《免疫学》（英文版） Chapter 07 Major Histocompatibility Complex

8536ach07-161-1848/15/028:41 PM Page161mac114Mac114:2 nightshift chapter 7 Major Histocompatibility npl om e VERY MAMMALIAN SPECIES STUDIED TO DATE possesses a tightly linked cluster of genes, the ma- jor histocompatibility complex(MHC), whose products play roles in intercellular recognition and in dis- crimination between self and nonself. The MHC partici- Presentation of Vesicular Stomatitis Virus Peptide(top) and Sendai Virus Nucleoprotein Peptide by Mouse MHC ates in the development of both humoral and Class I molecule H-2K mediated immune responses. While antibodies may react with antigens alone, most T cells recognize antigen only when it is combined with an mhc molecule. furthermore a General Organization and Inheritance of the mHc because MHC molecules act as antigen-presenting struc tures, the particular set of MHC molecules expressed by an MHC Molecules and genes individual influences the repertoire of antigens to which that a Detailed Genomic Map of MHC Genes individuals TH and Tc cells can respond. For this reason, the Cellular distribution of mhc molecules MHC Partly determines the response of an individual to antigens of infectious organisms, and it has therefore been a Regulation of MHC Expression implicated in the susceptibility to disease and in the devel a MHC and Immune Responsiveness opment of autoimmunity. The recent understanding that natural killer cells express receptors for MHC class I antigens a MHC and Disease Susceptibility and the fact that the receptor-MHC interaction may lead to inhibition or activation expands the known role of this gene family(see Chapter 14). The present chapter examines the genes'; their current designation as histocompatibility-2 molecules play in producing an immune response. recognized fully, Snell was awarded the Nobel prize in 1980 for this work The MHC Encodes Three Major General Organization and Classes of molecules Inheritance of the mHc The major histocompatibility complex is a collection of The concept that the rejection of foreign tissue is the result genes arrayed within a long continuous stretch of DNA on of an immune response to cell-surface molecules, now called chromosome 6 in humans and on chromosome 17 in mice histocompatibility antigens, originated from the work of The MHC is referred to as the HLA complex in humans and Peter Gorer in the mid-1930s Gorer was using inbred strains as the H-2 complex in mice. Although the arrangement of of mice to identify blood-group antigens In the course of genes is somewhat different, in both cases the MHc genes are these studies, he identified four groups of genes, designated organized into regions encoding three classes of molecules I through IV, that encoded blood-cell antigens. Work carried (Figure 7-1 out in the 1940s and 1950s by Gorer and George Snell estab- Class I MHC genes encode glycoproteins expressed o lished that antigens encoded by the genes in the group desig- the surface of nearly all nucleated cells; the major nated II took part in the rejection of transplanted tumors function of the class I gene products is presentation of and other tissue. Snell called these genes" histocompatibility peptide antigens to Tc cells

genes”; their current designation as histocompatibility-2 (H-2) genes was in reference to Gorer’s group II blood-group antigens. Although Gorer died before his contributions were recognized fully, Snell was awarded the Nobel prize in 1980 for this work. The MHC Encodes Three Major Classes of Molecules The major histocompatibility complex is a collection of genes arrayed within a long continuous stretch of DNA on chromosome 6 in humans and on chromosome 17 in mice. The MHC is referred to as the HLA complex in humans and as the H-2 complex in mice. Although the arrangement of genes is somewhat different, in both cases the MHC genes are organized into regions encoding three classes of molecules (Figure 7-1): ■ Class I MHC genes encode glycoproteins expressed on the surface of nearly all nucleated cells; the major function of the class I gene products is presentation of peptide antigens to TC cells. chapter 7 ■ General Organization and Inheritance of the MHC ■ MHC Molecules and Genes ■ Detailed Genomic Map of MHC Genes ■ Cellular Distribution of MHC Molecules ■ Regulation of MHC Expression ■ MHC and Immune Responsiveness ■ MHC and Disease Susceptibility Major Histocompatibility Complex E      possesses a tightly linked cluster of genes, the ma￾jor histocompatibility complex (MHC), whose products play roles in intercellular recognition and in dis￾crimination between self and nonself. The MHC partici￾pates in the development of both humoral and cell￾mediated immune responses. While antibodies may react with antigens alone, most T cells recognize antigen only when it is combined with an MHC molecule. Furthermore, because MHC molecules act as antigen-presenting struc￾tures, the particular set of MHC molecules expressed by an individual influences the repertoire of antigens to which that individual’s TH and TC cells can respond. For this reason, the MHC partly determines the response of an individual to antigens of infectious organisms, and it has therefore been implicated in the susceptibility to disease and in the devel￾opment of autoimmunity. The recent understanding that natural killer cells express receptors for MHC class I antigens and the fact that the receptor–MHC interaction may lead to inhibition or activation expands the known role of this gene family (see Chapter 14). The present chapter examines the organization and inheritance of MHC genes, the structure of the MHC molecules, and the central function that these molecules play in producing an immune response. General Organization and Inheritance of the MHC The concept that the rejection of foreign tissue is the result of an immune response to cell-surface molecules, now called histocompatibility antigens, originated from the work of Peter Gorer in the mid-1930s. Gorer was using inbred strains of mice to identify blood-group antigens. In the course of these studies, he identified four groups of genes, designated I through IV, that encoded blood-cell antigens. Work carried out in the 1940s and 1950s by Gorer and George Snell estab￾lished that antigens encoded by the genes in the group desig￾nated II took part in the rejection of transplanted tumors and other tissue. Snell called these genes “histocompatibility Presentation of Vesicular Stomatitis Virus Peptide (top) and Sendai Virus Nucleoprotein Peptide by Mouse MHC Class I Molecule H-2Kb 8536d_ch07_161-184 8/15/02 8:41 PM Page 161 mac114 Mac 114:2nd shift:

8536d_cho7 161-184 8/16/02 12: 09 PM Page 162 mac100 mac 100: 1?/8_tm: 8536d: Goldsby et al./Immunology Se 162 PART II Generation of B-Cell and T-Cell Response VISUALIZING CONCEPTS Mouse H-2 complex III H-2K INF-a products B aB Cproteins TNFB/H-2DH-2L Human HLA complex HLA MHC class DP DQDR C4, C2, BF B C Gene DP DQ DR C'proteins TNF-CL BaBc阝 TNF-B HLA-BHLACHLA-A GURE7-1Simplified organization of the major histocompat- (green) gene products. The class I and class ll gene products bility complex(MHC)in the mouse and human. The MHC is re- shown in this figure are considered to be the classical MHC mol ferred to as the H-2 complex in mice and as the HLA complex in ecules. The class lll gene products include complement(C"pro- humans. In both species the MHC is organized into a number of teins and the tumor necrosis factors(TNF-a and TNF B) a Class II MHC genes encode glycoproteins expressed antigens begin to appear)and from being rejected by ma- nting cells(macrophages, ternal Tc cells dendritic cells, and B cells), where they present processed The two chains of the class li mhc molecules are en- antigenic peptides to TH cells coded by the la and ie regions in mice and by the DP, DQ and DR regions in humans. The terminology is somewhat a Class IlI MHC genes encode, in addition to other confusing, since the D region in mice encodes class I MHC products, various secreted proteins that have immune molecules, whereas the D region(DR, DQ, DP)in humans functions, including components of the complement refers to genes encoding class II MHC molecules! Fortu- system and molecules involved in inflammation nately, the designation D for the general chromosomal loca Class I MHC molecules encoded by the K and D regions in today; the sequence of the entire MHC region is available o tion encoding the human class II molecules is seldom used mice and by the A, B, and C loci in humans were the first the more imprecise reference to region is seldom necessary discovered, and they are expressed in the widest range of As with the class I loci, additional class II molecules en- cell types. These are referred to as classical class I molecules. coded within this region have specialized functions in the complexes also encode class I molecules; these gene.A immune process Additional genes or groups of genes within the H-2 or HLA The class I and class II MHC molecules have common designated nonclassical class I genes Expression of the non- structural features and both have roles in antigen processing classical gene products is limited to certain specific cell By contrast, the class Ill MHC region, which is flanked by the types. Although functions are not known for all of these class I and II regions, encodes molecules that are critical gene products, some may have highly specialized roles in immune function but have little in common with class I or II immunity. For example, the expression of the class I HLA- molecules. Class Ill products include the complement com- G molecules on cytotrophoblasts at the fetal-maternal in- ponents CA, C2, BF(see Chapter 13), and inflammatory cy terface has been implicated in protection of the fetus from tokines, including tumor necrosis factor (TNF)and being recognized as foreign(this may occur when paternal heat-shock proteins(see Chapter 12)

