T Cell Receptors and MHC Molecules

Chapter 5 T Cell Receptors and MHC Molecules




Summary




The T cell antigen receptor (TCR) is located on the surface of T cells and plays a critical role in the adaptive immune system. Its major function is to recognize antigen and transmit a signal to the interior of the T cell, which generally results in activation of T cell responses.


TCRs are similar in many ways to immunoglobulin molecules. Both are made up of pairs of subunits (α and β or γ and δ), which are themselves members of the immunoglobulin superfamily, and both recognize a wide variety of antigens via N terminal variable regions. These antigen recognition subunits are associated with the invariant chains of the T cell receptor, the CD3 complex, which perform critical signaling functions.


The two types of TCR may have distinct functions. In humans and mice, the αβ TCR predominates in most peripheral lymphoid tissues, whereas cells bearing the γδ TCR are enriched at mucosal surfaces.


Like immunoglobulins, TCRs are encoded by several sets of genes, and a large repertoire of TCR antigen-binding sites is generated by V(D)J recombination during T cell differentiation. Unlike immunoglobulins, TCRs are never secreted and do not undergo class switching or somatic hypermutation.


Antigen recognition by the αβ TCR requires the antigen to be bound to a specialized antigen-presenting structure known as a major histocompatibility complex (MHC) molecule. Unlike immunoglobulins, TCRs recognize antigen only in the context of a cell–cell interaction.


Class I and class II MHC molecules bind to peptides derived from different sources. Class I MHC molecules bind to peptides derived from cytosolic (intracellular) proteins, known as endogenous antigens. Class II MHC molecules bind to peptides derived from extracellular proteins that have been brought into the cell by phagocytosis or endocytosis (exogenous antigens).


Class I and class II MHC present peptide antigens to the TCR in a cell–cell interaction between an antigen-presenting cell (APC) and a T cell.


In humans the gene loci HLA-A, HLA-B, and HLA-C gene loci encode class I MHC molecules.


HLA-DP, HLA-DQ, and HLA-DR gene loci encode class II MHC molecules.


An individual’s MHC haplotype affects susceptibility to disease.


CD1 is an MHC class I-like molecule that presents lipid antigens.



T cell receptors


As discussed in Chapter 1, the immune system of higher vertebrates can be divided into two components – humoral immunity and cell-mediated immunity.


Humoral immunity, of which antibodies are a key component, provides protection via the extracellular fluids. Antibodies deal quite effectively with extracellular pathogens:



If antibodies were our only defense, however, pathogens could escape immune surveillance simply by hiding within cells. In fact, many pathogens – all viruses, some bacteria, and certain parasites – do just that, carrying out substantial portions of their life cycles within host cells. Remarkably, some bacteria even thrive within macrophages after being phagocytosed. These considerations highlight the need for a second arm of the immune response – cell-mediated immunity – of which T cells are critical operatives.


T cells recognize antigen via specialized cell surface antigen receptors – T cell receptors (TCRs) – which are structurally and evolutionarily related to antibodies.


TCRs recognize antigen via variable regions generated through V(D)J recombination (see Chapter 3), much like immunoglobulins, but are much more restricted in their antigen recognition capabilities.





The αβ heterodimer is the antigen recognition unit of the αβ TCR


The αβ TCR is the predominant receptor found in the thymus and peripheral lymphoid organs of mice and humans. It is a disulfide-linked heterodimer of α (40–50 kDa) and β (35–47 kDa) subunits and its structural features have been determined by X-ray crystallography (Fig. 5.2).



Each polypeptide chain of the αβ TCR contains two extracellular immunoglobulin-like domains of approximately 110 amino acids, anchored into the plasma membrane by a transmembrane domain that has a very short cytoplasmic tail.



The extracellular portions of the α and β chains fold into a structure that resembles the antigen-binding portion (Fab) of an antibody (see Fig. 3.9). Indeed, as in antibodies, the amino acid sequence variability of the TCR resides in the N terminal domains of the α and β (and also the γ and δ) chains.


