Studies highlighting the key role of proinflammatory cytokines in RA have allowed the development of treatment targeting those molecules. IL-1band TNF inhibitors are the first step toward targeted therapies in RA. However, despite the high efficiency of these molecules, primary or acquired unresponsiveness are not uncommon, thus highlighting the necessity to elaborate alternate therapeutic means. In polygenic diseases such as RA, gene therapy can be used, to transfer a DNA fragment that encodes an anti-inflammatory mediator. Advantages of this strategy over direct administration of the protein include the possibility to obtain long term protein expressionin vivo and to control the kinetics and spatial distribution of protein delivery. In this context, we previously established the feasibility of electrotransfer (ET) of plasmids in collagen-induced arthritis (CIA), a model of RA, using IL-10 gene[5]or type I soluble receptor for TNF (sTNFRI) gene.Among anti-inflammatory candidates, IL-35 belongs to the IL-12 family of heterodimeric cytokines and is composed by two subunits, IL-12a and Epstein–Barr virus induced gene 3 (Ebi3). It was recently identified as an additional anti-inflammatory and immunosuppressive cytokine [8]. There is still relatively limited knowledge of this molecule which has been shown to induce the differentiation of CD4+ effector T cells into regulatory T cells (Treg) that in turn express IL-35 but lack expression of Foxp3, TGF-band IL-10 (iTreg35 cells). The iTreg35 cells generated in vitro can prevent various mouse models of autoimmune diseases. These include the systemic autoimmunity of Foxp3/ mice, experimental autoimmune encephalitis and inflammatory bowel disease (IBD) . Moreover, ectopic expression of IL-35 in pancreatic beta cells prevents autoimmune diabetes , and recombinant IL-35 injections protects against IBD [11] and CIA. Interestingly, in this model, exogenous IL-35 treatments can suppressed Th1 and Th17 cells, but promote CD39 expression by
CD4+ T cells.The need to develop new therapeutic strategies in RA led us to apply a gene therapy system to transfer IL-35 gene in CIA. We thus aimed at evaluating the therapeutic potential of IL-35 encoding plasmid electrotransfer in this model and at evaluating its cellular mode of action.