What is HLA?
HLA stands for Human Leukocyte Antigen: a molecule that is present on almost every cell type in the human body. HLA has a central role in coordinating immune responses through a variety of different mechanisms. The main function of HLA is to present small protein molecules (‘peptides’) to T cells, a type of immune cell. Peptides may be from inside the cell, or from within the cell’s environment. If the cell is infected with a microorganism such as a virus, HLA will present viral peptides to T cells and trigger an immune response (which includes the infected cell being killed). Similarly, the cell may absorb molecules or microorganisms from the environment and, when presenting these to the T cell, cause the T cell to become activated and to begin searching for the source of any ‘non-self’ molecules. Furthermore, because HLA is present on almost all cells, immune cells check for the presence of HLA and can kill cells where HLA is absent as this indicates the cell isn’t functioning correctly (it may be a tumour cell, or infected with virus).
What is Tissue Typing?
HLA is actually a family of molecules encoded by the HLA genes. ‘Classical’ HLA molecules are typically of interest clinically and in most research environments, and consist of the ‘Class I’ molecules HLA-A, HLA-B and HLA-C, and the ‘Class II’ molecules HLA-DR, HLA-DQ and HLA-DP. The Class I genes HLA-A, HLA-B and HLA-C encode the alpha chain of the corresponding molecule. HLA Class II is divided into alpha and beta chains encoded by separate genes, for example, HLA-DQA1 and HLA-DQB1. All individuals possess the HLA-DRB1 gene, but many individuals also possess additional HLA-DRB3, HLA-DRB4 and HLA-DRB5 genes. Class I and Class II HLA molecules have distinct functions and are present on different cell types.
HLA genes are highly polymorphic. This means that, unlike most genes in the body, there are many different variants of the HLA genes (‘alleles’). Because of this, unrelated individuals are very likely to have different HLA alleles. The process of identifying which HLA variants an individual possesses is colloquially known as Tissue Typing or, more accurately, HLA typing. Depending on the method used, the terms HLA genotyping or serological HLA typing may be used. HLA genotyping uses ‘molecular’ or DNA-based methods and is considered more reliable.
Why is HLA Important?
In the natural world, the variation seen in HLA genes means that certain variants provide more protection against infectious diseases than others. Differences in HLA between individuals means that, as a population, survival is more likely.
HLA is critically important in transplantation. Genetic differences between the donor and recipient can illicit an immune response and, as the HLA molecule is central in the immune response pathway, differences in HLA can have a dramatic impact on transplant outcome. Reactivity against donor HLA can develop post-transplant, leading to rejection of the transplant by the recipient’s immune system. Pre-transplant exposure to non-self HLA (e.g. via blood transfusion) can also cause antibodies to develop which can result in immediate loss of the transplant. HLA is important in both the solid organ (e.g. kidney) and blood and marrow transplant (BMT) settings. In BMT, transplanted donor cells mature into immune cells and can attack the recipient’s own cells unless a high-level HLA match is achieved.
Specific HLA types are also associated with an increased susceptibility to disease, or reactivity against particular drugs. For example, HLA has been linked to Coeliac Disease, Rheumatoid Arthritis and Narcolepsy, and HLA-B*57:01 is associated with hypersensitivity to Abacavir (an antiretroviral drug). Often the exact mechanism linking HLA to disease or drug hypersensivity is unknown.
Why Perform HLA Typing?
Clinical laboratories typically perform HLA typing to support solid organ transplantation, blood and marrow transplantation, and transfusion (effectively, a blood transplant). Typically, laboratories look to match donor and recipient HLA as closely as possible and avoid mismatches to which there are antibodies resulting from previous exposure. This field of clinical science is often referred to as Histocompatibility and Immunogenetics (H&I).
Studies may perform HLA typing to determine whether specific HLA types are associated with susceptibility or protection to specific diseases or drug reactions. For example, early in the SARS-CoV-2 pandemic research groups sought to identify whether specific HLA types were associated with or protective against severe COVID-19 disease.
Different HLA variants bind peptides differentially (this is referred to as the ‘peptide repertoire’). Because of this, characterised and consistent HLA types are important in studies of the immune response as differences can impact the magnitude and outcome. In particular, studies involving T cells and the intimate interaction between HLA and the T cell Receptor (TCR) require careful consideration of the HLA molecule.
Healthcare is also moving in the direction of personalised medicine. Cell-based therapies require consideration of HLA as incompatibility with the recipient can result in failure of the therapy or unwanted side effects. Cell and tissue banking services often HLA type their inventory for this reason.
How is HLA Typing Performed?
Historically, multiple methods have been used for HLA typing and many different methodologies are still in use today. Serological HLA typing has fallen out of favour as this method uses biological reagents which are often in limited supply, require rigorous QC and provide limited discrimination between different HLA variants. Typically, laboratories perform HLA genotyping using molecular (DNA-based) methods which differ in terms of their level of resolution. For example, VH Bio Ltd. performs intermediate-resolution genotyping by PCR-RSSO and high-resolution genotyping by Next Generation Sequencing (NGS). High-resolution genotyping tends to define HLA alleles unambiguously (e.g. HLA-A*02:01) or to a level considered functionally important, whereas intermediate-resolution genotyping defines group of related alleles (e.g. HLA-A*02:01/02:04/02:17). Different methodologies have pros and cons, e.g. cost, throughput and turnaround time, level of resolution. VH Bio Ltd. provide a bespoke service and endeavour to act as a laboratory partner: supporting decision-making, providing HLA genotyping services, and assisting in the understanding of HLA genotyping data.