Human leukocyte antigen (HLA) and tissue typing

The human leukocyte antigen (HLA) system is the human version of the major histocompatibility complex (MHC), a set of cell-surface proteins that present peptide fragments to T lymphocytes and that constitute the principal antigens responsible for immune recognition of transplanted tissue.[1][2] Tissue typing (histocompatibility testing) is the laboratory characterization of a donor's and a recipient's HLA molecules, together with detection of any antibodies the recipient carries against HLA, performed before transplantation to estimate immunologic risk and to support organ allocation and matching.[2][3] HLA matching and antibody assessment, alongside ABO blood-group compatibility, form the core of the pre-transplant immunologic workup described in Organ transplantation and Transplant immunology.[2]

This article describes the genetics and biology of the HLA system, the classes and nomenclature of HLA molecules, the laboratory methods used for typing and antibody detection, and the role of HLA matching in solid-organ and hematopoietic-cell transplantation. Related antibody-focused topics are covered in Panel-reactive antibody / cPRA and sensitization, Crossmatch (CDC, flow, virtual), and Donor-specific antibodies (DSA).

Genetics and the MHC region

The HLA genes are clustered on the short arm of human chromosome 6 (region 6p21) in the MHC, one of the most gene-dense and most polymorphic regions of the human genome.[1][4] The region is conventionally divided into class I, class II, and class III subregions. Because the loci are tightly linked and usually inherited together as a block, the set of HLA alleles on a single chromosome is called a haplotype; each person inherits one haplotype from each parent and the alleles are codominantly expressed, so both parental sets of HLA molecules appear on the cell surface.[1][2] This inheritance pattern means a full sibling has a 25 percent chance of being HLA-identical (sharing both haplotypes), a 50 percent chance of sharing one haplotype, and a 25 percent chance of sharing neither, which is the genetic basis for prioritizing sibling donors in hematopoietic-cell transplantation.[2][5]

The HLA system is the most polymorphic in the human genome: tens of thousands of HLA alleles have been described, catalogued by the international IPD-IMGT/HLA Database, and this diversity is the central practical obstacle to finding well-matched unrelated donors.[4][6]

Classes of HLA molecules

HLA molecules are grouped into two functionally distinct classes relevant to transplantation.[1][7]

• Class I molecules (encoded by HLA-A, HLA-B, and HLA-C) are expressed on essentially all nucleated cells. They present endogenous peptides to CD8+ cytotoxic T cells. A class I molecule consists of a polymorphic heavy chain non-covalently associated with the invariant β2-microglobulin.[7][8]
• Class II molecules (encoded by the HLA-DR, HLA-DQ, and HLA-DP loci) are expressed mainly on professional antigen-presenting cells (dendritic cells, macrophages, B cells) and on activated endothelium. They present exogenous peptides to CD4+ helper T cells and are heterodimers of an α and a β chain.[1][9]

The class III subregion encodes molecules such as complement components and tumor necrosis factor and does not encode classical transplantation antigens.[1] In clinical solid-organ matching, attention centers on the HLA-A, HLA-B, and HLA-DR loci historically, with HLA-C, HLA-DQ, and HLA-DP increasingly assessed as antibody targets.[2][3]

Nomenclature

HLA alleles are named under a standardized system maintained by the WHO Nomenclature Committee for Factors of the HLA System. A name such as *HLA-A\02:01:01:01 specifies the locus (A), then up to four colon-separated fields: the allele group (often corresponding to the historical serologic antigen), the specific protein, any synonymous coding-sequence variant, and any non-coding variant.[6] For clinical matching, low-resolution typing resolves the first field (the antigen group), whereas high-resolution** typing resolves the protein-level (second-field) allele; high resolution is standard in hematopoietic-cell transplantation and increasingly used in solid-organ work.[3][6]

Laboratory methods

HLA typing

Historically, HLA typing was performed serologically by the complement-dependent microcytotoxicity assay, in which lymphocytes were tested against panels of antisera of known specificity. Serologic typing has largely been replaced by DNA-based (molecular) typing, which is more precise and can achieve high resolution.[2][3] Molecular methods include sequence-specific oligonucleotide probe hybridization (SSO), sequence-specific primer PCR (SSP), Sanger sequence-based typing (SBT), and, increasingly, next-generation sequencing (NGS), which can resolve full-length alleles and phase ambiguities.[3][10]

HLA antibody detection

Because recipients may have been immunized to foreign HLA by prior transplantation, pregnancy, or transfusion, tissue typing also includes screening the recipient's serum for anti-HLA antibodies.[2][11] The dominant contemporary method is the single-antigen bead (SAB) solid-phase immunoassay on a multiplexed flow platform (commonly Luminex), in which microbeads each coated with a single recombinant HLA molecule are exposed to patient serum; bound antibody is detected with a fluorescent secondary reagent and reported as a mean fluorescence intensity (MFI) for each specificity.[11][12] These data define a candidate's antibody profile, drive the calculated panel-reactive antibody (cPRA), and enable the virtual crossmatch (see the dedicated articles).[11][13]

