Author: Gizem Tumer // Date: JUN.20.2018 // Source: Trainee Council in English
- Pathophysiology and Organ Systems,
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Hello, my name is Gizem Tumer. I am an Assistant Professor in Laboratory Medicine and Pathology and one of the medical directors of the HLA laboratory at the University of Minnesota. Welcome to this Pearl of Laboratory Medicine on “HLA: Basic Terminology and Nomenclature.”
What is the definition of HLA or Human Leukocyte Antigen?
HLA refers to a gene complex that encodes the major histocompatibility complex (MHC) proteins in humans. In humans, MHC is named HLA for Human Leukocyte Antigens because these gene products were first described as proteins expressed on leukocytes. These proteins were causing agglutination reactions when leukocytes were incubated with serum from multiparous women or people that have been transfused (similar to the erythrocyte agglutination observed on ABO typing assay). These proteins are cell surface glycoproteins expressed in almost all cells in the body. The HLA gene loci are located on the short arm of the chromosome 6.
What are the characteristic features of HLA?
HLA is highly polygenic meaning that it is composed of many genes, which broadly can be divided into three categories: Class I, Class II, and Class III.
HLA is also highly polymorphic. Polymorphism is a term that refers to the multiple variations of antigens or alleles. As an example: HLA B*27:05 vs B*27:08 are 2 distinct alleles. HLA class I and class II antigens have the most highly polymorphic structural genes found in humans. This
means that the sequence of amino acids in any given HLA molecule varies slightly from one person to the next. Literally, hundreds of different sequence variants are seen in the human population. This variation generates distinct HLA types and also causes an allograft rejection when tissues are transplanted.
It also has many functions. In other words it is Pleiotropic. The main function is the distinction of self from non-self. T lymphocytes interact with peptide antigens when Complementarity- Determining Region 3 (CDR3) of T-cell receptor (TCR) engages both the HLA molecule and the antigenic peptide present within the antigen-binding groove of the HLA molecule. Normally, TCRs recognize foreign antigenic peptides presented on the HLA molecule, but not self- antigens. TCRs can recognize peptide antigens only if they’re presented on particular MHC molecules, which is known as “MHC restriction. Thus, MHC functions as a major player in the immune response such as immune surveillance, vaccines, autoimmunity and transplantation with orchestration of T cells, B cells and macrophages.
The MHC genes that act as transplantation antigens are referred to as classical HLA genes. The classical HLA class I genes (HLA-A, -B and -C) are expressed in most of the somatic cells in the body. Classical HLA Class II genes (HLA-DR,-DQ, -DP) are expressed on antigen presenting cells such as B- cells, Activated T cells, Macrophages, Dendritic cells and Thymic epithelial cells. If interferon is present, other cell types can also express class II HLA molecules. The Class I molecule is composed of one polypeptide chain and a B2 microglobulin chain.
Whereas, the class II molecule has 2 polypeptide chains. Alpha1 and alpha2 domains form the peptide binding site for class I; alpha1 and beta1 domains form the peptide binding site for class II.
One important function of the HLA class I molecule is to display peptides that result from the degradation of cytosolic proteins to the cell surface where they can be recognized by the CD8+T cells. CD8 binds to the alpha3 region of the class I molecule and presents peptide antigens to CD8+T cells.
HLA class II molecules display peptides that result from the degradation of endocytosed proteins to the cell surface where they can be recognized by the CD4+ T cells. CD4 binds to the beta2 region of the class II molecule.
Class I and class II molecules also differ in the number of amino acids that they can accommodate in the peptide-binding groove.
This cartoon image shows the structure of Class I and Class II molecules. The B2 microglobulin chain of the class I molecule is encoded by a gene on chromosome 15. The alpha polypeptide of the class I molecule is encoded by the class I genes. Class I and Class II molecules are structurally and functionally different. The alpha chain has 5 domains: 2 peptide-binding domains (alpha1 & alpha2), one alpha3 domain, the transmembrane region, and the cytoplasmic tail. The alpha3 domain penetrates the cell membrane, whereas B2 microglobulin does not. Alpha1 and alpha2 domains contain the majority of polymorphic regions conferring HLA antigen specificity, which are the outermost domains.
The class II genes encode for the alpha and beta polypeptide chains of the class II molecules. Each alpha and beta chain of the class II molecule has four domains: the peptide-binding domain (either alpha1 or beta1), either alpha2 or beta2 domain, the transmembrane region, and the cytoplasmic tail. Both alpha and beta chains traverse the membrane. The extramembranous portion of each chain has two amino acid domains, of which the outermost domain contains the variable regions of the Class II alleles
After this brief review of the HLA molecule, let’s start understanding the historical nomenclature.
