Human leukocyte antigens (HLA) began as a list of antigens identified as a result of transplant rejection. The antigens were initially identified by categorizing and performing massive statistical analyses on interactions between blood types.Davis, Daniel M. The Compatibility Gene. How Our Bodies Fight Disease, Attract Others, and Define Our Selves. Oxford: Oxford UP, 2014. Print. This process is based upon the principle of serotypes. HLA are not typical antigens, like those found on surface of infectious agents. HLAs are alloantigens, they vary from individual to individual as a result of genetic differences. An organ called the thymus is responsible for ensuring that any T-cells that attack self proteins are not allowed to live. In essence, every individual's immune system is tuned to the specific set of HLA and self proteins produced by that individual; where this goes awry is when tissues are transferred to another person. Since individuals almost always have different "banks" of HLAs, the immune system of the recipient recognizes the transplanted tissue as non-self and destroys the foreign tissue, leading to transplant rejection. It was through the realization of this that HLAs were discovered.

Discovery

The thought that the mammalian body must have some way of identifying introduced foreign tissues first arose during World War II. It started with a plane crash in the height of the London Blitz. The pilot sustained severe burns requiring skin grafts; however, skin grafts were a risky business at the time, often being rejected for unknown reasons. Numerous theories were proposed and it wasn't until 1958 that the first of these "identifying" proteins was found.Irene Park, Paul Terasaki, Origins of the first HLA specificities, Human Immunology, Volume 61, Issue 3, March 2000, Pages 185-189 The first standardized naming system was established in 1968 by the WHO Nomenclature Committee for Factors of the HLA System."HLA Nomenclature @ Hla.alleles.org." HLA Nomenclature @ Hla.alleles.org. Anthony Nolan Research Institute, 10 Nov. 2013. Web. 08 Dec. 2013. HLA research didn't heat up until the 1980s when a group of researchers finally elucidated the shape of the HLA-A*02 protein (just one of many specific HLA proteins). Even more recently, in 2010, the WHO committee responsible for naming all HLA proteins revised their standards for naming to introduce more clarity and specificity in the naming system.

Identification of non-self

Peter Medawar was a zoologist turned clinician, who specialized in burn trauma. A plane crash near his home changed the path of his career, turning his work with burns from mere academia to a full on quest to save lives. Medawar and a Scottish surgeon, Tom Gibson, were tasked with working the Burns Unit of the Glasgow Royal Infirmary. The first insight came when the pair decided to experiment, and grafted part of a wound with the patient's skin, and another part with skin from the patient's brother. Within days the skin grafts from the brother were completely destroyed. Successive skin grafts from the brother were destroyed even faster, a fact that gave them the evidence they needed to implicate the immune system. Medawar later repeated this experiment on rabbits and 625 surgeries later validated their initial conclusions.Medawar, P. B. "A second study of the behaviour and fate of skin homografts in rabbits: a report to the War Wounds Committee of the Medical Research Council. Journal of Anatomy 1945; 79, 157-76 Medawar then set out in search of the reason why rabbits rejected non-self grafts.

Medawar continued his work, this time with a team of three at the University College London during the 1950s. Medawar's coworkers were Leslie Brent, a PhD student, and Rupert Billingham, Medawar's first graduate student at Oxford several years prior. Through carefully planned experimentation, the trio showed that mice exposed to cells of unrelated mice as fetuses did not reject skin grafts from those same mice.Billingham, R.E., Brent, L. and Medawar, P.B. "Quantitative studies on tissue transplantation immunity. iii. Actively acquired tolerance. Philosophical Transactions of the Royal Society of London B Biological Sciences 1956; 239, 357-414 For this discovery, Medawar and Australian scientist Macfarlane Burnet earned the 1960 Nobel Prize.

Learned self-tolerance

Burnet, independently of Medawar, came to the conclusion that the immune system must learn to tolerate any self cells, and hypothesized that this must occur during fetal development. For this, he jointly was awarded the Nobel Prize in 1960. Burnet's work continued and in 1957 along with Niels Jerne published a paper that modified and revolutionized antibody theory. "Burnet speculated that one cell makes one particular shape of antibody and that all our antibody-making immune cells together make an unimaginably vast repertoire of 10 billion antibodies, each having a slightly different shape".Davis, Daniel M. The Compatibility Gene. How Our Bodies Fight Disease, Attract Others, and Define Our Selves. Oxford: Oxford UP, 2014. Print. pg 34 Thus, whenever a non-self molecule appears in the human body, one of these antibodies will have an accurate enough shape to bind to that molecule. This idea is known as clonal selection theory. At the time, many leading scientists including Linus Pauling and James Watson completely rejected the idea, but repeated experimentation intended to disprove the theory actually served to build up a large body of evidence supporting Burnet and Jerne's theory.

