Reconstitution of a functional human type II IL-4/IL-13 receptor in mouse B cells: demonstration of species specificity

IL-13 is a Th2-derived pleiotropic cytokine that recently was shown to be a key mediator of allergic asthma. IL-13 mediates its effects via a complex receptor system, which includes the IL-4R alpha-chain, IL-4Ralpha, and at least two other cell surface proteins, IL-13Ralpha1 and IL-13Ralpha2, which specifically bind IL-13. IL-13 has been reported to have very limited effects on mouse B cells. It was unclear whether this was due to a lack of receptor expression, a disproportionate relative expression of the receptor components, or an additional subunit requirement in B cells. To determine the requirements for IL-13 signaling in murine B cells, we examined IL-13-dependent Stat6 activation and CD23 induction in the murine B cell line, A201.1. A201.1 cells responded to murine IL-4 via the type I IL-4R, but were unresponsive to IL-13, and did not express IL-13 receptor. B220(+) splenocytes also failed to signal in response to IL-13 and did not express IL-13 receptor. We transfected A201.1 cells with human IL-4Ralpha, IL-13Ralpha1, or both. Transfectants expressing either human IL-4Ralpha or human IL-13Ralpha1 alone were unable to respond or signal to IL-13. Thus, human IL-13Ralpha1 could not combine with the endogenous murine IL-4Ralpha to generate a functional IL-13R. However, cells transfected with both human IL-4Ralpha and IL-13Ralpha1 responded to IL-13. Thus, the relative lack of IL-13 responsiveness in murine B cells is due to a lack of receptor expression. Furthermore, the heterodimeric interaction between IL-4Ralpha and IL-13Ralpha1 is species specific.     

Level of expression of IL-13R alpha 2 impacts receptor distribution and IL-13 signaling

IL-13, a critical cytokine for allergic inflammation, exerts its effects through a complex receptor system including IL-4Ralpha, IL-13Ralpha1, and IL-13Ralpha2. IL-4Ralpha and IL-13Ralpha1 form a heterodimeric signaling receptor for IL-13. In contrast, IL-13Ralpha2 binds IL-13 with high affinity but does not signal. IL-13Ralpha2 exists on the cell surface, intracellularly, and in soluble form, but no information is available regarding the relative distributions of IL-13Ralpha2 among these compartments, whether the compartments communicate, and how the relative expression levels impact IL-13 responses. Herein, we investigated the distribution of IL-13Ralpha2 in transfected and primary cells, and we evaluated how the total level of IL-13Ralpha2 expression impacted its distribution. Our results demonstrate that the distribution of IL-13Ralpha2 is independent of the overall level of expression. The majority of the IL-13Ralpha2 protein existed in intracellular pools. Surface IL-13Ralpha2 was continually released into the medium in a soluble form, yet surface expression remained constant supporting receptor trafficking to the cell surface. IL-13Ralpha2 inhibited IL-13 signaling proportionally to its level of expression, and this inhibition could be overcome with high concentrations of IL-13.     

IL-13 receptor alpha2 selectively inhibits IL-13-induced responses in the murine lung

IL-13 is a critical cytokine at sites of Th2 inflammation. In these locations it mediates its effects via a receptor complex, which contains IL-4Ralpha and IL-13Ralpha1. A third, high-affinity IL-13 receptor, IL-13Ralpha2, also exists. Although it was initially felt to be a decoy receptor, this has not been formally demonstrated and the role(s) of this receptor has recently become controversial. To define the role(s) of IL-13Ralpha2 in IL-13-induced pulmonary inflammation and remodeling, we compared the effects of lung-targeted transgenic IL-13 in mice with wild-type and null IL-13Ralpha2 loci. We also investigated the effect of IL-13Ralpha2 deficiency on the OVA-induced inflammatory response. In this study, we show that in the absence of IL-13Ralpha2, IL-13-induced pulmonary inflammation, mucus metaplasia, subepithelial fibrosis, and airway remodeling are significantly augmented. These changes were accompanied by increased expression and production of chemokines, proteases, mucin genes, and TGF-beta1. Similarly, an enhanced inflammatory response was observed in an OVA-induced phenotype. In contrast, disruption of IL-13Ralpha2 had no effect on the tissue effects of lung-targeted transgenic IL-4. Thus, IL-13Ralpha2 is a selective and powerful inhibitor of IL-13-induced inflammatory, remodeling, and physiologic responses in the murine lung.     

