14. How a double-positive cell picks a side · Prof. Kwon's TCR-HLA and CD4/CD8, First Steps

While T cells are being made in the thymus, the intermediate stage wears both CD4 and CD8. This is the double-positive (DP) cell.

Which creates a problem. If this cell's TCR reads class I it should keep CD8, and if it reads class II it should keep CD4. But how would it know?

The TCR itself does not say what it reads

We saw this in lesson 10: TCRs that read class I and TCRs that read class II look almost identical. The sequence will not tell you.

The explanation that failed — the 'strength model'

The old story went: CD4 brings more Lck, so reading class II gives a strong signal and reading class I a weak one. Strong becomes CD4, weak becomes CD8.

Plausible, and broken. Signal strength also depends on the quality of the fragment. A cell that reads class I very well can signal more strongly than a cell that barely reads class II — and then the sides swap.

The current explanation — the kinetic signalling model

Alfred Singer's lab produced a far cleverer answer. The cell runs an experiment.

1️⃣

Positive selection begins

The TCR reads a self pMHC weakly and a signal switches on. Identical for both lineages so far.

2️⃣

CD8 is switched off first

Once signalling starts, the cell stops transcribing CD8 — no matter what it reads. This is the test.

3️⃣

Is the signal still there?

A cell reading class II still has its CD4, so the signal continues. A cell reading class I has lost CD8, so the signal stops.

4️⃣

The answer arrives

Signal continues → ThPOK on → CD4 cell.
Signal stops → IL-7 signalling → Runx3 on → back to a CD8 cell.

This is called coreceptor reversal. The cell does not find out by reading its own TCR. It finds out by cutting the wire.

Set the TCR's class and run the thymic test. You can watch how the cell works it out.

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The numbers are invented toys. Look only at the skeleton of the logic.

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# Why 'strength' fails and 'persistence' works — a toy model
import thla

# Two cells. A reads class I well; B barely reads class II.
cells = [("A · reads class I well", "I", 1.5),
         ("B · barely reads class II", "II", 1.0)]

print("(1) both still wear CD4 and CD8")
before = {}
for name, cls, t in cells:
    v = thla.proofread(thla.with_coreceptor(t, boost=3.0), steps=4)
    before[name] = v
    print(f"   {name:<28}signal {v:.3f}")
print("   -> choosing on strength makes A a CD4 cell. Wrong answer.")
print()

print("(2) switch off the CD8 gene, regardless of what is being read")
for name, cls, t in cells:
    boost = 1.0 if cls == "I" else 3.0        # the class I reader loses its helper
    v = thla.proofread(thla.with_coreceptor(t, boost=boost), steps=4)
    drop = 100 * (1 - v / before[name])
    print(f"   {name:<28}signal {v:.3f}   change {-drop:+.0f}%")
print("   -> A collapses, B does not. The cell watches the change, not the level.")

ThPOK and Runx3 — two switches that turn each other off

ThPOK

Master transcription factor of the CD4 fate.
Once on, it switches off Runx3.
So the CD8 gene never comes back on.

Runx3

Master transcription factor of the CD8 fate.
Once on, it switches off ThPOK and the CD4 gene.
It also switches on the cytotoxic programme.

Each switches the other off, so there is no middle. Once it tips, it goes all the way. This arrangement is called double-negative feedback, and it is a very common shape in cell fate decisions.

🤔 Check yourself

How does a double-positive thymocyte find out which class its TCR reads?

🤔 Check yourself

Why did the 'strength model' fail?

🤔 Check yourself

What is the relationship between ThPOK and Runx3?