FTY720 promotes accumulation of S1P₁^–/– T cells in PPs. TRITC- or CFSE-labeled thymocytes from S1P₁^+/ (A) or S1P₁^–/– (B) FL chimeras were adoptively transferred into control or FTY720-treated WT recipients (n = 4 mice/group). After 20 hours, mice were killed and PBL and SLOs were harvested. Single-cell suspensions were stained for CD4 and CD8α and analyzed by FACS for the presence of TRITC⁺ and CFSE⁺ donor SP cells. (C) TRITC- or CFSE-labeled thymocytes from S1P₁^+/ or S1P₁^–/– FL chimeras were adoptively transferred into control or FTY720-treated WT recipients (n = 5). After 2.5 hours mice were killed and PPs were harvested and single-cell suspensions were stained for CD4 and CD8α and analyzed by FACS. Error bars indicate SEM. *P < .05.