HLA matching in transplantation

Solid organ

The clinical importance of HLA matching varies by organ. In kidney transplantation, better HLA matching (fewer mismatches at HLA-A, -B, and -DR) is associated with improved long-term graft survival and reduced sensitization, and matching is a factor in U.S. deceased-donor kidney allocation; nonetheless, advances in immunosuppression have allowed successful transplantation across substantial HLA mismatch.[2][3][14] For heart, lung, and liver transplantation, the short ischemic tolerance of the organ and logistical urgency mean transplants generally proceed without prospective HLA matching, although HLA antibodies still matter for risk assessment.[2][3] More recently, attention has shifted from counting whole-antigen mismatches to molecular (epitope/eplet) mismatch analysis, which estimates the number of mismatched antibody-accessible epitopes and may predict the risk of developing de novo donor-specific antibody.[15]

Hematopoietic-cell transplantation

In allogeneic hematopoietic stem cell transplantation, the matching imperative is reversed and far more stringent: high-resolution matching at HLA-A, -B, -C, and -DRB1 (often expressed as an "8/8 match") is sought to minimize both graft rejection and graft-versus-host disease, in which donor T cells attack recipient tissue.[5][16] HLA-identical siblings are the preferred donors; when none is available, matched unrelated donors are identified through international registries, and HLA-haploidentical and cord-blood approaches extend access for patients lacking a fully matched donor.[5][16] See Graft-versus-host disease.

See also

• Transplant immunology
• Crossmatch (CDC, flow, virtual)
• Panel-reactive antibody / cPRA and sensitization
• Donor-specific antibodies (DSA)
• ABO compatibility and ABO-incompatible transplantation
• Transplant rejection
• Graft-versus-host disease

References

• Choo SY. The HLA system: genetics, immunology, clinical testing, and clinical implications. Yonsei Med J. 2007;48(1):11-23. PMID:17326240. https://pmc.ncbi.nlm.nih.gov/articles/PMC2628004/
• Justiz Vargas AN, et al. Transplantation Immunology. StatPearls. NBK538218. https://www.ncbi.nlm.nih.gov/books/NBK538218/
• Tait BD, et al.; and Williams TM. Human leukocyte antigen testing in transplantation (review of HLA typing and antibody methods). Methods/clinical practice summarized in StatPearls "Biochemistry, HLA Antigens." NBK546662. https://www.ncbi.nlm.nih.gov/books/NBK546662/
• Trowsdale J, Knight JC. Major histocompatibility complex genomics and human disease. Annu Rev Genomics Hum Genet. 2013;14:301-323. PMID:23875801. https://pmc.ncbi.nlm.nih.gov/articles/PMC4426292/
• Hematopoietic Stem Cell Transplantation. StatPearls. NBK536951. https://www.ncbi.nlm.nih.gov/books/NBK536951/
• Marsh SGE, et al. Nomenclature for factors of the HLA system; IPD-IMGT/HLA Database. https://www.ebi.ac.uk/ipd/imgt/hla/
• Physiology, MHC Class I. StatPearls. NBK556022. https://www.ncbi.nlm.nih.gov/books/NBK556022/
• Wieczorek M, et al. Major Histocompatibility Complex (MHC) Class I and Class II Proteins: Conformational Plasticity in Antigen Presentation. Front Immunol. 2017;8:292. PMID:28367149. https://pmc.ncbi.nlm.nih.gov/articles/PMC5355494/
• Roche PA, Furuta K. The ins and outs of MHC class II-mediated antigen processing and presentation. Nat Rev Immunol. 2015;15(4):203-216. PMID:25720354. https://pmc.ncbi.nlm.nih.gov/articles/PMC4495510/
• Profaizer T, Kumánovics A. Human Leukocyte Antigen Typing by Next-Generation Sequencing. Ann Lab Med. 2018;38(5):401-414. PMID:29797810.
• Tambur AR, et al. Sensitization in transplantation: Assessment of risk (STAR) consensus documents. Am J Transplant. (STAR 2017/2019). https://pubmed.ncbi.nlm.nih.gov/29603851/
• Biochemistry, HLA Antigens. StatPearls. NBK546662. https://www.ncbi.nlm.nih.gov/books/NBK546662/
• OPTN/UNOS. Calculated panel reactive antibody (CPRA) and histocompatibility policy. https://optn.transplant.hrsa.gov/
• Williams RC, Opelz G, et al. The Risk of Transplant Failure With HLA Mismatch in First Adult Kidney Allografts From Deceased Donors. Transplantation. 2016;100(5):1094-1102. PMID:26901078.
• Wiebe C, Nickerson PW. Strategic use of epitope matching to improve outcomes. Transplantation. 2016;100(10):2048-2052. PMID:27362314.
• Dehn J, et al. Selection of unrelated donors and cord blood units for HCT: NMDP/CIBMTR guidelines. Blood. 2019;134(12):924-934. PMID:31292117.