L18- Cytoxicity

Question 1

What are the types of cytotoxic lymphocytes?

Answer 1

Natural Killer cells- direct killing, ADCC in innate immunity
CD8+ T cells- direct killing in adaptive immunity

Question 2

What is the difference between direct killing and ADCC?

Answer 2

In direct killing, the NK cell/CD8+ T cell receptor binds directly to the MHC molecule of the target cell
In ADCC, the NK cells bind to target cell via a specific antibody

Question 3

How do naive CD8+ cells obtain IL-2?

Answer 3

1. Mature dendritic cells directly stimulate them to make IL-2
2. Need CD4+ T cell directly bound to the same APC, CD4+ T cell releases IL-2 and induces CD40 and 4IBB ligand on APC for extra co-stimulation

Question 4

What is an immune synapse?

Answer 4

The interface between an APC or target cell and a CD4+/CD8+ T cell or NK cell.
Consists of molecules that regulate T cell activation- ensure detailed antigen recognition and effective T cell responses

Question 5

How does necrosis occur?

Answer 5

• Chromatin clumping, swollen organelles, flocculent mitochondria
• Disintegration
• Release of intracellular contents
• Inflammation

Question 6

How does apoptosis occur?

Answer 6

• Condensation of cytoplasm, chromatin compaction and segregation
• Nuclear fragmentation, blebs, apoptotic bodies
• Phagocytosis of apoptotic bodies

Question 7

How do CD8+ T cells recognise and kill targets?

Answer 7

1. Initial interaction of CD8+ T cell with target is made by non-specific adhesion molecules.
2. Antigen-specific recognition: stable pairing and release of effector molecules
3. Death of target and release of the CD8+ T cell

Question 8

What molecules mediate initial non-specific adhesion of CD8 T cells with target cells?

Answer 8

LFA-1 or LFA-2 on CD8+ T cell

ICAM1 or LFA-3 on target cell

Question 9

What happens in the lytic immune synapse formation?

Answer 9

In the presence of specific TCR recognised antigen bound to class I MHC cells, cells adhere strongly and formation of LIS is initiated leading to clustering of cytolytic granules towards target cell.

Question 10

What happens to cell components when specific recognition occurs?

Answer 10

• T cell becomes polarised
• Microtubule organising centre (MTOC) formation occurs
• Clustering of granules at point of contact

Question 11

What is the role of actin rich lamellipodia?

Answer 11

Found on CTL and form interdigitated contact side with target cell
TCR-peptide-MHC interactions occur at the tips of the actin rich interdigitations

Question 12

What do the granules of CD8+ T cells contain?

Answer 12

• Perforin
• Granzymes
• Centrosomes
• Granulysin

Question 13

What does perforin do?

Answer 13

Aids in delivering contents of granules into the cytoplasm of the target cell. A pore forming protein

Question 14

What do granzymes do?

Answer 14

Serine proteases which activate apoptosis once in the cytoplasm of the target cell. Cleave substrates in cytoplasm of target cells triggering rapid apoptosis.

Question 15

What do centrosomes do?

Answer 15

Consist of two centrioles surrounded by pericentriolar material and are the only microtubule organising centres (MTOC’s) in the T cells.

Question 16

What does granulysin do?

Answer 16

Has antimicrobial actions and can induce apoptosis

Question 17

What are the mechanisms of action of perforin and granzymes?

Answer 17

• Perforin forms large transmembrane pores that enable diffusion of granzymes into target cytosol.
• It does this by binding to target cell plasma membrane and oligomerising in a Ca2+ dependent manner.
• Granzymes then initiate apoptosis of the target cell

Question 18

How is apoptosis induced?

Answer 18

• Granzyme delivered into cytosol and targets BID and pro-caspase 3.
• Truncated BID disrupts outer MT membrane and activated caspase 3 cleaves ICAD
• This releases caspase activated DNAse to fragment cell DNA

Question 19

What are the additional killing mechanisms of CD8+ T cells?

Answer 19

• Cytokines; IFN-g, TNF-a
• Fas-L receptor
• TNF-b receptor