A proposal to standardize terminology for weak D antigen

Rh flow cytometry: An updated methodology for D antigen density applied to weak D types 164 and 165

BACKGROUND

An original methodology for determining the D antigen density on red cells was published in 2000 and has been applied in many publications since. This flow cytometry-based assay remained largely unrevised utilizing monoclonal anti-Ds that are not readily available anymore. We updated the methodology to quantify erythrocyte D antigen sites using microspheres and monoclonal anti-Ds that are commercially available today.

METHODS

The absolute D antigen density of a frozen standard CcDEe cell, drawn in 2003, a fresh blood donation from the same individual, drawn in 2022, and an internal control CcDEe cell, was quantified by flow cytometry using fluorescence-labeled microspheres. The internal control CcDEe cell was used in conjunction with 9 commercial anti-Ds to determine D antigen densities of 7 normal D, 4 partial D, and 11 weak D type samples, including 2 novel alleles.

RESULTS

The reproducibility of the updated assay was evaluated with red cells of published D antigen densities. The current results matched the known ones closely. The new weak D types 164 and 165 carried 4500 and 1505 D antigens/red cell, respectively. The absolute D antigen density decreased from 27,231 to 26,037 in an individual over 19 years.

DISCUSSION

The updated assay gave highly reproducible results for the D antigen densities of Rh phenotypes. Readily available anti-Ds allowed for the determination of the D antigen densities of 7 weak D types. The assay is suitable to evaluate the effects of distinct amino acid substitutions on the RhD phenotype.

Implementation of Molecular RHD Typing at Two Blood Transfusion Institutes from Southeastern Europe

Introduction

Determination of RhD variants in blood donors, pregnant women, and newborns is important for transfusion strategies, in order to prevent RhD alloimmunisation and hemolytic disease of fetuses and newborns. Implementation of molecular RHD typing in two transfusion institutes is presented in this article, from Banja Luka (Bosnia and Herzegovina) and Belgrade (Serbia).

Study Design and Methods

Blood donors' RhD was checked by direct agglutination assays (tube) and indirect antiglobulin test (gel). Molecular RHD typing was performed by PCR-SSP with fluorometric signal detection in both centres. Donors were selected by weak RhD serological reactivity (Banja Luka, 85 samples; Belgrade, 62 samples) or serologically RhD-negative C/E-positive results (Banja Luka, 92 samples; Belgrade, 61 samples).

Results

Among serologically determined weak D donors from the institute from Banja Luka, weak D type 3 was the most frequent (58.8%), followed by type 1 (35.3%) and DNB (1.2%), whereas results obtained at the Belgrade institute were distributed between weak D type 1 (41.9%), type 3 (30.7%), type 14 (6.5%), type 15 (1.6%), and DNB with anti-D (1.6%). In 17.7% of serologically typed weak D samples from the Belgrade institute, the molecular typing result was standard D. Additionally, RHD presence was detected in 9.8% of serologically RhD-negative, C/E-positive samples from both institutes.

Conclusion

Rh molecular testing was successfully implemented in both blood transfusion institutes in Banja Luka and Belgrade. This study proved the efficiency of serological algorithms for weak D, as well as the presence of the RHD gene among serologically tested RhD-negative, C/E-positive samples.

Strategies to identify candidates for D variant genotyping

BACKGROUND

RhD variants have altered D epitopes and/or decreased antigen copies per red cell. Individuals carrying these variants may test antigen negative, weakly positive, or positive by serology, and may or may not be at risk of alloimmunisation after exposure. There have been recommendations to perform RHD genotyping of patients, pregnant women and females of childbearing potential with serological weak D phenotype, to guide prophylactic use of Rh immune globulin (RhIG), and better conserve D-negative blood products. The purpose of this study was to evaluate the performance of a set of empirical criteria to identify such patients.

MATERIALS AND METHODS

A two-method strategy of gel testing (GT) and tube testing (TT) was used for Rh typing of patients with no historical blood type in the present institution. A monoclonal-polyclonal blend anti-D was used for Rh typing by TT at immediate spin. Three empirical criteria were used to identify candidates for genotyping: C1: discrepancy between the two test methods and a GT reaction strength >2+ stronger than TT; C2: weak serological reaction, defined as reaction strength ≤2+ regardless of testing method if both GT and TT were performed or reaction strength ≤2+ if only GT was performed, or reaction strength ≤1+ if only TT was performed; C3: presence of anti-D in D-positive patients with no history of RhIG use in the preceding 3 months and in whom alloanti-D is suspected.

