Restriction in the usage of variable beta regions in T-cells infiltrating valvular tissue from rheumatic heart disease patients

Rheumatic Heart Disease (RHD) is a delayed consequence of a pharyngeal infection with group A streptococcus (GAS), usually ascribed to a cross-reactive immune response to the host's cardiac tissues. Several GAS proteins have been reported to be superantigens, also raising the possibility that T cells in RHD could be driven by superantigens. We therefore analysed the variable beta (V beta) repertoire of T cells infiltrating heart valves from chronic RHD patients undergoing elective valvular surgery. We analysed 15 valve specimens from patients with longstanding quiescent RHD and control valves from four non-rheumatic individuals. Total RNA was extracted from fresh valve tissue and employed to amplify 22 V beta genes by RT-PCR. In valvular tissue, a restricted number of only 2 to 9 V beta regions were detected as opposed to the findings in control valves. In 8 RHD valves, the expression of V beta1, 2, 3, 5.1, 7, 8, 9 or 14 was marked. These V beta regions have been related to GAS superantigens. Our results evidence the presence of a restricted set of T lymphocytes in valvular tissue from a majority of patients with chronic RHD and suggest that valvular sequelae in these patients might be related to a local antigen or superantigen driven inflammatory process that persists even many years after the initial triggering event.

Mhc class I genes of swordtail fishes, Xiphophorus: variation in the number of loci and existence of ancient gene families

Swordtail fishes and platies in the genus Xiphophorus (order Cyprinodontiformes, Teleostei) encompass 22 closely related species which are the products of a recent adaptive radiation in the streams of Central America. To investigate the evolution of the major histocompatibility complex (Mhc) genes in the period immediately following speciation, the class I genes from 20 of the 22 species were cloned and characterized by sequencing. The analysis revealed the existence of multiple loci (at least seven in some individuals) whose numbers vary among the different species and probably also among individuals of the same species. The variation does not seem to bear any relationship to the taxonomy of the genus. Genes at the different loci are distinguished by their intron sequences and by the presence of characteristic motifs in exons 2 and 3. The variation in copy number of loci may have been effected in part by unequal crossing over occurring between introns of misaligned closely related genes. The sequences of the genes fall into two groups, A and B, which represent ancient lineages. The groups define two families of loci, which diverged from each other an estimated 85 million years ago, before the separation of the Acanthopterygii from the Paracanthopterygii of the advanced bony fishes. Evolution of the genes within each family can be explained by the birth-and-death process driven by gene duplications and mutational differentiation.

Mhc class II B gene evolution in East African cichlid fishes

A distinctive feature of essential major histocompatibility complex (Mhc) loci is their polymorphism characterized by large genetic distances between alleles and long persistence times of allelic lineages. Since the lineages often span several successive speciations, we investigated the behavior of the Mhc alleles during or close to the speciation phase. We sequenced exon 2 of the class II B locus 4 from 232 East African cichlid fishes representing 32 related species. The divergence times of the (sub)species ranged from 6,000 to 8.4 million years. Two types of evolutionary analysis were used to elucidate the pattern of exon 2 sequence divergence. First, phylogenetic methods were applied to reconstruct the most likely evolutionary pathways leading from the last common ancestor of the set to the extant sequences, and to assess the probable mechanisms involved in allelic diversification. Second, pairwise comparisons of sequences were carried out to detect differences seemingly incompatible with origin by nonparallel point mutations. The analysis revealed point mutations to be the most important mechanism behind allelic divergences, with recombination playing only an auxiliary part. Comparison of sequences from related species revealed evidence of random allelic (lineage) losses apparently associated with speciation. Sharing of identical alleles could be demonstrated between species that diverged 2 million years ago. The phylogeny of the exon was incongruent with that of the flanking introns, indicating either a high degree of convergent evolution at the peptide-binding region-encoding sites, or intron homogenization.

Nonlinkage of major histocompatibility complex class I and class II loci in bony fishes

In tetrapods, the functional (classical) class I and class II B loci of the major histocompatibility complex (Mhc) are tightly linked in a single chromosomal region. In an earlier study, we demonstrated that in the zebrafish, Danio rerio, order Cypriniformes, the two classes are present on different chromosomes. Here, we show that the situation is similar in the stickleback, Gasterosteus aculeatus, order Gasterosteiformes, the common guppy, Poecilia reticulata, order Cyprinodontiformes, and the cichlid fish Oreochromis niloticus, order Perciformes. These data, together with unpublished results from other laboratories suggest that in all Euteleostei, the classical class I and class II B loci are in separate linkage groups, and that in at least some of these taxa, the class II loci are in two different groups. Since Euteleostei are at least as numerous as tetrapods, in approximately one-half of jawed vertebrates, the class I and class II regions are not linked.