The regions of greatest variability correspond to immunoglobulin hypervariable regions and are also known as complementarity determining regions (CDRs). They are clustered together to form an antigen-binding site analogous to the corresponding site on antibodies (see Fig. 5.2). Note, however, that:



The disulfide bond that links the α and β chains is in a peptide sequence located between the constant domain of the extracellular portion of the receptor and the transmembrane domain (shown as the C terminal residue in the α and β chain in Fig. 5.2).


One remarkable feature of the transmembrane portion of the receptor is the presence of positively charged residues in both the α and β chains. Unpaired charges would be unfavorable in a transmembrane segment. Indeed, these positive charges are neutralized by assembly of the complete TCR complex, which contains additional polypeptides bearing complementary negative charges (see below).



The CD3 complex associates with the antigen-binding αβ or γδ heterodimers to form the complete TCR


The αβ or γδ heterodimers must associate with a series of polypeptide chains collectively termed the CD3 complex for the antigen-binding domains of the TCR to form a complete, functional receptor that is stably expressed at the cell surface and is capable of transmitting a signal upon binding to antigen.


The four members of the CD3 complex (γ, δ, ε, and ζ) are sometimes termed the invariant chains of the TCR because they do not show variability in their amino acid sequences. (The γ and δ chains of the CD3 complex should not be confused with the quite distinct antigen-binding variable chains of the TCR that bear the same names.)


The CD3 chains are assembled as heterodimers of γε and δε subunits with a homodimer of ζ chains, giving an overall TCR stoichiometry of (αβ)2, γ, δ, ε2, ζ2. Current data suggest that the TCR complex exists as a dimer (Fig. 5.3).



The CD3 γ, δ, and ε chains are the products of three closely linked genes, and similarities in their amino acid sequences suggest that they are evolutionarily related.


Indeed, all three are members of the immunoglobulin superfamily, each containing an external domain followed by a transmembrane region and a substantial, highly conserved, cytopasmic tail of 40 or more amino acids.


As with the transmembrane domains of the variable chains of the TCR, the membrane-spanning regions of these CD3 chains contain charged amino acids.


The negatively charged residues in the transmembrane region of the CD3 chains interact with (and neutralize) the positively charged amino acids in the αβ polypeptides, leading to the formation of a stable TCR complex (see Fig. 5.3).


The CD3ζ gene is on a different chromosome from the CD3γδε gene complex, and the ζ protein is structurally unrelated to the other CD3 components. The ζ chains possess:



An alternatively spliced form of CD3ζ, called CD3η, possesses an even larger cytoplasmic tail (42 amino acids longer at the C terminus).



The cytoplasmic portions of ζ and η chains contain ITAMs


The ζ and η chains may associate in all three possible combinations (ζζ, ζη, or ηη) and play a critical role in signal transduction through the TCR. The cytoplasmic portions of these subunits contain particular amino acid sequences called immunoreceptor tyrosine-based activation motifs (ITAMs), and each chain contains three of these motifs.



The conserved tyrosine residues in the ITAM motifs are targets for phosphorylation by specific protein kinases. When the TCR is bound to its cognate antigen–MHC complex, the ITAM motifs become phosphorylated within minutes in one of the first steps in T cell activation (see Fig. 8.w1image).


ITAMs:



CD3ζ also functions in another signaling pathway, associating with the low-affinity FcγRIIIa receptor (CD16), which is involved in the activation of macrophages and natural killer (NK) cells (see Fig. 3.17).


Other subunits of the CD3 complex (γ, δ, ε), though lacking in ITAMs, may also become phosphorylated following TCR engagement. Phosphorylation of the CD3γ chain downregulates TCR expression on the cell surface via a mechanism involving increased receptor internalization.