RESULTS

Overall, 50 patients, ranging from newly born to 93 years old, were identified. Genomic testing confirmed D variants in 49/50 cases with a positive predictive value of 98%.

DISCUSSION

This two-method strategy is a powerful screening tool for identifying candidates for RHD genotyping. This strategy meets the current requirements of two blood type determinations/two specimens in pre-transfusion testing while simultaneously identifying candidates for RHD genotyping with a minimal increase in work load and cost.

Serological weak D phenotypes: a review and guidance for interpreting the RhD blood type using the RHD genotype

Approximately 0·2-1% of routine RhD blood typings result in a "serological weak D phenotype." For more than 50 years, serological weak D phenotypes have been managed by policies to protect RhD-negative women of child-bearing potential from exposure to weak D antigens. Typically, blood donors with a serological weak D phenotype have been managed as RhD-positive, in contrast to transfusion recipients and pregnant women, who have been managed as RhD-negative. Most serological weak D phenotypes in Caucasians express molecularly defined weak D types 1, 2 or 3 and can be managed safely as RhD-positive, eliminating unnecessary injections of Rh immune globulin and conserving limited supplies of RhD-negative RBCs. If laboratories in the UK, Ireland and other European countries validated the use of potent anti-D reagents to result in weak D types 1, 2 and 3 typing initially as RhD-positive, such laboratory results would not require further testing. When serological weak D phenotypes are detected, laboratories should complete RhD testing by determining RHD genotypes (internally or by referral). Individuals with a serological weak D phenotype should be managed as RhD-positive or RhD-negative, according to their RHD genotype.

[Molecular serological characteristics of weak D antigen types of the Rhesus system]

AIM

to estimate the spread of weak D antigen types of the Rhesus system in the citizens of the Russian Federation and a possibility of serologically identifying these types.

SUBJECTS AND METHODS

The red blood cells and DNA of people with weakened expression of D antigen were investigated using erythrocyte agglutination reaction in salt medium (2 methods); agglutination reaction in the gel columns containing IgM + IgG anti-D antibodies, indirect antiglobulin test with IgG anti-D antibodies (2 methods); polymerase chain reaction to establish the type of weak D.

RESULTS

A rhesus phenotype was determined in 5100 people in 2014-2015. The weakened agglutinable properties of red blood cells were detected in 102 (2%) examinees. 63 examinees underwent genotyping to identify the variants of the weak D antigen, which identified 6 weak D types. There were the most common weak D types 3 (n=31 (49.2%)) and weak D type 1 (n=18 (28.6%)), including weak D type 1.1 in one (1.6%) case. The other 4 weak D antigen types were as follows: weak D type 2 (14.3% (n=9)), weak D type 15 (4.8% (n=3)), weak D type 4.2 (DAR) (1.6% (n=1)) and weak D type 6 (1.6% (n=1)). The antiglobulin test in the gel column containing antiglobulin serum was the most sensitive serological assay to identify the weak D antigen. Only a molecular test could establish weak D type 15 in 2 samples of red blood cells with Ccdee and ccdEe phenotypes.

CONCLUSION

The weak D antigen could be serologically identified in 96.8% of cases. When testing for weak D, particular attention should be given to people with the D-negative phenotype who had the C or E antigens. Our investigations conducted for the first time in Russia will be able to improve the immunological safety of red blood cell-containing medium transfusions for patients.

Postpartum Rh immunoprophylaxis

The postpartum dose of Rh immune globulin varies according to an individual laboratory estimation of fetal red blood cells in each mother's peripheral blood. In the United States, a four-step procedure determines the postpartum dose (number of vials of 300 micrograms; 1,500 international units) of Rh immune globulin (anti-D) for each RhD-negative mother who has delivered an RhD-positive newborn and has not already formed anti-D. The first step is a rosette fetal red blood cell screen to determine whether an excessive (greater than 30 mL fetal whole blood) fetomaternal hemorrhage occurred. If the rosette screen is negative, the mother receives one vial of Rh immune globulin for Rh immunoprophylaxis. If the rosette screen is positive, the blood sample is retested by a quantitative method, typically an acid-elution (Kleihauer-Betke) assay. The result of the acid-elution assay is converted to an estimation of the volume of the fetomaternal hemorrhage, which is the basis for calculating the dose of Rh immune globulin. The acid-elution assay is subjective, imprecise, and poorly reproducible. As a result, the formula for calculating the dose includes a precautionary adjustment, adding an extra vial in borderline situations to prevent underdosing. Flow cytometry is a more precise method for quantifying a fetomaternal hemorrhage. However, few hospitals use flow cytometry, because it is not cost-effective to maintain an expensive, high-technology laboratory service for the relatively few occasions when a precise quantitative determination of fetomaternal hemorrhage is required.

