Supplement 1, Vol. 8, Aust. N.Z. J. Med. (1978), pp. 15-29
The Major Histocompatibility Complex in Man: HLA Antigens J. D. Wetherall and R. L. Dawkins
From the Department of Clinical Immunology, Royal Perth
The human major histocompatibility complex (MHC) comprises a group of genes located on autosomal chromosome VI. These genes determine the structure of cell surface glycoproteins (including HLA antigens) which are distributed on most tissues of the body. They collectively form the basis of a complex set of antigenic determinants which include the “markers of biological individuality” or histocompatibility antigens.’. The function of these antigens is presently the subject of much current investigation. It is known that these antigens constitute the strongest antigenic barrier to tissue transplantation between genetically non-identical individuals. However, their precise role has yet to be defined and it has been suggested recently that this may involve cell to cell recognition (reviewed in Australasian Tissue Typing Workshop, 1977).3 This paper briefly reviews the current status of the human MHC region and the significance of the glycoproteins determined by the genes within this region.
’
xpital, Perth, Western Australia
B and C loci have been detected by serological techniques and are consequently called serologically determined (SD) antigens. In contrast, HLA antigens determined by the D locus were initially detected by interactions between lymphocytes and have been called lymphocyte determined ( L D ) antigens. The method almost universally used for the detection of SD antigens is a microcytotoxicity test utilising HLA antigen specific typing sera to induce complement mediated killing of the test lymphocytes expressing the corresponding HLA THE
H I S T C C O I I P A T I B I L I T Y COMPLEX
f‘
PG5
!WC)
CHROMOSOME V I (homologous p a i r.)
‘J
” N
HLA-A
x
e
c4,cn
a
HLA-C HLA-B
-
C3b receDtor
Bf
-
Correspondence. Dr. J. D Wetherall, Department of Medical Technology, Western Australian Institute of Technology, Bentley. W A 6102
MAJOR
-A7
m
The M H C Region An operational view of the MHC complex is summarised in Fig. 1. This figure depicts a region on a pair of homologous chromosomes which contains the MHC region and several other oligomorphic loci helpful in mapping the chromosome. The relative positions of other genes within the MHC region are also shown. It can be seen that this chromosomal region codes for HLA antigens, some complement components, and receptors for the C3b component of complement. The loci which code for HLA antigens are designated A, B, C and D. Each of these loci is extremely polymorphic; that is, at each locus there are many alternative forms (alleles).The HLA antigens determined by the A.
HUMAN
x
a
l
GLO
HLA-D
t
/
“\ I CODE:
PGtI PG5 GLO Bf C2 cE:
cChido 2 blood group
phosphoglucomutase isoenzymes u r i n a r y pepsinogen glyoxylase a l t e r n a t e pathway factor B complement components recombination u n i t s centimorgans
-
FIGURE 1. A schematic diagram of the HLA system on chromosome VI. HLA-A and B loci are approximately I recombination unit (i.e. 1 centi-morgan, CM) apart. The HLA-D locus determines LD antigens and may be analogous to the I region of the mouse. Genes controlling the synthesis of complement components C4 and C8, alternate pathway factor B, and receptor for C3b. are also within the human M H C but have not been mapped precisely. The gene for C2 deficiency is known to be close to the HLA-D locus. Several non M H C chromosomal markers (glyoxylase, phosphoglucomutase 3 and urinary pepsinogen) are also shown.
