Immunology Lecture 4: the complement system

what is the complement system

system of soluble plasma proteins (aka complement proteins ) that act to opsonize and lyse pathogens

what is opsonization

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

where are complement proteins produced

in the liver

true or false: the complement system is apart of the innate immune system AND the adaptive immune system

FALSE: only part of the innate immune system

are complement proteins always active?

NO - circulate in inactive forms. activated in the presence of pathogens or antibodies bound to pathogens

what would happen if the complement sytsem was active all the time

would attack our healthy cells

what are the three different proteolytic pathways (cascade) that lead to complement activation

contact with pathogen → alternative pathway → lectin pathway → classical pathway

how does complement activation occur

via a cascade of enzymatic reactions in which one component activates the next

stages of complement action

pattern-recognition trigger → protease cascade amplification/C3 convertase → inflammation, phagocytosis, membrane attack

where do all 3 pathways converge

inflammation, phagocytosis, membrane attack

what are many of the complement proteins and what do they do

many are proteases - they successively cleave and activate each other

what are these proteases synthesized as

inactive pro-enzymes; or zymogens

what causes zymogens to become enzymatically active

only after proteolytic cleavage, usually by another complement protein

what are complement pathways triggered by

proteins that act as PRRs to detect the presence of pathogens

what does the detection of pathogens activate

an initial zymogen, triggering a cascade of proteolysis - other zymogens are activated sequentially - amplifying the signal as the cascade progresses

how does the complement system differentiate host cells from pathogens

labels foreign molecules and pathogens as non-self to allow detection by the immune system

complement fixation

activation of the complement proteins promotes the attachment of specific proteins to the pathogen surface that then serves as an opsonin for recognition by phagocytic cells

true or false: some components of the complement system are capable or directly damaging and destroying pathogens

TRUE

membrane attack complex:

insertion of complement components into the pathogen memrbane and formation of pores that in the membrane disrupt the osmotic balance across the membrane and promote pathogen death

what are complement proteins designated by

the letter C followed by a number

how do you label inactive complement proteins (aka zymogens)

with a simple number designation e.g. C1 and C2

named in what order

order of discovery: C1, C4, C2, C3, C5, C6, C7, C8 and C9

how are products of cleavage named

by adding a lowercase letter as a suffix

e.g. C3 cleaves to produce C3a (smaller fragment) and C3b (larger fragment) 

what are the exceptions to the products of cleavage

C3 cleaves to produce C3a (larger fragment) 

C1q, C1r, and C1s are not cleavage products of C1, but distinct proteins that together compose C1

how are proteins of the alternative pathway named

factor B and factor D

what does factor B cleave into

factor Ba (smaller) and factor Bb (larger)

C1 function, and what pathway is it present in

protease involved in cleaving C2 and C4; classical

C2 function and what pathway is it involved in

cleave dby either C1 or MASP-2 to form C2b fragment of classical C3 convertase; lectin and classical

C4 function and what pathway is it involved in

cleaved by either C1 OR MASP-2 to form C4b fragment of classical C3 convertase; lectin. and classical

what initiates the lectin pathway

initiated by soluble carbohydrate-binding proteins - mannose-binding lectin (MBL) and ficolins

what do mannose-binding lectins and ficolins bind to

particular carbohydrate structures on the microbial surfaces

what triggers the cleavage of complement proteins and activation of the lectin pathway

MASPs (MBL-associated serine proteases)

what do MASPs do

trigger the cleavage of complement proteins and activation of the lectin pathway

when is the classical pathway initiated

when C1 (complement component 1; comprised of C1q, C1r and C1s) either recognizes a microbial surface directly or bind to antibodies already bound to a pathogen

what initiates the alternative pathway

initiated by spontaneous hydrolysis and activation of C3, which can bind directly to microbial surface - C3 Tickover

what is the first pathway that gets activated

the alternative pathway

what do all pathways generate

a C3 convertase, which cleaves C3, leaving C3b bound to the microbial surface and releasing C3a

what happens to C3a

gets lost in fluids

when do all 3 pathways converge

at the central and most important step in complement activation - C3

what does C3b start to do

starts coating pathogen

what does cleavage of C3 lead to

directly or indirectly leads to all the effector activities of the complement system

what happens after all pathways converge at C3

• C3a and C5a recruit phagocytic cells to the site of infection and promote inflammation

• phagocytes with receptors engulf and destroy the pathogen 

• all pathogens generate a C5 convertase that leads to formation of a membrane-attack complex (MAC), which disrupts cell membranes

what is the most important factor of the complement system

complement component C3

what does C3 cleave into

C3a and C3b

what is C3a

and anaphylatoxin

what is C3b

an opsonin

what is an anaphylatoxin

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

what is an opsonin

covalently attaches to the pathogen surface. Marks the pathogen for destruction (acts as an opsonin) - renders pathogen more susceptible to phagocytosis

what enzyme causes the breakdown of C3

C3 convertase

how does C3 convertase activate C3 for covalent bonding to microbial surfaces

by cleaving it into C3a and C3b and exposing a highly reactive thioester bondin C3b

