Molecular Cell Biology (Lodish, 8th ed) Part 24 PDF

The Class I MHC Pathway Presents Cytosolic nonproductive interactions, or even act as dominant nega- Antigens tive versions of a protein. Properly folded proteins may also...

The Class I MHC Pathway Presents Cytosolic nonproductive interactions, or even act as dominant nega- Antigens tive versions of a protein. Properly folded proteins may also sustain damage that leads to their unfolding, completely Figure 23-25 summarizes the six steps in the class I MHC or in part, and necessitates their removal. The rate of cy- pathway using a virus-infected cell as an example. tosolic proteolysis of these dysfunctional proteins must be 1 Acquisition of Antigen: In the case of a viral infection, matched to the rate at which mistakes in protein synthesis acquisition of antigen is usually synonymous with the in- and folding occur. These proteins are an important source fected state. Viruses rely on host biosynthetic pathways to of the peptides destined for presentation by class I MHC generate new viral proteins. Protein synthesis, unlike DNA molecules. With the exception of a specialized process called replication, is an error-prone process, in which a fraction cross-presentation (discussed below), the class I MHC path- of newly initiated polypeptide chains are terminated pre- way results in the formation of peptide-MHC complexes in maturely or suffer from other errors (misincorporation of which the peptides are derived from proteins synthesized by amino acids, frameshifts, improper or delayed folding). the class I MHC-bearing cell itself. These mistakes in protein synthesis affect the host cell’s 2 Targeting Antigen for Destruction: For the most part, own proteins and those specified by viral genomes equally. polyubiquitinylation is responsible for targeting a protein for Such error-containing proteins must be rapidly removed so destruction (see Chapter 3, page 99). Polyubiquitinylation is as not to clog up the cytoplasm, engage partner proteins in a covalent modification that is tightly regulated. Exterior 6 Plasma membrane High error rate in translation targets X Error dysfunctional proteins 1 for Ub addition Cytosol and proteolysis X Golgi complex 2 X Vesicular transport G1 Proteasome 3 G2 G5 Aminopeptidases Peptide Calreticulin epitope ATP ADP + Pi ERp57 4 X Error Peptides Peptide epitope Rough ER 5 Calnexin Ubiquitin Class I (Ub) TAP1 MHC TAP2 Tapasin Peptide-loading complex FIGURE 2325 Class I MHC pathway of antigen processing and are replaced by interferon-induced immune-specific β subunits. Step 4 : presentation. Step 1 : Acquisition of antigen is synonymous with the pro- Peptides are delivered to the interior of the endoplasmic reticulum (ER) via duction of proteins with errors (premature termination, misincorporation). the dimeric TAP peptide transporter. Step 5 : Peptide is loaded onto newly Step 2 : Dysfunctional proteins are targeted for degradation by ubiquiti- made class I MHC molecules within the peptide-loading complex. Step 6 : nylation. Step 3 : Proteolysis is carried out by the proteasome. In cells ex- The fully assembled class I MHC–peptide complex is transported to the posed to interferon γ, the catalytically active β subunits of the proteasome cell surface via the secretory pathway. See text for details. 1110 CHAPTER 23 t Immunology  3 Proteolysis: Polyubiquitinylated proteins are destroyed 6 Display of Class I MHC–Peptide Complexes at the Cell by proteolysis in proteasomes. The proteasome is a protease Surface: Once peptide loading is complete, the class I MHC– that engages its substrates and, without the release of inter- peptide complex is released from the peptide-loading complex mediates, yields peptides in the size range of 3–20 amino and enters the constitutive secretory pathway (see Figure 14-1). acids as its final digestion products (see Figure 3-31). Dur- Transfer from the Golgi to the cell surface is rapid and completes ing the course of an inflammatory response and in response the biosynthetic pathway of a class I MHC–peptide complex. to interferon γ, the three catalytically active β subunits (β1, β2, β5) of the proteasome can be replaced by three immune- The entire sequence of events in the class I pathway occurs specific subunits: β1i, β2i, and β5i. The β1i, β2i, and β5i constitutively in all nucleated cells, all of which express class I subunits are encoded in the MHC region of the genome. MHC molecules and the other required proteins, or can be The net result of this replacement is the generation of an induced to do so. As we have seen, exposure to cytokines such immunoproteasome, the output (length of peptide products) as interferon γ can induce immune-specific proteasomal sub- of which is matched to the requirements for peptide binding units to generate immunoproteasomes with enhanced ability by class I MHC molecules. The immunoproteasome adjusts to produce the appropriate peptides for presentation by class the average length of the peptides produced as well as the I MHC molecules. In the absence of a viral infection, pro- sites at which cleavage occurs. Given the central role of the tein synthesis and proteolysis continuously generate a stream proteasome in the generation of the peptides presented by of peptides that are loaded onto class I MHC molecules. class I MHC molecules, proteasome inhibitors interfere po- Healthy, normal cells therefore display on their surfaces a rep- tently with antigen processing via the class I MHC pathway. resentative selection of peptides derived from their own pro- teins. There may be several thousand distinct MHC-peptide 4 Delivery of Peptides to Class I MHC Molecules: Protein combinations displayed at the surface of a typical nucleated synthesis, polyubiquitinylation, and proteasomal proteolysis cell. The display of MHC–self-peptide complexes on the sur- all occur in the cytoplasm, whereas peptide binding by class faces of normal, uninfected cells plays an essential role in the I MHC molecules occurs in the lumen of the endoplasmic immune system. It is not until a virus makes its appearance reticulum (ER). Thus peptides must cross the ER membrane that virus-derived peptides begin to make a contribution to to gain access to class I molecules, a process mediated by the the display of peptide-MHC complexes on cell surfaces. heterodimeric TAP complex, a member of the ABC superfam- As we noted above, a properly functioning immune system ily of ATP-powered pumps (see Figure 11-15). The TAP com- must be able to distinguish self (nonpathogenic) antigens from plex binds peptides on the cytoplasmic face of the ER and, in nonself (potentially pathogenic) antigens. The small organ a cycle that includes ATP binding and hydrolysis, translocates called the thymus—located near the sternum at the level of the them into the ER. The specificity of the TAP complex is such heart in humans—plays a critical role in controlling the ability that it can transport only a subset of all cytosolic peptides, pri- of the immune system to identify self and nonself. Develop- marily those in the length range of 5–10 amino acids, that are ing T cells in the thymus, referred to as thymocytes, calibrate compatible with the circumscribed length of peptides that can their antigen-specific receptors to the sets of MHC-peptide fit into the class I MHC molecules. The mouse TAP complex complexes generated on thymic epithelial cells and learn to shows a pronounced preference for peptides that terminate recognize self-MHC products as the guideposts—or restriction in leucine, valine, isoleucine, or methionine residues, which elements, in immunological parlance—on which they must match the binding preference of class I MHC molecules. The henceforth rely for antigen recognition. At the same time, the genes encoding the TAP1 and TAP2 subunits composing the display of self peptides by self MHC molecules in the thymus TAP complex are located in the MHC region. enables the developing T cell to learn which peptide-MHC 5 Binding of Peptides to Class I MHC Molecules: Within combinations are self-derived and must therefore be ignored the ER, newly synthesized class I MHC molecules are part to avoid a self-destructive autoimmune reaction. T-cell devel- of a multiprotein complex referred to as the peptide-loading opment is thus driven by self MHC molecules loaded with self complex. This complex includes two chaperones (calnexin peptides, a “template” on which a useful repertoire of T cells and calreticulin) and the oxidoreductase Erp57. Another