Questions: T Cell Receptor Structure, Diversity, and Recognition

Question 1

A CD4+ T cell has a TCR that binds strongly to a specific peptide-MHC class II complex. A mutation completely eliminates Lck expression in this T cell. What is the most likely outcome when the TCR engages peptide-MHC II?

• A. Normal T cell activation, because ZAP-70 can phosphorylate ITAMs independently of Lck
• B. No ITAM phosphorylation and no T cell activation, because Lck is required to initiate the signaling cascade
• C. Partial activation through the CD3 γε complex only, since some ITAMs remain accessible
• D. Normal activation, because TCR signaling does not require kinases — physical engagement suffices

Answer: (B)

Lck is the critical kinase that bridges TCR engagement to intracellular signaling. When CD4 binds MHC class II during TCR engagement, it brings Lck into proximity with the ITAMs on the CD3 and ζ chains. Lck phosphorylates those ITAMs, which then recruit ZAP-70. Without Lck, ITAMs remain unphosphorylated, ZAP-70 is never recruited, and the entire downstream cascade fails. The TCR can still physically bind peptide-MHC, but no activation signal is generated.

Question 2

A CD4+ T cell has a TCR specific for a peptide that happens to also fit in the groove of an MHC class I molecule. If the same peptide were displayed on MHC class I on the same antigen-presenting cell, what would happen when this T cell's TCR contacts the peptide-MHC I complex?

• A. Full T cell activation, because the TCR only recognizes the peptide, not the MHC class
• B. No productive activation, because CD4 cannot co-engage MHC class I, so Lck is not efficiently recruited
• C. Activation equal to MHC class II engagement, because the coreceptors are functionally interchangeable
• D. T cell death, because cross-class MHC engagement triggers apoptosis

Answer: (B)

The coreceptor is not interchangeable. CD4 binds specifically to the non-polymorphic region of MHC class II; CD8 binds MHC class I. If the CD4+ T cell's TCR contacts peptide on MHC class I, CD4 cannot co-engage the complex, Lck is not brought into position, ITAMs are not phosphorylated, and no productive activation occurs — even if the TCR binds the peptide portion perfectly. Coreceptor-MHC class matching is essential for activation, not optional.

Question 3

The α and β chains of the T cell receptor contain immunoreceptor tyrosine-based activation motifs (ITAMs) in their cytoplasmic tails that are phosphorylated upon peptide-MHC binding.

• T. True
• F. False

Answer: False

ITAMs are located on the cytoplasmic tails of the CD3 subunits (γε and δε dimers) and the ζ (zeta) chain homodimer — not on the TCR α or β chains themselves. The TCR α and β chains have extremely short cytoplasmic tails with no signaling capacity. This is why the TCR must associate with the CD3 complex and ζ chains to signal at all. The separation between recognition (TCR α/β) and signaling (CD3/ζ) is a fundamental architectural feature of the receptor complex.

Question 4

T cell receptor diversity, like antibody diversity, is generated through somatic recombination of V, D, and J gene segments.

• T. True
• F. False

Answer: True

Both TCRs and antibodies use V(D)J recombination — the same enzymatic machinery (RAG1 and RAG2) joins variable (V), diversity (D), and joining (J) gene segments to create diverse antigen-binding domains. For TCR β and δ chains, all three segments are used; for α and γ chains, only V and J are joined. Additional diversity from junctional imprecision gives the T cell repertoire the capacity to recognize an enormous range of peptide-MHC combinations.

Question 5

Why does the T cell receptor require associated CD3 and ζ chain subunits to signal, rather than carrying intracellular signaling domains directly on its own α and β chains?

Model answer: The TCR α and β chains are generated by V(D)J recombination, which diversifies their variable domains to recognize different peptide-MHC combinations. If each chain also carried signaling domains, those domains would have to be co-diversified — an unnecessary and potentially disruptive constraint. Instead, the system uses division of labor: the α/β heterodimer handles recognition, while the invariant CD3 and ζ chains handle signal transduction. This modular architecture allows the same optimized signaling pathway to work with every possible TCR specificity.

B cell receptors follow the same principle, associating with Igα/Igβ for signaling. The short TCR cytoplasmic tail is not an accident; it is a feature that decouples recognition diversity from signaling. It also allows regulatory flexibility — the CD3/ζ signaling module can be modulated independently of the TCR's binding specificity, enabling fine-tuned control over T cell activation thresholds.