[10] The discovery that the mechanism of action of FTY720 occurs via S1PR modulation[11] spurred interest in immunological functions of S1P signalling. Later studies demonstrated amelioration of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, with low-dose FTY720,[12] which has since been approved as a first-line oral agent for treatment of relapsing–remitting multiple sclerosis.[13-15] The pharmacology and biology of FTY720 are covered in great depth by other reviews.[16, 17] Studies to characterize the mechanisms underlying the induction of lymphopenia by FTY720 paved the

way to better this website understanding of the basic biological principles of lymphocyte circulation and revealed the importance of S1P1 in this process[4] (Fig. 1a). Using fetal liver from S1pr1−/− embryos to create bone marrow chimeric mice, Matloubian, et al. demonstrated that egress of lymphocytes from thymus and secondary lymphoid organs did not occur in the absence of S1P signalling, establishing

a requirement for S1P–S1P1 interaction in regulating lymphocyte egress. Additional MK-1775 molecular weight studies established that S1P1 expression was temporally regulated during T-cell development, culminating in high expression by mature single-positive CD4 or CD8 thymocytes and that conditional deletion of S1pr1 in T cells alone was sufficient to block their egress from the thymus. As S1P1 provides a critical chemotactic cue, and levels of S1P are high in the blood and lymph and low in most tissues,[7] it was postulated that this

‘S1P gradient’ would play a role in lymphocyte egress. Indeed, disruption of the S1P gradient by 2-acetyl-4-tetrahydroxyimidazole, an inhibitor of the S1P degradative enzyme S1P lyase, led to lymphopenia and blocked T-cell egress from the thymus.[18] This effect was mediated by increases in tissue concentrations of S1P and S1P-mediated down-regulation of surface S1P1, so impairing chemotactic responses.[18] Studies using conditional deletion of the S1P biosynthetic enzymes, sphingosine kinases 1 and 2 (Sphk1/2) demonstrated that an almost complete loss of S1P in the blood and lymph correlated with high cell surface expression Oxalosuccinic acid of S1P1 on naive T cells in the circulation. Lymphopenia was also evident, but infusion of S1P (in the form of S1P-producing erythrocytes) into sphingosine kinase-deficient mice, led to the release of lymphocytes into the blood concomitant with decreased cell surface expression of S1P1.[19] Mutant mice that express an internalization-defective S1P1 that is signalling competent have delayed lymphopenia kinetics in response to FTY720 or 2-acetyl-4-tetrahydroxyimidazole treatment, further supporting the premise that cell surface residency of S1P1 is a primary determinant of lymphocyte egress.[20] These observations combine to create a model whereby high concentrations of ligand lead to S1P1 surface down-regulation and so to non-responsiveness to S1P chemotactic cues.