can be molded. Simply put, any T cell that bears a receptor chaperone (tapasin) interacts with both the TAP complex that too strongly reacts with self-MHC–self-peptide complexes and the class I MHC molecule about to receive peptide. The is potentially dangerous when it leaves the thymus and must be physical proximity of TAP and the class I MHC molecule is eliminated. This process of selection will be discussed below. maintained by tapasin. Once peptide loading onto the class An exception to the usual mode of antigen presentation I MHC molecule has occurred, a conformational change re- that is nonetheless crucial in the development of cytotoxic leases the loaded class I MHC molecule from the peptide- T cells is cross-presentation. This term refers to the acquisition loading complex. This arrangement effectively ensures that by dendritic cells of apoptotic cell remnants, complexes com- only peptide-loaded class I MHC molecules are released posed of antigen bound to antibody, and possibly other forms from the ER and then transported to and displayed at the of antigen, by phagocytosis. By a pathway that has yet to be cell surface. The overall efficiency of this pathway is such fully understood, these materials escape from phagosomal or that approximately 4000 molecules of a given protein must endosomal compartments into the cytosol, where they are then be destroyed to generate a single MHC-peptide complex car- handled according to the steps described above. Dendritic cells rying a peptide from that particular polypeptide. are the most efficient at cross-presentation, and so allow the 23.4 The MHC and Antigen Presentation 1111 loading of class I MHC molecules complexed with peptides serve to guide CD4-bearing helper T cells to the cells with that derive from cells other than the APC itself. which they interact, primarily professional APCs. Activated helper T cells provide protection not only by helping B cells to produce antibodies, but also by means of the complex sets The Class II MHC Pathway Presents Antigens of cytokines they produce, which activate phagocytic cells Delivered to the Endocytic Pathway to clear pathogens or help set up an inflammatory response. Although class I MHC and class II MHC molecules show a As noted previously, class II MHC molecules are ex- striking structural resemblance, the manner in which the two pressed primarily by professional APCs: dendritic cells and classes acquire peptide and their function in antigen recogni- macrophages, which are phagocytic, and B cells, which are tion differ greatly. Whereas the primary function of class I not. Hence the class II MHC pathway of antigen process- MHC molecules is to guide CD8-bearing cytotoxic T cells to ing and presentation generally occurs only in these cells. The their target (usually infected) cells, class II MHC molecules steps in this pathway are depicted in Figure 23-26. Macrophages Dendritic cells B cells Receptor-mediated BCR-mediated Phagocytosis Pinocytosis endocytosis endocytosis Tubular 1a 1b 1c 1d endosome 2 3 pH-dependent Proteolysis unfolding, by lysosomal reduction of peptidases S-S bonds Peptide Peptides 6 epitope Transport to cell surface + CLIP F G Class II Invariant (FGIi)3 MHC chains (Ii) DM 4a Assembly 4b Transport 5 Peptide of class II via Golgi loading in MHC in ER complex endosomes FIGURE 2326 Class II MHC pathway of antigen processing and compartments by means of signals contained in the associated invari- presentation. Step 1 : Particulate antigens are acquired by phagocyto- ant (Ii) chain. This delivery targets late endosomes, lysosomes, and sis and nonparticulate antigens by pinocytosis or endocytosis. Step 2 : early endosomes, ensuring that class II MHC molecules are exposed Exposure of antigen to the acidic and reducing environment of to the products of proteolytic breakdown of antigen along the entire endosomes and lysosomes prepares the antigen for proteolysis. Step 3 : endocytic pathway. Step 5 : Peptide loading is accomplished with The antigen is broken down by various proteases in endosomal and the assistance of DM, a class II MHC–like chaperone protein. Step 6 : lysosomal compartments. Step 4 : Class II MHC molecules, assembled in Peptide-loaded class II MHC molecules are displayed at the the ER from their subunits, are delivered to endosomal and lysosomal cell surface. See text for details. 1112 CHAPTER 23 t Immunology 1 Acquisition of Antigen: In the class II MHC path- way, antigen is acquired by pinocytosis, phagocytosis, or =Fc receptor FcLR receptor-mediated endocytosis. Pinocytosis, which is rather nonspecific, involves the delivery, by a process of membrane =Class I MHC invagination and fission, of a volume of extracellular fluid restricted peptide Opsonized and the molecules dissolved therein. Phagocytosis, the inges- =Class II MHC pathogen tion of particulate materials such as bacteria, viruses, and restricted peptide remnants of dead cells, involves extensive remodeling of =Lipid antigen the actin-based cytoskeleton to accommodate the incom- ing particle. Although phagocytosis may be initiated by specific receptor-ligand interactions, these are not always 1 IgG-decorated required: even latex particles and other particulates such as bacterium binds to FcLR glass beads can be ingested very efficiently by macrophages. Pathogens decorated by antibodies and certain complement Phagocytic cell components are targeted to macrophages and dendritic cells, (macrophage; dendritic cell) which recognize them by means of cell-surface receptors for complement components or for the Fc portion of immuno- globulins, then phagocytose them (Figure 23-27). Macro- phages and dendritic cells also express several types of less selective receptors (e.g., C-type lectins, Toll-like receptors, 2 Active FcLR scavenger receptors) that recognize molecular patterns in stimulates phagocytosis both soluble and particulate antigens; these cells then inter- nalize the bound antigens by receptor-mediated endocytosis. B cells, which are not phagocytic, can also acquire antigens by receptor-mediated endocytosis using their antigen-specific B-cell receptors (Figure 23-28). Finally, cytosolic antigens may enter the class II MHC pathway via autophagy (see Figure 14-35). 3 Intracellular destruction 2 Targeting Antigen for Destruction: Proteolysis is re- of bacterium quired to convert intact protein antigens into peptides of a Release of size suitable for binding to class II MHC molecules. Protein contents antigens are targeted for degradation by progressive unfold- ing, brought about by the drop in pH as proteins progress along the endocytic pathway. The pH of the extracellular environment is around pH 7.2, and that in early endosomes Class I Class II is between pH 6.5 and 5.5; in late endosomes and lysosomes the pH may drop to pH 4.5. ATP-powered V-class proton 4 Presentation of CD1 pumps in the endosomal and lysosomal membranes are re- bacterial antigens sponsible for this acidification (see Figure 11-9). Proteins to T cells via class I that are stable at neutral pH tend to unfold when they are cross-presentation and class II MHC exposed to extremes of pH through rupture of hydrogen bonds and destabilization of salt bridges. Furthermore, the Lipid presentation environment in the endosomal or lysosomal compartment is via CD1 a reducing one, in which lysosomes attain a concentration of reducing equivalents in the millimolar range. Reduction of FIGURE 2327 Presentation of opsonized antigen by phagocytic the disulfide bonds that stabilize many extracellular proteins cells. By means of Fc receptors such as FcγR displayed on their cell sur- can also be catalyzed by a thioreductase inducible by expo- face, specialized phagocytic cells such as macrophages or dendritic cells sure to interferon γ. The combined action of low pH and can bind and ingest pathogens that have been decorated with antibod- reducing environment prepares the antigens for proteolysis. ies (opsonization). After digestion of the phagocytosed particle (e.g., immune complex, bacterium, virus), some of the peptides produced, 3 Proteolysis: Degradation of proteins in the class II including fragments of the pathogen (orange), are loaded onto class II MHC pathway is carried out by a large set of lysosomal MHC molecules (green). Class II MHC–peptide complexes displayed at proteases, collectively referred to as cathepsins, which are the surface allow activation of T cells specific for these MHC-peptide either cysteine or aspartyl proteases. Other proteases, such combinations. Lipid antigens are delivered to the class I MHC–like as asparagine-specific endoprotease, may also contribute to molecule CD1 (pink), whose binding site is specialized to accommodate proteolysis. A wide range of peptide fragments is produced, lipids. Certain pathogen-derived peptides (purple) may be delivered to including some that can bind to class II MHC molecules. class I MHC products (blue) by means of cross-presentation. The mecha- The lysosomal proteases operate optimally at the acidic pH nisms that underlie cross-presentation remain to be clarified. 