Expression of the sphingosine 1-phosphate receptor, S1P1, on T-cells controls thymic emigration

S1P1 is a widely distributed G protein-coupled receptor whose ligand, sphingosine 1-phosphate, is present in high concentrations in the blood. The sphingosine 1-phosphate receptor-signaling pathway is believed to have potent effects on cell trafficking in the immune system. To determine the precise role of the S1P1 receptor on T-cells, we established a T-cell-specific S1P1 knock-out mouse. The mutant mice showed a block in the egress of mature T-cells into the periphery. The expression of the S1P1 receptor was up-regulated in mature thymocytes, and its deletion altered the chemotactic responses of thymocytes to sphingosine 1-phosphate. The results indicated that the expression of the S1P1 receptor on T-cells controls their exit from the thymus and entry into the blood and, thus, has a central role in regulating the numbers of peripheral T-cells.