How Is the Process of Complement Activation?

Introduction

Complement system is a system of serum and cell surface proteins that interact with one another and with other molecules of the immune system to generate important effectors of innate and adaptive immune responses.

The complement system works by producing three effects:
- inflammation
- opsonization (attachment of certain complement proteins to microbial cell wall to target the microbe for phagocytosis)
- cell lysis (preferably microbes)

The complement system consists of three players:

- the complement proteins
plasma proteins that function as effectors of the complement system
- the receptors
proteins embedded in a cell surface that enable certain complement proteins to bind the cell
- the regulators
proteins that can alter the course of complement activation

To explain the process of complement activation, we will only discuss the complement proteins.

The process of complement activation

The complement proteins consist of
- C1 to C9
- certain lectins
- factor B
- factor D

The process of complement activation can be divided into three types:
- alternative pathway
- classical pathway
- lectin pathway

The three pathways share the same central events, which are:
- the creation of C3 convertase
  a compound that converts C3 into C3b and C3a
- the creation of C5 convertase
  a compound that converts C5 into C5b and C5a
- the creation of the membrane attack complex (MAC)
  a compound that lyses the cell it is attached to

However, they also differ in some aspects, which are summed below.

	Alternative	Classical	Lectin
Stimulus	Microbial cell wall	Antigen-antibody complex	Polysaccharide on microbial cell wall
1st complement protein to be activated	C3	C1q	Certain lectins
C3 convertase molecule	C3bBb	C4bC2b	C4bC2b
C5 convertase molecule	C3bBbC3b	C4bC2bC3b	C4bC2bC3b

The Alternative Pathway: EARLY PHASE

- C3 circulating in blood experiences spontaneous hydrolysis to form C3b and C3a (the letter “b” marks the larger product, while “a” marks the smaller)

- In the absence of microbes, C3b is hydrolyzed and rendered inactive
- If microbes are present, C3b binds to microbial cell wall, while C3a remains in the circulation
- Factor B circulating in blood can become attached to the C3b on microbial cell wall, forming the complex C3bB
- Factor D circulating in blood will cleave the attaching factor B, producing Bb which stays attached and Ba which goes in circulation
- The resulting complex C3bBb is the alternative pathway C3 convertase and is able to cleave more circulating C3 into C3b and C3a
- The resulting C3b can bind to the same or other microbial cell walls, while the resulting C3a remains in the circulation
- Some of the produced C3b will bind to the C3 convertase that created it, forming the complex C3bBbC3b
- The complex C3bBbC3b is the alternative pathway C5 convertase and is able to cleave C5 into C5b and C5a
- After C5b is formed, the alternative pathway enters its LATE PHASE which is the same for all the three pathways

The Classical Pathway: EARLY PHASE

- C1 is a protein complex consisting of C1q, C1r, and C1s
- C1q is a protein that binds to antigen-antibody complex on its immunoglobulin heavy chain, C1r and C1s are both proteases
- When C1q binds to two or more immunoglobulin heavy chains, it becomes activated

- C1q will activate C1r, which will subsequently activates C1s
- C1s cleaves C4 circulating in blood, producing C4b and C4a
- C4b can bind to microbial cell wall, while C4a remains in the circulation
- C2 circulating in blood can become attached to the C4b on microbial cell wall, forming the complex C4bC2
- C1s will cleave the attaching C2, producing C2b which stays attached and C2a which goes in circulation
- The resulting complex C4bC2b is the classical pathway C3 convertase and is able to cleave circulating C3 into C3b and C3a
- The resulting C3b can bind to the same or other microbial cell walls, while the resulting C3a remains in the circulation
- Some of the produced C3b will bind to the C3 convertase that created it, forming the complex C4bC2bC3b
- The complex C4bC2bC3b is the classical pathway C5 convertase and is able to cleave C5 into C5b and C5a
- After C5b is formed, the classical pathway enters its LATE PHASE which is the same for all the three pathways

The Lectin Pathway: EARLY PHASE

- Mannose binding lectin (MBL, a type of lectin) binds to mannose (a type of polysaccharide) on microbial cell wall

- MBL also binds to MBL-associated serine proteases (MASPs): MASP-1 and MASP-2

- MBL will activate MASP-1, which will subsequently activates MASP-2

- MASP-2 cleaves C4 circulating in blood, producing C4b and C4a

- C4b can bind to microbial cell wall, while C4a remains in the circulation

- C2 circulating in blood can become attached to the C4b on microbial cell wall, forming the complex C4bC2

- MASP-2 will cleave the attaching C2, producing C2b which stays attached and C2a which goes in circulation

- The resulting complex C4bC2b is the lectin pathway C3 convertase and is able to cleave circulating C3 into C3b and C3a

- The resulting C3b can bind to the same or other microbial cell walls, while the resulting C3a remains in the circulation

- Some of the produced C3b will bind to the C3 convertase that created it, forming the complex C4bC2bC3b

- The complex C4bC2bC3b is the lectin pathway C5 convertase and is able to cleave C5 into C5b and C5a

- After C5b is formed, the lectin pathway enters its LATE PHASE which is the same for all the three pathways

THE LATE PHASE

- The resulting C5b from the three pathways binds to C6 and the hydrophobic C7 forming a complex that inserts itself to cell membrane

- The complex has high affinity for C8 and will bind to it

- The addition of C8 to the complex stabilizes it and gives it limited lysing capability

- Monomers of C9 then bind to the complex, forming a pore like polymer called MAC

- The pore allows free movement of water and ion, resulting in osmotic swelling and rupture of the cell on which the MAC is deposited

Not all of C3b and C4b complements that bind to microbial cell wall will form more complex molecules (C3 convertase or C5 convertase). They can remain singly attached to microbial cell wall and act as opsonins. Mononuclear phagocytes have various receptors for C3b and C4b. Hence, the phagocytes can bind to microbes through C3b or C4b and phagocyte them.

The cleavage products C5a, C3a, and C4a also have a role. They induce inflammation by:

- attaching to mast cell and inducing degranulation

- stimulating neutrophils, increasing vascular permeability, and promoting neutrophil binding to vascular wall (C5a only)


References:

• Abbas AK, Lichtman AH, Pillai S. Cellular and molecular immunology. 6th ed. Philadelphia: Saunders; 2007. p. 329-46.