Apolipoprotein M forms a complex with sphingosine-1-phosphate (S1P) and is the carrier of S1P in high density lipoprotein particles and mediates its endothelial protective effect. Increasing the concentration of the apoM/S1P complex by administering it, either alone or in HDL particles, can prevent or treat diseases caused by endothelial cell injury, including inflammatory diseases, sepsis, atherosclerosis and acute lung injury, ischemic heart disease, stroke, vital organ failure after ischemic stress.