The role of the lysophospholipid sphingosine 1-phosphate in immune cell biology

Sphingosine 1-phosphate (S1P) has been shown to be a bioactive lipid mediator intimately involved in mediating a variety of immunological processes. In particular, S1P regulates lymphocyte cell trafficking between the lymphatic system and the blood. The lysophospholipid signals mainly through five related G protein-coupled receptor subtypes, termed S1P₁ to S1P₅. S1P₁ seems to play an essential role in cell trafficking, as this receptor subtype promotes the egress of T and B cells from secondary lymphatic organs. This S1P₁-mediated migratory response is a consequence of different S1P levels in the serum and lymphatic organs. In addition to its direct effects on lymphocyte motility, S1P strengthens cell barrier integrity in sinus-lining endothelial cells, thereby reducing lymphocyte egress out of lymph nodes. Furthermore, S1P modulates cytokine profiles in T and dendritic cells, resulting in an elevated differentiation of T helper-2 cells during the T cell activation process. It is of interest that the mode of molecular action of the novel immunomodulator FTY720 interferes with the signaling of S1P. After phosphorylation, FTY720 shares structural similarity with S1P, but in contrast to the natural ligand, phosphorylated FTY720 induces a prolonged internalization of S1P₁, resulting in an impaired S1P-mediated migration of lymphocytes.