Tumor necrosis factor-α (TNF), a proinflammatory cytokine, is critical to the pathogenesis of various inflammatory diseases. There are two subtypes of receptors for TNF, namely type I TNF receptor (TNFR1) and type II TNF receptor (TNFR2). Previous studies using animal models of diseases have demonstrated the predominant role of TNFR1 in the pathogenesis of inflammation. It has recently been proposed that TNFR2 is associated with anti-inflammatory function. This intriguing function of TNFR2 has implications from an immunological and pharmacological perspective. However, the mechanism of the TNFR2-mediated anti-inflammatory effect is not fully understood. In this context, we attempted to elucidate the TNFR2-mediated anti-inflammatory effect and other unknown biological functions of TNFR2 by utilizing our protein engineering technology to generate functional mutant cytokines. Our findings reveal the following. (1) TNFR2 is expressed on regulatory T cells (Tregs) but not conventional T cells (Tconvs) and TNFR2-mediated signals promote proliferation and activation of Tregs. (2) The crystal structure of TNF/TNFR2 complex was solved, which suggests a possible signal initiation mechanism via TNF/TNFR2 cluster formation on the cellular membrane. (3) A novel TNFR2-mediated signal molecule, aminopeptidase P3 (APP3/XPNPEP3), was identified that interacts with TNFR2 as an intracellular adaptor protein. APP3 is required for c-Jun N-terminal kinase (JNK) phosphorylation, the downstream molecule of TNFR2 signal transduction. These results are key to understanding the mechanism of immune regulation and will assist in the identification of immunomodulatory drugs targeting the TNFR2 signaling cascade as well as the function of Tregs.
Keywords: mutant cytokine; phage display library; regulatory T cell; type II tumor necrosis factor-α receptor.