8536d_chog_200-220 8/2/029: 49 AM Page 205 mac79 Mac 79: 45_Bw Apldsby et al./ Immunology 5e 205 Mouse tCR a-chain and 8-chain DNA(chromosome 14) (Jan=-50) L Val lva2 l Van L val L Van Dsl D82 J81J82 C8 L V55 Jal Jo2 Ja3 Jan Mouse TCR B-chain DNA(chromosome 6) DB1—JB1.1-JB1.7 LVR14 廿H[}[Hy Mouse TCR ?chain DNA(chromosome 13 L V75 LVr2 V A L V C 5HHHH∥w∥H∥HHy V1.3 0=Enhancer FICURE9-5 Germ-line organization of the mouse TCR ae,B. y, the various gene segments differs in some cases (see Table 9-2) and 8-chain gene segments. Each C gene segment is composed of a (Adapted from D. Raulet, 1989, Annu. Rev. Immunol. 7: 175, and M series of exons and introns, which are not shown. The organization Davis, 1990, Annu. Rev. Biochem. 59: 475 1 of TCR gene segments in humans is similar, although the number of of V segments has been observed in rearranged a- and TCR Variable-Region Genes Rearrange 8-chain genes. Two Ds and two Ja gene segments and one Cs in a Manner Similar to Antibody Genes of D, I, and C segments, each repeat consisting of one DB, six V, ), and C gene segments. The B chain, like the im- ye, and one CB. The y-chain gene family consists of seven v. munoglobulin H chain, is encoded by V, D, J, and C gene seg- segments and three different functional Jr-Cy repeats. The y s resuits in V) joining for the a chain and VDj join- arrangement of the tCr a- and B-chain gene organization of the TCR multigene families in humans is segmer generally similar to that in mice, although the number of seg- ing for the p chain(Figure 9-6) ments differs(Table 9-2) After transcription of the rearranged TCR genes, RNA processing, and translation, the a and B chains are expressed as a disulfide-linked heterodimer on the membrane of the t cell. Unlike immunoglobulins, which can be membrane TABLE9-2 TCR Multigene families in humans bound or secreted, the ap heterodimer is expressed only in a membrane-bound form; thus, no differential RNA process NO. OF GENE SEG MENTS ing is required to produce membrane and secreted forms Each TCR constant region includes a connecting sequence,a D) c transmembrane sequence, and a cytoplasmic sequence. The germ-line DNA encoding the tCR a and B chain 70 constant regions is much simpler than the immunoglobulin 8 Chain 333 1 heavy-chain germ-line DNA, which has multiple C gene seg- 57 2 13 2 ments encoding distinct isotypes with different effector fur 5 2 tions. TCR a-chain DNA has only a single C gene segment the B-chain DNA has two C gene segments, but their protein The 8-chain gene segments are located between the va and la segments. products differ by only a few amino acids and have no knor TThere are two repeats, each containing 1 DB, 6 or 7 JB, and 1 CB functional differences TThere are two repeats, each containing 2 or 3 hy and 1c MECHANISM OF TCR DNA REARRANGEMENTS SOURCE: Data from P. A.H. Moss et al. 1992. Annu. Rev. Immunol. 10-71 The mechanisms by which tCR germ-line DNA is re- arranged to form functional receptor genes appear to be
of V segments has been observed in rearranged - and -chain genes. Two D and two J gene segments and one C segment have also been identified. The -chain gene family has 20–30 V gene segments and two almost identical repeats of D, J, and C segments, each repeat consisting of one D, six J, and one C. The -chain gene family consists of seven V segments and three different functional J-C repeats. The organization of the TCR multigene families in humans is generally similar to that in mice, although the number of segments differs (Table 9-2). TCR Variable-Region Genes Rearrange in a Manner Similar to Antibody Genes The chain, like the immunoglobulin L chain, is encoded by V, J, and C gene segments. The chain, like the immunoglobulin H chain, is encoded by V, D, J, and C gene segments. Rearrangement of the TCR - and -chain gene segments results in VJ joining for the chain and VDJ joining for the chain (Figure 9-6). After transcription of the rearranged TCR genes, RNA processing, and translation, the and chains are expressed as a disulfide-linked heterodimer on the membrane of the T cell. Unlike immunoglobulins, which can be membrane bound or secreted, the heterodimer is expressed only in a membrane-bound