26
WETHERALL AND DAWKINS
antigens. This technique is standardised within the Australasian Region and further details may be obtained from the Australasian Tissue Typing Workshop, 1977.3The typing sera used to detect HLA antigens are not so well standardised. These sera are obtained from individuals who have been immunised during pregnancy or transplantation. In many cases, these individuals produce multispecific antibodies of little practical value. A small number however, produce an operationally oligo or monospecific antiserum which is of practical value for tissue typing. In Perth, it has been found that it is necessary to screen several thousand potential typing sera to obtain approximately 50 useful sera, and of these only about ten reach international standard. Laboratories within the Australasian Region are dependent upon mutual collaboration for obtaining acceptable standards of tissue typing.3 It should be emphasised that these typing sera are operationally defined. They react with the most immunogenic determinant present on the HLA antigen glycoprotein, and their pattern of reactivity does not reflect complete structural detail of the immunising molecule. It is probably for this reason that antisera which initially appeared homogeneous have subsequently been shown to be heterogeneous in their patterns of reactivity. For example, the previously well defined HLA antigens A9 and A10 have now been shown to consist of at least two immunoreactive components designated Aw23, 24 and Aw25, 26 respectively. Very recently, the antigen Bw17 has been “split”. It is quite possible that antigens at present considered homogeneous, for example B27, will eventually be split also. HLA antigens expressed at the D locus (LD antigens) have previously been detected by the mixed lymphocyte culture reaction. However, it is now clear that they can also be detected by serological means so long as B lymphocytes are used as the target cell. Separation of T and B lymphocytes can now be achieved by rosette sedimentation and most laboratories perform this technique r o ~ t i n e l y . ~ The allelic forms of HLA antigens expressed at each of the four loci is shown in Table I . Antigens recognised internationally are designated by a number following the locus designation. Anti-
1,
SUPPLEMENT
8
VOL.
TABLE I HLA antigens* A locus -~ ___
c locus
B locus
D locus
~~-
HLA-A1 A2 A3 A9 A10 A1 1 A28 A29 Aw23 AwZ4 Aw25 Aw26 Aw30 Aw31 Aw32 Aw33 Aw34 Aw43
HLA-B 87 B8 B12 B13 B14 B18 B27 Bw15 Bw16 Bw17 BwZl Bw22 Bw35 Bw37 Bw38 Bw39 Bw4O Bw4 I Bw42 TT KSO Hs
HLA-Cwl CW2 cw3 cw4 cw5
~~
HLA-Dwl Dw2 Dw3 Dw4 Dw5 Dw6
T7 LD107 LDI08
*Identifiable by antisera presently available. The 19 A locus antigens account for approximately 95”,, of the A and B locus genes. The more recently defined C and D loci antigens account for about half the alleles at these loci.
gens in the process of being defined internationally are designated by a “w”. The phenotypic and gene frequencies for HLA-A and B antigens are shown in Table 2. TABLE 2 Phenotypic and gene frequencies of HLA-A and B locus an:igens in an Australian population* HLA-A antigen
Phenotypic frequency
(”,J ~~
Al A2 A3 A9 A10 All A19 S28
33 8 52-6 26.1 17.9 8.3 7-2 14 1 2 4
*BusselLon, WA in
=
Gene frequency
HLA-B antigen
Phenotypic frequency (‘“J
-~
0.1866 Obi116 0 1405 n ,0940 0 0426 0.0367 0-0734 0 0121
Gene
frequency ~~
B5 87 B8 B12 813 B14 Bw15 Bwl6 Bw17 B18 Bw2l Bw22 B27 Bw35 Bw40 KSO
8.3 26 0 25 6 33.0 4.3 5.1
9 6 3 9 7.8 8. I 3.1 3-9 6.I 11.6
10.4 0.09
2745) from Hawklns. B R (unpublished)
n 0424 n- 1396 0.1377 0.1815 0.0219 0-025s 0 0493 0 0195
wo4on 0-0411 0-0158 0.0199 0.0311 0.0595 0-0535 0.0044
~
JUNE
1978
MAJOR HISTOCOMPAT 'IBILITY COMPLEX IN MAN
TABLE 3 Tissue distribution of HLA-A. B. C. D antigenst Tissue
HLA-A, B, C
~
H LA-D ~
~
...
+++ +++ +++ +++ +++ ++
T lymphocytes R lymphocytes Macrophages Epidermal cells Endothelial cells Spermatozoa Erythrocytes Platelets Liver Brain
-
*
+++ +++ ++ + ++
I
-
i t
+
i
*Not well expi-essed in resting T lymphocytes but can be expressed in activated lymphocytes. ?Modified from Wernet (1976).
