Rev. sci. tech. Off. int. Epiz., 1998,17 (1), 95-107 · Rev. sci. tech. Off. int. Epiz., 1998,17 (1), 95-107 Porcine major histocompatibility complex ... with the standar d haplotype

Rev. sci. tech. Off. int. Epiz., 1998,17 (1), 95-107

Porcine major histocompatibility complex M. Vaiman P. C h a r d o n ( 2 ) & M.F. Rothschild

(1) L a b o r a t o i r e de r a d i o b i o l o g i e e t d ' é t u d e du g é n o m e . C o m m i s s a r i a t à l ' éne rg ie a t o m i q u e / I n s t i t u t n a t i o n a l de la r e c h e r c h e a g r o n o m i q u e , D i r e c t i o n des Se rv i ces v é t é r i n a i r e s . D é p a r t e m e n t d e r a d i o b i o l o g i e e t d e r a d i o p a t h o l o g i e , 7 8 3 5 0 J o u y - e n - J o s a s , France

(2) L a b o r a t o i r e m i x t e de r a d i o b i o l o g i e a p p l i q u é e , I n s t i t u t n a t i o n a l de la r e c h e r c h e a g r o n o m i q u e / C o m m i s s a r i a t à l ' é n e r g i e a t o m i q u e . D i r e c t i o n g é n é r a l e d e l ' a l i m e n t a t i o n . D é p a r t e m e n t de g é n é t i q u e a n i m a l e . D o m a i n e de V i l v e r t , 7 8 3 5 0 J o u y - e n - J o s a s , F rance

(3) N a t i o n a l Pig G e n o m e C o o r d i n a t o r , D e p a r t m e n t o f A n i m a l S c i e n c e , I o w a S t a t e U n i v e r s i t y , 2 2 5 K i l dee H a l l , A m e s , l o w a 5 0 0 1 1 - 3 1 5 0 , U n i t e d S t a t e s o f A m e r i c a

The major histocompatibility complex in swine (swine leucocyte antigen: SLA) is located on chromosome 7 with the class I and class III regions separated by the centromere from the class II region. The overall molecular organisation of the class I and III regions is well known, but further research is needed to establish that of the class II region. Approximately sixty genes have been characterised to date, including ten tightly packed SLA class I sequences. The exact number of functional polymorphic class I genes, as defined by serology, probably varies from one to four, depending on the haplotype. At least two other distantly class I-related gene families exist. The numerous and significant associations reported between SLA haplotypes and physiological traits are described. These traits include immune responsiveness to a variety of microbes and metazoan parasites, and male and female production and reproduction performance. The results obtained suggest that selection for specific SLA haplotypes may assist in the improvement of porcine production.

Keywords Genetics - Immune response - Major histocompatibility complex - Maps - Production traits - Swine.

Summary

The swine major histocompatibility complex (MHC), named SLA (swine leucocyte antigen) was identified in 1970 ( 7 6 , 8 1 ) , several decades after identification of the mouse H-2 complex (22) and the human HLA (human leucocyte antigen) system (12). Early experiments emphasised the role of MHC as a strong histocompatibility system, while serological tests revealed both a high level of polymorphism and genetic complexity. At present, the molecular organisation of the SLA complex is partly unravelled and represents the best-defined genetic region in swine. The MHC specialised class I and class II proteins are essential in the development and control of specific immune responses. Other genes of the MHC region are involved in the non-specific branch of the immune defence system, such as the C2-Bf and C4 complement components. Similarly, the tumour necrosis factor (TNF) gene family plays a prominent role in these mechanisms as well as in the development of the lymphoid tissues (46) . Finally, the

Introduction SLA region has been shown to affect a number of biological characteristics including productive and reproductive performance, thus the SLA region is of great importance for selection in the swine industry.

Chromosomal map of the swine leucocyte antigen complex The SLA complex was mapped near the centromere of chromosome 7 (19, 52) : the SLA class I and class III regions were assigned to the 7 p l . l band on the short arm while the SLA class II region was assigned to the 7 q l . 1 band on the long arm (73) . The current view of SLA region is shown diagrammatically in Figure 1. This diagram is based on serological, genetical and molecular biological results obtained in National Institutes of Health (NIH) Miniature swine and swine from commercial breeds, by pulse field gel electrophoresis (PFGE), cosmids and yeast artificial

96 Rev. sci. tech. Off. int. Epiz., 17 (1)

Fig. 1

Ch

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of th

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leuco

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Rev. sci. tech. Off. int. Epiz., 17 (11 97

chromosomes (YAC) alignments (9, 50 , 84 ) . The comparison with the human HLA region is also shown. The approximate size of the SLA complex without the centromeric structure covers about 2 megabase (Mb) of DNA, which equals half of the HLA region.

