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Background Caspases form an evolutionarily conserved family of cytosolic, aspartate-specific, cysteine proteases. Beyond its role in apoptosis, certain caspases (caspase-1, -4, -5, -11 and -12) exert non-apoptotic functions, including cytokine maturation during innate immunity, cell differentiation, proliferation, and NF-κB activation. Objectives This study investigated expression of caspase-5 and effect of Z-WEHD-fmk, a selective caspase-5 inhibitor, on pro-inflammatory gene expressions and cellular proliferation in fibroblast-like synoviocyte (FLS) in patients with rheumatoid arthritis (RA). Methods Synovial tissues were obtained from patients with RA during total knee replacement surgery then FLS were isolated and cultured. Expression of caspase-5 in rheumatoid FLS were measured after stimulation either without or with pro-inflammatory agents IL-1β (2 ng/mL), TNF-α (20 ng/mL), lipopolysaccharide (1000 ng/mL) by real-time quantitative RT-PCR. The involvement of caspase-5 in inflammation and cellular proliferaion was further examined by treating the cells with caspase-5 inhibitor Z-WEHD-fmk. The mRNA expressions of IL-1β, IL-6, IL-18, COX-2, MMP-9 and CCL-2 after caspase-5 inhibitor Z-WEHD-fmk treatment were determined by real-time quantitative RT-PCR, and FLS proliferation in response to caspase-5 inhibitor Z-WEHD-fmk treatment was measured by MTT assay. Results Caspase-5 mRNA expression in RA FLS was increased by all the pro-inflammatory agents compared to control. Caspase-5 inhibitor Z-WEHD-fmk treatment induced a reduction in mRNA expressions of IL-1β, IL-6, IL-18, COX-2, MMP-9 and CCL-2 from RA FLS. But, cellular proliferation of rheumatoid FLS was not suppressed by caspase-5 inhibitor Z-WEHD-fmk. Conclusions Caspase-5 inhibitor treatment inhibited suppressed various pro-inflammatory gene expressions. These results suggest that inhibition of Caspase-5 can be a novel therapeutic approach in human RA. References Martinon F, Tschopp J. Inflammatory caspases and inflammasomes: master switches of inflammation. Cell Death Differ. 2007;14:10 –22. Scott AM, Saleh M. The inflammatory caspases: guardians against infections and sepsis. Cell Death Differ. 2007;14:23–31. Lamkanfi M, Kalai M, Vandenabeele P. Caspase-12: an overview. Cell Death Differ. 2004;11:365–368. Schwerk C, Schulze-Osthoff K. Non-apoptotic functions of caspases in cellular proliferation and differentiation. Biochem Pharmacol 2003;66:1453-8. Disclosure of Interest None Declared