Blood groups: the past 50 years

Since the first issue of TRANSFUSION in 1961, there has been a tremendous expansion in not only the number of blood group antigens identified but also in our knowledge of their biochemical basis, function, and more recently, associated DNA changes. As certain techniques became available, our ability to discover and elucidate blood group antigens and appreciate their contribution to biology became possible. In particular, Western blotting, monoclonal antibodies, cloning, and polymerase chain reaction-based assays have led to an explosion of our knowledge base. The study of blood groups has had a significant effect on human genetics where they serve as useful markers in genetic linkage analyses. Indeed blood groups have provided several "firsts" in certain aspects of genetics. Blood group-null phenotypes, as natural human knockouts, have provided valuable insights into the importance of red blood cell membrane components. This review summarizes key aspects of the discovery of blood groups; the inconsistent terminology that has arisen; and the contribution of blood groups to genetics, safe transfusion, transplantation, evolution, and biology.

How I manage donors and patients with a weak D phenotype

PURPOSE OF REVIEW

Since the adoption of molecular blood-group typing, the considerable heterogeneity of the serologic entities weak D and DEL at the molecular level has come to light. I offer an approach to the management of donors and patients expressing D antigen weakly and carrying any of the various molecular types of weak D and DEL.

RECENT FINDINGS

More than 50 distinct weak D alleles have been described. An internet-based survey of anti-D immunizations occurring in D-positive transfusion recipients reveals that no allo-anti-D has been observed in patients carrying prevalent weak D types. Allo-immunizations are documented for weak D types 4.2 (also known as DAR), 11 and 15. Anti-D immunizations have been reported in D-negative persons transfused with weak D and DEL red blood cells.

SUMMARY

Patients carrying any of the prevalent weak D types 1, 2, 3 or 4.1 are not prone to allo-anti-D immunization and may safely be transfused with D-positive red blood cells. Pregnant women with these weak D types need not receive RhIg. We should pay attention to weak D- or DEL-positive blood units that are labelled D-negative. The clinical benefit of removing DEL blood units from our supply of D-negative red blood cell units should be determined.

Outliers in RhD membrane integration are explained by variant RH haplotypes

BACKGROUND

Variations in a multipass transmembrane protein may affect its membrane integration. To study this effect, the systematic molecular characterization of variant D antigen density is a suitable model. Unlike most other membrane proteins, the expression of the D antigen is often determined by a single allele, because it occurs frequently in hemizygous form.

STUDY DESIGN AND METHODS

The D antigen density distribution of 530 CcDee, 475 ccDEe, and 514 ccDee random samples was established by flow cytometry. The molecular bases of samples with D antigen densities outside a bell-shaped peak was investigated.

RESULTS

The antigen densities of 499 CcDee, 437 ccDEe, and 480 ccDee samples formed bell-shaped peaks. Three, 10, and 12 samples, respectively, had decreased antigen densities and carried variant RHD alleles. Weak D type 19, RHD(I204T); weak D type 20, RHD(F417S); and the partial D DYU (also known as DQC), RHD(R234W) were new RHD alleles. Twenty-eight CcDee, 28 ccDEe, and 22 ccDee samples had increased antigen densities; 53 of them lacked a hybrid Rhesus box and were thus predicted to be RHD homozygous. Eight ccDee samples were predicted to be heterozygous despite a large relative dose of RHD to RHCE alleles in quantitative polymerase chain reaction. One of these samples was further investigated and carried an RHD-CE hybrid transcript characteristic for a -D- haplotype.

CONCLUSIONS

Unusual little and large RhD protein integration into the membrane could be traced to a host of distinct protein variants. Weak expression of D antigen was invariably associated with variant RHD alleles. Larger than normal D antigen density may often be caused by the presence of two D encoding alleles, which may be located in cis, and confounding zygosity testing that is solely based on gene copy number.

