Genetics of atopy in a mouse model: polymorphism of the IL-5 receptor alpha chain

To study the genetics of atopy systematically we established a mouse model that provides the general phenotype of atopy: the early response characteristic of IgE-dependent eczema or atopic dermatitis, and the diagnostic test of atopy, the skin-prick test. Using an immediate cutaneous hypersensitivity test (ICHS) against birch pollen extract we could classify A/J and C57BL/6 (B6) inbred mouse strains respectively as high responder and low responders. The F1 hybrids were found to be high responders with incomplete penetrance. Backcrossing F1 mice to the low responder B6 strain yielded three classes of responders, high, intermediate, and low. A genome-wide microsatellite screen of the backcross progeny disclosed suggestive linkage to a microsatellite marker on chromosome 6 close to the locus of the IL-5 receptor alpha chain. Its allelic variation in A/J and B6 strains was investigated and two major differences were detected. Firstly, a nucleotide exchange in the 5' untranslated region of B6 mRNA resulted in increased transcription/translation of a reporter construct. Higher expression of the receptor on the cell surface would be expected to favor an allergic immune response. Secondly, the two alleles are differentially spliced so as to yield two soluble isoforms in A/J mice versus one in B6 mice. Higher expression of soluble IL-5R would be expected to reduce the level of allergy through capture of IL-5. Thus both findings conform to the expectation based on susceptibility to atopy and thus identify the IL-5R alpha chain as a likely contributor to the genetics of atopy.