Integrins are important adhesion receptors in all Metazoa that transmit conformational change bidirectionally across the membrane. Integrin α and β subunits form a head and two long legs in the ectodomain and span the membrane. Here, we define with crystal structures the atomic basis for allosteric regulation of the conformation and affinity for ligand of the integrin ectodomain, and how fibrinogen-mimetic therapeutics bind to platelet integrin α IIb b β3 . Allostery in the β 3 I domain alters three metal binding sites, associated loops and a α1-and α7-helices. Piston-like displacement of the a 7-helix causes a 62° reorientation between the β 3 I and hybrid domains. Transmission through the rigidly connected plexin/semaphorin/integrin (PSI) domain in the upper β 3 leg causes a 70Å separation between the knees of the α and β legs. Allostery in the head thus disrupts interaction between the legs in a previously described low-affinity bent integrin conformation, and leg extension positions the high-affinity head far above the cell surface.Integrins are adhesion receptors that transmit signals bidirectionally across the plasma membrane 1-4 . Rearrangements in integrin extracellular, transmembrane and cytoplasmic domains underlie diverse biological processes, including cell migration, morpho-genesis, immune responses and vascular haemostasis. The platelet-specific integrin α IIb β 3 is important in both the arrest of bleeding at sites of vascular injury and pathological thrombosis leading to heart attacks and stroke. Loss of the vascular endothelium results in platelet deposition, and receptors for collagen, thrombin and other agonists then initiate

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Vascular occlusion is the major cause of morbidity and mortality in sickle cell disease but its mechanisms are poorly understood. We demonstrate by using intravital microscopy in mice expressing human sickle hemoglobin (SS) that SS red blood cells (RBCs) bind to adherent leukocytes in inflamed venules, producing vasoocclusion of cremasteric venules. SS mice deficient in P-and E-selectins, which display defective leukocyte recruitment to the vessel wall, are protected from vasoocclusion. These data uncover a previously unsuspected paradigm for the pathogenesis of sickle cell vasoocclusion in which adherent leukocytes play a direct role and suggest that drugs targeting SS RBC-leukocyte or leukocyte-endothelial interactions may prevent or treat the vascular complications of this debilitating disease. Sickle cell disease, the first molecular disease identified in humans, is among the most common inherited hematological disorder in the United States. It results from a single amino acid substitution in the ␤-chain of hemoglobin (Hb␤ S ). Hb␤ S polymerizes on deoxygenation, producing less deformable sickle red blood cells (SS RBCs) that can obstruct blood vessels (1). More than 20 years ago, it was demonstrated that SS RBCs displayed increased adherence to endothelial cells (2, 3). Subsequent studies recognized the importance of several adhesion pathways in these interactions (4-10), and revealed that young (lowdensity) SS RBC were more adherent to the endothelium than dense (often irreversibly sickled) SS RBCs (11-13). Collectively, these observations led to the current multistep model for sickle cell vasoocclusion in which light-density cells first adhere in postcapillary venules and secondary trapping of dense cells produces vascular obstruction and ischemia.To test this model in vivo, we applied the technique of intravital microscopy, which allows direct visualization of the microvasculature, to mice genetically engineered to exclusively express human (h) Hb␤ S . Such mice have circulating irreversibly sickled cells and manifest several of the cardinal features of sickle cell disease, including anemia, reticulocytosis, and organ damage (14, 15). To overcome the limitations imposed by the high perinatal mortality and low breeding efficiency, we generated sickle cell mice by transplantation of sickle cell bone marrow precursors into lethally irradiated normal adult recipients. Unexpectedly, we observed that SS RBCs interacted primarily with adherent leukocytes in postcapillary and collecting venules rather than the endothelial surface, and that these interactions led to vascular occlusion. These data indicate a direct role for leukocytes in the pathogenesis of sickle cell vasoocclusion and provide a potential explanation for the clinical association between leukocytosis and poor prognosis in sickle cell disease. Bone Marrow Transplantation. Femoral bone marrow nucleated cells from WT C57BL͞6, SA, and SS mice were harvested and injected (1.5 ϫ 10 6 cells͞recipient) via the lateral tail vein of lethally irradiated (1,2...