Alloimmunization To HLA Class I Antigens But Not H-Y Antigens Is Associated With Transfusion In Children With Sickle Cell Disease

Patients with sickle cell disease (SCD) are at high risk for graft rejection after hematopoietic stem cell transplant (HSCT) especially when using reduced intensity conditioning (RIC). Alloimmunization from red blood cell (RBC) transfusions, critical to the management of SCD, may predispose these patients to HSCT rejection. Alloimmunization to major or minor histocompatibility antigens (mHA) may occur after RBC transfusion due to residual white blood cells or platelets present in RBC units even after leukoreduction. In mice, despite extensive leukoreduction, RBC transfusions caused alloimmunization and subsequent HSCT rejection after RIC.

We conducted a cross-sectional study to determine if RBC transfusions are associated with alloantibody formation to human leukocyte antigens (HLA) and mHA, the target of the host versus graft rejection in HLA-identical transplantation. For mHA, we assessed immunity to H-Y antigens, mHA encoded on the Y chromosome. We have previously demonstrated the importance of H-Y antibodies in a variety of settings including chronic graft versus host disease (in male recipients of female grafts) and rejection of renal transplants (in female recipients of male grafts).

We enrolled 114 nulliparous pediatric SCD (SS or Sβ⁰) patients: 58 females on chronic transfusion, 32 males on chronic transfusion, and 24 females who had never been transfused. Serum was evaluated for HLA antibodies using the FlowPRA® (panel reactive antibody) screening test which detects IgG antibodies to the majority of HLA class I and II antigens. Serum was also tested for IgG antibodies against 5 H-Y antigens (DDX3Y [DBY], EIF1AY, RPS4Y1, UTY, ZFY) using protein microarray technology with each H-Y-seropositive threshold determined as a microarray mean fluorescence intensity (MFI) > the median MFI + 2.5 quartiles measured in 60 adult healthy males.

Chronically transfused patients had a higher prevalence of HLA class I antibodies than never transfused patients (33.3% vs. 12.5%, p=0.074). This increased prevalence of HLA class I antibodies in the chronic transfusion group was significant when examining the number of patients who tested positive to more than 25% of the panel (17.8% vs. 0%, p=0.022). Few patients had detectable HLA class II antibodies with no significant differences between chronically transfused and non-transfused patients.

Few SCD patients had detectable H-Y alloantibodies with no significant differences in chronically transfused females, chronically transfused males, and non-transfused females.

Leukocyte-reduced RBC transfusions appear to engender immunization to class I HLA but not class II HLA or H-Y antigens in pediatric SCD patients. Since hematopoietic progenitor cells express class I HLA, transfusion-related immunity to these antigens could pose a barrier to engraftment in HLA-disparate transplants. Since these antigens are also expressed on platelets, class I HLA antibodies in SCD could have important implications for platelet transfusion support during HSCT. While RBC transfusion does not appear to be an adequate stimulus for H-Y alloantibody formation, it is possible that exposure to mHA during transfusion could cause a cellular rather than a humoral immune response that may contribute to future HSCT graft rejection. Our result that few (6%) nulliparous female children with SCD had detectable H-Y alloantibodies is also a significant finding considering that 19-41% of healthy adult females had antibodies to at least one H-Y antigen in previous studies. Finally our finding that 25% of never transfused children with SCD had a positive HLA PRA is unexpected for an “unsensitized” group; its significance is being further investigated.

No relevant conflicts of interest to declare.