IFN-γ Directly Controls IL-33 Protein Level through a STAT1- and LMP2-dependent Mechanism

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-γ, and IL-4 proteins, recombinant replication-deficient adenoviruses were produced and used for dual expression of IL-33 and IFN-γ 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-γ 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-γ in the lungs led to attenuation of IL-33 protein levels. Purified IFN-γ also attenuated IL-33 protein in fibroblast culture, suggesting that IFN-γ controls IL-33 protein degradation. Specific inhibition of caspase-1, -3, and -8 had minimal effect on IFN-γ-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-γ on IL-33. Stimulation with IFN-γ strongly elevated the levels of the LMP2 proteasome subunit, known for its role in IFN-γ-regulated antigen processing. siRNA-mediated silencing of LMP2 expression abrogated the effect of IFN-γ on IL-33. Thus, IFN-γ, IL-4, and IL-33 are engaged in a complex interplay. The down-regulation of IL-33 protein levels by IFN-γ 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.