■ Class II MHC genes encode glycoproteins expressed primarily on antigen-presenting cells (macrophages, dendritic cells, and B cells), where they present processed antigenic peptides to TH cells. ■ Class III MHC genes encode, in addition to other products, various secreted proteins that have immune functions, including components of the complement system and molecules involved in inflammation. Class I MHC molecules encoded by the K and D regions in mice and by the A, B, and C loci in humans were the first discovered, and they are expressed in the widest range of cell types. These are referred to as classical class I molecules. Additional genes or groups of genes within the H-2 or HLA complexes also encode class I molecules; these genes are designated nonclassical class I genes. Expression of the non￾classical gene products is limited to certain specific cell types. Although functions are not known for all of these gene products, some may have highly specialized roles in immunity. For example, the expression of the class I HLA￾G molecules on cytotrophoblasts at the fetal-maternal in￾terface has been implicated in protection of the fetus from being recognized as foreign (this may occur when paternal antigens begin to appear) and from being rejected by ma￾ternal TC cells. The two chains of the class II MHC molecules are en￾coded by the IA and IE regions in mice and by the DP, DQ, and DR regions in humans. The terminology is somewhat confusing, since the D region in mice encodes class I MHC molecules, whereas the D region (DR, DQ, DP) in humans refers to genes encoding class II MHC molecules! Fortu￾nately, the designation D for the general chromosomal loca￾tion encoding the human class II molecules is seldom used today; the sequence of the entire MHC region is available so the more imprecise reference to region is seldom necessary. As with the class I loci, additional class II molecules en￾coded within this region have specialized functions in the immune process. The class I and class II MHC molecules have common structural features and both have roles in antigen processing. By contrast, the class III MHC region, which is flanked by the class I and II regions, encodes molecules that are critical to immune function but have little in common with class I or II molecules. Class III products include the complement com￾ponents C4, C2, BF (see Chapter 13), and inflammatory cy￾tokines, including tumor necrosis factor (TNF) and heat-shock proteins (see Chapter 12). 162 PART II Generation of B-Cell and T-Cell Responses VISUALIZING CONCEPTS FIGURE 7-1 Simplified organization of the major histocompat￾ibility complex (MHC) in the mouse and human. The MHC is re￾ferred to as the H-2 complex in mice and as the HLA complex in humans. In both species the MHC is organized into a number of regions encoding class I (pink), class II (blue), and class III (green) gene products. The class I and class II gene products shown in this figure are considered to be the classical MHC mol￾ecules. The class III gene products include complement (C) pro￾teins and the tumor necrosis factors (TNF- and TNF-). II III Complex MHC class Region Gene products IA αβ H–2K H–2L C′ proteins H–2D IE αβ TNF-α TNF-β TNF-α TNF-β H–2 I I K IA IE S D III Complex MHC class Region Gene products DQ αβ C′ proteins HLA-B HLA-C HLA-A DR αβ HLA II I DP DQ DR C4, C2, BF B C A Human HLA complex Mouse H-2 complex DP αβ 8536d_ch07_161-184 8/16/02 12:09 PM Page 162 mac100 mac 100: 1268_tm:8536d:Goldsby et al. / Immunology 5e-:

8536d_ cho7 161-184 8/16/02 8: 28 AM Page 163 mac100 mac 100: 129Atm: 8536d: Goldsby et al. Immunology 5e Major Histocompatibility Complex CHAPTER 7 Allelic Forms of mhc genes are Inherited presses both parental alleles at each MHC locus. For exam in Linked Groups Called Haplotypes ple, if an H-2 strain is crossed with an H-2 then the F, in herits both parental sets of alleles and is said to be H-2 As described in more detail later, the loci constituting the ( Figure 7-2a) Because such an Fi expresses the MHC pro MHC are highly polymorphic; that is, many alternative teins of both parental strains on its cells, it is histocompatible forms of the gene, or alleles, exist at each locus among the with both strains and able to accept grafts from either population. The genes of the MHC loci lie close together; for parental strain(see example in Figure 7-2b)However,nei- example, the recombination frequency within the H-2 com- ther of the inbred parental strains can accept a graft from the plex (i.e, the frequency of chromosome crossover events Fi mice because half of the MHC molecules will be foreign to during mitosis, indicative of the distance between given gene the parent. segments)is only 0.5%-crossover occurs only once in every The inheritance of HLA haplotypes from heterozygous 200 mitotic cycles. For this reason, most individuals inherit human parents is illustrated in Figure 7-2c In an outbred the alleles encoded by these closely linked loci as two sets, one population, each individual is generally heterozygous at each from each parent. Each set of alleles is referred to as a haplo- locus. The human hLA complex is highly polymorphic and type. An individual inherits one haplotype from the mother multiple alleles of each class I and class ll gene exist. How- and one haplotype from the father In outbred populations, ever, as with mice, the human MHC loci are closely linked the offspring are generally heterozygous at many loci and will and usually inherited as a haplotype. When the father and express both maternal and paternal MHC alleles. The alleles mother have different haplotypes, as in the example shown are codominantly expressed; that is, both maternal and pater- (Figure 7-2c)there is a one-in-four chance that siblings will nal gene products are expressed in the same cells. If mice are inherit the same paternal and maternal haplotypes and inbred(that is, have identical alleles at all loci), each H-2 lo- therefore be histocompatible with each other; none of the cus will be homozygous because the maternal and paternal offspring will be histocompatible with the parents haplotypes are identical, and all offspring therefore express Although the rate of recombination by crossover is low identical haplotypes. within the HLA, it still contributes significantly to the diver Certain inbred mouse strains have been designated as sity of the loci in human populations.Genetic recombina prototype strains, and the MHC haplotype expressed by tion generates new allelic combinations(Figure 7-2d),and these strains is designated by an arbitrary italic superscript the high number of intervening generations since the ap- (e.g,H-2,H-2). These designations refer to the entire set of pearance of humans as a species has allowed extensive re- inherited H-2 alleles within a strain without having to list combination, so that it is rare for any two unrelated each allele individually(Table 7-1). Different inbred strains individuals to have identical sets of HLa genes may have the same set of alleles, that is the same MHC hap lotype, as the prototype strain. For example, the CBA, AKR, MHC Congenic Mouse Strains Are ldentical The three strains differ, however, in genes outside the H-2 at All Loci Except the MHC Detailed analysis of the H-2 complex in mice was made If two mice from inbred strains having different MHC possible by the development of congenic mouse strains. In haplotypes are bred to one another, the Fi generation inher- bred mouse strains are syngeneic or identical at all genetic its haplotypes from both parental strains and therefore ex- loci. Two strains are congenic if they are genetically identical TABLE 7 2 Haplotypes of some mouse strains H-2 ALLELE Prototype strain Other strains with the same haplotype Haplotype K CBA AKR C3H. B10.BR. C57 k k DBA/2 BALB/C, NZB, SEA, YBR kdbkss Ekdbkskq d C57BL/10(B10) C57BL/6, C57L, C3H SW, LP, 129 b b B10.s S儿L t1 DBA/ STOLI, B10.Q