23.4 The MHC and Antigen Presentation 1113  of binding peptide because the peptide-binding cleft in the class II molecule is occupied by the invariant chain (Ii). For the same reason, newly assembled (αβIi)3 complexes do not Protein antigen compete for class I MHC–destined peptides delivered to the ER via TAP: their peptide-binding site is already occupied by Ii. Recall that the ER is where both class I and class II MHC T cell molecules assemble. The presence of Ii in the nascent class II 1 Surface Ig epitope captures B MHC complex ensures that class II MHC molecules do not antigen bind peptide in the ER. The same proteases in endosomes and lysosomes that act on internalized antigens and degrade them into peptides also act on the (αβIi)3 complex, resulting in removal of the Ii molecule from the complex with the ex- B cell ception of a small portion called the CLIP segment. Because epitope it is firmly lodged in the class II MHC peptide-binding cleft, 2 Complex CLIP is resistant to proteolytic attack. The class II MHC internalized molecules themselves are also resistant to unfolding and proteolytic attack under the conditions that prevail in the endocytic pathway. The CLIP segment is removed from the αβ heterodimer by the chaperone DM. The newly vacated 3 Complex peptide-binding cleft of the class II MHC molecule may now destroyed bind the peptides that are abundantly present in the endo- and T-cell epitope cytic pathway. Although the DM protein is MHC encoded presented by T and structurally very similar to class II molecules, it does class II MHC not itself bind peptides. However, newly formed class II MHC–peptide complexes are themselves susceptible to fur- ther “editing” by DM, which may dislodge the peptide al- 4 T cell provides help to B cell in ready bound, until the class II molecule acquires a peptide antigen-specific that binds so strongly that it cannot be removed by DM. fashion The resulting class II MHC–peptide complexes are extremely FIGURE 2328 Antigen presentation by B cells. B cells bind stable, with estimated half-lives in excess of 24 hours. antigen, even if present at low concentration, to their B-cell receptors, 6 Display of Class II MHC–Peptide Complexes at the Cell or surface Ig. The immune complex that results is internalized and Surface: The newly generated class II MHC–peptide com- then delivered to endosomal or lysosomal compartments, where it is plexes are localized mostly in late endosomal compartments, destroyed. Peptides liberated from the immune complex, including which include multivesicular endosomes (or bodies) (see fragments of the protein antigen, are displayed as class II MHC–peptide complexes at the cell surface. Helper T cells specific for the displayed Figure 14-33). Recruitment of the internal vesicles of the complex can now provide help to the B cell. This help is MHC restricted multivesicular bodies to the delimiting membrane expands and antigen specific. their surface area: by formation of tubular membranes, laid down along tracks of microtubules, these compartments elongate and ultimately deliver class II MHC–peptide com- plexes to the surface by membrane fusion. These events are within lysosomes. Consequently, agents that inhibit the ac- tightly regulated: tubulation and delivery of class II MHC tivity of the V-class proton pumps that maintain their acidi- molecules to the surface are enhanced in dendritic cells and fication interfere with antigen processing, as do inhibitors of macrophages following their activation by signals generated lysosomal proteases. in response to infection, such as bacterial lipopolysaccharide, 4 Encounter of Peptides with Class II MHC Molecules: which is detected by Toll-like receptors on the surfaces of these Recall that most class II MHC molecules synthesized in the professional APCs, as well as inflammatory cytokines, such as endoplasmic reticulum are directed to late endosomes. The interferon γ, produced by CD4-expressing helper T cells. peptides generated by proteolysis reside in the same topologi- For professional APCs, the above steps are constitutive— cal space as the class II MHC molecules themselves—they do happening all the time—but they can be modulated by ex- not have to cross a membrane, as is the case for peptides des- posure to microbial agents and cytokines. In addition to tined to bind to class I MHC molecules (see Figure 23-25). the pathways described here for class I and class II MHC To allow peptides and class II MHC molecules to meet, the products, there is a category of class I MHC–related mol- (αβIi)3 complex is transported via the secretory pathway to ecules, the CD1 proteins, that are specialized in the presen- endosomal compartments. tation of lipid antigens. The structure of a CD1 molecule 5 Binding of Peptides to Class II Molecules: The (αβIi)3 resembles that of a class I MHC molecule: a larger subunit complex delivered to endosomal compartments is incapable complexed with β2-microglobulin. Many species of bacteria 1114 CHAPTER 23 t Immunology produce lipids whose chemical structures are not found in r The process by which protein antigens are acquired, pro- their mammalian hosts. These lipids can serve as antigens cessed into peptides, and converted into surface-displayed when presented by CD1 molecules, to which they bind via MHC-peptide complexes is referred to as antigen processing a lipid-binding pocket that is conceptually similar to that and presentation. This process operates continuously in cells of most MHC molecules. Signals in the cytoplasmic tail of that express the relevant MHC molecules, yet can be modu- the larger CD1 subunit target these molecules to endosomal lated in the course of an immune response. or lysosomal compartments, where loading with antigenic lipids occurs. The CD1-lipid complexes engage a relatively r Antigen processing and presentation can be divided into rare class of T cells, referred to as NKT cells, as well as γδ six discrete steps: (1) acquisition of antigen; (2) targeting of T cells, both described below. NKT cells fulfill an important the antigen for destruction; (3) proteolysis; (4) encounter role in cytokine production and help initiate and orchestrate of peptides with MHC molecules; (5) binding of peptides adaptive immune responses via their cytokine outputs. to MHC molecules; and (6) display of the peptide-loaded MHC molecules on the cell surface (see Figure 23-27). KEY CONCEPTS OF SECTION 23.4 23.5 T Cells, T-Cell Receptors, and T-Cell The MHC and Antigen Presentation Development r The MHC, discovered as the genetic region responsible for acceptance or rejection of grafts, encodes many dif- T lymphocytes recognize antigen through specific interactions ferent proteins involved in the immune response. Two with MHC molecules. The diverse, antigen-specific T-cell re- of these proteins, class I and class II MHC molecules, are ceptors entrusted with this task are structurally and biosyn- highly polymorphic, occurring in many allelic variations (see thetically related to the F(ab) portion of immunoglobulins. Figure 23-21). To generate a large repertoire of antigen-specific T-cell recep- tors, T cells rearrange the genes encoding the T-cell receptor r The function of the class I and class II MHC proteins is subunits by mechanisms of somatic recombination essentially to bind peptide antigens and display them on the surfaces of identical to those used by B cells to rearrange immunoglobu- cells so that the antigen–MHC protein complex can interact lin genes. And the development of T cells, like that of B cells, with antigen-specific T-cell receptors on T cells. When an is strictly dependent on successful completion of these somatic antigen–MHC protein complex on an antigen-presenting cell gene rearrangements to yield a functional T-cell receptor. In binds to its complementary T-cell receptor on a T cell, the this section, we describe the receptor subunits that mediate T cell is activated to assume effector