This past year we have determined a key role for the S1P1 receptor in the peripheral expression of natural killer T (NKT) cells:[unreadable] [unreadable] The S1P1 receptor, on the surface of lymphocytes and endothelial cells, regulates the unique trafficking behavior of certain lymphocyte populations. We have examined whether the S1P1 receptor also dictates the distinctive tissue distribution of Valpha14-Jalpha18 natural killer T (NKT) cells, whose trafficking pattern is not well understood. Mice (TCS1P1KO) were established with a conditional deletion of the S1P1 receptor in thymocytes that included precursors of NKT cells. Within the thymus, NKT cells were found at normal or increased levels, indicating that S1P1 receptor expression was dispensable for NKT cell development. However, substantially reduced numbers of NKT cells were detected in the peripheral tissues of the TCS1P1KO mice. Short-term S1P1 deletion after NKT cells had established residence in the periphery did not substantially alter their distribution in tissues, except for a partial decrease in the spleen. FTY720, a S1P1 receptor ligand that has potent effects on the trafficking of conventional T cells, did not alter the preexisting distribution of NKT cells within peripheral tissues of wild-type mice. Our results indicate that the S1P1 receptor expression on NKT cells is dispensable for development within thymus but is essential for the establishment of their tissue residency in the periphery.[unreadable] [unreadable] Our collaborative efforts have yielded the following results:[unreadable] [unreadable] 1. Sphingosine kinases are crucial determinants of mast cell responsiveness and regulate anaphylaxis.[unreadable] [unreadable] 2. S1P1 receptor has a key role in IgG plasma cell homing to bone marrow and retention in secondary lymphoid organs.[unreadable] [unreadable] 3. Sphingosine kinase 1 is dispensable for inflammatory responses.