The Complement System
Features of the Complement System
- Comprises more than 30 circulating and membrane-expressed proteins
- Is an important effector arm of both innate and acquired immune responses
- Named from early observations of its activity
 Material of serum that “complemented” the ability of antibody to kill bacteria
- Complement components play a major role in defense against infectious organisms
 Production of opsonins
o Molecules that enhance the ability of macrophages and neutrophils to phagocytose material
 Production of anaphylatoxins
o Peptides that induce local and systemic inflammatory responses
 Direct killing (lysis) of pathogens
- Components are synthesized mostly in the liver and cells involved in the inflammatory response
 Circulatory concentration of all complement proteins is ~3mg/ml
 Some components are found at higher concentrations than others
Complement Activation
- The first step of complement activation is recognition of a pathogen
- There are 3 different mechanisms by which the complement system recognizes microorganisms
 Each pathway results in the attachment of complement proteins to microbial surfaces
 Each pathway begins with an enzyme cascade
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Classical pathway
o This pathway is a major effector pathway of the adaptive immune response
 Triggered by antibodies bound to antigens on a pathogen
 Antigen-antibody complexes (immune complexes) are the main pathway activators
o Component proteins are designated C1 – C9 (order in which they were discovered)
o First step of pathway is the binding of C1 to the Fc region of an antibody
 C1 is a macromolecular complex consisting of three proteins
- C1q, C1r, C1s
 Each C1 must bind ≥ 2 Fc sites for a stable C1-antibody interaction to occur
o Series of interactions between C1q, C1r, and C1s leads to generation of C1 protease
 C1 protease has two substrates (C4, C2)
o C1 protease acts on C4 to yield two fragments
 C4a (smaller)
 C4b (larger) that binds covalently to the cell surface of a target
o C1 protease also acts on C2 to form two products
 C2a
 C2b that remains attached to C4b on the pathogen’s cell surface (C4b2b)
o C4b2b complex acts on complement component C3 to become C3 convertase
o C3 convertase cleaves C3 to produce two fragments
 C3a (smaller)
 C3b (larger)
o Some C3b binds to C4b2b to form a trimolecular complex (C4b2b3b)
o C4b2b3b complex acts on complement component C5 to become C5 convertase
Alternative pathway
o This pathway is an effector arm of the innate immune response
 Activation of the alternative pathway occurs in the absence of specific antibody
o Triggered directly by macromolecules on pathogen cell surfaces
o Triggered by almost any foreign substance
 Lipopolysaccharide (LPS) from cell walls of gram-negative bacteria is widely studied
o Component proteins can be unique (i.e. serum factors B, D, P)
o Component proteins can be common to other pathways (i.e. C3)
o Low-level C3 conversion (via slow spontaneous hydrolysis) makes complement activation
possible without the participation of classical pathway components
o C3 converts into two products
 C3a
 C3b
o C3b binds to antigens on foreign pathogen surfaces
o C3b then combines with serum protein Factor B to form C3bB
o Serum protein factor D cleaves C3bB into C3bBb (alternative pathway C3 convertase)
 C3bBb interacts with Factor P to stabilize and extend the life of C3 convertase
o C3 convertase generates C5 convertase (C3bBb3b complex)
Lectin pathway
o This pathway is an effector arm of the innate immune response
 Activation occurs in the absence of antibody
o Activated by terminal mannose residues on the surface of pathogens
 Terminal mannose residues are not found on the surfaces of mammalian cells, so this
pathway is thought to discriminate between self and non-self
o Mannan-binding lectin (MBL) binds to mannose on the surface of pathogens
 Binding activates two associated proteases
- Mannose-associated serine protease (MASP) 1 and 2
o Activated MASP 1 and 2 leads to C2 and C4 cleavage
o C2b and C4b form C3 convertase (as in the classical pathway)
o C3 convertase cleaves C3 into C3a and C3b
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 Leads to the formation of the C5 convertase (C4b2b3b)
All complement activation pathways converge at the formation of the membrane attack complex (MAC)
 C5 convertase cleaves C5 to form two product
o C5a, C5b
 C5b binds to microbial cell membranes and bind C6 (C5b6 complex)
 C5b6 complex combines spontaneously with C7 (C5b67) and inserts into the membrane
 C8 binds to the complex C5b67 and initiates the binding and polymerization of C9
 Incorporation of many C9 molecules causes further membrane penetration that results in the formation
of a larger transmembrane channel that disturbs the cell’s osmotic equilibrium
 Ions pass through the channel and water enters cell causing the cell to swell and lyse
Regulation of Complement Activity
- Complement activation generates activated fragments (especially cleavage products of C3, C4, C5) that induce
potent inflammatory responses that remove infectious agents but can also damage the host
