Erythrophagocytosis by angiogenic endothelial cells is enhanced by loss of erythrocyte deformability.

OBJECTIVE: Angiogenic endothelial cells can function as phagocytes, and phagocytosis is initiated via the opsonin lactadherin. In this study, we examined the interaction between lactadherin-opsonized erythrocytes with reduced deformability and angiogenic endothelium, as loss of deformability is characteristic for suicidal and aged erythrocytes. MATERIALS AND METHODS: We used the Arg-Gly-Asp (RGD)-modified erythrocyte model and investigated the deformability parameter by cross-linking erythrocyte membranes through treatment with glutaraldehyde. Association in vitro with primary endothelial cells was detected by flow cytometry and visualized by light, fluorescent, and electron microscopy. Involvement of two crucial factors in phagocytosis, alpha(v)-integrins and Rho guanosine triphosphatase family member Rac1, was studied using small interfering RNA technology. Modified erythrocytes were administered in vivo into tumor-bearing mice to detect phagocytosis by endothelial cells. RESULTS: Glutaraldehyde-treated (rigid) RGD-modified erythrocytes showed a strongly enhanced endothelial cell association compared to flexible RGD-modified erythrocytes. Knockdown by small interfering RNA lipoplexes of alpha(v)-integrins and Rac1 confirmed classical tethering and internalization of rigid RGD-erythrocytes. Upon in vivo administration, tumor endothelium showed pronounced erythrophagocytosis. CONCLUSION: The pronounced phagocytosis of opsonized erythrocytes with reduced deformability by angiogenic growth factor-activated endothelial cells evokes new insights in endothelial cell function and suggests a role for these endothelial cells in (hematological) disorders because of their capacity to clear disordered erythrocytes.