Differences in Expression, Affinity, and Function of Soluble (s)IL-4R and sIL-13R2 Suggest Opposite Effects on Allergic Responses 1 Marat Khodoun,* † Christina Lewis, ‡ Jun-Qi Yang,* † Tatyana Orekov,* † Crystal Potter,* † Thomas Wynn, ¶ Margaret Mentink-Kane, ¶ Gurjit K. Khurana Hershey, § Marsha Wills-Karp, ‡ and Fred D. Finkelman 2 * †‡ IL-4 and IL-13 are each bound by soluble receptors (sRs) that block their activity. Both of these sRs (sIL-4R and sIL-13R2) are present in low nanogram per milliliter concentrations in the serum from unstimulated mice, but differences in affinity and half-life suggest differences in function. Serum IL-4/sIL-4R complexes rapidly dissociate, releasing active IL-4, whereas sIL- 13R2 and IL-13 form a stable complex that has a considerably longer half-life than uncomplexed IL-13, sIL-13R2, IL-4, or sIL-4R. Approximately 25% of sIL-13R2 in serum is complexed to IL-13; this percentage and the absolute quantity of sIL- 13R2 in serum increase considerably during a Th2 response. sIL-13R2 gene expression is up-regulated by both IL-4 and IL-13; the effect of IL-4 is totally IL-4R-dependent while the effect of IL-13 is partially IL-4R-independent. Inhalation of an IL-13/ sIL-13R2 complex does not affect the expression of IL-13-inducible genes but increases the expression of two genes, Vnn1 and Pira-1, whose products activate APCs and promote neutrophilic inflammation. These observations suggest that sIL-4R predom- inantly sustains, increases, and diffuses the effects of IL-4, whereas sIL-13R2 limits the direct effects of IL-13 to the site of IL-13 production and forms a stable complex with IL-13 that may modify the quality and intensity of an allergic inflammatory response. The Journal of Immunology, 2007, 179: 6429 – 6438. A llergic immunopathology and host protection against hel- minth parasites are both mediated, in large part, by the type 2 cytokines IL-4, IL-5, IL-9, and IL-13 (1–14). Of these, a particularly prominent role is played by two related cytokines, IL-4 and IL-13. Both of these cytokines bind to cell membrane receptors that contain an IL-4R-chain and activate the transcription factor Stat6 (15–17), and both have prominent effects on multiple cell types that contribute to allergic inflammation (3, 5, 7, 12, 14, 18 –27). There are, however, important differences between the biological effects of these two cytokines. Some of these differences stem from the failure of IL-13 to signal through the type 1 IL-4 receptor (IL-4R), which is composed of IL-4R and the cytokine receptor common -chain, c (28 –30). Because this is the only IL-4R expressed on T cells and mast cells, these cell types respond to IL-4 but not IL-13 (30). This prob- ably accounts for the greater importance of IL-4 than IL-13 in the promotion of a Th2 response. In contrast, the type 2 IL-4R, which is composed of the IL-4R and IL-13R1 polypeptides (15, 29, 31, 32), is expressed by some bone marrow-derived cells, including macro- phages, as well as most non-bone marrow-derived cells and is acti- vated by both IL-4 and IL-13 (28, 29, 33, 34). Signaling through this receptor appears to be responsible for many proallergic effects of IL-4 and most proallergic effects of IL-13 (7, 10, 20, 28, 33–35), although IL-13 signaling through a cell membrane form of an additional IL- 13-binding protein, cell membrane IL-13R2, may contribute to the profibrotic effects of this cytokine (36). Additional differences between IL-4 and IL-13 function may reflect: 1) the ability of IL-4, but not IL-13, to stimulate NK and NK T cell production of IFN- (37), presumably through effects mediated by the type 1 IL-4R; 2) 1 IL-4R-dependent stimulation of other compounds that inhibit IL-13 effects (38), 3) the production of IL-13, but not IL-4, by some cell types (39 – 41), and 4) the production of more IL-13 than IL-4 during immune responses to allergens or worms (C. Perkins and F. D. Finkelman, unpublished data). The studies described in this article were performed to de- termine whether differences in the function of IL-4 and IL-13 might also be influenced by differences between the expression and function of soluble (s) 3 forms of IL-4R and IL-13R2, proteins in blood and lymph that bind IL-4 and IL-13, respectively (42– 45). To address this possibility, we have evaluated the expression and functional characteristics of sIL-4R and IL-13R2 in mouse se- rum. We find that both of these soluble receptors (sRs) are present in low nanogram per milliliter quantities in the serum of naive BALB/c mice. However, the in vivo affinity of sIL-4R for IL-4 is considerably lower than that of sIL-13R2 for IL-13; IL-4/sIL- 4R complexes rapidly dissociate while IL-13/sIL-13R2 com- plexes are very stable and have a considerably longer half-life than uncomplexed IL-13 or sIL-13R2. These differences suggest *Cincinnati Veterans Affairs Medical Center, Cincinnati, OH 45220; † Department of Medicine, University of Cincinnati College of Medicine, Cincinnati, OH 45267; ‡ Di- vision of Immunobiology and § Division of Allergy and Immunology, Cincinnati Chil- dren’s Hospital Medical Center, Cincinnati, OH 45229; and ¶ National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892 Received for publication July 5, 2007. Accepted for publication August 28, 2007. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 This work was supported by a Merit Award (to F.D.F.) from the U.S. Department of Veterans Affairs, by National Institutes of Health grants to F.D.F. (R01 AI052099 and R01 AI55848) and G.K.H. (R01AI58157), and a National Institutes of Health P01 grant to M.W-K., G.K.K.H., and F.D.F. (HL076383). 2 Address correspondence and reprint requests to Dr. Fred D. Finkelman, Cincinnati Veterans Authority, Medical Center, 3200 Vine Street, Cincinnati, OH 45220. E-mail address: ffinkelman@pol.net 3 Abbreviations used in this paper: s, soluble (prefix); GaKLH, goat antiserum to keyhole limpet hemocyanin; GaMD, goat anti-mouse IgD antiserum; i.t., intratrache- al(ly); sR, soluble receptor. The Journal of Immunology www.jimmunol.org