Allelic Forms of MHC Genes Are Inherited in Linked Groups Called Haplotypes As described in more detail later, the loci constituting the MHC are highly polymorphic; that is, many alternative forms of the gene, or alleles, exist at each locus among the population. The genes of the MHC loci lie close together; for example, the recombination frequency within the H-2 com￾plex (i.e., the frequency of chromosome crossover events during mitosis, indicative of the distance between given gene segments) is only 0.5%—crossover occurs only once in every 200 mitotic cycles. For this reason, most individuals inherit the alleles encoded by these closely linked loci as two sets, one from each parent. Each set of alleles is referred to as a haplo￾type. An individual inherits one haplotype from the mother and one haplotype from the father. In outbred populations, the offspring are generally heterozygous at many loci and will express both maternal and paternal MHC alleles. The alleles are codominantly expressed; that is, both maternal and pater￾nal gene products are expressed in the same cells. If mice are inbred (that is, have identical alleles at all loci), each H-2 lo￾cus will be homozygous because the maternal and paternal haplotypes are identical, and all offspring therefore express identical haplotypes. Certain inbred mouse strains have been designated as prototype strains, and the MHC haplotype expressed by these strains is designated by an arbitrary italic superscript (e.g., H-2a , H-2b ). These designations refer to the entire set of inherited H-2 alleles within a strain without having to list each allele individually (Table 7-1). Different inbred strains may have the same set of alleles, that is the same MHC hap￾lotype, as the prototype strain. For example, the CBA, AKR, and C3H strains all have the same MHC haplotype (H-2k ). The three strains differ, however, in genes outside the H-2 complex. If two mice from inbred strains having different MHC haplotypes are bred to one another, the F1 generation inher￾its haplotypes from both parental strains and therefore ex￾presses both parental alleles at each MHC locus. For exam￾ple, if an H-2b strain is crossed with an H-2k , then the F1 in￾herits both parental sets of alleles and is said to be H-2b/k (Figure 7-2a). Because such an F1 expresses the MHC pro￾teins of both parental strains on its cells, it is histocompatible with both strains and able to accept grafts from either parental strain (see example in Figure 7-2b). However, nei￾ther of the inbred parental strains can accept a graft from the F1 mice because half of the MHC molecules will be foreign to the parent. The inheritance of HLA haplotypes from heterozygous human parents is illustrated in Figure 7-2c. In an outbred population, each individual is generally heterozygous at each locus. The human HLA complex is highly polymorphic and multiple alleles of each class I and class II gene exist. How￾ever, as with mice, the human MHC loci are closely linked and usually inherited as a haplotype. When the father and mother have different haplotypes, as in the example shown (Figure 7-2c) there is a one-in-four chance that siblings will inherit the same paternal and maternal haplotypes and therefore be histocompatible with each other; none of the offspring will be histocompatible with the parents. Although the rate of recombination by crossover is low within the HLA, it still contributes significantly to the diver￾sity of the loci in human populations. Genetic recombina￾tion generates new allelic combinations (Figure 7-2d), and the high number of intervening generations since the ap￾pearance of humans as a species has allowed extensive re￾combination, so that it is rare for any two unrelated individuals to have identical sets of HLA genes. MHC Congenic Mouse Strains Are Identical at All Loci Except the MHC Detailed analysis of the H-2 complex in mice was made possible by the development of congenic mouse strains. In￾bred mouse strains are syngeneic or identical at all genetic loci. Two strains are congenic if they are genetically identical Major Histocompatibility Complex CHAPTER 7 163 TABLE 7-1 H-2 Haplotypes of some mouse strains H-2 ALLELES Prototype strain Other strains with the same haplotype Haplotype K IA IE S D CBA AKR, C3H, B10.BR, C57BR k k k k kk DBA/2 BALB/c, NZB, SEA, YBR d d d d dd C57BL/10 (B10) C57BL/6, C57L, C3H.SW, LP, 129 b b b b bb A A/He, A/Sn, A/Wy, B10.A a k k k dd A.SW B10.S, SJL s s s s ss A.TL t1 s k k kd DBA/1 STOLI, B10.Q, BDP q q q q qq 8536d_ch07_161-184 8/16/02 8:28 AM Page 163 mac100 mac 100: 1268_tm:8536d:Goldsby et al. / Immunology 5e-:

8536d_cho7_161-184 8/16/02 12: 09 PM Page 164 mac100 mac 100: 1/8_tm: 8536d: Goldsby et al./ Immunology 5e (a) Mating of inbred mouse strains with different MHC haplotypes Homologous chromosomes with MHC loci The letters b/b designate a mouse homozy H-2b parent H-2 parent gous for the H-2b MHC haplotype, k/k ho- mozygous for the H-2 haplotype, and b/ka heterozygote. Because the MHC loci are dosely linked and inherited as a set, the MHC haplotype of F1 progeny from the mat ing of two different inbred strains can be pre dicted easily.(b) Acceptance or rejection of FI progeny (H-2b/) skin grafts is controlled by the MHC type of b/k the inbred mice. The progeny of the cross be tween two inbred strains with different mhc haplotypes(H-2 and H-25) will express both (b) Skin transplantation between inbred mouse strains with same or different MHC haplotypes haplotypes(H-2b/)and will accept grafts from either parent and from one anoth Parental recipient Skin graft donor Progeny recipient Neither parent strain will accept grafts from the offspring.(c) Inheritance of HLA haplo- types in a hypothetical human family. In hu- mans, the paternal HLA haplotypes are arbitrarily designated A and B, maternal C and D. Because humans are an outbred b/k species and there are alleles at each HLA locus, the alleles comprising the haplo- types must be determined by typing parents and progeny.(d)The genes that make up aplotype in the hypothec family in(c)are shown along with a new hap- type that arose from recombination(R)of →口 b/k Paren Progeny (c) Inheritance of HLA haplotypes in a typical human family (d) A new haplotype (R) arises from recombination Parents o HLA Alleles B C DR DQ DP C/D A17 Haplotypes c3 44 w4 4 13 /C A/D B/R B/C B/D

