5.2 T cell receptor structure and signaling

T Cell Receptor Structure and Function

T cell receptors (TCRs) are the molecular sensors that allow T cells to detect specific antigens. Each TCR pairs with CD3 signaling proteins to form a complex that both recognizes peptide-MHC and transmits activation signals into the cell. Understanding this receptor system is central to grasping how adaptive immunity achieves both specificity and diversity.

Structure of the TCR and CD3 Complex

The TCR itself is a heterodimer of α and β chains (a small subset of T cells uses γ and δ chains instead). Each chain has a variable (V) region at the tip that contacts antigen and a constant (C) region closer to the membrane. The extracellular domains handle antigen recognition, while the transmembrane segments anchor the receptor. Critically, the TCR's cytoplasmic tails are very short, so the receptor alone can't signal. That's where CD3 comes in.

The CD3 complex provides the signaling machinery. It consists of CD3γ, CD3δ, CD3ε, and CD3ζ chains, each containing immunoreceptor tyrosine-based activation motifs (ITAMs) on their cytoplasmic tails. ITAMs are the docking sites that get phosphorylated to launch intracellular signaling cascades.

The fully assembled complex has a defined stoichiometry:

• TCR αβ (one heterodimer for antigen binding)
• CD3 γε (one heterodimer)
• CD3 δε (one heterodimer)
• CD3 ζζ (one homodimer, contributing 6 ITAMs total)

All of these subunits must assemble correctly in the ER before the complex reaches the cell surface. If any component is missing, the complex gets degraded.

TCR Gene Rearrangement and Diversity

TCR diversity is generated through somatic recombination of gene segments, similar to how immunoglobulin genes rearrange in B cells.

• The α chain locus contains V and J gene segments.
• The β chain locus contains V, D, and J gene segments (the extra D segment adds another layer of diversity).

V(D)J recombination proceeds through these steps:

1. RAG1 and RAG2 enzymes recognize recombination signal sequences (RSSs) flanking each gene segment.
2. RAG proteins introduce DNA double-strand breaks between the coding segment and its RSS.
3. The broken ends are joined by non-homologous end joining (NHEJ), a somewhat imprecise repair process.

Three main mechanisms generate the enormous TCR repertoire:

• Combinatorial diversity: Multiple V, D, and J segments can combine in many different ways.
• Junctional diversity: During NHEJ, nucleotides are randomly added (by TdT) or deleted at the junctions between segments. This is actually the largest source of diversity.
• Chain pairing: Different α and β chains can pair together, multiplying the total combinations.

Together, these mechanisms produce an estimated $10^{15}$ to $10^{18}$ possible TCR sequences.

Allelic exclusion ensures that each T cell expresses only one functional TCR. The β chain rearranges first; once a productive rearrangement occurs, the second β allele is silenced. This guarantees that each T cell has a single antigen specificity, which is essential for maintaining self-tolerance.

Role of CD4 and CD8 Co-receptors

Co-receptors don't bind antigen directly. Instead, they bind to conserved regions on MHC molecules, stabilizing the TCR-peptide-MHC interaction and amplifying the signal.

• CD4 is expressed on helper T cells and binds the β2 domain of MHC class II molecules.
• CD8 (typically an αβ heterodimer) is expressed on cytotoxic T cells and binds the α3 domain of MHC class I molecules.

Both co-receptors recruit the tyrosine kinase Lck to the TCR-CD3 complex via their cytoplasmic tails. This is functionally important because Lck is the kinase responsible for phosphorylating CD3 ITAMs, the very first step in TCR signaling. Co-receptor engagement increases sensitivity to antigen by roughly 100-fold, meaning a T cell can respond to far fewer peptide-MHC complexes than it could without co-receptor help.

Co-receptors also play a role during thymic selection: whether a developing thymocyte expresses CD4 or CD8 determines its lineage commitment (helper vs. cytotoxic).

TCR Signaling Pathways and Activation

Once the TCR engages peptide-MHC, signaling proceeds through a well-defined cascade.

Early signaling events:

1. Lck (recruited by CD4 or CD8) phosphorylates the ITAMs on CD3ζ and other CD3 chains.
2. Phosphorylated ITAMs recruit ZAP-70, a cytoplasmic tyrosine kinase, which binds via its tandem SH2 domains.
3. Lck then phosphorylates and activates ZAP-70.
4. Active ZAP-70 phosphorylates the adaptor protein LAT and SLP-76, which together nucleate a large signaling platform called the signalosome.

Downstream pathways branch from the signalosome into three major routes:

• Calcium/NFAT pathway: PLCγ1 cleaves PIP2 into IP3 and DAG. IP3 triggers calcium release from the ER, which activates calcineurin, which dephosphorylates NFAT so it can enter the nucleus.
• MAPK/AP-1 pathway: DAG and adaptor proteins activate the Ras-MAPK cascade, ultimately activating the transcription factor AP-1.
• PKC-θ/NF-κB pathway: DAG also activates PKC-θ, which triggers a signaling chain leading to NF-κB activation.

All three transcription factors (NFAT, AP-1, NF-κB) cooperate to drive expression of genes required for T cell activation, including IL-2 (the major T cell growth factor), IFN-γ, and receptors needed for proliferation and differentiation.

Co-stimulation is required for full activation. TCR signaling alone (Signal 1) is not sufficient. The T cell also needs Signal 2, most commonly provided by CD28 on the T cell binding B7 (CD80/CD86) on the antigen-presenting cell. CD28 signaling enhances IL-2 production and promotes cell survival. Without co-stimulation, TCR engagement leads to anergy, a state of functional unresponsiveness that serves as a peripheral tolerance mechanism.

The combined result of proper TCR signaling plus co-stimulation is T cell proliferation and differentiation into effector cells (which carry out immune functions) and memory cells (which provide long-lasting immunity).