Surface expression of tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors on rheumatoid arthritis synovial fibroblasts (RASFs). (a) Staining of TRAIL receptors, as analysed by flow cytometry, showed constitutive surface expression of TRAIL-R1 and TRAIL-R2 on RASFs in vitro. TRAIL-R3 and TRAIL-R4 were not detectable. The expression levels did not change in confluent RASFs. (b) Furthermore, cell surface expression of TRAIL-R1 and TRAIL-R3 on RASFs remained unchanged in RASFs synchronised with hydroxyurea (HU). Representative histograms of three separate experiments are shown.