Edited by Peter CresswellTumor necrosis factor-␣ (TNF) exerts its biological effect through two types of receptors, p55 TNF receptor (TNFR1) and p75 TNF receptor (TNFR2). An inflammatory response is known to be induced mainly by TNFR1, whereas an anti-inflammatory reaction is thought to be mediated by TNFR2 in some autoimmune diseases. We have been investigating the use of an antagonistic TNF mutant (TNFR1-selective antagonistic TNF mutant (R1antTNF)) to reveal the pharmacological effect of TNFR1-selective inhibition as a new therapeutic modality. Here, we aimed to further improve and optimize the activity and behavior of this mutant protein both in vitro and in vivo. Specifically, we examined a trimeric structural fusion of R1antTNF, formed via the introduction of short peptide linkers, as a strategy to enhance bioactivity and molecular stability. By comparative analysis with R1antTNF, the trimeric fusion, referred to as single-chain R1antTNF (scR1antTNF), was found to retain in vitro molecular properties of receptor selectivity and antagonistic activity but displayed a marked increase in thermal stability. The residence time of scR1antTNF in vivo was also significantly prolonged. Furthermore, molecular modification using polyethylene glycol (PEG) was easily controlled by limiting the number of reactive sites. Taken together, our findings show that scR1antTNF displays enhanced molecular stability while maintaining biological activity compared with R1antTNF.Recently, TNF inhibitors in various molecular formats such as a neutralizing antibody (infliximab or adalimumab), soluble receptor (etanercept), or a PEGylated antibody Fab fragment (certolizumab pegol), have been clinically used as anti-TNF drugs to treat autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, and ulcerative colitis (1-3). These drugs elicit a highly beneficial therapeutic effect, but this treatment may lead to serious complications including bacterial or viral infection (4, 5), demyelination (6), and lupus-like syndrome (7). Therefore, biopharmaceutical development of this treatment with a new modality is sought.TNF binds to two receptor subtypes to exert its biological functions: TNFR1 3 and TNFR2 (8). TNFR1 is expressed in diverse cell types and raises the inflammatory response, whereas TNFR2, which is limited to expression in immune cells, endothelial cells, and neuronal cells, induces cell survival and proliferation. For example, it has been reported that the incidence and severity of arthritis were lower and milder in TNFR1 knock-out mice than in wild-type mice (9). Moreover, TNFR1 knock-out mice or TNFR1/TNFR2 double knock-out mice display less severe symptoms of experimental autoimmune encephalomyelitis symptoms than wild-type mice (10). Previous studies demonstrated that transgenic mice overexpressing human TNF develop severe arthritis (11, 12) or demyelination (13). Thus, the involvement of TNFR1 in autoimmune diseases is strongly implicated as human TNF is unable to signal