IL-33Rα, also known as ST2 or IL-1RL1, is a member of the Toll/IL-1 receptor family. It binds IL-33 and is structurally similar to IL-1R1. Two forms of the protein exist - a soluble form known as ST2 and a membrane anchored form known as ST2L. The membrane form is expressed by Th2 cells and bone marrow derived mast cells, whereas the soluble form is expressed by serum-stimulated fibroblasts.
Blocking with anti-ST2 antibodies has been shown to alleviate experimental arthritis and airway inflammation. The IL-33-ST2 axis has been reported to be important across a range of diseases including asthma, allergies, and cardiac disease.