and C.D.J. Estropipate transfusion of KEL RBCs that express low KEL antigen levels. Intriguingly, unlike RBC clearance, loss of the KEL antigen from the transfused RBCs occurred at a similar rate regardless of the KEL antigen density following an incompatible transfusion. In addition to antigen density, anti-KEL antibody levels also regulated RBC removal and KEL antigen loss, suggesting that antigen density and antibody levels dictate incompatible RBC transfusion outcomes. These results demonstrate that antibody-induced antigen loss and RBC clearance can occur at distinct antigen density thresholds, providing important insight into factors that may dictate the outcome of an incompatible RBC transfusion. Visual Abstract Open in a separate window Introduction Red blood cell (RBC) transfusion represents a common therapeutic intervention in patients with chronic anemia.1-3 Although RBC transfusion is often beneficial, incompatible transfusions can result in the rapid removal of transfused RBCs with potential adverse consequences.4,5 Rapid, antibody-mediated removal of RBCs following an incompatible transfusion is rare. However, recent results suggest that this outcome may be more common in certain patient populations.4,6-12 Patients with transfusion-dependent conditions can present with life-threatening anemia and a complex-alloantibody profile that precludes prompt identification of compatible RBCs.13-19 In these situations, patients occasionally receive RBCs that are not fully compatible. 20-22 Prior alloimmunization events can also prime patients to develop a recrudescent alloantibody response, which can result in the accelerated clearance of the transfused RBCs days to weeks following transfusion.23-31 Recent reports suggest that patients with sickle cell disease can be particularly prone to develop amnestic alloantibody responses.12,32 Often, these responses are accompanied by accelerated clearance of the transfused RBCs, potentially resulting in serious complications.12,32 Despite the potential consequences of incompatible RBC transfusions,33-37 the factors that determine the overall consequences of an incompatible RBC transfusion remain incompletely understood.38 RBC removal is not necessarily the absolute outcome of an incompatible transfusion.39 Although sophisticated in vitro approaches (such as the monocyte monolayer test40,41) exist that may aid in identifying incompatible transfusion scenarios where RBC clearance is likely to occur, the unexpected presentation of patients with life-threatening anemia can make it difficult to effectively use this platform in acute cases.42 When only incompatible RBCs are available and life-threatening anemia necessitates transfusion, optimal strategies that may prevent the removal of incompatible RBCs are needed but remain incompletely defined.43 Factors that regulate RBC removal following incompatible transfusion remain inadequately understood largely because of challenges studying incompatible RBC transfusions clinically and lack of suitable animal models to study incompatible transfusion biology in vivo. Given the unpredictability in outcomes following incompatible RBC transfusions, deliberately transfusing incompatible RBCs in human subjects to define factors that may regulate RBC removal is not ethical. To overcome this challenge, recently developed preclinical models have provided an opportunity to begin examining the consequences of antibody engagement and the factors that may govern the outcomes of incompatible transfusion.33-37 Results obtained using these models suggest that the consequences of antibody engagement may be more complicated than simple antibody-mediated removal of target RBCs. Instead, competing processes appear to regulate the sensitivity of RBCs to antibody-mediated removal following an incompatible transfusion. These processes include common antibody effector functions (which may result in RBC clearance) and competing antibody-mediated removal of the target antigen from the RBC surface (antigen modulation).37,44-46 Antibody-mediated antigen modulation appears to render cells less sensitive to antibody-mediated removal and, therefore, may protect incompatible RBCs from antibody-mediated destruction.37 Complementary studies in patients who have received antibody-based drugs that target RBCs appear to corroborate these results, suggesting that antibody engagement of antigens on the RBC surface may alter the target antigen.47,48 However, a more complete understanding Rabbit polyclonal to DDX6 of the factors that regulate antigen modulation and subsequent RBC clearance is still needed. Estropipate Among factors that may regulate the consequences of incompatible RBC transfusion, the surface density of the target antigen has been suggested as a variable that may influence overall incompatible transfusion outcomes.49 However, although antibody engagement of different antigens may be more likely to result in RBC clearance, a variety of other variables (including other characteristics of the antigens themselves) may likewise contribute to these correlations. As Estropipate a result, it is difficult to determine the potential impact of antigen density itself on the outcome of an incompatible transfusion. In this report, we use a novel murine system that couples RBCs that express varying levels of.