The biggest weakness in Burnet's theory was that he had no explanation for how the body selected for immune cells that only identified non-self. In 1961, Jacques Miller published a paper offering an explanation. Miller was a PhD student at the Chester Beatty Research Institute in London. His discovery centered on the thymus. The thymus had long been regarded as nothing more than a repository for dead cells. Miller didn't buy this hypothesis. By removing the thymus of leukemic mice early in life, he found that the mice had a drastically weakened immune system. Taking inspiration from Medawar's skin transplant work, he performed a series of skin-graft experiments that showed that these immunocompromised mice didn't reject skin grafts from non-genetically identical mice. Miller then hypothesized that the thymus was essential in the construction and maintenance of the immune system. At this point Burnet came back into the picture, extending the hypothesis to specify that the dead cells found in the thymus are not any old immune cells, but instead the cells that are activated by self molecules. In other words, any cell that binds to and hence "recognizes" a self molecule is killed before exiting the thymus. These cells were later found to be one of the three types of Lymphocytes, the T-cells (named for their origin, the thymus).

Identification of the first HLAs

In 1958 Jean Dausset, Jon van Rood and Rose Payne published papers in which they described antibodies in human sera that reacted with the leucocytes from many but not all other tested individuals. In particular, Jean Dausset studied sera from patients who had received multiple blood transfusions and found seven sera that had a very similar behavior, in that they agglutinated leucocytes from 11 of 19 tested individuals. Thus he detected an alloantigen on human leucocytes which he subsequently named MAC after the initials of three important volunteers for his experiments. Antigen MAC (later known as HLA-A2) was present in approximately 60% of French population. For his discovery, Dausset received the Nobel Prize in 1980.

In the follow-up research, Jon van Rood and Rose Payne continued studying sera from multiple women who had given birth multiple times and identified more leucocyte antigens. Namely, in 1962 van Rood analyzed reaction patterns of 60 sera against leucocytes from 100 donors and detected a seemingly diallelic system of two leucocyte antigens which he called 4a and 4b (later known as HLA-Bw4 and HLA-Bw6see also ), while Payne with collaborators in 1964 detected two leucocyte antigen LA1 and LA2 (later HLA-A1 and HLA-A2), seemingly controlled by alleles, and suggested the existence of at least one additional antigen which would be controlled by an additional allele at the same gene locus. Also, multiple other investigators started identifying more leucocyte antigens around that time.

At this point the researchers all realized that the sheer quantity of data they were capable of obtaining was vastly greater than that of any previous study and so collaboration would be essential. The first international meeting, in 1964, highlighted the difficulties of such massive collaborative work. Different experimental methods and inconsistency in the execution of the same tests and a non-homogeneity of naming systems added together to make collaboration incredibly difficult.

World Health Organization steps in

In 1967 the World Health Organization (WHO) decided that the HLA research needed an official naming system. This in turn would aid in organization and would more easily facilitate the unification of data being collected at numerous laboratories across the world. This committee is still in existence today and vastly accelerated the rate of HLA research. The first meeting of this committee in 1968 set forth guidelines and rules that govern HLAs. First, compatibility genes were divided into two types, class I and class II. Class I molecules were identified via reactions between blood serum and cells. Class II molecules were identified by mixtures of white blood cells. Second, the compatibility genes were renamed Human Leukocyte Antigens (HLA). Despite this clarification and the ever-increasing number of identified HLAs, nobody knew how they worked.

MHC restriction

Late in 1973 a pair of researchers in Australia, Rolf Zinkernagel and Peter Doherty made a revelatory discovery that altered the thinking of immunologists forever. The pair was doing research on viral infections in mice and noticed that T-cells that prevented viral infections in some mice wouldn't always prevent the same infection in other mice. After looking at the MHCs present in the mice, they realized that cytotoxic T-cells could only identify virus infections in cells with the right Class I compatibility gene. Traditional thinking was that the immune system identified infections directly but this discovery turned that theory on its head. Compatibility genes were essential in immune system mediated viral clearing. The pair coined the term "MHC Restriction" to describe this relationship between T-cells, specific MHC proteins, and viral detection. In 1975, in an article in the journal Lancet, they introduced the idea of "altered self", meaning that viruses alter the MHC proteins and this alteration is detected by T-cells.Doherty, P.C. and Zinkernagel, R.M. A biological role for the major histocompatibility antigens. Lancet I, 1406-9 (1975). For their work they won the 1996 Nobel Prize. It took the work of many others to determine how T-cells made this identification.

Discovering the protein shape