The T-Cell Receptor
It had been evident for many years that T lymphocytes have a surface receptor for antigen, with roughly similar properties to the antibody on B lymphocytes, but furious controversy raged as to whether the two molecules were in fact identical. The T-cell receptor (TCR) was finally identified unambiguously in 1983–4 by the use of monoclonal antibodies to study the fine structure of the molecule and use of DNA probing to identify the corresponding genes.
The TCR has the typical domain structure of an immunoglobulin-family molecule. Its three-dimensional structure is rather similar to that of one arm of an antibody molecule (see Fig. 14), and is made up of two major chains (α, β) each of two domains. A second (γδ) combination is found on some T cells instead of αβ. However, instead of interacting directly with intact macromolecules as does antibody, the TCR recognizes very short stretches of peptide antigen bound to an MHC molecule (as illustrated in the right-hand part of the figure for a T cell of the helper variety). The α and β chains associate on the cell membrane with other transmembrane proteins to form the CD3 complex. This complex, in association with other molecules (e.g. CD4, CD8), is responsible for transducing an activation signal into the T cell. An unusual feature of the αβ chains of the TCR, which is shared with the heavy and light chain of the antibody molecule, is that the genes for different parts of each polypeptide chain do not lie together on the chromosome, so that unwanted segments of DNA, and subsequently of RNA, have to be excised to bring them together. This process is known as gene rearrangement and occurs only in T and B cells, so that in all other cells the genes remain in their non-functional ‘germline’ configuration. Once this rearrangement has occurred in an individual lymphocyte, that cell is committed to a unique receptor, and therefore a unique antigen-recognizing ability. In this and the following figure, the portions of genes and proteins that are coloured blue are those thought to have evolved from the primitive V region, although they do not all show the same degree of variability.
TCR The T-cell receptor. It is made up of one α (MW 50 000) and one β (MW 45 000) chain, each with an outer variable domain, an inner constant domain and short intramembrane and cytoplasmic regions. Some T cells, especially early in fetal life and in some organs such as the gut and skin, express the alternative γδ receptor and seem to recognize a different set of antigens including some bacterial gly- colipids. γδ T cells are rare in humans, but are a major proportion of T cells in other animals including cows, pigs and sheep. The way in which individual T cells are first positively and then negatively selected in the thymus to ensure they only recognize self-MHC plus a foreign peptide is described in Fig. 16.
CD3 A complex of three chains, γ (MW 25 000), δ (MW 20 000) and ε (MW 20 000), essential to all T-cell function. Also associated with the TCR–CD3 complex are two other signalling molecules, ζ and η. All these molecules contain sequences known as immunoreceptor tyrosine-based activation motifs (ITAMs), which allow them to bind to phosphorylating enzymes in the cell and hence lead to T-cell activa- tion. Interaction of antigen (i.e. MHC plus peptide) with this whole complex causes many TCR complexes to cluster together on the surface of the cell, forming an ‘immunological synapse’.
CD4 A single-chain molecule (MW 60 000) found on human helper T cells. It interacts with MHC class II molecules (as shown in the figure), and is therefore recruited into the vicinity of the TCR, bringing with it a T-cell-specific kinase, lck, which binds to its cytoplasmic portion and which facilitates the process of T-cell activation. CD4 is also the major receptor which HIV uses to enter the T cell (see Fig. 28).
CD8 A molecule (MW 75 000) found on most cytotoxic T cells. In humans it is composed of two identical chains, but the equivalent in the mouse has two different chains (Ly2/3). It is involved in interacting with MHC class I molecules. Because of their close association with the TCR, CD4 and CD8 are sometimes known as ‘coreceptors’.