The swina leucocyte antigen class I region Current understanding of the overall physical organisation of the SLA complex is a result of the extensive use of molecular biology techniques. It is nevertheless important to emphasise the historical as well as the practical importance of the SLA class I serology, the understanding of which has led to knowledge of the SLA region. Furthermore, SLA class I serology still remains a powerful, quick and inexpensive tool for analysing large numbers of individuals.

Serological comparison tests The anti-SLA reagents used in serological comparison tests are produced essentially by full thickness dermo-epidermal allografts or injections of peripheral blood leucocytes, lymph node or spleen cells. Following continuous exchanges of reagents and information between a limited number of laboratories, the first official international comparison test was held in 1986. The joint report issued in 1988 presented the results of the comparison of 157 selected alloantisera tested on lymphocytes from 2 6 4 unrelated Landrace or Large White pigs (55). Among the 31 SLA specificities characterised, eighteen (designated W l to W 1 8 ) were defined internationally. Most of these specificities were tentatively assigned to one of the three class I series which are presumed to exist and were based on results from serological, genetical and biochemical studies.

In addition to the conventional allo-anti-SLA reagents, a number of monoclonal antibodies have been produced. In general, these were shown to recognise monomorphic determinants common to most class I molecules or semi-public specificities shared by several SLA class I molecules (29) . In fact, monoclonals capable of identifying SLA private specificities have not been produced so far.

The swine leucocyte antigen haplotype chart and polymorphism A haplotype consists of a combination of alleles of genes located on the same chromosome. Analysis of more than 5 0 0 SLA serologically informative families revealed the repeated occurrence of at least 68 haplotypes in numerous swine commercial breeds world-wide. An updated version of the previous SLA haplotype chart is given in Table I. The NIH Miniature a haplotype was found to share class I specificities with the standard haplotype H10, while the d haplotype

resembled the haplotype H04. The NIH c haplotype did not correlate with any of the haplotypes of the chart. Unlike other haplotypes, the SLA H04 haplotype was found in a majority of breeds and is considered to be one of the ancestral haplotypes in swine. Furthermore, there is clear haplotype-to-haplotype variation in the number of SLA class I genes expressed. Although most SLA haplotypes code for at least two or three distinct class I molecules, some haplotypes appear to control one or even four - and exceptionally five - SLA class I molecules.

The overall level of swine MHC polymorphism is not known, as SLA tests have been carried out thus far on about twenty breeds out of the 3 0 0 or more breeds which exist throughout the world.

The genomic swine leucocyte antigen class I region Like the MHC class I genes in other species, the SLA class I molecules consist of a heavy glycoprotein chain of about 45 kiloDaltons (kDa) which is non-covalently associated with the swine |32-microglobulin, a polypeptide of 11.7 kDa. The heavy chain is a transmembrane molecule, encoded within the SLA complex, which expresses all the polymorphisms observed. The -microglobulin is monomorphic and is coded for by a gene which has been mapped to the chromosome l q l . 7 region (59) .

The SLA class I classical molecules are expressed ubiquitously although there is wide variation in the level of expression among tissues and cells.

Number of swine leucocyte antigen class I genes The first SLA functional class I gene to be characterised, designated PD1, was recovered in 1982 from a NIH Miniature pig homozygous for SLA haplotype d genomic library (72) . Further screening of dd genomic libraries in phage and cosmid led to the characterisation of six more non-allelic SLA class I sequences named PD14, PD7, PD6, PD15, PD8 and PD4 (16) . Thus, a total of seven class I sequences were found to be present on haplotype d. More recenüy, a laboratory in Germany has identified seven to nine class I genes in a different miniature swine breed by screening a cosmid library with an HLA cDNA (complementary DNA) probe (68) .

Restriction fragment length polymorphism (RFLP) analyses by Southern blotting of the SLA class I genes in both the NIH Miniature lines (72) and in commercial breeds in the United States of America (USA) and Europe revealed a limited number of bands compatible with the cloning findings (8, 69) . Thus, the swine carries a significantly lower number of class I sequences than rodents and humans, in which 20 to 4 0 class-I bands are observed, among which only a minority of bands correspond to classical class I genes.

98 Rev. sci. tech. Off. int. Epiz., 17 (1)

Table I Swine leucocyte antigen haplotype chart

C o d e n o . A

L o c i

C B C o d e N o .

A

L o c i

C B C o d e N o .

