Sphingosine-1-phosphate receptor 1 (S1P1) was recently shown to be required for lymphocyte egress from lymphoid organs. Here we have examined the relationship between S1P1 abundance on the cell and egress efficiency. Using an integrin neutralization approach to separate the processes of entry and exit, we show that pertussis toxin treatment reduces lymphocyte egress from lymph nodes. Retrovirally mediated S1P1 overexpression is sufficient to reduce B cell accumulation in the splenic white pulp and to promote egress of activated T cells from lymph nodes, whereas S1P1/cells have reduced lymph node exit efficiency. Furthermore, lymphocyte S1P1 is down-regulated in the blood, up-regulated in lymphoid organs, and down-regulated again in the lymph. We propose that cyclical ligand-induced modulation of S1P1 on circulating lymphocytes contributes to establishing their lymphoid organ transit time.

After entering a secondary lymphoid organ from the blood, naive lymphocytes travel to separate subcompartments where they survey for antigen. In the absence of antigen encounter, the cells leave the organ via the efferent lymphatics or in the case of the spleen, via the red pulp. Timely egress ensures that the cells travel rapidly to further lymphoid organs to continue their antigen-surveillance process. The molecular mechanisms of lymphocyte entry into secondary lymphoid organs have been well characterized over the past several decades; however, the molecules involved in lymphocyte exit are just starting to be realized. Integrins are essential for lymphocyte entry into peripheral lymphoid organs (1–3). The integrin L 2 (LFA-1) plays a major role and 2-deficient lymphocytes exhibit 50% reduced homing to peripheral lymph nodes. 4 integrins, 4 1 and 4 7, account for the remaining integrin requirement during entry, and when both L 2 and 4 integrins are blocked with neutralizing antibodies, lymphocyte entry into lymph nodes is inhibited by 98%(1). Integrin function during entry into