Objective To investigate the interplay between IL-32 and tumour necrosis factor alpha (TNFα) during the chronic inflammation of rheumatoid arthritis (RA) and to assess whether anti-TNFα treatment of RA patients modulates synovial IL-32 expression.

Methods Induction of IL-32γ by Pam3Cys, lipopolysaccharide, IL-1β or TNFα was investigated in human fibroblast-like synoviocytes (FLS). Stimulation of TNFα production by IL-32γ was studied by adenoviral overexpression of IL-32γ (AdIL-32γ) and lipopolysaccharide stimulation of THP1 cells. Silencing of endogenous IL-32 was employed to study cytokine regulation in FLS. AdIL-32γ followed by TNFα stimulation was performed in FLS to investigate cytokine induction. Immunohistochemistry was applied to study IL-32 expression in synovial biopsies from RA patients.

Results TNFα potently induced IL-32γ expression in FLS. Increased TNFα, IL-1β, IL-6 and CXCL8 production was observed after IL-32γ overexpression and lipopolysaccharide stimulation of THP1 cells. TNFα stimulation of FLS after silencing IL-32γ resulted in diminished IL-6 and CXCL8 production, whereas IL-32γ overexpression resulted in enhanced IL-6 and CXCL8 levels. Remarkably, the mechanism through which IL-32γ overexpression induced TNFα, IL-1β and CXCL8 was by counteracting messenger RNA decay. Importantly, treatment of RA patients with anti-TNFα resulted in significant reduction of IL-32 protein in synovial tissue.

Conclusions TNFα is a potent inducer of endogenous IL-32 expression and IL-32 itself contributes to prolonged TNFα production, thus inducing an important auto-inflammatory loop. Treatment of RA patients with anti-TNFα antibodies diminished IL-32 expression in synovial tissue. The potent anti-inflammatory effect of TNFα blockade in RA patients may be partly due to the reduction of synovial IL-32 expression.