Immunology III: Innate Immunity & Inflammation 2 (Part 1: The Complement System)

Question 1

Complement system: What is it? What does it aid? How is it activated?

Answer 1

A blood-borne molecular defense system that “complements” the immunity provided by antibodies
Complement aids:
Phagocytosis  some complement components are opsonins
Destruction of microorganisms  the end result of complement activation is formation of a large, anti-microbial protein complex
Inflammation  some complement components are potent inflammatory mediators
Activated through a tightly-controlled enzyme-triggered cascade
“Cascade” = enzymatic reactions that activate protein effectors in a sequence
These proteins could be enzymes or anti-microbial proteins
normally only turned on at certain localized sites

Question 2

3 Pathways of Complement Activation:

Answer 2

Alternative Pathway
Complement component 3 (C3) acts as a pattern-recognition receptor – binds to the surface of a pathogen

Mannose-Binding-Lectin (MBL) Pathway
Triggered by binding of MBL (another pattern-recognition receptor) to mannose-containing CHO on bacteria/viruses

Classical Pathway
When antibodies (Ab) bind to a pathogen, C1q binds to the Fc portion of those antibodies

MBL and classical have a similar activation mechanism

Question 3

Complement system – general principles

Answer 3

All pathways lead to the production of a stable C3 convertase
C3 convertase cleaves C3  C3b and C3a

C3b is an important opsonin and also causes progression through the rest of the complement cascade
C3b forms a part of the C5 convertase
C5 convertase usually formed when C3b binds to the C3 convertase
C5 convertase cleaves C5  C5a and C5b
C5b becomes associated with the cell wall/membrane of the microbe and causes lysis as it activates other components of complement (C6 – C9)
C5b+C6+C7+C8+C9 all associate and form a large pore in the microbial membrane and cause lysis

Question 4

Complement – an overall view CHART

Answer 4

Question 5

The alternative pathway – the first responder

Answer 5

C3 forms C3a and C3b spontaneously in the bloodstream, but is degraded quickly under normal conditions
Factor B, a circulating protein, is also spontaneously cleaved to a protein known as Bb
Bb complexes with C3b to form C3bBb
C3bBb can convert C3  C3a + C3b
It’s a C3 convertase

C3bBb is rapidly inactivated in the uninfected host

If a bacterium is present, then C3bBb binds to the bacterial membrane, (C3b is a pattern-recognition receptor)

C3Bb bound to the bacterial membrane is a stable C3 convertase

As more and more C3b is generated, then more and more C3Bb forms
Another circulating protein, properdin, helps stabilize the entire complex to form the stable C3 and C5 convertases

C5 convertase of the alternative pathway:
C3bBbC3b + properdin
Needs to bind to the bacterial membrane to stay stable and keep converting C5  C5a + C5b

Question 6

The lectin and classical pathways

Answer 6

C1q is a complement protein that will bind to the Fc portion of an Ab that is activated (bound to an antigen)
Mannose-binding lectin (MBL) is a circulating pattern-recognition receptor
Recognizes mannose residues on bacterial membranes
When C1q detects a bound antibody or MBL detects mannose on a membrane:
1.) They bind complement-activating proteins
2.) These proteins cleave C2 and C4

C3 convertase:
C4b2a
formed from cleavage of C4 and C2 upon C1q or MBL activation
As the C3 convertase acts, C3b accumulates and binds to the complex
C5 convertase: C4b2aC3b

The C5 convertase cleaves C5  C5a + C5b

Question 7

Cell lysis: What does the Membrane Attack Complex (MAC) do? What is the sequence of events?

Answer 7

Generates a pore in lipid bilayer membranes

Sequence of events:
C5b triggers assembly of complex of C5b, C6, C7 & C8
Upon binding to C7, C8 will insert into membrane
Induces polymerization of C9 (n = 10-16) forming pore in membrane

Question 8

What are the MAC and pro-inflammatory effects of C5a and C3a?

Answer 8

The MAC and the pro-inflammatory effects of C5a and C3a can be extremely damaging to cells if they are not tightly regulated (and limited to microbes)

There are many proteins that down-regulate or degrade complement components

Question 9

What else can C3a and C5a do?

Answer 9

They are both cause vasodilation, increased vascular permeability, smooth muscle contraction (i.e. bronchoconstriction) and histamine release from mast cells
C5a is a chemotactic agent for a wide variety of cells (neutrophils & macrophages in particular)

Question 10

What does a deficiency in complement proteins cause?

Answer 10

Yes – it’s a rare cause of immunodeficiency
Inadequate complement proteins (C2, C3, C4, C5, MBL, MAC complex) tend to make patients vulnerable to bacterial infection

Question 11

What does a deficiency in C1q cause?

Answer 11

Deficiencies in C1q highly predispose patients to systemic lupus erythematosus (discussed later in immunology)

Photosensitivity rash, arthritis of small joints, renal failure, neurological vasculitis, neuropathies, and diverse effects on heart and lungs

Prevalence: 50 – 100/100,000

C1q helps macrophages to clear apoptotic bodies as well as initiate the classical pathway (C1q recognizes phosphatidylserine)
Thought that continual presence of self-antigens (especially nuclear material in the extracellular space) from dying cells increases the likelihood of developing autoimmunity
C1q may also have complex immunomodulatory roles with Th cells

Question 12

Why is the alternative pathway the first responder, and the lectin/classical pathways more effective later?

Answer 12

C3 is always present in the bloodstream – it’s constantly being produced by the liver, cleaved, and degraded

If a microbe is present, C3b instantly binds (non-specifically) to the cell wall/membrane – if properdin and Bb also bind, then the stable C3 convertase forms very quickly

Mannose-binding lectin does not circulate in high concentrations unless it is secreted by the liver in response to pro-inflammatory signals
Significant quantities of antibodies take days – weeks to produce

Question 13

QUESTIONS to answer

Answer 13

How are the 3 pathways of complement activated?
What is the C3 convertase for each pathway?
Remember – C3 convertase is the same for the classical and lectin pathway
What is needed to stabilize the C3 convertase in the alternative pathway?
What is the C5 convertase for each pathway?
Remember – it’s usually just C3b attached to the C3 convertase
What is the role of MAC? How (in general) is it activated?
What are the other functions of complement components other than forming MAC?
Relate these to inflammatory processes
How can complement deficiency be linked to immunodeficiency and autoimmunity?