RH Gene Polymorphisms Are Risk Factor for Alloimmunization in Patients with Sickle Cell Disease.

Transfusion therapy for treatment of sickle cell disease (SCD) is predicted to increase, following the significant benefit of chronic red cell transfusion demonstrated by the stroke prevention trial (STOP). Despite progress in mitigating negative effects of transfusion therapy with the use of iron chelating agents, alloimmunization remains a significant problem. Patients with SCD have a higher incidence of antibody production compared to other patient groups undergoing chronic transfusion. To limit alloimmunization, many programs transfuse SCD patients with RBCs that are phenotype-matched for the most immunogenic blood groups, Rh and K, and some programs also supply RBCs from African-American (AA) donors. Although this approach reduces the incidence of alloantibody production, it is resource- and cost-prohibitive for many programs, and, importantly, some patients (∼5%) still become alloimmunized. The development of high-throughput genotyping for blood group antigens will make antigen-matching cost effective; therefore, it is important to determine why some patients become alloimmunized despite antigen-matching. We investigated the antibody specificity and sequenced the RH genes in 46 SCD patients who were alloimmunized, despite having received Rh and K matched units. The antibodies identified included anti-D, or -C, or -e, and/or antibodies to high-prevalence antigens. None had anti-E. RH gene sequencing revealed that 20 patients had a RHD-CE(3–7)-D hybrid gene in which RHD exons 3 through 7 are replaced with reciprocal exons from RHCE. The resulting Rh protein encodes an altered C antigen. This D-CE-D gene was also linked to an RHce allele encoding altered e antigen. These patients had anti-C and/or anti-e in their serum (i.e.-hrB). We screened healthy AA donors and found that the prevalence of the hybrid RHD-CE(3–7)-D gene in this population is 5–8%. The remaining 26 patients were homozygous for mutations in RHce and had produced anti-e, and some also had mutations in RHD and had produced anti-D. These results suggest that inheritance of a RHD-CE-D gene or altered RHce, with or without altered RHD, underlies Rh alloimmunization in SCD. The altered Rh proteins are not distinguished with current serologic typing reagents. Therefore, these patients are not truly Rh antigen matched. The development of RH genotyping platforms offers a potential solution to prevent alloimmunization by identifying SCD patients who are homozygous for variant alleles and at risk for production of alloantibodies to Rh antigens. The 5–8% of donor units with the same RH genotype could be directed to these high risk patients.