Redistribution of Sphingosine 1-Phosphate by Sphingosine Kinase 2 Contributes to Lymphopenia

Abstract

Sphingosine kinases (SKs) 1 and 2 produce high concentrations of sphingosine 1-phosphate (S1P) in blood and lymph. In contrast, S1P concentrations in lymphoid tissues are kept low by the S1P-degrading activity of the S1P-lyase. These differences in S1P concentrations drive lymphocyte circulation. Inhibition of the S1P-lyase prevents lymphocyte egress and causes lymphopenia because of increased S1P levels in lymphoid tissues. In this study, we investigated the source of this accumulating S1P in lymphoid tissues by using SK2-deficient (SK2−/−) mice. In contrast to wild-type mice, SK2−/− mice exhibited attenuated lymphopenia after S1P-lyase inhibition by 4-deoxypyridoxine (DOP). Consistently, S1P concentrations were only modestly increased in lymphoid tissues of SK2−/− mice compared with a significantly higher increase in wild-type mice after DOP treatment. Low S1P concentrations in lymphoid tissues of DOP-treated SK2−/− mice were accompanied by higher S1P concentrations in blood, suggesting that SK2−/− mice display defective S1P transport from blood into lymphoid tissues. To investigate this potential new role of SK2, RBCs loaded with traceable C17-S1P were transfused into wild-type and SK2−/− mice, resulting in much higher C17-S1P concentrations in blood of SK2−/− mice compared with wild-type mice 2 h after transfusion. Moreover, cocultures of RBCs with mouse splenocytes and endothelial cells demonstrated that SK2 regulated cellular uptake of S1P from RBCs. Collectively, our data suggest that S1P in lymphoid tissues derives from blood and point to an essential role of SK2 in S1P transport.

Sphingosine-1-phosphate lyase expression in embryonic and adult murine tissues

“…The relative SPL expression of each cell population is given below, as determined by ImageJ quantifi cation of autoradiogram signal and shown as SPL signal normalized to actin signal. Similarly, SPL is highly expressed in the thymus, corroborating previous reports ( 61,62 ). SPL expression is restricted to the epithelial cells of the thymic stroma.…”

Local Inactivation of Sphingosine 1-Phosphate in Lymph Nodes Induces Lymphopenia

“…1C). The reason for these differences is the extracting activity of Sphingomab for RBC-associated S1P (14,15). In contrast to isotype control Ab, Sphingomab was able to bind and dissociate RBC-bound S1P in cell culture experiments with whole blood, transferring RBCassociated S1P in its Ab-bound form into plasma (Fig.…”

“…Inhibition of this enzyme by the vitamin B 6 antagonist 4-deoxypyridoxine (DOP) abrogates lymphocyte egress by extensive accumulation of S1P in lymphoid organs (2). Cellular S1P transport from erythrocytes in blood to lymphocytes in tissues predominantly accounts for accumulating cellular S1P after S1P-lyase inhibition (14,15). The resulting high S1P concentrations in the thymus and lymph nodes neutralize proposed S1P gradients between lymphoid organs and blood and lymph, prematurely downregulate the S1P-gradient-sensing S1P 1 receptor in lymphocytes, and activate the S1P 1 receptor on lymphatic endothelial cells to close suggested endothelial cell barriers, resulting in peripheral blood lymphopenia (7,16).…”

Sources, metabolism, and regulation of circulating sphingosine-1-phosphate

“…In order to prevent S1P Pharmacological inhibition or knockout of SPHK1 decreases the S1P level in whole blood and plasma by approximately 50%, suggesting that both SPHK isoforms are equally important in generation of the blood pool of this sphingolipid ( 77,78 ). Unexpectedly, genetic or pharmacological inhibition of SPHK2 results in a striking increase in plasma and blood cell S1P concentration ( 77,79 ). This observation indicates that regulation of plasma S1P level is more complex than initially presumed.…”

“…This observation indicates that regulation of plasma S1P level is more complex than initially presumed. Sensken et al ( 79 ) proposed that the accumulation of S1P in blood observed in mice lacking SPHK2 is a consequence of impaired intracellular rephosphorylation of sphingosine released from blood-borne S1P degraded by LPP1. Their results strongly suggest that SPHK2 plays a key role in regulation of S1P redistribution from erythrocytes into ECs.…”