MPO and Systemic capillary leak syndrome: NO promotes relaxation of vascular smooth muscle and endothelial contraction and has been implicated as a therapeutic target for SCLS‐associated hypotension.19 Rodent studies show that MPO localizes at the vascular endothelium and extracellular matrix under inflammatory stress and affects vascular smooth muscle and endothelial functions.20 ROS may also induce EC apoptosis; however, we did not observe apoptosis of HMVECs incubated with acute or convalescent SCLS sera.2