- Uncontrolled complement activation can rapidly diminish complement components, leaving the host unable to
remove infectious agents
- Inappropriate complement activation does not normally occur due to regulatory mechanisms at most steps
- Many molecules that regulate complement activation are expressed only on mammalian cell surfaces
 Not found on microbial cells
 Consequently, regulators of complement activity limit damage to the individual while allowing
activated components to focus on removing microbial pathogens
- Important regulators of the classical pathway
 C1 protease inhibitor (C1INH) inhibits the first step in the activation of the classical pathway
o C1INH binds to C1r and C1s, causing them to dissociate from C1q
 Proteins inhibit C3 convertase assembly via displacement of C2b from the C4b2b complex and
cleavage of C4b to inactive forms (C4d, C4c)
o C4b-binding protein (C4BP)
o Decay accelerating factor (DAF, CD55)
o Cofactor protein (MCP, CD46)
o Complement receptor 1 (CR1)
 CD59 prevents final assembly of the MAC at the C8 and C9 stages
- Important regulators of the alternative pathway
 Factor H and Factor I regulate and inhibit C3 convertase assembly
Biological Activities of Complement
- Opsonization
 Macrophages and neutrophils express cell-surface receptors (CRI) that binds to C3b
o Microorganisms coated with C3b can then be phagocytosed
- Induction of local inflammation
 C3a, C4a and C5a fragments cause inflammation at a site of complement activation via 2 mechanisms
o Fragments act as chemoattractants that direct phagocytic cell migration to the activation site
o Fragments can cause degranulation of mast cells and basophils to release inflammatory
mediators (histamine, other vasoactive substances) that increase capillary permeability
 If degranulation is too strong, anaphylactic shock occurs
 Thus, fragments are often referred to as anaphylatoxins
- Pathogen lysis
 The membrane attack pathway causes hole formation in pathogen membranes (leads to pathogen lysis)
- Viral neutralization
 MBL binds to a variety of viruses, leading to opsonization and lysis of the organism
 MBL can interfere with virus – membrane receptor interaction, preventing viral entry into the host cell
- Clearance of immune-complexes
 Soluble immune complexes coated with C3b become bound to complement receptors (CRI) present on
phagocytic cells and especially on circulating RBCs
o RBCs carry complexes to the liver and spleen where they are removed by macrophages
Complement Deficiencies
- The complement system is particularly important in defense against pyogenic (pus-forming) bacteria
 Neisseria species (responsible for meningitis and some STDs)
 Staphlococcus aureus
 Streptococcus pneumoniae
 Haemophilus influenzae
- The major pathways of defense include the production of IgG antibody that binds to bacteria, opsonization,
compliment activation, phagocytosis, and intracellular killing
 If any one of these functions is diminished (i.e. from genetic deficiencies or acquired conditions), the
individual becomes susceptible to pathogens
- Because the compliment system is vital for removal of immune complexes from circulation, defective or
deficient components can result in deposition of these complexes in tissues (results in autoimmune conditions)
 Deficiencies are rare (1 in 10,000 people) and not always associated with disease
Complement Deficiency
C1q, C1r, C2, C4
C3
C5
C1 INH
Factor I
Factor D
Factor H
Clinical Presentation
Systemic Lupus Erythematosus (SLE)
Recurrent infection
Recurrent infection
Hereditary Angioedema (HAE)
Recurrent infection
Recurrent infection
Recurrent infection
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Deficiencies in any of the classical pathway components (C1, C4, C2) are associated with the inability
to clear immune complexes, resulting in increased susceptibility to autoimmune disease (e.g. SLE)
 A C3 deficiency is severe (even life-threatening) because C3 is central to all compliment pathways
o C3-deficient individuals are susceptible to recurrent pyogenic infections and may also develop
inflammatory disorders associated with circulating immune complexes
 Deficiency of the alternative pathway’s properdin (factors B or D) results in pyogenic infections
 Deficiency of MBL can be a major problem in early life, manifesting as severe recurrent infections
Hereditary angionedema (HAE)
 Uncontrolled cleavage of C2 and C4 that results from a deficiency of C1INH, the regulatory control
protein for classical pathway components C1r and C1s
o C1INH inhibitory functions
 Inhibits complement activation
 Inhibits activity of enzymes in other serum cascades
- Serum pathways that form kinins (e.g. bradykinin), potent vasodilators and
inducers of vascular permeability and smooth muscle contraction
 Symptoms of HAE
o Localized edemas in skin and mucosa due to dilatation and increased capillary permeability
o Pain in the abdomen
o Swelling of the larynx that can compromise breathing
How can pathogens avoid being killed by these multiple complement activation pathways?
Schistosomes – Extracellular parasitic blood worms that infect several wildlife species, livestock and people