T-cell receptors (TCRs) have a similar structure to the Fab fragment of an immunoglobulin molecule, they are heterodimers made up two polypeptide chains: a (40-50kDa) and b (35-47kDa), which are linked by a disulphide bond; which makes up the large extracellular variable and constant regions for binding antigen. This a:b arrangement makes up for the majority of antigen recognition as it is present on >95% of peripheral blood T-cells and majority of thymocytes, however a minority of T-cells present an alternative receptor with a different pair of polypeptide chains; g and d, this set of TCRs are rare in the secondary lymphoid tissues and peripheral blood but predominate at epithelial surfaces; as this subtype of TCR is not seen in the anatomical locations which would normally support the classical mechanism of antigen presentation and lymphocyte proliferation, suggesting that g:d TCRs can recognise antigen in an MHC-independent manner. TCR heterodimers (a:b) form a complete, stably expressed antigen-binding domain that is capable of transmitting a signal upon ligand binding, by the constitutive association of multimeric hypervariable regions known as the CD3 complex which associate with the TCR variable domains, the TCR itself does not have intrinsic intracellular signalling motifs, the CD3 does and can thus cause intracellular signal transduction mediated by the immunoreceptor tyrosine-based activation motif(ITAM) sequencesi, which leads to T-lymphocyte activation. The CD3 complex comprises of 5 invariant chains; ?, ?, ?, ?, ?; these chains are organised into 2 heterodimers (g:? and d:? ) and one homodimer (? ?), which have 4 and 6 ITAMs respectively. The 10 copies of this motif within the cytoplasmic domains of the TCR:CD3 complex become phosphorylated on the conserved tyrosine residues when the TCR: peptide(MHC) interaction occurs, this allows a docking site for adaptor molecules which cause the mitogenic effects which result in polyclonal T-cell expansion.
TCR rearrangement: TCRs are expressed on the surface of T-cells as a result of somatic gene rearrangement of the TCR locus during thymic ontogeny, VDJ recombination occurs in the developing T and B lymphocytes. This generates a diverse repertoire of antigen-specific receptors with varying specificities, this provides the versatility that is needed for the recognition of nearly all pathogens including altered self-cells seen in cancer and thus required for normal immune system functioning. TCR genes; Tcra, Tcrb, Tcrg and Tcrd each have constant and variable regions which are assembled through this site specific recombination mediated by recombination activating genes, RAG-1/2. Rearrangements in the TCR a:b gene segments results in VJ joining for the a and g chain and VDJ joining for the b and d chains. In addition, T-cell repertoire diversity is increased via the input of P and N nucleotides and deletion at junctional regions, these junctional regions encodes the hypervariable region CDR3, the site responsible for antigen binding.
Mechanism of action: Although signals generated by the TCR determine the specificity of the T-cell response to antigen, co-stimulatory receptors: CD4 specific for MHC class II, CD8 specific for MHC class I, CD45 and integrin molecules enhance the avidity of the TCR antigen binding and the co-receptors, this signalling is known as ‘signal 2’. Signal 2 also brings the cytoplasmic domains of the TCR-CD3 and APC co-receptors into close proximity with MHC molecules, this helps bring about the cascade of intracellular events aswell as the formation of the immunological synapse. The CD45 receptor tyrosine phosphatase activates the src-family members, Lck and Fyn, these kinases then phosphorylate the 6 ITAMs (3 per polypeptide) within the ?? CD3-homodimer. A second tyrosine kinase, ZAP-70 docks at the phosphorylated ? chain, this additional docking results in receptor clustering (Lipid raft) which promotes the recruitment of signalling proteins such as the Linker protein of Activated T-cells (LAT), this becomes phosphorylated by ZAP-70, these phosphorylated tyrosine residues become docking sites for enzymes bearing Sh2 domains which will subsequently cause the downstream signalling to occur. The phosphorylated LAT can initiate many downstream pathways, one of which is the Ras-mitogen-activated protein kinase pathwayii. All intracellular cascade pathways ultimately lead to cytokine secretion, transcription factor upregulation to cause mitogenic effects and thus T-cell proliferation. Therefore, the functional activity of T-cells correlates with TCR cell-surface expression
TCR and related disease states: TCR rearrangements is a fundamental process in T-cell ontogeny, therefore any dysregulation within this process will result in a diseased state. TCR rearrangement during the ontogeny phase produces a reactive polyclonal T-cell population, where no single clonotypic population predominates the population; however, in peripheral T-cell lymphoma (non-Hodgkin lymphoma)iii, a monoclonal expansion of the T-cell population is a characteristic hallmark of malignancy; one specific T-cell population with a unique TCR gene rearrangement.