Red cell alloimmunization remains a frequent and potentially serious complication of transfusion for patients with Sickle Cell Disease (SCD). RBC antigen disparities between African American patients and European American blood donors, particularly in the Rh system, are thought to contribute to the elevated frequency of alloimmunization in SCD. Extended RBC antigen-matching has been shown in single institutional and prospective multi-center experiences to reduce the incidence of alloantibody production in SCD. Some transfusion services also supply RBCs from African American donors when possible, but its effect on alloimmunization is not well-studied. Despite a program of providing D, C, E, and K-matched RBCs from primarily African-American donors, we have continued to observe Rh alloimmunization. Many patients produced anti-D, -C and –e, even though their RBCs type D+, C+, and e+, respectively. To examine whether Rh alloimmunization was associated with inheritance of diverse RH alleles that encode altered Rh proteins, we characterized RHD and RHCE in 50 patients with SCD: 27 who were alloimmunized to at least one Rh antigen, and 23 patients who had received transfusions but were not Rh alloimmunized. RH testing consisted of PCR-RFLP, AS-PCR and/or amplification and sequencing of exons and Rh-cDNA. In the Rh alloimmunized group, the average number of unit exposures was 275, and the median was 189. Twelve patients had anti-e despite typing e+ serologically, and all 12 had genotypes associated with weak or altered e antigen suggesting the antibody formed in response to exposure to conventional e antigen. Three of these patients subsequently formed anti-E when placed on e-negative transfusion protocols. Of 12 patients with anti-C, 7 typed serologically as C+. Of these, 3 had a hybrid RHD-CE-D that expresses an altered C antigen. However, the remaining 5 had at least one RHCE encoding conventional C antigen. Interestingly, anti-D was reported in 14 patients who typed serologically as D+. Genotyping revealed weak and/or partial D variants in 3, but 11 patients had at least one conventional RHD. These Rh antibodies were almost all transient, particularly unusual for anti-D. In the non-Rh-alloimmunized group, the average number of unit exposures was 189, and the median was 120. RH genotyping of the non-alloimmunized group revealed significant genetic diversity as well, with most patients carrying altered RH alleles. Of 23 e+ patients, 15 had only variant RHce, while 8 had at least one conventional RHce (only 3 were homozygous for conventional RHce). All 4 C+ patients had conventional RHCe genotypes. All 23 non-Rh-alloimmunized patients typed serologically D+, of which 15 had at least one conventional RHD and 8 had altered RHD. In conclusion, we demonstrate that some but not all Rh alloimmunization in patients with SCD transfused with D, C, E, and K phenotype-matched RBCs from primarily African American donors is due to recipients with variant RH genes who are exposed to conventional Rh proteins. In this small study of 50 patients, all 12 allo anti-e occurred in patients with only variant RHce, likely due to mismatched e antigen. Three cases of anti-C in C+ patients and 3 anti-D in D+ patients were also explained by variant RH. However, we found a number of cases of anti-D and -C in patients who had at least one conventional allele and would not be expected to make alloantibodies. More studies are needed, but one possible explanation is that variations in Rhce or RhD proteins may alter the entire Rh antigenic complex in the RBC membrane, as most of these patients had changes in at least one RH allele. Alternatively, these transient Rh antibodies could be stimulated by exposure to units from African American donors expressing variant Rh antigens, and these antibodies could represent “mimicking” or “cross-reactive” specificities. Since the RH genotypes of most of the non-alloimmunized patients also demonstrated variant RH alleles, future studies should evaluate which Rh variants are immunogenic. These data suggest 1) RH genotyping is a valuable complement to serologic phenotyping, 2) it may become important to use molecularly characterized reagent cells for antibody identification, and 3) knowledge of donor genotypes will be important for understanding Rh alloimmunization and possibly for selecting blood for patients with SCD.
Westhoff:Immucor scientific advisory board: Membership on an entity's Board of Directors or advisory committees.
Asterisk with author names denotes non-ASH members.