Complement Pathways: Types, Functions and Regulation

Complement Pathways: Types, Functions and Regulation

• Complement was discovered by Jules Bordet as a heat-labile component of normal plasma that causes the opsonisation and killing of bacteria.
• It helps antibodies and phagocytic cells to clear pathogens and damaged cells; promote inflammation and attack pathogen’s plasma membrane.
• Proteins that take part in the complement system are called complements that collectively work as a biological cascade
• The complement system refers to a series of >20 proteins, circulating in the blood and tissue fluids. Most of the proteins are normally inactive, but in response to the recognition of molecular components of microorganisms they become sequentially activated in an enzyme cascade – the activation of one protein enzymatically cleaves and activates the next protein in the cascade.
• Complements are mainly denoted by the capital letter C with numbers; like, C1, C2, C3, and so on. Some have only alphabet, like, B, D. Some are simply represented by names, like, homologous restriction factor.
• C1 has three sub-units; C1q, C1r and C1s. C2-C5 have two components, a and b. Larger subunits are denoted by b whereas the smaller are denoted by a (except C2a, which is larger than C2b).
• The early step of complement system varies in different pathways. However, all the pathways form enzyme complexes; C3 convertase, which cleaves C3 into C3a and C3b; and the C5 convertase, which cleaves C5 into C5a and C5b.C3b, thus formed, binds C3 convertase to form C5 convertase.C5 convertase, generated by the alternative, classical, or lectin pathway, initiates the activation of late components of the complement system to form membrane attack complex (MAC) and ultimately kills the pathogen.

Types and Activation of Complement Pathways

The complement activation occurs via three pathways; which are:

1. Classical pathway
2. Alternative pathway
3. Lectin pathway (or mannose binding lectin pathway)

1. Classical pathway

• Complement activation by the classical pathway commonly begins with the formation of soluble antigen-antibody complexes (immune complexes),IgM and certain subclasses of IgG (human IgG1, IgG2, and IgG3) can activate the classical complement pathway.
• The formation of an antigen-antibody complex induces conformational changes in the Fc portion of the IgM molecule that expose a binding site for the C1 component of the complement system.This forms a trimolecular complex (C1qrs + antibody + antigen), which has esterase activity.
• C1 is a large, multimeric, protein complex composed of one molecule of C1q and two molecules each of C1r and C1s subunits. C1q binds to the antigen bound antibody (Fc portion). C1r and C1s are proteases which help to cleave C4 and C2.
• The immune complex bound to C1 calls another protein C4 which is cleaved into C4a and C4b. C4a goes away whereas activated C4b attaches to the target surface near C1q. Now, C4b attracts C2 which is also cleaved into C2a and C2b.C2b binds C4b forming the C4b2b complex whereas C2a goes away.
• The active C4bC2b activates C3. The C4b2b complex is also known as C3 convertase as this converts C3 into an active form by separating C3a and C3b. One molecule of C4b2b can cleave a large number of C3 molecules. C3b binds to the microbial surface or to the convertase itself, and forms C4bC2bC3b (C5 convertase) which activates C5.
• C5 convertase cleaves C5 into C5a and C5b. C5a diffuses away but C5b is stabilized by binding C6. Then C5bC6 binds to C7. C5bC6C7 complex is then inserted into the phospholipid bilayer of the cell membrane which further binds C8. These all (C5b678) activate C9 to form a macromolecular structure called the membrane attack complex (MAC). 
• This makes hole in the bacterium, as a result, the intracellular contents leak out and unwanted substances get in. Thus, the cell cannot maintain its osmotic stability and the lysis occurs by an influx of water and loss of electrolytes.
• This is more effective in Gram negative bacteria than in Gram positive bacteria because MAC formation is easy in the outer membrane in Gram negatives whereas it is difficult in the rigid thick layer of peptidoglycan in Gram positives.
• Smaller complement subunits diffuse from the site and can initiate localized inflammatory responses by binding to specific receptors.

2. Alternative Pathway

• Unlike classical pathway, alternative pathway, does not require Ag-Ab complex for the initiation of complement pathway. It is initiated by cell surface constituents that are foreign to the host. These surface molecules may be lipopolysaccharide etc. 
• This major pathway of complement activation involves four serum proteins: C3, factor B, factor D, and properdin.
• This pathway initiated by the cleavage of C3 into C3a and C3b molecules(this molecules are produces in slow rate and doesn't initiate next activation step),with the help of blood containing enzyme.Next the c3b binds with the LPS of bacterial cell and form a bimolar complex.
• A protein in blood known as the Factor B is cleaved into large subunit Bb and small subunit Ba,by the factor D.and the Bb sbunit bind to the C3b and form an complex is called C3bBb.
• Next this bi-molar complex is cleaved the C3 complement into C3b and C3a,the c3a subunit is leaved and c3b bind to the C3bBb and formed trimolar complex C3bBb3b.Here c3bBb function as C3 convertase.
• Next this complex molecule cleaved the c5 into c5b and c5a,that is why this  complex molecule is called c5 convertase.
• C5a diffuses away but C5b is stabilized by binding C6. Then C5bC6 binds to C7. C5bC6C7 complex is then inserted into the phospholipid bilayer of the cell membrane which further binds C8. These all (C5b678) activate C9 to form a macromolecular structure called the membrane attack complex (MAC).

3. Mannose binding Lectin (MBL) Pathway 

• The lectin complement pathway (also called the mannosebinding lectin pathway) also begins with the activation of C3 convertase. However, in this case, a lectin—a protein that binds to specific carbohydrates—initiates the proteolytic cascade.A lectin of major importance is mannose-binding protein (MBP).
• This protein is produced when phagocytic cells called macrophages ingest viruses, bacteria, or other foreign material.
• MBP, as its name implies, binds mannose, a sugar that is a major component of bacterial cell walls, some virus envelopes, and antigen antibody complexes.
• MBP enhances phagocytosis and is therefore an opsonin.
• MBP forms a complex with another blood protein known as mannose-binding lectin-associated serine protease (MASP). When the MBP component of the complex binds to mannose on pathogens, MASP is activated.
• MASP cleaves the C4 and C2 complement proteins, also circulating in the blood, to produce a C3 convertase. The C3 convertase is the same as that used in the classical complement pathway, discussed previous. 

There is present another complement pathway:

Lytic Pathway

                               This pathway is initiated by the splitting of C5, and attachment of C5b to a target. C6, C7, C8 and C9 unite with C5b, and this membrane-attack complex (MAC), when inserted into the outer membrane of some bacteria, can contribute to their death by lysis. Red cells which have antibody bound to the cell surface can also activate the classical and lytic pathways, and become susceptible to lysis.

 Function: 

After initial activation, the various complement components interact, in a highly regulated cascade, to carry out a number of basic functions (Figure 13-1) including:

1. Lysis of cells, bacteria, and viruses
2. Opsonization, which promotes phagocytosis of particulate antigens. 
   

   Opsonization (Greek opson, to prepare victims for) coats microorganisms or inanimate particles with serum components, thereby preparing them for recognition and ingestion by phagocytic cells.

3. Binding to specific complement receptors on cells of the immune system, triggering specific cell functions, inflammation, and secretion of immunoregulatory molecules
4. Immune clearance, which removes immune complexes from the circulation and deposits them in the spleen and liver .