Evidence that natural murine soluble interleukin 4 receptors may act as transport proteins

The present studies were undertaken to determine whether the interleukin 4 binding proteins (IL4BPs) previously identified in the biological fluids of mice are soluble forms of IL4Rs. We also studied the binding properties of IL4BPs in order to gain insight into their physiological role in vivo. Affinity-purified IL4BPs and recombinant soluble IL4Rs generated similar onedimensional (Cleveland) peptide maps after digestion with either Staphylococcus aureus V8 protease or trypsin, indicating structural similarities. Furthermore, a rat mAb directed against the murine IL4Rs immunoprecipitated the IL4BPs and completely inhibited binding of 1251-IL4 to a purified preparation of IL4BPs. Taken together these data indicate that the IL4BPs are soluble IL4Rs. At 4°C the IL4BPs competitively inhibited the binding of IL-4 to membrane IL4Rs but their ability to prevent binding of IL4 to cells at 37°C, at the same concentrations, was significantly reduced. Kinetic binding studies of soluble IL4BPs vs. membrane IL4Rs disclosed important differences in their rates of dissociation from IL-4. Whereas dissociation at 4°C was slow for both, dissociation of 114 from ILBPs at 37°C was considerably faster (t1i2 of 2 min) than dissociation of IL4 from membrane IL4Rs (t_1/2 of ∼69 min). Temperature-dependent changes in dissociation kinetics were reversible, and could not be accounted for by either inactivation of the IL4BPs at 37°C or receptor internalization. Additional experiments also demonstrated that when IL4BPs bind to IL4 at 37°C, the IL4/IL4BPs complex can rapidly dissociate, allowing IL-4 to bind to membrane IL4Rs. In addition, binding of 114 by the IL4BPs protects IIA from proteolytic degradation. Taken together, these results suggest that the IL4BPs are naturally occurring forms of soluble II, r4Rs and that some of their properties (fast dissociation kinetics and protection of 1174 from proteolysis) are consistent with a potential role as carrier proteins for IL-4 in the circulation.