4 - The Complement System

Front 1

How does the complement system work and what are the 3 outcomes?

Back 1

It is a biochemical cascade that:

1. releases inflammatory and chemotactic mediators

2. opsonization (coats microbes with a "tag")

3. directly lyses microbes

Front 2

Which immune systems activate the complement system?

Back 2

BOTH innate and adaptive immune responses

Front 3

Why is the complement system so important?

Back 3

Consists of 16 different serum poteins and glycoproteins (mostly made in liver) that comprise about 10% of total serum protein (a lot!!)

Important because a small inciting stimulus can cause the biochemical cascade to amplify a huge, billion-fold response

Also because it can cause many effects to happen at the same time in a coordinated fashion

Front 4

What are the 3 complement activation pathways?

Back 4

1. Classical - initiated by IgG or IgM (acquired immune system)

2. Lectin - initiated by mannose-binding lectin (an extracellular PRR) (innate immune system)

3. Alternative - initiated by microbial surface vicinity (innate immune system)

Front 5

What are 3 common features of the complement activation pathways?

Back 5

1. All consist of a proteolytic cascade that allows for signal amplification

2. result in creation of a C3 and a C5 convertase which can cleave C3 and C5 complement proteins respectively (BUT the C3 convertase used in alternative pathway is different than the C3 convertase used by classical and lectin pathways)

3. all end in a common terminal pathway (membrane attack pathway)

Front 6

What does the C in C3a/C5a stand for?

What is the number?

What is the second letter?

Back 6

Complement

Historical order of cascade

Cleavage product or subunit (a is the soluble one and is usually smaller than b)

Front 7

How is the classical pathway triggered?

Back 7

Antigen-antibody complex produced after adaptive humoral immune response

specifically by IgG₁, IgG₃ and IgM

*these antibodies may take several days to appear after a new infection

- At least two antibodies need to be bound close to each other on the surface of the microbe

- If at least 2 bound, then C1 complex recognizes this using it's C1q subunit

- This causes conformational changes in C1 complex, and triggers autocatalysis (cleaves its own proteins)

This is all Ca²⁺ dependent!

Front 8

What are the next steps in the classical pathway after it has been activated?

Back 8

- The cleaved proteins from the C1 complex (C1s) cleaves C4 and C2 to form C4b and C2a

- These two join together to form the C3 convertase C4b2a

- This cleaves C3 into C3a and C3b

- Both C4b and C3b covalently attach to activating surface (e.g. microbe) via thiolester linkage (weak bond, doesn't last long so the microbe must be very close by)

- Part of C3b will form C5 converting complex

- Will go on to the terminal membrane attack pathway

In general terms:

Once the C1 complex attaches to 2 antibodies, the C3 convertase is created, then cleaved and it's parts attach to the surface of a microbe. One of those parts will also help make the C5 convertase which will go on to make the membrane attack complex

Front 9

How is the Lectin pathway triggered?

Back 9

Antibody Independent (non-specific)

Mannose-binding lectin (MBL) is similar C1

- MBL binds to PRR's found on microbial surfaces (mannose, fucose and N-acetyl glucosamine (NAG))

- This causes a conformational change in MBL that induces autocatalysis of MASPS that then cleave C4 and C2

Front 10

How is the alternative pathway triggered?

Back 10

Antibody and lectin independent (non-specific)

Relies on low-level serum hydrolysis of C3 (steady state, there is always a low level of C3 cleavage happening all the time)

- Hydrolyzed C3 must be really close to an "activator surface" (e.g. bacteria), binds serum factors to form C3 convertase (different convertase than the lectin/classical pathways)

- C3 convertase hydrolyses C3 into C3a and C3b, estbalishes an amplification loop in absence of regulators

- C3b can be used to form C5 convertase which is used to make membrane attack complex

Front 11

How does the host control the alternative activation pathway?

Back 11

Host cells possess surface carbs and regulators that inactivate C3b so that they act only on bacterial membranes and not on host cells

Factor H and Factor I inactivate and degrade inappropriately bound C3b

Spontaneously bound C3b on host cells is destroyed as fast as it is deposited

Front 12

How is the alternative pathway inherent to the classical and lectin pathways?

