Introduction

MHC class I or major histocompatibility complex type 1 is a diverse set of cell surface receptors expressed on all nucleated cells in the body, as well as platelets. In humans, MHC also has the name HLA (human leukocyte antigen), and these are three subtypes, HLA-A, HLA-B, and HLA-C. These molecules play a vital role in the immune system recognizing self from non-self, presenting foreign antigens to other immune cells. HLA class I alleles are extremely polymorphic among the world population, which presents issues relating to human tissue transplants.[1][2]

Issues of Concern

MHC class I cross-reactivity is often the mediator in transplant reactions. A host T-cell may bind the MHC molecule on the donor's grafted tissue and recognize the graft as non-self, after that mounting an immune attack on the graft via a cascade of immune cell activation. This type of reaction is labeled a T-cell mediated reaction. T-cell mediated reactions are responsible for acute transfusion reactions with symptoms arising in days to weeks after transplant. The host may have also become sensitized via a previous transplant, blood transfusion, or pregnancy. The sensitization process resulting in anti HLA antibodies is called alloimmunization and is not currently well understood. Anti-HLA antibodies are responsible for hyperacute transfusion reactions, which arise minutes to hours after transfusion.[3][1]

Before performing the transplant of any hemopoietic precursors such as stem cells or solid tissue transplants of kidney or livers, etc., a pretransplant crossmatch test is necessary to evaluate the reactivity of the recipient's HLA-antibodies against the donor's HLA proteins.[4]

HLA class I proteins are highly immunogenic. If HLA types do not match, this can lead to hyperacute reactions in which the recipient's immune system recognizes the graft as foreign and signals the body's immune system to destroy the allograft.

Another area of concern is the association of specific HLA alleles leading to a genetic predisposition to developing some disease conditions. While it is often unclear exactly how these HLA class I subtype have implications in the pathogenesis of the disease, their presence may aid in the diagnosis of particular diseases. Further research is needed to discover how these alleles contribute to disease and any therapies that could prevent their action.

Cellular Level

MHC class 1 molecules are protein structure consisting of three alpha domain and a beta two macroglobulin domain. The HLA class I gene is located on the chromosome, while beta-2 macroglobulin encoding is on chromosome 15.[5] The HLA class 1 alleles are codominant expressed, and inheritance is via simple Mendelian inheritance patterns.[6]

The alpha 1 and 2 domains serve as the binding cleft for various peptides, which will then be presented to a T cell receptor. One end of the alpha domain also serves as the binding site for an inhibitory receptor located on NK cells. The beta-2 macroglobulin act to stabilize the peptide binding.[5]The binding cleft of MHC class I is flanked by tyrosine residues and creates closed ends that limit the peptide size; it can bind to around 8 to 10 amino acids. These amino acids are of cytosolic origin. Self or foreign cytosolic proteins are degraded via the proteasome and transported into the lumen of the ER. In the ER, the peptides are loaded onto an MHC class 1 via the aid of a chaperone protein named tapasin. The peptide bound MHC class I is then transported to the cell’s plasma membrane, where it presents the peptide to CD8+ T cell receptors.[7]

Function

HLA class I function as part of the adaptive immune system and play a vital role in recognizing self from non-self as well as providing protection against viruses and tumors. In a healthy person HLA class 1 bind degraded cytosolic self-proteins and then transport these fragments to the cell membrane where they present to CD8+ T lymphocytes. When the presented peptide is of foreign origin, such as peptide derived from a virus-infected cell, the CD8+ T cell works to eliminate the infected cell.

MHC class 1 presenting self-proteins also serves as an inhibitory signal to NK cells; this prevents NK killing of healthy cells.

Related Testing

HLAs are identifiable via multiple different types of detection methods.

