IL1B and rheumatoid arthritis: We have previously reported that MTX promotes gene expression of the cell cycle regulators CDKN1A (cyclin-dependent kinase inhibitor 1A, p21), TP53 (tumor protein p53) and LBH (Limb-bud and heart development) in activated (PDGF + IL-1β-stimulated) RA-FLS, with most pronounced effects after 48 h [17].