Plasma Sphingosine-1-Phosphate Orchestrates the Reverse Transendothelial Migration of Aortic Intimal Myeloid Cells
Background: Myeloid cells (MCs) reside in the aortic intima at regions predisposed to atherosclerosis. Systemic inflammation triggers reverse transendothelial migration (RTM) of intimal MCs into arterial blood, which orchestrates a protective immune response that clears intracellular pathogens from the arterial intima. Molecular pathways that regulate RTM remain poorly understood. Sphingosine-1-phosphate (S1P) is a lipid mediator that regulates immune cell trafficking by signaling via five G protein-coupled receptors (S1PRs). We investigated the role of S1P in the RTM of aortic intimal MCs.  Methods: Intravenous injection of lipopolysaccharide (LPS) was used to model a systemic inflammatory stimulus that triggers RTM. CD11c+ intimal MCs in the lesser curvature of the ascending aortic arch were enumerated by en face confocal microscopy. Local gene expression was evaluated by transcriptomic analysis of microdissected intimal cells.  Results: In wild-type C57BL/6 mice, LPS induced intimal cell expression of S1pr1, S1pr3, and sphingosine kinase 1 (Sphk1, a kinase responsible for S1P production). Pharmacological modulation of multiple S1PRs blocked LPS-induced RTM and modulation of S1PR1 and S1PR3 reduced RTM in an additive manner. Cre-mediated deletion of S1pr1 in MCs blocked LPS-induced RTM, confirming a role for myeloid-specific S1PR1 signaling. Global or hematopoietic deficiency of Sphk1 reduced plasma S1P levels, the abundance of CD11c+ MCs in the aortic intima and blunted LPS-induced RTM. In contrast, plasma S1P levels, the abundance of intimal MCs, and LPS-induced RTM were rescued in Sphk1-/- mice transplanted with Sphk1+/+ or mixed Sphk1 +/+ and -/- bone marrow. Stimulation with LPS increased endothelial permeability and intimal MC exposure to circulating factors such as S1P.  Conclusions: Functional and expression studies support a novel role for S1P signaling in the regulation of LPS-induced RTM as well as the homeostatic maintenance of aortic intimal MCs. Our data provides insight into how circulating plasma mediators help orchestrate intimal MC dynamics. Overall design: Cells were isolated by micro-dissection of the lesser curvature aortic arch intima of C57BL/6J wildtype mice (control or injected with 30 ug ultrapure LPS i.v. 18 hours prior to harvest). Cells from 5-10 mice were pooled for each sample, and were subjected to scRNAseq.