functions, such as the antigen-specific recognition, how they pair up with mem- production of cytokines or the ability to kill a virus-infected brane glycoproteins essential for signal transduction, and cell. Class I MHC molecules are found on most nucleated how these complexes recognize MHC-peptide combinations. cells, whereas the expression of class II MHC molecules is As pointed out in the preceding section, an individu- confined largely to professional APCs such as dendritic cells, al’s T cells recognize peptide antigens only when they are macrophages, and B cells. bound to the polymorphic MHC molecules present in that r The organization and structure of class I and class II MHC individual. In the course of T-cell development, T cells must molecules is similar and includes a peptide-binding cleft that “learn” the identity of these “self” MHC molecules and re- is specialized for binding a wide variety of peptides (see ceive instructions about which MHC-peptide combinations Figure 23-23). to ignore, so as to avoid potentially catastrophic reactions r Different allelic variants of MHC molecules bind differ- of newly generated T cells with the individual’s own tissues ent sets of peptides because the differences that distinguish (i.e., autoimmunity). one allele from another include residues that define the architecture of the peptide-binding cleft (see Figure 23-24). The Structure of the T-Cell Receptor Resembles Allelic variation also includes residues in the MHC mole- cule that directly contact the corresponding T-cell receptor. the F(ab) Portion of an Immunoglobulin Thus different allelic variants of an MHC molecule, even Much as B cells use the B-cell receptors on their surfaces if they bind the identical peptide, do not usually react with to recognize antigens and generate intracellular signals that the same T-cell receptor. This phenomenon is called MHC lead to clonal expansion, T cells depend on their T-cell restriction. receptors (TCRs) to initiate their participation in immune r Class I and class II MHC molecules bind to the peptides in responses. T cells that have been activated via these antigen- different intracellular compartments: class I molecules bind specific receptors proliferate and acquire the capacity to kill predominantly to cytosolic materials, whereas class II mol- antigen-bearing target cells (in the case of cytotoxic T cells) ecules bind to extracellular materials internalized by phago- or to secrete cytokines that will assist B cells in their differ- cytosis, pinocytosis, or receptor-mediated endocytosis. entiation (in the case of helper T cells). The TCR recognizes antigenic peptides bound to MHC molecules. 23.5 T Cells, T-Cell Receptors, and T-Cell Development 1115  The TCR is composed of two glycoprotein subunits (a) (Figure 23-29), each of which is encoded by a somatically T cell T cell rearranged gene. The receptor is composed of either an α and a β subunit or a γ and a δ subunit. The structure of these subunits is similar to that of the F(ab) portion of an TCR TCR immunoglobulin: at the N-terminal end is a variable region, CD4 followed by a constant region and a transmembrane seg- G F G F ment. The cytoplasmic tails of the TCR subunits are short and do not directly interact with cytoplasmic signal trans- duction molecules. Instead, the TCR associates with the CD3 complex, a set of membrane glycoproteins composed of γ, δ, ε, and ζ chains. (The TCR γ and δ subunits are not to be confused with the similarly designated subunits of the CD8 CD3 complex.) The ε chain forms a noncovalent dimer with Class I Class II the γ or the δ chain to yield δε and γε complexes. The ex- MHC MHC tracellular domains of the CD3 subunits are homologous to immunoglobulin domains, and the cytoplasmic domain APC APC in each contains an ITAM (immunoreceptor tyrosine-based activation motif), by which adapter proteins may be recruited upon phosphorylation of its tyrosine residues. The ζ chain is integrated into the CD3-TCR complex as a disulfide-bonded (b) homodimer, and each ζ chain contains three ITAMs. TCR Genes Are Rearranged in a Manner Similar to Immunoglobulin Genes Virtually all antigen-specific receptors generated by somatic recombination contain a subunit that is the product of V-D-J G recombination (e.g., Ig heavy chain; TCR β chain) and an- other that is the product of V-J recombination (e.g., Ig light TCR chain; TCR α chain). The mechanism of V-D-J and V-J recombination for TCRs is essentially identical to that de- scribed for immunoglobulin genes and requires all the com- F ponent proteins composing the nonhomologous end-joining machinery: RAG1, RAG2, Ku70, Ku80, the catalytic subunit of DNA-dependent protein kinase, XRCC4, DNA ligase IV, and Artemis. Recombination signal sequences (RSSs) are required, and recombination obeys the 12/23-bp spacer rule HA peptide (Figure 23-30). A number of noteworthy features characterize the or- ganization and rearrangement of the TCR loci. First, the organization of the RSSs is such that D-to-D rearrange- ments are allowed, unlike the case for Ig. Second, terminal FIGURE 2329 Structure of the T-cell receptor and its co-receptors. (a) The antigen-specific T-cell receptor (TCR) is composed of two chains, the α and β subunits, which are produced by V-J and Class II MHC V-D-J recombination, respectively. The α and β subunits must associ- molecule ate with the CD3 complex (see Figure 23-31) to allow the transduction of signals. The formation of a full TCRαβ–CD3 complex is required for surface expression. The T-cell receptor further associates with a co-receptor, CD8 (light blue) or CD4 (light green), which allows interac- tion with conserved features of class I MHC or class II MHC molecules, respectively, on antigen-presenting cells. (b) Structure of the T-cell receptor bound to a class II MHC–peptide complex as determined by x-ray crystallography. [Part (b) data from J. Hennecke, 2000, EMBO J. 19:5611, PDB ID 1 fyt.] 1116 CHAPTER 23 t Immunology  TCR G chain TCR F chain Germ-line DNA SS VG1 SSVGn DG1 JG1 CG1 DG2 JG 2 CG2 GEnh SS VF1 SS VFn JF(~50-100) CF GEnh 5ⴕ 3ⴕ 5ⴕ 3ⴕ Rearranged DNA Somatic recombination: D-J joining SS VG1 SS VGn DG1JG CG1 CG2 Somatic recombination: V-J joining 5ⴕ 3ⴕ V-DJ joining SS VF1JF CF VGnDG1JG CG1 CG2 SS 5ⴕ 3ⴕ 5ⴕ 3ⴕ Primary RNA transcript Transcription Transcription SS VG1DGJG CG2 SS VF1JF CF 5ⴕ 3ⴕ 5ⴕ 3ⴕ RNA processing (splicing) RNA processing (splicing) Messenger RNA (mRNA) SS VF1 J CF SS VGDGJG CG1 F AAA AAA V C V C Translation Translation Assembled TCR molecule V C V C FIGURE 2330 Organization and recombination of TCR loci. the recombination signals is such that not only is D-J joining allowed, The organization of TCR loci is in principle similar to that of immuno- but also V-D-J joining. Direct V-J joining in the TCRβ locus is not globulin loci (see Figure 23-15). Left: The TCR β-chain locus includes a observed. Right: The TCR α-chain locus is composed of a cluster of cluster of V segments, a cluster of D segments, and several J segments, V segments and a large number of J segments. SS = exon encoding downstream of which are two constant regions. The arrangement of signal sequence; Enh = enhancer. deoxynucleotidyltransferase (TdT) is active at the time the considered separate lineages with distinct functions. Among TCR genes are rearranged, and therefore N nucleotides can the T cells expressing γδ receptors are some capable of rec- be present in all rearranged TCR genes. Third, in humans ognizing the CD1 molecule, which is specialized for the pre- and mice, the TCR δ locus is embedded within the TCR α sentation of lipid antigens. The γδ T cells are programmed to locus. This organization results in complete excision of the home in on distinct anatomic sites (e.g., the epithelium lining interposed δ locus when TCRα rearrangement occurs, so a the genital tract, the skin) and probably play a role in host choice of the TCR α locus for rearrangement precludes use defense against pathogens commonly found at these sites. of the δ locus, which is lost by deletion. T cells that express Deficiencies in the key components of the recombina- the αβ receptor and those that express the γδ receptor are tion apparatus, such as the RAG recombinases, preclude 23.5 T Cells, T-Cell Receptors, and T-Cell Development 1117 rearrangement of TCR genes. As we have seen for B cells, Signaling via Antigen-Specific Receptors development of lymphocytes is strictly dependent on the re- Triggers Proliferation and Differentiation arrangement of the antigen-receptor