Identification, Purification, and Characterization of a Soluble Interleukin (IL)-13-binding Protein

Interleukin-4 (IL-4) and interleukin-13 (IL-13) are structurally and functionally related cytokines which play an important role in the regulation of the immune response to infection. The functional similarity of IL-4 and IL-13 can be explained, at least in part, by the common components that form their cell surface receptors, namely the IL-4 receptor α-chain (IL-4Rα) and the IL-13 receptor α-chain (IL-13Rα). Soluble forms of the IL-4Rα have also been described and implicated in modulating the effect of IL-4. In this paper we describe the presence of a 45,000-50,000 M r IL-13-binding protein (IL-13BP) in the serum and urine of mice. This protein binds IL-13 with a 100-300-fold higher affinity ( K D = 20-90 p M ) than does the cloned IL-13Rα ( K D = 3-10 n M ). In addition to this functional difference, the IL-13BP appears to be structurally and antigenically distinct from the IL-13Rα. Finally, unlike the cloned receptor, the IL-13BP acts as a potent inhibitor of IL-13 binding to its cell surface receptor, raising the possibility that it may be used to modulate the effects of IL-13 in vivo