What is a CTL?
Cytotoxic T Lymphocyte
A CD8+ lymphocyte that has recognize its antigen and been activated.
Part of adaptive immunity.
What is an NK cell?
Natural Killer cell
Contains granules like a CTL but is not antigen specific.
Part of innate immunity.
Identify this cell type
Probably an NK cell or a CTL.
Note the large granules in the cytoplasm. Together, NK cells and CTLs are called "large granular lymphocytes."
What is the major function of CTLs and NK cells?
Protect against intracellular pathogens
What process is occurring in this image?
Cell with pointer is undergoing apoptosis.
Note the chromatin clumping and membrane blebbing that are characteristic of a cell undergoing programmed death.
Learning Objective 1
Describe, in general terms, how apoptosis in induced and what happens in a cell that is induced to undergo apoptosis.
[Wow, that's just terrible wording]
- FasL (for Fas ligand) on the killer cell binds to the Fas receptor (the death receptor) on the target cell, inducing apoptosis.
- TNF molecule binds to TNF receptor on target cell, inducing apoptosis.
- Release of cytochrome C induces apoptosis.
Common to both pathways: activation of caspase 9.
What is a granzyme?
Proteolytic enzyme released by a killer cell and taken up by a target cell.
Initiates the intrinsic apoptotic pathway.
What is a caspase and how does initiate cell breakdown?
Caspases are "executioner" molecules initiated by both apoptotic pathways.
- Cleave proteins, causing breakdown of the cytoskeleton.
- Activate endonucleases, fragmenting DNA and causing chromatin clumping.
Why does apoptosis not cause inflammation?
The process of apoptosis causes formation of cytoplasm blebs, which contain surface molecules for phagocytic cells. The blebs are digested by phagocytic cells with no leakage, so inflammation is not initiated.
Learning Objective 2
Explain the differences between apoptosis and necrosis.
- Cell degrades into membrane-bound blebs, which are phagocytosed. No inflammation.
- Individual or clusters of cells
- Violent and uncontrolled
- Membrane degrades, releasing cytoplasmic contents. Inflammation.
- Larger area involved
What is the function of perforin?
An activated CTL will release granules of perforin onto its target cell. These polymerize and insert into the target cell membrane, creating a hole (similar to the MAC or defensins).
This allows granzymes to enter the target cell.
What is the Fas receptor?
Fas is described as the "death receptor."
Fas is expressed on somatic cell surfaces. FasL (Fas ligand) is expressed on the surface of CTLs. If a CTL is targeting a somatic cell, it will upregulate the amount of FasL expressed in its membrane. Binding of FasL to Fas triggers apoptosis.
What is TNF?
Tumor necrosis factor
Binding of TNF to the TNF receptor on a target cell will initiate apoptosis.
Learning objective 3
List 3 mechanisms that CTLs use to kill target cells. For each, include the molecules and functions involved - for binding and killing.
- Perforin and granzyme pathway - perforin creates a hole in the cell membrane and granzymes enter to initiate the intrinsic apoptosis cascade.
- Fas/FasL pathway - FasL expressed on the CTL binds with Fas on the target cell, inducing the extrinsic apoptosis cascade.
- TNF produced byt the CTL binds with TNF receptor on the target cell, initiating the extrinsic apoptosis cascade.
CTLs bind to which molecule on the target cell?
TCR (t cell receptor) binds to target peptide presented on MHC I.
CD8 molecule binds to MHC I itself.
Describe the activation process of a CTL
CTL leaves the blood stream in response to upregulated inflammatory markers.
CTL binds to MHC I on target cell and recognizes its target peptide.
CTL redistributes its cytoskeleton to polarize killing granules toward the target cell.
CTL releases its granules onto target cell (the immunological synapse), initiating apoptosis.
What is the difference in surface molecules between a CTL and NK cell?
