Erythrocyte-derived ectosomes have immunosuppressive properties

Several clinical studies have suggested that blood transfusions are immunosuppressive. Whereas there have been reports describing immunosuppression induced by leukocytes or fragments thereof, the possibility that microparticles, released by erythrocytes during storage, are also involved was not investigated. We present evidence here that such microparticles have all the properties of ectosomes including size, the presence of a lipid membrane, and the specific sorting of proteins. These erythrocyte-derived ectosomes (Eecto) fixed C1q, which was followed by activation of the classical pathway of complement with binding of C3 fragments. Similarly to ectosomes released by PMN, they express phosphatidylserine on their surface membrane, suggesting that they may react with and down-regulate cells of the immune system. In vitro, they were taken up by macrophages, and they significantly inhibited the activation of these macrophages by zymosan A and LPS, as shown by a significant drop in TNF-alpha and IL-8 release (respectively, 80% and 76% inhibitions). In addition, the effect of E-ecto was not transient but lasted for at least 24 h. In sum, E-ecto may interfere with the innate immune system/inflammatory reaction. Therefore, E-ecto transfused with erythrocytes may account for some of the immunosuppressive properties attributed to blood transfusions. J. Leukoc. Biol. 84: 1316-1325; 2008.