The human B cell response to IL-13 is dependent on cellular phenotype as well as mode of activation.

Normal mature quiescent human B lymphocytes, isolated as a function of buoyant density, require activation for up-regulation of IL-13R constituents. Cell activation through a combination of surface Ig and CD40 receptor ligation leads to the most substantial message production for IL-13Ralpha1. Functional consequences of this receptor variation, in initially quiescent cells, includes demonstrable effects on cellular proliferation in response to ligand exposure. Variations in the method of surface activation, with particular emphasis on the CD40 receptor, reveals that immobilized CD40 ligand may be sufficient, in and of itself, to up-regulate IL-13Ralpha1, which may bear significance for B-lymphocyte bystander proliferation. Regulation of the IL-13Ralpha1 protein and message also differs as a function of cellular phenotype. Although values are greater in memory than naive B cells, as they are initially isolated from extirpated tonsils, variations in the magnitude of message and protein, as a function of surface stimulation, are more substantial in the naive subset. The magnitude of variation in message production in naive cells is associated with a more vigorous proliferative response to IL-13 than seen in memory lymphocytes. The cellular response to IL-13, as a function of activation and phenotype, is the converse of that demonstrated for IL-2. Evaluation of proliferation, receptor message, ligand binding protein production, and the response to putatively synergistic cytokines reveals that IL-2 is the predominant lymphokine utilized by memory cells. This is in contradistinction to IL-13, which along with IL-4, are the predominant moieties for naive lymphocytes.     

Functional expression of the interleukin-11 receptor alpha-chain and evidence of antiapoptotic effects in human colonic epithelial cells

A tissue-protective effect of interleukin-11 (IL-11) for the intestinal mucosa has been postulated from animal models of inflammatory bowel disease (IBD). Despite the fact that the clinical usefulness of the anti-inflammatory effects of this cytokine is presently investigated in patients with IBD, there are no data available regarding the target cells of IL-11 action and the mechanisms of tissue protection within the human colonic mucosa. IL-11 responsiveness is restricted to cells that express the interleukin-11 receptor alpha-chain (IL-11Ralpha) and an additional signal-transducing subunit (gp130). In this study, we identified the target cells for IL-11 within the human colon with a new IL-11Ralpha monoclonal antibody and investigated the functional expression of the receptor and downstream effects of IL-11-induced signaling. Immunohistochemistry revealed expression of the IL-11Ralpha selectively on colonic epithelial cells. HT-29 and colonic epithelial cells (CEC) constitutively expressed IL-11Ralpha mRNA and protein. Co-expression of the signal-transducing subunit gp130 was also demonstrated. IL-11 induced signaling through triggering activation of the Jak-STAT pathway without inducing anti-inflammatory or proliferative effects in colonic epithelial cells. However, IL-11 stimulation resulted in a dose-dependent tyrosine phosphorylation of Akt, a decreased activation of caspase-9, and a reduced induction of apoptosis in cultured CEC. In HLA-B27 transgenic rats treated with IL-11, a reduction of apoptotic cell numbers was found. This study demonstrates functional expression of the IL-11Ralpha restricted on CEC within the human colonic mucosa. IL-11 induced signaling through triggering activation of the Jak-STAT pathway, without inducing anti-inflammatory or proliferative effects. The beneficial effects of IL-11 therapy are likely to be mediated by CEC via activation of the Akt-survival pathway, mediating antiapoptotic effects to support mucosal integrity.     

A novel mechanism by which interferon-gamma can regulate interleukin (IL)-13 responses. Evidence for intracellular stores of IL-13 receptor alpha -2 and their rapid mobilization by interferon-gamma.

