Syringin inhibits the crosstalk between macrophages and fibroblast-like synoviocytes to treat rheumatoid arthritis via PDE4

Background: Syringin (SRG) is well-known for its anti-inflammatory effects. However, its pharmacological mechanisms against rheumatoid arthritis (RA) are not fully understood.

Materials and methods: We assessed the anti-RA effects of SRG using a collagen-induced arthritis (CIA) rat model. And, we employed single-cell RNA sequencing (scRNA-seq) to analyze the changes in cell types and gene expression in the synovial tissues. Building on these observations, we investigated the effects of SRG on M1 macrophage polarization and RA-fibroblast-like synoviocytes (FLS) proliferation.

Results: Our findings highlighted the anti-RA effects of SRG on CIA rat. scRNA-seq of rat synovial tissues revealed significant changes in M1 and RA-FLS. Specifically, SRG decreased the levels of inflammatory factors in the supernatants of LPS and IFN-γ induced THP-1 cells and downregulated M1-polarized markers in these cells. Further analysis indicated that SRG's regulation of phosphodiesterase 4 (PDE4) and its associated factors was crucial for its anti-M1 polarization effects. Besides, we found that SRG inhibited the activation of FLS in vivo but showed no direct effects on RA-FLS in vitro. However, in RA-FLS, co-cultured with supernatant from SRG-treated M1-polarized THP-1 cells exhibited lower ability of cell proliferation and activation as compared to co-cultured with supernatant from M1-polarized THP-1 cells.

Conclusion: By integrating scRNA-seq analysis with in vivo and in vitro validations, our study revealed that SRG achieved its anti-RA effects by blocking the interaction between macrophages and RA-FLS, with PDE4 playing a central role. This study may provide a novel research paradigm in studying the multi-cell regulatory mechanisms of natural compounds.