Tissue distributions of HLA-A, B, C antigens as opposed to HLA-D antigens are shown in Table 3." The tnost important distinction between the A, B, C antigens and the D antigens is the lack of D antigens on platelets and resting T lymphocytes. A practical application of this difference is the use of platelets to absorb antibodies reactive with A, B and C locus INHERITANCE
OF
HLA
ANTIGENS
FIGURE 2. Mode of inheritance of HLA antigens. Genotypes of Down family. "Haplotype"-the combination of antigens determined by the M H C of a single chromosome. The "genotype" of an individual is made up of t w o haplotypes. The "phenotype" is the combination of antigens expressed by an individual. It can be seen that the mother had six specificities detected and has the same phenotype and genotype (A2, 3; 87, 12; Dw2, w4). Her eldest daughter inherited A3; 87; D w 2 and the A2; 87; D w 2 haplotype from the father so that only four specificities were detected (phenotype A2, 3; 87; Dw2). Because of the information provided by the family study the genotype can be surmised, 1.e. A2, 3: 9. 7, 7; Dw2. w2. Note that the three daughters are H I A identical whereas the elder son is quite different. The younger son shares a haplotype with his siblings. The possible genotype A2, 2; 87, 12; Dw2, w 4 is not represented.
1 I
27
antigens. In this way, useful typing sera defining D antigens can be obtained, The mode of inheritance of HLA antigens is illustrated in Fig. 2. It can be seen that each individual inherits a particular HLA-A, B, C and D antigen from each parent. Antigens from each parent are expressed so that the genes responsible are considered to be co-dominant. As shown in the family, the M H C is inherited as a whole, although occasionally crossovers occur so that an individual may inherit HLA-A and C antigens from one paternal chromosome but HLA-B and D antigens from another paternal chromosome. The presence of other loci on chromosome VI allows such crossovers to be mapped. It can be seen from this figure that one parent has genes for A3, B7 and Dw2 expressed on one chromosome. From population studies it has been found that these antigens are found more frequently together than would be expected from their frequency in the population as a whole. Such a haplotype is often referred to as a superhaplotype and exhibits the phenomenon of linkage disequilibrium. There are a number of instances of linkage disequilibrium within the major histocompatibility complex and their presence is thought to indicate a survival advantage of particular haplotypes. Structure of H L A Antigens It is possible to extract HLA glycoproteins from cell membranes with the aid of detergents and specific antisera. The tertiary structure of these molecules and their amino acid sequences can then be ' Murine histocompatibility antigens have been analysed in detail but some information on the composition of human HLA antigens has been obtained. The HLA-A and B antigens have been found to consist of two non-covalently linked polypeptide chains of molecular weight 12,000 and 44,000 daltons respectively. The postulated orientation of these polypeptides in the cell membrane is shown in Fig. 3. The smaller polypeptide (12.000 daltons; p12) has been identified as p2 microglobulin-a molecule shown to share amino acid sequence homology with some domains of immunoglobulin molecules. It has been demonstrated that the synthesis of p2 microglobulin is controlled by a gene on
28
WETHERALL AND DAWKINS OF HLA-A AND B A N T i G E d S
STRUCTURE
SUPPLEMENT
COMPOSITION
1,
O F T I i E MOUSE MHC
VOL.
8
(H-2)
N
I
-
ALLOANTIGENIC HEAVY (44003 ? k L ) CHAIN dal f m 5 ; ~
CARBOHYDRATE
[
~
~
~
~
‘
~
~
~
~
~ ~ ~ ~ CHROMOSOME X Vp I I
,
2
~
-
/$
s
NON COVALENT BONDING
HY D R W H O B IC
C E L L MEMBRANE
(I
1
CENTROMERE
H-2K
CELL IllTERlOR
FIGURE 3. After Springer and Strominger.’ A schematic diagram showing the postulated tertiary structure of HLA-A and B locus antigens and their arrangement relative to the cell membrane. The positions of the carbohydrate residues and 8, microglobulin relative to the alloantigenic chain are not known. 8, microglobulin synthesis is determined by a gene on chromosome XV. The structure of C locus antigens has not been investigated. D locus antigens are known to have different structures (reference 4).