A

L o c i

C B

H01 15 1 18 H24 13 5 2 2 H47 5 3 5

H 0 2 10 14 H25 20 .8 2 H 4 8 8 31

H03 2 0 2 2 3 H26 13 19 6 H49 20 .8 1

H 0 4 13 9 H27 5 11 H50 1

HÛ5 5 21 .4 H28 13 6 H51 5 - 1 4 11

HOB 20 5 4 H29 13 H52 1 3

H07 20 .8 2 11 H30 9 H53 13 9 H06

H08 17 H31 8 16 11 H 5 4 20

H09 5 4 .4 H32 1 H 5 5 13 9 3

H10 33 12 H33 1 1 H56 5 6

H11 20 .7 2 H34 15 1 2 3 H57 20 .7 2 3

H12 13 1 3 H35 8 9 H58 20 14 2 3

H13 13 3 H36 5 -14 H59 US U N E C

H14 33 16 11 H37 27 H60 20 .8

H 1 5 2 5 H38 3 3 28 H61 39 14

H16 13 19 3 5 H39 17 16 H62 13 5 11

H17 19 H40 S B | a | H63 3 6 6

H18 5 H41 2 3 H 6 4 20 2 37

H19 6 H42 29 H65 3 8

H20 24 .13 2 11 H43 15 1 H66 8 2 5

H21 20 2 11 H 4 4 20 .7 2 11 H67 2 0 - 1 3 2 11

H22 20 .8 14 H 4 5 10 H 6 8 13 18 3 6

H23 18 .26 H46 20.7 12

a) Belgian haplotype

Structure of the swine leucocyte antigen class I gene Swine leucocyte antigen classical class I genes The genomic structure of the MHC PD1, PD7 and PD14 genes

corresponds to the canonical model found for all classical

class 1 genes, and consists of a leader sequence, three exons

encoding corresponding extracellular domains, a trans­

membrane exon and three intracytoplasmic exons. The PD1

and PD14 sequences were highly homologous in both coding

and non-coding regions, with an average homology of 8 8 %

and 8 0 % , respectively (66) . The deduced amino acid residue

differences were essentially concentrated in exon 2 and more

specifically clustered in positions 63 to 77. By contrast, the

exon 3 sequences differed by 14 substitutions scattered

throughout the whole exon. Although the PD7 gene is closely

related to PD1 and PD14, with 8 1 % and 8 5 % homology in

the coding regions, respectively, its expression level was very

low when compared to PD1 and PD14.

Swine leucocyte antigen class I divergent gene The PD6 gene represents a divergent monomorphic member

of the SLA class I family and has an overall homology of only

5 5 % with PD1 or PD14 (13). The size and the eight-exon

organisation of PD6 are similar to the other class I genes with

open reading frames for exons 1 to 6: however, PD6 has a

codon stop in exon 7. A human or mouse counterpart of

swine PD6 sequence has not been found. In vivo expression of

PD6 products could not be revealed, while PD6 transcripts

were present mainly in secondary lymphoid tissues and

preferentially in peripheral T cells as compared to B cells. The

PD6 gene was also transcribed in mouse cells and upregulated

by interferon treatment. Among the remaining sequences,

PD15 represented a pseudogene while PD4 and PD8 were not

characterised further.

Molecular organisation of the swine leucocyte antigen class I region

Pulse field gel electrophoresis has shown that the three

classical genes PD1, PD14 and PD7 are located on a 320

kilobase (kb) genomic fragment, while the PD6 and PD15

sequences could not be located more than 5 0 0 kb apart, as

quoted in Schook et al. (67). More recently, the use of

evolutionary conserved anchor genes together with SLA class I

genes permitted the construction of a contig of overlapping

YAC clones spanning about 1.2 Mb of the SLA class I region

(9) of a Large White male (Fig. 1). Within this contig there

was perfect conservation of the order of the anchor genes

between swine and humans. Actually, the conservation of the

synteny could be extended possibly far beyond the MHC

region as it includes the butyrophilin sequence, which is

Rev. sci. tech. Off. int. Epiz., 17 (1) 99

located presumably several tens of megabases away from the MHC. As shown in Figure 1, from the centromeric region onwards there are the class III pig counterpart of the HLA-B-associated transcript 1 (BATÍ) genes, at least two divergent members of the class I gene family including SLA-6, the cell growth-regulated gene S C I , the octamer transcription factor 3 gene POU5F1, the skin-associated protein gene S, the guanosine-5'-triphosphate (GTP)-binding protein gene HSR1, the ring-finger-protein gene RFB30, the myelin oligodendrocyte glyco-protein gene MOG, and finally an olfactory receptor gene cluster. Of the seven class I sequences included here, five (SLA-1, SLA-3, SLA5, SLA-9 and SLA-10) were found within a segment of less than 180 kb. Partial sequencing of these genes suggested that the SLA-2 and SLA-3 genes were probably alleles of the NIH classical PD14 and PD7 genes, respectively. The SLA-1 and SLA-10 sequences closely resembled the NIH PD1 gene, while SLA-5 and SLA-9 where slightly less homologous. The SLA-4 gene was found to contain a stop codon in exon 4. Among the two most centromeric class I-related genes were SLA-6 (which is identical to the NIH PD6 gene) and the SLA-7 gene, which appeared not to be related to any of the described sequences. Remarkably, in contrast to the anchor genes framework, the class I gene organisation and spatial location differed greatiy between humans and swine. Thus, while all functional SLA class I genes are clustered in Jess than 180 kb, the HLA classical class I genes are spread over a 2 Mb segment.