Ancient allelism at the cytosolic chaperonin-alpha-encoding gene of the zebrafish

The T-complex protein 1, TCP1, gene codes for the CCT-alpha subunit of the group II chaperonins. The gene was first described in the house mouse, in which it is closely linked to the T locus at a distance of approximately 11 cM from the Mhc. In the zebrafish, Danio rerio, in which the T homolog is linked to the class I Mhc loci, the TCP1 locus segregates independently of both the T and the Mhc loci. Despite its conservation between species, the zebrafish TCP1 locus is highly polymorphic. In a sample of 15 individuals and the screening of a cDNA library, 12 different alleles were found, and some of the allelic pairs were found to differ by up to nine nucleotides in a 275-bp-long stretch of sequence. The substitutions occur in both translated and untranslated regions, but in the former they occur predominantly at synonymous codon sites. Phylogenetically, the alleles fall into two groups distinguished also by the presence or absence of a 10-bp insertion/deletion in the 3' untranslated region. The two groups may have diverged as long as 3.5 mya, and the polymorphic differences may have accumulated by genetic drift in geographically isolated populations.

Fish trypanosomes: their position in kinetoplastid phylogeny and variability as determined from 12S rRNA kinetoplast sequences

Fish trypanosomes have traditionally been classified according to the host species from which they were isolated, each isolate being regarded as a distinct species. To test the soundness of this practice, the genetic variabilities of the kinetoplast 12S rRNA-encoding genes of different fish trypanosomes isolates were compared. The DNAs were extracted from trypanosomes cloned from blood samples of 15 donors representing ten different fish species in four orders from waters of three major river systems of Central and Northern Europe. Comparison with other trypanosomatid sequences revealed that the fish trypanosomes form a monophyletic group with Trypanosoma brucei as a sister group. Pairwise comparisons of genetic distances yielded a wide range of continuous variation with no indication of any discontinuities attributable to barriers to gene flow. The genetic distances did not correlate with either the identity of the host species or geography. The host specificity of fish trypanosomes appears to be limited.

Phylogeny of Darwin's finches as revealed by mtDNA sequences

Darwin's finches comprise a group of passerine birds first collected by Charles Darwin during his visit to the Galápagos Archipelago. The group, a textbook example of adaptive radiation (the diversification of a founding population into an array of species differentially adapted to diverse environmental niches), encompasses 14 currently recognized species, of which 13 live on the Galápagos Islands and one on the Cocos Island in the Pacific Ocean. Although Darwin's finches have been studied extensively by morphologists, ecologists, and ethologists, their phylogenetic relationships remain uncertain. Here, sequences of two mtDNA segments, the cytochrome b and the control region, have been used to infer the evolutionary history of the group. The data reveal the Darwin's finches to be a monophyletic group with the warbler finch being the species closest to the founding stock, followed by the vegetarian finch, and then by two sister groups, the ground and the tree finches. The Cocos finch is related to the tree finches of the Galápagos Islands. The traditional classification of ground finches into six species and tree finches into five species is not reflected in the molecular data. In these two groups, ancestral polymorphisms have not, as yet, been sorted out among the cross-hybridizing species.

cDNA sequence coding for the alpha'-chain of the third complement component in the African lungfish

cDNA clones coding for almost the entire C3 alpha-chain of the African lungfish (Protopterus aethiopicus), a representative of the Sarcopterygii (lobe-finned fishes), were sequenced and characterized. From the sequence it is deduced that the lungfish C3 molecule is probably a disulphide-bonded alpha:beta dimer similar to that of the C3 components of other jawed vertebrates. The deduced sequence contains conserved sites presumably recognized by proteolytic enzymes (e.g. factor I) involved in the activation and inactivation of the component. It also contains the conserved thioester region and the putative site for binding properdin. However, the site for the interaction with complement receptor 2 and factor H are poorly conserved. Either complement receptor 2 and factor H are not present in the lungfish or they bind to different residues at the same or a different site than mammalian complement receptor 2 and factor H. The C3 alpha-chain sequences faithfully reflect the phylogenetic relationships among vertebrate classes and can therefore be used to help to resolve the long-standing controversy concerning the origin of the tetrapods.

