Natural Killer Cells and Cytotoxic T Cells

Question 1

Recap the two types of immunity

Answer 1

• Innate immunity = Non-specific, immediate response
• Active immunity = Highly specific, delayed response

Question 2

Where do natural killer and T cells arise from?

Answer 2

• Both arise from a common lymphoid progenitor cell
• Both part of the lymphocyte lineage

Question 3

What is the role of cytotoxic lymphocytes (aka T cells and NK cells)?

Answer 3

• Destroy cells infected with bacteria, viruses or parasites
• And tumour cells
  
  • Requires a cell-surface mechanism to display what is going on in a cell (MHC)

Question 4

Compare CTLs and NK cells?

Answer 4

Question 5

What do MHC proteins do?

Answer 5

• MHC proteins will display what is going on inside the cells
• They are found at the cell surface and form a structure that holds antigenic peptides for surveillance by T cells
• HOWEVER = all intracellular proteins (mutated, infectious and healthy) can be presented at the cell surface by MHC class I

Question 6

By what type of lymphocyte is MHC-Class I specifically recognised?

Answer 6

CD8+ Cytotoxic T cells

Question 7

Recap (yr 1) How antigens from the cytosol are presented on MHC I?

Answer 7

1. Viral proteins will be ubiquinated to the proteasome where it will be broken down into peptides
2. These peptides will be transported to the ER via a transporter called TAP
3. In the ER the MHC class I molecules are generated, here the peptides from the virus will be sampled if it forms a stable complex it will leave
4. It will leave the cell via the golgi where it forms an exocytic vesicle which will fuse with the plasma membrane

Question 8

Describe the MHC class I structure

Answer 8

• Alpha 3 domain and beta-2-microglobulin will form the base and provide support for the peptide binding groove
• The two alpha helices (alpha 1 and alpha 2) will form the groove where the peptides will bind

Question 9

MHC class I proteins are central to anti-viral immune responses, so why dont we see many pathogens that have mutated to avoid antigen presentation?

Answer 9

• We have multiple MHC genes (HLA-A, B and C)
  
  • There is a high genetic variabillity in these genes

Question 10

How do MHC polymorphisms impact on MHC binding?

Answer 10

• Polymorphisms in the MHC are in the upper peptide binding part of MHC protein
  
  • The peptide binding groove will create pockets where the bound peptide can ‘anchor’
    
    • By substituting different amino acids, you will get different positive and negative charges as well as a varied size and shape of the binding pockets
    • This will mean different peptides will bind to different MHCs due to the different alleles

Question 11

What does the T cell receptor recognise?

Answer 11

1. MHC protein itself
2. Antigenic peptide presented by MHC protein

It is able to recognise this by binding with a diagonal footprint which cuts across both alpha helices and the peptide inbetween

Question 12

How do we overcome the weak binding of MHC and T cell receptors?

Answer 12

• CD8 on T cells act as a co-receptor and strengthens the interaction between the T cell and MHC Class I
• CD8 will bind near the structural support region (a3 specifically)
  
  • We do not see polymorphisms in this area but the upper peptide binding part as we could lose CD8 binding!!!
• Whereas TCR will bind to the a1-a2 heterodimer ligand and checks for antigenicity

Question 13

How can pathogens adapt to stop being presented on MHC I?

Answer 13

• Microbes may subvert MHC upregulation
• Inhibit MHC-I transcription (adenovirus)
• Block TAP activity (HSV)
• Retain MHC-I in endoplasmic reticulum (adenovirus, HCMV)
• Target MHC-I for disposal from ER (HCMV)
• Downregulate MHC-I from cell surface (HIV)

Question 14

What are natural killer cells?

Answer 14

• Large, granular lymphocytes (NOT B or T cells)
• They do not express T cell receptor (CD3) or B cell receptor
• They do express the cell surface marker CD56

Question 15

What is the function of natural killer cellls?

