Antigen Recognition by Lymphocytes II

How are T cells activated?

By interactions with MHC

What are the three signals that T cells have to see to be activated?

1. Peptide it has affinity for on MHC

2. Costimulation

3. Cytokine signal

How are CD8+ T cells activated? What is the function of CD8?

CD8 T cell will recognize viral antigens presented by MHC Class I on virus-infected cell. CD8 will kill the virus-infected epithelial cell

What are the two ways that CD4 T cells can be activated? What is the function of CD4?

1. Recognizes bacterial antigens presented by MHC class II on a macrophage. CD4 will secrete cytokines that activate the macrophage increasing its capacity to kill bacteria

2. Recognize bacterial antigens presented by MHC class II on a B cell. CD4 secretes cytokines that drive the differentiation of the B cell into a plasma cell making bacteria-specific antibodies

What are the 3 features of the TCR?

1. Two polypeptide chains linked by disulfide bonds

2. TCR complex has CD3(epsilon, delta, gamma) and zeta chains

3. TCR complex contains CD4 or CD8 glycoproteins

What are the two types of polypeptide chains that TCR can have? Which one is most prevalent in humans? What is the structure?

1. Alpha and beta chains (humans)

2. Gamma and delta chains

Has a single variable region and a single constant region

What do the CD3(epsilon, delta, gamma) and zeta chains do?

They are involved in stabilizing the TCR and signal transduction. The zeta chains are at the bottom and bring intracellular signalling domains.

Are all T cells CD3? What do they further divide into?

Yes they are and then they further divide into CD4 and CD8 subsets

Describe the structure of CD8

It contains a alpha and a beta chain that is held together with disulfide bonds and has two transmembrane domains

Describe the structure of CD4

One chain molecule with four domains and a hinge region in the middle with one transmembrane domain

What is the function of CD4 and CD8? Where are they located in relation to the TCR? What do they deliver?

They are co-receptors that aid in binding MHC II or MHC I. They are floating around the TCR and get brought into the complex. They deliver kinases on some of their intracellular tails that are important for early activation

Where do CD4 and CD8 bind? Are they conserved in everybody?

On a conserved side of the MHC molecule (not where the peptide binds). Yes

What is a good way to stop activating T cells that you don’t want to be activated?

If you prevent CD3 and CD4 T cells from developing as if they are not mature then they can’t be activated

How is a T cell supposed to find an MHC molecule with its correct peptide if it is not being pulled towards it?

Can be activated by SMAC (supramolecular activating cluster)

What are the 2 different SMACs involved in activation of a T cell?

1. c-SMAC (central SMAC)

2. p-SMAC (peripheral SMAC)

What are p-SMAC formed by? What are examples?

Adhesion molecule interactions. LFA-1, ICAM-1.

What is the c-SMAC formed by? What are examples?

TCR and related costimulatory and co-activating molecules are clustered together. It is where MHC is interacting with the TCR and getting the second signal with CD28 and other accessory molecules. TCR, CD4, CD8

Is a low or high threshold interaction required to bring the T cells and MHC molecules together?

Low

Does the APC have the same peptide binding to MHC repeated over its surface? If no, what does this mean?

No and this means that the T cell needs to inspect and crawl on the surface of APC to see if it is a match or not

What facilitates the crawling of membranes between APC and T cell?

LFA-1 binding to ICAM-1

What happens if the APC and T cell are not a match?

Then they will dissociate

What happens if there is a match between the APC and T cell?

The p-SMAC will stabilize the TCR against the MHC molecule and all the molecules come together to initiate the signal transaction that activates the T cell

Describe the steps that occur for activation of a T cell at the adhesion phase

1. Naive T cell binds dendritic cell with low affinity LFA-1 ICAM-1 interactions

2. Subsequent binding of TCR sends a signal to LFA-1

3. Conformational change in LFA-1 increases affinity (LFA-1 and ICAM-1 can interact more strongly) and prolongs cell-cell contact

4. T cell and APC are now committed to hanging out together and allows for the formation of SMAC

What is adhesion key to forming? What is it?

An immunologic synapse and this is the region where the TCR can interact with MHC and co-stimulatory interactions can occur

What are the steps following adhesion for T cell activation?

1. In a resting T cell the ITAMs of the CD3 complex are not phosphorylated

2. Binding of the MHC to the TCR leads to ITAM phosphorylation by Lck

3. CD4 or CD8 will bind to the conserved side of the MHC molecule

4. ZAP70 binds phosphorylated zeta chain ITAMs and is phosphorylated by Lck

5. This causes a signal to be transmitted down to the nucleus that initiates an activation cascade in the TCR which leads to T cell activation for helper function, cytokine production, and cytotoxicity

6. The T cell will now proliferate rapidly

What TCR is the most common in the human T cell repertoire?

Alpha and Beta TCR

Which TCR receptor is the most diverse antigen recognition repertoire?

Alpha and Beta TCR

What do alpha and beta TCRs recognize?

MHC and short peptides

What do gamma delta TCRs recognize? Where are they found at? Do they have to wait to be activated? Why?

A wide variety of antigens with less requirement for MHC and can see proteins in a native structure. Mucosal surfaces. No they don’t because they are less diverse

What is the antigen location of TCR vs BCR?

TCR: MHC/HLA molecular, intracellular

BCR: Extracellular

What are the antigen features of TCR vs BCR?

TCR: Peptides (aB) and some native antigens (gamma delta)

BCR: Antigens in their native configurations

What is the composition of the TCR vs BCR?

TCR: 2 chains: alpha and beta or gamma and delta

BCR: 2 light chains and 2 heavy chains

Are TCR secreted? BCR?

TCR: No

BCR: Yes as antibodies but they can be retained as a BCR

Binding sites/receptor for TCR? BCR?

TCR: 1 binding site/receptor

BCR: More than 1

Do TCR and BCR undergo gremlin rearrangement?

Yes

What are the associated molecules for BCR and TCR?

TCR: CD3, CD4, CD8

BCR: CD79a and CD79b