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.