Introduction: Sphingosine-1-phosphate (S1P) is a signaling sphingolipid that regulates cell egress from lymphoid and non-lymphoid tissues. While S1P has been described as a chemoattractant, its activity is very low in in vitro migration assays. Since moving from tissue to lymph, and egress from lymphoid organs, involves interactions with lymphatic endothelium, we examined T cell migration to S1P across biologically relevant endothelial monolayers.

Methods: Induced Treg (iTreg) were generated using Foxp3GFP mice, and flow-purified based on CD25 and Foxp3GFP expression, and non Treg CD4 conventional T cells (Tconv) were isolated directly from C57BL/6 mice using CD4 and CD25 bead selection. T cells were used in transmigration assays across SVEC (lymphatic endothelial cell (LEC) line) and MS-1 (blood endothelial cell line) monolayers in vitro. Inverted monolayers allowing assessment of migration from the endothelial basal side to luminal side are termed iSVEC or iMS-1. In some experiments T cells, endothelial cells, or both were treated with pertussis toxin (PTX), S1P, the S1P analogue FTY720, or anti-S1P1 Ab.

Results: Migration of Tconv and iTreg to S1P across plastic was poor (∼2-5% of input cells), while migration to S1P across LEC in the basal to luminal direction was much greater (∼2-5 fold). In contrast, migration of iTreg and Tconv across blood MS-1 and iMS-1 monolayers was no different than across plastic. In contrast to S1P, migration of Tconv across LEC to CCL19 was worse than across plastic. Tconv migration to S1P + CCL19 across LEC was additive. Enhanced migration of iTreg and Tconv to S1P across LEC was partially blocked by treating T cells and LEC with anti-S1P1 receptor Ab. PTX, which inhibits many G-coupled chemotaxis receptors, only inhibited migration if LEC but not T cells were treated. This suggests that T cells depended on a PTX-insensitive pathway for migration to S1P across LEC, while LEC were partially PTX sensitive.

Conclusions: LEC are specially adapted to mediate S1P-driven transmigration of T cells, while blood endothelial cells are not. Chemotaxis to S1P and CCL19 across LEC is fundamentally different. T cell migration to S1P across LEC is a process that requires active participation from both T cells and LEC, and involves PTX sensitive and insensitive components. These results identify novel regulatory pathways for Treg, LEC, and lymphatic migration.

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