- History and naming of human leukocyte antigens
In these instances, the A1/A2, A2/A3, A1/A3 are matched, decreasing the probability of a rejection because many are linked to a given haplotype. Occasionally the 'recombinant' A1-Cw7-B7(rare), B7 becomes the alloantigen in a recipient with A1-Cw7-B8(common).
This linkage disequilibrium in Europeans explains why A1, A2, A3, "A7" [B7] , and "A8" [B8] were identified, first. It would have taken substantially longer to identify other alleles because frequencies were lower, and haplotypes that migrated into the European population had undergone equilibration or were from multiple sources.
This is the genetic background against which scientists tried to uncover and understand the histocompatibility antigens.
A list of antigens created
In the late 1960s, scientist began reacting sera from patients with rejecting transplants to donor or 'third party' tissues. Their
sera(the liquid part of the blood when blood clots) was sensitized to the cells from donors - it was "alloreactive". By testing different anti-sera from recipients they were able to uncover some with unique reactivities. As a result, scientists were able to a few antigens. At first the first antigens were called the Hu-1 antigenscite journal | author = Bach FH, Amos DB | title = Hu-1: Major histocompatibility locus in man | journal = Science | volume = 156 | issue = 781 | pages = 1506–8 | year = 1967 | pmid = 4887739 | doi = 10.1126/science.156.3781.1506 ] and tentatively tagged as gene products of the Human equivalent of the mouse histocompatibility locus (H2). In 1968, it was discovered that matching these antigens between kidney donor and recipient improved the likelihood of kidney survival in the recipient.cite journal | author = Patel R, Mickey MR, Terasaki PI | title = Serotyping for homotransplantation. XVI. Analysis of kidney transplants from unrelated donors | journal = N. Engl. J. Med. | volume = 279 | issue = 10 | pages = 501–6 | year = 1968 | pmid = 4876470 ] The antigen list still exists, although it has been reorganized to fit what we have since learned about genetics, refined, and greatly expanded.
Lymphocyte bearing antigens recognized
As the study of these 'rejection'
seraand "allo"-antigens progressed, certain patterns in the antibody recognition were recognized. The first major observation, in 1969, was that an allotypic antibodies to "4" ("Four") was only found on lymphocytes, while most of the antigens, termed "LA", recognized most cells in the body.cite journal | author = Mann DL, Rogentine GN, Fahey JL, Nathenson SG | title = Molecular heterogeneity of human lymphoid (HL-A) alloantigens | journal = Science | volume = 163 | issue = 874 | pages = 1460–2 | year = 1969 | pmid = 5773111 | doi = 10.1126/science.163.3874.1460 ]
This group "4" antigen on lymphocytes would expand into "4a", "4b" and so on, becoming the "D" series (HLA-D (Class II) antigens) DP, DQ, and DR. This is an interesting history in itself.
The Hu-1 antigens were renamed the Human-lymphoid (HL) allo-antigens (HL-As). Allo-antigen comes from the observation that a tolerated protein in the donor becomes antigenic in the recipient. This can be compared with an
autoantigen, in which a person develops antibodies to one or more of their own proteins. This also suggested the donor and recipient have a different genetic makeup for these antigens. The "LA" group thereafter was composed of HL-A1, A2, A3, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14 and A15 until further divisions and renaming were necessary. Some of the antigens above, for example HL-A1, are similar to HLA-A1, as they are the same serotype. Some of the above, like A5, are not mentioned within the last few years, as they have been renamed.
During these early studies it became known that there were associations with many autoimmune diseases. And the
HLA A1-B8 haplotypeis linked to a very long piece of conserved chromosome 6 variant called AH8.1 haplotype. In these studies "HL-A1,8" were frequently found co-linked to disease. This linkage is not neccesarily a function of either gene, but a consequence of the way AH8.1 evolved.
ubclassification of lymphoid antigens
A series of tests on cultured cells revealed that, within the "LA" group, a donor tissue might have some antigens but not others. For example, an antiserum may react with patterns (on a given tissue):
* A1, A2, A7, A12
* A1, A3, A7, A8
* A1, A11, A8, A5
* A1, A8
But fail to react in the following patterns:
* A1, A2, A3, ...
* A1, A2, A11, ....
* A2, A3, A11, ....
* . . . A7, A8, A12
The HLA serotype series
If 2 members of the series (A1, 2, 3, 9, 10, 11) were typed, a reaction with a third member of the series to the donor was not observed. This 'exclusivity' identified series "A". [Bach ML, Bach FH. "The genetics of histocompatibility".(1970) Hosp. Practice 5(8): 33-44] One might notice the similarities of this numeric series with the , as series "A" antigens are the first six members of
HLA-A. Inadvertently, the scientist had discovered an antibody set that recognized only gene products from one locus,HLA-A gene the "antigens" being the gene products. The implication is that an alloreactive anti-sera can be a tool for genetic identification.
Not long after the series A antigens were separated from the (rapidly expanding) list of antigens, it was determined another group also could be separated along the same "logical" lines. This group included HL-A5, A7, A8, A12. This became the series "B".Note the similarity of Series "B" to the first few members . The names of these antigens were necessarily changed to fit the new putative series they were assigned to. From HL-A# to HLA-B#. The problem was that the literature was using "A7" and would soon be using "B7" as short hand for
Since it was now certain, by the early 1970s, that the "antigens" were encoded by different series, implicit loci, numeric lists became somewhat cumbersome. Many groups were discovering antigens. In these instances an antigen was assigned a temporary name, like "RoMa2" and after discussion, the next open numeric slot could be assigned, but not to an "A" or "B" series until proper testing had been done. To work around this problem a 'workshop' number "w#" was often assigned while testing continued to determined which series the antigen belonged to.
