Human embryonic kidney cells HEK293 cells were obtained from the American Type Culture Collection (ATCC, Manassas, VA), and maintained in Dulbecco’s modified Eagle’s medium (DMEM; Life Technologies) supplemented with 10% fetal bovine serum (FBS; Life Technologies), penicillin (100 U ml−1), and streptomycin (100 mg ml−1) at 37 °C, 5% CO2. Spodoptera frugiperda cells (purchased from Life Technologies) were maintained at 28 °C in Grace insect medium supplemented with 10% FBS and penicillin and streptomycin (Life Technologies). FLS were obtained from synovial tissue of RA patients undergoing joint surgery who fulfilled the ACR/EULAR criteria for RA20. Human and rat FLS were isolated by enzyme digestion and cultured in DMEM supplemented with 10% FBS and antibiotics and used between passages 4 and 9, as previously described13. All cell lines were routinely checked for mycoplasma every 6 months.