Project Summary Blood transfusion is a life-saving intervention for subjects with acute blood loss or hematological disturbance, and approximately 5 million people per year receive red blood cell transfusions annually in the US. While blood transfusions clearly help those in need, the immune and inflammatory side-effects of transfusions may have detrimental consequences in transfusion recipients. Recent clinical studies suggest that older RBC units may be associated with worsened outcome in some patient populations. The purpose of this research proposal is to discover changes that occur in stored RBC units and test methods of reversing or preventing these changes. The thrust of the research will be to define how RBC units affect innate and adaptive immune responses and vascular reactivity in transfusion recipients and how storage of RBC units can alter these transfusion effects. In addition to detailed in vitro studies, the proposal will explore these same parameters in participants of a clinical trial correlating age of blood with clinical outcome in critically ill transfusion recipients. The broad hypothesis behind this proposal is that storage of RBCs increases their ability to modulate immune responses and to activate vascular endothelial cells in transfusion recipients. Leukoreduced RBC units will be characterized throughout storage for the ability to modulate innate and adaptive immune responses. RBC- endothelial cell interaction will be measured using cutting-edge flow cell technology to measure the frequency and strength of RBC adhesion to endothelial cells and to measure the activation of endothelial cells exposed to fresh and stored RBC units. After defining the changes in the immunomodulatory and vasoactivating properties of stored RBCs, methods of preventing these changes will be explored. Sophisticated immunology measurements will be made after RBC exposure, including multiplex measurement of cytokine/chemokine induction in T cells and neutrophils, induction of proliferative responses, and skewing of regulatory and IL-17 secreting T cell subsets. To test the in vivo relevance of the study findings, the immune parameters measured will be extended to subjects receiving RBC units stored for short vs. long periods in a randomized clinical trial, and relevant immune parameters will be correlated with disease outcome (e.g. is immune suppression linked with infectious complications?). This research proposal joins a team of investigators with complementary expertise to significantly advance our knowledge of potentially harmful immunomodulatory and vasoactivating effects of transfusion and their dependence on storage of RBCs. Importantly, the proposal will validate the knowledge gained and systems developed in a human clinical trial and will evaluate methods of preventing harmful effects of RBC storage using in vitro systems.