The swine leucocyte antigen class II region Swine leucocyte antigen class II serology Characterisation of SLA class II serology has met with limited success because of the existence of a wide range of class II cross-reactions which precluded its use in routine tests. A number of monoclonal antibodies produced mainly against MHC class II specificities in the mouse and other species were found to react with swine cells. An updated list of these reagents was provided recently (36) . Some of these monoclonals recognise either SLA-DR or SLA-DQ antigens, but most appeared to react with monomorphic determinants. Only two monoclonals have been reported to recognise private polymorphic determinants of swine class II antigens.

Swine leucocyte antigen class II proteins Biochemical and serological studies demonstrated the expression of SLA-DQ and DR molecules but not the swine counterpart of the human DP third series (7, 49 ) . As in other species, these molecules are heterodimers which consist of a heavy chain (alpha) with a molecular weight of approximately 34 kDa and a light chain (beta) of about 29 kDa. Both chains, which associate non-covalently, have a transmembrane and a cytoplasmic tail and are encoded by genes in the MHC complex.

SLA class II antigens are found mainly on lymphoreticular B and macrophage cells and on a significant fraction (60% to 70%) of circulating T cells. Parenchymatous cells, such as kidney cells, also express class II molecules physiologically.

Swine leucocyte antigen class II molecular analysis Southern blot analyses of swine class II genes revealed the existence of one to two DQA and DRA genes but more DQB and DRB sequences per haplotype (8, 65) . These analyses also revealed extensive cross-hybridisation between the SLA-DQB and DRB genes. Cloning experiments subsequendy demonstrated a unique DRA monomorphic gene in the NIH lines. On the other hand, the DQA gene (and especially the DQB and DRB genes) were shown to be highly polymorphic.

The predicted structure of the miniature swine DQA and of the DQB genes confirmed the overall organisation established for their counterpart genes in rodents and man. Thus, the swine DQA sequence consists of a leader peptide, two extracellular domains (exons 2 and 3) the transmembrane domain (exon 4) and the cytoplasmic tail. Domain sizes are conserved, and so the locations of cysteine residues are found at positions 111 and 167 and the two glycosylations sites at positions 82 -84 and 122-124. Similar structures were deduced from the sequences of the other swine class II genes. The comparison of the NIH swine DQB c and d alleles revealed 18 nucleotide substitutions in the first domain, and only one was silent. The remaining part of the two sequences differed by only three nucleotides, with one leading to the replacement of an amino acid in the second domain. The alignment of the allelic sequences DRB c and d revealed a total of 42 nucleotide substitutions of which twelve were located in the first extracellular domain (residues 1-94), three in the region encoding the second extracellular domain (residues 95 -188) and one in the exon encoding the transmembrane portion (residues 198-220) . Remarkably, ten of the substitutions in the first domain consisted of amino acid replacements and two were silent, as were all substitutions in the second domain. Of the ten predicted replacement substitutions, six (60%) were situated in positions corresponding to the putative antigen recognition site (24, 25 ) .

SLA class II expression and DRB and DQA exon 2 sequences have also been analysed recently in commercial breeds of swine in Norway and the USA by the reverse transcriptase-polymerase chain reaction (RT-PCR) and the PCR-RFLP techniques (70, 75) . Addition of the results obtained in these breeds to those of the NIH Miniatures gives a total of seven DQB and thirteen DRB alleles, thus confirming the polymorphism of the class II region revealed earlier by cellular tests. Preliminary results indicate that the SLA class II region may cover at least 4 5 0 kb (67) .

The swine leucocyte antigen class III region As shown in Figure 1, the genomic organisation of the SLA class III region spans about 700 kb of DNA and contains 33 characterised genes (50) . Comparison with the corresponding human region confirmed the good overall conservation of this segment of the MHC between mammal species. The biggest difference concerned the 21-hydroxylase-complement C4 component (CYP21-C4) locus, with only one CYP21 and one

100 Rev. sci. tech. Off. int. Epiz., 17 (1)

C4 gene present in swine, whereas in humans, mice and probably in ruminants species, the CYP21-C4 group underwent independent tandem duplications. Interestingly, the majority of well-documented SLA recombinants occurred within the class III region which suggests that recombination hot spots may exist in this chromosomal segment. There is limited information on the polymorphism of SLA class III genes except some RFLP data concerning the complement C4 component and CYP21. For the latter, six allelic patterns were obtained in 31 Large White pigs bearing a large panel of SLA haplotypes (20). Similarly, a dinucleotide repeat (TG) 23 close to the TNF-a locus has been isolated, which allowed the characterisation of six alleles (34).

