Background: Acute pain is the primary symptom of tendinopathy, and mast cells activation is a nonnegligible cause. Currently, safe and effective treatment for acute pain in tendinopathy is still lacking. This study was aimed to explore the analgesic effect of iPSC-derived mesenchymal stem cell-derived small extracellular vesicles (iMSC-sEVs) on acute pain and further investigate the underlying mechanisms in tendinopathy. 

Methods: A rat tendinopathy model was established and then treated with iMSC-sEVs or PBS for 4 weeks. The pathology of tendinopathy was accessed by H&E staining and immunohistochemical analysis 2 and 4 weeks after treatment. Then we measured pain-related behaviors 4 weeks after treatment. Double immunofluorescent staining on tendon sections for tryptase and PGP9.5 was conducted to assess whether iMSC-sEVs inhibit mast cells activation in tendinopathy. To further investigate the potential mechanism, RBL-2H3 cells were stimulated with substance P to mimic tendinopathy condition in vitro. The effect of iMSC-sEVs on inhibiting mast cells activation was measured by b-Hexosaminidase release assay, RT-qPCR, Toluidine blue staining and ELISA. RNA-seq was performed to analyze the related global changes and discover the underlying mechanism. 

Results: iMSC-sEVs effectively reduced inflammation, thereby alleviating acute pain in tendinopathy as reflected by histological analysis and pain-related behaviors. Moreover, iMSC-sEVs inhibited activated mast cell infiltration and interactions with nerve fibers in tendinopathy. In vitro study showed that the expression of proinflammatory cytokines and the degranulation of mast cells induced by substance P were reduced upon iMSC-sEVs treatment. Transcriptome analysis revealed that iMSC-sEVs treatment down-regulated the expression of genes involved in the HIF-1 signaling pathway. 

Conclusion: Overall, this study demonstrated that iMSC-sEVs relieved tendinopathy-related pain through inhibiting mast cells activation partly via the HIF-1 signaling pathway. These findings provide a novel treatment strategy for pain derived from tendinopathy and unravel the molecular mechanism underlying the application of iMSC-sEVs on mast cells.