Patrolling monocytes in sickle cell hemolytic conditions.

Patients with sickle cell disease (SCD) suffer from intravascular hemolysis associated with vascular injury and dysfunction. Painful vaso-occlusive crisis (VOC) involving increased attachment of sickle erythrocytes and activated leukocytes to damaged vascular endothelium is a hallmark of SCD. Patrolling monocytes, which normally scavenge damaged cells and debris from the vasculature, express higher levels of anti-inflammatory heme oxygenase 1 (HO-1), a heme degrading enzyme with anti-cytotoxic and anti-inflammatory properties. Recent data show that patients with SCD have a novel subset of patrolling monocytes expressing very high levels of HO-1 (HO-1hi) which are decreased in numbers in patients who had a recent VOC episode. This patrolling monocyte subset was responsible for protection of endothelium against sickle RBC stasis in an experimental model. This raises the possibility that patrolling monocytes may also offer protection against vascular endothelium damage in hyperhemolytic conditions in SCD.