The role of RhD agglutination for the detection of weak D red cells by anti-D flow cytometry

Anti-D flow cytometry is an accurate method for quantifying feto-maternal haemorrhage (FMH). However, weak D red cells with <1000 RhD sites are not detectable using this methodology but are immunogenic. As quantitation of RhD sites is not practical, an alternative approach is required to identify those weak D fetal red cells where anti-D flow cytometry is inappropriate. We describe a simple algorithm based on RhD agglutination and flow cytometry peak separation. All weak D (n = 34) gave weak agglutination with RUM-1 on immediate spin (grading </=2.5). In Diamed-ID Diaclon ABO/D or ABO/Rh for Newborn cards two subgroups of weak D were observed. In one subgroup, weak agglutination (grading 3) was observed and the red cells were undetectable by flow cytometry. In the second subgroup, agglutination was strong (grading 4) and the red cells were detectable by anti-D flow cytometry. The accuracy of the quantitation was dependent on adequate separation of the weak D and RhD-negative peaks as in seven of 11 samples <1.11% of an expected 2% red cells were detectable. Monitoring RhD agglutination and flow cytometric peak separation are pivotal if anti-D flow cytometry is to be maintained as the primary technique for FMH quantitation in the routine laboratory.

Policies and procedures related to weak D phenotype testing and Rh immune globulin administration. Results from supplementary questions to the Comprehensive Transfusion Medicine Survey of the College of American Pathologists

OBJECTIVE

To determine and evaluate policies and procedures related to weak D phenotype testing and terminology and the administration of Rh immune globulin in selected clinical situations. Design, Setting, and Participants.-Institutions participating in the College of American Pathologists 1999 J-A Comprehensive Transfusion Medicine Survey program were asked to respond to a series of supplementary questions related to weak D phenotype testing and Rh immune globulin administration. More than 3500 institutions and transfusion services participated.

RESULTS

Most supplementary questions elicited more than 3000 responses. Despite no clinical or regulatory mandate, 58. 2% of transfusion services routinely perform an antiglobulin test for the weak D phenotype in patients who test negative with anti-D reagents. Significant differences were found concerning the transfusion of blood components to patients with the weak D phenotype and the administration of Rh immune globulin to these individuals. At least one patient with the weak D phenotype with anti-D alloantibody formation was observed during a 12-month period by 31.8% of transfusion services.

CONCLUSIONS

Significant variability concerning policies and procedures related to weak D typing and terminology was found in this survey. Transfusion of blood components to patients with the weak D phenotype and the administration of Rh immune globulin also demonstrated variations. Anti-D alloantibody formation by patients with the weak D phenotype may not be as rare as previously thought. Additional study related to the clinical significance of these results is warranted.

Weak D alleles express distinct phenotypes

The weak D phenotype is caused by many different RHD alleles encoding aberrant RhD proteins, raising the possibility of distinct serologic phenotypes and of anti-D immunizations in weak D. We reported 6 new RHD alleles, D category III type IV, DIM, and the weak D types 4.1, 4.2.1, 4.2.2, and 17. The immunohematologic features of 18 weak D types were examined by agglutination and flow cytometry with more than 50 monoclonal anti-D. The agglutination patterns of the partial D phenotypes DIM, DIII type IV, and DIVtype III correlated well with the D epitope models, those of the weak D types showed no correlation. In flow cytometry, the weak D types displayed type-specific antigen densities between 70 and 4000 RhD antigens per cell and qualitatively distinct D antigens. A Rhesus D similarity index was devised to characterize the extent of qualitative changes in aberrant D antigens and discriminated normal D from all tested partial D, including D category III. In some rare weak D types, the extent of the alterations was comparable to that found in partial Ds that were prone to anti-D immunization. Four of 6 case reports with anti-D in weak D represented auto-anti-D. We concluded that, in contrast to previous assumptions, most weak D types, including prevalent ones, carry altered D antigens. These observations are suggestive of a clinically relevant potential for anti-D immunizations in some, but not in the prevalent weak D types, and were used to derive an improved transfusion strategy in weak D patients.