Involvement of the major histocompatibility complex in physiology and pathological syndromes In higher vertebrate species including the chicken, laboratory rodents, farm animals and humans, the MHC region was shown to affect a variety of biological parameters more or less profoundly (30, 71) . In humans, the MHC complex has been associated significantly with disease susceptibility in various syndromes, often in conjunction with other unlinked genes (10, 14). Similarly, specific HLA haplotypes were found to increase resistance towards life-threatening infectious pathogens including bacteria, protozoa and nematodes (1, 26, 47) . In swine, investigations of the role of the SLA complex have concerned the immune responsiveness, disease resistance and associations with reproduction and production performance traits.

The swine leucocyte antigen complex and cutaneous malignant melanoma Segregation analyses of the occurrence of melanocytic lesions in the American miniature pig Sinclair line suggested a two-loci model involving an undefined major initiator gene and a second locus located within the SLA region (4, 27, 74) . One particular haplotype appeared to be necessary for the tumour initiator locus to be fully penetrant. In a herd of pigs with Sinclair origins in the Czech Republic, the reappearance of the malignant melanoma also seemed to correlate with an ancestral SLA haplotype (28) . On the other hand, segregation studies of melanocytic lesions in crosses with the Munich miniature swine Troll in Germany showed no SLA-complex influence (48).

Swine leucocyte antigen and immune responsiveness The role of the SLA complex in allograft tissues and organs has been fully assessed and therefore represents a very well characterised model for self versus non-self recognition experiments. The first evidence that the SLA complex was responsible for at least pan of the genetic control of the immune response against conventional antigens was obtained from a herd of related Large White pigs in which only the haplotypes SLA H10 and H12 were segregating. Pigs homozygous for haplotype H10 and H12 had lower primary immune responses to both low and high doses of hen egg

white lysozyme (HEWL) than heterozygous H10/H12 pigs. Challenged homozygous H12 and heterozygous H10/H12 had significantly better secondary responses than the homozygous H10 group (77) . Similar studies performed in the NIH Miniature lines by a group in the USA confirmed that the SLA complex affected the immune response against HEWL and the synthetic peptide (T,G)-A--L (37) . Thus, the SLA aa, dd and gg animals (the g haplotype is a recombinant haplotype comprising the class I genes from haplotype c and the class II region from haplotype d), were high responders for HEWL while the cc pigs were low responders. Conversely, dd- and gg-bearing pigs did not respond to (T,G)-A-—L, while aa and cc pigs produced specific antibodies.

Scientists in Canada have also performed extensive investigations of the influence of the SLA complex on a number of immune parameters against a wide range of antigens in the NIH Miniature lines, using both in vivo and in vitro tests. Overall, the conclusion of these research efforts was that the SLA complex exhibited some role (although usually a moderate one) in the immune response. Thus, in 8-week-old piglets the SLA haplotypes contributed slightly (P < 0.1) to the variation in serum immunoglobulin (IgG) concentration while the sire, dam and litter effects were predominant (44) . Pigs with the dd, dg and gg genotypes were associated with higher serum IgG levels. Similarly, the pigs bearing these haplotypes also produced more antibody to sheep red blood cells and (T,G)-A--L, while dg and gg pigs developed higher primary antibody response to HEWL (42) . The antibody avidity tests to HEWL after primary and booster immunisations revealed no significant influence of the SLA genotype, while pigs of the dd genotype had greater avidity maturation between primary and secondary responses than other genotypes (2) .

Cellular activity was measured using the delayed hypersensitivity test to bacillus Bilié-Calmette-Guérin (BCG). The challenge injection of purified tuberculin derivative 21 days after showed most marked reactions in the dd, dg and gg pigs. In contrast, hypersensitivity to dinitrochlorobenzene was lower in the dg and gg pigs compared to the other genotypes (42) .

In vitro tests of phagocytic and bactericidal action of peripheral blood monocytes of 4- and 8-week-old pigs against Salmonella Typhimurium and Staphylococcus aureus showed that the effects of the SLA haplotypes on both bacteria were generally significant (33) . Furthermore, serum agglutinating antibody titre and O-polysaccharide (O-ps) specific peripheral blood lymphocyte blastogenesis were measured following two parenteral vaccinations with an aromatic-dependent mutant of 5. Typhimurium and an oral challenge with virulent bacteria (35) . While in most cases only litter significantly influenced both parameters, the SLA complex influenced significantly the degree of O-ps specific lymphocyte proliferation six days after the second vaccination (P < 0.004) . In addition, the dd and gg homozygous and dg

Rev. sci. tech. Off. int. Epiz.. 17 (1) 101

heterozygous pigs generally behaved as a group distinct from

the other genotypes. In Yorkshire pig lines divergent for the

antibody and cell-mediated immune response, the same

authors found no apparent effect of selection for high and low

responsiveness in swine on monocyte 0 - 2 production and

SLA-DR and SLA-DQ expression (23) .

