Cytokines and Complement

Question 1

Innate Immune system vs Adaptive Immune system

Answer 1

Innate Immune system:

• Detects DANGER
• Rapid, generic response
• Communicates danger to adaptive immune system
• Uses: Neutrophils, macrophages, complement, dendritic cells, natural killer (NK) cells, eosinophils

Adaptive Immune system:

• Differentiates between self and non-self
• Slow, highly specific response
• Has “memory” to antigens it has seen before
• Uses: T cells, B cells

Question 2

Concepts - “Danger” and “non-self”

Answer 2

The immune system attacks things that are “non-self” and if there is “danger”.

Danger = signals indicating there is harm to the body, and/or that infectious agents are present.
Recognised by innate immune response.

Self/non-self = The immune system can recognise your own proteins (= self) and knows not to attack. Anything it doesn’t recognise (= non-self) it will kill.
Recognised by adaptive immune response.

Question 3

Danger Signals:

Answer 3

PAMPs = Pathogen Associated Molecular Patterns
types of molecules only produced by infectious agents and not host tissue
Example = bacterial cell wall constituents (lipopolysaccharide - LPS)

DAMPs = Damage Associated Molecular Patterns
Molecules released from injured/dead cells
Examples = DNA, RNA, ATP, breakdown products of extracellular matrix

Question 4

Pattern Recognition Receptors Recognise PAMPs and DAMPs

TRUE OR FALSE

Answer 4

TRUE

Question 5

How does the body determine between “self” and “non-self”?

Answer 5

During development, the adaptive immune system randomly samples everything in its environment, and decides that that is “self”.

Negative selection removes these adaptive immune cells that bind with “self” cells, leaving only cells that bind to “non-self” cells.

Now they can react to anything new (non-self), but not your own molecules (self).

Question 6

Complement:

Answer 6

• Series of soluble proteins in the blood.
• Called C1, C2, C3 etc, to C9.

Pathogens lead to the activation of complement by one or more of three pathways:

• Classical pathway
• Mannose-binding lectin pathway
• Alternative pathway

Activation of complement can lead to :

• Anaphylatoxins (Inflammation)
• Membrane Attack Complex (Lysis)
• Opsonisation

Question 7

Classical Pathway of Complement Activation:

Answer 7

Only occurs when there are antibodies present specific to a foreign antigen

Antibody complexes bound by complement component “C1q”

Activates subsequent complement components (C3)

Question 8

Mannose-binding lectin pathway of Complement Activation:

Answer 8

Activation through mannose-binding lectin binding mannose (or similar carbohydrates) on bacteria

Mannose not present on surface of host cells.

Activates subsequent complement components (C3)

Question 9

Alternative pathway of Complement Activation:

Answer 9

Complement component C3:

• Spontaneously activates and binds to nearby membranes
• Host cells have control proteins on their surface to prevent further complement activation
• Bacterial cells do not, causing the complement cascade to be activated

Question 10

Complement lysis:

Answer 10

Membrane Attack Complex (MAC) forms in membrane of bacteria
Barrel-like structure formed from multiple late complement components (C6-C9)

Water rushes in, ions rush out, bacteria swells and bursts

Can also happen to host/foreign cells marked for killing

Question 11

Complement opsonisation:

Answer 11

Membrane-bound complement components also opsonise pathogens:

• bind to the surface of a bacteria
• phagocytes have Complement Receptors which bind membrane-bound complement
• encourage phagocytosis and killing

Question 12

Complement anaphylatoxins:

Answer 12

Fragments of complement components (C3a, C5a) are released on complement activation
“Anaphylatoxins” = toxins that can cause anaphylaxis

Their release results in blood vessels becoming leaky (oedema), resulting in recruitment of immune cells (e.g. neutrophils) and activation of mast cells.

Question 13

Cytokines:

Answer 13

Small proteins that mediate cell – cell communication during immune reactions

Predominately produced by cells in the immune system

Main action is on the immune system but in some circumstances target non immune cells

Stimulate cells via specific cytokine receptors

Can act to modulate immune cell function or attract cells to specific locations

Can be divided into different classes (based on the types of receptors and signaling pathways they activate):

• Chemokines
• Interleukins
• TNF family
• TGF superfamily

Some cytokines act to regulate immune cell development and haemostatis (eg IL-7) other act in pro-inflammatory (TNF, IL-6, IL-1) or anti-inflammatory (IL-10, TGF-β) responses
Some cytokines are monomeric (eg IL-1), others are dimers (eg IL-12) or trimers (eg TNF)

Question 14

Chemokines:

Answer 14

Can have “homeostatic” or “inflammatory effects” on leukocyte migration

Divided into 4 groups based on the position of cysteine residues that mediate the formation of disulphide bridges in the three dimensional structure:

• XCL
• CCL
• CXCL
• CX3CL

Chemokine receptor-expressing cell moves up the chemokine gradient to the chemokine-producing cell

Chemokines organize the immune system- Homeostatic
Chemokine and neutrophil recruitment - inflammatory

Question 15

Shared subunits and shared receptors of cytokines:

Answer 15

IL-12 family – dimeric cytokines
shared cytokine and receptor subunits, different signaling and effects.

IL-4 and IL-13 can bind and signal through the same receptor:

• IL2Rγc receptor (“common gamma chain”) subunit required for IL-2, IL-4, IL-7, IL-9, IL-13, IL-15 and IL-21 signaling
• IL2Rγc-deficient mice profoundly immunodeficient

Question 16

Cytokine release:

Answer 16

Cytokine release requires de-novo transcription and translation of cytokine genes (closely regulated at the levels of transcription, mRNA stability and mRNA translation).

Most cytokines are made with a pro-sequence (signal peptide) that allows their transport into the ER and their subsequent secretion via secretory vesicles:

• IL-1b and TNF are exceptions
• IL-1b requires processing by the ‘inflammasome’
• TNF is initially expressed as a plasma membrane protein, but requires cleavage by the protease TACE (Adam17) to release the mature TNF cytokine

Question 17

Promiscuity, redundancy and complexity:

Answer 17

Redundancy in cytokine’s effects:
- Block one cytokine and another may compensate

Promiscuity in receptor binding:
- IL2Rγc-deficiency blocks not only IL-2 signaling but many other cytokines too.

The release of a cytokine can be post-transcriptionally or post-translationally controlled:
- E.g. active IL-1β release requires cleavage by the inflammasome.

Predicting a cytokine’s effects can be complex.

Question 18

Cytokine release by macrophages:

Answer 18

• Activation of immune cells at the site of inflammation

- Release into the plasma promotes systemic changes in response to infections

Question 19

T helper cell polarisation:

Answer 19

Three signals required to activate and differentiate a CD4+ T cell response:

Signal 1: Antigen presentation: Antigen presented on MHC class II molecules on an antigen presenting cell, recognized by the T cell receptor (TCR) of a naïve CD4 T cell.

Signal 2: Co-stimulation: dendritic cells which have been activated through their PRRs upregulate surface receptors which ligate receptors on the surface of the T cell.

Signal 3: Polarization: in response to cytokines secreted by the APC or other nearby immune cells the Th cell will polarize to take on specific characteristics