IL-33 is a member of the IL-1 family of cytokines produced importantly by endothelial cells. Nuclear IL-33 protein acts as a transcriptional repressor. Secreted IL-33 is involved in Th2-mediated inflammatory responses in allergic diseases such as asthma. IL-33 acts through activation of the ST2L receptor recruiting of adaptor proteins and leading to formation of an E3 ligase complex, which introduces Lys 48-linked polyubiquitin chains in ST2 leading to its degradation in the cytoplasm. The increased level of IL-33 in allergic disorders and its correlation with disease severity and the effectiveness of anti-IL-33 antibody and soluble form of ST2 in alleviating symptoms of asthma in mice support the view of IL-33 as a therapeutic target in allergic diseases. IL-33 is synthesized as a full-length form migrating at 30–34kDa on gels. Different proteases can process full-length IL-33 to biologically active forms.

Cells were transfected with a construct expressing an EGFP-IL-33 protein and analyzed by cytometry in presence or absence of proteasome inhibitor or E1 enzyme inhibitor. Ubiquitinated IL-33 proteins were analyzed by western blot. Proteasome and NFκB activity involved in the allergen-dependent induction of IL-33 was also evaluated.

We show that in human and murine lung tissues, proteolytic products or products of shorter splice variants can be found in addition to higher molecular weight forms. However, in endothelial cells, the full-length and shorter forms of IL-33 are poorly detected suggesting that the IL-33 protein may be labile in proliferating cells. These experiments demonstrate that IL-33 is a target for ubiquitination but the role of this modification is unclear. Moreover, we identified high molecular weight forms of IL-33 probably corresponding to post-translational modified forms or complexes with other proteins, which could stabilize IL-33 protein.

Altogether these studies suggest that post-translational modifications of IL-33 occur in endothelial cells which may have functional consequences in numerous diseases including allergic diseases.