BACKGROUND: Histone deacetylase inhibitors (HDACi) display potent therapeutic efficacy in animal models of arthritis and suppress inflammatory cytokine production in rheumatoid arthritis (RA) synovial macrophages and tissue. OBJECTIVES: To determine the molecular mechanisms contributing to the suppressive effects of HDACi on RA synovial cell activation, using interleukin 6 (IL-6) regulation as a model. METHODS: RA fibroblast-like synoviocytes (FLS) and healthy donor macrophages were treated with IL-1$β$, tumour necrosis factor (TNF)$α$, lipopolysaccharide or polyinosinic:polycytidylic acid (poly(I:C)) in the absence or presence of the HDACi trichostatin A (TSA) or ITF2357 (givinostat). IL-6 production and mRNA expression was measured by ELISA and quantitative PCR (qPCR), respectively. Protein acetylation and the activation of intracellular signalling pathways were assessed by immunoblotting. The DNA-binding activity of nuclear factor $κ$B (NF$κ$B) and activator protein 1 (AP-1) components was measured by ELISA-based assays. RESULTS: HDACi (0.25-1.0 $μ$M) suppressed RA FLS IL-6 production induced by IL-1$β$, TNF$α$ and Toll-like receptor ligands. Phosphorylation of mitogen-activated protein kinases and inhibitor of $κ$B$α$ (I$κ$B$α$) following IL-1$β$ stimulation were unaffected by HDACi, as were AP-1 composition and binding activity, and c-Jun induction. TSA induced a significant reduction in nuclear retention of NF$κ$B in FLS 24 h after IL-1$β$ stimulation, but this did not reduce NF$κ$B transcriptional activity or correlate temporally with reductions in IL-6 mRNA accumulation. HDACi significantly reduced the stability of IL-6 mRNA in FLS and macrophages. CONCLUSIONS: Our study identifies a novel, shared molecular mechanism by which HDACi can disrupt inflammatory cytokine production in RA synovial cells, namely the promotion of mRNA decay, and suggests that targeting HDAC activity may be clinically useful in suppressing inflammation in RA.