Interleukin (IL)-13 mediates its activities via a complex receptor system. Interleukin-13 receptor alpha-1 chain (IL-13Ralpha1) binds IL-13 with low affinity, but does not signal. However, when IL-13Ralpha1 combines with IL-4 receptor alpha (IL-4Ralpha), a signaling high affinity receptor complex for IL-13 is generated. In contrast, IL-13Ralpha2 alone binds IL-13 with high affinity, but does not signal and has been postulated to be a decoy receptor. Herein, we investigated the cellular localization of IL-13Ralpha2 and the regulation of its expression by confocal microscopy and flow cytometry in primary and cultured cells. Our results demonstrate that IL-13Ralpha2 is largely an intracellular molecule, which is rapidly mobilized from intracellular stores following treatment with interferon (IFN)-gamma. Up-regulation of IL-13Ralpha2 surface expression in response to IFN-gamma was rapid, did not require protein synthesis, and resulted in diminished IL-13 signaling. These results provide the first evidence that the IL-13Ralpha2 is predominantly an intracellular molecule and demonstrate a novel mechanism by which IFN-gamma can regulate IL-13 responses.     

TNF-alpha and IL-4 regulate expression of IL-13 receptor alpha2 on human fibroblasts

Two interleukin 13 receptors (IL-13Rs) have been identified as IL-13Ralpha1 and IL-13Ralpha2. IL-13Ralpha1 is composed of a heterodimer consisting of IL-13Ralpha1 and IL-4 receptor alpha (IL-4Ralpha) as a signaling subunit. In contrast, IL-13Ralpha2 is known as a decoy receptor for IL-13. In this study, we investigated the expression of IL-13Rs on human fibroblasts. IL-13Ralpha2 was significantly up-regulated after stimulation with tumor necrosis factor-alpha (TNF-alpha) and/or IL-4. In contrast, IL-13Ralpha1 was constitutively detectable and was not up-regulated. After the induction of IL-13alpha2 by IL-4, STAT6 phosphorylation through IL-13Ralpha1 by IL-13 was inhibited. We also detected large intracellular pools of IL-13Ralpha2 in fibroblasts quantitatively. Furthermore, mobilization of the IL-13Ralpha2 protein stores from the cytoplasm to the cell surface was prevented by an inhibitor of protein transport, brefeldin-A. These results indicate that TNF-alpha and IL-4 synergistically up-regulate the expression of IL-13Ralpha2 decoy receptor on human fibroblasts by inducing gene expression and mobilizing intracellular receptors, and thus may down-regulate the IL-13 signaling.     

IgE generation and mast cell effector function in mice deficient in IL-4 and IL-13

IL-4 and IL-13 are potent cytokines that drive production of IgE, which is critical to the development of atopic disease. In this study, we directly compared IgE generation and IgE-dependent mast cell effector function in mouse strains lacking IL-4, IL-13, IL-4 + IL-13, or their common receptor component, IL-4Ralpha. Although serum IgE was undetectable under resting conditions in most animals deficient in one or both cytokines, peritoneal mast cells from mice lacking IL-4 or IL-13 had only partial reductions in surface IgE level. In contrast, peritoneal mast cells from IL-4/13(-/-) and IL-4Ralpha(-/-) animals were severely deficient in surface IgE, and showed no detectable degranulation following treatment with anti-IgE in vitro. Surprisingly, however, intradermal challenge with high concentrations of anti-IgE Ab induced an ear-swelling response in these strains, implying some capacity for IgE-mediated effector function in tissue mast cells. Furthermore, upon specific immunization with OVA, both IL-4/IL-13(-/-) and IL-4Ralpha(-/-) mice produced detectable levels of serum IgE and Ag-specific IgG1, and generated strong ear-swelling responses to intradermal administration of anti-IgE. These findings suggest that a mechanism for IgE production exists in vivo that is independent of IL-4 or IL-13.     

Characterization of the interaction between interleukin-13 and interleukin-13 receptors