Recall that an NK cell is part of innate immunity and cannot recognize specific peptides. So, NK cells do not express CD3, CD4, or CD8.
(CTLs express CD3 and CD8)
True or False
NK cells have similar granule content to CTLs
Thought they do not recognize specific antigens, NK cells have similar killing mechanisms to CTLs.
What is MICA receptor?
MICA receptor is and MHC I - like molecule that is expressed by stressed cells.
If the NK cell binds to the MICA receptor on a target cell, it will induce apoptosis.
What is the Fc receptor?
Recall that Fc is a component of antibody. Like many other leukocytes, NK cells have Fc receptors to recognize when a molecule is opsonized by antibody.
If a target cell is opsonized, an NK cell will induce apoptosis in the target cell.
Which cytokines activate NK cells?
IFNalpha and IL12 - produced by cells infected with virus.
Th1 cytokines (IL2, IFNgamma, TNFalpha)
These cytokines make NK cells better at killing.
What is antibody-dependent cell-mediated cytotoxicity (ADCC)?
If a cell is coated by antibody, this will be detected by the Fc receptor of the NK cell. The NK cell will then initiate apoptosis in the opsonized cell.
Describe the MHC I inhibitory receptor mechanism in NK cells.
When an NK cell binds to MHC I on the target cell, it is inhibited from killing the target cell.
If the target cell is not expressing MHC I, it will be killed by the NK cell.
Suppose a tumor cell is not expressing MHC I normally. What will the result be?
If an NK cell fails to bind to MHC I on a target cell, it will initiate apoptosis.
Some viruses prevent antigen presentation on MHC molecules, so viral particles cannot be recognized by CTLs. Can the infected cell still be killed by the immune system?
NK cells act as a "backup" for CTLs
If the virally-infected cell is not expressing MHC I in the normal fashion, it will be killed by the NK cell (even though the virus was not recognized by the CTL).
Learning Objective 4
Describe how an NK cell recognizes a cell to kill and describe the killing mechanisms of the NK cell.
NK cells do not recognize specific peptides.
- When an NK cell binds to an MHC I molecule on the target cell, it is inhibited from killing. If the cell is not expressing MHC I, it will be killed.
- If the target cell is expressing the MICA receptor (a stress protein), it will be killed.
- If a cell is coated in antibody, it will be killed.
Killing mechanism: NK cells have granules containing perforins and granzymes, just like CTLs. They also possess the Fas ligand. NK cells can be made more effective at killing by cytokine activation.
Learning Objective 5
Explain how the CTL and NK cell are complimentary, including timing, targets, and the benefit to the host to have both cell types.
Both protect against intracellular pathogens.
CTLs recognize antigenic peptides presented on MHC I. They are important in acting against specific infections.
NK cells are part of innate defense. They respond to antibodies, stress proteins, and cells not correctly expressing MHC I.
In the case of a viral infection, a CTL could recognize specific viral proteins to respond (NK cell could not recognzie specific proteins). In the case of a tumor that stopped expressing MHC I, NK cells could induce apoptosis (CTL could not).
Learning Objective 6
Compare and contrast the various killing mechanisms used by the immune system.
- Release of toxic granules, oxidative burst, netosis
- Phagocytosis, nitric oxide
NKs and CTLs
- Induction of apoptosis
- Defensins, complement, antibody inactivation
Learning Objective 7
Give an example of an infection that CTLs would be important in controlling.
CTLs are important for controlling intracellular infections. Peptides in the cytoplams are processed and presented on MHC I through the endogenous pathway.
Examples: viral infections, intracellular bacteria
Learning Objective 8
List a few important properties of a vaccine that would stimulate CTLs.
Remember that CTLs respond to peptides presented through the endogenous pathway. This means that the pathogen must be located intracellularly.
Vaccine properties: modified live virus that replicates intracellularly.
Can also use an ISCOM (immune-stimulatory complex) - antigen encased in lipid can enter into the cytoplasm.