Natural born killers: NK cells and CD8+ T lymphocytes

Question 1

What is the origin of Natural Killer cells and T cells?

Answer 1

Both arise from common lymphoid progenitor cell Both part of the lymphocyte lineage The 3 cells that can arise from common lymphoid progenitor cell: T cells B cells Innate lymphoid cells Natural killer cells are a subset of innate lymphoid cells

Question 2

What is the role of cytotoxic lymphocytes?

Answer 2

We need cytotoxic cells to be able to destroy: 1.Cells infected with bacteria, viruses or parasites 2.Tumour cells Lymphocytes scanning a target cell surface need to detect changes in protein production inside that target cell

Question 3

What is the role of Cytotoxic T cells (CD8)?

Answer 3

Cytotoxic adaptive immune cells (part of the adaptive response) Kill virally infected targets Kill tumour cells Controlled by T cell receptor recognition, with CD8 acting as a co-receptor very Highly specific to the antigen

Question 4

What is the role of innate Natural Killer cells (NK cells)?

Answer 4

Cytotoxic innate immune cells Kill virally infected targets Kill tumour cells Controlled by a balance of signals between different activating and inhibitory receptors on their surface Broad specificity for target cells instead of having one specific receptor on their surface like CD8 cells, they have many receptors

Question 5

Why do we need more than 1 type of cytotoxic lymphocyte?

Answer 5

1. To combat infection in the period before a T cell response develops (NK cells respond to a viral infection entrance much earlier than CD4 and CD8 cells/before antibody levels rise, all to lower viral load as early as possible)
2. To provide an alternative system when a tumour or infected cells evade Cytotoxic T cell responses (so NK cells provide a back up)
3. To provide an additional mechanism for killing infected targets via antibody recognition

Question 6

What does a low NK cell level activity correlate with?

Answer 6

Low NK cell activity correlates with severe disseminating herpesvirus infections ie NK cell deficiency

Question 7

What is the role of MHC class I in terms of antigen presentation?

Answer 7

Lymphocytes scanning a target cell surface need to detect changes in protein production inside that target cell MHC class I proteins are found at the cell surface Form a structure that presents protein fragments (peptides) at the cell surface for immune surveillance Recognised by CD8+ cytotoxic T cells

Question 8

How are intracellular proteins presented at the cell surface by MHC class I? Describe a summary example of what happens as a result of virus entering cell.

Answer 8

Proteins expressed within a cell are processed and presented on MHC class I proteins Samples include ‘normal’, mutated or viral proteins. These proteins enter a protease complex called a proteasome, which chops the proteins up into smaller fragments. These fragments then enter the endoplasmic reticulum and come into contact with empty MHC Class I. If a peptide fragment binds MHC Class I, then the whole molecule goes towards the surface and the MHC class I holds the peptide where a T cell receptor can see it EXAMPLE: Virus enters/infects cell Viral proteins are synthesised in the cytoplasm Peptide fragments of viral proteins bound by MHC Class I in the endoplasmic reticulum Bound peptides transported by MHC class I to cell surface. Cytotoxic T cell recognises complex of the viral peptide with MHC Class I and kills the infected cells

Question 9

What is the structure of the MHC Class I?

Answer 9

Tissue distribution: found on all nucleated cells two polypeptides, non-covalently bound: each MHC Class I molecule has a peptide binding cleft- where the peptides bind made up of 2 alpha helices and a platform beneath them, beta pleated sheet forms base

Question 10

In humans, what are proteins also known as? Where is the human MHC gene located?

Answer 10

In humans, proteins are also known as HLA (Human Leucocyte Antigens). The human MHC gene complex is located on chromosome 6 and contains 3 MHC class I proteins and 3 MHC class II proteins (technically found in pairs so 6?) Each gene is highly polymorphic – There are 100s of different genetic variants

Question 11

Why do we have HLA polymorphism?

Answer 11

Pathogens can evolve to evade immune responses Variation in MHC class I proteins - Multiple genes (e.g. two copies each of HLA-A, B and C) and high genetic variability within these genes may counteract this across populations

Question 12

Where do you find polymorphisms in the MHC Class I structure?

Answer 12

polymorphisms are found in the upper peptide-binding part of the MHC protein so that’s where we see the variation- in the peptide binding groove Amino acids in the MHC peptide binding groove create pockets where the bound peptide can “anchor the size, shape or charge will determine what peptide can fit in to the pockets on the peptide binding groove

Question 13

What can variations between different MHC alleles lead to?

Answer 13

Variations between different MHC alleles = Variations in size, charge and location of pockets = Variations in peptides that can anchor within the pockets

Question 14

What do MHC Class I proteins play a central role in? What 2 things do T cell receptors recognise?

Answer 14

MHC-I proteins play a central role in the ability of the immune system to distinguish self from non-self T Cell Receptors (TCR) recognise two things MHC protein itself (hence compatibility…) Antigenic peptide presented by MHC protein TCR recognises both the MHC protein and the peptide antigen being presented by it Binds with a diagonal footprint that cuts across both alpha helices with the peptide in between

Question 15

What do distant binding sites allow with regards to CD* and TCRs?

