Complement system

Activities of complement system

Lysis, opsonisation, activation of inflammatory response, clearance of immune complexes. They are produced by liver hepatocytes and small amount by blood monocytes, macrophage, and epithelial cells. Zymogens are inactive form of complement proteins and they are activated by proteolytic cleavage which removes the inhibitory fragment and exposes the active site of molecule.

There are three pathways of complement activity

1. Classical pathway
2. Alternative pathway
3. Lectin pathway

The common among all these pathways is the formation of MAC (membrane attacking complex). 

1. Classical pathway

The activation starts with formation of soluble antigen antibody complexes or with the binding of antibody to antigen on a suitable target. IgM and IgG activate the complement pathway and initial stage of activation involves components C1, C2, C3, and C4 which are present in plasma in functionally inactive forms.

Fc portion of IgM conformational change on binding of antigen. Exposure site on IgM for C1 complement protein to bind. C1 consists of C1qr₂s₂ held together in a complex stabilized by calcium ions.

C1q molecule à 18 polypeptide chains that associate to form six collagen like triple helical arms, tips of which bind to exposed C1q binding sites in C_H2 domain of the antibody molecule.

Each C1r and C1s monomer contains a catalytic domain and an interaction domain, the latter facilitates interaction with C1q or with each other.

After the C1q is bound to Fc binding sites, a conformational changes induce C1r to become a active serine protease enzyme C1r* which then cleaves C1s to a similar active enzyme C1s*.

C1s* à two substrates C4 and C2. The C4 is glycoprotein contains three polypeptide chains α, β, and γ.

C4 is activated when C1s* hydrolyzes a small fragment C4a from the amino terminus of α chain, exposing a binding site on the larger fragment (C4b).

The C4b fragment attaches to the target surface in the vicinity of C1 and the C2 proenzyme then attaches to the exposed binding site on C4b, where the C2 is then cleaved by the neighbouring C1s*, the small fragment C2b diffuses away.

The resulting C4b2a* complex is called C3 convertase.

C3 convertase converts C3 into active form. C4a is an anaphylatoxin or mediator of inflammation which does not participate directly in the lytic function of the complement cascade. Similarly C3a and C5a is also an anaphylatoxin.

The C3 component consists of two polypeptide chains α and β. Hydrolysis of a short fragment C3a from the amino terminus of α chain by the C3 convertase generates C3b.

Some C3b component binds to C4b2a* complex to form a trimolecular complex called C5 convertase (C4b2a3b*).

C3b component of this complex binds C5 and alters its conformation allowing C4b2a* component to cleave C5 into C5a, which diffuses away and C5b which attaches to C6 and initiates formation of the membrane attack complex.

C3b sometimes function as opsonin and promote phagocytosis. C3b may also bind directly to cell membranes.

2. Alternative pathway

It happens similar to the classical pathway, but it does without the antigen antibody complexes for initiation. Since, it is a component of innate immunity.

It involves four serum proteins C3, factor B, factor D and properdin.

Serum C3 gets cleaved into C3a and C3b on the microbial cell surface.

The C3b bind to either foreign surface antigens or to the host cell. But on the host cell is not affected by C3b binding because host cell surface is having high sialic acid which contributes to rapid inactivation of bound C3b molecules on host cells.

Then another serum protein called factor B forms complex with C3b and it is stabilized by Mg²⁺.

Now this complex serve as a substrate for factor D, which convert them into C3bBb* complex.

The complex is having C3 convertase activity which is limited half life unless the serum protein properdin binds to it. 

Now the C3 convertase cleaves the C3 into C3b and C3a. C5 convertase is formed by the binding of C3b to the C3 convertase (C3bBbC3b). C5 convertase generates C5a and C5b. C5b is used for MAC. 

3. Lectin pathway

Lectins are proteins that recognize and bind to specific carbohydrate targets. Lectin activates complements binds to mannose residues.

Lectin pathway is activated by the binding of mannose binding lectin to mannose residues on glycoproteins or carbohydrates on the surface of microorganisms, including bacteria such as salmonella, listeria and neisseria strains.

MBL is a collectin family acute phase proteins and its concentration increases during inflammatory responses. Its function in the complement pathway is similar to that of C1q, which it resembles in structure.

After MBL binds to the carbohydrates residues on the surface of cell or pathogen, MBL associated serine proteases, MASP-1 and MASP-2 bind to MBL.

Now the active complex formed by this association causes cleavage and activation of C4 and C2. MASP-1 and MASP-2 are structurally similar to C1r and C1s and mimic their activity. 

MAC formation 

C5b, C6, C7, C8, and C9 which interact sequentially to form a macromolecular structure called the membrane attack complex (MAC). It forms a large channel on the membrane of the target cell enabling ions and small molecules to diffuse freely across the membrane.

C3b and C4b binding facilitates opsonisation.

Complement system neutralize viral infectivity. 

Regulation of complement system

C1 inhibitor à form complex with C1r2s2 causing it to dissociate from C1q and preventing further activation of C4 or C2.

C3b is hydrolysed once it moves away at 40 nm distance from the C4b2a* or C3bBb* convertase enzymes.

C3 convertase activity is regulated by proteins which contain repeating amino sequences of about 60 residues termed short consensus repeats (SCRs). All this proteins are encoded in single location on chromosome 1 in humans, known as the regulators of complement activation (RCA) gene cluster.

RCA proteins prevent C3 convertase assembly and it includes proteins like C4b binding protein (C4bBP), and two membrane bound proteins, complement receptor type 1 (CR1) and membrane cofactor protein (MCP).

Each of these regulatory proteins binds to C4b and prevents its association with C2a. Once the regulatory proteins binds to C4b, another regulatory protein factor I cleaves the C4b into bound C4d and soluble C4c.

In other case, MCP, CR1 and Factor H binds to C3b and prevents it association with factor B. After this, Factor I cleaves C3b into a bound iC3b and release C3c fragment.

Decay accelerating factor (DAF or CD55) which is a glycoprotein covalently anchored to a glycophospholipid membrane protein has the ability to dissociate C3 convertase.

Homologous restriction factor (HRF) or membrane inhibitor of reactive lysis (MIRL or CD59). CD59 protects cells from nonspecific complement mediated lysis by binding to C8 preventing assembly of poly C9 and its insertion into plasma membrane.