Interleukin-13 (IL-13) possesses two types of receptor: the heterodimer, composed of the IL-13Ralpha1 chain (IL-13Ralpha1) and the IL-4Ralpha chain (IL-4Ralpha), transducing the IL-13 signals; and the IL-13Ralpha2 chain (IL-13Ralpha2), acting as a nonsignaling "decoy" receptor. Extracellular portions of both IL-13Ralpha1 and IL-13Ralpha2 are composed of three fibronectin type III domains, D1, D2, and D3, of which the last two comprise the cytokine receptor homology modules (CRHs), a common structure of the class I cytokine receptor superfamily. Thus far, there has been no information about the critical amino acids of the CRHs or the role of the D1 domains of IL-13Ralpha1 and IL-13Ralpha2 in binding to IL-13. In this study, we first built the homology modeling of the IL-13.hIL-13 receptor complexes and then predicted the amino acids involved in binding to IL-13. By incorporating mutations into these amino acids, we identified Tyr-207, Asp-271, Tyr-315, and Asp-318 in the CRH of human IL-13Ralpha2, and Leu-319 and Tyr-321 in the CRH of human IL-13Ralpha1, as critical residues for binding to IL-13. Tyr-315 in IL-13Ralpha2 and Leu-319 in IL-13Ralpha1 are positionally conserved hydrophobic amino acid residues. Furthermore, by using D1 domain-deleted mutants, we found that the D1 domain is needed for the expression of IL-13Ralpha2, but not IL-13Ralpha1, and that the D1 domain of IL-13Ralpha1 is important for binding to IL-13, but not to IL-4. These results provide the basis for a precise understanding of the interaction between IL-13 and its receptors.     

Structure,binding, and antagonists in the IL-4/IL-13 receptor system

Interleukin-4 (IL-4) and IL-13 are the only cytokines known to bind to the receptor chain IL-4Ralpha. Receptor sharing by these two cytokines is the molecular basis for their overlapping biological functions. Both are key factors in the development of allergic hypersensitivity, and they also play a major role in exacerbating allergic and asthmatic symptoms. Knowledge of structure and function of this system has allowed the development of inhibitors that block the interaction between the cytokines and their shared receptor. Mutational analysis of IL-4 has revealed variants with high-affinity binding to IL-4Ralpha but no detectable affinity for the second receptor subunit, which is either (gamma)c or IL-13Ralpha1. These IL-4 antagonists fail to induce signal transduction and block IL-4 and IL-13 effects in vitro. IL-4 antagonists prevent the development of allergic disease in vivo and an antagonistic variant of human IL-4 is now in clinical trials for asthma. Detailed knowledge of the site of interaction of IL-4 and IL-4Ralpha has been gained by structure analysis of the complex of these two proteins and through functional studies employing mutants of IL-4 and its receptor subunits. Based on these new data, the hitherto elusive goal of designing small molecular mimetics may be feasible.     

Lysophosphatidic acid induces interleukin-13 (IL-13) receptor alpha2 expression and inhibits IL-13 signaling in primary human bronchial epithelial cells

Interleukin-13 (IL-13), a Th2 cytokine, plays a pivotal role in pathogenesis of bronchial asthma via IL-13 receptor alpha1 (IL-13Ralpha1) and IL-4 receptor alpha (IL-4Ralpha). Recent studies show that a decoy receptor for IL-13, namely IL-13Ralpha2, mitigates IL-13 signaling and function. This study provides evidence for regulation of IL-13Ralpha2 production and release and IL-13-dependent signaling by lysophosphatidic acid (LPA) in primary cultures of human bronchial epithelial cells (HBEpCs). LPA treatment of HBEpCs in at imedependent fashion increased IL-13Ralpha2 gene expression without altering the mRNA levels of IL-13Ralpha1 and IL-4Ralpha. Pretreatment with pertussis toxin (100 ng/ml, 4 h) or transfection of c-Jun small interference RNA or an inhibitor of JNK attenuated LPA-induced IL-13Ralpha2 gene expression and secretion of soluble IL-13Ralpha2. Overexpression of catalytically inactive mutants of phospholipase D (PLD) 1 or 2 attenuated LPA-induced IL-13Ralpha2 gene expression and protein secretion as well as phosphorylation of JNK. Pretreatment of HBEpCs with 1 microM LPA for 6 h attenuated IL-13-but not IL-4-induced phosphorylation of STAT6. Transfection of HBEpCs with IL-13Ralpha2 small interference RNA blocked the effect of LPA on IL-13-induced phosphorylation of STAT6. Furthermore, pretreatment with LPA (1 microM, 6 h) attenuated IL-13-induced eotaxin-1 and SOCS-1 gene expression. These results demonstrate that LPA induces IL-13Ralpha2 expression and release via PLD and JNK/AP-1 signal transduction and that pretreatment with LPA down-regulates IL-13 signaling in HBEpCs. Our data suggest a novel mechanism of regulation of IL-13Ralpha2 and IL-13 signaling that may be of physiological relevance to airway inflammation and remodeling.     

