Classical complement pathway

Introduction to Complement

• Complement is the name given to a system of some non-specific proteins present in normal human and animal serum.
• These proteins have got the ability to lyse or damage cells (bacteria, virus, tumour cells) and to stimulate some antigen and antibody reactions.
• This system consists of approximately 20 serum proteins which include components of complement (C1-C9), properdin system and regulatory proteins.
• Some of these proteins are enzymes, some are control molecules while others are structural proteins without any enzymatic action.
• Complement in general is heat-labile and inactivated at 56^oC for 30 minutes.
• Components of the complement are activated sequentially through a cascade manner either by classical or alternative pathway and both the ways have same results.

                                                                       Image source: sciencedirect

Classical Pathway

• This pathway is initiated by binding of complement to aggregates of antibody and antigen on the cell surface or an immune complex.
• The model explains the lysis of erythrocyte by antibody in the presence of complement.
• In lytic action, all the nine components of the complement (C1-C9) comprising of 11 proteins are involved.
• These reacts together one after the other in a cascade or sequential manner once the reaction has been triggered.
• Only IgG and IgM possess complement binding sites and capable of sensitizing red cells to complement lysis.
• Other immunoglobulins (IgA, IgD and E) do not have a binding site for C1q.
• IgM is a better complement binding antibody than IgG.
• The complement components are grouped under three functional units: C1, the recognition unit; C4, C2, C3, the activation unit; and C5-C9, the membrane attack unit.

A) C1- the recognition unit

• C1 is a complex of three subunits; C1q, C1r and C1s held together by calcium ions.
• One molecule each of C1q and C1s with two molecules of C1r, compose the C1 complex or recognition unit.
• The complement binding site is not exposed in the native state of Ig molecule and it gets exposed only after configurational changes in the molecule following antigen-antibody interaction.

1. C1q facilitates binding of the recognition unit (C1 complex) to the exposed site on the Fc region of the cell surface antigen-antibody complexes.
2. Activation of classical complement cascade requires linkage of C1q with two molecules of IgG or one molecules of IgM through their Fc regions.
3. Binding of C1q, activates C1r (referred to now as C1r*).the enzymes C1r* then converts C1s to an active serine esterase (C1s*, C1s esterase).

B) Activation Stage

• C1s esterase activates C4 which gets split into C4a (anaphylatoxin) and reactive C4b, and C2 to C2a and C2b.
• C4a is liberated into fluid phase, and C4b attaches to erythrocyte surfaces, bacterial cell membranes and other antigens.
• C2a (larger fragments) binds with C4b forming cell bound C4b2a complex, known as the classical pathway C3 convertase.
• C2b (minor fragment) is released into fluid phase.
• C3 is a heart of complement cascade.
• C4b2a (C3 convertase) binds to the cell membrane and acts on C3 and splits into a small 3a and a large C3b subunit on the surface of the cell.
• C3a is an anaphylatoxin.
• The C3 convertase amplifies the response by splitting many C3 molecules.
• A small amount of C3b generated will bind to the activating surface and act as focus for further complement activation.

C) Membrane Attack Complex

• The terminal stage of the complement pathway involves creation of the membrane attack complex, which is known as lytic unit.
• The C5 convetases are generated from C4b2a of the classical and C3bBb of the alternative pathway by addition of another C3b molecules.
• These membrane bound tri-molecular complexes selectively bind C5 and cleave it into a small C5a released in body fluids (anaphylatoxic and chemotactic) and a large membrane bound C5b.
• The formation of rest of the membrane attack complex is non-enzymatic.
• C6 binds to C5b and the joint complex C5b6 is released from the C5 convertase.
• C7 binds to the C5b6 complex forming a heat stable tri-molecular complex C5b67.
• The C5b67 is highly lipophilic and binds to membrane bilayer in the vicinity of initial activation site, where it lies as a high affinity receptor for C8.
• C5b678 binds and polymerizes C9 forming the membrane attack complex (MAC).
• As many as 12-16 C9 molecules can be polymerized by one C5b678 complex.
• A (C5b678)1(C9)n complex forms and drills a hole in the cell membrane, which leads to the hypotonic lysis of cells.

References: 

i) https://www.sinobiological.com/research/complement-system/complement-activation-classical-pathway

ii) https://microbenotes.com/classical-pathway-of-complement-activation/

Classical complement pathway