The complement system consists of more than 30 soluble and cell-bound proteins.
It participates in both innate and adaptive immunity.
Components are mainly produced by hepatocytes, monocytes, and epithelial cells of the gastrointestinal and genitourinary tracts.
Many components are proenzymes (zymogens), which are functionally inactive until proteolytic cleavage removes an inhibitory fragment and exposes the active site.
The reaction starts with an enzyme cascade.
Cascade System
The complement system functions as a cascade system.
A cascade is when one reaction triggers another, which triggers others, leading to exponential growth.
Functions of Complement
Lysis:
Lysis of cells, bacteria, and viruses is a major effector of the humoral branch of the immune system.
Opsonization:
Promotes phagocytosis of particulate antigens.
Activation:
Binding to specific complement receptors on cells of the immune system triggers specific cell functions, inflammation, and secretion of immunoregulatory molecules.
Immune Clearance:
Removes immune complexes from the circulation and deposits them in the spleen and liver.
Visual Representation of Complement Functions
Lysis: Complement leading to target cell lysis.
Opsonization: Complement opsonizing bacteria for phagocytosis by a phagocyte via complement receptors.
Activation of Inflammatory Response: Complement receptors triggering degranulation and extravasation, leading to inflammation in tissues.
Clearance of Immune Complexes: Phagocytes clearing antigen-antibody complexes.
Complement Activation Pathways
Classical Pathway:
Activated by antigen-antibody complexes.
Alternative Pathway:
Activated by aggregated immunoglobulins and microbial products.
Mannan-Binding Lectin Pathway:
Activated by microbial products.
Classical Pathway
Activators: Antigen-antibody complexes.
Antibodies Involved: IgG and IgM.
Activation involves nine major protein components: C1-C9.
Products of activation are enzymes that catalyze subsequent steps.
C1 Complex
C1 complex consists of C1q, C1r, and C1s.
C1qr2s2
C1 Activation
C1 becomes activated when it binds to the ends of antibodies.
Soluble IgM (inaccessible Fc) does not activate complement.
Soluble IgG (Fc portions not adjacent) does not activate complement.
Antigen-bound IgM and IgG can activate complement.
IgA and IgE cannot activate complement.
Building a C3 Activation Complex
Activated C1 activates C2 and C4 by cleaving them.
C2 is cleaved into C2a and C2b.
C4 is cleaved into C4a and C4b.
C2a and C4b bind together on the surface of bacteria.
C2b and C4a diffuse away.
C3 Activation Complex Function
The C3 activation complex (C2a and C4b) activates C3 proteins by cleaving C3 into C3a and C3b.
C3b
Many C3b molecules are produced by the C3 activation complex.
C3b binds to and coats the surface of bacteria.
C3b is an opsonin.
Opsonins bind both to bacteria and phagocytes.
Opsonization increases phagocytosis by 1,000 fold.
Inflammatory Response
C3a increases the inflammatory response by binding to mast cells and causing them to release histamine.
Building the C5 Activation Complex
The surface of the bacteria becomes saturated with C3b.
C2a and C4b (C3 convertase) have a slight affinity for C3b, and C3b binds to them.
When C3b binds to C2a and C4b, it forms a new complex referred to as the C5 convertase.
C5 Activation Complex
The C5 convertase (C2a, C4b, C3b) activates C5 proteins by cleaving them into C5a and C5b.
Many C5b proteins are produced by the C5 convertase, which begin to coat the surface of the bacteria.
Function of C5a
C5a disperses away from the bacteria.
Binds to mast cells and increases inflammation.
It is the most powerful chemotactic factor known for leukocytes.
Building the Membrane Attack Complex (MAC)
C5b on the surface of bacteria binds to C6.
The binding of C6 to C5b activates C6 so that it can bind to C7.
C7 binds to C8, which in turn binds to many C9s.
Together, these proteins form a circular complex called the Membrane Attack Complex (MAC).
Membrane Attack Complex (MAC) Details
The MAC comprises C5b, C6, C7, C8, and multiple C9 molecules.
The pore size is approximately 70-100 Å.
The contents of the cell leak out through the MAC pore, and the cell dies.
Mechanism of Cytolysis
The MAC causes cytolysis.
The circular membrane attack complex acts as a channel through which cytoplasm can rush out and water rushes in.
Functional Overview
Opsonization: Enhancement of phagocytosis by coating with C3b.
Cytolysis: Loss of cellular contents through a transmembrane channel formed by the membrane attack complex C5-C9.
Inflammation: Increase of blood vessel permeability and chemotactic attraction of phagocytes.
Hydrolysis and Binding
C4b2a (C3 convertase) hydrolyzes C3 into C3b and C3a.
C3b binds to C4b2a and forms C4b2a3b (C5 convertase).
C4b2a3b cleaves C5 into C5b and C5a causing inflammatory responses and opsonization.
Hydrolysis of C3 by C3 Convertase C4b2a generates > 200 C3b molecules.
Bound C3b exhibits various biological activities, e.g., binding of C5 and binding to C3b receptors on phagocytes.
Formation of a labile internal thioester bond in C3 binds to free – OH or – NH2 groups on a cell membrane.
Alternative Pathway
Four components: C3, factor B, factor D, and properdin.
Triggering substances may be pathogens or non-pathogens, such as bacterial cell wall components, fungi, viruses, parasites, immune complexes, RBCs, and polymers.
