Inflammation-dependent secretion and splicing of IL-32γ in rheumatoid arthritis

Different splice variants of the proinflammatory cytokine IL-32 are found in various tissues; their putative differences in biological function remain unknown. In the present study, we report that IL-32 gamma is the most active isoform of the cytokine. Splicing to one less active IL-32 beta appears to be a salvage mechanism to reduce inflammation. Adenoviral overexpression of IL-32 gamma (AdIL-32 gamma) resulted in exclusion of the IL-32 gamma-specific exon in vitro as well as in vivo, primarily leading to expression of IL-32 beta mRNA and protein. Splicing of the IL-32 gamma-specific exon was prevented by single-nucleotide mutation, which blocked recognition of the splice site by the spliceosome. Overexpression of splice-resistant IL-32 gamma in THP1 cells or rheumatoid arthritis (RA) synovial fibroblasts resulted in a greater induction of proinflammatory cytokines such as IL-1 beta, compared with IL-32 beta. Intraarticular introduction of IL-32 gamma in mice resulted in joint inflammation and induction of several mediators associated with joint destruction. In RA synovial fibroblasts, overexpression of primarily IL-32 beta showed minimal secretion and reduced cytokine production. In contrast, overexpression of splice-resistant IL-32 gamma. in RA synovial fibroblasts exhibited marked secretion of IL-32 gamma. In RA, we observed increased IL-32 gamma expression compared with osteoarthritis synovial tissue. Furthermore, expression of TNF alpha and IL-6 correlated significantly with IL-32 gamma expression in RA, whereas this was not observed for IL-32 beta. These data reveal that naturally occurring IL-32 gamma can be spliced into IL-32 beta, which is a less potent proinflammatory mediator. Splicing of IL-32 gamma into IL-32 beta is a safety switch in controlling the effects of IL-32 gamma and thereby reduces chronic inflammation.