Neutrophil Elastase-mediated proteolysis activates the anti-inflammatory cytokine IL-36 Receptor antagonist

The interleukin-36 receptor antagonist (IL-36Ra) which regulates IL-36α, -β and -γ is linked to psoriatic inflammation, especially loss-of-function mutations in pustular psoriasis subtypes. As observed with other IL-1 superfamily proteins, the IL-36 members require N-terminal cleavage for full biological activity but the mechanisms of IL-36Ra activation remain poorly defined. Using different blood leukocyte and skin resident cell preparations, and recombinant proteins, we have identified that neutrophil elastase, but not other neutrophil derived proteases, cleaves IL-36Ra into its highly active antagonistic form. The activity of this processed form of IL-36Ra was confirmed in human primary dermal fibroblasts and keratinocytes and in skin equivalents. A significant dose dependent reduction of IL-36γ induced IL-8 and chemokine ligand 20 (CCL20) levels were detected following addition of the cleaved IL-36Ra compared to full length IL-36Ra. By activating IL-36Ra, the neutrophil derived protease can inhibit IL-36 induced chemokine production, including IL-8 and CCL20, and reduce further inflammatory cell infiltration. These findings strongly indicate neutrophil elastase to be a key enzyme in the biological function of IL-36Ra and that neutrophils can play a regulatory role in psoriatic inflammation with regard to balancing the pro-inflammatory activity of IL-36.