Figure 2. Cellular sources of plasma S1P.

Endothelial cells and rbc release S1P, which is picked up and chaperoned by ApoM on the HDL particles and albumin. Chaperone-bound S1P interacts with S1P₁ on the endothelial cells to promote vascular barrier function. When endothelial cells are damaged, platelet activation and aggregation release S1P, which leads to local release of S1P that aids in the repair of vascular injury.