Conclusion
Our findings indicate that S1P1 signaling in ECs modulates vascular responses to IC deposition. S1P1 agonists and ApoM-Fc enhance the EC barrier, limit leukocyte escape from capillaries, and provide protection against inflammatory injury. The S1P/S1P1 axis is a newly identified target to attenuate tissue responses to IC deposition and mitigate end-organ damage.
Methods
In vitro barrier function was assessed in human umbilical vein endothelial cells (HUVECs) by electrical cell-substrate impedance sensing. Phosphorylation of myosin light chain 2 (p-MLC-2) and VE-cadherin staining in HUVECs were assessed by immunofluorescence. A reverse Arthus reaction (RAR) was induced in the skin and lungs of mice with S1P1 deleted from ECs (S1P1 EC-knockout [ECKO] mice) and mice treated with S1P1 agonists and antagonists.
Objective
Immune complex (IC) deposition activates polymorphonuclear neutrophils (PMNs), increases vascular permeability, and leads to organ damage in systemic lupus erythematosus and rheumatoid arthritis. The bioactive lipid sphingosine 1-phosphate (S1P), acting via S1P receptor 1 (S1P1 ), is a key regulator of endothelial cell (EC) barrier function. This study was undertaken to investigate whether augmenting EC integrity via S1P1 signaling attenuates inflammatory injury mediated by ICs.
Results
S1P1 agonists prevented loss of barrier function in HUVECs treated with IC-activated PMNs. S1P1 ECKO and wild-type (WT) mice treated with S1P1 antagonists had amplified RAR, whereas specific S1P1 agonists attenuated skin and lung RAR in WT mice. ApoM-Fc, a novel S1P chaperone, mitigated EC cell barrier dysfunction induced by activated PMNs in vitro and attenuated lung RAR. Expression levels of p-MLC-2 and disruption of VE-cadherin, each representing manifestations of cell contraction and destabilization of adherens junctions, respectively, that were induced by activated PMNs, were markedly reduced by treatment with S1P1 agonists and ApoM-Fc.