Answer 15

• Cytokine secretion (esp. interferon gamma)
• Killing targets

Question 16

Why are NK cells important in up-regulation of immunity?

Answer 16

• Medium to high cytolytic function was associated with reduced cancer risk; low cytolytic function was associated with increased cancer risk
• Low NK cell infection correlates with severe disseminating herpes virus infection

Question 17

What are the two types of receptors found on natural killer cells?

Answer 17

1. Killer Ig-like receptors (KIR) = innate immune receptors which regulate the activity of NK cells
   
   • Recognise MHC-I and inhibit NK cell from releasing lytic granules
2. Leukocyte Ig-like receptors (LILR) = innate immune receptors which regulate function of NK cells
3. Natural Cytotoxicity Receptors (NCRS)
   
   • Provide activating signals to NK cells

Question 18

What are Killer Ig like receptors (KIR) and leucocyte Ig-like receptors encoded by?

Answer 18

• Encoded in a gene complex (leukocyte receptor complex or LRC) on chromosome 19

Question 19

Describe the function of KIRs on NK cells

Answer 19

• When KIR recognise MHC-I they will inhibit NK cells from releasing lytic granules
  
  • Some viruses will downregulate MHC as an evasion mechanism so loss of MHC-I is a common feature of tumour cells
  • If a target cell does not express MHC-I then there is no KIR inhibition = lytic granule released to lyse the target
    
    • Known as ‘missing self’

Question 20

Describe NCRs

Answer 20

• These are natural cytoxicity receptors located on natural killer cells
• They will provide activating signals to NK cells
  
  • however they are not well characterised (dont know everything about them)
    
    • NCR 1 - binds viral haemagglutinin
    • NCR2 – binds a ligand that is expressed on tumour cells and upregulated by viral infection

Question 21

What is natural killer cell activity controlled by?

Answer 21

• Control by the balance between inhibitory signals (when recognising MHC) and activating signals it is recieving from other receptors

Question 22

How can antibodies activate NK cells?

Answer 22

• Natural killer cells express an Fc receptor (FcγRIII, CD16)
• FcγRIII will bind to IgG antibodies that have bound to antigens on an infected cell
• Cross-linking of Fc receptors will send an activating signal to the NK cell to kill the target cell by releasing its granule proteins
• This process is called ANTIBODY-DEPENDANT CELL MEDIATED CYTOTOXICITY (ADCC)
• This is a super activating signal of NK cells and will outweigh the inhibitory MHC signal on NK cells

Question 23

How can NK cells kill tumour cells?

Answer 23

• Similar to pathogens, tumour cells will escape the adaptive immune system by downregulating the expression of MHC class I
  
  • This will make them more susceptible to NK cells

Question 24

Describe how cytotoxic T cells and natural killer cells will kill via cytotoxic granules?

Answer 24

• NK cells and T cells will carry granules filled with cytotoxic proteins
• Release cytotoxic granules at site of contact with target cell
  
  • ! Must be directed in order to avoid damaging innocent bystander cells

Question 25

Give examples of three protein granules of cytoxic T cells

Answer 25

* **Perforin** = released to aid delivering contents of granules into cytoplasm target * **Granzymes** = serine proteases which apoptosis once in cytoplasm of target cells * **Granulysin** = Has antimicrobial actions and can induce apoptosis

Question 26

How can CD8 cells trigger apoptosis through non-cytotoxic granule dependant pathway?

Answer 26

* PROCESS DOES NOT DEPEND CYTOXIC GRANULES * Cytoxic cells will express **FasL (Fas ligand)** which will bind to **death inducing receptor Fas** on **target cells** and trigger apoptosis * Fas/FasL triggered apoptosis is used to **dispose of unwanted lymphocytes**

Question 27

What can a loss of Fas result in?

Answer 27

Autoimmune lymphoproliferative syndrome (ALPS) Body cannot regulate the number of lymphocytes due to mutated Fas on target cells

Question 28

What is the difference in the target recognition of natural killer cells vs cytotoxic T cells?

Answer 28