Serious complications and mortality after cardiac surgery are increased when transfused red cells are stored for more than 2 weeks (Koch, N Engl J Med. 2008). However, the mechanism remains unclear and the relationship of procoagulant activity to PS exposure on banked RBCs has not been clarified, although prior studies suggest that banked cells went to apoptosis. Procoagulant activity of RBCs was measured in a modified prothrombin time in which RBCs replaced thromboplastin. The presence of PS in neutrophils and mononuclear cells from healthy donors and banked RBCs was investigated by flow-cytomety and confocal microscopy. The assembly of extrinsic tenase, intrinsic tenase, and prothrombinase complexes on these cells was studied using enzymatic assays employing plasma or purified proteins. Lactadherin, a glycoprotein, was utilized as a probe to track PS exposure and as an agent to block exposed PS. Coagulant activity increased approx 5-fold after cells were banked for 14 days and progressively increases. The percent of phosphatidylserine (PS)-positive cells increased from 7.4% on day 14 to 30% on day 42. RBCs with exposed PS were nearly absent in samples at 7 days but readily evident at 14 days on confocal microscopy. Banked RBCs exhibited patched, a rim, and diffuse-pattern that stained with lactadherin but neutrophils and MNC did not, indicating more PS exposure on banked RBCs from 7 days than the other two cell types. These data reveal a positive correlation between PCA and PS exposure on stored RBCs. Inhibition of PCA by lactadherin further confirms that PS supports the PCA in stored RBCs. Thus, we propose the possible role of PCA due to increased PS exposure on aged RBCs as a potential pathogenetic factor leading to organ injury, aggravated infections and finally higher mortality.

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