The pathway of interferon-gamma (IFN-gamma)-induced suppression in tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-mediated apoptosis of fibroblast-like synovial cells (FLS) was investigated. rTRAIL triggered FLS apoptosis in a type II cell death manner, whereas IFN-gamma pretreatment significantly inhibited TRAIL-mediated apoptosis. As disruption of mitochondrial transmembrane potential (DeltaPsim), Leu-Glu-His-Asp ase (IETD ase) activity, and the appearance of hypodiploid DNA + cells were markedly suppressed in IFN-gamma-treated FLS in response to TRAIL, IFN-gamma-induced suppression was supposed to achieve at upstream of caspase-8. IFN-gamma rapidly phosphorylated signal transducers and activators of transcription 1 (STAT1), STAT3, and STAT6 as well as ERK, whereas enhanced neither phosphorylation of Akt nor nuclear translocation of nuclear factor kappaB (NF-kappaB) p65. Janus kinase (JAK)-induced phosphorylation of STAT1/3/6, which acts at translational regulation, seemed to be crucial because chemical inhibition of JAK as well as cycloheximide (CHX) abolished both the phosphorylation of STAT1/3/6 and the IFN-gamma-induced inhibitory effect. Although ERK was phosphorylated through IFN-gamma, chemical inhibition of ERK by PD98059 did not abolish the IFN-gamma-induced inhibitory effect. The authors tried to determine the responsible molecules; however, expression of TRAIL receptors; pro-caspase-3/-8/-9; Fas-associated death domain protein (FADD); tumor necrosis factor receptor 1-associated death domain protein (TRADD); silencer of death domain (SODD); FLICE inhibitory protein (FLIP); and Bcl-2, Bcl-xL, and Bax in FLS was not modulated by IFN-gamma. Although the authors have not yet clarified the precise mechanism, these data suggest that IFN-gamma/JAK/STAT pathway, which is supposed to be activated in inflammatory rheumatoid arthritis (RA) synovial tissues, contributes to form apoptosis resistance phenotype of the cells in situ, leading to a marked increase in cellularity of synovial cells.