IL33 receptor activation is IL33 isoform and receptor genotype specific

Introduction: Interleukin 33 (IL33) and its receptor (IL1RL1/ST2) play important roles in asthma development. Multiple IL33 isoforms have been identified, while IL1RL1 contains coding region polymorphisms associated with asthma. Aim: To characterise the ability of IL33 isoforms to activate IL1RL1 signalling and determine the role of IL1RL1 TIR signalling domain haplotypes. Methods: A NFKB/AP1 promoter driven secretory alkaline phosphatase (SEAP) cell line was engineered to express asthma risk or protective IL1RL1 TIR domain haplotype receptors. Cells were treated with recombinant IL33 isoforms (95-270aa, 99-270aa, 102-270aa, 112-270aa, 113-270aa, Exon 3-4 deletion & oxidisation resistant) at 1, 10, 25 & 50ng/ml for 24 hours. Activity was measured by colorimetric assay (SEAP) & ELISA (IL8). Results: All IL33 variants activated IL1RL1, with elevated SEAP/IL8 responses observed in risk haplotype carriers. Only the 99-270aa & oxidisation resistant forms activated the protective haplotype. The two longest isoforms gave the greatest maximal response, while the Exon 3-4 deletion variant had limited activation. Oxidisation resistant IL33 had the largest SEAP signal, but limited IL8 response. Conclusion: The IL1RL1 asthma protective haplotype attenuates IL33 driven inflammation irrespective of isotype. The increased signalling observed for longer IL33 isoforms generated in vivo by proteolytic cleavage via e.g. allergens may be asthma-relevant. The limited activity of the Exon 4-5 deletion suggests a protective role for this variant as a competitive antagonist. Overall, we demonstrate that activation of the IL33/IL1RL1 axis is complex, with response governed by a combination of IL33 isoform and IL1RL1 receptor polymorphisms.