Molecular Testing

Sequence-specific primer polymerase chain reaction: this method makes use of various primers that are complementary to specific HLA DNA sequences. The DNA is plated into a multi-well plate with different primers. If the DNA extracted from that cell is complementary to the primer, it will be amplified, and the product can be run on a gel via electrophoresis. The band can be identified as a particular primer and matched to candidate HLA alleles that are known.[8]

Sequence-specific oligonucleotide probes

One way to detect the high polymorphism seen in these genes is via polymerase chain reaction paired with sequence-specific oligonucleotides. This method involves amplifies the gene via PCR and then probing DNA with a fluorescent tag. The HLA type is determined using known HLA alleles as a reference, and that gene may undergo sequencing.[9][8]

Direct DNA sequencing

Another technique is to use sanger sequencing or next-gen sequencing to sequence the entire gene of a specific HLA variant after amplification via PCR. Once the sequence is known, it can be compared to previously published know HLA alleles.[8]

Serological Testing

Serological testing generally uses a recipient's lymphocytes obtained from their sera and incubated with anti-sera containing antibodies against various HLA class 1 subtypes. The solution is then incubated with rabbit sera, providing a source of complement; a dye is also added, which allows the identification of dead cells. This assay progresses serially with different HLA antibodies put into each well of a tray. Through the elimination of those wells with a positive result allows for the determination of the HLA type. This method provides a relatively quick and easy way to determine general subtypes of HLA's present but does not offer an in-depth analysis of the true molecular identity of the HLA's. This method is less common today because of its inability to detect small changes in HLA types that may make an immunological difference and cause a transfusion reaction.[10][8]

Antibody Testing

Cytotoxic cell-based antibody testing can also be an effective way to measure the recipient's risk of having a positive crossmatch. In this method, a set of 30-40 donor cell lymphocytes are mixed with dye and complement with the serum of the recipient. If the serum of the recipient contains a high enough level of HLA specific antibodies against a particular donor lymphocyte complement will be activated, the cell will die, and the dye will be taken up, and that well in the plate will be visually identifiable as a positive result. This test yields a value known as a percentage panel reactive antibody. Its result measures the recipient's risk of a positive crossmatch in a similar population of donors. This test does lack the ability to take race and different HLA frequencies in a population into account, weakening its value.[8]

Clinical Significance

Transplant rejection remains one of the most clinically relevant issues surrounding the HLA class 1 molecule. Proper identification of the donor and recipient’s HLA status is imperative to prevent transplantation reactions. Continuing education on the different types of serological, molecular, and cellular tests available to clinicians, and their proper use and healthcare professionals must learn how to interpret them.

Spondylosis Ankylosis

Research has shown a strong correlation between HLA B27 and ankylosing spondylitis. Among those with the HLA B27 allele, 5 to 6 percent develop ankylosing spondylitis (AS).[11] AS is a seronegative spondyloarthropathy, which leads to progressive stiffness of the spine and sacroiliac joints. This stiffness is the result of improper bone deposition leading to fused vertebrae. The exact role of HLA B27 in AS is still unknown.[12]

Behcet Disease

Research has found the HLA B51 allele to be the single greatest risk factor in developing Behcet disease. Behcet disease is an autoinflammatory condition that results in normally immune-privileged sites such as the brain, eye, and joints to become infiltrated with neutrophils. The link between the pathogenesis of Behcet disease and HLA B51 is still unclear. More research is needed to ascertain the true connection between the HLA B21 allele and Behcet disease. Researchers postulate that it could be due to antigen presentation to CD8 + cell or HLA B51’s interaction with NK receptor KIR3DL1.[13]

Psoriasis

Psoriasis, an autoimmune condition, is highly linked to specific HLA alleles. HLA A01, HLA A02, B13, B17 B39, B57, Cw06, Cw07 are all HLA class one molecules associated with psoriasis. The role of HLA in psoriasis is currently unknown.[14]

Birdshot Chorioretinopathy

Birdshot chorioretinopathy, a form of posterior uveitis, correlates with HLA A29, with 85 to 97.5 percent of those diagnosed carrying the allele.[15]

HIV

HLA B27, B51, HLA C06, and HLA B5701 all confer protection against HIV infection. The reason appears to be that the peptides presented by these alleles are structurally resistant to mutation. Another idea is that those CD8 positive T cells that interact with these specific alleles have higher functionality in riding the body of HIV infected cells.[15]