chromosome XV. The heavier polypeptide chain (44,000 daltons; p44) is the glycoprotein which exhibits the allogenic antigenic HLA specificities. The p12 polypeptide chain, although associated with the p44 glycoprotein, is not thought to contribute any amino acid residues to the antigenic specificity of the bimolecular complex. The p44 chain extends through the cell membrane where the internal portion makes contact with the cytoskeleton. The region of the molecule associated with the cell membrane is hydrophobic whereas the remainder of the molecule is primarily hydrophilic. Some carbohydrate is associated with p44 chain, but does not contribute to the antigenic specificities. Recently, preliminary amino acid sequencies for both the p12 and p44 chain have been obtained. Apart from a surprising degree of similarity between the HLA antigens and the analogous murine antigens, the p44 chain shows some degree of homology with one of the variable regions in human immunoglobulin gamma chains. This homology raises some interesting questions in terms of T cell receptors for antigen and the regulation of immune responses.
;ipl-
s
#
region
H-2G ti-2D
1:
FIGURE 4. A schematic diagram of the mouse H - 2 region (MHC). The H-2 region is bounded by the D and K regions (analogous to HLA-A and B respectively). The I region (immune region associated antigens) includes five subloci which determine la antigens and contain genes controlling immune responsiveness and immune suppression. The S region contains at least two genes one of which determines the structure of complement component C2. There is also evidence that the synthesis of other complement components is governed by genes within the MHC. The H-2G locus determines antigens expressed on mouse erythrocytes. The thymus leukaemia (TL) region governs the expression of antigens on thymic and leukaemic lymphocytes.
Future Directions The availability of a large number of strains of inbred mice has made possible a more complete description of the Major Histocompatibility Complex in this species (H-2 The composition of the H-2 system is shown in Fig. 4.
JUNE
1978
MAJOR HISTOCOMPATIBILITY COMPLEX IN MAN
The I region of the mouse MHC has been shown to be responsible for determining immune region associated antigens, that is Ia antigens, and patterns of immune responsiveness in this species. The postulated immune respone genes (Ir genes) located within the I region appear to determine whether an individual mouse can respond to certain antigens. However, it would appear that the ability to mount a complete immune response especially to more complex antigens is dependent upon multiple genes some ofwhich are not within the MHC region and may occur on other chromosomes. Analogous immune response genes may occur in man but have not been unequivocally demonstrated to date. Some believe HLA-D represents the human I a 3 It would seem that certain patterns of immune response are associated with particular H L A specificities and this may be explained in terms of linkage disequilibrium between HLA and immune response genes, although other explanations are possible. Extrapolation from mouse to man has helped to define important questions but should be accepted with some caution since the murine system has some features which may be unique to this species. Work on the mouse has shown that the H-2 '
39
system determines the susceptibility of a virus infected target cell to killing by cytotoxic T lymphocytes.' Recent work has suggested that antigens coded for by the murine MHC may be involved in antigen recognition.' The implications for human disease remain to be determined. On the other hand there can be little doubt that the association between particular HLA specificities and some diseases will be explained in terms of antigen receptors, recognition and patterns of immune responsiveness. References 1. BACH, F H. and Vah R(ivn, J. J (1476): The major histocompatibility compler. NPM.€ n f / . J. M t d 295. ROh. 428
2
3. 1
5. b.
7. 8
3.
