Gene set enrichment analysis (GSEA) revealed that pathways involved in immunity and tumor-like transformation of RA-FLSs, such as interferon response, TNF signaling via NF-κB, inflammatory response, KRAS signaling, epithelial mesenchymal transition and interleukins (IL6 and IL2), were significantly downregulated after propionate treatment (Fig. 5c and Fig. S6). Here, IL2 is linked to rheumatoid arthritis.