Relationship of Soluble Adhesion Receptors to Red Cell Apoptosis In SCD.

Abstract 1654

The interaction of sickle red cells (SS RBCs) with endothelium can cause endothelial injury, leading to increased expression of soluble endothelial cell adhesion molecules and plasma markers of endothelial activation and dysfunction. Among such plasma markers, sVCAM-1 levels are increased in SCD and rise further during vaso-occlusive crisis (Duits et al. 1993), and several groups have shown that sVCAM-1 levels are positively related to LDH levels, a marker of hemolysis. Perfusion of endothelial cells with SS RBCs has been shown to increase the expression of the cellular adhesion molecules ICAM-1 and VCAM-1 by endothelial cells (Shiu et al. 2000). Furthermore, sVCAM-1, sICAM-1, and sE-selectin have been independently associated with risk of mortality (Kato et al. 2005). We were interested in determining whether identifiable characteristics of SS RBCs themselves might be associated with the degree to which soluble adhesion molecules are expressed in SCD patient plasma. Therefore, we measured sVCAM-1, sICAM-1, sCD40 ligand (sCD40L) and sP-selectin in SCD patient plasma by ELISA (n=46) and looked at the relationship of these biomarkers of disease activity to SS RBC characteristics. We specifically examined phosphatidylserine exposure (as measured by annexin V binding analyzed by flow cytometry) and the degree of SS RBC adhesion to laminin and endothelial cells, measured in vitro in a graduated height flow chamber. We found that the degree of red cell damage measured by annexin V binding correlated with the level of soluble adhesion receptors sVCAM-1 (p=0.036) and sICAM-1 (p=0.026). However, annexin V binding did not correlate with sCD40L or sP-selectin expression in plasma, and neither annexin V binding nor levels of soluble adhesion molecules correlated with SS RBC adhesion to laminin or endothelial cells in vitro. In addition, the concentrations of sVCAM-1 and sICAM-1 in plasma were strongly correlated with each other (p<0.0001), suggesting that both of these adhesion molecules may be released from activated endothelial cells in SCD patients as a result of the same mechanism involving contact with phosphatidylserine-exposing SS RBCs. These data support the hypothesis that phosphatidylserine exposure on SS RBCs is a critical factor in SS RBC interaction with the endothelium (Setty et al., 2002), including the release of soluble cellular adhesion markers that indicate endothelial activation and/or injury.