S A s A m h i , T.. MCDEVIIT.H.0. and GRI'MET, F. C . 11977): lheassoctation between genes in the major histricornpatibility complex and dised3e susceptibility, 4 n n . rc'i' M e d 28, 42.5 DAWKINS,R. L. and W E T H L R A I S , J . D (1977): The Australasian Tissue Typing Workahop. University of Western Australia. Perth. WERWT. P. (IY76). H u m m lo-type a i l o a n t l p m Methods of defection. J > ~ C C ~ofS chernihtrq and biology, markers for disease stater. Tiun$plunl. Rci, 36,27 I. PEILKSi>N. P. A , , RASK, L., SEtiL, I
The Major Histocompatibility Complex in Man: HLA Antigens J. D. Wetherall and R. L. Dawkins
From the Department of Clinical Immunology, Royal Perth
The human major histocompatibility complex (MHC) comprises a group of genes located on autosomal chromosome VI. These genes determine the structure of cell surface glycoproteins (including HLA antigens) which are distributed on most tissues of the body. They collectively form the basis of a complex set of antigenic determinants which include the “markers of biological individuality” or histocompatibility antigens.’. The function of these antigens is presently the subject of much current investigation. It is known that these antigens constitute the strongest antigenic barrier to tissue transplantation between genetically non-identical individuals. However, their precise role has yet to be defined and it has been suggested recently that this may involve cell to cell recognition (reviewed in Australasian Tissue Typing Workshop, 1977).3 This paper briefly reviews the current status of the human MHC region and the significance of the glycoproteins determined by the genes within this region.
’
xpital, Perth, Western Australia
B and C loci have been detected by serological techniques and are consequently called serologically determined (SD) antigens. In contrast, HLA antigens determined by the D locus were initially detected by interactions between lymphocytes and have been called lymphocyte determined ( L D ) antigens. The method almost universally used for the detection of SD antigens is a microcytotoxicity test utilising HLA antigen specific typing sera to induce complement mediated killing of the test lymphocytes expressing the corresponding HLA THE
H I S T C C O I I P A T I B I L I T Y COMPLEX
f‘
PG5
!WC)
CHROMOSOME V I (homologous p a i r.)
‘J
” N
HLA-A
x
e
c4,cn
a
HLA-C HLA-B
-
C3b receDtor
Bf
-
Correspondence. Dr. J. D Wetherall, Department of Medical Technology, Western Australian Institute of Technology, Bentley. W A 6102
MAJOR
-A7
m
The M H C Region An operational view of the MHC complex is summarised in Fig. 1. This figure depicts a region on a pair of homologous chromosomes which contains the MHC region and several other oligomorphic loci helpful in mapping the chromosome. The relative positions of other genes within the MHC region are also shown. It can be seen that this chromosomal region codes for HLA antigens, some complement components, and receptors for the C3b component of complement. The loci which code for HLA antigens are designated A, B, C and D. Each of these loci is extremely polymorphic; that is, at each locus there are many alternative forms (alleles).The HLA antigens determined by the A.
HUMAN
x
a
l
GLO
HLA-D
t
/
“\ I CODE:
PGtI PG5 GLO Bf C2 cE:
cChido 2 blood group
phosphoglucomutase isoenzymes u r i n a r y pepsinogen glyoxylase a l t e r n a t e pathway factor B complement components recombination u n i t s centimorgans
-
FIGURE 1. A schematic diagram of the HLA system on chromosome VI. HLA-A and B loci are approximately I recombination unit (i.e. 1 centi-morgan, CM) apart. The HLA-D locus determines LD antigens and may be analogous to the I region of the mouse. Genes controlling the synthesis of complement components C4 and C8, alternate pathway factor B, and receptor for C3b. are also within the human M H C but have not been mapped precisely. The gene for C2 deficiency is known to be close to the HLA-D locus. Several non M H C chromosomal markers (glyoxylase, phosphoglucomutase 3 and urinary pepsinogen) are also shown.