Examples of Initiators
Pathogens: Gram-negative bacteria, lipopolysaccharides, gram-positive bacteria, teichoic acid, fungal and yeast cell walls, some viruses and virus-infected cells, some tumor cells, parasites.
Nonpathogens: Human IgG, IgA, and IgE in complexes; rabbit and guinea pig IgG in complexes; cobra venom factor; heterologous erythrocytes; anionic polymers; pure carbohydrates.
Independence of Antibodies
The alternative pathway is antibody-independent and thus a component of the innate immune system.
C1, C4, and C2 are not involved in the alternative pathway.
Process Overview
C3 hydrolyzes spontaneously; C3b fragment attaches to a foreign surface.
Factor B binds C3a, exposes site acted on by factor D. Cleavage generates C3bBb, which has C3 convertase activity.
Binding of properdin stabilizes convertase.
Convertase generates C3b; some binds to C3 convertase, activating C5 convertase. C5b binds to antigenic surface.
Spontaneous Hydrolysis
Plasma C3, with an unstable thioester bond, can be hydrolyzed spontaneously into C3a and C3b.
C3b attaches to the surface of bacteria, yeasts, viruses, or even host’s own cells.
Stabilization of C3bBb requires Mg++.
Lectin Pathway
Lectin is a protein that binds to carbohydrate.
MBL (mannose-binding lectin) binds to mannose on many bacterial cells.
MBL is produced by the liver during acute-phase inflammatory reactions.
Once MBL binds to the target cell, two serine proteases (MASP-1, MASP-2) bind.
Acts like C1.
Details of MBL
MBL is induced during inflammatory responses.
After MBL binds to the surface of a microbe, MBL-associated serine proteases, MASP-1 and MASP-2, bind to MBL.
The MBL-MASP-1/2 complex mimics the activity of C1r and C1s and causes cleavage and activation of C4 and C2.
Thus, the lectin pathway is antibody-independent.
It is an important innate defense mechanism comparable to the alternative pathway but utilizes the elements of the classical pathway, except for the C1 proteins.
Specificity of MBL
Mammalian cells normally have sialic acid residues covering the sugar groups recognized by MBL and are not a target for binding.
Convergence of Pathways
The three complement pathways converge at the production of an active C5 convertase, leading to the formation of the C5b6789 membrane-attack complex (MAC).
Lectin Pathway: Mannose-binding lectin (MBL) binds to the foreign surface; MBL-associated proteases (MASP1 + 2) bind MBL, generating an activated C1-like complex.
Alternative Pathway: Spontaneous, slow, small amounts of C3.
Common Steps
All pathways lead to the formation of C3 convertase.
Major amplification step with C3b.
Formation of C5 convertase.
Assembly of the Membrane-Attack Complex.
Formation of the Membrane-Attack Complex (MAC)
C5b attaches to C6, then to C7, and the C5b67 complex inserts into the membrane.
Binding of C8 to membrane-bound C5b67 induces a 10 Å pore.
Binding and polymerization of C9, a perforin-like molecule, to C5b678.
The completed membrane-attack complex (MAC) has a tubular form and a functional pore size of 70 – 100 Å.
General Functions of Complement Pathways
Classical Pathway: Antigen:antibody complexes activate the pathway.
MB-Lectin Pathway: Mannose-binding lectin binds mannose on pathogen surfaces.
Alternative Pathway: Pathogen surfaces activate the pathway.
Overview of Complement Functions
Complement activation.
Recruitment of inflammatory cells.
Opsonization of pathogens.
Killing of pathogens.
Biological Effects Mediated by Complement
Cell Lysis
The membrane-attack complex can lyse a broad spectrum of cells: G(-) bacteria, parasites, viruses, erythrocytes, and nucleated cells (tumor cells).
The activation of alternative and lectin pathways is Ab-independent, making these pathways important innate immune defenses against infectious microorganisms.
Inflammatory Response
Peptides generated during activation of complement play a decisive role in the development of an effective inflammatory response.
C3a, C4a, C5a (anaphylatoxins) bind to complement receptors on mast cells and basophils and induce degranulation with the release of histamine and other mediators.
Anaphylatoxins also induce smooth-muscle contraction, increased vascular permeability, extravasation, and chemoattraction (induced by C5a, C3a, and C5b67).
Opsonization by Ab and Complement
C3b opsonizes bacteria, enhancing phagocytosis.
Phagocytes have CR1 and Fc receptors that bind C3b and IgG, respectively.
Viral Neutralization
Formation of larger viral aggregates reduces the net number of infectious viral particles.
The deposits of Ab and complement on viral particles neutralize viral infectivity by blocking attachment to susceptible host cells and facilitate binding of the viral particle to cells possessing FcR or CR1.
Clearance of Immune Complexes
Soluble immune complexes are bound by C3b.
C3b-bound complexes bind to CR1 on erythrocytes.
Erythrocytes transport the complexes to the liver and spleen, where phagocytes clear them.
Consequences of Complement Deficiency
C3 and Factor B Deficiency: Severe bacterial infections.
C3b-INA, C6, and C8 Deficiency: Severe Neisseria infections.
Deficiencies of Early C Components (C1, C4, C2): Systemic lupus erythematosus (SLE), glomerulonephritis, and polymyositis.