consists of C1q, C1r, and C1s subunits. C1q has six globular heads attached to collagen-like stalks.

To trigger the cascade, at least two Fc regions of antibodies must bind simultaneously to two globular heads of the C1q molecule on the antigen surface. IgM, due to its pentameric structure, is more efficient at activating complement compared to IgG. IgA (dimer) can also activate the classical pathway. IgE and IgD are less efficient.

Cascade Initiation

When at least two Fc regions bind to C1q, a conformational change occurs in C1q. This change leads to the activation and association of C1r and C1s, forming a catalytically active complex.

This active C1 complex then cleaves both C2 and C4 into their respective fragments, designated 'a' and 'b'.

C3 Convertase Formation

The cleavage of C2 and C4 leads to the generation of C4b and C2a, which combine to form the C3 convertase (C4b2a).

The C3 convertase cleaves C3 into C3a and C3b. This cleavage is a central event in the complement cascade, leading to various effector roles. C3b can then bind to C4b2a to form C4b2a3b, which acts as C5 convertase.

Formation of the Membrane Attack Complex (MAC)

The C5 convertase cleaves C5 into C5a and C5b. C5b initiates the assembly of the membrane-attack complex (MAC).

Deposition of C5b on the target surface leads to the sequential assembly of MAC from C5b, C6, C7, C8, and multiple C9 molecules. The MAC forms a pore in the target cell membrane, leading to cell lysis.

Electron micrographs show the MAC as a funnel-shaped lesion inserted into the cell membrane. The pore typically consists of 16 C9 molecules.

MB-Lectin Pathway

In the MB-Lectin pathway, mannose-binding lectin (MBL) recognizes and binds to mannose and other carbohydrate sugar residues on the pathogen surface. This binding activates the complement cascade, leading to the formation of C3 convertase.

Alternative Pathway

The alternative pathway is initiated by direct contact with certain microbes and does not require antibodies. Initiators include:

  • Many strains of gram-negative bacteria

  • Lipopolysaccharides from gram-negative bacteria

  • Many strains of gram-positive bacteria

  • Teichoic acid from gram-positive cell walls

  • Fungal and yeast cell walls (zymosan)

  • Some viruses and virus-infected cells

  • Some tumor cells (Raji)

  • Parasites (trypanosomes)

Biological Roles of Complement

The complement system has several critical biological roles:

  1. Opsonization: Enhances phagocytosis by coating microorganisms and immune complexes with C3b.

  2. Activation and Attraction of Phagocytes: The components C3a and C5a activate and attract phagocytes, including macrophages and neutrophils.

  3. Degranulation of Mast Cells: C3a and C5a induce degranulation of mast cells, leading to the release of inflammatory mediators.

  4. Lysis of Target Cells: Formation of the MAC (C5b-C9) leads to the lysis of target cells.

Effector Functions

  • C5b-C9 (MAC): Lysis of target cells.

  • C3b: Opsonization.

  • C3a and C5a: Activation and attraction of phagocytes, degranulation of mast cells (anaphylatoxins).

  • C3b (especially in liver and spleen): Clearance of immune complexes.

Role of Antibody and Complement in Combating Infections

Antibodies and complement work synergistically to combat bacterial infections through:

  1. Toxin neutralization

  2. Complement-mediated lysis

  3. Opsonization and phagocytosis

  4. Anaphylatoxins mediate mast cell degranulation

  5. Chemotaxis

Antibodies act as opsonins and also activate the complement system (complement C3b also functions as an opsonin). The combined effect leads to more rapid clearance of microbes.

Anaphylatoxins: C3a and C5a

C3a and C5a are anaphylatoxins that promote mast cell degranulation. They trigger the release of granules and activate:

  • Phospholipase A2

  • Arachidonic acid

  • Lipoxygenase pathway

  • Cyclo-oxygenase pathway

This leads to the production of inflammatory mediators and extravasation.

Opsonization Enhances Phagocytosis

Opsonization of antigens (e.g., bacteria) greatly enhances phagocytosis. The process involves:

  • Pseudopodia extension

  • Phagosome formation

  • Lysosome fusion (phagolysosome)

  • Antigenic peptide presentation via Class II MHC molecules

  • Exocytosis of degraded material

Complement Deficiencies and Disease

Deficiencies in complement components can lead to increased susceptibility to infections:

  • Deficiency in early components (C1, C2, C4): Recurrent disseminated infections with pyogenic bacteria (e.g., streptococci, staphylococci). These bacteria are normally dealt with by opsonization, phagocytosis, and local inflammation. The lytic pathway is less critical as gram-positive bacteria are resistant to lysis.

  • Deficiency in lytic MAC components (C5-C8): Disseminated infections with Neisseria bacteria (e.g., N.N. gonorrhoeae, N.N. meningitides). These bacteria are normally eliminated by the lytic pathway.

  • Deficiency in complement receptors (on phagocytic cells): Recurrent infections with pyogenic bacteria due to impaired chemotaxis, opsonization, and phagocytosis.

Specific Infections Associated with Complement Deficiencies Based on specific complement component deficiencies:

  • C1 (r/q): Gram-positive infections, mainly respiratory.

  • C2: Gram-positive, recurrent respiratory infections; meningitis, sepsis, tuberculosis.

  • C3: Gram-positive, recurrent infections.

  • C4: Gram-positive infections; sepsis, meningitis.

  • C5: Meningitis (N.N. meningitidis), disseminated gonococcal infection.

  • C6: Meningitis (N.N. meningitidis), disseminated gonococcal infection.

  • C7: Meningitis (N.N. meningitidis).

  • C8: Meningitis (N.N. meningitidis), disseminated gonococcal infection.

  • C9: Meningitis (N.N. meningitidis).

Summary of Complement System Functions

The major functions of the complement system in host defense are:

  • Opsonization and phagocytosis: Cell-bound C3b promotes phagocytosis of coated cells.

  • Stimulation of inflammatory reactions: The proteolytic products C5a, C3a, and (to a lesser extent) C4a stimulate leukocyte recruitment and inflammation.

  • Complement-mediated cytolysis: The MAC lyses cells.

Effective Collaboration

The complement system plays a key role in the effective collaboration between the innate and adaptive immune responses to facilitate microbe destruction and/or elimination.