26
WETHERALL AND DAWKINS
antigens. This technique is standardised within the Australasian Region and further details may be obtained from the Australasian Tissue Typing Workshop, 1977.3The typing sera used to detect HLA antigens are not so well standardised. These sera are obtained from individuals who have been immunised during pregnancy or transplantation. In many cases, these individuals produce multispecific antibodies of little practical value. A small number however, produce an operationally oligo or monospecific antiserum which is of practical value for tissue typing. In Perth, it has been found that it is necessary to screen several thousand potential typing sera to obtain approximately 50 useful sera, and of these only about ten reach international standard. Laboratories within the Australasian Region are dependent upon mutual collaboration for obtaining acceptable standards of tissue typing.3 It should be emphasised that these typing sera are operationally defined. They react with the most immunogenic determinant present on the HLA antigen glycoprotein, and their pattern of reactivity does not reflect complete structural detail of the immunising molecule. It is probably for this reason that antisera which initially appeared homogeneous have subsequently been shown to be heterogeneous in their patterns of reactivity. For example, the previously well defined HLA antigens A9 and A10 have now been shown to consist of at least two immunoreactive components designated Aw23, 24 and Aw25, 26 respectively. Very recently, the antigen Bw17 has been “split”. It is quite possible that antigens at present considered homogeneous, for example B27, will eventually be split also. HLA antigens expressed at the D locus (LD antigens) have previously been detected by the mixed lymphocyte culture reaction. However, it is now clear that they can also be detected by serological means so long as B lymphocytes are used as the target cell. Separation of T and B lymphocytes can now be achieved by rosette sedimentation and most laboratories perform this technique r o ~ t i n e l y . ~ The allelic forms of HLA antigens expressed at each of the four loci is shown in Table I . Antigens recognised internationally are designated by a number following the locus designation. Anti-
1,
SUPPLEMENT
8
VOL.
TABLE I HLA antigens* A locus -~ ___
c locus
B locus
D locus
~~-
HLA-A1 A2 A3 A9 A10 A1 1 A28 A29 Aw23 AwZ4 Aw25 Aw26 Aw30 Aw31 Aw32 Aw33 Aw34 Aw43
HLA-B 87 B8 B12 B13 B14 B18 B27 Bw15 Bw16 Bw17 BwZl Bw22 Bw35 Bw37 Bw38 Bw39 Bw4O Bw4 I Bw42 TT KSO Hs
HLA-Cwl CW2 cw3 cw4 cw5
~~
HLA-Dwl Dw2 Dw3 Dw4 Dw5 Dw6
T7 LD107 LDI08
*Identifiable by antisera presently available. The 19 A locus antigens account for approximately 95”,, of the A and B locus genes. The more recently defined C and D loci antigens account for about half the alleles at these loci.
gens in the process of being defined internationally are designated by a “w”. The phenotypic and gene frequencies for HLA-A and B antigens are shown in Table 2. TABLE 2 Phenotypic and gene frequencies of HLA-A and B locus an:igens in an Australian population* HLA-A antigen
Phenotypic frequency
(”,J ~~
Al A2 A3 A9 A10 All A19 S28
33 8 52-6 26.1 17.9 8.3 7-2 14 1 2 4
*BusselLon, WA in
=
Gene frequency
HLA-B antigen
Phenotypic frequency (‘“J
-~
0.1866 Obi116 0 1405 n ,0940 0 0426 0.0367 0-0734 0 0121
Gene
frequency ~~
B5 87 B8 B12 813 B14 Bw15 Bwl6 Bw17 B18 Bw2l Bw22 B27 Bw35 Bw40 KSO
8.3 26 0 25 6 33.0 4.3 5.1
9 6 3 9 7.8 8. I 3.1 3-9 6.I 11.6
10.4 0.09
2745) from Hawklns. B R (unpublished)
n 0424 n- 1396 0.1377 0.1815 0.0219 0-025s 0 0493 0 0195
wo4on 0-0411 0-0158 0.0199 0.0311 0.0595 0-0535 0.0044
~
JUNE
1978
MAJOR HISTOCOMPAT 'IBILITY COMPLEX IN MAN
TABLE 3 Tissue distribution of HLA-A. B. C. D antigenst Tissue
HLA-A, B, C
~
H LA-D ~
~
...
+++ +++ +++ +++ +++ ++
T lymphocytes R lymphocytes Macrophages Epidermal cells Endothelial cells Spermatozoa Erythrocytes Platelets Liver Brain
-
*
+++ +++ ++ + ++
I
-
i t
+
i
*Not well expi-essed in resting T lymphocytes but can be expressed in activated lymphocytes. ?Modified from Wernet (1976).
