S1P: the elixir of life for naive T cells

During their life, T cells are constantly circulating throughout the body. Sphingosine-1-phosphate (S1P), a sphingolipid metabolite, triggers T cell egress from the thymus and secondary lymphoid organs (SLO) into the lymph and the blood. This phenomenon is dependent on the S1P gradient between lymphoid organs (including thymus and SLO) and the lymphatic and blood vessels, which exhibit low, intermediate and high S1P levels, respectively. T cells follow the S1P gradient via the engagement of the G-protein-coupled receptor S1P receptor 1 (S1P1) expressed on T cells.1 S1P production is regulated by various sphingolipid-metabolizing enzymes. The sphingosine kinases 1 and 2 (SKs), encoded by Sphk1 and Sphk2, phosphorylate the sphingosine (Sph) into S1P. Conversely, S1P can be dephosphorylated to Sph by S1P phosphatases 1 and 2, encoded by Sgpp1 and Sgpp2. Alternatively, S1P can be irreversibly degraded by the S1P Lyase (SPL), encoded by Sgpl1, into phosphoethanolamine and hexadecenal. The secretion of S1P was described to involve two transporters, the ATP-binding cassette (ABC) transporter, in a non-specific manner, and the recently identified specific transporter Spinster 2 (SPNS2), encoded by the Spns2 gene.2 As a result, Spns2-deficient mice exhibit decreased concentrations of S1P into the lymph but not blood.3 In a recent study, Mendoza et al. reported that S1P, which is secreted by lymphatic endothelial cells via SPNS2, favors naive T-cell survival by maintaining their mitochondrial content in an S1P1-dependent manner.4 Herein, we discuss the main findings and pathophysiological implications of the latter study as regard to the state of the art.