C3 convertase of the lectin and classical pathway

C4b2a

what is the C3 convertase of alternative pathway

C3bBb

what is the C5 convertase of the lectin and classical pathway

C4b2a3b

what is the C5 convertase of the alternative pathway

C3b₂Bb

consists of two C3b subunits and on Bb subunit

duties of C3a

recruitment of phagocytes

granulocyte activation

duties of C3b

pathogen opsonization

pathogen lysis

what kickstarts the lectin pathway

mannose-binding lectin and ficolins form complexes with serine proteases and recognize particular carbohydrates on microbial surfaces

how do MBLs primarily exist

as trimers and tetramers, with each MBL molecule consisting of two to six trimers

MBL binds with high _____ to repetitive carbohydrate structures on microbes due to its carbohydrate-recognition domains targeting mannose, fucose, and GlcNAc, but it avoids binding to sialic acid residues found on host cells

avidity

MBL in plasma binds to five proteins: what are they?

three MBL-associated serine proteases, MASP-1, MASP-2, MASP-3 and two nonenzymatic proteins, MAp19, MAp44

what do ficolins bind to 

oligosaccharides containing acetylated sugars

what do MBL bind to with high avidity

mannose and fucose residues

the actions of the _______ result in the binding of large numbers of ______ molecules to the pathogen surface

C3 convertase; C3b

what is the first protein in the classical pathway of complement activation 

C1, a complex of C1q, C1r, and C1s

C1q function

binds directly to pathogen surfaces or indirectly to antibody bound to pathogens, thus allowing autoactivation of C1r

C4 gets cleaved into C4a and C4b, C4a gets lost

C2 gets cleaved into C2a and C2b, C2b gets lost 

C2a amd C4b form C4bC2a - the classical C3 convertase

this cleaves C3 into C3a and C3b - C3a gets lost

how is the alternative pathway activated

water joins C3 producing iC3[C3(H₂O)], the production of this is accelerated in the presence of a pathogen 

factor B binds to iC3[C3(H₂O)]

factor D proteolytically cleaves factor B activating it

iC3Bb is formed (fluid phase C3 convertase)

C3 is then cleaved into C3a and C3b 

Factor B then joins C3b 

Factor D then cleaves factor B into Bb and Ba 

Ba leaves and C3bBb is formed (alternative phase C3 convertase) 

C3 is then cleaved into C3b and C3a

The alternative pathway is an amplification loop for C3b formation that is accelerated by ______ in the presence of pathogens

properdin

what is properdin

plasma protein that enhances the activity of the alternative C3 convertase to aid in the complement activation and fixation (aka factor P) - made by neutrophils

how can the alternative pathway benefit the classical or lectin pathway

the alternative pathway of complement activation can amplify the classical or the lectin pathway by forming an alternative C3 convertase and depositing more C3b molecules on the pathogen

what do human cells express to ensure that C3b fixation on host surfaces does not result in targeting these cells for destruction

membrane proteins that inhibit complement

examples of membrane proteins on human cells that prevent complement fixation

DAF (decay accelerating factor) - breakdown the alternative C3 convertase

MCP (membrane cofactor protein) - binds to C3b and enhances its cleavage to inactive iC3b by factor I 

Factor H

plasma protein that enhances the cleavage of C3b into iC3b by Factor I 

how do plasma proteins bind to cell membranes

by interacting with sialic acid, a common carbohydrate on the surface of eukaryotic cells 

what is the defense mechanism that some bacteria have evolved

incorporating sialic acid on their surfaces, thus inhibiting complement activation

cell-surface receptors bind to ________ fixed on the surface of a pathogen, allowing for ______ and ______

complement proteins; recognition; phagocytosis

iC3b is a ligand for

CR3 and CR4

C3dg is a ligand for

CR2

what does Factor I do

serine protease that inactivates C3b through its cleavage into a smaller fragment known as iC3b, which cannot function as a component of C3 convertase

what is Membrane-Attack Complex (MAC)

a complex of 5 complement proteins C5, C6, C7, C8, and C9 that work in concert to form holes in bacterial and eukaryotic membranes

what functions as an initiating factor in the formation of the MAC

C5b

what plasma proteins are involved in regulating MAC

S protein, clusterin, and factor J

what Human-cell surface proteins are involved in regulating MAC

CD59, HRF

functions of anaphylatoxins C3a and C5a

act on blood vessels to increase vascular permeability and cell-adhesion molecules

what does increased vascular permeability allow for

increased fluid leakage from blood vessels and extravasation of immunoglobulin and complement molecules

what happens to migration of macrophages, neutrophils, and lymphocytes due to increased vascular permeability

it is increased, so is microbicidal activity of macrophages and neutrophils

what happens when the complement system malfunctions

• abnormal clearance of bacteria and fungi 

• hypersensitivity responses and autoimmune disorders (lupus)- due to improper clearance of soluble immune complexes

• increased infection rate due to malfunction of MAC