In a similar quantitative immunological testing approach

carried out in several commercial swine breeds in Germany,

significant breed differences were found for most of the traits

tested, some which were related to the MHC region (5) .

Similarly, the immune response in commercial pig breeds in

the USA to inactivated Bordetella bronchiseptica vaccine was

affected mainly by the breed and dam, while the SLA

haplotype also had some limited effect (60) . More recently, a

possible disease susceptibility and resistance pattern in

porcine reproductive and respiratory syndrome virus

associated with the SLA complex has been reported (32) .

Regarding the biology of the SLA complex in virally infected

swine, the expression in spleen of SLA class I and II antigens

was followed in Yorkshire pigs which were inoculated with

either a highly virulent or a less virulent African swine fever

virus (ASFV) isolate (21) . Spleen staining with specific

anti-SLA class I and anti class II monoclonals revealed a

general decrease of SLA molecule expression at three days

post inoculation. However, pigs inoculated with the

moderately virulent isolate showed an upregulated expression

of both SLA classes on day four (when compared to the

controls) which paralleled the recovery of the spleen

macrophage population and the major increase of spleen

T cells. As the pigs which recover from ASFV infection have

circulating virus-specific cytotoxic T lymphocytes which

readily lyse viral infected target cells upon in vitro

restimulation, the role of the SLA as a restriction element has

been investigated in infected NIH Miniature swine lines (45) .

The results were less clear than expected. Thus, although cc

effectors in general preferentially lysed cells bearing cc SLA

class 1 antigens, the restriction effect was less clear-cut with

the dd and aa effector cells, which appeared to have broader

lysis capacities.

The role of the swine MHC loci in natural resistance against

the nematode Trìchinella spiralis has also been studied in SLA

homozygous NIH Miniature lines (38) . Preliminary studies

revealed that after a low inoculum dose, swine homozygous

for the SLA haplotype c exhibited a lower burden of muscular

larvae than the dd and aa swine (40) . Although not statistically

significant, this possible resistance was correlated with the

development of an earlier antibody response and perhaps

with a higher cellular response too. When challenged by a

secondary infection, all three lines were protected, but the aa

pigs showed a significant reduction in the number of encysted

muscular larvae compared to the two other lines. Further

research revealed that just one 'a' haplotype was enough to

ensure a higher resistance level in about half (50%) of the pigs

following a challenge inoculation in comparison to 8% in pigs

without this haplotype (39) . Altogether, these results were

interpreted as evidence for the role of one gene of the SLA

region and another elsewhere in the genome.

C o m p l e m e n t act ivi ty

Despite the presence of numerous potential important genes

within the SLA class III region, no pathologies have been

related to this region so far. Evidence for the control of global

serum haemolytic complement levels by the SLA complex was

obtained by measuring complement haemolytic 50 activity in

Large White related pigs previously tested for their immune

response capability against HEWL (78) . The SLA H12

homozygous pigs and the SLA H10/H12 heterozygous pigs

displayed significantly lower haemolytic activity than the SLA

H10 homozygous animals. The role of SLA control was not

confirmed in the NIH Miniature lines (43).

T h e s w i n e l eucocy te ant igen complex and product ion p e r f o r m a n c e s

With regard to investigations carried out to assess the

influence of the SLA region on swine production

performance, some scientists concluded an absence of (or at

best a small) effect (83) . The majority of reports, however,

revealed significant associations with one or several traits. As

shown in Table II, a wide variety of productive and

reproductive traits was affected by SLA specific haplotypes as

reviewed recently (67, 82) . In particular, phenotypically

identical haplotypes may have opposite effects in different

breeds: for example, haplotype H04 and carcass fat content.

Conversely, the haplotype H12 influenced growth rate

favourably in both Large White and Meishan piglets. A

haplotype may exhibit pleiotropic effects such as the

'Landrace' haplotype SLA H23, which was found to be

strongly associated with carcass leanness and muscle malic

enzyme activity and moderately associated with ham

development.

A number of reproduction traits were also affected by the SLA

complex, including the ovulation rate, embryo development

and litter size. The latter trait is generally affected negatively in

fitters where SLA homozygous piglets are expected, though

occasionally the opposite was observed. Thus, in a Swiss

study of Landrace families, larger litters were found with SLA

haplotypes H16 and H24. In a separate study, SLA H16

homozygous sows had significantly more mummies than

non-H16 homozygous dams. Investigation of male

reproductive traits revealed that several SLA haplotypes were

associated with the genital tract development and with the

tissue androstenone level.

