CD47 Regulates Red Blood Cell Alloimmunization in Mice

Background: Exposure to red blood cell (RBC) alloantigens during pregnancy or transfusion can lead to the development of alloantibodies and result in transfusion-related complications, including hemolytic transfusion reactions and hemolytic disease of the fetus and newborn. However, the factors that regulate RBC alloimmunization remain incompletely understood. Several studies suggest that alterations in factors that regulate RBC clearance may impact RBC uptake and antigen presentation, directly influencing the likelihood of RBC alloimmunization. To test this, we directly examined the potential role of CD47, a master regulator of RBC removal previously shown to be altered during RBC senescence and cold storage. To accomplish this, we crossed transgenic mice that express the model HOD antigen (a fusion protein consisting of hen egg lysozyme fused to ovalbumin and human Duffy b) with CD47 knock out (KO) mice to generate HOD RBC donors with wild type, heterozygous or homozygous KO levels of CD47 and used these donors to define the impact of CD47 on antibody formation following RBC transfusion.

Methods: HOD transgenic mice expressing the HOD antigen exclusively on RBCs were crossed with CD47^-/- mice to produce HOD CD47^+/- or HOD CD47^-/- mice. HOD and CD47 levels were assessed by flow cytometric analysis using anti-HOD and anti-CD47 antibodies. HOD CD47^+/+, HOD CD47^+/- or HOD CD47^-/- RBCs were transfused into C57BL6 recipients, followed by serum collection on days 14 and 28 post transfusion and evaluation of anti-HOD antibodies by flow cytometry crossmatch. To determine the CD4 T cell response to transfusion, TCR transgenics specific to ovalbumin (OTII) were labeled with CFSE, followed by adoptive transfer, transfusion of HOD CD47^+/+, HOD CD47^+/- or HOD CD47^-/- RBCs and evaluation of T cell proliferation, activation and cytokine secretion. Cellular removal of HOD RBCs was determined by flow cytometric examination of CFSE-labeled HOD RBCs. Finally, antigen levels on HOD RBCs was determined by staining cells with anti-HEL antibodies followed by flow cytometric examination. All three groups were subjected to one-way ANOVA analysis with a p value <0.05 considered significant.

Results: While HOD CD47^+/+ RBCs expressed levels of CD47 comparable to WT RBCs, HOD CD47^+/- RBCs exhibited significantly reduced CD47 levels (nearly half that observed on WT HOD RBCs) and HOD CD47^-/-RBCs failed to express any detectable CD47 (p < 0.0001). Following transfusion, HOD CD47^+/- and HOD CD47^-/- RBCs produced significantly higher levels of IgG anti-HOD antibodies than HOD CD47^+/+ RBCs on days 14 and 28 post-transfusion (p < 0.001). However, while HOD CD47^-/- RBCs displayed increased clearance consistent with the possible enhancement of a CD4 T cell response (p < 0.0001), HOD CD47^+/- RBCs failed to exhibit any different in clearance when compared to HOD CD47^+/+ RBCs overtime. To examine the potential impact of differences in HOD RBC clearance on CD4 T cell activation, OTII T cell proliferation was evaluated. While HOD CD47^-/- RBC transfusion resulted in significantly increased 33D1⁺ dendritic cell uptake, OTII proliferation, activation and cytokine secretion (p < 0.05), no difference in 33D1⁺ dendritic cell uptake or T cell response was observed following HOD CD47^+/+ RBC or HOD CD47^+/- RBC transfusion. Instead, HOD CD47^+/- RBCs exhibited enhanced antigen removal, while also displaying an increased ability to activate HEL specific B cells when compared to HOD CD47^+/+ or HOD CD47^-/- RBC transfusion.

Conclusions: These results demonstrate that alterations in CD47, which occur during normal RBC senescence and cold storage, directly influence RBC alloimmunization through different mechanisms depending on the extent of CD47 loss. While complete loss of CD47 results in enhanced RBC clearance, antigen presentation and CD4 T cell activation, reductions in CD47 to half WT levels failed to impact RBC clearance or T cell activation, but instead enhanced antigen specific B cell activation. These results demonstrate that even partial loss of CD47 is capable of significantly enhancing alloantibody formation completely independent of its role in regulating RBC clearance. In doing so, these findings provide novel insight into the role of CD47 as a key regulator of RBC alloimmunization.