Lecture 4 - The Complement System

Where are complement proteins mainly produced

Liver

Opsonization

Coating of a pathogen with antibodies/complement proteins so that it can be more readily taken and destroyed by phagocytic cells

Complement system is composed of more than how many proteins

30

Main function of the complement system

Soluble plasma proteins that act to opsonize and lyse pathogens

Complement proteins circulate in inactive forms, they are activated in the presence of

Pathogens or antibodies bound to pathogens

3 different proteolytic pathways that lead to complement activation in order

Alternative pathway, Lectin pathway, Classical pathway

Zymogen

An inactive complement protein in your body

Proteases

Enzyme that breaks proteins

When do zymogens become enzymatically active

After proteolytic cleavage by another complement protein

Complement pathways are triggered by proteins that act as ___ to detect the presence of pathogens

PRRs

What are the 3 distinct effector pathways, end product

Inflammation, phagocytosis, membrane attack

Complement fixation

Activation of complement proteins promotes the attachment of specific proteins to the pathogen surface to serve as opsonin to be recognized for phagocytosis

Inactive complement protein nomenclature

Simple letter and number, ex. C1 and C2

Order of complement protein discovery

C1, C4, C2, C3, C5, C6, C7, C8, C9

Is C3a or C3b the larger fragment

C3b

Exceptions to the nomenclature of cleaved proteins

C2a is the larger fragment, C1 is made of C1q, C1r, C1s

What are the complement proteins that are not involved in all 3 pathways and which ones are they in

C1 - Classical

C2 - Classical and Lectin

C4 - Classical and Lectin

Lectin pathway: What are the soluble carbohydrate-binding proteins that initiate the pathway

Mannose-binding lectin (MBL) and ficolins

Lectin pathway: What do MASPs do (MBL-associated serine proteases)

Trigger the cleavage of complement proteins and activation of the pathway

Classical pathway: What initiates it

When C1 recognizes a microbial surface directly or binds to antibodies already bound to a pathogen

Alternative pathway: What initiates it

Spontaneous hydrolysis and activation of C3, which can bind directly to microbial surface - C3 Tickover

What step do all 3 pathways of complement activation converge at

C3 Convertase

What is C3 convertase

Enzyme that cleaves C3 into C3a and C3b to activate it

What are the functions of C3a and C3b

C3b is an opsonin coating the surface of the pathogen (opsonization), C3a is the lost smaller fragment that aids in inflammation

All pathogens generate ____ to lead to the formation of ___ to disrupt cell membranes

C5 convertase, membrane-attack complex (MAC)

What is an anaphylatoxin (C3a)

Molecules capable of activating an inflammatory response by triggering degranulation of cells capable of inducing inflammation

What bond do opsonins attach to the pathogen surface

Covalently

What kind of bond does inactive C3 have

Thioester bond

What happens when C3s Thioester bond is exposed after breaking

Pathogen will use the Thioester bond to attach

What if the pathogen doesn’t attach to the Thioester bond right away

Water will attack to make it soluble and get lost in the blood

What are the C3 and C5 convertases of the Alternative pathway

C3bBb and C3b(2)Bb

What is CRP (classical pathway)

Acute-phase protein made mainly by the liver during inflammation

Lectin pathway: Mannose-binding lectin and ficolins form complexes with what and recognize what

Serine proteases to recognize particular carbohydrates on microbial surfaces

Lectin pathway: What does MBL recognize on microbes

Repetitive carbohydrates like mannose, fucose, GlcNAc

Lectin pathway: Why does MBL avoid binding sialic acid residues on host cells

Pathogens don’t have it, complement distinguishes self vs. non-self

Lectin pathway: Ficolins have ___-like domains and recognize ____

Fibrinogen-like domains and recognize acetylated sugars

Lectin pathway: What are the five proteins MBL in plasma binds to

MBL-associated serine proteases: MASP-1, MASP-2, MASP-3

Non-enzymatic proteins: MAp19, MAp44

First MASP-2 associated with MBL or ficolin to cleave what

C4 to C4a and C4b

After C4 is cleaved, C4b binds to what to form what

C2 to be cleaved and form C4b2a complex

Once C4b2a is formed, what is it and what does it do

Active C3 convertase to cleave C3 and create multiple C3b for pathway and binding to pathogen surface

Lectin pathway: What is the key enzyme for initiating complement

MASP-2

Classical pathway: What does C1q do in its active form

Bind directly to pathogen surfaces or indirectly to antibody body to pathogens to allow autoactivation of C1r

Alternative pathway: C3 is not completely stable so it undergoes spontaneous hydrolysis to form

iC3

Alternative pathway: What is a Tickover

Production of iC3 is accelerated in the presence of a pathogen

Alternative pathway: After iC3 is formed, what does Factor B do to it

Hydrolyzes it

Alternative pathway: What happens to iC3 after it is hydrolyzed

Become iC3Bb - fluid phase convertase in plasma

Alternative pathway: What cleaves Factor B to activate it and become a subunit of C3

Factor D

Alternative pathway: iC3Bb acts as what to continue cleaving more C3

C3 convertase

Alternative pathway: Surface bound C3b binds another molecule of Factor B, what happens and what is formed

Factor D cleaves it again to leave Bb attached - forming C3bBb

Alternative pathway: What is C3bBb

Alternative C3 convertase bound to surface of pathogen

Which pathway is an amplification loop for C3b formation accelerated by properdin in the presence of pathogens

Alternative pathway

What is properdin

Plasma protein that helps stabilize C3 convertase of the Alternative pathway since C3bBb on the surface of the pathogen is not very stable

What makes Properdin

Neutrophils

C3bBb convertase activity in the Alternative pathway is the equivalent of what

C4b2a in the lectin and classical pathways

How does the Alternative pathway amplify the classical or lectin pathway

Forming an alternative C3 convertase and depositing more C3b molecules on the pathogen

Why do human cells express membrane proteins that inhibit complement proteins

To ensure C3b fixation on host cell surface does not result in targeting these cells for destruction

DAF (decay-accelerating factor) function

Breakdown alternative C3 convertase, C3bBb → C3b + Bb

MCP (membrane cofactor protein) function

Binds to C3b and enhances its cleave to inactive iC3b by factor I, C3b → iC3b

DAF associates with C3bBb so that Bb is let go and the reaction stops

MCP associated to C3bBb so Bb is lost and tehn factor I enhances cleavage to inactive iC3b

iC3b is inactive and can’t do downstream reactions but it can still act as

An opsonin

What does Factor H do to regulate complement activation

Enhances cleave of C3b into iC3b by Factor I

What does Factor H interact with to bind to cell membranes

Sialic acid

How do some pathogens reduce complement activation

Mimic host surfaces by displaying Sialic acid and recruit Factor H through microbial proteins

What binds to complement proteins fixed on the surface of a pathogen to allow for recognition and phagocytosis

Cell-surface receptors

iC3b is a ligand for which complement receptors (PRRs)

CR3 and CR4

What are the 5 complement proteins that make up the Membrane-Attack Complex (MAC)

C5, C6, C7, C8, C9

Which complement protein functions as an initiating factor in the formation of the MAC

C5b

If plasma proteins block insertion and assembly, at which complement protein will MAC stop

C7

Examples of plasma proteins

S protein, clusterin, factor J

What step do human cell-surface proteins block

Final pore-forming step of complement

Human cell-surface protein exmaples

CD59, HRF