STAT3 and rheumatoid arthritis: costus</i> and to determine the contribution of reactive sesquiterpene lactones to this effect.<h4>Materials and methods</h4>Extracts from Formulation C plants were screened in cell-based assays targeting RA-relevant pathways (NF-κB, NFAT, STAT3/STAT5), prostanoid formation in a synovial fibroblast cell line, and cytokine production in primary B cells.