Background Nucleotide-binding and oligomerization domain NOD-like receptors (NLRs) have been associated with several human diseases including infections, malignancies, and inflammatory disorders. These innate immune pattern recognition molecules paly critical roles in innate immunity and inflammation. Some inflammasome-forming NLRs including NLRP1, NLRP2, NLRP3, NLRC4, NLRC5, NLRP6, and NLRP7, can induce the activation of mitogen-activated protein kinase and the transcription factor NF-κB. A different set of NLRs, such as NLRP12, NLRX1, and NLRC3, negatively regulate diverse biological pathways associated with inflammation.
Objectives This study was performed to determine the expression pattern of NLRs in rheumatoid synovium and to investigate whether their regulation can influence the inflammatory response in fibroblast-like synoviocytes (FLS) from patients with rheumatoid arthritis (RA).
Methods We explored the expression of several inflammasome-forming and -nonforming NLRs, including NLRP1, NLRP2, NLRP3, NLRC5, NLRX1, and NLRC3, in synovium obtained from patients with RA and patients with osteoarthritis (OA) by immunohistochemistry. The expression of NLRs was also studied in FLS derived from joint tissues of RA patients and OA patients using real time RT-PCR. RNA interference (RNAi) plasmids for NLRs were transfected to abrogate specific NLR expression in RA FLS, and adenovirus containing the NLR transcript was delivered into RA FLS to strengthen its gene expression. Levels of pro-inflammatory genes and their protein products were determined using real-time RT-PCR and ELISA in RA FLS.
Results By immunohistochemistry and real time RT-PCR, we were unable to distinguish RA from OA synovium on the basis of their level of RNA expression and protein production of NLRP2, NLRP3, NLRC5, and NLRX1. In synovial tissues from RA patients, NLRC1 was not detected, though RA FLS contained higher levels of NLRP1 compared to OA FLS. NLRC3 expression was detected at the RNA and protein levels in RA synovium and FLS, but its levels were much lowered that those from OA samples. Gene expression and production of IL-1β, IL-6, chemokine ligand 2 (CCL-2), CCL-7, cyclooxygenase 2 and MMP-9 were markedly increased in NLRC3 RNAi plasmid-transfected RA FLS, while transfection with adenoviral vectors encoding NLRC3 induced reductions in those levels.
Conclusions NLRC3 is down-regulated in RA synovium and the regulation of its expression showed anti-inflammatory activity in RA FLSs. These findings provide evidence for the anti-inflammatory effect of NLRC3 in RA.
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Disclosure of Interest None declared