Complement: kills cells by making pores on cell membranes

But how?Molecular mechanisms of complement-mediated killing anddiseases associated with the failure of these pathways are discussed in this presentation

OBJECTIVES

To understand:

1. That complement plays an important role as an effector of innate and adaptive immune responses.

2. The mechanisms of activation and function of the complement system.

3. That complement is able to distinguish between self (host cells) and non-self (pathogens).

4. That deficiency of complement and/or complement regulatory proteins can result in disease.

Complement is a system of proteases that mostly exists in plasma but some components are found on the surface of cells.

These proteases circulate as inactive precursor proteases (or zymogens) that can be activated by proteolytic cleavage.

Activation of the complement system is triggeringof a cascade of activated proteases. An activated protease generated by cleavage of its zymogen precursor cleaves its substrate, the next protease precursor.

Complement is a Protease Cascade

A cascade of proteases can be an exponential biological response amplifier

Protease cascades as biological amplifiers

10,000 molecules

100 molecules

1 molecule

FUNCTIONS OF COMPLEMENT

1. Opsonization and pathogen clearance: C’-coated bacteria and immune complexes are marked for engulfment by phagocytic cells such as macrophages.

2. Inflammatory cell stimulation: Activation and recruitment of phagocytic cells (including macrophages neutrophils, granulocytes, mast cells) mediated by anaphylotoxins (small mediators) released as a result of proteolytic digestion of complement components.

3. Direct lysis of pathogens via the terminal membrane attack complex (MAC).

Three major biological activities of the Complement system

OPSONIZATION

LYSIS CELL ACTIVATION

There are 3 pathways of complement activation

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The classical pathway of complement activation

The Classical Activation Pathway

The large fragments function by binding to membranes (esp. pathogen membranes)

The small fragments act as soluble mediators

C1C4

Ab

C4a

C4b

C3

C3a

C5

C5a

C5b C6 C7 C8 C9

C6 C7 C8 C9

MAC

Membrane

Classical Activation Pathway

C2

C2b

C2a C3b

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Structure of C1

EMSchematic structure

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Structure of IgM in solution Structure of IgM bound to bacterial surface

First event: Binding of C1 complex via C1q globular domains to antibody bound to the pathogen is the firstevent in the Classical pathway of complement activation.

Second event: Binding of C1q to antibody causes a conformational change in the (C1r:C1s)2 complex leading to activation of C1r molecules by autocatalysis.

Third event: Activated C1r cleaves C1s to generatean active serine protease C1s.

Early events of the Classical Pathway of Complement Activation-1

a

b

a

C3 convertase

Activation of C4

C4b is covalently bound to the pathogen surface

The Fourth event : classical pathway The Fifth event : classical pathway

Pathogen

C4

C4a

C4b

C4b

THIOESTER-MEDIATED BINDING OF C4b TO THE SURFACE OF A PATHOGEN

Thioester mediated binding of C4b

a

b

a

C3 convertase

Activation of C4 Activation of C2 and formationof C4bC2a, a C3 convertase.

Fourth and Fifth events: Activated C1s acts to cleave C4 and C2 molecules to give rise to the pathogen-bound central core molecule, C3 convertase, C4bC2a.

Early events of the Classical Pathway of Complement Activation-2

Deficiency of C1, C4, C2 Components

C1, C4, C2 deficiency leads to a failure to clear immune complexes. This leads to immune complex disease [Systemic Lupus Erythematosus (SLE), Glomerulonephritis].

The MB Lectin pathway of complement activation

The early events in the Classical and the Mannose binding-Lectin (MB-Lectin or MBL) pathways of complement activation are very similar

Classical Pathway MB-Lectin Pathway C1 (complex): MBL (complex):

C1q (Collectin family of MBL (Collectin familyproteins with 6 globular of proteins with 2-6

globularheads) heads)

C1r (serine protease) MASP1 (serine protease)C1s (serine protease) MASP2 (serine protease)

Identical C3 convertase: C4bC2a

Structure of the Mannan Binding Lectin complexed with serine Proteases, MASP-1 and MASP-2

People deficient in MB-Lectin experience a substantial increase in infections during early childhood.

MB-Lectin Deficiency

1. The C3 convertase cleaves C3 into anopsonin,C3b, which serves to coat the surface of the pathogen, and a small fragment C3a, an anaphylotoxin, which serves as a mediator of inflammation.

2. The activation/formation of the C3 convertase is the most critical step in complement activation!!

The C3 Convertase, C4b2a

a

a

a a

C2a part of the C3 convertase, C4b2a, is the active serine protease, that cleaves C3.

Activation of C3 byC3 Convertase

Since most of the C3 convertase, C4bC2a, is bound to the pathogen surface, C3 is cleavedon the same surface. The exposed thio-ester bonds of the newly formed C3b are thus Available to react with the pathogen surface –OH or -NH2 residues.

Many molecules of C3b coat the surface of thepathogen.

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The Alternate Pathway of Complement Activation

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Spontaneous Hydrolysis of C3 and formation of a C3 convertase

Alternate pathway C3 Convertase depositing C3b On the Pathogen Surface

Factor P: stabilizes C3bBbon the pathogen surface

P

Amplification Loop

C3b B

C3bB

D

Ba

C3bBbC3

C3a

(C3 convertase)

C3

C3

One C3 convertase molecule cleaves many C3 molecules.

Properdin stabilization

C3b interacts with factor B, and more C3 convertase is generated.

Amplification Loop

The Alternate Pathway of Complement ActivationEarly Events-1

First event: This pathway is initiated by spontaneous hydrolysis of the thio-ester bond in C3 to form “activated” C3i [or C3(H2O)]. C3 is abundant in plasma and C3b is generated at a significant rate by such a spontaneous cleavage. Second event: C3i binds to a plasma protein, factor B, to give rise to C3iB [or C3(H2O)B].

Third event: Factor B bound to C3i (in C3iB), is a substrate for a plasma protease, factor D. Factor D cleaves B to give rise to C3iBb and Ba (small mediator of inflammation). C3iBb is a fluid phase C3 convertase.

Fourth event: C3iBb cleaves additional molecules of C3 to C3b and C3a. Some of the C3b binds the pathogen surface.

Fifth Event is an amplification loop: (a) Pathogen-bound C3b binds factorB to give rise to

C3bB. (b) Factor D cleaves B bound to C3b to give rise to C3bBb, a pathogen-bound C3 convertase.

This starts a loop in which additional C3b makes more C3 convertase, leading to amplified formation of C3b and the C3 convertase C3bBb.

The Alternate Pathway of Complement ActivationEarly Events-2

C3 deficiency:

C3 deficiency, whether genetic, or due to incessantconsumption, leads to susceptibility to infections with pyogenic bacteria and Neisseria spp.

Sometimes also leads to immune complex disease.

Late events in Complement activation

a

a

a

a

a

a

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2

Late events in Complement activation

Formation of C5 convertase: C3b, in addition to opsonizing pathogens, complexes with C3 convertases (the classical and the alternate) to give rise to C5 convertase.

Cleavage of C5: C5 is cleaved by C5 convertase to release C5b and C5a.

Formation of membrane attack complex (MAC): C5b anchors the MAC by first forming “C5bC6C7C8” complex. CD8 part of this complex induces polymerization of 10-16 molecules of C9 into pore forming structure called MAC.

Complement Receptors

Anaphylotoxins

C5a and C3a are potent anaphylotoxins produced by the pathogen-triggered complement cascade and play a role in the recruitment of polymorphonuclear leucocytes for the phagocytosis of C3b or C4b opsonized pathogens via CR1.

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Actions of anaphylotoxins

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