genes. A deficiency in either RAG1 or RAG2 thus prevents both B-cell and T-cell of T and B Cells development. Mice with homozygous RAG gene knockouts The immune responses mediated by T cells and B cells are are frequently used to assess the roles of B and T cells in initiated when their antigen-specific cell-surface receptors physiological and pathophysiological processes. (TCRs or BCRs) are activated by binding to their respective ligands. The ligands for TCRs are MHC-peptide complexes Many of the Variable Residues of TCRs Are expressed on the surfaces of APCs. The ligands for BCRs are antigens that bind to the receptors without the need for Encoded in the Junctions Between V, D, and MHC intervention and do not need to be associated with a J Gene Segments presenting cell. The activation of TCRs and BCRs by their The diversity created by somatic recombination of TCR antigens is similar to the activation of the signaling receptors genes is estimated to exceed 1010 unique receptors. Combi- we have already considered (G protein–coupled receptors, natorial use of different V, D, and J gene segments makes an tyrosine kinase receptors; see Chapters 15 and 16) in that important contribution to this diversity, as do the mecha- signal transduction cascades are activated. Several integral nisms of junctional imprecision and N-nucleotide addition membrane proteins, as well as soluble cytosolic proteins, already discussed for immunoglobulin gene rearrangements. participate in TCR and BCR signaling. In some cases, these The net result is a degree of variability in the V regions that membrane-associated proteins can be thought of as auxiliary matches that of the immunoglobulins (see Figure 23-13). subunits of the receptors. Examples of how such auxiliary Indeed, each of the TCR’s variable regions includes three proteins participate in signaling are shown in Figure 23-31. hypervariable regions (CDRs), equivalent to those in the The cytosolic portions of the antigen-specific receptors BCR. Unlike immunoglobulin genes, however, the TCR themselves are very short, do not protrude much beyond the genes do not undergo somatic hypermutation. Therefore, cytosolic leaflet of the plasma membrane, and are incapable TCRs exhibit nothing equivalent to the affinity maturation of recruitment of downstream signaling molecules. Instead, of antibodies during the course of an immune response, nor as discussed previously, the antigen-specific receptors associ- is there the option of class-switch recombination or the use ate with auxiliary subunits that contain ITAMs. Engagement of alternative polyadenylation sites to create soluble and of the antigen-specific receptors by ligand initiates a series of membrane-bound versions of the receptors. receptor-proximal events: kinase activation, modification of The crystal structures of a number of TCRs bound to ITAMs, and subsequent recruitment of adapter proteins that class I MHC–peptide or class II MHC–peptide complexes serve as scaffolds for recruitment of yet other downstream have been determined. These structures show variation signaling molecules. in how the TCR docks with the MHC-peptide complex, As outlined in Figure 23-31, engagement of the antigen- but the most extensive contacts in the somatically diverse specific receptors by ligand activates Src-family tyrosine CDR3 region are made with the central peptide-containing kinases (e.g., Lck in helper T cells; Lyn and Fyn in B cells). portion of the complex, with the germ line–encoded CDR1 These kinases are found in close proximity to or physically and CDR2 contacting the α helices of the MHC molecules. associated with the antigen-specific receptors. The active Many of the TCRs for which a structure has been solved kinases phosphorylate the ITAMs in the antigen-specific dock diagonally across the peptide-binding portion of the receptors’ auxiliary subunits. In their phosphorylated MHC-peptide complex. As a result, the TCR makes exten- forms, these ITAMs recruit and activate non-Src-family sive contacts with the peptide as well as with the α helices tyrosine kinases (ZAP-70 in T cells, Syk in B cells) as well of the MHC molecule to which it binds. The positions at as other adapter proteins. This recruitment and activation which allelic MHC molecules differ from one another are involves phosphoinositide-specific phospholipase Cγ and frequently those residues that directly contact the TCR, PI-3 kinases. Subsequent downstream events parallel those thus precluding tight binding of unrelated allelic MHC described in Chapter 16 for signaling from receptor tyro- products. sine kinases. Antigen-specific receptors on B and T cells Amino acid differences that distinguish one MHC al- are perhaps best characterized as “modular” receptor tyro- lele from another also affect the architecture of the peptide- sine kinases, with the ligand recognition units and kinase binding cleft. Even if the MHC residues that interact directly domains carried by separate molecules. Ultimately, signal- with the TCR were shared by two allelic MHC molecules, ing via antigen-specific receptors initiates transcription pro- their peptide-binding specificity would probably differ be- grams that determine the fate of the activated lymphocyte: cause of amino acid differences in the peptide-binding cleft. proliferation and differentiation. Consequently, the TCR contact residues provided by bound T cells depend critically on the cytokine interleukin 2 peptide, which are essential for stable interaction with a (IL-2) for clonal expansion. Following antigen stimulation TCR, would be absent from the “wrong” MHC-peptide of a T cell, one of the first genes to be turned on is that combination. A productive interaction with the TCR would for IL-2. The T cell responds to its own initial burst of IL-2 then be unlikely to occur. and proceeds to make more IL-2, an example of autocrine 1118 CHAPTER 23 t Immunology  TCR BCR Antigen CD4 MHC-peptide IgG L J J I IgF Exterior 1 Cytosol _ _ Binding of ligand ITAM to receptor activates Src kinases (Lck, Fyn, Lyn) ITAM Fyn, Lyn Lck 2 P P P P Src kinases P phosphorylate ITAMs P P P P Fyn, Lyn Lck 3 P P P P P Phosphorylated ITAMs recruit non-Src kinases P P via SH2 domain (ZAP-70, P P SH2 Syk), which are activated domain Syk by Lck, Fyn, Lyn ZAP-70 SH2 Fyn, Lyn domain Lck SLP65 4 P LAT P P P P Activated non-Src P kinases recruit and P P P P phosphorylate P P multiple adaptor proteins (LAT, SLP65) P Signaling pathways activated (Ras, Jnk, PKC, NF-AT) FIGURE 2331 Signal transduction from the T-cell receptor downstream signaling events lead to changes in gene expression that (TCR) and B-cell receptor (BCR). The signal transduction pathways result in proliferation and differentiation of the antigen-stimulated used by the antigen-specific receptors of T cells (left) and B cells (right) lymphocytes. See text for further discussion. are conceptually similar. The initial stages are depicted in this figure; 23.5 T Cells, T-Cell Receptors, and T-Cell Development 1119 stimulation and part of a positive feedback loop. An impor- Figure 23-32 illustrates the analogous steps in the develop- tant transcription factor required for the induction of IL-2 ment of T and B cells. synthesis is the NF-AT protein (nuclear factor of activated How is the newly emerging repertoire of T cells, with T cells). This protein is sequestered in the cytoplasm in phos- their diverse pre-TCRs, further differentiated so that a pro- phorylated form and cannot enter the nucleus unless it is ductive interaction with self-MHC–peptide complexes can dephosphorylated first. The phosphatase responsible is cal- occur? The random nature of the gene rearrangement pro- cineurin, a Ca2+-activated enzyme. The initial rise in cyto- cess and the enormous variability engendered as a conse- solic Ca2+ leading to activation of calcineurin results from quence produces a large and diverse set of TCRs, the vast mobilization of ER-resident Ca2+ stores triggered by hydro- majority of which cannot interact productively with the lysis of PI(4,5)P2 and the concomitant generation of IP3 (see host MHC products, and are therefore useless. The im- Figure 15-34, steps 2 – 4 ). mune system has developed selection processes to eliminate those T cells that make TCRs incapable of a productive in- The immunosuppressant drug cyclosporine inhib- teraction with self-MHC–peptide complexes. Selection in its calcineurin activity through formation of a the thymus also removes those T cells with TCRs that can cyclosporine-cyclophilin