Back 12

It can activate the other pathways because C3 is the main player in all the pathways, and the alternative pathway provides an amplification loop for C3 deposition

Front 13

How does the terminal membrane attack pathway work?

Back 13

Classical, Lectin and Alternative pathways all end with formation of C5 convertase complex

C5 convertase cleaves, initiates a pathway involving C6, C7, C8 and C9 to form a rigid pore (the membrane attack complex - MAC)

This pore allows influx of solute and electrolytes across the microbe membrane which causes osmotic swelling and sometimes lysis

Front 14

How do proteins help regulate the complement system?

Back 14

The complement system is highly regulated, inhibitory factors in the blood help control it:

CD55 regulates alternative pathway - accelerates decay of C3 convertase

CD35 regulates alternative, lectin and classical pathways - inhibits C3 convertases

CD59 regulates terminal membrane attack - prevents MAC formation

Front 15

Why is cobra venom so toxic?

Back 15

Forms a stable C3 convertase that is not recognized by regulatory proteins such as Factor H and I

It then continuously hydrolyzes C3 and C5 which leads to complement depletion and severe local tissue damage

Front 16

What are the 4 outcomes of complement activation?

Back 16

1. Inflammation and chemotaxis

2. Enhancement of phagocytosis (opsonization)

3. Promoting humoral immunity (talking to adaptive immune system)

4. Clearance of immune complexes

Front 17

What is opsonization?

Back 17

Making microbes "more tasty"

C3b deposition enhances binding and uptake of microbes via CR1 and CR3 in professional phagocytes

- causes highly enhanced clearance

Front 18

What is chemotaxis and how does it work in the complement system?

Back 18

Polymorphonuclear leukocytes and macrophages migrate along C3a and C5a gradients to site of C' activation

C3a and C5a are small and not part of the MAC complex - their job is to increase vascular permeability

Front 19

How does complement activation cause increased vascular permeability?

Back 19

- C3a and C5a trigger degranulation of Mast cells

- Mast cells release vasoactive amines which are vascular dilaters

- this causes increased vascular permeability and blood flow so blood cells can get to the inflamed tissue

Front 20

How does complement activation enhance adaptive immunity?

Back 20

- Breakdown of C3b gives off C3dg

- When B-cells find an antigen, they are activated and C3dg helps to significantly enhance their activity, which in turn enhances generation of antibodies directed against that antigen

- C3dg also helps retain the antigen in the B-cell follicles of lymph nodes, which promotes prolonged antigen - B-cell interaction, enhancing the humoral response to that antigen even more

Front 21

How does the complement system work in clearing immune complexes?

Back 21

- C3b can bind to the immune complexes, which allows them to be bound to the surface of erythrocytes

- Once bound, immune complexes are safely removed by phagocytosis or erythrocytes in the liver and spleen

Front 22

Why are C3a and C5a considered anaphylotoxins?

Back 22

C3a and C5a help recruit circulating leukocytes by prompting endothelial cells lining local vasculature to express adhesion molecules

when generated in large amounts systematically, C3a and C5a can cause systemic anaphylactic shock

Front 23

What happens if there are deficiencies in the classical activation pathway?

Back 23

Little or no increased risk of infection

- suggests that alternative and lectin pathways are sufficient for controlling infection = compensatory mechanisms

- but deficiency in classic pathway components are associated with increased risk of SLE-like autoimmune disease, probably due to impaired clearance of immune complexes by monocyte-macrophage system

*SLE = systemic lupus erythematosus

This can lead to kidney problems, since immune complexes that shouldn't be there go into glomeruli of kidneys and cause inflammation there

Front 24

What happens if there is a factor H deficiency?

Back 24

Factor H part of the alternative pathway - regulates complement activation that blocks formation of C3 convertase and is a cofactor of the cleavage of C3b with factor I

Deficiency of factor H results in unregulated activation of alternative pathway

Norwegian Yorkshire pigs develop type II membranoproliferative glomerulonephritis due to deposition of C3 components in glomerulus of kidney