P-selectin–deficient mice to study pathophysiology of sickle cell disease

Sickle cell disease (SCD) is an autosomal-recessive genetic disorder that affects millions of people worldwide.1,2 Vaso-occlusion and hemolysis are the 2 predominant vascular events that contribute to the pathogenesis of SCD.3 Vaso-occlusion is believed to trigger acute systemic painful vaso-occlusive episode, which is the primary reason for emergency medical treatment of SCD patients.1,4 A role for P-selectin in promoting vaso-occlusion in the cremaster venules of SCD mice was first demonstrated by Kaul and Hebbel.5 Later, sickle erythrocytes were shown to adhere to P-selectin in vitro and undergo P-selectin–mediated rolling adhesion in postcapillary venules of mice in vivo, and P-selectin deletion or inhibition was shown to prevent adhesion of adoptively transferred sickle erythrocytes and vaso-occlusion in nonsickle mice in vivo.6-9 Also, platelet P-selectin–dependent neutrophil-platelet-erythrocyte heterocellular aggregates were shown to be significantly elevated in SCD patient blood.10 Epidemiological evidence suggests that a vaso-occlusive episode is often an antecedent to acute chest syndrome, a type of acute lung injury and one of the leading causes of mortality among SCD patients.1,11-13 Recently, we found that vaso-occlusive episode in transgenic humanized SCD mice triggered microembolism of precapillary pulmonary arterioles by platelet-neutrophil aggregates, which led to loss of blood flow in the lung microvasculature.13 Remarkably, platelet-neutrophil aggregates were attenuated, lung vaso-occlusion was prevented, and pulmonary blood flow was rescued in SCD mice following therapeutic blockade of P-selectin.13 A role for P-selectin in vaso-occlusion was further supported by a recent phase 2 study that reported a significant reduction in painful vaso-occlusive episodes among SCD patients receiving the P-selectin–blocking antibody crizanlizumab.14 Altogether, these findings suggest that SCD mice genetically deficient in P-selectin would be protected from vaso-occlusion. Such a mouse would also be useful in identifying the role of P-selectin in SCD-associated morbidities other than painful vaso-occlusive episode or acute chest syndrome.15 A role for P-selectin in systemic vaso-occlusion has been investigated using chimeric SCD mice lacking P-selectin only in the endothelium (intact in platelets),6,16 because SCD mice with global deletion of P-selectin did not exist. Here, we introduce the first SCD mice genetically lacking P-selectin in hematopoietic and nonhematopoietic compartments. Using our recently developed quantitative fluorescence intravital lung microscopy (qFILM)13,17 technique, we show that P-selectin deficiency protects SCD mice from lung vaso-occlusion.