Independent duplications of Bf and C3 complement genes in the zebrafish

As part of an ongoing project aimed at the characterization of the MHC system in bony fishes, we have attempted to identify the class III region in the zebrafish (a teleost), a region that in higher vertebrates contains genes coding for complement proteins C2, Bf and C4. We obtained several genomic PAC clones by hybridization with a zebrafish Bf probe, previously identified in our laboratory, and searched these for the presence of other class III genes. We were able to obtain a second Bf-like gene, however, we were unable to detect any C2- or C4-like genes. By using highly degenerated primers, we extended our search to a hepatopancreas cDNA library and amplified from it clones corresponding to three different C3-like genes, and also the Bf genes, but not any C2- or C4-like genes. The zebrafish therefore contains two Bf and three C3 loci but apparently no C2 and C4 loci. Independent duplications of the Bf and C3 genes in bony fishes suggest that complement plays a prominent part in the immune response of this class of vertebrates.

Identification and characterization of amelogenin genes in monotremes, reptiles, and amphibians

Two features make the tooth an excellent model in the study of evolutionary innovations: the relative simplicity of its structure and the fact that the major tooth-forming genes have been identified in eutherian mammals. To understand the nature of the innovation at the molecular level, it is necessary to identify the homologs of tooth-forming genes in other vertebrates. As a first step toward this goal, homologs of the eutherian amelogenin gene have been cloned and characterized in selected species of monotremes (platypus and echidna), reptiles (caiman), and amphibians (African clawed toad). Comparisons of the homologs reveal that the amelogenin gene evolves quickly in the repeat region, in which numerous insertions and deletions have obliterated any similarity among the genes, and slowly in other regions. The gene organization, the distribution of hydrophobic and hydrophilic segments in the encoded protein, and several other features have been conserved throughout the evolution of the tetrapod amelogenin gene. Clones corresponding to one locus only were found in caiman, whereas the clawed toad possesses at least two amelogenin-encoding loci.

Cloning of major histocompatibility complex (Mhc) genes from threespine stickleback, Gasterosteus aculeatus

The threespine stickleback Gasterosteus aculeatus is an important model in evolutionary and ethologic studies. Its utility would greatly be increased by the availability of molecular markers distinguishing individuals and populations. Such markers can be provided by the major histocompatibility complex (Mhc) genes, which are well known for their extensive polymorphism. In the present study, both class I and class II B Mhc genes have been identified and sequenced. Fifteen distinct class I exon 2 and exon 3 sequences were obtained and assigned to 12 loci on the basis of intron 2 length differences. Some of the loci appear to be related to class I loci identified previously in cichlid fish. The intron 2 sequences and insertions/deletions in exon 2 group the loci into three families (with one family divided further into two subfamilies) derived from different ancestral genes. The ancestors presumably diverged from one another before the divergence of Gasterosteiformes from Perciformes. The 12 distinct class II B sequences may be derived from six loci, which are, however, closely related to one another in both exonic and intronic parts and may have diverged from a single common ancestor after the divergence of Gasterosteiformes from Perciformes. The intron 2 of some of the class I genes contains two microsatellites that can be used as markers, in addition to the polymorphism of the Mhc genes in their exonic regions.

Linkage relationships and haplotype polymorphism among cichlid Mhc class II B loci

The species flocks of cichlid fishes in the Great East African Lakes are paradigms of adaptive radiation and hence, of great interest to evolutionary biologists. Phylogenetic studies of these fishes have, however, been hampered by the lack of suitable polymorphic markers. The genes of the major histocompatibility complex hold the promise to provide, through their extensive polymorphism, a large number of such markers, but their use has been hampered by the complexity of the genetic system and the lack of definition of the individual loci. In this study we take the first substantial step to alleviate this problem. Using a combination of methods, including the typing of single sperm cells, gyno- or androgenetic individuals, and haploid embryos, as well as sequencing of class II B restriction fragments isolated from gels for Southern blots, we identify the previously characterized homology groups as distinct loci. At least 17 polymorphic class II B loci, all of which are presumably transcribed, have been found among the different species studied. Most of these loci are shared across the various cichlid species and genera. The number of loci per haplotype varies from individual to individual, ranging from 1 to 13. A total of 21 distinct haplotypes differing in the number of loci they carry has thus far been identified. All the polymorphic loci are part of the same cluster in which, however, distances between at least some of the loci (as indicated by recombination frequencies) are relatively large. Both the individual loci and the haplotypes can now be used to study phylogenetic relationships among the members of the species flocks and the mode in which speciation occurs during adaptive radiation.

