The risk and frequency of anti-D alloimmunization in SCD patients with weak RHD variant alleles is not well characterized. The aim of this study was to assess the risk for alloimmunization in transfused SCD patients with variant RHD alleles encoding D antigens with weak expression.

Thirty-seven transfused SCD patients typed as weak D were enrolled in this study. Among them, 11 patients had anti-D. Serological studies were performed to assess the D antigen expression through hemmagglutination rections in microplate, tube and gel methods. Molecular analyses were performed to identify the RHD allele encoding the D antigen by using the RHD BeadChip (Immucor) or Sanger sequencing.

Patient RBC samples showed weak-strength reactions varying from 1+ to 2+ with monoclonal anti-D by hemmaggglutination. RHD variant alleles found in the patients with weak D expression included: 8 RHD*weak D type 4.0, 6 RHD*DAU0, 16 RHD*DAR, 3 RHD*DOL, 1 RHD*DAU4 and 3 RHD*DAU5. Among the 11 patients with anti-D, 7 had the RHD*DAR allele, 2 had RHD*DOL, 1 had RHD*DAU4 and 1 had the RHD*DAU5 allele. All these alleles are currently designated weak partial RHD alleles. Anti-D formed in individuals with weak partial RHD alleles DAR, DOL and DAU5 occurred after fewer D+ unit exposures, and remained detectable for longer than the other weak partial D types. Among all anti-D, 2 patients with weak partial D DAR and one patient with weak partial D DOL had clinical or laboratory evidence of poor transfused red cell survival.

RH diversity among patients with SCD contributes to RhD immunization but not all RHD variant alleles are prone to alloimmunization and not all anti-D developed are clinically significant. To minimize RBC transfusion of D-negative blood or genotype-matched transfusions in patients with weak D expression it is important to better understanding of who would actually make clinically significant anti-D. In this study we identified variant RHD alleles encoding weak D antigens in patients with SCD and their risk of anti-D alloimmunization. We also explored their potential clinical significance showing that patients with weak partial D DAR and weak partial D DOL develop clinically significant anti-D and may benefit from prophylactic D- RBC units or RH genotype-matched transfusions to prevent anti-D.

Our findings suggest that SCD patients with weak D type 4.0 and DAU0 receiving chronic transfusions could be managed as D+ but the other currently designated weak partial RHD alleles should still be considered potentially at risk for alloimmunization.