(a) Mating of inbred mouse strains with different MHC haplotypes b/b b/b b/b b/b b/k b/k k/k k/k k/k b/k F1 progeny (H-2b/k) H-2 H-2k parent b parent Homologous chromosomes with MHC loci (b) Skin transplantation between inbred mouse strains with same or different MHC haplotypes Parental recipient Skin graft donor Parent Progeny recipient b/b b/k b/k k/k Progeny b/b k/k b/k k/k Parent (c) Inheritance of HLA haplotypes in a typical human family Parents Progeny A/B C/D A/C A/D B/C B/D B/R (d) A new haplotype (R) arises from recombination of maternal haplotypes 1 7 w3 2 1 1 ABC HLA Alleles DR DQ DP 2 8 w2 3 2 2 3 44 w4 4 1 3 11 35 w1 7 3 4 3 A Haplotypes B C D 5e R 44 w4 734 2 FIGURE 7-2 (a) Illustration of inheritance of MHC haplotypes in inbred mouse strains. The letters b/b designate a mouse homozy￾gous for the H-2b MHC haplotype, k/k ho￾mozygous for the H-2k haplotype, and b/k a heterozygote. Because the MHC loci are closely linked and inherited as a set, the MHC haplotype of F1 progeny from the mat￾ing of two different inbred strains can be pre￾dicted easily. (b) Acceptance or rejection of skin grafts is controlled by the MHC type of the inbred mice. The progeny of the cross be￾tween two inbred strains with different MHC haplotypes (H-2b and H-2k ) will express both haplotypes (H-2b/k) and will accept grafts from either parent and from one another. Neither parent strain will accept grafts from the offspring. (c) Inheritance of HLA haplo￾types in a hypothetical human family. In hu￾mans, the paternal HLA haplotypes are arbitrarily designated A and B, maternal C and D. Because humans are an outbred species and there are many alleles at each HLA locus, the alleles comprising the haplo￾types must be determined by typing parents and progeny. (d) The genes that make up each parental haplotype in the hypothetical family in (c) are shown along with a new hap￾lotype that arose from recombination (R) of maternal haplotypes. 8536d_ch07_161-184 8/16/02 12:09 PM Page 164 mac100 mac 100: 1268_tm:8536d:Goldsby et al. / Immunology 5e-:

8536d_cho7 161-184 8/16/02 12: 09 PM Page 165 mac100 mac 100: 1?/8_tm: 8536d: Goldsby et al./Immunology Se Major Histocompatibility Complex CHAPTER 7 URE 7-3 Production of congenic mouse Cross AB, which has the genetic background of arental strain a but the H-2 complex of strain B Crossing inbred strain A(H-2)with strain B(H-2) generates Fi progeny that are heterozygous(a/b) at all H-2 loci. The Fi progeny are interbred to pro interbreeding duce an F2 generation, which includes a/a, a/b, and b/b individuals. The F2 progeny homozygous r the B-strain H-2 complex are selected by their ability to reject a skin graft from strain A; any prog from future breeding. The selected b/b homozy Strain-A skin grafts gous mice are then backcrossed to strain A; the re- nd selection for ability reject an A-strain graft is repeated for at least 12 backcross restored at all loci except the H-2 locus, which for the b strain Interbreed. select and Strain a. B except at a single genetic locus or region. Any pheno- typic differences that can be detected between congenic strains are related to the genetic region that distinguishes ABA邱 the strains. Congenic strains that are identical with each Parental other except at the mHc can be produced by a series of crosses, backcrosses, and selections. Figure 7-3 outlines the Congenic B10.A steps by which the H-2 complex of homozygous strain B B10.A(2R) b2 an be introduced into the background genes of homozy- gous strain a to generate a congenic strain, denoted A B B10.A(4R)b4 The first letter in a congenic strain designation refers to the B10.A(18R)i8 strain providing the genetic background and the second letter to the strain providing the genetically different MHC FIGURE 7.4 Examples of recombinant congenic mouse strains region. Thus, strain A.B will be genetically identical to generated during production of the B10. A strain from parental strain 10(H-2)and parental strain A(H-2).Crossover events within the strain A except for the MHC locus or loci contributed by H-2 complex produce recombinant strains, which have a-haplotype strain B During production of congenic mouse strains, a crossover alleles(blue)at some H-2 loci and b-haplotype alleles (orange)at event sometimes occurs within the H-2 complex, yielding a other loci recombinant strain that differs from the parental strains or the congenic strain at one or a few loci within the H-2 duction of a B10. A congenic strain. Such recombinant complex Figure 7-4 depicts haplotypes present in several re- strains have been extremely useful in analyzing the MHC be- binant congenic strains that were obtained during pro- cause they permit comparisons of functional differences

except at a single genetic locus or region. Any pheno￾typic differences that can be detected between congenic strains are related to the genetic region that distinguishes the strains. Congenic strains that are identical with each other except at the MHC can be produced by a series of crosses, backcrosses, and selections. Figure 7-3 outlines the steps by which the H-2 complex of homozygous strain B can be introduced into the background genes of homozy￾gous strain A to generate a congenic strain, denoted A.B. The first letter in a congenic strain designation refers to the strain providing the genetic background and the second letter to the strain providing the genetically different MHC region. Thus, strain A.B will be genetically identical to strain A except for the MHC locus or loci contributed by strain B. During production of congenic mouse strains, a crossover event sometimes occurs within the H-2 complex, yielding a recombinant strain that differs from the parental strains or the congenic strain at one or a few loci within the H-2 complex. Figure 7-4 depicts haplotypes present in several re￾combinant congenic strains that were obtained during pro￾duction of a B10.A congenic strain. Such recombinant strains have been extremely useful in analyzing the MHC be￾cause they permit comparisons of functional differences Major Histocompatibility Complex CHAPTER 7 165 F2 a/a b/b × a/b a /b × Strain-A skin grafts Cross Interbreeding Select for b/b at H-2 complex F1 a/a a/b a/b b/b Strain A × a/b a/b × Backcross Interbreed, select, and backcross for ≤ 10 cycles ≤ Strain A•B a/a FIGURE 7-3 Production of congenic mouse strain A.B, which has the genetic background of parental strain A but the H-2 complex of strain B. Crossing inbred strain A (H-2a ) with strain B (H-2b ) generates F1 progeny that are heterozygous (a/b) at all H-2 loci. The F1 progeny are interbred to pro￾duce an F2 generation, which includes a/a, a/b, and b/b individuals. The F2 progeny homozygous for the B-strain H-2 complex are selected by their ability to reject a skin graft from strain A; any prog￾eny that accept an A-strain graft are eliminated from future breeding. The selected b/b homozy￾gous mice are then backcrossed to strain A; the re￾sulting progeny are again interbred and their offspring are again selected for b/b homozygosity at the H-2 complex. This process of backcrossing to strain A, intercrossing, and selection for ability to reject an A-strain graft is repeated for at least 12 generations. In this way A-strain homozygosity is restored at all loci except the H-2 locus, which is homozygous for the B strain. Strain Parental Congenic Recombinant congenic A B10 B10.A B10.A (3R) B10.A (2R) B10.A (4R) B10.A (18R) H-2 haplotype a b a i3 h2 h4 i18 KA A E E S D H-2 loci β β α α FIGURE 7-4 Examples of recombinant congenic mouse strains generated during production of the B10.A strain from parental strain B10 (H-2b ) and parental strain A (H-2a ). Crossover events within the H-2 complex produce recombinant strains, which have a-haplotype alleles (blue) at some H-2 loci and b-haplotype alleles (orange) at other loci. 8536d_ch07_161-184 8/16/02 12:09 PM Page 165 mac100 mac 100: 1268_tm:8536d:Goldsby et al. / Immunology 5e-:

8536d_cho7 161-184 8/16/02 12: 09 PM Page 166 mac100 mac 100: 1?/8_tm: 8536d: Goldsby et al./Immunology Se 166 PART I1 Generation of B-Cell and T-Cell Response between strains that differ in only a few genes within the Class I Molecules Have a Glycoprotein Heavy MHC. Furthermore, the generation of new H-2 haplotypes Chain and a Small Protein Light Chain under the experimental conditions of congenic strain devel opment provides an excellent illustration of the means by Class I MHC molecules contain a 45-kilodalton(kDa)a which the MHC continues to maintain heterogeneity even in chain associated noncovalently with a 12-kDa B2-microglob populations with limited diversity. brane glycoprotein encoded by polymorphic genes within the A,B, and C regions of the human HLA complex and within the K and D/L regions of the mouse H-2 complex(see Figure MHC Molecules and Genes 7-1). B2-Microglobulin is a protein encoded by a highly con- served gene located on a different chromosome. Association Class I and class II MHC molecules are membrane-bound of the a chain with B2-microglobulin is required for expres- glycoproteins that are closely related in both structure and sion of class I molecules on cell membranes. The a chain is function. Both class I and class II MHC molecules have been anchored in the plasma membrane by its hydrophobic trans isolated and purified and the three-dimensional structures membrane segment and hydrophilic cytoplasmic tail. of their extracellular domains have been determined by x Structural analyses have revealed that the a chain of class I ay crystallography. Both types of membrane glycoproteins MHC molecules is organized into three external domains function as highly specialized antigen-presenting molecules (al, a2, and a3), each containing approximately 90 amino that form unusually stable complexes with antigenic pep- acids; a transmembrane domain of about 25 hydrophobic tides, displaying them on the cell surface for recognition by amino acids followed by a short stretch of charged(hy T cells. In contrast, class III MHC molecules are a group of drophilic)amino acids; and a cytoplasmic anchor segment of unrelated proteins that do not share structural similarity 30 amino acids. The B2-microglobulin is similar in size and and common function with class I and II molecules. The organization to the a3 domain; it does not contain a trans- class lll molecules will be examined in more detail in later membrane region and is noncovalently bound to the class I chapters glycoprotein Sequence data reveal homology between the a3 Class i molecule Class ll molecule membrane. distal Membrane-proximal a3 β2 microglobulin a2 β2 domains (g-fold structure) Transmembrane segment Cytoplasmic tail FIGURE7-5Schematic diagrams of a class I and a class ll MHc membrane-proximal domains possess the basic immunoglobulin- molecule showing the external domains, transmembrane segment, fold structure; thus, class l and class ll MHC molecules are classified and cytoplasmic tail. The peptide-binding cleft is formed by the mem. as members of the immunoglobulin superfamily. brane- distal domains in both dass i and class ll molecules. The

between strains that differ in only a few genes within the MHC. Furthermore, the generation of new H-2 haplotypes under the experimental conditions of congenic strain devel￾opment provides an excellent illustration of the means by which the MHC continues to maintain heterogeneity even in populations with limited diversity. MHC Molecules and Genes Class I and class II MHC molecules are membrane-bound glycoproteins that are closely related in both structure and function. Both class I and class II MHC molecules have been isolated and purified and the three-dimensional structures of their extracellular domains have been determined by x￾ray crystallography. Both types of membrane glycoproteins function as highly specialized antigen-presenting molecules that form unusually stable complexes with antigenic pep￾tides, displaying them on the cell surface for recognition by T cells. In contrast, class III MHC molecules are a group of unrelated proteins that do not share structural similarity and common function with class I and II molecules. The class III molecules will be examined in more detail in later chapters. Class I Molecules Have a Glycoprotein Heavy Chain and a Small Protein Light Chain Class I MHC molecules contain a 45-kilodalton (kDa) chain associated noncovalently with a 12-kDa 2-microglob￾ulin molecule (see Figure 7-5). The chain is a transmem￾brane glycoprotein encoded by polymorphic genes within the A, B, and C regions of the human HLA complex and within the K and D/L regions of the mouse H-2 complex (see Figure 7-1). 2-Microglobulin is a protein encoded by a highly con￾served gene located on a different chromosome. Association of the chain with 2-microglobulin is required for expres￾sion of class I molecules on cell membranes. The chain is anchored in the plasma membrane by its hydrophobic trans￾membrane segment and hydrophilic cytoplasmic tail. Structural analyses have revealed that the chain of class I MHC molecules is organized into three external domains (1, 2, and 3), each containing approximately 90 amino acids; a transmembrane domain of about 25 hydrophobic amino acids followed by a short stretch of charged (hy￾drophilic) amino acids; and a cytoplasmic anchor segment of 30 amino acids. The 2-microglobulin is similar in size and organization to the 3 domain; it does not contain a trans￾membrane region and is noncovalently bound to the class I glycoprotein. Sequence data reveal homology between the 3 166 PART II Generation of B-Cell and T-Cell Responses α1 α2 β1 β2-microglobulin β2 Transmembrane segment Cytoplasmic tail α2 α1 α3 S Class I molecule Class II molecule S S S S S S S S S S S Peptide-binding cleft Membrane-distal domains Membrane-proximal domains (Ig-fold structure) FIGURE 7-5 Schematic diagrams of a class I and a class II MHC molecule showing the external domains, transmembrane segment, and cytoplasmic tail. The peptide-binding cleft is formed by the mem￾brane-distal domains in both class I and class II molecules. The membrane-proximal domains possess the basic immunoglobulin￾fold structure; thus, class I and class II MHC molecules are classified as members of the immunoglobulin superfamily. 8536d_ch07_161-184 8/16/02 12:09 PM Page 166 mac100 mac 100: 1268_tm:8536d:Goldsby et al. / Immunology 5e-:

8536d_cho7 161-184 8/16/02 12: 09 PM Page 167 mac100 mac 100: 1?/8_tm: 8536d: Goldsby et al./Immunology Se Major Histocompatibility Complex CHAPTER 7 Peptide-binding cleft a1 domain a 2 domain Sheets a2 domain g a3 domain FIGURE7-6Representations of the three-dimensional structure of munoglobulin- fold structure of the a3 domain and B2-microglobulin the external domains of a human class I MHC molecule based on x. (b) The al and a2 domains as viewed from the top, showing the ray crystallographic analysis. (a) Side view in which the B strands are peptide-binding cleft consisting of a base of antiparallel p strands depicted as thick arrows and the a helices as spiral ribbons. Disulfide and sides of a helices. this cleft in class I molecules can accommo- bonds are shown as two interconnected spheres. The al and a2 do- date peptides containing 8-10 residues ding cleft. Note the im- domain,B2-microglobulin, and the constant-region domains which is not surprising given the considerable sequence sim- in immunoglobulins. The enzyme papain cleaves the a chain ilarity with the immunoglobulin constant regions, class I just 13 residues proximal to its transmembrane domain, re- MHC molecules and B2-microglobulin are classified as leasing the extracellular portion of the molecule, consisting of members of the immunoglobulin superfamily( see Figure al,a2, a3, and B2-microglobulin. Purification and crystal- 4-20). The a3 domain appears to be highly conserved among lization of the extracellular portion revealed two pairs of in- class I MHC molecules and contains a sequence that interacts teracting domains: a membrane-distal pair made up of the al with the CD8 membrane molecule present on Tc cells and a2 domains and a membrane-proximal pair composed of B2-Microglobulin interacts extensively with the a3 do- the a3 domain and B2-microglobulin( Figure 7-6a) main and also interacts with amino acids of the al and a2 The al and o2 domains interact to form a platform of domains. The interaction of B2-microglobulin and a peptide eight antiparallel B strands spanned by two long a-helical re- with a class I a chain is essential for the class I molecule to gions. The structure forms a deep groove, or cleft, approxi- reach its fully folded conformation As described in detail in mately 25 A X 10 A X 11 A, with the long a helices as sides Chapter 8, assembly of class I molecules is believed to occur nd the B strands of the B sheet as the bottom( Figure 7-6b). by the initial interaction of B2-microglobulin with the fold This peptide-binding cleft is located on the top surface of the ing class I a chain. This metastable"empty dimer is then sta- class I MHC molecule, and it is large enough to bind a peptide bilized by the binding of an appropriate peptide to form the of 8-10 amino acids. The great surprise in the x-ray crystallo- native trimeric class I structure consisting of the class I a graphic analysis of class I molecules was the finding of small chain, B2-microglobulin, and a peptide. This complete mole peptides in the cleft that had cocrystallized with the protein. cular complex is ultimately transported to the cell surface These peptides are, in fact, processed antigen and self-pep In the absence of B2-microglobulin, the class I MHC tides bound to the al and a2 domains in this deep groove. chain is not expressed on the cell membrane. This is illus The a3 domain and B2-microglobulin are organized into trated by Daudi tumor cells, which are unable to synthesize two p pleated sheets each formed by antiparallel p strands of B2-microglobulin. These tumor cells produce class I MHCa amino acids. As described in Chapter 4, this structure, known chains, but do not express them on the membrane. However, as the immunoglobulin fold, is characteristic of im- if Daudi cells are transfected with a functional gene encoding munoglobulin domains. Because of this structural similarity, B2-microglobulin, class I molecules appear on the membrane

