Caspase 3 Inactivates Biologically Active Full Length Interleukin-33 As a Classical Cytokine but Does Not Prohibit Nuclear Translocation

IL-33 is a member of the IL-1 family of cytokines with dual function which either activates cells via the IL-33 receptor in a paracrine fashion or translocates to the nucleus to regulate gene transcription in an intracrine manner. We show that full length murine IL-33 is active as a cytokine and that it is not processed by caspase 1 to mature IL-33 but instead cleaved by caspase 3 at aa175 to yield two products which are both unable to bind to the IL-33 receptor. Full length IL-33 and its N-terminal caspase 3 breakdown product, however, translocate to the nucleus. Finally, bioactive IL-33 is not released by cells constitutively or after activation. This suggests that IL-33 is not a classical cytokine but exerts its function in the nucleus of intact cells and only activates others cells via its receptor as an alarm mediator after destruction of the producing cell.