Tumour necrosis factor alpha-driven IL-32 expression in rheumatoid arthritis synovial tissue amplifies an inflammatory cascade

Objective To investigate the interplay between IL-32 and tumour necrosis factor alpha (TNF alpha) during the chronic inflammation of rheumatoid arthritis (RA) and to assess whether anti-TNF alpha treatment of RA patients modulates synovial IL-32 expression. Methods Induction of IL-32 gamma by Pam3Cys, lipopolysaccharide, IL-1 beta or TNF alpha was investigated in human fibroblast-like synoviocytes (FLS). Stimulation of TNF alpha production by IL-32. was studied by adenoviral overexpression of IL-32 gamma (AdIL-32 gamma) and lipopolysaccharide stimulation of THP1 cells. Silencing of endogenous IL-32 was employed to study cytokine regulation in FLS. AdIL-32 gamma followed by TNF alpha stimulation was performed in FLS to investigate cytokine induction. Immunohistochemistry was applied to study IL-32 expression in synovial biopsies from RA patients. Results TNF alpha potently induced IL-32 gamma expression in FLS. Increased TNF alpha, IL-1 beta, IL-6 and CXCL8 production was observed after IL-32 gamma overexpression and lipopolysaccharide stimulation of THP1 cells. TNFa stimulation of FLS after silencing IL-32 gamma resulted in diminished IL-6 and CXCL8 production, whereas IL-32 gamma overexpression resulted in enhanced IL-6 and CXCL8 levels. Remarkably, the mechanism through which IL-32 gamma overexpression induced TNF alpha, IL-1 beta and CXCL8 was by counteracting messenger RNA decay. Importantly, treatment of RA patients with anti-TNF alpha resulted in significant reduction of IL-32 protein in synovial tissue. Conclusions TNF alpha is a potent inducer of endogenous IL-32 expression and IL-32 itself contributes to prolonged TNF alpha production, thus inducing an important autoinflammatory loop. Treatment of RA patients with anti-TNF alpha antibodies diminished IL-32 expression in synovial tissue. The potent anti-inflammatory effect of TNF alpha blockade in RA patients may be partly due to the reduction of synovial IL-32 expression.