domain, 2-microglobulin, and the constant-region domains in immunoglobulins. The enzyme papain cleaves the chain just 13 residues proximal to its transmembrane domain, re￾leasing the extracellular portion of the molecule, consisting of 1, 2, 3, and 2-microglobulin. Purification and crystal￾lization of the extracellular portion revealed two pairs of in￾teracting domains: a membrane-distal pair made up of the 1 and 2 domains and a membrane-proximal pair composed of the 3 domain and 2-microglobulin (Figure 7-6a). The 1 and 2 domains interact to form a platform of eight antiparallel  strands spanned by two long -helical re￾gions. The structure forms a deep groove, or cleft, approxi￾mately 25 Å  10 Å  11 Å, with the long helices as sides and the  strands of the  sheet as the bottom (Figure 7-6b). This peptide-binding cleft is located on the top surface of the class I MHC molecule, and it is large enough to bind a peptide of 8–10 amino acids. The great surprise in the x-ray crystallo￾graphic analysis of class I molecules was the finding of small peptides in the cleft that had cocrystallized with the protein. These peptides are, in fact, processed antigen and self-pep￾tides bound to the 1 and 2 domains in this deep groove. The 3 domain and 2-microglobulin are organized into two  pleated sheets each formed by antiparallel  strands of amino acids. As described in Chapter 4, this structure, known as the immunoglobulin fold, is characteristic of im￾munoglobulin domains. Because of this structural similarity, which is not surprising given the considerable sequence sim￾ilarity with the immunoglobulin constant regions, class I MHC molecules and 2-microglobulin are classified as members of the immunoglobulin superfamily (see Figure 4-20). The 3 domain appears to be highly conserved among class I MHC molecules and contains a sequence that interacts with the CD8 membrane molecule present on TC cells. 2-Microglobulin interacts extensively with the 3 do￾main and also interacts with amino acids of the 1 and 2 domains. The interaction of 2-microglobulin and a peptide with a class I chain is essential for the class I molecule to reach its fully folded conformation. As described in detail in Chapter 8, assembly of class I molecules is believed to occur by the initial interaction of 2-microglobulin with the fold￾ing class I chain. This metastable “empty” dimer is then sta￾bilized by the binding of an appropriate peptide to form the native trimeric class I structure consisting of the class I chain, 2-microglobulin, and a peptide. This complete mole￾cular complex is ultimately transported to the cell surface. In the absence of 2-microglobulin, the class I MHC chain is not expressed on the cell membrane. This is illus￾trated by Daudi tumor cells, which are unable to synthesize 2-microglobulin. These tumor cells produce class I MHC chains, but do not express them on the membrane. However, if Daudi cells are transfected with a functional gene encoding 2-microglobulin, class I molecules appear on the membrane. Major Histocompatibility Complex CHAPTER 7 167 (b) α1 domain α2 domain α3 domain α2 domain α1 domain β2-microglobulin α helix β sheets (a) Peptide-binding cleft FIGURE 7-6 Representations of the three-dimensional structure of the external domains of a human class I MHC molecule based on x￾ray crystallographic analysis. (a) Side view in which the  strands are depicted as thick arrows and the helices as spiral ribbons. Disulfide bonds are shown as two interconnected spheres. The 1 and 2 do￾mains interact to form the peptide-binding cleft. Note the im￾munoglobulin-fold structure of the 3 domain and 2-microglobulin. (b) The 1 and 2 domains as viewed from the top, showing the peptide-binding cleft consisting of a base of antiparallel  strands and sides of helices. This cleft in class I molecules can accommo￾date peptides containing 8–10 residues. 8536d_ch07_161-184 8/16/02 12:09 PM Page 167 mac100 mac 100: 1268_tm:8536d:Goldsby et al. / Immunology 5e-:

8536d ch07161-184 8/16/02 12: 09 PM Page 168 mac100 mac 100: 1258 tm: 8536d: Goldsby et al. Immunology 5e 168 PART I1 Generation of B-Cell and T-Cell Response Class ll molecules have two nonidentical Glycoprotein Chains Class II MHC molecules contain two different polypeptide hains, a 33-kDa a chain and a 28-kDa B chain, which asso ciate by noncovalent interactions(see Figure 7-5b) Like class I a chains, class II MHC molecules are membrane-bound glycoproteins that contain external domains, a transmem- brane segment, and a cytoplasmic anchor segment. Each chain in a class ii molecule contains two external domains. I and a2 domains in one chain and Bl and B2 domains in the other. The membrane-proximal a2 and B2 domains, like the membrane-proximal a3/B2-microglobulin domains of class I MHC molecules, bear sequence similarity to the im- munoglobulin-fold structure; for this reason, class II MHC (b) molecules also are classified in the immunoglobulin super- family. The membrane-distal portion of a class II molecule is composed of the al and Bl domains and forms the antigen binding cleft for processed antigen X-ray crystallographic analysis reveals the similarity of lass II and class I molecules, strikingly apparent when the molecules are surperimposed( Figure 7-7). The peptide binding cleft of HLA-DRl, like that in class I molecules, is composed of a floor of eight antiparallel B strands and side of antiparallel a helices. However, the class Il molecule lacks the conserved residues that bind to the terminal residues of short peptides and forms instead an open pocket; class I pre sents more of a socket, class ll an open-ended groove. Thas functional consequences of these differences fine structure will be explored beloy An unexpected difference between crystallized class I and class li molecules was observed for human dri in that the FIGURE7-8Antigen-binding cleft of dimeric class ll DR1 molecule in(a) top view and (b) side view. This molecule crystallized as a dimer of the ap heterodimer. The crystallized dimer is shown with one dri molecule in red and the other dri molecule in blue. the bound peptides are yellow. The two peptide- binding clefts in the dimeric molecule face in opposite directions. From H Brown et al 1993. Nature364:33J latter occurred as a dimer of aB heterodimers, a" dimer of dimers"(Figure 7-8). The dimer is oriented so that the two peptide-binding clefts face in opposite directions. While it has not yet been determined whether this dimeric form exists in vivo, the presence of CD4 binding sites on opposite sides of the class Il molecule suggests that it does. These two sites on the a2 and B2 domains are adjacent in the dimer form and a CD4 molecule binding to them may stabilize class ll dimers The Exon/Intron Arrangement of Class I and FIGURE 7.7 The membrane-distal, peptide-binding cleft of a hu. ll Genes Reflects Their Domain Structure man class Il MHC molecule, HLA-DR1 (blue perimposed over Separate exons encode each region of the class I and ll pro the corresponding regions of a human class I MHC molecule, HLA- teins( Figure 7-9). Each of the mouse and human class I A2(red). From/ H. Brown et aL., 1993, Nature 364: 33. J genes has a 5' leader exon encoding a short signal peptide