form; thus, no differential RNA processing is required to produce membrane and secreted forms. Each TCR constant region includes a connecting sequence, a transmembrane sequence, and a cytoplasmic sequence. The germ-line DNA encoding the TCR and chain constant regions is much simpler than the immunoglobulin heavy-chain germ-line DNA, which has multiple C gene segments encoding distinct isotypes with different effector functions. TCR -chain DNA has only a single C gene segment; the -chain DNA has two C gene segments, but their protein products differ by only a few amino acids and have no known functional differences. MECHANISM OF TCR DNA REARRANGEMENTS The mechanisms by which TCR germ-line DNA is rearranged to form functional receptor genes appear to be T-Cell Receptor CHAPTER 9 205 FIGURE 9-5 Germ-line organization of the mouse TCR -, -, -, and -chain gene segments. Each C gene segment is composed of a series of exons and introns, which are not shown. The organization of TCR gene segments in humans is similar, although the number of the various gene segments differs in some cases (see Table 9-2). [Adapted from D. Raulet, 1989, Annu. Rev. Immunol. 7:175, and M. Davis, 1990, Annu. Rev. Biochem. 59:475.] 5′ 3′ L Vα1 Mouse TCR α-chain and δ-chain DNA (chromosome 14) L Vα2 L L Vαn L Dδ1 Dδ2 Jδ1 J Vδ1 Vδn δ2 Cδ L Vδ5 J Jα1 α2 Jα3 Jαn Cα 5′ 3′ L Vβ1 Mouse TCR β-chain DNA (chromosome 6) L Vβ2 L Jβ1.1−J Vβn Dβ1 D β1.7 Cβ1 β2 Cβ2 LVβ14 5′ 3′ L Vγ5 Mouse TCR γ-chain DNA (chromosome 13) L Vγ2 L Vγ4 L Vγ3 Cγ1 L Cγ2 L Vγ1.1 Jγ4 Vγ1.2 Jγ1 Jγ3 Cγ3 Jγ2 L Cγ4 Vγ1.3 ψ ψ ψ ψ Jβ2.1−Jβ2.7 ψ (Vαn = ~100 ; Vδn = ~10) (Vβn = 20 − 30) ( Jαn = ~50) = Enhancer = pseudogene TABLE 9-2 TCR Multigene families in humans NO. OF GENE SEGMENTS Chromosome Gene location V D J C Chain 14 50 70 1 Chain* 14 3 3 3 1 Chain† 7 57 2 13 2 Chain‡ 7 14 5 2 * The -chain gene segments are located between the V and J segments. † There are two repeats, each containing 1 D, 6 or 7 J, and 1 C. ‡ There are two repeats, each containing 2 or 3 J and 1 C. SOURCE: Data from P. A. H. Moss et al., 1992, Annu. Rev. Immunol. 10:71. 8536d_ch09_200-220 8/2/02 9:49 AM Page 205 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e:
8536d_chog_200-220 8/2/02 9: 49 AM Page 206 mac79 Mac 79: 45_Bw Glasby et al. Immunology 5e 206 PART II Generation of B-Cell and T-Cell Respons VISUALIZING CONCEPTS L V85 JalJa2 Jall Germ-line a-chain DNA 5H 辽 Rearranged a-chain DNA Vaja cox Protein product aB heterodimer T cell CR2 LV814 Rearrangedβ- chain dna 口x L VR14 Germ-line B-chain DNA 口∥HH} FIGURE 9-6 Example of gene rearrangements that yield a func. bound TCR. The leader sequence is cleaved from the nascent tional gene encoding the ap T-cell receptor. The a-chain DNA, polypeptide chain and is not present in the finished protein. As analogous to immunoglobulin light-chain DNA, undergoes a no secreted TCR is produced, differential processing of the pri- variable-region Va la joining The B-chain DNA, analogous to im- mary transcripts does not occur. Although the B-chain DNA con- unoglobulin heavy-chain DNA, undergoes two variable-region tains two C genes, the gene products of these two C genes exhibit joinings: first DB to Js and then Vs to DBB. Transcription of the re- no known functional differences. The C genes are composed of arranged genes yields primary transcripts, which are processed to several exons and introns, which are not individually shown here give mRNAS encoding the a and B chains of the membrane- similar to the mechanisms of Ig-gene rearrangements. For immunoglobulin genes, RAG-1/2 introduces a nick on one ample, conserved heptamer and nonamer recombination DNA strand between the coding and signal sequences. The signal sequences(RSSs), containing either 12-bp(one-turn) recombinase then catalyzes a transesterification reaction or 23-bp(two-turn)spacer sequences, have been identified that results in the formation of a hairpin at the coding flanking each V, D, and j gene segment in TCR germ-line sequence and a flush 5 phosphorylated double-strand DNA(see Figure 5-6). All of the TCR-gene rearrangements break at the signal sequence. Circular excision products follow the one-turn/two-turn joining rule observed for the lg thought to be generated by looping-out and deletion dur genes, so recombination can occur only between the two dif- ing TCR-gene rearrangement have been identified in thy ferent types of RSSs. mocytes(see Figure 5-8) Like the pre-B cell, the pre-T cell expresses the recombi Studies with SCID mice, which lack functional t and B nation-activating genes(RAG-I and RAG-2). The RAG-1/2 cells, provide evidence for the similarity in the mechanisms recombinase enzyme recognizes the heptamer and non- of Ig-gene and TCR-gene rearrangements As explained in amer recognition signals and catalyzes V-J and V-D-J join- Chapter 19, SCID mice have a defect in a gene required for ing during TCR-gene rearrangement by the same deletional the repair of double-stranded DNA breaks. As a result of this or inversional mechanisms that occur in the Ig genes defect, D and j gene segments are not joined during re (see Figure 5-7). As described in Chapter 5 for the rangement of either Ig or TCR DNA (see Figure 5-10).This
similar to the mechanisms of Ig-gene rearrangements. For example, conserved heptamer and nonamer recombination signal sequences (RSSs), containing either 12-bp (one-turn) or 23-bp (two-turn) spacer sequences, have been identified flanking each V, D, and J gene segment in TCR germ-line DNA (see Figure 5-6). All of the TCR-gene rearrangements follow the one-turn/two-turn joining rule observed for the Ig genes, so recombination can occur only between the two different types of RSSs. Like the pre-B cell, the pre-T cell expresses the recombination-activating genes (RAG-1 and RAG-2). The RAG-1/2 recombinase enzyme recognizes the heptamer and nonamer recognition signals and catalyzes V-J and V-D-J joining during TCR-gene rearrangement by the same deletional or inversional mechanisms that occur in the Ig genes (see Figure 5-7). As described in Chapter 5 for the immunoglobulin genes, RAG-1/2 introduces a nick on one DNA strand between the coding and signal sequences. The recombinase then catalyzes a transesterification reaction that results in the formation of a hairpin at the coding sequence and a flush 5 phosphorylated double-strand break at the signal sequence. Circular excision products thought to be generated by looping-out and deletion during TCR-gene rearrangement have been identified in thymocytes (see Figure 5-8). Studies with SCID mice, which lack functional T and B cells, provide evidence for the similarity in the mechanisms of Ig-gene and TCR-gene rearrangements. As explained in Chapter 19, SCID mice have a defect in a gene required for the repair of double-stranded DNA breaks. As a result of this defect, D and J gene segments are not joined during rearrangement of either Ig or TCR DNA (see Figure 5-10). This 206 PART II Generation of B-Cell and T-Cell Responses VISUALIZING CONCEPTS FIGURE 9-6 Example of gene rearrangements that yield a functional gene encoding the T-cell receptor. The -chain DNA, analogous to immunoglobulin light-chain DNA, undergoes a variable-region V-J joining. The -chain DNA, analogous to immunoglobulin heavy-chain DNA, undergoes two variable-region joinings: first D to J and then V to DJ. Transcription of the rearranged genes yields primary transcripts, which are processed to give mRNAs encoding the and chains of the membranebound TCR. The leader sequence is cleaved from the nascent polypeptide chain and is not present in the finished protein. As no secreted TCR is produced, differential processing of the primary transcripts does not occur. Although the -chain DNA contains two C genes, the gene products of these two C genes exhibit no known functional differences. The C genes are composed of several exons and introns, which are not individually shown here (see Figure 9-7). Protein product αβ heterodimer 5 3′ ′ Vα1 Vαn Dδ1Dδ2 Cδ Jδ1Jδ2 Jα1Jα2 Jαn Cα Germ-line α-chain DNA 5′ 3′ Vα1 Vα2 J Vα αn T cell Jα Cα Rearranged α-chain DNA L L L Vα Jα Cα S S VβDβJβCβ L 5′ 3′ Vβ1 Vβ Rearranged β-chain DNA L L L Vβn L V Vδ1 L Vδn L δ5 DβJβ Cβ Dβ2 Cβ2 L Vβ14 Germ-line β-chain DNA 5′ 3′ L Vβ Jβ Jβ L Dβ Jβ Cβ1 Dβ2 Cβ2 VL β14 8536d_ch09_200-220 8/2/02 9:49 AM Page 206 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: