T-cell Receptor: Structure, Organisation and Genes

Back to blogging about Immunology!

Understand TCR binding to MHC complex
Understand the structure and formation of the TCR

TCRs (T-cell receptors), unlike BCRs, aren't antibodies. No offence to TCRs, but they're not as exciting as BCRs: rather than the Y-shape of antibodies, TCRs just have two small, straight chains (usually an α-chain and a β-chain), each containing Ig (immunoglobulin) domains. Each chain has a constant region and a variable region. Just like the BCR, the variable region of the TCR is where antigens bind.

Just as a refresher, MHC-I binds to TCRs on CD8 (cytotoxic) T-cells and MHC-II binds to TCRs on CD4 (helper) T-cells. In fact, CD8 and CD4 act as co-receptors that facilitate this: CD8 binds to α₂ and α₃ on MHC-I, whereas CD4 binds to β₁ and β₂ on MHC-II. CD4 is a monomer that has four domains in a long chain, whereas CD8 has two short chains, called amazeballs A and amazeballs B. (Just kidding. They're just called α and β.)

Anyway, back to TCRs. As I mentioned earlier, most TCRs have an α-chain and a β-chain, but some have a γ-chain and a δ-chain instead. These γδ TCRs are more common in early foetal development, but later on they are overtaken by the αβ variety. A T-cell can only have one kind of receptor (either αβ or γδ). Both kinds of TCRs develop in the thymus and associate with a co-receptor called CD3, but there are a lot of other major differences: T-cells expressing γδ generally do not have CD4 or CD8, and they don't interact with MHCs. Instead they are a bit more like PRRs in that they bind to ligands that are common across microbes, such as phospholipids and certain intact proteins. They are considered to be innate lymphocytes (probably because of this), but there's still a lot about them that we don't really know.

One last thing about γδ T-cells before I move on: when I said they don't have CD8, I kinda lied. Intraepithelial lymphocytes (iELs), which are γδ T-cells found in the gut, have a different form of CD8. Instead of an α-chain and a β-chain, the CD8s on iELs have two α-chains (and thus these receptors are known as CD8αα+). iELs still aren't considered CD8 T-cells, though, since they don't have the normal kind of CD8.

Now I'm going to talk about the TCR signalling complex! I'm going to talk about it in regards to the αβ T-cells, but I assume it probably works in a similar way in γδ T-cells. The TCR signalling complex consists of the TCR, as well as extracellular CD3 molecules and intracellular ζ (zeta) chains. CD3 molecules are comprised of three different proteins (γ, δ and ε) that can dimerise in different ways (γ/ε or δ/ε). CD3 molecules help out with the assembly, expression and stability of TCRs, whereas ζ-chains help out with the signalling.

Finally, a note on complementarity determining regions of TCRs, specifically αβ TCRs. This is a bit random, but I really didn't know where else to put this. The binding site of the TCR has complementarity determining regions (CDRs), imaginatively named CDR-1, CDR-2 and CDR-3. CDR-1 interacts with both the peptide and the MHC, CDR-2 interacts with the MHC only and CDR-3 interacts with the peptide only.

Understand the organisation of the TCR genes

The organisation of the T-cell receptor genes is somewhat similar to the organisation of the B-cell receptor genes, which I've gone over here.

Organisation of Genes

The α-chain, in some ways, is analogous to the light chain of the B-cell receptor in that it has leader, variable, joining and constant sequences, whereas the β-chain is more like the BCR heavy chain in that it also has diversity sequences. The β-chain genes are a bit unique, however, in that the D, J and C sequences are arranged in two separate "chunks": following the LV sequences, there's a D sequence, some J sequences and a C sequence, and then a second D sequence, more J sequences and a second C sequence.

The γ- and δ-chains have the same regions as the α- and β-chains (aside from the δ-chain lacking leader sequences), but there is less diversity (i.e. fewer variable, diversity and joining sequences) and the genes are arranged a little bit differently. In the γ-chain, which has L, V, J and C sequences, the LV sequences are followed by three joining sequences and a constant sequence, and then two more joining sequences and a constant sequence (like the "chunks" making up the β-chain genes). The δ-chain has the sequences more or less in order- three variable sequences, three diversity sequences, three joining sequences and a constant sequence- but the locus is rounded off by a final variable sequence at the end. Another important thing you need to know is that the δ-chain genes are located between the variable and joining segments of the α-chain locus, so once the VJ rearrangement is done in the α-chain, all of the δ-chain genes become unavailable.

Receptor Editing and Allelic Exclusion

Just like in BCRs, there are ways of ensuring that only one receptor is made, and that the receptor genes are productively arranged.

First, let's look at the β-chain. As I mentioned earlier, the β-chain genes are in two separate "chunks." Essentially, the first "chunk" gets rearranged first. If that's unsuccessful, then the second "chunk" is rearranged, deleting the locus of the first "chunk" in the process. (Presumably, if that's also unsuccessful, the cell dies.) To ensure that only one chain is made overall, β-chains are subject to allelic exclusion, a process I described in my post about B-cell development. (If you don't remember what that is, basically it's the process where only one chromosome is trialled at a time.)

Now let's look at the α-chain! α-chain troubleshooting is a little different. Essentially, if a rearrangement is non-productive, then a different VJ pair will join. If that's non-productive, then another VJ pair will join, and so on until either a productive rearrangement is made or the cell runs out of V and/or J segments. Unlike β-chains and BCRs, α-chains do not undergo allelic exclusion. Instead, they express TWO α-chains during development (one from each chromosome). Eventually one is selected.

Aside from these, there are a few more similarities and differences between TCR and BCR development that I need to go over. Firstly, TCRs, unlike BCRs, do not undergo somatic hypermutations. They do, however, have P- and N-nucleotide additions, particularly in CDR3 regions.

TCR and Thymocyte Development

Now it's time to see how this all fits in with the bigger picture of thymocyte development! (For the uninitiated: thymocytes are basically precursor T-cells, so-called because they grow in the thymus.)

Thymocytes first progress through four double-negative stages, so called because they lack CD4 and CD8. They then progress through the double-positive phase, where they have both CD4 and CD8. Eventually, they mature into single-positive cells expressing only CD4 or CD8.

That's not to say that other CD molecules aren't important! In some of the double-negative stages, thymocytes express CD44 and/or CD25. CD44 is a glycoprotein that plays roles in cell adhesions, whereas CD25 is part of the IL-2 receptor. Cells in the DN1 (double-negative 1) stage have only CD44, cells in DN2 have CD44 and CD25, cells in DN3 have low expression of CD44 and regular expression of CD25, and cells in DN4 lack both. Also, from the DN3 stage onwards, cells begin to express a "pre-TCR" which has a β-chain and a pre-Tα-chain (which I guess is kinda like the surrogate light chain in the early BCRs). The pre-TCR allows for the formation of a pre-TCR CD3 complex, which in turn allows for signalling.

So when does the β-chain actually start forming? Well, in the DN2 stage, the DJ rearrangement occurs, and in DN3, the VDJ rearrangement occurs. (Just like BCRs, the VDJ-C rearrangement isn't done in the DNA, but rather as a part of RNA processing.) When the cell enters the double-positive (DP) stage, the VJ rearrangement in the α-chain occurs, and cells begin to express a real α-chain which replaces the pre-α-chain.

I'm not going to go into too much more detail about thymocyte development here, as that's the topic of a later post. Last fun fact for this post is that a receptor called Notch commits new cells to the T-cell lineage. Now get out there and win those trivia games with that new fact!