Class II Molecules Have Two Nonidentical Glycoprotein Chains Class II MHC molecules contain two different polypeptide chains, a 33-kDa chain and a 28-kDa  chain, which asso￾ciate by noncovalent interactions (see Figure 7-5b). Like class I chains, class II MHC molecules are membrane-bound glycoproteins that contain external domains, a transmem￾brane segment, and a cytoplasmic anchor segment. Each chain in a class II molecule contains two external domains: 1 and 2 domains in one chain and 1 and 2 domains in the other. The membrane-proximal 2 and 2 domains, like the membrane-proximal 3/2-microglobulin domains of class I MHC molecules, bear sequence similarity to the im￾munoglobulin-fold structure; for this reason, class II MHC molecules also are classified in the immunoglobulin super￾family. The membrane-distal portion of a class II molecule is composed of the 1 and 1 domains and forms the antigen￾binding cleft for processed antigen. X-ray crystallographic analysis reveals the similarity of class II and class I molecules, strikingly apparent when the molecules are surperimposed (Figure 7-7). The peptide￾binding cleft of HLA-DR1, like that in class I molecules, is composed of a floor of eight antiparallel  strands and sides of antiparallel helices. However, the class II molecule lacks the conserved residues that bind to the terminal residues of short peptides and forms instead an open pocket; class I pre￾sents more of a socket, class II an open-ended groove. These functional consequences of these differences in fine structure will be explored below. An unexpected difference between crystallized class I and class II molecules was observed for human DR1 in that the latter occurred as a dimer of  heterodimers, a “dimer of dimers” (Figure 7-8). The dimer is oriented so that the two peptide-binding clefts face in opposite directions. While it has not yet been determined whether this dimeric form exists in vivo, the presence of CD4 binding sites on opposite sides of the class II molecule suggests that it does. These two sites on the 2 and 2 domains are adjacent in the dimer form and a CD4 molecule binding to them may stabilize class II dimers. The Exon/Intron Arrangement of Class I and II Genes Reflects Their Domain Structure Separate exons encode each region of the class I and II pro￾teins (Figure 7-9). Each of the mouse and human class I genes has a 5 leader exon encoding a short signal peptide 168 PART II Generation of B-Cell and T-Cell Responses FIGURE 7-7 The membrane-distal, peptide-binding cleft of a hu￾man class II MHC molecule, HLA-DR1 (blue), superimposed over the corresponding regions of a human class I MHC molecule, HLA￾A2 (red). [From J. H. Brown et al., 1993, Nature 364:33.] (a) (b) FIGURE 7-8 Antigen-binding cleft of dimeric class II DR1 molecule in (a) top view and (b) side view. This molecule crystallized as a dimer of the  heterodimer. The crystallized dimer is shown with one DR1 molecule in red and the other DR1 molecule in blue. The bound peptides are yellow. The two peptide-binding clefts in the dimeric molecule face in opposite directions. [From J. H. Brown et al., 1993, Nature 364:33.] 8536d_ch07_161-184 8/16/02 12:09 PM Page 168 mac100 mac 100: 1268_tm:8536d:Goldsby et al. / Immunology 5e-:

8536d_cho7 161-184 8/16/02 12: 09 PM Page 169 mac100 mac 100: 1?/8_tm: 8536d: Goldsby et al./Immunology Se Major Histocompatibility Complex CHAPTER 7 Tn CC mRNA mRNA LILLA a chain B chain Class I MHC Class II MHC nolecule B? COOH HN→S DDDyCCOOH H2N H2N mRNA L al (2 TmcC m+C FIGURE7-9Schematic diagram of(a)class I and(b)class l MHC codes a separate domain of the MHC molecule. The leader peptides genes, mRNA transcripts, and protein molecules. There is corre- are removed in a post-translational reaction before the molecules are spondence between exons and the domains in the gene products; expressed on the cell surface. The gene encoding B2-microglobulin is note that the mRNA transcripts are spliced to remove the intron se. located on a different chromosome. Tm=transmembrane; C uences. Each exon, with the exception of the leader(L)exon, en- cytoplasmic followed by five or six exons encoding the a chain of the class Class I and ll Molecules Exhibit I molecule(see Figure 7-9a). The signal peptide serves to fa- Polymorphism in the Region That cilitate insertion of the a chain into the endoplasmic reticu lum and is removed by proteolytic enzymes in the Binds to Peptides ndoplasmic reticulum after translation is completed. The Several hundred different allelic variants of class I and II MHC ext three exons encode the extracellular al, a 2, and a3 do- molecules have been identified in humans. Any one individual, mains, and the following downstream exon encodes the however, expresses only a small number of these molecules transmembrane(Tm)region; finally, one or two 3-terminal up to 6 different class I molecules and up to 12 different class ll exons encode the cytoplasmic domains(C) lecules. Yet this limited number of mhc molecules must be Like class I MHC genes, the class II genes are organized able to present an enormous array of different antigenic pep- into a series of exons and introns mirroring the domain struc- tides to T cells, permitting the immune system to respond ture of the a and B chains(see Figure 7-9b) Both the a and specifically to a wide variety of antigenic challenges. Thus, pep- the B genes encoding mouse and human class II MHC mole- tide binding by class I and ll molecules does not exhibit the fine cules have a leader exon, an al or Bl exon, an a2 or B2 exon, specificity characteristic of antigen binding by antibodies and a transmembrane exon, and one or more cytoplasmic exons. T-cell receptors. Instead, a given MHC molecule can bind

followed by five or six exons encoding the chain of the class I molecule (see Figure 7-9a). The signal peptide serves to fa￾cilitate insertion of the chain into the endoplasmic reticu￾lum and is removed by proteolytic enzymes in the endoplasmic reticulum after translation is completed. The next three exons encode the extracellular 1, 2, and 3 do￾mains, and the following downstream exon encodes the transmembrane (Tm) region; finally, one or two 3-terminal exons encode the cytoplasmic domains (C). Like class I MHC genes, the class II genes are organized into a series of exons and introns mirroring the domain struc￾ture of the and  chains (see Figure 7-9b). Both the and the  genes encoding mouse and human class II MHC mole￾cules have a leader exon, an 1 or 1 exon, an 2 or 2 exon, a transmembrane exon, and one or more cytoplasmic exons. Class I and II Molecules Exhibit Polymorphism in the Region That Binds to Peptides Several hundred different allelic variants of class I and II MHC molecules have been identified in humans. Any one individual, however, expresses only a small number of these molecules— up to 6 different class I molecules and up to 12 different class II molecules.Yet this limited number of MHC molecules must be able to present an enormous array of different antigenic pep￾tides to T cells, permitting the immune system to respond specifically to a wide variety of antigenic challenges. Thus, pep￾tide binding by class I and II molecules does not exhibit the fine specificity characteristic of antigen binding by antibodies and T-cell receptors. Instead, a given MHC molecule can bind Major Histocompatibility Complex CHAPTER 7 169 DNA 5′ 3′ α1 α2 α3 Tm C C (a) COOH H2N α chain DNA 5′ 3′ β1 β2 CC Tm+C (b) Class I MHC molecule mRNA mRNA mRNA (A)n (A)n (A)n DNA 5′ 3′ α1 α2 α1 α2 Tm+C C S S S S S S L α1 α2 α1 α3 β2 - microglobulin L CC α2 α3 Tm L C β1 β2 C L L Tm+C L C Tm+C COOH COOH H2N H2N β chain α chain Class II MHC molecule S S S S S S β1 β2 α1 α2 FIGURE 7-9 Schematic diagram of (a) class I and (b) class II MHC genes, mRNA transcripts, and protein molecules. There is corre￾spondence between exons and the domains in the gene products; note that the mRNA transcripts are spliced to remove the intron se￾quences. Each exon, with the exception of the leader (L) exon, en￾codes a separate domain of the MHC molecule. The leader peptides are removed in a post-translational reaction before the molecules are expressed on the cell surface. The gene encoding 2-microglobulin is located on a different chromosome. Tm  transmembrane; C  cytoplasmic. 8536d_ch07_161-184 8/16/02 12:09 PM Page 169 mac100 mac 100: 1268_tm:8536d:Goldsby et al. / Immunology 5e-:

8536d_cho7161-184 8/16/02 1: 49 PM Page 170 mac100 mac 100: 12F+\tm: 8536d: Goldsby et al./ Immunology 5e- 170 PART II Generation of B-Cell and T-Cell Response TABLE 7-2 Peptide binding by class I and class II MHC molecules Class I molecules Class ll molecules Peptide-binding domain a1/1 Nature of peptide-binding cleft Closed at both end Open at both ends General size of bound peptides 8-10 amino acids 13-18 amino acids Peptide motifs involved in Anchor residues at both ends of Anchor residues distributed along binding to MHC molecule peptide; generally hydrophobe the length of the peptide carboxyl-terminal anchor Nature of bound peptide Extended structure in which both ends Extended structure that is held nteract with MHC cleft but middle at a constant elevation above arches up away from MHC molecule the floor of mhc cleft numerous different peptides, and some peptides can bind to the MHC molecules expressed on the membrane of a cell several different MHC molecules. Because of this broad speci- will be associated with a peptide of self or nonself origin. icity, the binding between a peptide and an MHC molecule is often referred to as"promiscuous Given the similarities in the structure of the peptide-bind- CLASS I MHC-PEPTIDE INTERACTION ing cleft in class I and II MHC molecules, it is not surprising Class I MHC molecules bind peptides and present them to that they exhibit some common peptide-binding features CD8+ T cells. In general, these peptides are derived from en- (Table 7-2). In both types of MHC molecules, peptide lig. dogenous intracellular proteins that are digested in the cy- ands are held in a largely extended conformation that runs toso. The peptides are then transported from the cytosol the length of the cleft. The peptide-binding cleft in class I into the cisternae of the endoplasmic reticulum, where they molecules is blocked at both ends, whereas the cleft is open in interact with class I MHC molecules. This process, known as class II molecules(Figure 7-10). As a result of this difference, the cytosolic or endogenous processing pathway, is discussed lass I molecules bind peptides that typically contain 8-10 in detail in the next chapter. amino acid residues, while the open groove of class II mole Each type of class I MHC molecule(K, D, and L in mice cules accommodates slightly longer peptides of 13-18 amIno rA, B, and C in humans) binds a unique set of peptides. In cids. Another difference, explained in more detail below, is addition, each allelic variant of a class I MHC molecule(e.g that class I binding requires that the peptide contain specific H-2K and H-2K also binds a distinct set of peptides.Be- amino acid residues near the n and C termini; there is no cause a single nucleated cell expresses about 10 copies of such requirement for class II peptide binding. each class I molecule, many different peptides will be ex- The peptide-MHC molecule association is very stable pressed simultaneously on the surface of a nucleated cell by (Kd-" )under physiologic conditions; thus, most of class I MHC molecules. (a Class I MHC (b) Class II MHC FIGURE 7-10 MHC class I and class ll molecules with bound pep. those below from a2.(b)Space-filling model of human class ll mol- tides. (a) Space-filling model of human class I molecule HLA-A2 ecules HLA-DR1 with the DRa chain shown in white and the DRB ite)with peptide(red) from HIV reverse transcriptase(amino chain in blue. The peptide(red) in the binding groove is from in- acid residues 309-317) in the binding groove. B2-microglobulin is fluenza hemagglutinin(amino acid residues 306-318) From D. A shown in blue. Residues above the peptide are from the al doma Vignali and. Strominger, 1994, The Immunologist 2: 112. 1

numerous different peptides, and some peptides can bind to several different MHC molecules. Because of this broad speci￾ficity, the binding between a peptide and an MHC molecule is often referred to as “promiscuous.” Given the similarities in the structure of the peptide-bind￾ing cleft in class I and II MHC molecules, it is not surprising that they exhibit some common peptide-binding features (Table 7-2). In both types of MHC molecules, peptide lig￾ands are held in a largely extended conformation that runs the length of the cleft. The peptide-binding cleft in class I molecules is blocked at both ends, whereas the cleft is open in class II molecules (Figure 7-10). As a result of this difference, class I molecules bind peptides that typically contain 8–10 amino acid residues, while the open groove of class II mole￾cules accommodates slightly longer peptides of 13–18 amino acids. Another difference, explained in more detail below, is that class I binding requires that the peptide contain specific amino acid residues near the N and C termini; there is no such requirement for class II peptide binding. The peptide–MHC molecule association is very stable (Kd ~ 106 ) under physiologic conditions; thus, most of 170 PART II Generation of B-Cell and T-Cell Responses TABLE 7-2 Peptide binding by class I and class II MHC molecules Class I molecules Class II molecules Peptide-binding domain 1/2 1/1 Nature of peptide-binding cleft Closed at both ends Open at both ends General size of bound peptides 8–10 amino acids 13–18 amino acids Peptide motifs involved in Anchor residues at both ends of Anchor residues distributed along binding to MHC molecule peptide; generally hydrophobic the length of the peptide carboxyl-terminal anchor Nature of bound peptide Extended structure in which both ends Extended structure that is held interact with MHC cleft but middle at a constant elevation above arches up away from MHC molecule the floor of MHC cleft (a) Class I MHC (b) Class II MHC FIGURE 7-10 MHC class I and class II molecules with bound pep￾tides. (a) Space-filling model of human class I molecule HLA-A2 (white) with peptide (red) from HIV reverse transcriptase (amino acid residues 309–317) in the binding groove. 2-microglobulin is shown in blue. Residues above the peptide are from the 1 domain, those below from 2. (b) Space-filling model of human class II mol￾ecules HLA-DR1 with the DR chain shown in white and the DR chain in blue. The peptide (red) in the binding groove is from in￾fluenza hemagglutinin (amino acid residues 306–318). [From D. A. Vignali and J. Strominger, 1994, The Immunologist 2:112.] the MHC molecules expressed on the membrane of a cell will be associated with a peptide of self or nonself origin. CLASS I MHC–PEPTIDE INTERACTION Class I MHC molecules bind peptides and present them to CD8 T cells. In general, these peptides are derived from en￾dogenous intracellular proteins that are digested in the cy￾tosol. The peptides are then transported from the cytosol into the cisternae of the endoplasmic reticulum, where they interact with class I MHC molecules. This process, known as the cytosolic or endogenous processing pathway, is discussed in detail in the next chapter. Each type of class I MHC molecule (K, D, and L in mice or A, B, and C in humans) binds a unique set of peptides. In addition, each allelic variant of a class I MHC molecule (e.g., H-2Kk and H-2Kd ) also binds a distinct set of peptides. Be￾cause a single nucleated cell expresses about 105 copies of each class I molecule, many different peptides will be ex￾pressed simultaneously on the surface of a nucleated cell by class I MHC molecules. 8536d_ch07_161-184 8/16/02 1:49 PM Page 170 mac100 mac 100: 1268_tm:8536d:Goldsby et al. / Immunology 5e-: