Pathways of Complement System | Immunology

In this article we will discuss about the classical and alternative pathways of complement system.

1. The Classical Pathway of Complement:

The classical pathway of complement is ini­tiated by the interaction of antibody with antigen directly (soluble antigen-antibody complexes or immune complexes).

The gradual progress of classical pathway can be mediated by these successive stages called:

(i) Activation of C1 component

(ii) Production of C3 convertase

(iii) Production of C5 convertase and

(iv) Action of membrane attack complex (MAC)

(i) Activation of CI component:

The initial stage of activation involves C1, C2, C3 and C4. The soluble antigen-antibody complex induces a conformational changes in the fragment crystalized (Fc) portion of the antibody molecule that exposes a binding site for the C1 component of the complement system.

1. C1 is a complex macromolecular protein present in serum in inactive condition. It is a com­plex of three proteins named—C1q, C1r and C1s, out of which C1q recognizes and binds to the Fc region of the antibody and C1r and C1s remain as inactive proteases with their two subunits each. C1q and two molecules of each C1r and C1s held together is a complex called C1qr₂s₂ which is stabilized by Ca²⁺ ions.

2. The structure of C1 is mainly exhibited by C1q; a large molecule composed of 18 polypeptide chains that associate in such a way that forms six collagen-like triple helical arms. The amino-terminal two-thirds of the polypep­tides form the stalk and the carboxy-terminal one-third of the polypeptides form the globular flower, which contains the binding site for anti­body.

3. Normally, C1r₂s₂ complex remains in inactive form and never binds with C1q at that time and shows the configuration ‘S’. Each C1r and C1s includes two domains named catalytic domain and interaction domain. Due to action of interaction domain in presence of antigen- antibody complex in the serum it binds with C1q.

4. C1q binds to an antibody Fc region by its globular heads, in terms, activates serine pro­teases C1r and C1s which are proteolytic enzymes gives serine residues at the active site after being activated.

On binding to antibody, one molecule of C1r is induced to cleave itself, becomes enzymatically active. Gradually it cleaves and acti­vates the second C1r and both C1s molecules. The activated serine protease C1s binds, cleaves and activates the next two components of the classical pathway i.e. serine protease C4 and C2. Ultimately active CI component is called C1qr₂s₂ (Fig. 7.9 and 10).

Activation of classical pathway via IgM and IgG:

The cascade reaction of complement system is only initiated when antibody binds to multiple sites on a cell surface, normally that of a pathogen. When IgM (pentameric) is bound to antigen on a target surface, it requires at least three binding sites for C1q attachment.

In case of IgG molecule, it contains a single C1q binding site in the CH₂ domain of the Fc. As C1q globular head requires at least two Fc sites for a stable C1-antibody reaction, it indicates that two IgG are required to be present on a target surface.

The structural differences between IgM and IgG exert the effect on their activation level. At the acti­vation of Clq binding, IgG requires less amount of time but a good number of IgG molecules are to be present. Whereas IgM activation is delayed one but it is more efficient, even a single IgM molecule can initiate the process (Fig. 7.11).

(ii) Production of C3 convertase:

Active serine protease enzyme C1qr₂s₂ has two distinct substrates, C4 and C2. C4 component is a large globular glycoprotein containing three polypeptide chains named α, β and γ. C4 is activated when C1s hydrolyzes a small fragment C4a from the amino terminus of the chain, exposed a binding site on the larger fragment C4b. The C4b fragment attaches to the target surface of the C1 bound to antibody on the pathogen surface.

Besides, active C4 component, the activated C1s protease acts on C2 serine protease, as a result the smaller fragment C2b will be cleaved away from the site of action and C2a larger fragment will remain active at the active site. After that C4b2a active complex is formed which in turn act on the substrate C3 component. C4b2a is called C3 convertase of the classical pathway.

(iii) Production of C5 convertase:

C3 is almost very similar to C4. C3 compo­nent is with two types of polypeptide chains — α and β. C3 convertase (C4b2a) helps to cleave the smaller fragment C3a from the amino terminus of the a chain of C3 component.

Even a single C3 convertase molecule can accelerate the production of more than 200 molecules of C3b, and the result is amplification. In due course produced C3b binds with C4b2a to form a tri-molecular complex called C4b2a3b i.e. C5 convertase.

(iv) Action of membrane attack complex (MAC):

C5 convertase acts on C5 protein component, cleaves C5a from the action site and C5b attaches to the antigenic surface. This bound C5b initiates formation of membrane-attack complex (MAC) by taking participation of C6, C7, C8 and C9 compo­nents gradually and ultimately forms C5b6789 (MAC) which makes a large pore in the membrane of the antigen and accelerates lysis of it (Fig. 7.12).

2. The Alternative Pathway:

Besides the classical pathway, complement system can be initiated by another method called alternative pathway. Unlike classical pathway the alternative pathway is initiated by the cell-wall constituents of both gram-positive and gram- negative bacteria as foreign particles.

Microbial surfaces directly affect the serine protease C3, gradually cleaving of C3 into C3a and C3b. This conformational change extends its effect on another factor i.e. factor B. In turn Ba removed from active site keeping Bb towards the C3b in presence of Mg⁺⁺; forms C3bBb, and consi­dered as C3 convertase of alternative pathway.

Binding of C3b exposes a site on factor B that again serves as the substrate for an enzymatically active serum protein called factor D. Actually factor D cleaves the C3b bound factor B, and helps to form C3bBb. The action of C3bBb is very unsta­ble, becomes stabilized by the presence of another exclusive serum protein properdin in this pathway, helps to increase the convertase activity period.

Formation of C3bBb accelerates the auto- catalyse of more C3 component and forms C3bBb3b as C5 convertase. Though structural basis of C3 and C5 convertase vary in these two path­ways of complement system but their mode of action is alike.

Here, C3bBb3b subsequently hydro­pses the bound C5, C6, C7, C8 and C9 respectively, resulting in Membrane Attack Complex (MAC) formation which binds to the antigenic surfaces of microbes (antigen). MAC gradually displaces the membrane phospholipids, forms a large trans­-membrane channel and gradually destroys the membrane and lysis of the antigen occurs.

The Lectin mediated pathway:

The third pathway of complement system is lectin-mediated pathway. Lectin-mediated path­way is activated by the binding of mannose-binding protein present in blood plasma to mannose containing proteoglycans on the surfaces of the bacteria and yeast, it forms MBP-MASP (Mannose-binding protein-mannose-associated serum protease). In lectin pathway MBP-MASP acts on the substrate C₄ and C₂ component protein.

Three different pathways of complement acti­vation is shown in the Fig. 7.13.