Functional Analysis of Protective IL1RL1 Variants Associated with Asthma Risk

GWAS studies have identified polymorphisms in both IL33 and IL1RL1, the gene encoding ST2, the high affinity chain of the IL-33 receptor, that associate with asthma susceptibility. We identified amino acid changing variants in IL1RL1 associating with asthma incidence and found these SNPs to be protective from asthma risk in our study population. These variants result in coding changes to the intracellular region of ST2, which contains the TIR domain of the receptor that is critical for signaling downstream of IL-1 cytokine family and TLRs. Mutations or deletions to this region can inhibit ligand-induced responses. IL-33 responses were diminished in cell lines expressing all 4 IL1RL1 missense variants. To further elucidate how this haplotype could affect IL-33 activity, we compared IL-33 activity and ST2 expression between donors carrying either haplotype. We observed reduced IL-33 mediated IL-8 secretion from purified blood eosinophils derived from individuals carrying the protective haplotype. We also observed greater soluble ST2 expression in these individuals. Additionally, we have recently observed that expression of other IL1R family members may also be affected by this haplotype due to the tight linkage disequilibrium at this locus. Expression of IL18R is also elevated in these individuals, suggesting a link between IL33 and IL18 responses in vivo. Our results provide a link between the genetic predisposition to asthma and IL-33 mediated responses. Given IL-33 promotes Th2 immunity, perturbations that diminish this response may provide protection from asthma risk.