complex, which binds and inhibits strongly interact with self-MHC–self-peptide complexes calcineurin. If dephosphorylation of NF-AT is suppressed, because such T cells have the potential to be self-reactive, NF-AT cannot enter the nucleus and induce transcription of damaging normal healthy tissue (autoimmunity). T cells the IL-2 gene. This precludes clonal expansion of antigen- whose TCRs recognize peptide-MHC complexes in the thy- stimulated T cells and so leads to immunosuppression, argu- mus with an affinity that falls between these two extremes ably the single most important intervention that contributes receive survival signals and are positively selected. Recall to the success of organ transplantation involving unrelated that antigen processing and presentation are constitutive donors and recipients (individuals who are genetically differ- processes, so that in the thymus, all self-MHC molecules ent and therefore express different MHC products), referred are necessarily occupied with peptides derived from self- to as allogeneic tissue transplantation. Although the success proteins. These combinations of self-peptides complexed to of transplantation varies with the organ used, the availability class I and class II MHC molecules constitute the substrate of strong immunosuppressants such as cyclosporine has used by the set of newly generated T-cell receptors to deter- expanded the possibilities of clinical transplantation enor- mine what is “self” and ought to be ignored. mously. The heterogeneity of peptide-MHC complexes presented to T cells undergoing selection makes it highly probable that T Cells Capable of Recognizing MHC Molecules the T-cell receptor interprets signals not only in a qualitative (strength, duration) manner, but also in additive fashion: the Develop Through a Process of Positive and summation of the binding energies of the different MHC– Negative Selection self-peptide combinations it encounters helps determine the The rearrangement of the gene segments that are assem- outcome of selection. This phenomenon is called the avidity bled to encode a functional T-cell receptor is a stochastic model of T-cell selection. event, completed on the part of the T cell without any prior T cells are killed off by apoptosis only if the appropriate knowledge of the MHC molecules with which its receptors self antigen is represented adequately in the thymus in the must ultimately interact. As in somatic recombination of Ig form of MHC-peptide complexes. How does the immune heavy-chain loci in B cells, the first gene segments to be re- system ensure that T cells generated in the thymus learn to arranged in the TCR β chain are the D and J elements; a ignore self antigens that are not normally expressed at that V segment is then joined to the newly recombined DJ (see location? Proteins that are expressed in tissue-specific fash- Figure 23-30). At this stage of T-cell development, produc- ion or after the development of the thymus, such as insulin tive rearrangement allows the synthesis of the TCR β chain, in the β cells of the pancreas or the components of the my- which is incorporated into the pre-TCR through association elin sheath in the nervous system, obviously fit this category. with the pre-T α subunit. This pre-TCR fulfills a function However, a factor called AIRE (autoimmune regulator) al- strictly analogous to that of the pre-BCR in B-cell develop- lows expression of such tissue-specific antigens in a subset of ment: it tells the T cell that it has successfully completed epithelial cells in the thymus. How AIRE accomplishes this is a productive rearrangement, with no need for further rear- not known, but it is widely suspected of directly regulating rangements in the genes on the homologous chromosome. the transcription of the relevant genes in the thymus and at The pre-TCR allows clonal expansion of the pre-T cells that select sites in secondary lymphoid organs. Defects in AIRE successfully underwent rearrangement, and it imposes al- lead to a failure to express these tissue-specific antigens in lelic exclusion to ensure that, as a rule, a single functional the thymus. In individuals who do not express AIRE, de- TCR β subunit is generated for a given T cell and its descen- veloping T cells fail to receive the full set of instructions in dants. RAG expression subsides until the expansion phase the thymus that lead to the elimination of potentially self- of the pre-T cells is complete, after which it is re-initiated to reactive T cells. As a consequence, these individuals show a allow rearrangement of the TCR α locus, ultimately lead- bewildering array of autoimmune responses, causing wide- ing to the generation of T cells with a fully assembled TCR. spread tissue damage and disease. 1120 CHAPTER 23 t Immunology  Differentiated Stem Immature Mature effector cell Pro-lymphocyte Pre-lymphocyte lymphocyte lymphocyte lymphocyte Pre-BCR BCR B lineage T lineage Pre-TCR TCR Cytokines Anatomic Bone marrow or thymus Periphery site Antigen Self antigen Foreign No dependent antigen Completion of antigen Pre-antigen Major Early maturation receptor; selection of Performance of receptor events and expansion receptor repertoire; effector functions expression differentiation FIGURE 2332 Comparison of T-cell and B-cell development. λ5 (orange, green) for the pre-BCR; pre-T α (blue) for the pre-TCR. Cell fate decisions are executed by receptors composed of either the Upon completion of the expansion phase, expression of the gene newly rearranged μ chain (pre-BCR) or the newly rearranged β chain encoding the remaining subunit of the antigen-specific receptor (pre-TCR). The pre-BCRs and pre-TCRs serve similar functions: signaling begins: Ig light chain (light blue) for the BCR; TCR α chain (light red) clonal expansion of cells that have successfully undergone rearrange- for the TCR. Lymphocyte development and differentiation occur at ment and allelic exclusion. This phase of lymphocyte development distinct anatomic sites, and only fully assembled antigen-specific does not require antigen recognition. Both the pre-BCR and pre-TCR receptors (BCR, TCR) recognize antigen. Mature lymphocytes are include subunits unique to each receptor type and absent from the strictly dependent on antigen recognition for their activation. antigen-specific receptors found on mature lymphocytes: VpreB and express ThPOK will commit to the CD4 lineage and repress T Cells Commit to the CD4 or CD8 Lineage Runx3 expression. On the other hand, if ThPOK expression in the Thymus is not induced, Runx3 expression is high, and cells commit TCR gene rearrangement coincides with the acquisition of to the CD8 lineage. In mice, a loss-of-function mutation in co-receptors. A key intermediate in T-cell development is the ThPOK gene abrogates CD4 T-cell development, and all a thymocyte that expresses both of the TCR co-receptors, thymocytes become CD8-expressing T cells. CD4 and CD8, as well as a functional TCR-CD3 complex. A third type of CD4 T cells also develop in the thymus, These cells, called double positive (CD4CD8 +) cells, are named natural (or thymically derived) regulatory T cells found only as developmental intermediates in the thymus. As (Tregs), but their function differs from that of the classic, the T cells mature, they lose either CD4 or CD8 to become conventional CD4 helper T cells, as will be described below. single-positive cells. The choice of which co-receptor (CD4 The development and function of natural Tregs requires the or CD8) to express determines whether a T cell will recog- transcription factor FoxP3, which is also regulated to some nize class I or class II MHC molecules. The question of how a extent by TCR signaling. While the avidity model of T-cell CD4CD8+ cell is instructed to become a CD8 (class I MHC- selection also applies to the development of natural Tregs, restricted) T cell or a CD4 (class II MHC-restricted) T cell is the threshold for negative selection seems to be higher for not entirely settled, but we know that the transcription fac- natural Tregs: thymocytes that recognize self antigen with tors ThPOK and Runx3 play fundamental roles. ThPOK and high affinity yet escape negative selection further commit to Runx3 are regulated by TCR signaling. Cells that transiently the natural Treg lineage. Finally, the thymus gives rise to 23.5 T Cells, T-Cell Receptors, and T-Cell Development 1121 unconventional (and less numerous) types of T cells, such TCR as invariant natural killer T cells (iNKT) that express the