The role of ceramide synthase 2 in sphingosine-1-phosphate-mediated thymic egress

Abstract

A key element for effective adaptive immunity against invading pathogens is an everchanging peripheral TCR repertoire, which requires the constant release of newly formed T cells from the thymus. The process of thymic egress into the circulation is of chemotactic nature and is mediated by a soluble gradient of the lipid mediator sphingosine-1- phosphate (S1P), which results from low S1P levels in the thymic interstitium, but high S1P levels at the thymic exit sites and in the blood. Despite its importance for peripheral T cell homeostasis, the exact architecture of the thymic S1P gradient, as well as the underlying mechanisms, which maintain the spatial differences in S1P concentration are incompletely understood.<br /> In the present study, we investigated if ceramide synthase 2 (CERS2), as regulator of the S1P precursor sphingosine, is involved in the maintenance of the thymic S1P gradient. We found that CERS2 -deficient mice not only exhibited increased amounts of sphingosine, but also elevated levels of S1P within the thymic parenchyma, in the plasma and in RBCs, which resulted in a robust distortion of the S1P gradient between thymus and blood. Consistently, thymic egress was impaired in CERS2 -deficient mice, indicated by a selective accumulation of egress-competent mature SP thymocytes in the thymus and a corresponding T cell lymphopenia in the periphery. We, therefore, concluded that the CERS2-dependent regulation of sphingosine is an important mechanism to facilitate ef- ficient thymic egress of newly formed T cells into the periphery, as it prevents aberrant S1P synthesis by the sphingosine kinases (SPHKs) and thus stabilizes the chemoattractive S1P gradient between thymus and blood. CERS2 is ubiquitously expressed in the hematopoietic and non-hematopoietic cells of the thymus. However, we observed that neither the exclusive ablation of CERS2 in hematopoietic cells, nor the specific absence of CERS2 in thymic stromal cells could phenocopy the impaired S1P-dependent thymic egress, which was observed in CERS2 -deficient mice. These results indicate that CERS2 shapes the thymic S1P gradient not by regulating S1P production in the thymus itself, but in extrathymic non-hematopoietic cells and thus exclusively by limiting the levels of S1P in the circulation. Based on our CERS2 expression data and the current understanding how S1P levels in blood are regulated, we hypothesize that vascular endothelial cells are the most promising cell type, in which CERS2 might be required to compete with the SPHKs to limit the production and secretion of S1P into the plasma.<br /> Taken together, our study establishes for the first time a requirement of CERS2 for S1P gradient regulation between thymus and blood. Thus, we advance the understanding how egress from primary lymphoid organs and peripheral lymphocyte homeostasis are regulated in vivo. Consequently, CERS2 might be a promising pharmacological target to treat autoimmune diseases or other immunological disorders by manipulating lymphocyte trafficking.