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Circulation of mature lymphocytes between blood and secondary lymphoid tissues plays a central role in the immune system. Homing of lymphocytes from blood into secondary lymphoid tissues beyond high endothelial venules is highly dependent on the interaction between the chemokines CCL19, CCL21, CXCL12, and CXCL13,and their receptors CCR7, CXCR4 and CXCR5. However, the molecular mechanism(s) of lymphocyte egress from secondary lymphoid tissues to lymph remained unclear. We have found a new class of immunomodulator, FTY720 by chemical modification of vegetative wasp-derived natural product, ISP-I (myriocin). FTY720 has been shown to be highly effective in experimental allograft and autoimmune disease models. A striking feature of FTY720 is the induction of a marked decrease in peripheral blood lymphocytes at doses that show immunomodulating activity in these models. The reduction of circulating lymphocytes by FTY720 is caused by sequestration of iymphocytes into secondary lymphoid tissues and thymus. FTY720 is rapidly converted to (S)-enantiomer of FTY720-phosphate[(S)-FTY720-P] by sphingosine kinase 2 in vivo. (S)-FTY720-P acting as a potent agonist of S1P receptor type 1(S1P1), induces long-term down-regulation of S1P1 on lymphocytes, and thereby inhibits the migration of lymphocytes toward S1P. Thus, it is presumed that FTY720-induced lymphocyte sequestration is due to the inhibition of S1P/S1P1-dependent lymphocyte egress from secondary lymphoid tissues and thymus by its active metabolite (S)-FTY720-P. Throughout the analysis of the mechanism of action of FTY720, it is clarified that S1P/S1P1 interaction plays an important role for lymphocyte egress from secondary lymphoid tissues and thymus.