APC NK cell marker NK1.1 and are selected on the nonclassic 1 T MHC molecule CD1, which presents lipid antigens, as well Signal via TCR as intraepithelial lymphocytes that will colonize the mucosal (Signal 1) Signal 1 surfaces of the intestine. After the final stages of maturation, T cells of all types are exported to the peripheral lymphoid organs. MHC T Cells Require Two Types of Signals for CD80, CD86 CD28 Full Activation All T cells require a signal via their TCR for activa- 2 tion, but that signal is not sufficient: the T cell also needs CTLA4 CD28 on T cell co-stimulatory signals. To perceive these co-stimulatory interacts with signals, T cells carry on their surface several additional CD80, CD86 on Signal 2 APC (Signal 2) receptors, of which the CD28 molecule is the best-known example. CD28 interacts with CD80 and CD86, two surface glycoproteins on the professional APCs with which the T cell interacts. Expression of CD80 and CD86 increases when Induction of CTLA4 these APCs have themselves received the proper stimulatory signals, for example, by engagement of their Toll-like recep- CD28 Autocrine tors (TLRs). The signals delivered to T cells via CD28 syner- IL-2 loop gize with signals that emanate from the TCR when bound to its cognate self-MHC–peptide antigen complex, all of which 3 are required for full T-cell activation (Figure 23-33). Activation and T cells, once activated, also express receptors that pro- proliferation of T cell vide an attenuating or inhibitory signal upon recognition Activation of these very same co-stimulatory molecules, providing CTLA4 negative feedback regulation. The CTLA4 protein, whose expression in T cells is induced only upon activation, com- petes with CD28 for binding of CD80 and CD86. Because CTLA4 recruited the affinity of CTLA4 for the CD80 and C86 proteins is CTLA4 to surface higher than that of CD28, the inhibitory signals provided CD28 through CTLA4 will ultimately overwhelm the stimulatory signals coming via CD28. Co-stimulatory molecules can thus be stimulatory or—as was discovered later without adjusting 4 the nomenclature—inhibitory, and they therefore provide CTLA4 out- an important means of controlling the activation status and competes CD28, duration of a T-cell response. termination of response Termination of signal Cytotoxic T Cells Carry the CD8 Co-receptor and Are Specialized for Killing As we have seen, cytotoxic T cells (CTLs) generally express on their surfaces the TCR co-receptor glycoprotein called CD8. These CD8+ T cells kill target cells that display their FIGURE 2333 Signals involved in T-cell activation and its cognate class I MHC–peptide combinations and do so with termination. The two-signal model of T-cell activation involves recognition of an MHC-peptide complex by the T-cell receptor, which exquisite sensitivity: a single MHC-peptide complex suffices constitutes signal 1 (step 1 ), along with recognition of co-stimulatory to allow a properly activated CTL to kill the target cell that molecules (CD80, CD86) on the surface of an antigen-presenting cell, bears it. which constitutes signal 2 (step 2 ). If co-stimulation is not provided, the The mechanism of killing by CTLs involves two classes newly engaged T cell becomes unresponsive (anergic). The provision of of proteins that act synergistically: perforins and granzymes both signal 1 via the T-cell receptor and signal 2 via engagement of CD80 (Figure 23-34). Perforins, which exhibit homology to the and CD86 by CD28 allows full activation. Full activation, in turn, leads terminal components of the complement cascade compos- to increased expression of CTLA4 (step 3 ). After moving to the T-cell ing the membrane attack complex, form pores up to 20 nm surface, CTLA4 binds CD80 and CD86, leading to inhibition of the T-cell across in membranes to which they attach. The destruc- response (step 4 ). Because the affinity of CTLA4 for CD80 and CD86 is tion of an intact permeability barrier, which leads to loss of greater than that of CD28, T-cell activation is eventually terminated. 1122 CHAPTER 23 t Immunology  4b Target Caspase activation Cleft T Target cell Death 5 Class I MHC Endosome 4a 1 3 TCR Synaptic cleft 2 Cytotoxic T cell Granzyme Perforin Cytotoxic granule (lysosome-related) FIGURE 2334 Perforin- and granzyme-mediated cell killing by they adsorb, and granzymes are serine proteases that enter through cytotoxic T cells. Upon recognition of a target cell (step 1 ), a cytotoxic the perforin pores (step 3 ). Perforins are believed to act not only at the T cell forms tight antigen-specific contact with the target cell. Tight surface of the target cell, but also at the surface of its endosomal com- contact results in the formation of a synaptic cleft, into which the partments after the perforin molecules have been internalized from the contents of cytotoxic granules, including perforins and granzymes, are cell surface (step 4 ). Once in the cytoplasm, the granzymes activate released (step 2 ). Perforins form pores in the membranes onto which caspases, which initiate programmed cell death (step 5 ). electrolytes and other small solutes, contributes to cell death. of a lymphocyte and initiating a transcriptional program that Granzymes are delivered to and are presumed to enter the allows the lymphocyte to either proliferate or differentiate into target cell, probably via the pores generated by perforin. an effector cell ready to exert cytotoxic (cytotoxic T cells), Granzymes are serine proteases that activate caspases and helper (helper T cells), or antibody-secreting activity (B cells). so propel the target cell on a path of programmed cell death Cytokines that are produced by or act primarily on leukocytes (apoptosis; see Chapter 21). Perforins and granzymes are are called interleukins; at least 35 interleukins have been rec- packaged into cytotoxic granules, which are stored inside ognized and molecularly characterized. Each type of interleu- the cytotoxic T cell. Upon binding of the T-cell receptor to kin receptor has some structural similarity to the others; those its cognate class I MHC–antigen complex, signal transduc- interleukins whose structures are most closely related can be tion from the TCR leads to release of the cytotoxic granules recognized by their cognate receptors. The interleukin-2 re- and their contents into the extracellular space that is formed ceptor is particularly well characterized. Interleukin 2 (IL-2), between the cytotoxic T cell and the target cell, called the a T-cell growth factor, is one of the first cytokines produced immunological synapse. How the T cell avoids being killed when T cells are stimulated. IL-2 acts as an autocrine (self- upon release of granzymes and perforins into the synapse is acting) and paracrine (acting on neighboring cells) growth unknown. Natural killer cells also exert cytotoxic activity factor and drives clonal expansion of activated T cells. and likewise rely on perforins and granzymes to kill their Interleukin 4 (IL-4), which is produced by helper T cells, targets (see Figure 23-6). induces activated B cells to proliferate and to undergo class- switch recombination and somatic hypermutation. Interleu- kin 7 (IL-7), produced by stromal cells in the bone marrow, T Cells Produce an Array of Cytokines That is essential for development of T and B cells. Both IL-7 and Provide Signals to Other Immune-System Cells IL-15 play a role in the maintenance of memory cells, which Many lymphocytes and other cells in lymphoid tissue produce are antigen-experienced T cells that may be called upon when cytokines. These small secreted proteins control lymphocyte re-exposure to antigen occurs. These memory cells then rap- activity by binding to specific cytokine receptors on the surface idly proliferate and deal with the re-invading pathogens. 