Tissue distributions of HLA-A, B, C antigens as opposed to HLA-D antigens are shown in Table 3." The tnost important distinction between the A, B, C antigens and the D antigens is the lack of D antigens on platelets and resting T lymphocytes. A practical application of this difference is the use of platelets to absorb antibodies reactive with A, B and C locus INHERITANCE
OF
HLA
ANTIGENS
FIGURE 2. Mode of inheritance of HLA antigens. Genotypes of Down family. "Haplotype"-the combination of antigens determined by the M H C of a single chromosome. The "genotype" of an individual is made up of t w o haplotypes. The "phenotype" is the combination of antigens expressed by an individual. It can be seen that the mother had six specificities detected and has the same phenotype and genotype (A2, 3; 87, 12; Dw2, w4). Her eldest daughter inherited A3; 87; D w 2 and the A2; 87; D w 2 haplotype from the father so that only four specificities were detected (phenotype A2, 3; 87; Dw2). Because of the information provided by the family study the genotype can be surmised, 1.e. A2, 3: 9. 7, 7; Dw2. w2. Note that the three daughters are H I A identical whereas the elder son is quite different. The younger son shares a haplotype with his siblings. The possible genotype A2, 2; 87, 12; Dw2, w 4 is not represented.
1 I
27
antigens. In this way, useful typing sera defining D antigens can be obtained, The mode of inheritance of HLA antigens is illustrated in Fig. 2. It can be seen that each individual inherits a particular HLA-A, B, C and D antigen from each parent. Antigens from each parent are expressed so that the genes responsible are considered to be co-dominant. As shown in the family, the M H C is inherited as a whole, although occasionally crossovers occur so that an individual may inherit HLA-A and C antigens from one paternal chromosome but HLA-B and D antigens from another paternal chromosome. The presence of other loci on chromosome VI allows such crossovers to be mapped. It can be seen from this figure that one parent has genes for A3, B7 and Dw2 expressed on one chromosome. From population studies it has been found that these antigens are found more frequently together than would be expected from their frequency in the population as a whole. Such a haplotype is often referred to as a superhaplotype and exhibits the phenomenon of linkage disequilibrium. There are a number of instances of linkage disequilibrium within the major histocompatibility complex and their presence is thought to indicate a survival advantage of particular haplotypes. Structure of H L A Antigens It is possible to extract HLA glycoproteins from cell membranes with the aid of detergents and specific antisera. The tertiary structure of these molecules and their amino acid sequences can then be ' Murine histocompatibility antigens have been analysed in detail but some information on the composition of human HLA antigens has been obtained. The HLA-A and B antigens have been found to consist of two non-covalently linked polypeptide chains of molecular weight 12,000 and 44,000 daltons respectively. The postulated orientation of these polypeptides in the cell membrane is shown in Fig. 3. The smaller polypeptide (12.000 daltons; p12) has been identified as p2 microglobulin-a molecule shown to share amino acid sequence homology with some domains of immunoglobulin molecules. It has been demonstrated that the synthesis of p2 microglobulin is controlled by a gene on
28
WETHERALL AND DAWKINS OF HLA-A AND B A N T i G E d S
STRUCTURE
SUPPLEMENT
COMPOSITION
1,
O F T I i E MOUSE MHC
VOL.
8
(H-2)
N
I
-
ALLOANTIGENIC HEAVY (44003 ? k L ) CHAIN dal f m 5 ; ~
CARBOHYDRATE
[
~
~
~
~
‘
~
~
~
~
~ ~ ~ ~ CHROMOSOME X Vp I I
,
2
~
-
/$
s
NON COVALENT BONDING
HY D R W H O B IC
C E L L MEMBRANE
(I
1
CENTROMERE
H-2K
CELL IllTERlOR
FIGURE 3. After Springer and Strominger.’ A schematic diagram showing the postulated tertiary structure of HLA-A and B locus antigens and their arrangement relative to the cell membrane. The positions of the carbohydrate residues and 8, microglobulin relative to the alloantigenic chain are not known. 8, microglobulin synthesis is determined by a gene on chromosome XV. The structure of C locus antigens has not been investigated. D locus antigens are known to have different structures (reference 4).
