T-cell receptor
The T-cell receptor is a protein complex, located on the surface of T cells. They are responsible for recognizing fragments of antigen as peptides bound to major histocompatibility complex molecules. The binding between TCR and antigen peptides is of relatively low affinity and is biologically degenerate.
The TCR is composed of two different protein chains. In humans, in 95% of T cells the TCR consists of an alpha chain and a beta chain, whereas in 5% of T cells the TCR consists of gamma and delta chains. This ratio changes during ontogeny and in diseased states. It also differs between species. Orthologues of the 4 loci have been mapped in various species. Each locus can produce a variety of polypeptides with both constant and variable regions.
When the TCR engages with antigenic peptide and MHC, the T lymphocyte is activated through signal transduction. Based on the initial receptor-triggering mechanism, the TCR is classified as belonging to the family of non-catalytic tyrosine-phosphorylated receptors.
History
In 1982, Nobel laureate James P. Allison first discovered a clonally expressed T-cell surface epitope in murine T lymphoma. In 1983, Ellis Reinherz first defined the structure of the human T-cell receptor using anti-idiotypic monoclonal antibodies to T-cell clones, complemented by studies in the mouse by Philippa Marrack and John Kappler. Then, Tak Wah Mak and Mark M. Davis identified the cDNA clones encoding the human and mouse TCR respectively in 1984. These findings allowed the entity and structure of the elusive TCR, known before as the "Holy Grail of Immunology", to be revealed. This allowed scientists from around the world to carry out studies on the TCR, leading to important studies in the fields of CAR-T, cancer immunotherapy and checkpoint inhibition.Structural characteristics
The TCR is a disulfide-linked membrane-anchored heterodimeric protein normally consisting of the highly variable alpha and beta chains expressed as part of a complex with the invariant CD3 chain molecules. T cells expressing this receptor are referred to as α:β T cells, though a minority of T cells express an alternate receptor, formed by variable gamma and delta chains, referred as γδ T cells.Each chain is composed of two extracellular domains: Variable region and a Constant region, both of Immunoglobulin superfamily domain forming antiparallel β-sheets. The Constant region is proximal to the cell membrane, followed by a transmembrane region and a short cytoplasmic tail, while the Variable region binds to the peptide/MHC complex.
The variable domain of both the TCR α-chain and β-chain each have three hypervariable or complementarity-determining regions. There is also an additional area of hypervariability on the β-chain that does not normally contact antigen and, therefore, is not considered a CDR.
The residues in these variable domains are located in two regions of the TCR, at the interface of the α- and β-chains and in the β-chain framework region that is thought to be in proximity to the CD3 signal-transduction complex. CDR3 is the main CDR responsible for recognizing processed antigen, although CDR1 of the alpha chain has also been shown to interact with the N-terminal part of the antigenic peptide, whereas CDR1 of the β-chain interacts with the C-terminal part of the peptide.
CDR2 is thought to recognize the MHC. HV4 of the β-chain is not thought to participate in antigen recognition as in classical CDRs, but has been shown to interact with superantigens.
The constant domain of the TCR consists of short connecting sequences in which a cysteine residue forms disulfide bonds, which form a link between the two chains.
The TCR is a member of the immunoglobulin superfamily, a large group of proteins involved in binding, recognition, and adhesion; the family is named after antibodies. The TCR is similar to a half-antibody consisting of a single heavy and single light chain, except the heavy chain is without its crystallizable fraction. The two main subunits of TCR are twisted together. CD3 and zeta subunits are required to carry out the signal transduction. The MHC-TCR-CD3 interaction for T cells is functionally similar to the antigen-immunoglobulin-FcR interaction for myeloid leukocytes, and Ag-Ig-CD79 interaction for B cells.
Generation of the TCR diversity
The generation of TCR diversity is similar to that for antibodies and B-cell antigen receptors. It arises mainly from genetic recombination of the DNA-encoded segments in individual somatic T cells by somatic VJ recombination using RAG1 and RAG2 recombinases. Unlike immunoglobulins, however, TCR genes do not undergo somatic hypermutation, and T cells do not express activation-induced cytidine deaminase. The recombination process that creates diversity in BCR and TCR is unique to lymphocytes during the early stages of their development in primary lymphoid organs.Each recombined TCR possess unique antigen specificity, determined by the structure of the antigen-binding site formed by the α and β chains in case of αβ T cells or γ and δ chains on case of γδ T cells.
- The TCR alpha chain is generated by VJ recombination, whereas the beta chain is generated by VDJ recombination.
- Likewise, generation of the TCR gamma chain involves VJ recombination, whereas generation of the TCR delta chain occurs by VDJ recombination.
It is the unique combination of the segments at this region, along with palindromic and random nucleotide additions, which accounts for the even greater diversity of T-cell receptor specificity for processed antigenic peptides.
Later during development, individual CDR loops of TCR can be re-edited in the periphery outside thymus by reactivation of recombinases using a process termed TCR revision and change its antigenic specificity.
