Complement in acute infection

The Complement System 1. Introduction 2. Activation 3. Complement receptors 4. Regulation 5. Functions

Complement in acute infection — an ancient but important defence system

1. Introduction to the complement system Approximate 30 proteins forming one of the major defence systems in the body, that are normally inactive but become activated under certain circumstances Accounting for ~10% of serum proteins, C3 is the predominant component. Sensitive to physical and chemical factors, the complement activity can be inactivated by incubation at 56℃ for 30 min Most complement components are produced by hepatocytes and macrophages

Complement Components soluble membrane-bound

2. Complement activation Complement doesn’t function until being activated 2.1. Activation pathways 2.1.1 Classical 2.1.2. MBL 2.1.3. Alternative 2.2. Terminal stage of activation All 3 pathways have a terminal activation in common — formation of the MAC

2.1. Activation pathways There are 3 pathways - differ in how C3 convertase is generated Classical pathway - activation by antibody-antigen complexes (adaptive) MBL pathway - activation by carbohydrates present on microbes in absence of Ig (innate) Alternative pathway - activation by microbes’ ‘protected’ surface in absence of Ig (innate)

Central step of complement activation — activation of the C3 thioester bond

2.1.1. Classical pathways of complement activation The classical pathway is activated by immune complexes This activation pathway begins with the C1q binding to the Fc part of antibody (IgM and IgG) attached to antigens.

Classical Activation Components (a)

Classical Activation Components (b)

Classical pathways of complement activation After formation of antigen-antibody complex, and C1q binds to the Fc fragment of the antibody This results in activation of C1. The activated C1 then catalyzes the cleavage of C4 and C2. Pieces of C4 and C2 bind to the surface of antigen, forming a new enzyme — C3 convertase (C4b2a) C3 convertase cleavages C3 into C3a and C3b. C3b binds to the antigen surface, forming a C4b, C2a and C3b complex — C5 convertase (C4b2a3b)

Classical pathway starts from the C1q binding to antigen-antibody complex

Activation of complement by IgG and IgM antibodies

Classical pathway of complement activation

2.1.2. MBL pathway of complement activation The MBL pathway is activated by bacterial carbohydrates This activation pathway is initiated by the MBL binding to the terminal mannose groups on bacterial carbohydrates. MBL (mannan-binding lectin) belongs to the same family as C1q, collectin family. MBL is associated with 2 proenzymes MASP-1 and MASP-2, analogous to C1r and C1s. (MASP: MBL-associated serine protease)

Mannan-Binding Lectin (MBL) a C1q like molecule, but binding to the terminal mannose groups on bacterial carbohydrates

MBL pathway of complement activation MBL binding to the terminal mannose groups on bacterial carbohydrates. MBL-associated MASP-1 and MASP-2 are activated. Activated MASP-1 and MASP-2 catalyzes the cleavage of C4 and C2. Pieces of C4 and C2 bind to the surface of antigen, forming C3 convertase (C4b2a). C3 convertase cleavages C3 into C3a and C3b. C3b binds to the antigen surface, forming a C4b, C2a and C3b complex — C5 convertase

2.1.3. Alternative pathway of complement activation The alternative pathway is activated near ‘protected’ surface Normally, the alternative pathway is continuously be activated at very low rate (spontaneous breakdown of C3), and brought under control by regulatory molecules. In the presence of suitable activator surfaces (microbe walls), the alternative pathway increases dramatically, for the surfaces protect the newly activated complement products from the inactivation by regulatory molecules.

Alternative Path Components

Alternative pathway of complement activation The amplification loop and its regulation

C3 convertase: C3bBb, C5 convertase: C3bnBb (n ≧ 2) Alternative pathway of complement activation C3 convertase: C3bBb, C5 convertase: C3bnBb (n ≧ 2)

2.2. Terminal stage of complement activation — formation of the membrane attack complex (MAC) C5 convertase formed from either pathway  C5 — C5b + C5a   C6 and C7 C5b67 (insert into the cell membrane)   C8 C5b678 (into the cell membrane further)   (≧ 6 of C9) 5b678poly9 (a doughnut-shaped pore across the membrane) (MAC: the complex consists of the C5b, 6, 7, 8, and a variable number of C9) After the formation of C5 convertase, C5 is cleaved into C5a and C5b, the C5b binds C6 and C7 to form a complex which is able to insert into the cell membrane, then C8 joins the complex and unwinding into the cell membrane, causing the disruption and lysis of the membrane, this effect is largely enhanced by the incorporation of C9, more than 6 C9 molecules will a doughnut-shaped pore across the membrane.

Terminal Complement

Late step of complement activation Formation of the membrane attack complex (MAC)

Summery of the complement activation pathways

4. Regulation of complement activation The activation of complement is tightly controlled The activated complement components are unstable, and lose binding capacity rapidly after activation. There are a number of inhibitor/regulatory proteins

Inhibitor/regulatory proteins

3. Receptors for Complement Proteins Complement Receptors (CR): cell membrane bound proteins binding to complement activation products, mediating different biologic activities

Complement Receptors (CR) Many Functions & Biologic Effects

After the activation, complement system can perform: 5. Functions of the complement system After the activation, complement system can perform: 5.1. Opsonization C3b, C4b bind to the receptors on phagocytes, help to engulf the C3b and C4b coated microbes 5.2. Initiation of acute inflammation C3a, C5a (anaphylatoxin) bind to the receptors on mast cells, causing degranulation and release inflammatory mediators 5.3. Chemotaxis C3a, C5a bind to the receptors on phagocytes, attracting them into the site of inflammation 5.4. Lysis of the target cells The MAC pores the membrane of microbes or target cells.

Functions of complement system (1) Opsonization

Functions of complement system (2) Actions of anaphylatoxins — C3a and C5a

Functions of complement system (3) — Recruitment and activation of leukocytes

Functions of complement system (4) — Complement-mediated cytolysis