Initially serology methodology was used to describe the polymorphism of the class I and class II loci.
New HLA antigens were defined at the international workshops with the extensive exchange of typing reagents between laboratories and HLA scientists exhaustively examining the reactivity of antisera, which were submitted as defining a particular specificity.
When an HLA antigen was established as reproducibly defined, it was given a formal designation to be used by all laboratories which consisted of a number preceded by a workshop ”w” designation.
When sufficient antisera were available for a particular specificity and most laboratories in the world had gained experience in accurately defining the specificity, the “w “designation was removed from all loci.
The “w” designation was also removed from the C locus alleles but will remain part of the antigen nomenclature to avoid confusion with the complement factors in the most current nomenclature.
What are the challenges of converting serologic typing into molecular typing?
Due to the improvements in serologic techniques, some of the antigens defined as combinations of closely related antigens, which could be distinguished serologically. For example, HLA-B15 consists of 8 closely related specificities, HLA-B62, B63, B70, B71, B72, B75, B76, and B77.
Serologic supertypes are the broad specificities. Eg: B15
“Splits” or Subtypes are the finer specificities that comprised the supertype. Eg: B62.
As the splits were discovered, they were given number designations that again give no indication as to the supertype to which they belong. Eg: Molecular B*15:12 is equivalent of HLA- B76.
The assigned numbers reflect the chronological order of discovery rather than a systematic numbering system.
To the uninitiated, the HLA numbering system is not something that can be deduced by studying the designated numbers of the supertypes and subtypes, and to those who work in the field, it is something that must be learnt.
Let’s now move on to learn more about the current HLA nomenclature. All HLA antigens are uniformly named starting with the locus, antigenic specificity, and molecularly typed allele group. The asterisk “*” sign indicates that typing is performed by a molecular method and the colon “: “is a field separator. Now, I will walk you through this example. A*03 is low resolution typing by molecular method. In most cases this is equivalent to the antigen.
A*03:01 is the allele and the 1st field (A*03) refers to a group of alleles that encode for the A3 antigen. 2nd field (:01) refers to an allele, which encodes a unique HLA protein (A*03:01).
Next we will describe the 3rd field designation. This 3rd field refers to a synonymous mutation. This is a change in the DNA sequence that codes for amino acids in a protein sequence, but does not change the encoded protein. A*03:01:01 vs A*03:01:02 difference is due a synonymous (silent) mutation.
The 4th field is reserved for non-coding regions. Finally, expression modifiers follow after the 4th field. An example includes Null alleles, which are not expressed are noted as capital N.
We finished discussing the nomenclature. Now let’s learn about a few common serologic terms utilized to identify the HLA antibodies and define their serum reactivity.
Epitope (Antigenic determinant): is the minimum structural unit composed of a few amino acids in the HLA antigen that can be recognized by a B or T cell receptors.
Private Epitopes: Epitopes that are present only on a single gene product such as HLA-A2.
Public Epitopes: Refers to epitopes that are shared by more than one HLA antigen. HLA antibodies will show reactivity with the antigens that contains the public epitope.
Cross-reactive groups (CREGs): is basically a group of antigens that share a public epitope, as demonstrated by the ability of a specific antibody to react with all of them.
HLA is an important barrier for hematopoietic stem cell (HPC) transplantation. Therefore, HLA matching and compatibility between the donor and the recipient is required for a successful HPC transplantation. HLA antigens also have role in graft- versus-host disease (GVHD), a potentially serious complication of allogeneic stem cell transplantation. GVHD occurs when donor T cells react to host antigens on antigen-presenting cells (APCs) and attack host tissues, with sequential activation of donor T cells and monocytes/macrophages.
In solid organ transplantation, HLA antibodies play an important role and HLA matching is not as important. When patients are exposed to foreign HLA antigens due to pregnancy or blood transfusions or transplantation, they can make antibodies against epitopes of those foreign HLA molecules. This is called HLA alloimmunization.
Presence of HLA antibodies against graft can cause antibody mediated rejection and graft loss.
HLA alloimmunization can also causes platelet refractoriness. This condition creates difficulty in finding compatible platelet units especially in transfusion dependent HPC transplant patients. In platelet refractoriness HLA Class I antibodies especially antibodies against HLA-A and HLA-B play an important role.
Slide 14: References
- Klein J, Sato A. The hla system. First of two parts. Engl J Med 2000;343:702-9.