Before too long, a series "C" was uncovered. Series C has proved difficult to serotype, and the alleles in the series still carry the "w" tag signifying that status; in addition, it reminds us that Series C were not assigned names the same way as Series A and B, it has its own numeric list Cw1, Cw2, Cw3.
Serotypegroup expansion and refinement
By the mid 1970s, genetic research was finally beginning to make sense of the simple list of antigens, a new series "C" had been discovered and, in turn genetic research had determined the order of HLA-A, C, B and D encoding loci on the human 6p.cite journal | author = Yunis EJ, Dupont B, Hansen J | title = Immunogenetic aspects of allotransplantation | journal = Adv. Exp. Med. Biol. | volume = 73 Pt B | issue = | pages = 231–51 | year = 1976 | pmid = 136874 ] With new series came new antigens; Cw1 and 2 were quickly populated, although Cw typing lagged. Almost half of the antigens could not be resolved by serotyping in the early 90's. Currently genetics defines 18 groups.
At this point, Dw was still being used to identify DR, DQ, and DP antigens. The ability to identify new antigens far exceeded the ability to characterize those new antigens.
As technology for transplantation was deployed around the world, it became clear that these antigens were far from a complete set, and in fact hardly useful in some areas of the world (eg, Africa, or those descended from Africans). Some serotyping antibodies proved to be poor, with broad specificities, and new serotypes were found that identified a smaller set of antigens more precisely. These broad antigen groups, like A9 and B5, were subdivided into "split" antigen groups, A23 & A24 and B51 & B52, respectively. As the HL-A serotyping developed, so did identification of new antigens.
In the early 1980s, it was discovered that a restriction fragment segregates with individuals who bear the
HLA-B8serotype. By 1990, it was discovered that a single amino acid sequence difference between HLA-B44 (B*4401 versus B*4402) could result in allograft rejection. This revelation appeared to make serotyping based matching strategies problematic if many such differences existed. In the case of B44, the antigen had already been split from the B12 broad antigen group. In 1983, the cDNA sequences of HLA-A3and Cw3cite journal | author = Strachan T, Sodoyer R, Damotte M, Jordan BR | title = Complete nucleotide sequence of a functional class I HLA gene, HLA-A3: implications for the evolution of HLA genes | journal = EMBO J. | volume = 3 | issue = 4 | pages = 887–94 | year = 1984 | pmid = 6609814 | doi = ] All three sequences compared well with mouse MHC class I antigens. The Western European HLA-B7antigen had been sequenced (although the first sequence had errors and was replaced). In short order, many HLA class I alleles were sequencedincluding 2 Cw1 alleles.cite journal | author = Parham P, Lomen CE, Lawlor DA, "et al" | title = Nature of polymorphism in HLA-A, -B, and -C molecules | journal = Proc. Natl. Acad. Sci. U.S.A. | volume = 85 | issue = 11 | pages = 4005–9 | year = 1988 | pmid = 3375250 | doi = 10.1073/pnas.85.11.4005 ]
By 1990, the full complexity of the HLA class I antigens was beginning to be understood. At the time new serotypes were being determined, the problem with multiple alleles for each serotype was becoming apparent by nucleotide sequencing.
RFLPanalysis helped determine new alleles, but sequencing was more thorough. Throughout the 1990s, PCR kits, called SSP-PCR kits were developed that allowed, at least under optimal conditions, the purification of DNA, PCR and Agarose Gel identification of alleles within an 8 hour day. Alleles that could not be clearly identified by serotype and PCR could be sequenced, allowing for the refinement of new PCR kits.
Serotypes like B*4401, B*4402, B*4403, each abundant within those with B44 serotypes could be determined with unambiguous accuracy. The molecular genetics has advanced HLA technology markedly over serotyping technology, but serotyping still survives. Serotyping can help to reveal which primers for sequencing may best work for new sequences. Serotyping had identified the most similar antigens that now form the HLA subgroups.
* Lymphoid "antigens" became an experimental
artifactof medical techniques (i.e., of transplantation). Simply, as scientist gained familiarity with the human immune system they learned more about graft rejection, the cause was antibody production to proteins in donor tissue. The key word is allo - which means of different origin. 'Allo'typic proteins in 'allo'grafts developed immune responses in recipients. What makes these proteins different?
* From a more modern perspective, HLA
gene products (i.e., antigen-presenting, cell-surface receptors) did not evolve to be transplantation antigens, nor to interfere with transplantation, organ transplantation being unknown until 1960. The HLA genes are much older. Variation in HLA major antigens is the cause of transplant rejection, but variation at HLA is under preservative selection (Called heterozygous selection or balancing selection). Variation of HLA has led to an estimate that they are at least 60 million years in age for humans (DRB1).cite journal | author = Ayala FJ | title = The myth of Eve: molecular biology and human origins | journal = Science | volume = 270 | issue = 5244 | pages = 1930–6 | year = 1995 | pmid = 8533083 | doi = 10.1126/science.270.5244.1930 ] In humans, the number of HLA alleles is expanding, even with many genes, many more are still tolerable as immune presentation antigens.
The scientific problem has been to explain the natural function of a molecule, such as a self cell-surface receptor involved in
immunity. It also seeks to explain how variation developed (perhaps by evolutionary pressure), and how the genetic mechanisms works (dominant, codominant, semidominant, or recessive; purifying selectionor balancing selection).
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