The Complement System

Complement activities - interface between adaptive and innate immunity

-Augmentation of antibody responses by C3b and C4b. -Enhancement of immunologic memory by C3b and C4b and their fragments bound to immune complexes and antigen; receptors for complement components on follicular dendritic cells. -Enhancement of antigen presentation.

Complement activities - innate defence against infection

-Lysis of bacterial and cell membranes by MAC. -Opsonization by covalently bound C3b, C4b. -Induction of inflammation and chemotaxis by anaphylatoxins C3a, C4a and C5a and their receptors on leukocytes.

Complement System

-Part of the innate and adaptive immunity - only adaptive if specific antibody is present. -Refers to a set of serum proteins that cooperates with the innate and adaptive immunity to eliminate blood and tissue pathogens. -Serum proteins circulate in their inactive form. Once activated, they produce multiple outcomes.

3 complement pathways:

1. Classical pathway - antigen-antibody immune complexes 2. Lectin pathway (innate) - PAMP recognition by lectins 3. Alternative pathway (innate) - spontaneous hydrolysis on pathogenic surfaces

Classical complement pathway

1. Initiated by antibody (IgM or IgG) binding to a multivalent antigen (allows the binding of C1q to antibody complex, beginning the process of complement deposition). 2. C1q binds antigen-bound antibody, inducing a conformational change in one C1r molecule, activating it. 3. This C1r then activates the second C1r and the two C1s molecules. 4. C1s cleaves C4 and C2. -C4 is cleaved first and C4b binds to the membrane close to C1. -C4b binds C2 and exposes it to the action of C1s. -C1s cleaves C2, creating the C3 convertase, C4b2a, which is bound to the cell surface. 5. C3 convertase hydrolyzes many C3 molecules. Some combine with C3 convertase to form C5 convertase. 6. The C3b component of C5 convertase binds C5, permitting C4b2a to cleave C5. 7. C5 triggers pathway to produces C5b6789 (MAC).

Alternative tickover complement pathway

1. Initiated when C3 undergoes spontaneous hydrolysis - usually inactivated if nothing around to bind to. 2. Activated C3b binds to membrane of target cell. 3. Factor B binds C3b molecules and is cleaved by Factor D, generating C3bBb complexes (C3 convertase). 4. Properdin binds and stabilizes the C3 convertase. 5. C3 convertase cleaves more C3 molecules. 6. Newly active C3b binds to C3bBb to form C3bBbC3b (C5 convertase - stabilized by binding to Factor P). 7. C5 convertase cleaves C5, which goes on to form MAC.

Lectin complement pathway

1. Initiated when soluble proteins (lectins) recognize microbial antigens. -Lectins recognize specific carbohydrate components found on microbial surfaces (e.g. MBL binds mannose). 2. Lectins can serve as docking sites for MBL-associated serine proteases (MASPs): MASP-1 and MASP-2. 3. MASP-2 is structurally related to the serine protease C1s, and cleaves C4 and C2 to form the C3 convertase. 4. Continues on into the classical pathway.

3 stages of complement pathways:

1. Initiation - varies between different pathways. 2. Amplification - via production of C3 and C5 convertases. 3. Termination - results in inflammation, opsonization and lysis.

Proteins involved in the complement system & their functions

1. Initiators (C1q, MBL, ficolins) 2. Convertase activators (C1r, C1s, C2a) and enzymatic mediators (C3 convertase, C5 convertase). 3. Opsonins --> enhance phagocytosis 4. Anaphylatoxins --> inflammation & chemotaxis 5. Membrane attack complex --> lysis of cell membrane 6. Complement receptor --> amplify immune response 7. Regulatory proteins --> degrade complement components or prevent deposition of components - if not regulated properly, can cause damage to host.

3 main classes of complement activity:

1. Innate defence against infection. 2. Interface between innate immunity and adaptive immunity. 3. Complement in the contraction phase of the immune response. -->Complement components are highly regulated.

Alternative properdin-activated pathway

1. Properdin can directly bind to a microbial surface, which can then recruit C3b and Factor B. 2. Factor D is recruited and cleaves Factor B into Bb. 3. The resultant C3bBb is an active C3 convertase. 4. Subsequent steps are identical to the tickover pathway.

Generation of Membrane Attack Complex (MAC)

1. The 3 complement pathways converge at the formation of the C5 convertase, which cleaves C5 to C5a and C5b. 2. C5 initiates the generation of MAC. 3. MAC is the result of the deposition of C5b, C6, C7, C8, and C9 in target cell membranes, forming a pore. -C5b binds to membrane and initiates formation of MAC. -C5b678 creates a small pore. -Addition of C9 polymerization makes pore larger. 4. Pore structure disrupts osmotic integrity of the cell, resulting in cell death.

3 alternative complement pathway initiation methods:

1. Tickover 2. Properdin-activated 3. Protease-activated

Complement activities - complement in the contraction phase of the immune response example

Clearance of immune complexes from tissues - overproduction of immune complexes may cause disease.

Alternative protease-activated pathway

Initiation of clotting cascades has been shown to stimulate cleavage of complement proteins. 1. Thrombin cleaves C3 and C5 in vitro. 2. Platelet activation releases ATP, calcium ions and serine/threonine kinases that could stabilize C3b in fluid phase. --> Strong inflammatory reactions can potentially activate complement systems.

Where do all complement pathways converge?

The generation of C3 convertases - an enyzme complex capable of cleaving C3 molecules into two fragments, C3a and C3b.