The γδ TCR structurally resembles the αβ TCR but may function differently


The overall structure of the γδ TCR is similar to that of its αβ counterpart. Each chain is organized into:



One indication that the two types of T cell (i.e. T cells with αβ TCRs and T cells with γδ TCRs) might perform different functions comes from their anatomic distribution:



It is further believed that there are distinct subsets of γδ T cells that can perform different functions.



Antigen recognition by γδ T cells is unlike that of their αβ counterparts


The fact that γδ T cells are rare in anatomic locations known to support the classical mechanisms of antigen presentation and lymphocyte clonal expansion suggests the possibility that γδ cells might not need to rely upon normal antigen presentation mechanisms for their activation.


Several lines of evidence support the hypothesis that γδ T cells can recognize antigen in an MHC-independent fashion, for example:



γδ T cells therefore appear to be able to follow a different paradigm for T cell recognition of antigen than that employed by αβ T cells.


γδ T cells recognize at least two classes of ligand:



For example, human intraepithelial γδ T cells have been found to respond to MHC class I-related antigens (MICA and MICB) expressed on the surface of stressed cells.


In addition, some human γδ T cells recognize small organic compounds secreted by mycobacteria, such as monoethylphosphate and isopentenyl pyrophosphate. These ligands are secreted by a number of bacteria and may also be produced by some eukaryotic pathogens.


The γδ T cell arm of the adaptive immune system therefore appears to share some key characteristics of innate immune responses.




TCR variable region gene diversity is generated by V(D)J recombination


As with antibody genes, a highly diverse repertoire of TCR variable region genes is generated during T cell differentiation by a process of somatic gene rearrangement termed V(D)J recombination (see Chapter 3). Variable (V), joining (J), and sometimes diversity (D) gene segments are joined together to form a completed variable region gene.


Junctional diversity (imprecise joining of V, D, and J with loss and/or addition of nucleotides) contributes an enormous amount of variability to the TCR repertoire in addition to the variation that results from combinatorial assortment of the various gene segments.





TCRs are encoded by several sets of genes


The general arrangement of the genes encoding the α, β, γ, and δ chains of the TCR is remarkably similar to that of the immunoglobulin heavy chain genes (see Figs 3.21–3.23), suggesting a common origin from a primordial rearranging antigen receptor locus.


Figure 5.w1 illustrates the murine TCR genes, which are similar to those of humans. All four TCR gene families have been strongly conserved across more than 400 million years of evolution of the jawed vertebrates, which suggests a strong selective pressure for the preservation of both αβ and γδ T cell functions.



The nomenclature of the TCR genes is simple – the TCRA locus encodes the α gene, TCRB the β gene, and so on. Interestingly, the TCRD locus is nested within the TCRA cluster.


The α and γ loci have sets of V and J gene segments (analogous to immunoglobulin light chain loci), whereas the β and δ loci have V, D, and J gene segments (analogous to immunoglobulin heavy chains).








MHC molecules



Recognition by the αβ TCR requires antigen to be bound to an MHC molecule


As discussed in Chapter 3, antibodies recognize intact proteins, binding either to linear epitopes derived from contiguous amino acids or discontinuous epitopes produced by amino acids that are not near one another in the primary structure but are brought together in the three dimensional structure of the protein. In contrast, T cell receptors only recognize linear epitopes present in the form of short peptides which are generated by degradation of intact proteins within the cell (a process termed antigen processing). This property of T cell receptors is critical to the way the immune system recognizes intracellular pathogens and tumor antigens. Such antigens present a special challenge to the immune system: intracellular antigens are hidden within the cell, and are not available for recognition by antibodies. How can intracellular antigens be recognized by extracellular receptors? The immune system has solved this problem by evolving an elegant means of displaying internal antigens (including those of intracellular pathogens) on the cell surface, allowing their recognition by T cells:


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Jun 18, 2016 | Posted by in IMMUNOLOGY | Comments Off on T Cell Receptors and MHC Molecules

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