Costimulation Binding of the TCR to the MHC–peptide antigen is not, by itself, sufficient to activate T cells efficiently. T cells need simultaneously to receive signals via other cell-surface receptors, which bind ligands on the antigen-presenting cell. Two examples of such ‘costimulatory’ interactions are those between CD2 on the T cell and LFA-3 (CD58) on the antigen-presenting cell, and between CD28 on the T cell and CD80 (B7.1) or CD86 (B7.2) on the antigen-presenting cell. This is often called the ‘two-signal’ model of T-cell activation (although in reality there are many more than two signals involved). It has important implications for the induction of tolerance (see Fig. 22) because when T cells recognize antigen in the absence of the right costimulation they can become unresponsive to future encounters with antigen (such T cells are described as tolerant, or sometimes anergic). Other costimulatory molecules (e.g. CTLA-4 and PD1 on the T cell which interact with their respective ligands on the antigen-presenting cells) transmit negative signals that are important to prevent over- activation of T cells. Blocking these negative interactions is showing promising results as a way of improving immune responses to chronic viral infections (see Fig. 27) and cancer (see Fig. 42).
CD45 This transmembrane protein was originally known as ‘leucocyte common antigen’ because it is found on all white blood cells. However, on T cells it distinguishes ‘memory’ T cells (those that have already encountered antigen) from ‘naive’ T cells (those that have yet already encountered antigen) from ‘naive’ T cells (those that have yet to encounter antigens). The extracellular portion of CD45 exists in a number of variant forms. The shortest form (known as CD45Ro) is found on activated and memory T cells, but not on most naive T cells (see Fig. 15). In contrast, one of the longer forms (CD45RA) is found predominantly on naive T cells. The intracellular portion codes for a tyrosine phosphatase, which plays a key part in TCR regulation via regulation of the tyrosine kinase lck (see above).
Gene rearrangement The TCR genes contain up to 100 V genes and numerous J and D genes, so that to make a single chain, one of each must be linked up to the correct C gene. This is done by excision of intervening DNA sequences and further excision in the mRNA, eventually producing a single V–D–J–C RNA to code for the polypeptide chain. When all the possible combinations of α and β chains are taken into account, the number of different TCR molecules available to an individual may be as high as 1015 (see also Fig. 10).
Antigen Shown in the figure as a short peptide, in this case bound by an MHC molecule and then recognized by the TCR (for details see Fig. 18). The strength of interaction between one TCR and one MHC– peptide complex is relatively weak, but the combined effect of many simultaneous interactions on the T cell, aided by CD4–MHC or CD8– MHC interactions, results in T-cell activation. Interestingly, some antigen peptides (antagonist peptides) can have the opposite effect, in that they somehow turn off T-cell activation and make the T cells unresponsive to further stimulation. Such peptides might have possible therapeutic uses in regulating unwanted immune reactions such as allergies or autoimmunity.
T-cell activation ultimately results in the transcription of several hundred genes that determine T-cell proliferation, differentiation and effector function. A key early event is the movement of many TCR molecules on the surface of the T cell into the contact area between the T cell and the antigen-presenting cell (the immunological synapse). This increased local concentration leads to tyrosine phosphorylation of ITAMs on the cytoplasmic tails of several of the CD3 chains. This in turn recruits further tyrosine kinases (e.g. zap) and ultimately leads to activation of transcription factors, proteins that bind specific sites on DNA and hence regulate transcription of particular sets of genes. One key step in T cells is the activation of the transcription factor NF-AT, and it is this step that is inhibited by cyclosporine and FK506, important immunosuppressants used clinically to block transplant rejection (see Fig. 39).
IL-2 One of the main events that follows recognition of antigen by T cells is that the responding T cells undergo several rounds of cell division (a phenomenon known as clonal expansion). T-cell proliferation is driven largely by secretion from the T cells themselves of the cytokine interleukin 2 (IL-2). IL-2 was one of the first of the family of cytokines to be identified. As well as its major role in inducing T-cell proliferation, it has effects on B lymphocytes, macrophages, eosinophils, etc. (see Fig. 24). T-cell activation also results in secretion of many other cytokines (see Figs 21, 23 and 24).
Superantigens There is one exception to the very high specificity of T cell–peptide–MHC interactions: certain molecules, e.g. some viruses and staphylococcal enterotoxins, have the curious ability to bind to both MHC class II and the TCR β chain outside the peptide-binding site. The result is that a whole ‘family’ of T cells respond, rather than a single clone, with excessive and potentially damaging over-production of cytokines.