T-cell Receptor: Activation

Last post about TCRs!

Identify the cell surface molecules on T-cells and APC

There are a lot of cell surface molecules that you're going to need to know about, because the TCR can't initiate intracellular signalling all by itself. Instead, it needs help from some other proteins. This might be because while an MHC binds a peptide with high affinity, it binds to a TCR with relatively low affinity. Hence, we need other stuff to help hold things together!

Firstly, I'm going to give you an overview of the T-cell activation process, just to give you a framework for everything else that's going to follow. The first signal occurs when the TCR and MHC bind. Next co-stimulatory molecules on the TCR bind to molecules on the antigen-presenting cell (APC). Finally, the T-cell receives cytokine signalling, directing its differentiation into different kinds of effector cells.

The first thing that needs to happen is that the cells need to come into contact. This is helped by some of the cell-surface molecules. LFA-1 (on T-cell) binds to ICAM-1 (on APC) and CD2 (on T-cell) binds to LFA-3 (on APC) (and yes, you do need to know these pairings- just remember that the 1s go together). CD2, LFA-3 and ICAM-1 are all members of the Ig superfamily, but LFA-1 is an integrin instead (a molecule that facilitates cell-to-cell attachment and signal transduction).

Next up the TCR recognises a peptide bound to MHC. Since, as stated before, this is not a strong interaction, it is aided along by the ζ-chains in the TCR signalling complex, as well as by another little molecule called P56^lck (sometimes just called P56 or lck for short). Also aiding the binding is the formation of an immunological synapse. This is basically a cluster of proteins: the TCR/MHC pair are in the middle and are surrounded by LFA-1/ICAM-1 and CD2/LFA-3 pairs. This helps to increase the avidity of the T-cell/APC interaction. The TCR/MHC pair in the middle are sometimes also known as the cSMAC (central supramolecular activating complex), whereas the other pairs are sometimes known as the pSMAC (peripheral supramolecular activating complex).

Finally some co-stimulatory receptors begin to kick in! CD22 on the APC binds to CD45R on the T-cell, which then goes off to activate signalling via P56^lck (yup, it's a very versatile little guy). B7 (a.k.a. CD80) on the APC binds to CD28 on the T-cell, activating signalling via PKC (protein kinase C). This ultimately leads to effector activities, such as cytotoxic cells killing an infected cell or helper cells producing cytokines.

Explain the intracellular signalling molecules and signal transduction pathways

Now we get onto the intracellular stuff, which is kinda complex, but we'll get through it, I promise!

Firstly, just a refresher on the TCR signalling complex, which I mentioned in an earlier post. Recall that TCRs associate with CD3 proteins as well as with zeta-chains (which are homodimers). All of this is held together by opposing charges on the molecules. Another tidbit that you need to know is that the intracellular portions of CD3 and zeta-chains have ITAMs, short for "immunoreceptor tyrosine-based activation motifs," which have tyrosine residues that can be phosphorylated. Ultimately, it's this phosphorylation that kicks off the whole chain of events.

I've mentioned P56^lck a few times (which I'll call Lck from now on because it's easier to type) and now I'm going to mention it again, because it is just that important. Lck is a tyrosine kinase that associates with CD4/CD8, but as mentioned above, it is activated by CD45R following binding to CD22. When activated, it phosphorylates ITAMs on the CD3 and zeta chains. Another protein, ZAP-70 (zeta-associated protein), can come in and "dock" to the new phosphate groups on the ITAMs. ZAP-70 can also be activated by Lck, and when active, ZAP-70 phosphorylates phospholipase Cγ (a.k.a. PLCγ). That's not the end though- we're pretty much only beginning!

The PLCγ signalling pathway is not unique to T-cells. In fact, I've mentioned it before in a PHAR2210 post about G-protein coupled receptors.

Phospholipase Cγ, once activated by ZAP-70, can cleave PIP₂ (phosphatidylinositol-4,5-biphosphate) into IP₃ (inositol triphosphate) and DAG (diacylglycerol). DAG remains in the membrane, while IP₃ diffuses through the cytoplasm.

Now I'm going to split off and talk about the activation of the three main transcription factors in T-cell activation separately, as I find that works better for me.

Activation of NF-AT

IP₃ is able to induce the release of Ca²⁺ ions from intracellular stores. Calcium can then bind a protein called calmodulin, which can then activate calcineurin phosphatase. Calcineurin phosphatase then dephosphorylates NF-AT (remember, phosphatases remove phosphate groups), which activates NF-AT.

An interesting thing about this pathway is that it can be blocked by an immunosuppressant called cyclosporine. Cyclosporine inhibits calcineurin phosphatase so that NF-AT cannot be activated. Since all three main transcription factors are ultimately required, this drug suppresses the immune system by blocking T-cell proliferation, and is thus useful in treating autoimmune diseases and preventing rejection following transplantation.

Activation of NFκB

Once again, this starts with IP₃ inducing the release of Ca²⁺. Calcium sends protein kinase C (PKC) to diacylglycerol, which as you may recall is still hanging around in the membrane. This ultimately leads to activation of NFκB.

Activation of AP-1

This one's a bit different in that it's induced by CD28 on the T-cell binding to B7 (a.k.a. CD80) on the APC. This binding activates the MAPK cascade, as mentioned here (but we don't need to know it in so much detail for this course). This ultimately leads to transcription of AP-1.

So what happens after all three transcription factors have been activated? Well, activated transcription factors can bind to the promoter region for the IL-2 gene, inducing IL-2 transcription. IL-2 is an important cytokine for T-cell proliferation: as I said a little earlier, if transcription is blocked due to blocked production of NF-AT (via cyclosporine or otherwise), then T-cells cannot proliferate.

Last post on the T-cell receptor! The next Immunology post will cover humoral immunity, which is the last topic that will be covered on the next midterm.