- Marsh SG, Albert ED, Bodmer WF, Bontrop RE, Dupont B, Erlich HA, et al. Nomenclature for factors of the hla system, 2010. Tissue Antigens 2010;75:291-455.
- Robinson J, Halliwell JA, Hayhurst JD, Flicek P, Parham P, Marsh SG. The ipd and imgt/hla database: Allele variant databases. Nucleic Acids Res 2015;43:D423-31.
- Fung MK, Eder AF, Spitalnik SL, Westoff CM, editors. Technical Manual. 19th Ed. Bethesda (MD): AABB Press; 2017. p.435-44.
- Rodey GE. HLA Beyond Tears: Introduction to Human Histocompatibility. 2nd Ed. Durango (CO): De Nova; 2000. p.17-25.
- Abul K. Abbas, Andrew H. Lichtman, Shiv Pillai Cellular and molecular immunology updated 6th Ed page 102.
- B.D. Tait The ever-expanding list of HLA alleles: changing HLA nomenclature and its relevance to clinical transplantation. Transplantation Reviews 25 (2011) 1–8
Slide 15: Disclosures
Slide 16: Thank You from www.TraineeCouncil.org
Thank you for joining me on this Pearl of Laboratory Medicine on “HLA: Basic Terminology and Nomenclature.”
Nomenclature of HLA Alleles.
|HLA-DRB1*13:01||a specific HLA allele|
Human leukocyte antigen (HLA) typing is used to match patients and donors for bone marrow or cord blood transplants. HLA are proteins -- or markers -- found on most cells in your body. Your immune system uses these markers to recognize which cells belong in your body and which do not.What are the 3 methods of HLA typing? ›
For typing of the HLA antigens there are used cellular, serological, biochemical and DNA methods.What is HLA typing PPT? ›
HLA TYPING In this test, donor's antigens expressed on the surface of leukocytes or their genes are matched with that of the recipient. The closer the HLA antigens on the transplanted organ match the recipient, the more likely that the recipient's body will not reject the transplant.How many HLA types does a person have? ›
There are three general groups of HLA, they are HLA-A,HLA-B and HLA-DR. There are many different specific HLA proteins within each of these three groups.How many HLA are alleles? ›
For humans, as in most mammalian populations, MHC Class I molecules are extremely variable in their primary structure, and HLA-A is ranked among the genes in humans with the fastest-evolving coding sequence. As of March 2022, there are 7,452 known HLA-A alleles coding for 4,305 active proteins and 375 null proteins.Why is HLA important? ›
The most important function of HLA molecule is in the induction and regulation of immune responses. T cells recognize foreign antigen in combination with HLA molecules. In an immune response, foreign antigen is processed by and presented on the surface of a cell (e.g. macrophage).How is HLA type determined? ›
The blood or cheek swab is tested in a lab to figure out your HLA type. Your HLA type will be compared to potential donors to see if there is a match. Your doctor will decide which donor has the best HLA match for you.What type of test is HLA? ›
The primary use for human leukocyte antigen (HLA) testing is to match organ and tissue transplant recipients with compatible donors. HLA testing also includes screening transplant recipients for the presence of antibodies that might target the donated tissue or organ as part of an immune response.Who is HLA nomenclature? ›
What is the definition of HLA or Human Leukocyte Antigen? HLA refers to a gene complex that encodes the major histocompatibility complex (MHC) proteins in humans. In humans, MHC is named HLA for Human Leukocyte Antigens because these gene products were first described as proteins expressed on leukocytes.
