ObjectiveTo investigate the role of sphingosine-1-phosphate （S1P） in abnormal ossification in ankylosing spondylitis （AS）， clarify the relationship between S1P and “angiogenesis-osteogenesis” coupling， and provide new strategies for AS treatment. MethodsFemoral heads from AS patients and patients undergoing routine hip replacement were collected for immunohistochemical （IHC） staining to evaluate osteogenesis and H-type vessel formation. In vitro， ELISA was used to quantify the synthesis of S1P and analyze the expression changes of S1P signaling pathway-related molecules during the osteogenic differentiation of mesenchymal stem cells derived from patients with ankylosing spondylitis （ASMSCs） and those from healthy donors （HDMSCs）， to evaluate the activation status of S1P pathway during osteogenesis. Sphingosine kinase 1 （SK1） expression was knocked down in MSCs， and the S1P receptor inhibitor FTY720 was applied to block S1P signaling. Alkaline phosphatase （ALP） activity and Alizarin Red S （ARS） quantification were used to assess the effect of S1P on ASMSCs osteogenesis. Conditioned medium from osteogenically induced MSCs was used to treat human umbilical vein endothelial cells （HUVECs） to evaluate the effect of S1P on angiogenesis. An AS mouse model （SKG mice） was treated with FTY720 or the SK1 inhibitor PF-543 citrate. IHC staining and micro-CT scanning were used to assess abnormal ossification and spinal fusion， and immunofluorescence was used to evaluate H-type vessel formation. ResultsCompared with Osteonecrosis of the Femoral Head（ONFH） patients， AS patients exhibited excessive osteogenesis and H-type vessel formation （OCN P＜0.001， CD31 P＜0.001， EMCN P＜0.001）. During osteogenic differentiation， S1P expression and secretion were significantly higher in ASMSCs than in HDMSCs （P=0.0179）. Inhibition of S1P signaling with FTY720 or SK1 knockdown significantly suppressed osteogenic differentiation （compared with ASMSC， ARS： HDMSC P=0.001 8， FTY720 P＜0.001， si-SK1 P＜0.001； ALP： HDMSC P=0.032 8， FTY720 P=0.001 6， si-SK1 P＜0.001） of ASMSCs and the angiogenesis of HUVEC（compared with ASMSC， cell-covered area， total loops， total tube length and total branch points P＜0.001）. Treatment with FTY720 or PF-543 markedly inhibited abnormal ossification and spinal fusion（compared with Curdlan， arthritis index score， P＜0.001； OCN：control P=0.002， PF-543 P=0.010 7， FTY720 P=0.015 9 ） in AS mice and reduced H-type vessel formation （CD31⁺EMCN⁺： compared with curdlan， control P＜0.001， PF-543 P=0.001 7， FTY720 P=0.002 1）. ConclusionIncreased S1P synthesis in ASMSCs promotes osteogenic differentiation via autocrine mechanisms and further enhances ossification by facilitating H-type angiogenesis. Inhibiting S1P secretion in ASMSCs significantly suppresses abnormal ossification in AS.