The Complement System Andrew e thompson md frcpc

The Complement System

• Andrew E Thompson MD FRCPC

• Fellow in Rheumatology

• University of British Columbia

Overview

• The complement system is part of the innate immune system (vs adaptive)

• It is named “complement system” because it was first identified as a heat-labile component of serum that “complemented” antibodies in the killing of bacteria

• It is now known that it consists of over 30 proteins and contributes 3 g/L to overall serum protein quantities

“Classical” Pathway

• Begins with antibody binding to a cell surface and ends with the lysis of the cell

• The proteins in this pathway are named C1-C9 (the order they were discovered and not the order of the reaction)

• When complement is activated it is split into two parts

• a – smaller of the two
• B – larger part and usually the active part (except with factor 2)

• Remember 3 Key Words

• ACTIVATION
• AMPLIFICATION
• ATTACK

“Classical Pathway”

• ACTIVATION

• C1q portion of C1 attaches to the Fc portion of an antibody
• Only IgG and IgM can activate complement
• Once activated C1s is eventually cleaved which activates C4 and C2
• C4b & C2a come together to form the C4b2a which is the C3 convertase
• C3 convertase activates C3 to C3a and C3b

“Classical Pathway”

• ACTIVATION

• C3a binds to receptors on basophils and mast cells triggering them to release there vasoactive compounds (enhances vasodilation and vasopermeability)
• C3a is called an anaphylatoxin
• C3b serves as an opsonin which facilitates immune complex clearance

“Classical Pathway”

• AMPLIFICATION

• Each C1s creates many C4b and C2b fragments
• Each C4bC2a creates many C3b (activated C3)
• Each C3b goes on to create many Membrane Attack Complexes
• Example
• 1 C1S makes 100 C4bC2b
• 100 C4bC2b makes 10,000 C3b
• 10,000 C3b makes 1,000,000 MAC

“Classical Pathway”

• ATTACK

• Most C3b serves an opsonin function
• Some C3b binds to C4bC2a to form the C5 convertase C4bC2aC3b
• C5 convertase cleaves C5 leading to the formation of the Membrane attack Complex (C5-C6-C7-C8-C9)
• The MAC “punches holes” in cell walls resulting in lysis

“Alternative Pathway”

• Requires no specific recognition of antigen in order to cause activation

“Alternative Pathway”

• ACTIVATION

• Spontaneous conversion from C3 to C3b occurs in body
• Normally, C3b is very short lived and quickly inactivated by proteins on the surface of the body’s own cell walls
• However, bacteria or other foreign material may lack these surface proteins allowing C3b to bind and stay active

“Alternative Pathway”

• AMPLIFICATION

• Factor B binds to C3b
• Factor B is then cleaved by factor D into Ba and Bb
• C3bBb remains which acts as a C3 convertase (C3  C3a and C3b)
• C3bBbC3b is formed which acts as a C5 convertase

“Alternative Pathway”

• ATTACK

• C5 is cleaved to C5a and C5b
• C5b then starts the assembly of the Membrane Attack Complex

Summary - Activation

• Complement can be activated by the binding of antibody (Classical) or by the adherance of C3b to foreign material (Alternative)

• The two pathways converge at the formation of the C5 convertase (C4b2a3b or C3bBbC3b)

• The final common pathway is the formation of the membrane attack complex

Summary - Function

• Opsonization – C3b

• Chemotaxis – C5a (attracts neutrophils)

• Increases vasodilation & permeability of capillary beds via mast cell and basophil activation – C3a & C5a (Anaphylatoxins)

• Cellular Lysis via the MAC

Laboratory Measures

• C3 – Quantitative measure (nephelometry)

• C4 – Quantitative measure (nephelometry)

• CH50

• Functional assay of entire Classical Pathway
• Measures the ability of the patients serum to lyse 50% of a standard suspension of sheep erythrocytes coated with rabbit antibody
• A low CH50 suggests either
• CONSUMPTION OF COMPLEMENT COMPONENTS
• DEFICIENCY OF COMPLEMENT COMPONENTS

• AH50

The Role of Complement in the Rheumatic Diseases

• The Double Edged Sword!

• Needed for proper handling of immune complexes

• Also mediates tissue damage, especially in the setting of autoantibodies and excessive immune complex formation

• Just as the complement system can destroy a microbe, it can lyse and erythrocyte, phagocytose a platelet, or disrupt a basement membrane

Autoimmunity and Inherited Complement Deficiencies

• Inherited deficiencies of complement components can result in autoimmunity, especially SLE

• C1q deficiency – 90% have SLE
• C4 deficiency – 75-80% have SLE
• C2 deficiency – 10-40% have SLE

• Early age of onset, prominent cutaneous manifestations, and presence of anti-Ro antibodies are features suggestive of a complement deficiency

Autoimmunity and Inherited Complement Deficiencies

• How does SLE form with complement deficiencies?

• Failure to clear autoantigens (apoptotic cells)
• Immature dendritic cells uptake the antigen in the presence of inflammatory cytokines causing them to mature into antigen presenting dendritic cells – Presents to T-Cell
• Autoreactive B-Cells take up antigen from apoptotic cells and (with the help CD4+ Th2-Cells) transform into plasma cells that secrete autoantibody

Indirect Laboratory uses of Complement – Detection of Immune Complexes

• C1q binding Assay

• Normally C1q has a very weak affinity for monomeric IgG and IgM
• When IgG or IgM are part of an immune complex the Fc portion undergoes a conformational change
• This results in a much higher affinity for C1q
• This test is an ELISA which looks for immune complexes in a patients serum capable of binding C1q

Indirect Laboratory uses of Complement – Detection of Immune Complexes

• Raji Cell Preparation

• Raji cells are a human lymphoblastoid cell derived from a patient with Burkitt’s lymphoma
• They are unique because
• They have surface receptors for C1q, C3b, C3bi, and C3d
• Lack of surface immunoglobulin
• Surface IgG receptors are low in number and avidity
• Therefore, immune complexes containing complement can bind to surface receptors on Raji Cells!
• This can then help to detect immune complexes capable of binding complement
• Sensitive test, however, warm reactive anti-lymphocyte antibodies and anti-ds-DNA antibodies may cause false positive results