Polymorphism of the HLA class II loci in Siberian populations

The populations that colonized Siberia diverged from one another in the Paleolithic and evolved in isolation until today. These populations are therefore a rich source of information about the conditions under which the initial divergence of modern humans occurred. In the present study we used the HLA system, first, to investigate the evolution of the human major histocompatibility complex (MHC) itself, and second, to reveal the relationships among Siberian populations. We determined allelic frequencies at five HLA class II loci (DRB1, DQA1, DQB1, DPA1, and DPB1) in seven Siberian populations (Ket, Evenk, Koryak, Chukchi, Nivkh, Udege, and Siberian Eskimo) by the combination of single-stranded conformational polymorphism and DNA sequencing analysis. We then used the gene frequency data to deduce the HLA class II haplotypes and their frequencies. Despite high polymorphism at four of the five loci, no new alleles could be detected. This finding is consistent with a conserved evolution of human class II MHC genes. We found a high number of HLA class II haplotypes in Siberian populations. More haplotypes have been found in Siberia than in any other population. Some of the haplotypes are shared with non-Siberian populations, but most of them are new, and some represent "forbidden" combinations of DQA1 and DQB1 alleles. We suggest that a set of "public" haplotypes was brought to Siberia with the colonizers but that most of the new haplotypes were generated in Siberia by recombination and are part of a haplotype pool that is turning over rapidly. The allelic frequencies at the DRB1 locus divide the Siberian populations into eastern and central Siberian branches; only the former shows a clear genealogical relationship to Amerinds.

[Effects of bromocriptine in patients with active rheumatoid arthritis]

BACKGROUND

Neuroendocrine factors play an important role in the expression of autoimmune diseases. Prolactin (PRL) can induce T-cell proliferation and macrophage activation. Elevated PRL levels have been described in patients with rheumatoid arthritis (RA).

AIM AND METHODS

We studied immunological and clinical effects of PRL suppression in 9 RA patients with active disease, treated for 3 months with bromocriptine (BRC), an inhibitor of PRL secretion.

RESULTS

BRC induced a significant depression of the peripheral blood mononuclear cells response to antigen (p = 0.008) and mitogen (p = 0.008) which was significantly correlated with improvements in the HAQ disability index (r = 0.68; p = 0.04) and grip strength (r = 0.7; p = 0.02). Also, the in-vitro production of IL-2, nitric oxide and poliamines--that are critical for the proliferative response of lymphoid cells--decreased significantly. The group experienced significant improvement of grip strength (p = 0.028) and the HAQ disability index (p = 0.025), whereas 4 individuals achieved clinical improvement according to the American College of Rheumatology preliminary definition. We conclude that BRC treatment induces a significant depression of in-vitro immune function in RA patients and that these changes are related to parameters of disease activity. The effects of BRC on immune function and disease activity in RA patients warrant further investigation.

Complex origin of the HLA-DR10 haplotype

The region of the HLA complex occupied by the DRB genes has undergone many rearrangements in the course of primate evolution. The rearrangements have produced a number of haplotypes differing from one another in the number and composition of the DRB genes. Some of the rearrangements also affected the DRB genes themselves. Selective intron sequencing has revealed the DR10 haplotype to be composed of at least three segments, each of different origin. The haplotype carries three DRB genes (gene fragments): DRB1*10, DRB6, and DRB9. The 5' end of the DRB1*10 gene, from the promoter region to a site in intron 1 approximately 500 bp from the beginning of exon 2, is derived from a DRB1*03-like gene. The segment of the DR10 haplotype encompassing the rest of the DRB1*10 gene and extending to the region between the DRB1 and DRB6 genes is of independent origin; it diverged from other DRB genes (DRB1*01 and DRB1*03) approximately 30 million years ago. Finally, the third segment encompassing the remainder of the DR10 haplotype is derived from a DR1-like haplotype. Since the functional part of the DR10 haplotype is of independent origin, there is little justification for the currently common practice of placing the haplotype together with DR1 in the group of DR1 haplotypes. The rearrangements in the DR haplotypes may constitute one of several mechanisms for increasing diversity at the DRB loci. The region of high instability seems to be flanked by conservatively evolving regions.