Weak D alleles express distinct phenotypes

The weak D phenotype is caused by many different RHD alleles encoding aberrant RhD proteins, raising the possibility of distinct serologic phenotypes and of anti-D immunizations in weak D. We reported 6 new RHD alleles, D category III type IV, DIM, and the weak D types 4.1, 4.2.1, 4.2.2, and 17. The immunohematologic features of 18 weak D types were examined by agglutination and flow cytometry with more than 50 monoclonal anti-D. The agglutination patterns of the partial D phenotypes DIM, DIII type IV, and DIVtype III correlated well with the D epitope models, those of the weak D types showed no correlation. In flow cytometry, the weak D types displayed type-specific antigen densities between 70 and 4000 RhD antigens per cell and qualitatively distinct D antigens. A Rhesus D similarity index was devised to characterize the extent of qualitative changes in aberrant D antigens and discriminated normal D from all tested partial D, including D category III. In some rare weak D types, the extent of the alterations was comparable to that found in partial Ds that were prone to anti-D immunization. Four of 6 case reports with anti-D in weak D represented auto-anti-D. We concluded that, in contrast to previous assumptions, most weak D types, including prevalent ones, carry altered D antigens. These observations are suggestive of a clinically relevant potential for anti-D immunizations in some, but not in the prevalent weak D types, and were used to derive an improved transfusion strategy in weak D patients.

Molecular Basis of Weak D Phenotypes

A Rhesus D (RhD) red blood cell phenotype with a weak expression of the D antigen occurs in 0.2% to 1% of whites and is called weak D, formerly Du. Red blood cells of weak D phenotype have a much reduced number of presumably complete D antigens that were repeatedly reported to carry the amino acid sequence of the regular RhD protein. The molecular cause of weak D was unknown. To evaluate the molecular cause of weak D, we devised a method to sequence all 10RHD exons. Among weak D samples, we found a total of 16 different molecular weak D types plus two alleles characteristic of partial D. The amino acid substitutions of weak D types were located in intracellular and transmembraneous protein segments and clustered in four regions of the protein (amino acid positions 2 to 13, around 149, 179 to 225, and 267 to 397). Based on sequencing, polymerase chain reaction-restriction fragment length polymorphism and polymerase chain reaction using sequence-specific priming, none of 161 weak D samples investigated showed a normal RHD exon sequence. We concluded, that in contrast to the current published dogma most, if not all, weak D phenotypes carry altered RhD proteins, suggesting a causal relationship. Our results showed means to specifically detect and to classify weak D. The genotyping of weak D may guide Rhesus negative transfusion policy for such molecular weak D types that were prone to develop anti-D.

Molecular Basis of Weak D Phenotypes

A Rhesus D (RhD) red blood cell phenotype with a weak expression of the D antigen occurs in 0.2% to 1% of whites and is called weak D, formerly Du. Red blood cells of weak D phenotype have a much reduced number of presumably complete D antigens that were repeatedly reported to carry the amino acid sequence of the regular RhD protein. The molecular cause of weak D was unknown. To evaluate the molecular cause of weak D, we devised a method to sequence all 10RHD exons. Among weak D samples, we found a total of 16 different molecular weak D types plus two alleles characteristic of partial D. The amino acid substitutions of weak D types were located in intracellular and transmembraneous protein segments and clustered in four regions of the protein (amino acid positions 2 to 13, around 149, 179 to 225, and 267 to 397). Based on sequencing, polymerase chain reaction-restriction fragment length polymorphism and polymerase chain reaction using sequence-specific priming, none of 161 weak D samples investigated showed a normal RHD exon sequence. We concluded, that in contrast to the current published dogma most, if not all, weak D phenotypes carry altered RhD proteins, suggesting a causal relationship. Our results showed means to specifically detect and to classify weak D. The genotyping of weak D may guide Rhesus negative transfusion policy for such molecular weak D types that were prone to develop anti-D.

STUDY OF WEAK D PHENOTYPE IN HETEROGENEOUS POPULATION

Weak D is the current phenotypic term used to denote a weakened expression of Rhesus (Rh) D antigen on red cells due to a quantitative or qualitative difference in Rh antigen. The present study was undertaken from January 1986 to August 1997. Out of a total of 5042 Rh D confirmation tests 22 (0.43%) weak D phenotypes were detected. Fifteen (68.3%) weak D phenotypes were from blood group O. Five (22.7%) from blood group A and one (4.5%) each from blood group B and AB. There were 15 males and 7 females. There were no cases of haemolytic disease of the newborn or Rh incompatibility.