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Abstract
Complement impacts innate and adaptive immunity. Using a model in which the human KEL glycoprotein is expressed on murine red blood cells (RBCs), we have shown that polyclonal immunoprophylaxis (KELIg) prevents alloimmunization to transfused RBCs when a recipient is in their baseline state of heath but with immunoprophylaxis failure occurring in the presence of a viral-like stimulus. As complement can be detected on antibody coated KEL RBCs following transfusion, we hypothesized that recipient complement synergizes with viral-like inflammation to reduce immunoprophylaxis efficacy. Indeed, we found recipient C3 and C1q were critical to immunoprophylaxis failure in the setting of a viral-like stimulus, with no anti-KEL IgG alloantibodies generated in C3-/- or C1q-/- mice following KELIg treatment and KEL RBC transfusion. Differences in RBC uptake were noted in mice lacking C3, with lower consumption by splenic and peripheral blood inflammatory monocytes. Finally, no alloantibodies were detected in the setting of a viral-like stimulus following KELIg treatment and KEL RBC transfusion in mice lacking complement receptors (CR1/2-/-), narrowing key cells for immunoprophylaxis failure to those expressing these complement receptors. In-vitro studies showed complement fixed opsonized RBCs were significantly less likely to bind to B-cells from CR1/2-/- than wild type mice, potentially implicating lowered B-cell activation threshold in the presence of complement as being responsible for these findings. We thus propose a two-hit model for inflammation-induced immunoprophylaxis failure, where the first “hit” is recipient inflammation and the second “hit” is complement production/sensing. These results may have translational relevance to antigen-antibody interactions in humans.
Highlights
Alloimmunization to red blood cells (RBCs) can be significant in transfusion, transplantation, and pregnancy settings
Following KEL IgG (KELIg) infusion, KEL RBCs labeled with a lipophilic dye were transfused and RBCs recovered one hour later were evaluated for bound complement proteins (Figure 1A shows the experimental design)
Complement C3, C4, and Factor Bb were detected bound to the recovered dihexad ecyloxacarbocyanine perchlorate (DiO) labeled KEL RBCs in mice treated with KELIg, but not in control mice treated with saline (Figure 1B)
Summary
Alloimmunization to red blood cells (RBCs) can be significant in transfusion, transplantation, and pregnancy settings. Polyclonal anti-D (RhIg) immunoprophylaxis has been used for over half a century as an effective preventive measure for alloimmunization to RBCs expressing the RhD antigen in pregnancy [1]. Immune regulation by antibody-antigen interactions is well known in different immunological settings; the mechanism(s) by which RhIg works remains controversial. It is not understood why breakthrough anti-D cases occur even when RhIg is administered properly. We have previously described a murine model in which polyclonal anti-KEL (KELIg) immunoprophylaxis prevents alloimmunization to transfused RBCs expressing the human KEL glycoprotein when administered to recipient mice in their baseline state of heath [2]. The mechanism(s) leading to this breakthrough alloimmunization are only partially understood
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