Antigen Density Impacts RBC Survival and Antigen Modulation Following Incompatible RBC Transfusion
Authors: Yoo Justin , Arthur Connie Maridith , Zerra Patricia , Girard-Pierce Kathryn , Hendrickson Jeanne E. , Stowell Sean R. 
Published: October 2019

Abstract
Background: While chronic transfusion therapy can signficantly reduce morbidity and mortality in patients with hemoglonbinopathies, transfusion is not without risk. Differences in red blood cell (RBC) antigen distributions between donors and recipients can result in the development of RBC alloantibodies, which can make it difficult, if not impossible, to find compatible blood. As a result, heavily alloimmunized individuals experience greater morbidity and mortality than those who do not possess RBC-induced alloantibodies. While incompatible RBC transfusion can result in fatal hemolytic transfusion reactions (HTRs), these adverse events are not the invariable outcome of incompatible transfusion. Although a variety of factors likely contribute to the likilihood of developing a HTR following incompatible RBC transfusion, variability in the density of the donor antigen may contribute to different outcomes. Using a novel model that utilizes RBC donors with different levels of the KEL antigen, we sought to directly examine the impact of antigen density on RBC clearance following incompatible RBC transfusion.
Methods: Wild type C57BL/6 mice (WT) were passively immunized with anti-KEL serum, generated following immunization of WT recipients with KEL RBCS, 2 hours prior to incompatible KEL RBC transfusion. RBC collected from previously generated KEL transgenic mice that express high (KELhi), intermediate (KELmed), and low (KELlo) levels of human KEL glycoprotein antigen, were labeled with chloromethylbenzamido 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), while WT RBC were labeled with fluorescently distinct 3,3'-dihexadecyloxacarbocyanine perchlorate (DiO) to facilitate detection post-transfusion. Passively immunized recipient WT mice were transfused with labeled KEL RBC to WT RBC. Percent KEL RBC survival was measured using flow cytometric analysis at various time points post transfusion by comparing the DiI to DiO ratio. Bound antibody, complement and the KEL antigen were assessed by staining cells with anti-Ig, anti-C3 and anti-KEL followed by flow cytometric analysis.
Results: Transfusion of RBCs expressing low, intermediate or high levels of the KEL antigen into non-immunized mice failed to result in any detectable alteration in RBC survival or KEL expression. However, transfusion of KELlo, KELmed and KELhi RBCs into anti-KEL immunized recipients resulted in differential clearance of each population, with KELhi experiencing the most clearance, followed by KELmed and KELlo. Similarly, the level of detectable C3 and bound antibody on KEL positive cells likewise correlated with the initial level of KEL antigen expression, with more C3 and IgG being detected on the surface of KELhi following transfusion into immunized recipients, than KELmed or KELlo. However, KELhi RBCs also experience the most significant alterations in KEL antigen levels following antibody engagement. Regardless of the KEL positive RBC transfused, each population developed a relatively rapid state of resistance to additional anti-KEL-mediated RBC removal.
Conclusions: Our results indicate that lower antigen densities may be less efficient in not only facilitating rapid clearance of RBCs following incompatible transfusion, but also antibody-induced antigen modulation. While KELhi RBCs displayed the greatest degree of clearance and antigen modulation, each population developed resistance to antibody-mediate removal, suggesting that once RBC antigen levels fall below a certain threshold, clearance discontinues, regardless of the initial levels of antigen prior to incompatible transfusion. Taken together, these results suggest that antigen density may provide an additional feature of RBC transfusion that may impact the outcome of incompatible transfusion in heavily alloimmunized recipients.

Disclosures
No relevant conflicts of interest to declare.