Erythroblastic islands are anatomical units consisting of a central macrophage surrounded by erythroblasts. We studied the adhesion molecules involved in the formation of these structures. Central macrophages of erythroblastic islands isolated from the spleens of phlebotomized mice were clearly stained for vascular cell adhesion molecule 1 (VCAM-1). The surrounding erythroblasts of the erythroblastic islands strongly expressed the α4 integrin of very late activation antigen 4 (VLA-4: α4βl integrin), the counter receptor of VCAM-1, whereas most reticulocytes and erythrocytes did not. Both monoclonal antibodies (mAbs) against α4 integrin and VCAM-1 disrupted the erythroblastic islands cultured in the presence of erythropoietin. Moreover, adhesion of splenic erythroblasts to tumor necrosis factor α-stimulated mouse splenic endothelial cells, which showed high expression of VCAM-1 but not intercellular adhesion molecule 1, was inhibited by the anti-VCAM-1 and anti-α4 mAbs. These findings suggest that VLA-4- VCAM-I interaction plays a crucial role in the formation of erythroblastic islands.
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