Conclusion A number of significant advances have been achieved recently

concerning the molecular organisation of the swine

MHC-chromosomal region. The development of new tools for

class I and above all class II allele studies is currendy

102 Rev. sci. teck Off. int. Epiz., 17 (1 |

Table II Associations between production, carcass and reproduction traits and the swine leucocyte antigen complex

Traits Breed Swine leucocyte antigen haplotypes P Reference No.

Average daily gain Duroc or Hampshire Positive or negative effects depending on the class I RFLP bands 0.05 or 0.01 31 French Large White Decreased in H06 haplotype 0.01 6

Backfat thickness French Large White Increased in H12 haplotype 0.05 B Decreased in H04 haplotype 0.01 79

French Landrace Decreased in H14 and H23 haplotypes 0.05 79 Increased in H01 haplotype 0.05 79

Belgian Landrace Increased in HO1, H14 and H16 haplotypes 0.01 80 Decreased in H10 and H64 haplotypes 0.05 80

Carcass fatness French Large White Decreased in H04 haplotype 0.01 79 French Landrace Decreased in H23 haplotype 0.001 79

Increased in H04 haplotype 0.05 79 Duroc and Hampshire Positive or negative effects depending on the class I RFLP bands 0.05 31

Ham development French Landrace Increased in H23 haplotype 0.05 79 Belgian Landrace Decreased in H01 haplotype 0.01 79 Belgian Landrace x Large White Decreased in BM37 haplotype 0.01 80

Muscle malic enzyme activity Large White x Landrace Decreased in H23 haplotype 0.001 57 Ovulation rate American synthetic line Increased in NIH a- and c-like haplotypes 0.01 61

Decreased in d-liké haplotype 0.001 61 NIH Miniature lines Increased in d haplotype 0.01 11

Embryonic development French Large White Decreased in the H02 homozygous expected conceptus 0.05 56 Mummies Netherlands Large White Increased in H16 homozygous sows 0.05 58 Litter size at weaning Swiss Landrace Decreased in H19 haplotype 0.1 18

NIH Miniature lines Decreased with a homozygous piglets 0.05 41 Litter size at birth with SLA French Large White Decreased in H02 haplotype 0.01 79 Homozygous expected piglets Danish Landrace Overall decrease 0.05 15

Swiss Landrace and Increased in H24 0.05 18 Large White Decreased in H12,H14,H19 and H16 0.05 18 NIH Miniature lines Increased in d haplotypes 0.05 11

Homozygous piglets at birth French Large White Decreased in H01 haplotype 0.05 54 Decreased in H02 haplotype 0.01 53

Birth weight Swiss Large White Decreased in H01 haplotype 0.1 17 Swiss Landrace Decreased in litter with H16 homozygous piglets 0.05 17 French Large White Increased in H12 haplotype 0.01 62

Weaning weight French Large White Increased in H12 haplotype 0.01 64 Meishan Increased in H12 haplotype 0.01 64

Segregation distortion Danish Landrace Increased In H07 0.05 51 Male genital tract development French Large White Decreased in H04 0.05 63

Increased in H15and H16 0.01 63 Tissue androstenone level French Large White Low in H10 0.05 63

High in H02 0.05 63 Meishan x Large White F2 Low in H04 0.05 3

RFLP : restriction fragment length polymorphism NIH : National Institutes of Health SLA : swine leucocyte antigen

underway and should contribute to an improvement in the precision of the tests. In addition to the class I and class II processed-peptide presenting molecules whose function is crucial in the immune response, more than fifty genes have been identified in the SLA region. These include the divergent class-I-related sequences whose functions are totally unknown, as are those of many of the remaining genes of this region. Future work should be devoted to the characterisation of some of these functions, which represents a necessary step

for a fuller comprehension of SLA region involvement in swine production performance. Nevertheless, it appears possible to use the available information even now in selection programmes. Thus, a policy of not mating parents with common SLA haplotypes might be advisable to avoid piglet deficits. Similarly, selection for some specific SLA haplotypes related to carcass traits and disease resistance may make sense in certain breeds and lines. Continued analysis of the SLA region at both the molecular level and in the context of

Rev. sci. tech. Off. int. Epiz., 17 (1) 103

production, reproduction and disease-related traits is needed

to unravel further this important complex of genes and their

functions.

Acknowledgements This article has received funding as Journal Paper

No. J . -17507 of the Iowa Agriculture and Home Economics

Experiment Station, Ames, Iowa, Project No. 3043 and has

also been supported by Hatch Act and State of Iowa funds for

M.F. Rothschild.

