Transfusion of stored blood impairs host defenses against Gram‐negative pathogens in mice

Administration of iron supplements, particularly to children in developing nations, increases the risk of infection and mortality.1–4 For example, the risk of Gram-negative sepsis increased by sixfold in a trial of iron dextran administration to newborns in Polynesia.2 Furthermore, in a meta-analysis of randomized studies of intravenous (IV) iron therapy, although iron administration reduced the need for red blood cell (RBC) transfusions, this practice is associated with a significantly increased risk of infection.5 Human tissues are iron restricted, and there is fierce competition between host and pathogen for this essential nutrient.6 In a meta-analysis of controlled trials,7 a restrictive transfusion strategy reduced the relative risk of infection (0.76; 95% confidence interval, 0.60–0.97). By US Food and Drug Administration (FDA) criteria, RBCs destined for transfusion can be refrigerator stored for up to 42 days before transfusion. Furthermore, in many observational studies,8–15 transfusions of RBCs stored for longer durations are an independent risk factor for infectious complications. Thus, both transfusion per se and iron therapy used to prevent RBC transfusions are associated with increased infectious complications. We previously showed in mice,16 dogs,17 and humans18 that transfused, storage-damaged RBCs are rapidly cleared from the circulation by mononuclear phagocytes; the accompanying hemoglobin (Hb) is rapidly catabolized, and the associated iron is returned to plasma at a pace that can exceed the rate of uptake by transferrin, the physiologic iron transporter, thereby producing circulating non–transferrin-bound iron.16,18 After administration of iron dextran, most of the colloid circulates in plasma for several hours to days and is progressively cleared by mononuclear phagocytes that split the complex and then return the iron to plasma.19,20 Malaria infection continuously delivers iron to macrophages by intra- and extravascular hemolysis.21 Interestingly, certain pathogens, such as Salmonella sp., exhibit seasonality along with malaria infection, and coinfection with malaria increases morbidity and mortality.22–24 Thus, we hypothesize that iron administration, whether by older, stored RBC transfusions, by iron dextran, or by malaria infection, exacerbates infection with model Gram-negative pathogens (i.e., Salmonella typhimurium and Escherichia coli). S. typhimurium is a facultative intracellular pathogen that survives and replicates inside mononuclear phagocytes.25 Salmonella infection is also associated with hemolytic disorders, such as malaria26–29 and sickle cell disease.30,31 In addition, in Africa, nontyphoidal Salmonella is frequently isolated, representing 18% of all isolates in children more than 60 days old.32 Although the prevalence of coinfection varies by region,24 nontyphoidal Salmonella is the most frequent,27–29 or among the most frequent,32 isolates from malaria-infected children. Finally, coinfections of malaria and nontyphoidal Salmonella are associated with increased morbidity and mortality in humans and animal models.27,33,34 Infection is a leading cause of morbidity and mortality in hospitalized patients. E. coli, an extracellular Gram-negative bacterium, frequently causes infection in intensive care units.35 Furthermore, animal models of hemolysis suggest that, like S. typhimurium, E. coli infections are exacerbated by hemolysis.36 Thus, using murine models,16,37 we examined the effects of RBC transfusion, iron dextran administration, or malaria on infections with model intracellular and extracellular bacterial pathogens: S. typhimurium and E. coli, respectively. Finally, to provide a potential explanation of why the effect of transfusion on infection risk is not more striking in epidemiologic studies of hospitalized patients, many of whom are on antibiotics, we treated mice with empiric antibiotic therapy and infected them with E. coli with or without RBC transfusion or iron dextran therapy.