The TCR complex
In the plasma membrane the TCR receptor chains α and β associate with six additional adaptor proteins to form an octameric complex. The complex contains both α and β chains, forming the ligand-binding site, and the signaling modules CD3δ, CD3γ, CD3ε and CD3ζ in the stoichiometry TCR α β - CD3εγ - CD3εδ - CD3ζζ. Charged residues in the transmembrane domain of each subunit form polar interactions allowing a correct and stable assembly of the complex. The cytoplasmic tail of the TCR is very short, hence the CD3 adaptor proteins containing the signaling motifs are needed for propagating the signal from the triggered TCR into the cell.The signaling motifs involved in TCR signaling are tyrosine residues in the cytoplasmic tail of these adaptor proteins that can be phosphorylated in the event of TCR-pMHC binding. The tyrosine residues reside in a specific amino acid sequence of the signature Yxxx6-8Yxx, where Y, L, I indicate tyrosine, leucine and isoleucine residues, x denotes any amino acids, the subscript 6-8 indicates a sequence of 6 to 8 amino acids in length. This motif is very common in activator receptors of the non-catalytic tyrosine-phosphorylated receptor family and is referred to as immunoreceptor tyrosine-based activation motif. CD3δ, CD3γ and CD3ε each contain a single ITAM, while CD3ζ contains three ITAMs. In total the TCR complex contains 10 ITAMs. Phosphorylated ITAMs act as binding site for SH2-domains of additionally recruited proteins.
Antigen discrimination
Each T cell expresses clonal TCRs which recognize a specific peptide loaded on a MHC molecule, either on MHC class II on the surface of antigen-presenting cells or MHC class I on any other cell type.A unique feature of T cells is their ability to discriminate between peptides derived from healthy, endogenous cells and peptides from foreign or abnormal cells in the body. Antigen-presenting cells do not discriminate between self and foreign peptides and typically express a large number of self-derived pMHCs on their cell surface and only a few copies of any foreign pMHC. For example, cells infected with HIV have only 8–46 HIV-specific pMHCs, compared with 100,000 total pMHCs, per cell.
Because T cells undergo positive selection in the thymus, there is a non-negligible affinity between self-pMHC and the TCR. Nevertheless, the T-cell receptor signaling should not be activated by self-pMHC so that endogenous, healthy cells are ignored by T cells.
However, when these very same cells contain even minute quantities of pathogen-derived pMHC, T cells must get activated and initiate immune responses. The ability of T cells to ignore healthy cells but respond when these same cells express a small number of foreign pMHCs is known as antigen discrimination.
To do so, T cells have a very high degree of antigen specificity, despite the fact that the affinity to the peptide/MHC ligand is rather low in comparison to other receptor types. The affinity, given as the dissociation constant, between a TCR and a pMHC was determined by surface plasmon resonance to be in the range of 1–100 μM, with an association rate of 1000 -10000 M−1 s−1 and a dissociation rate of 0.01 -0.1 s−1. In comparison, cytokines have an affinity of KD = 10–600 pM to their receptor.
It has been shown that even a single amino acid change in the presented peptide that affects the affinity of the pMHC to the TCR reduces the T-cell response and cannot be compensated by a higher pMHC concentration. A negative correlation between the dissociation rate of the pMHC-TCR complex and the strength of the T-cell response has been observed. That means, pMHC that bind the TCR for a longer time initiate a stronger activation of the T cell.
Furthermore, T cells are highly sensitive; interaction with a single pMHC is enough to trigger activation. T cells move on quickly from antigens that do not trigger responses, rapidly scanning pMHC on an antigen-presenting cell to increase the chance of finding a specific pMHC. On average, a T cell encounters 20 APCs per hour.
Different models for the molecular mechanisms that underlie this highly specific and highly sensitive process of antigen discrimination have been proposed. The occupational model simply suggests that the TCR response is proportional to the number of pMHC bound to the receptor. Given this model, a shorter lifetime of a peptide can be compensated by higher concentration such that the maximum response of the T cell stays the same. However, this cannot be seen in experiments and the model has been widely rejected.
The most accepted view is that the TCR engages in kinetic proofreading.
The kinetic proofreading model proposes that a signal is not directly produced upon binding but a series of intermediate steps ensure a time delay between binding and signal output. Such intermediate "proofreading" steps can be multiple rounds of tyrosine phosphorylation. These steps require energy and therefore do not happen spontaneously, only when the receptor is bound to its ligand. This way only ligands with high affinity that bind the TCR for a long enough time can initiate a signal. All intermediate steps are reversible, such that upon ligand dissociation the receptor reverts to its original unphosphorylated state before a new ligand binds.
This model predicts that maximum response of T cells decreases for pMHC with shorter lifetime. Experiments have confirmed this model.
However, the basic kinetic proofreading model has a trade-off between sensitivity and specificity. Increasing the number of proofreading steps increases the specificity but lowers the sensitivity of the receptor. The model is therefore not sufficient to explain the high sensitivity and specificity of TCRs that have been observed. Multiple models that extend the kinetic proofreading model have been proposed, but evidence for the models is still controversial.
The antigen sensitivity is higher in antigen-experienced T cells than in naive T cells. Naive T cells pass through the process of functional avidity maturation with no change in affinity. It is based on the fact that effector and memory T cell are less dependent on costimulatory signals and higher antigen concentration than naive T cell.