Answer 15

Distant binding sites allow CD8 and TCR to bind MHC-I at the same time CD8 acts as a co-receptor for MHC-I, and is required for the T cell to make an effective response TCR binds to the a1a2 domains CD8 binds to the support domains (a3 and b2m)

Question 16

How can pathogens subvert (essentially prevent) presentation by MHC Class I? (and evade adaptive immune response)

Answer 16

Inhibit MHC-I transcription (adenovirus) Block peptide transport into the endoplasmic reticulum (HSV) Retain MHC-I in endoplasmic reticulum (adenovirus, HCMV) Target MHC-I for disposal from the endoplasmic reticulum (HCMV) Downregulate MHC-I from cell surface (HIV)

Question 17

What is the function of Killer Ig-like receptors?

Answer 17

When KIR recognise MHC-I they negatively inhibit NK cells from releasing lytic granules (so KIRs send signal to NK cells to stop them killing the cell if its healthy) Some viruses down-regulate MHC-I as a means to evade cytotoxic T cells, loss of MHC-I is also a common feature of tumour cells If a target cell does not express MHC-I then there is no KIR inhibition, lytic granules will be released to lyse the target This mechanism is known as “missing self” Inhibitory KIR bind to the same face of MHC-I as the T cell receptor recognise subsets of MHC-I alleles KIR are also polymorphic, as well as being polymorphic individual KIR genes vary in their presence between individuals Different MHC-I/KIR combinations show disease associations e.g. in HIV infection (eg Martin et al 2002 Nat Genet 31:429) Each KIR has a different target set of MHC alleles

Question 18

There is a variety of receptors on Natural Killer cells, some are activating receptors eg Natural Cytotoxicity receptors- what do these do?

Answer 18

These provide activating signals to NK cells, but their range of ligands are not well characterised NKp46 is known to bind viral hemagglutinin NKp44 – binds a ligand that is expressed on tumor cells and upregulated by viral infection Ligand for NKp30 is a stress induced protein so when the natural cytotoxicity receptor sees its ligand, gives activating signals for natural killer cell to kill

Question 19

What ultimately determines whether a natural killer cell kills or not

Answer 19

the balance of activating and inhibiting signals it receives which determines the level of activation overall

Question 20

Why do NK cells kill tumour cells?

Answer 20

Similar to many pathogens, tumor cells can escape the adaptive immune system, by downregulating the expression of MHC class I. This makes them more susceptible to NK cells.

Question 21

What is Antibody-dependent cell-mediated cytotoxicity (ADCC)?

Answer 21

A mechanism for natural killer cells- recognition of antibody complexes on target cells it is a way of natural killer cells interfacing with the adaptive immune response, they are synergising with an antibody response and allowing the killing of cells that the antibody binds to- this is known as ADCC

Question 22

How do cytotoxic granules allow for the process of lysis to take place?

Answer 22

NK cells and T cells carry granules filled with cytotoxic proteins within their cytoplasm and when they are activated, these cells release cytotoxic granules at site of contact with target cell (must be directed in order to avoid damaging innocent bystander cells) Included in these granules is: perforin- punches holes in the membrane of the target cell, which allows the contents of the granules on the other proteins within them to get into the cytoplasm of the target cell. Other proteins delivered in the granules of cytotoxic T cells are granzymes which are proteases- which get into the target cell and trigger activation of apoptosis where the cell destroys itself. Another protein example is granulysin, which can induce apoptosis.

Question 23

What happens at the immunological synapse?

Answer 23

explains how granules reach their exact target cell: T cell receptors and co-receptors cluster at the site of cell-cell contact This polarises the T cell to release effector molecules(the granules) at the point of contact so granules are delivered to the exact site where the T cell is in contact with its T cell receptor with its target

Question 24

Another way to cytotoxic granules, how can CD8 cells trigger apoptosis of target through Fas/FasL interaction?

Answer 24

This process is slower than cytotoxic granules (2 hour lag time) Fas ligand (FasL) on T cells engages Fas on target cells to trigger apoptotic pathway Fas/FasL triggered apoptosis is used to dispose of unwanted lymphocytes Loss of Fas can result in autoimmune lymphoproliferative syndrome (ALPS)

Question 25

Compare cytotoxic T cells and natural killer cells

Answer 25

Cytotoxic T-cells:
1. Using CD8 as co-receptor for MHC class 1
2. Each express a unique and highly specific T cell receptor with CD8 acting as co-receptor
3. Presence of appropriate MHC class 1 ligand results in activation of killing response
4. Absence of appropriate MHC class 1 ligand - no recognition and cell moves on
5. Response requires initial activation of naive T-cells, followed differentiation and proliferation

Natural killer cells:
1. Do not use co-receptor for MHC class 1, do not express CD8
2. Express multiple receptors including activating and inhibitory receptors
3. Presence of appropriate MHC class 1 ligand results in inhibition of killing
4. Absence of appropriate MHC class 1 ligand results in activation of killing
5. Ready to act on encountering targets in the periphery