Results. The most common alleles found in Caucasians were HLA-A*02, 24, 01; HLA-B*35, 44, 51; DRB1*11, 13, 07; for Afro-Brazilians they were HLA-A*02, 03, 30; HLA-B*35, 15, 44; DRB1*13, 11, 03; and for Asians they were: HLA-A*24, 02, 26; HLA-B*40, 51, 52; DRB1*04, 15, 09.What is the full form of HLA? ›
Human leukocyte antigen tests are done before a donor stem cell or organ transplant, to find out if tissues match between the donor and the person receiving the transplant. Also called HLA and human lymphocyte antigen.Which dye is used in HLA typing? ›
Microbead that is fluorescent dye conjugated is then added to detect the presence of HLA antibody in the serum which is bound to the antigen.Where is HLA found? ›
The human major histocompatibility complex HLA is located on the short arm of chromosome 6. It is known to be the most polymorphic genetic system in humans. The biological role of the HLA class I and class II molecules is to present processed peptide antigens.Where are HLA antigens found? ›
A histocompatibility antigen blood test looks at proteins called human leukocyte antigens (HLAs). These are found on the surface of almost all cells in the human body. HLAs are found in large amounts on the surface of white blood cells.What is HLA positive? ›
A positive test means HLA-B27 is present. It suggests a greater-than-average risk for developing or having certain autoimmune disorders. An autoimmune disorder is a condition that occurs when the immune system mistakenly attacks and destroys healthy body tissue.Which HLA is most significant in transplantation? ›
HLA class I and II matching is important in organ transplantation  especially in kidney and bone marrow transplantation. In heart and lung transplantation, HLA match at the DR locus is important but there is some difficulties like ischemic times, availability of donors and clinical need of recipients.How many genes are in HLA? ›
Of the 45 HLA-like genes, 20 were identified as protein coding genes, 4 were NC genes and 21 were pseudogenes.Are all HLA genes expressed? ›
HLA class I antigens are expressed on all nucleated cells and platelets (except those of the central nervous system) while the HLA class II antigens are expressed on antigen presenting cells (APC) such as B lymphocytes, dendritic cells, macrophages, monocytes, Langerhans cells, endothelial cells, and thymic epithelial ...What is the role of HLA genes? ›
The HLA complex helps the immune system distinguish the body's own proteins from proteins made by foreign invaders such as viruses and bacteria. HLA is the human version of the major histocompatibility complex (MHC), a gene family that occurs in many species.
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You do not need to do anything if you test positive for HLA antibodies. Your HLA antibodies pose absolutely no risk to you. You will remain eligible to donate red blood cells. Unfortunately, you will no longer be eligible to donate plasma or plasma blood products, such as platelets collected by apheresis.WHO Nomenclature Committee for factors of the HLA system report? ›
Nomenclature for factors of the HLA system, 1991
In: Tsuji T, Aizawa M, Sasazuki T, eds. HLA 1991. Oxford: Oxford University Press, 1992: 17–31.
Jean Dausset (1916-2009) discovered the first HLA antigen in 1958, which he named “MAC”, the three letters based on the first names of three of his volunteers.Is HLA typing genetic testing? ›
Human leukocyte antigen (HLA) typing is a genetic test used to identify immune system variations. HLA typing is essential for people donating and receiving bone marrow, cord blood, or organ transplants.What diseases are associated with HLA? ›
- Rheumatoid arthritis (RA) ...
- Multiple sclerosis (MS) ...
- Systemic lupus erythematosus (SLE) ...
- Type 1 diabetes mellitus (T1D) ...
- Sjögren's syndrome (SS) ...
- Celiac disease (CD) ...
- Ankylosing spondylitis (AS)
To start the HLA typing process: HLA typing is a blood test that requires approximately 10mL of blood. The results typically take two to three weeks to return and be processed. The blood sample can be collected in the Bone Marrow Transplant Clinic at Children's National Health System.What is HLA made of? ›
The human leukocyte antigen (HLA) system or complex is a complex of genes on chromosome 6 in humans which encode cell-surface proteins responsible for the regulation of the immune system. The HLA system is also known as the human version of the major histocompatibility complex (MHC) found in many animals.What is HLA crossmatch? ›
The crossmatch is thought to be a miniature test transplant performed in the laboratory. To complete this test a sample of blood is taken from the donor and recipient. The blood must be drawn at the same time. In the laboratory, the donor blood cells are mixed with the recipient serum.How are alleles named? ›
Allele designations begin with a letter and contain alphanumeric characters. Punctuation marks occur in certain cases where they convey meaning. Allele designations appear as superscripted short alphanumeric strings following the gene symbol of which they are an allele and serve as an acronym for the allele name.
It includes the HLA gene name, the HLA allele group for a specific gene, a specific non-synonymous allele (i.e. one that changes a protein amino acid sequence), a specific synonymous allele (i.e. one that does not change a protein amino acid sequence), non-CDS (coding sequence) alleles (i.e. introns, 5′-UTR's, 3′-UTR's ...What is HLA Bw4? ›
Bw4 is an epitope present on several HLA-A and HLA-B antigens that can be divided into 8 subtypes based on amino acid differences (Tissue Ag 2000, 56:363).What are HLA cross reactive groups? ›
HLA Class I “cross-reactive” antigen groups were initially defined by serologic reagents binding to expressed molecules on the cell surface. CREG can now be assigned by computer analyses on the basis of shared determinants encoded for by variants in the nucleotide sequence.