The vaso-occlusive process in patients with sickle cell disease (SCD) is complex and involves interactions both between hemoglobin S red blood cells (SS RBC) and vascular endothelium, and between SS RBC and leukocytes adherent to endothelium. Vaso-occlusive events lead to recurrent pain and cerebrovascular accidents in both children and adults, as well as other types of end-organ damage, including pulmonary hypertension and renal failure. However, the physiologic triggers inducing SS RBC adhesion and vaso-occlusion are poorly understood, and elucidation of these mechanisms at the molecular level would allow development of new preventive and treatment strategies to abrogate vaso-occlusive events. We have begun to explore the role of SS RBC cAMP signal transduction pathways in upregulating SS RBC adhesion to endothelial cells (EC). We have found that treatment of SS RBC with agents such as epinephrine, that lead to elevation of intracellular cAMP, induced enhanced SS RBC adhesion to EC and that this signaling pathway is also dependent on tyrosine phosphorylation. We have also shown that interaction between SS RBC and EC is mediated at least primarily by the adhesion receptor LW (ICAM-4) on SS RBC, which binds to alphav-beta3 integrin on EC. LW also binds to leukocyte integrins. We therefore hypothesize that abnormal circulating SS RBC adhesion to endothelium and to adherent leukocytes may at least in part be due to SS RBC adhesion receptor activation in vivo by processes involving upregulation of endogenous cAMP, leading to activation of LW adhesion receptor on SS RBC. We propose therefore to explore the molecular basis of SS RBC adhesion to both endothelium and leukocytes. Our specific aims are 1) to characterize the effects of physiologic adrenergic agonists known to lead to increased cAMP on LW-mediated SS RBC adhesion to endothelial cells both in vitro and ex vivo; 2) to characterize the mechanism of activation of the LW receptor; and 3) to investigate the possible role of LW in the interaction of SS RBC with leukocytes. Overall, these experiments will elucidate the molecular mechanisms of LW-mediated SS RBC adhesion to endothelium and leukocytes, and thus how physiologic stress and stress hormones may contribute to vaso-occlusion.