23.5 T Cells, T-Cell Receptors, and T-Cell Development 1123 The receptors for IL-2, IL-4, IL-7, and IL-15 all rely on a potentially self-reactive T cells and are important in main- common subunit for signal transduction, the common γ taining peripheral tolerance (the absence of an immune chain (γc), with α (IL-2, IL-15) and β subunits (IL-2, IL-4, response to self antigens), whereas induced Tregs are IL-7, IL-15) providing ligand specificity. Genetic defects in believed to regulate excessively strong immune responses the γc result in nearly complete failure of lymphocyte devel- against foreign antigens. TH17 cells are important in defense opment, illustrating the importance of these cytokines not against bacteria (extracellular bacteria in particular) and only during the effector phase of an immune response, but also play a pathogenic role in autoimmune diseases. also in the course of lymphocyte development, where IL-7 in particular plays a key role. The mechanism of signal transduction by cytokine recep- Leukocytes Move in Response to Chemotactic tors through the JAK/STAT pathway is described in Chapter 16 Cues Provided by Chemokines (reviewed in Figure 16-1). Among the many genes under the Interleukins tell lymphocytes what to do by eliciting a tran- control of interleukins and the STAT pathway are those that scriptional program that allows lymphocytes to acquire spe- encode suppressors of cytokine signaling, or SOCS proteins. cialized effector functions. Chemokines, on the other hand, These proteins, which are themselves induced by cytokines, tell leukocytes where to go. Many cells emit chemotactic bind to the activated form of JAKs and target them for pro- cues in the form of chemokines. When tissue damage occurs, teasomal degradation (see Figure 16-13b). resident fibroblasts produce a chemokine, IL-8, that attracts neutrophils to the site of damage. The regulation of lympho- Helper T Cells Are Divided into Distinct Subsets cyte traffic within lymph nodes is essential for dendritic cells to attract T cells, and for T cells and B cells to meet. These Based on Their Cytokine Production and trafficking steps are all controlled by chemokines. Expression of Surface Markers There are approximately 40 distinct chemokines and CD4-expressing T cells are helper T cells that provide more than a dozen chemokine receptors. One chemokine assistance to B cells and guide their differentiation into plasma may bind to more than one receptor, and a single receptor cells. This function requires both the production and secretion can bind several different chemokines. This flexibility creates of cytokines such as IL-4 as well as direct contact between the the possibility of generating a combinatorial code of chemo- helper T cell and the B cell to which it provides help. tactic cues of great complexity. This code is used to guide the A second class of helper T cell has as its major function navigation of leukocytes from where they are generated, in secreting the cytokines that contribute to the establishment the bone marrow, into the bloodstream for transport to their of an inflammatory environment. Multiple subtypes of such target destination. inflammatory T cells are categorized based on the spectrum of Some chemokines direct lymphocytes to leave the circula- different cytokines they produce and their respective roles in tion and take up residence in lymphoid organs. These migra- regulating immune responses. Whereas all activated T cells can tions contribute to the population of lymphoid organs with produce IL-2, other cytokines are produced only by particu- the required sets of lymphocytes. Because these movements lar helper T-cell subsets. These helper T cells are classified as occur as part of normal lymphoid development, such che- TH1 cells, which secrete interferon γ and tumor necrosis factor mokines are referred to as homeostatic chemokines. Those (TNF), and TH2 cells, which secrete IL-4 and IL-10. TH1 cells, chemokines that serve the purpose of recruiting leukocytes through production of interferon γ, can activate macrophages to sites of inflammation and tissue damage are referred to as and stimulate an inflammatory response. Referred to also as inflammatory chemokines. inflammatory T cells, TH1 cells nonetheless play an important Chemokine receptors are G protein–coupled receptors role in antibody production, notably facilitating the produc- that function as an essential component of the regulation tion of complement-fixing antibodies such as IgG1 and IgG3. of cell adhesion and cell migration. Leukocytes that travel TH2 cells, through production of IL-4, play an important role through blood vessels do so at high speed and are exposed to in B-cell responses that involve class switching to the IgG1 high hydrodynamic shear forces. For a leukocyte to traverse and IgE isotypes (discussed above). Recall that in B cells, the the endothelium and take up residence in a lymph node or induction of activation-induced deaminase (AID) prepares the seek out a site of infection in tissue, it must first slow down, B cell for class-switch recombination and somatic hypermuta- a process that requires interactions of glycoprotein surface tion. This induction is a consequence of the precise mixture receptors called selectins with their ligands on the surfaces of cytokines produced by helper T cells and the binding of a of leukocytes, which are mostly carbohydrate in nature. If surface membrane protein on the activated T cell, CD40, to a chemokines are adsorbed to the extracellular matrix, and protein on the B-cell surface, CD40 ligand (CD40L). if the leukocyte possesses a receptor for those chemokines, Conventional helper T cells can also differentiate into activation of its chemokine receptor elicits a signal that TH17 cells, which produce IL-17, and into induced regula- allows integrins carried by the leukocyte to undergo a con- tory T cells (induced Tregs, distinct from the natural Tregs formational change. This change results in an increase in the generated in the thymus). Both types of Treg cells attenu- affinity of the integrin for its ligand and causes firm arrest of ate immune responses by exerting a suppressive effect on the leukocyte. The leukocyte may now exit the blood vessel other types of T cells. Natural Tregs restrain the activity of by a process known as extravasation (see Figure 20-40). 1124 CHAPTER 23 t Immunology  23.6 Collaboration of Immune-System KEY CONCEPTS OF SECTION 23.5 Cells in the Adaptive Response T Cells, T-Cell Receptors, and T-Cell An effective adaptive immune response requires the pres- Development ence of B cells, T cells, and APCs. For B cells to execute r The antigen-specific T-cell receptors are dimeric proteins class-switch recombination and somatic hypermutation— consisting of α and β subunits or γ and δ subunits. T cells prerequisites for production of high-affinity antibodies— occur in at least two major classes defined by their expres- they require help from activated T cells. These T cells, in sion of the glycoprotein co-receptors CD4 and CD8 (see turn, can be activated only by professional APCs such as Figure 23-29). dendritic cells. Dendritic cells sense the presence of patho- gens through TLRs and other pattern-recognition receptors, r Cells that use class I MHC molecules as the molecular such as the C-type lectins that can recognize polysaccha- guideposts for antigen recognition (restriction elements, in rides and carbohydrate determinants. The interplay between immunological parlance) carry CD8; those that use class components of the innate and adaptive immune systems is II MHC molecules carry CD4. These classes of T cells are therefore a very important aspect of adaptive immunity. This functionally distinct: CD8 T cells are cytotoxic T cells; CD4 layered, interwoven nature of innate and adaptive immunity T cells provide help to B cells and are an important source both ensures a rapid early response of immediate protective of cytokines. value and primes the adaptive immune system for a specific r Genes encoding the TCR subunits are generated by response to any persisting pathogen. In this section, we de- somatic recombination of V and J segments (α chain) and of scribe how these various elements are activated and how the V, D, and J segments (β chain); their rearrangement obeys relevant cell types interact. the same rules as does rearrangement of Ig genes in B cells (see Figure 23-30). Rearrangement of TCR genes occurs when the lymphocytes are present in the thymus and only in Toll-Like Receptors Perceive a Variety of those cells destined to become T lymphocytes. Pathogen-Derived Macromolecular Patterns r A complete T-cell receptor includes not only the α and An important part of the innate immune system is its ability β subunits responsible for antigen and MHC recognition, to immediately detect the presence of a microbial invader but also the accessory subunits referred to as the CD3 com- and respond to it. This response includes direct elimination plex, which is required for signal transduction. Each subunit of the invader, but it also prepares the mammalian host for of the CD3 complex carries in its cytoplasmic tail one or a proper adaptive immune response, particularly through thre