Procoagulant and proinflammatory effects of red blood cells on lipopolysaccharide-stimulated monocytes

Background: We aimed to evaluate the mechanisms underlying the effects of red blood cells (RBCs) on the reactivity of monocytes to lipopolysaccharide (LPS) stimulation. Methods: Measurements of tissue factor (TF) antigen and activity were performed on freshly isolated white blood cells (WBCs)/platelets resuspended in heparinized plasma, as well as cultured monocytic cells. Results: In a dose-dependent manner, RBCs significantly enhanced LPS-induced TF activity and antigen levels in blood monocytes; potentiation of TF activity by both human and murine RBCs did not require the presence of neutrophils and/or platelets. We also measured the levels of monocyte chemotactic protein-1 (MCP-1), the key proinflammatory chemokine that binds to duffy antigen receptor for chemokines (DARC) on RBC surface, in plasma and RBC lysates after the incubation of RBCs with WBC/platelets; at the concentrations corresponding to normal blood counts, RBCs exerted a significant influence on the free plasma levels of MCP-1, with about two-thirds of detectable MCP-1 post-LPS stimulation being associated with RBCs. Critically, DARC-deficient murine RBCs failed to enhance LPS-induced TF activity, confirming the mechanistic significance of RBC-DARC. Conclusions: Our study reports a novel mechanism by which RBCs promote procoagulant and proinflammatory sequelae of WBC exposure to LPS, likely mediated by RBC-DARC in the microenvironment(s) that bring monocytes and RBCs in close proximity.