Dynamic Binding to Chromatin Regulates the Extracellular Release of Interleukin-33 (IL-33)

Rationale: Interleukin-33 (IL-33) is a pro-atopy, epithelial-derived cytokine released following necrosis. Before extracellular release, IL-33 is retained within the nucleus bound to chromatin, but the role of this binding remains largely understudied. Herein, we examined the molecular characteristics of IL-33 binding to chromatin in esophageal epithelial cells and its functional significance in regulating IL-33 extracellular release.   

Methods: Immunofluorescence (IF) was performed on esophageal biopsies from patients with eosinophilic esophagitis or control individuals. Biochemical fractionation, IF, fluorescence recovery after photobleaching (FRAP), and enzyme-linked immunosorbent assay (ELISA) were performed on an esophageal epithelial cell line (TE-7) overexpressing IL-33 via lentiviral transduction.

Results: IF for IL-33 revealed only nuclear staining in esophageal epithelial cells in vitro and during allergic inflammation in vivo. Biochemical fractionation demonstrated IL-33 enrichment in the chromatin fraction compared with heat shock protein 90.  Assessment of nuclear mobility by FRAP demonstrated that GFP-tagged full-length IL-33 had a 70% recovery time of 29 seconds compared to 3 seconds for IL-1 alpha. A truncated form of IL-33 (IL-33^112-270) which lacks the chromatin binding domain was highly mobile within the nucleus similar to the GFP-alone control.  Functionally, IL-33^112-270   demonstrated increased extracellular release following induction of necrosis with calcium ionophore or hydrogen peroxide (8 to 20-fold, p < 0.0001) compared with chromatin-bound full-length IL-33.  

Conclusions: In summary, herein we have demonstrated that IL-33 binding to chromatin is dynamic and regulates its availability for release during necrosis. We propose that intracellular retention of IL-33 during necrosis may serve as a mechanism to curtail inflammatory responses in allergic disease.