Linkage of LMP, TAP, and RING3 with Mhc class I rather than class II genes in the zebrafish

The LMP2 and LMP7 genes code for subunits of the proteasome, a multimeric enzymatic complex that degrades proteins into peptides. The two subunits replace corresponding constitutively expressed subunits during the immune response. Some of the peptides generated by the proteasome in the cytosol are transported by the products of the TAP1 and TAP2 genes into the lumen of the endoplasmic reticulum and are loaded onto the assembling MHC class I molecules. In mammals, the LMP2, LMP7, TAP1, and TAP2 genes reside in the class II region of the Mhc, closely linked to the RING3 gene. In the present study we identified, cloned, and sequenced the LMP, TAP2, and RING3 genes of the zebrafish, Danio rerio. We identified variants of these genes and used them in a segregation analysis of haploid embryos derived from heterozygous mothers. The analysis revealed that in zebrafish, the LMP2, LMP7, TAP12, and RING3 loci are closely linked but, in contrast to mammals, the LMP/TAP/RING3 cluster resides not in the Mhc class II but in the class I region. We also confirmed that in the zebrafish, the class I and class II regions are not linked to each other. In this species, therefore, the LMP/TAP/RING3 genes are clustered with the class I genes on a chromosome that apparently does not contain any class II genes. The linkage of LMP/TAP/RING3/class I may be the original and the LMP/TAP/RING3/class II a derived arrangement of these genes.

Complex origin of the HLA-DR10 haplotype

The region of the HLA complex occupied by the DRB genes has undergone many rearrangements in the course of primate evolution. The rearrangements have produced a number of haplotypes differing from one another in the number and composition of the DRB genes. Some of the rearrangements also affected the DRB genes themselves. Selective intron sequencing has revealed the DR10 haplotype to be composed of at least three segments, each of different origin. The haplotype carries three DRB genes (gene fragments): DRB1*10, DRB6, and DRB9. The 5' end of the DRB1*10 gene, from the promoter region to a site in intron 1 approximately 500 bp from the beginning of exon 2, is derived from a DRB1*03-like gene. The segment of the DR10 haplotype encompassing the rest of the DRB1*10 gene and extending to the region between the DRB1 and DRB6 genes is of independent origin; it diverged from other DRB genes (DRB1*01 and DRB1*03) approximately 30 million years ago. Finally, the third segment encompassing the remainder of the DR10 haplotype is derived from a DR1-like haplotype. Since the functional part of the DR10 haplotype is of independent origin, there is little justification for the currently common practice of placing the haplotype together with DR1 in the group of DR1 haplotypes. The rearrangements in the DR haplotypes may constitute one of several mechanisms for increasing diversity at the DRB loci. The region of high instability seems to be flanked by conservatively evolving regions.

Linkage of LMP, TAP, and RING3 with Mhc class I rather than class II genes in the zebrafish

The LMP2 and LMP7 genes code for subunits of the proteasome, a multimeric enzymatic complex that degrades proteins into peptides. The two subunits replace corresponding constitutively expressed subunits during the immune response. Some of the peptides generated by the proteasome in the cytosol are transported by the products of the TAP1 and TAP2 genes into the lumen of the endoplasmic reticulum and are loaded onto the assembling MHC class I molecules. In mammals, the LMP2, LMP7, TAP1, and TAP2 genes reside in the class II region of the Mhc, closely linked to the RING3 gene. In the present study we identified, cloned, and sequenced the LMP, TAP2, and RING3 genes of the zebrafish, Danio rerio. We identified variants of these genes and used them in a segregation analysis of haploid embryos derived from heterozygous mothers. The analysis revealed that in zebrafish, the LMP2, LMP7, TAP12, and RING3 loci are closely linked but, in contrast to mammals, the LMP/TAP/RING3 cluster resides not in the Mhc class II but in the class I region. We also confirmed that in the zebrafish, the class I and class II regions are not linked to each other. In this species, therefore, the LMP/TAP/RING3 genes are clustered with the class I genes on a chromosome that apparently does not contain any class II genes. The linkage of LMP/TAP/RING3/class I may be the original and the LMP/TAP/RING3/class II a derived arrangement of these genes.

HLA-DRB9--possible remnant of an ancient functional DRB subregion

The DRB subregion of the HLA complex contains, in addition to the functional genes, a number of pseudogenes and gene fragments. Fourteen kilobases of DNA were sequenced from the segment upstream of the DRB9 gene fragment, as well as shorter segments from different HLA and corresponding ape haplotypes. The analysis of the sequences and restriction fragments indicates that the segment is a remnant of an ancient DRB subregion which may have been functional before the primate radiation and which later became the source of extant functional DRB genes in various primate groups, different ones in different groups. The remnant segment has remained constant in its organization for at least 4 million years. This constancy contrasts with the variability of the adjacent functional part of the DRB subregion occupied by the DRB1 and other loci. The constancy may be related to the monomorphism and evolutionary conservation of the DRA locus.

