All in one place: the future of blood group typing Free

In this issue of Blood, Gleadall et al¹ report the performance of an automated, high-throughput genotyping array to support matching of blood products. The array simultaneously types for human erythroid (red cell), platelet, leukocyte, and neutrophil antigens, and includes markers of genetic ancestry, donor health, and product quality (such as hemochromatosis-associated HFE variants).

Matching beyond ABO and the D (RH1) antigen is not available in all settings, despite being a cost-effective way to limit alloimmunization and improve patient outcomes.² High-income, and an increasing proportion of middle-income countries, strive to match beyond ABO/D for selected groups of patients, including female patients of childbearing age, patients with sickle cell anemia (SCA) or thalassemia, and patients with hematological malignancies.

For female patients, when hemolytic disease of the fetus and newborn is prevented with anti-D prophylaxis and transfusion of D^– red blood cells (RBCs), the incidence of antibodies to other antigens may surpass that of anti-D.³ Western European countries routinely match for the Rh (C/c, E/e) and K antigens and some are actively pursuing strategies beyond that, for prevention of fetal and neonatal alloimmune thrombocytopenia caused by antibodies to platelet antigens.⁴ 

For patients with SCA, the American Society of Hematology recommends prophylactic matching of RBCs for the Rh (C, E or C/c, E/e) and K antigens.⁵ Preventing alloimmunization is of particular importance in these patients considering the risk of delayed hemolytic transfusion reaction, resulting in hyperhemolysis in the most severe cases.⁶ Extended antigen matching (Jk^a/Jk^b, Fy^a/Fy^b, and S/s) and detection of blood group variants (the so-called “partial” antigens) provides further protection from alloimmunization, although studies have failed to consistently confirm the benefit of these strategies.⁷ 

For patients with hematological malignancies in need of transfusion support, or during the course of bone marrow transplant, antibodies to HLA antigens may cause platelet refractoriness. With few HLA-typed blood donors, finding HLA-compatible platelets is a challenge, especially in admixed populations.

Less often discussed but occurring in all populations, are the challenges posed by rare erythroid phenotypes.⁸ Low availability or total lack of compatible blood may delay or impede treatment for patients. Depending on donor typing policies, even uncommon phenotypes may pose a significant challenge, such as the e^– phenotype (in the Rh system), which has a prevalence of 2%. Extensive donor typing strategies significantly increase the likelihood of identifying compatible donors.

Until recently, donor typing was limited to serological typing for a selection of erythroid antigens, from ABO/D alone to 15 to 20 antigens at most for a subset of donors. This is only a fraction of the 393 currently known erythrocyte antigens, distributed in 47 blood group systems (when the gene has been identified), collections, and series (when the underlying molecular mechanisms are still unknown). Leukocyte and platelet antigen typing is limited to a small subset of platelet or bone marrow donors, and neutrophil typing to transfusion-related acute lung injury investigations. Genotyping methods are reliable, increasingly cost-effective, and easier to scale up than serological testing, and some recent donor typing strategies primarily rely on genotyping.⁹ Remarkably, the assay presented by Gleadall et al¹⁰ is the first to include simultaneous typing of erythroid, platelet, leukocyte, and neutrophil antigens.

As the result of an international collaborative effort (the Blood transfusion Genomics Consortium [BGC]) to improve the safety and efficiency of blood transfusion by introducing genomics technology into routine clinical practice, the array was designed for, and tested on, samples from 6946 donors of diverse genetic ancestry. All samples were tested in duplicate at Sanquin (The Netherlands) and the New York Blood Center (United States), and 3938 in triplicate at the National Health Service Blood and Transplant (United Kingdom). Gleadall et al report high concordance rates of the array results with historical donor phenotypes on record, and between laboratories. The discrepancies are discussed in detail. Of the discrepancies observed with clinical platelet typing (<0.5%), the array-generated type was confirmed for 66.6%. The concordance between HLA types was 99.1%, with highest concordance in European samples and lowest in East Asian samples. Of the discrepancies observed with clinical typing for erythroid antigens (0.10%), the array-generated type was confirmed for 54.2%. Absence of conclusive typing by the array was observed for antigens of the highly polymorphic erythrocyte systems RH (C/c) and MNS (M/N), especially in admixed American and African samples, highlighting the need for future development of the algorithm or refinement of the probes to improve performance. Some major RH antigens (C^X, C^W, V, and VS) and MNS rare blood types (U^– and U^var) are included in the array, but one may hope that the next steps by the BGC enable detection and calling of additional variants of clinical relevance in transfusion medicine. Fortuitously, 73 new donors negative for a high frequency erythroid antigen were identified.

Only a few discrepancies in reproducibility between laboratories were observed, some of which were because of absence of typing by the array (missing types) or contamination of samples. However, the high performance of the array should not sidetrack potential users from observing good laboratory practices, validating the assay in specific populations and considering carefully how such an array will fit into existing practices. Strategies to limit errors may include duplicate testing of a sample or donor and confirmation by parallel or follow-up serological typing, especially for rare types (absence of a high frequency antigen).

The assay presented here was made possible by the multinational BGC, whose collaborative efforts must be commended. The simultaneous typing of erythroid, platelet, leukocyte, and neutrophil antigens by a single assay will alleviate the significant conundrum of donor typing strategies in which cost and scalability have always been the limiting factors. The array, designed by specialists in immunohematology, anticipates future research endeavors. If used for patients, side by side erythroid and leukocyte typing may significantly advance the understanding of the association between HLA alleles and alloimmunization. The additional presence in the array of markers of genetic ancestry, donor health, and product quality may enable correlation with recipient outcomes in the future. Regulatory implications may differ by country for donor testing of disease-associated genetic variants (such as hemochromatosis-associated HFE variants). Fortunately, such variants may be hidden to accommodate user-specific requirements.

Conflict-of-interest disclosure: The authors declare no competing financial interests.