chromosome XV. The heavier polypeptide chain (44,000 daltons; p44) is the glycoprotein which exhibits the allogenic antigenic HLA specificities. The p12 polypeptide chain, although associated with the p44 glycoprotein, is not thought to contribute any amino acid residues to the antigenic specificity of the bimolecular complex. The p44 chain extends through the cell membrane where the internal portion makes contact with the cytoskeleton. The region of the molecule associated with the cell membrane is hydrophobic whereas the remainder of the molecule is primarily hydrophilic. Some carbohydrate is associated with p44 chain, but does not contribute to the antigenic specificities. Recently, preliminary amino acid sequencies for both the p12 and p44 chain have been obtained. Apart from a surprising degree of similarity between the HLA antigens and the analogous murine antigens, the p44 chain shows some degree of homology with one of the variable regions in human immunoglobulin gamma chains. This homology raises some interesting questions in terms of T cell receptors for antigen and the regulation of immune responses.
;ipl-
s
#
region
H-2G ti-2D
1:
FIGURE 4. A schematic diagram of the mouse H - 2 region (MHC). The H-2 region is bounded by the D and K regions (analogous to HLA-A and B respectively). The I region (immune region associated antigens) includes five subloci which determine la antigens and contain genes controlling immune responsiveness and immune suppression. The S region contains at least two genes one of which determines the structure of complement component C2. There is also evidence that the synthesis of other complement components is governed by genes within the MHC. The H-2G locus determines antigens expressed on mouse erythrocytes. The thymus leukaemia (TL) region governs the expression of antigens on thymic and leukaemic lymphocytes.
Future Directions The availability of a large number of strains of inbred mice has made possible a more complete description of the Major Histocompatibility Complex in this species (H-2 The composition of the H-2 system is shown in Fig. 4.
JUNE
1978
MAJOR HISTOCOMPATIBILITY COMPLEX IN MAN
The I region of the mouse MHC has been shown to be responsible for determining immune region associated antigens, that is Ia antigens, and patterns of immune responsiveness in this species. The postulated immune respone genes (Ir genes) located within the I region appear to determine whether an individual mouse can respond to certain antigens. However, it would appear that the ability to mount a complete immune response especially to more complex antigens is dependent upon multiple genes some ofwhich are not within the MHC region and may occur on other chromosomes. Analogous immune response genes may occur in man but have not been unequivocally demonstrated to date. Some believe HLA-D represents the human I a 3 It would seem that certain patterns of immune response are associated with particular H L A specificities and this may be explained in terms of linkage disequilibrium between HLA and immune response genes, although other explanations are possible. Extrapolation from mouse to man has helped to define important questions but should be accepted with some caution since the murine system has some features which may be unique to this species. Work on the mouse has shown that the H-2 '
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system determines the susceptibility of a virus infected target cell to killing by cytotoxic T lymphocytes.' Recent work has suggested that antigens coded for by the murine MHC may be involved in antigen recognition.' The implications for human disease remain to be determined. On the other hand there can be little doubt that the association between particular HLA specificities and some diseases will be explained in terms of antigen receptors, recognition and patterns of immune responsiveness. References 1. BACH, F H. and Vah R(ivn, J. J (1476): The major histocompatibility compler. NPM.€ n f / . J. M t d 295. ROh. 428
2
3. 1
5. b.
7. 8
3.
S A s A m h i , T.. MCDEVIIT.H.0. and GRI'MET, F. C . 11977): lheassoctation between genes in the major histricornpatibility complex and dised3e susceptibility, 4 n n . rc'i' M e d 28, 42.5 DAWKINS,R. L. and W E T H L R A I S , J . D (1977): The Australasian Tissue Typing Workahop. University of Western Australia. Perth. WERWT. P. (IY76). H u m m lo-type a i l o a n t l p m Methods of defection. J > ~ C C ~ofS chernihtrq and biology, markers for disease stater. Tiun$plunl. Rci, 36,27 I. PEILKSi>N. P. A , , RASK, L., SEtiL, I
