IFN-γDirectly Controls IL-33 Protein Level through a STAT1and LMP2-dependent Mechanism

Background: IL-33 levels are regulated through poorly understood cytokine-dependent mechanisms. Results: IFN- but not IL-4 down-regulates IL-33 protein by activating STAT1 and LMP2 proteasome, without engaging caspase-1, -3, or -8. Conclusion: Down-regulation of IL-33 protein by IFN- requires STAT1 and non-canonical involvement of LMP2 proteasome. Significance: Understanding the mechanisms of IL-33 regulation is important for the development of IL-33-targeting therapies. IL-33 contributes to disease processes in association with Th1 and Th2 phenotypes. IL-33 mRNA is rapidly regulated, but the fate of synthesized IL-33 protein is unknown. To understand the interplay among IL-33, IFN-, and IL-4 proteins, recombinant replication-deficient adenoviruses were produced and used for dual expression of IL-33 and IFN- or IL-33 and IL-4. The effects of such dual gene delivery were compared with the effects of similar expression of each of these cytokines alone. In lung fibroblast culture, co-expression of IL-33 and IFN- resulted in suppression of the levels of both proteins, whereas co-expression of IL-33 and IL-4 led to mutual elevation. In vivo, co-expression of IL-33 and IFN- in the lungs led to attenuation of IL-33 protein levels. Purified IFN- also attenuated IL-33 protein in fibroblast culture, suggesting that IFN- controls IL-33 protein degradation. Specific inhibition of caspase-1, -3, and -8 had minimal effect on IFN--driven IL-33 protein down-regulation. Pharmacological inhibition, siRNA-mediated silencing, or gene deficiency of STAT1 potently up-regulated IL-33 protein expression levels and attenuated the down-regulating effect of IFN- on IL-33. Stimulation with IFN- strongly elevated the levels of the LMP2 proteasome subunit, known for its role in IFN--regulated antigen processing. siRNA-mediated silencing of LMP2 expression abrogated the effect of IFN- on IL-33. Thus, IFN-, IL-4, and IL-33 are engaged in a complex interplay. The down-regulation of IL-33 protein levels by IFN- in pulmonary fibroblasts and in the lungs in vivo occurs through STAT1 and non-canonical use of the LMP2 proteasome subunit in a caspase-independent fashion.