Polymorphism of the HLA-DRB1 locus in Colombian, Ecuadorian, and Chilean Amerinds

We have characterized the DRB1 genotypes in a sample of 64 South American Indians drawn from populations in Chile, Colombia, and Ecuador. No novel DRB1 alleles were found in the total of 17 different alleles characterized, indicating that rapid allelic generation does not occur at the DRB1 loci, in contrast to HLA-B. Comparison between Chilean and Colombian/Ecuadorian samples revealed no major differences in their allelic frequencies. In the combined Amerind sample the HLA-DRB1*0407 and HLA-DRB1*1402 alleles occurred in the highest frequencies (38% and 22%, respectively). Genetic distance measurement showed the HLA-DRB1 frequencies reported here to agree with findings in other Amerind groups. The high frequencies of both HLA-DRB1*0407 and HLA-DRB1*1602 alleles, in conjunction with their absence in Siberian samples, suggest that migratory groups other than Siberians may have been involved in the peopling of the Americas.

Class I mhc genes of cichlid fishes: identification, expression, and polymorphism

Cichlid fishes of the East African Rift Valley lakes constitute an important model of adaptive radiation. Explosive speciation in the Great Lakes, in some cases as recently as 12 400 years ago, generated large species flocks that have been the focus of evolutionary studies for some time. The studies have, however, been hampered by the paucity of biochemical markers for phylogenetic reconstruction. Here, we describe a set of markers which should help to alleviate this problem. They are the class I genes of the major histocompatibility complex. We provide evidence for the existence of at least 17 class I loci in cichlid fishes, and for extensive polymorphism of three of these loci. Since the polymorphism has a trans-species character, it will be possible to use it in investigating the founding events of the individual species. The sequences of the cichlid class I fishes support the monophyly of actinopterygian fish on the one hand, and of tetrapods on the other.

Mapping of mhc class I and class II regions to different linkage groups in the zebrafish, Danio rerio

The mammalian major histocompatibility complex (Mhc) consists of three closely linked regions, I, II, and III, occupying a single chromosomal segment. The class I loci in region I and the class II loci in region II are related in their structure, function, and evolution. Region III, which is intercalated between regions I and II, contains loci unrelated to the class I and II loci, and to one another. There are indications that a similar Mhc organization exists in birds and amphibians. Here, we demonstrate that in the zebrafish (Danio rerio), a representative of the teleost fishes, the class II loci are divided between two linkage groups which are distinct from the linkage group containing the class I loci. The beta2-microglobulin-encoding gene is loosely linked to one of the class II loci. The gene coding for complement factor B, which is one of the region III genes in mammals, is linked neither to the class I nor to the class II loci in the zebrafish. These results, combined with preliminary data suggesting that the class I and class II regions in another order of teleost fish are also in different linkage groups, indicate that close linkage of the two regions is not necessary either for regulation of expression or for co-evolution of the class I and class II loci. They also raise the question of whether linkage of the class I and class II loci in tetrapods is a primitive or derived character.

The HLA-DRB9 gene and the origin of HLA-DR haplotypes

HLA-DRB9 is a gene fragment consisting of exon 2 and flanking intron sequences. It is located at the extreme end of the DRB subregion, whose other end is demarcated by the DRB1 locus. We sequenced approximately 1400 base pairs of the segment encompassing the DRB9 locus from eight human haplotypes (DR1, DR10, DR2, DR3, DR5, DR6, DR8, and DR9, the DR4 and DR7 having been sequenced by others earlier), as well as two chimpanzee, five gorillas, one orangutan and one macaque haplotype. The analysis of these sequences indicates that the DRB9 locus, which we estimate to be more than 58 million years (my) old, has been coevolving with the DRB1 locus for the last 4.2 my. As a consequence of this coevolution, the human DRB9 alleles fall into groups that correlate with the DRB1 allelic groups and with the gene organization of the human haplotypes. This observation implies that the present-day HLA-DR haplotype groups (DR1, DR51, DR52, DR8, and DR53) were founded more than 4 my ago and have remained intact (barring minor internal rearrangements that did not recombine the DRB1 and DRB9 genes) for this period of time. The haplotypes have been transmitted during speciations from ancestral to emerging species just like allelic lineages at the DRB1 locus. Thus not only allelic but also haplotype polymorphism evolves trans-specifically.