HLA Typing

HLA typing defines an individual's alleles at the human leucocyte antigen loci on chromosome 6p21. Class I genes (HLA-A, B, C) are expressed on all nucleated cells, and class II genes (HLA-DR, DQ, DP) on antigen presenting cells.

Typing is done by molecular methods: sequence specific oligonucleotide probes (SSO), sequence specific primers (SSP), real time PCR, and next generation sequencing, which gives high resolution (allele level, 2-field) results. Serological and cellular methods are obsolete. Results are reported as alleles (for example A*02:01) at low, intermediate or high resolution.

Typing has two main purposes: matching for transplantation (haematopoietic stem cell, kidney, heart, lung, liver, cornea) and disease association or drug hypersensitivity prediction. The chance that a sibling is a full match is 25%, and about 30% of patients needing stem cell transplantation have a matched sibling.

Unrelated donor registries now hold over 40 million donors, with match probability of 75% for white patients but under 20 to 30% for patients of African descent, so haploidentical donors are increasingly used.

Indications. Haematopoietic stem cell transplantation (patient, siblings, unrelated donors, cord blood, haploidentical relatives). Solid organ transplant candidates and donors (kidney in particular, for allocation and matching). Platelet refractoriness due to HLA antibodies (selection of HLA-matched platelets).

Disease association testing: HLA-DQ2/DQ8 in coeliac disease, HLA-B27 in spondyloarthritis, HLA-B*57:01 before abacavir, HLA-B*15:02 before carbamazepine in people of Asian ancestry, HLA-B*58:01 before allopurinol in high risk groups, HLA-DQB1*06:02 in narcolepsy type 1, HLA-DRB1 in narcolepsy or rheumatoid arthritis research. Paternity and forensic work.

Specimen. EDTA whole blood 3 to 5 mL (or buccal swab for DNA). Stable at room temperature for 48 hours and at 2 to 8 °C for a week. No fasting. For recent transfusion or post-transplant chimerism, a pre-transplant sample or buccal swab is used.

Reference values. There are no numerical reference ranges. The result is a list of alleles for each locus. Loci typed depend on purpose: HLA-A, B, C, DRB1 (and DQB1, DPB1) for stem cell transplantation; A, B, DR (and DQ) for kidney; single alleles for drug and disease associations.

Interpretation.

• Haematopoietic stem cell transplantation: a full match is 8/8 (A, B, C, DRB1) or 10/10 (adding DQB1) alleles. A single mismatch (7/8) is acceptable in some settings, with moderate increases in graft-versus-host disease and mortality. Haploidentical donors share one haplotype (5/10) and use post-transplant cyclophosphamide. Donor selection also considers donor age, sex, cytomegalovirus status and donor specific antibodies.
• Kidney transplantation: matching at HLA-A, B and DR affects graft survival, and zero mismatch grafts have the best outcomes (10-year survival 10 to 15% higher than with 6 mismatches). Matching is used with antibody screening and crossmatch for allocation.
• Drug hypersensitivity: HLA-B*57:01 positive means abacavir must not be given (hypersensitivity in 50 to 60% of carriers, negative predictive value near 100%). HLA-B*15:02 positive in people of Southeast Asian ancestry means carbamazepine and phenytoin are avoided (Stevens-Johnson syndrome). HLA-B*58:01 positive increases allopurinol hypersensitivity risk several hundred fold in Han Chinese, Thai and Korean patients and calls for avoidance.
• Disease association: HLA-DQ2/DQ8 negative excludes coeliac disease (negative predictive value over 99%). HLA-DQB1*06:02 positive supports narcolepsy type 1 (present in 90 to 95%, and in 12 to 25% of the general population).
• Platelet refractoriness: HLA class I typing of the patient allows selection of matched platelets.
• High resolution typing resolves ambiguities, and differences at the allele level (even a single amino acid) can influence transplant outcome.

Critical values and findings needing immediate action.

• Positive HLA-B*57:01 in an HIV patient planned for abacavir: do not give abacavir, and record an abacavir hypersensitivity alert in the notes.
• Positive HLA-B*15:02 before carbamazepine in a patient of Asian ancestry: avoid carbamazepine and oxcarbazepine.
• Urgent matched search for a patient with acute leukaemia or aplastic anaemia needing stem cell transplantation: typing of patient and family within days, and registry search started at diagnosis.
• Heavily sensitised kidney candidate with a rare haplotype: early registry and paired exchange options.

Interferences and pitfalls.

• Resolution matters: low resolution typing may hide allele level mismatches that matter, and confirmatory typing on a second sample is required for transplant donors and recipients to prevent sample mix up.
• Null alleles (not expressed on the cell surface) and new alleles can be missed by limited probe panels.
• Recent transfusion or transplant can contaminate the result with donor DNA (chimerism), so a buccal swab or fibroblast sample is used for a post-transplant patient.
• Ancestry affects allele frequencies and the chance of finding a donor, and carrying disease associated alleles differs by population, so drug hypersensitivity testing is population specific.
• A positive association allele is not diagnostic, and many carriers have no disease (HLA-DQ2 in 30% of the population, HLA-B27 in 8%).
• Typing does not detect donor specific antibodies, and antibody screening and crossmatch are required.
• Typing of the donor's relatives is performed before, not after, informed consent and counselling about unexpected parentage.
• Old serological types are not equivalent to molecular alleles, and conversion tables may give errors.

Special groups.

• Children: typing is done at diagnosis in children with leukaemia or bone marrow failure, together with siblings, since a sibling donor is preferred.
• Pregnancy: foetomaternal HLA incompatibility has no role in routine care, and sensitisation from pregnancy matters in later transplant candidates.
• Older adults: matching matters less for kidney transplantation in elderly recipients receiving older donor kidneys, and waiting time is prioritised.
• Regions: limited donor registries for African, South Asian and Middle Eastern populations lower the chance of a matched unrelated donor, and haploidentical transplants and family studies are valuable. HLA-B*15:02 and HLA-B*58:01 are especially relevant for drug safety in Asia.

Follow up tests. HLA antibody screening and single antigen bead assay, lymphocytotoxic or flow cytometric crossmatch, donor specific antibody testing, chimerism analysis after stem cell transplantation, cytomegalovirus and Epstein-Barr virus serology of donor and recipient, ABO and Rh grouping, KIR genotyping in haploidentical transplantation, and confirmatory typing of donor and recipient on a second sample.