What is T cell receptor (TCR) signalling?

T Cell Receptor (TCR) Signaling

Overview of TCR Structure and Function

TCR signaling is initiated when the T cell receptor recognizes peptide antigens presented by MHC molecules on antigen-presenting cells, triggering a cascade that requires two distinct signals for full T cell activation and proliferation. 1

The TCR is a multisubunit protein complex composed of six different polypeptides that must assemble correctly to function 2, 3. The receptor consists of:

• Antigen recognition components: Most T cells express αβ subunits with immunoglobulin-like variable domains that directly bind peptide-MHC complexes 3, 4
• Signal transduction machinery: The CD3 complex (γ, δ, ε, and ζ subunits) associates with the TCR αβ chains and transmits activation signals into the cell 2, 3
• Variable regions: The complementarity-determining region 3 (CDR3) undergoes V(D)J recombination to create diverse antigen recognition capabilities 5

The Two-Signal Model of T Cell Activation

Signal 1: TCR-Peptide-MHC Interaction

Signal 1 results from the direct interaction between the TCR and peptide antigen presented on MHC molecules by antigen-presenting cells. 1

• The TCR recognizes antigens in an MHC-restricted manner, meaning it must simultaneously engage both the peptide and the MHC molecule 6, 4
• The physical dimensions of the TCR-pMHC complex are critically important—elongating this interaction reduces TCR triggering even when binding affinity remains intact 7
• TCR triggering depends on size-based segregation at the T cell-APC interface, where small molecules (TCR-pMHC) are separated from large molecules (CD45, CD148 phosphatases) 7

Signal 2: Costimulatory Signals

Signal 2 results from the interaction between costimulatory receptors (such as CD28 or 4-1BB) on T cells and their corresponding ligands on APCs. 1

• Without costimulation, TCR engagement alone can lead to T cell anergy or tolerance rather than activation 1
• CD28 and 4-1BB are the primary costimulatory molecules that enhance T cell activation, proliferation, and antitumor activity 1
• The combination of both signals enables complete T cell activation with proliferation and effector function 1

Proximal Signaling Events

Following TCR-pMHC engagement and costimulation, several intracellular signaling cascades are initiated 4:

• CD3ζ activation domain: All functional TCR complexes use the CD3ζ signaling domain to transduce the initial activation signal 1
• Immunological synapse formation: The interface between T cells and APCs reorganizes to create a specialized signaling platform where TCR-pMHC complexes cluster 4, 7
• Phosphatase exclusion: Large phosphatases like CD45 are depleted from the T cell-APC interface, allowing kinase activity to predominate and initiate downstream signaling 7

Antigenicity and TCR Recognition

The ability of TCR signaling to drive adaptive immunity depends on antigenicity—the expression of antigens that escaped central tolerance and for which naïve T cell clones are available. 1

• Healthy cells have limited ability to drive immune responses because their antigens induce clonal deletion during thymic T cell development 1
• Infected and malignant cells display sufficient antigenicity through neoepitopes (highly immunogenic mutated peptides) and non-mutated epitopes with gaps in tolerance 1
• Some self-reactive low-affinity TCRs escape thymic selection, allowing potential recognition of self-antigens under conditions of peripheral tolerance breakdown 1

TCR Repertoire Diversity and Clonality

The TCR repertoire exhibits extraordinary diversity through combinatorial rearrangements 5:

• The TCRA locus on chromosome 14q11 undergoes V(D)J recombination to generate diverse receptors 5
• A healthy individual may have up to 10^19 different T cell clones in peripheral blood 1, 5
• Oligoclonal T cell populations can converge with consistent CDR3 motifs to recognize the same antigen, indicating that one-to-one mapping between T cell clones and antigens is unlikely 1, 5

Clinical Implications

TCR signaling analysis has become essential for cancer immunotherapy, where changes in TCR repertoire clonality indicate antitumor T cell responses. 5

• Single-cell RNA-seq approaches can identify neoantigen-specific TCRs by culturing tumor-infiltrating lymphocytes with peptide-pulsed autologous APCs 1, 5
• TCR clonotyping characterizes T cell responses following personalized cancer vaccination or checkpoint blockade therapy 5
• Tools like MixCR and TraCeR enable identification of paired alpha-beta TCR sequences from single cells, revealing clonally expanded populations 1, 5

Key Pitfalls in Understanding TCR Signaling

• Bulk sequencing limitations: Bulk TCR sequencing cannot pair TRA and TRB sequences to define specific clonotypes due to high repertoire diversity 1, 5
• Sampling bias: The extreme diversity of the TCR repertoire means that blood draws and sequencing samples capture only a fraction of the total repertoire 1, 5
• Context dependency: TCR signaling outcomes depend on the specific host context, including MHC haplotype and the presence of naïve T cell clones capable of recognizing presented antigens 1