Le complexe majeur d'histocompatibilité des porcins M . Vaiman, P. Chardon & M.F. Rothschild

Résumé Le c o m p l e x e m a j e u r d 'h is tocompat ib i l i té des porc ins ( a n t i g è n e l e u c o c y t a i r e du

porc ; en ang la is : swine leucocyte antigen : SLA) est por té par le c h r o m o s o m e 7,

les rég ions des c l a s s e s I e t III é t a n t s é p a r é e s par le c e n t r o m e r e de la rég ion de la

c lasse I I . A lors que l 'organisat ion m o l é c u l a i r e g é n é r a l e des rég ions des c lasses I

e t III est p a r f a i t e m e n t c o n n u e , les t r a v a u x de r e c h e r c h e do iven t ê t re poursuiv is

pour m i e u x c o m p r e n d r e ce l le de la rég ion de la c lasse I I . Environ so ixante g è n e s

ont é té c a r a c t é r i s é s à c e jour, y compr is dix s é q u e n c e s SLA de c lasse 1

é t r o i t e m e n t g r o u p é e s . Le n o m b r e e x a c t des g è n e s p o l y m o r p h e s f o n c t i o n n e l s de

la c lasse 1 , te l que déf ini p a r l a s é r o l o g i e , va r ie p r o b a b l e m e n t de un à q u a t r e , se lon

l 'haplotype. Il ex is te , au moins , d e u x au t res fami l les de g è n e s a y a n t une re lat ion

lo inta ine a v e c la c lasse I. Les au teurs d é c r i v e n t les assoc ia t ions n o m b r e u s e s e t

i m p o r t a n t e s o b s e r v é e s en t re les h a p l o t y p e s SLA et les c a r a c t è r e s

phys io log iques . Ces c a r a c t è r e s c o m p r e n n e n t la qual i té de la r é p o n s e i m m u n e à

divers m i c r o b e s e t p a r a s i t e s m é t a z o a i r e s , ainsi que les p e r f o r m a n c e s en m a t i è r e

de p roduct ion et de r e p r o d u c t i o n des m â l e s et des f e m e l l e s . Les résul ta ts o b t e n u s

m o n t r e n t q u e la sé lec t ion en fonc t ion d 'hap lo types SLA s p é c i f i q u e s p e u t

c o n t r i b u e r à a m é l i o r e r la p r o d u c t i o n porc ine .

Mots-clés Caractères de production - Cartographie - Complexe majeur d'histocompatibilité -Génétique - Porcins - Réponse immune.

El complejo mayor de histocompatîbilidad del cerdo M. Va iman, P. Chardon & M.F. Rothschild

Resumen El c o m p l e j o m a y o r de h is tocompat îb i l idad del c e r d o , o an t ígeno leucoc i ta r io

p o r c i n o [swine leucocyte antigen, S L A ) , s e e n c u e n t r a e n el c r o m o s o m a 7. Las

r e g i o n e s de la c lase I y la c lase I I I e s t á n s e p a r a d a s de la reg ión de la c lase II por

el c e n t r ò m e r o . A u n q u e se c o n o c e bien la o r g a n i z a c i ó n m o l e c u l a r g e n e r a l de las

reg iones de la c lase I y la c lase I I I , es n e c e s a r i o es tud ia r con m á s pro fund idad la

de la reg ión de la c lase I I . Has ta la f e c h a se han c a r a c t e r i z a d o a p r o x i m a d a m e n t e

s e s e n t a g e n e s , e n t r e el los un c o m p a c t o a g r e g a d o de diez s e c u e n c i a s que

cod i f i can m o l é c u l a s S L A de la c lase I. El n ú m e r o e x a c t o de g e n e s pol imórf icos

f u n c i o n a l e s de la c lase I, ta l y c o m o p u e d e n c a r a c t e r i z a r s e por s e r o l o g i a , var ía

104 Rev. sci. tech. Off. int. Epiz.. 17 (1)

p r o b a b l e m e n t e , según el haplot ipo, en t re uno y cua t ro . Existen por lo m e n o s otras dos fami l ias de g e n e s l e j a n a m e n t e r e l a c i o n a d a s con la c lase I. S e e x p o n e n aquí los numerosos y signif icat ivo casos descr i tos de c o r r e l a c i ó n e n t r e hap lo t ipos del SLA y rasgos f is iológicos. Entre t a l e s rasgos f iguran la c a p a c i d a d de r e s p u e s t a inmuni tar ia a d iversos m i c r o o r g a n i s m o s y parás i tos m e t a z o o s , as í c o m o la ef ic ienc ia de producc ión de m a c h o s y h e m b r a s y el rend imien to reproductor . Los resul tados obtenidos sug ie ren que la s e l e c c i ó n de c ier tos haplot ipos c o n c r e t o s de l SLA podría resul tar útil para m e j o r a r la p r o d u c c i ó n p o r c i n a .

Palabras clave Cartografía - Cerdo - Complejo mayor de h is tocompat ib i l i dad - Genét ica - Rasgos de

producción - Respuesta inmuni tar ia .

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