Abstract
Interleukin 23 (IL-23) regulates the development of TH17 cells, which are important for antimicrobial and antifungal responses and autoimmune and chronic inflammatory diseases. IL-23-induced Jak/STAT signaling is mediated via the heterodimeric IL-23 receptor (IL-23R)-IL-12 receptor β1 (IL-12Rβ1) complex. The typical signal-transducing receptor of the IL-6/IL-12 family contains three extracellular-membrane-proximal fibronectin type III (FNIII) domains, which are not involved in cytokine binding but are mandatory for signal transduction. In place of FNIII-type domains, IL-23R has a structurally undefined stalk. We hypothesized that the IL-23R stalk acts as a spacer to position the cytokine binding domains at a defined distance from the plasma membrane to enable signal transduction. Minor deletions of the murine, but not of the human, IL-23R stalk resulted in unresponsiveness to IL-23. Complete deletion of the human IL-23R stalk and the extended murine IL-23R stalk, including a 20-amino-acid-long duplication of domain 3, however, induced ligand-independent, autonomous receptor activation, as determined by STAT3 phosphorylation and cell proliferation. Ligand-independent, autonomous activity was caused by IL-23R homodimers and was independent of IL-12Rβ1. Our data show that deletion of the stalk results in biologically active IL-23R homodimers, thereby creating an as-yet-undescribed receptor complex of the IL-6/IL-12 cytokine family.