Prior Immunization to an Intracellular Antigen Enhances Subsequent Red Blood Cell Alloimmunization in Mice

Antibodies to red blood cell (RBC) alloantigens can increase morbidity and mortality in transfusion recipients. However, alloimmunization rates can vary dramatically, with some patients never generating alloantibodies following transfusion, while others not only become alloimmunized, but may be prone to generating additional alloantibodies following subsequent transfusion. Previous studies suggest that CD4 T cell responses that drive alloantibody formation recognize the same alloantigen engaged by B cells. However, as RBCs express numerous antigens, both internally and externally, it is possible that CD4 T cell responses directed against intracellular antigens may facilitate subsequent alloimmunization to a surface RBC antigen. Here we show that B cells can acquire intracellular antigens from RBCs. Using a mouse model of donor RBCs expressing two distinct alloantigens, we demonstrate that immune priming to an intracellular antigen, which would not be detected by any currently used RBC compatibility assays, can directly influence alloantibody formation following exposure to a subsequent distinct surface RBC alloantigen. These findings suggest a previously under-appreciated mechanism whereby transfusion recipient responders may exhibit an increased rate of alloimmunization due to prior immune priming toward intracellular antigens.