TCR’s… T-cell receptors… Some fundamentals… T-cells have T-cell receptors. Nota bene: The previous statement is ambiguous. One T-cell has one type of TCR. One T-cell has one TCR with a wholly unique specificity. One T-cell has as many as 100,000 identical TCR’s.

A T-cell receptor is a heterodimer. A heterodimer is made from two different protomers. T-cell heterodimers are either: –  (alphabeta) –  (gammadelta) There are two types of T-cells: T H & T C. Can they have the same TCR? (Yes.) So, T H & T C must be distinguished some other way. (More about that later.)

A T-cell receptor is a heterodimer. A heterodimer is made from two different protomers. T-cell heterodimers are either: –  (alphabeta) –  (gammadelta) Either is an important word in that previous statement. There must be a genetic mechanism for having one heterodimer and excluding the other. (More about that later, too!)

What do T-cell receptors do? Two things: Respond to MHC Respond to Ag More exactly, they see one histotope and many, many types of processed antigens. Implication: There is some constancy (careful here) and huge amounts of variety, too. A genetic mechanism is needed.

OK. Enough premises! Let’s look at some pictures! Hmmm… Not very pretty but very infor- mative. What’s being said? Consider all the symbolic notations.

What do the “symbolic notations” convey? Ig is bivalent. TCR is mono- valent. Lots of immuno- globulin folds ( domains ) present. variable component constant component T m component. Short “CT’s”

So, we have these complicated proteins… i.e., these complicated gene products… What is the genetic organization of the loci for these products?

So, let’s look at the protein again… Two views will be informative… and, this…. First, this already seen…

The previous slide implies rearrangements… Let’s take a look:

There’s something odd in this slide. Did you see it?

Let’s look at this image and put on our thinking caps…

There are two genetic problems to solve: How is it that only  or  are expressed? How does exclusion of one occur? After a “V” cassette has been selected, –Downstream cassettes are “deleted” –But what about upstream cassettes? –They’re still there. –Why are they not expressed?

How are the questions in the previous slide answered?

There are two genetic problems to solve: Determinants for delta chain are – in mouse – tucked within the segments for the alpha chain. If an alpha V segment is fused with an alpha J segment, the delta cassettes are deleted (assuring that delta is not expressed if alpha is expressed.) If a delta V segment is fused with a delta J segment, the alpha cassettes are not expressed because they remain too far from the ENHANCER. Thus, the “embedded” organization of delta within alpha assures that only alpha-beta or gamma- delta heterodimers are produced.

TCR DNA templates (and primary transcripts) for the alpha, beta, gamma, and delta chains have nucleotide sequences specifying variable components (V, [D], and J) and constant components (constant domain, connecting sequence, transmembrane region, and cytoplasmic tail.) Sounds familiar… Let’s take a look:

Diversity (and constancy…) TCR’s have CDR1, CDR2, and CDR3. The variability in CDR3 comes from combinations of V-J and V-D-J joining, variable numbers of D segments joining together, junctional flexibility, N-region nucleotide addition, P-nucleotide addition, and combinatorial association of chains. ( There is no somatic hypermutation.) So, there is a huge variety in CDR3; CDR1 and CDR2 obtain their variety from the selection of V segments.