IL-4 receptor alpha is an important modulator of IL-4 and IL-13 receptor binding: implications for the development of therapeutic targets

IL-4 is a key cytokine associated with allergy and asthma. Induction of cell signaling by IL-4 involves interaction with its cognate receptors, a complex of IL-4Ralpha with either the common gamma-chain or the IL-13R chain alpha1 (IL-13Ralpha1). We found that IL-4 bound to the extracellular domain of IL-4Ralpha (soluble human (sh)IL-4Ralpha) with high affinity and specificity. In contrast with the sequential mechanism of binding and stabilization afforded by IL-4Ralpha to the binding of IL-13 to IL-13Ralpha1, neither common gamma-chain nor IL-13Ralpha1 contributed significantly to the stabilization of the IL-4:IL-4Ralpha complex. Based on the different mechanisms of binding and stabilization of the IL-4R and IL-13R complexes, we compared the effects of shIL-4Ralpha and an IL-4 double mutein (R121D/Y124D, IL-4R antagonist) on IL-4- and IL-13-mediated responses. Whereas IL-4R antagonist blocked responses to both cytokines, shIL-4Ralpha only blocked IL-4. However, shIL-4Ralpha stabilized and augmented IL-13-mediated STAT6 activation and eotaxin production by primary human bronchial fibroblasts at suboptimal doses of IL-13. These data demonstrate that IL-4Ralpha plays a key role in the binding affinity of both IL-13R and IL-4R complexes. Under certain conditions, shIL-4Ralpha has the potential to stabilize binding IL-13 to its receptor to augment IL-13-mediated responses. Thus, complete understanding of the binding interactions between IL-4 and IL-13 and their cognate receptors may facilitate development of novel treatments for asthma that selectively target these cytokines without unpredicted or detrimental side effects.     

IL-4 receptor signaling in Clara cells is required for allergen-induced mucus production

Excessive mucus production is an important pathological feature of asthma. The Th2 cytokines IL-4 and IL-13 have both been implicated in allergen-induced mucus production, inflammation, and airway hyperreactivity. Both of these cytokines use receptors that contain the IL-4Ralpha subunit, and these receptors are expressed on many cell types in the lung. It has been difficult to determine whether allergen-induced mucus production is strictly dependent on direct effects of IL-4 and IL-13 on epithelial cells or whether other independent mechanisms exist. To address this question, we used a cell type-specific inducible gene-targeting strategy to selectively disrupt the IL-4Ralpha gene in Clara cells, an airway epithelial cell population that gives rise to mucus-producing goblet cells. Clara cell-specific IL-4Ralpha-deficient mice and control mice developed similar elevations in serum IgE levels, airway inflammatory cell numbers, Th2 cytokine production, and airway reactivity following OVA sensitization and challenge. However, compared with control mice, Clara cell-specific IL-4Ralpha-deficient mice were nearly completely protected from allergen-induced mucus production. Because only IL-13 and IL-4 are thought to signal via IL-4Ralpha, we conclude that direct effects of IL-4 and/or IL-13 on Clara cells are required for allergen-induced mucus production in the airway epithelium.     

Localization of human interleukin 13 receptor in non-haematopoietic cells

Although the functional roles of interleukin (IL-)13 in haematopoietic cells are well investigated, those in non-haematopoietic cells remain to be addressed. IL-13 exerts its actions by binding to the IL-13 receptor (IL-13R) on target cells, which is composed of IL-13Ralpha1 and the IL-4 receptor alpha chain (IL-4Ralpha). However, there has been no study of localization of IL-13R in each tissue. To address this question, we generated monoclonal anti-IL-13Ralpha1 antibody, and performed immunohistochemistry using this antibody and anti-IL-4Ralpha antibody. Distribution of these two components was the same in all examined tissues. Staining was positive in keratinocytes, hair follicles, and sebaceous and sweat glands in skin; in ciliated respiratory epithelial cells in nasal tissue; in heart muscle cells; in foveola cells, gastric glands, and the smooth muscle layer in stomach; and in hepatocytes in liver. However, staining was undetectable in brain and bone marrow. Fibroblasts and endothelial cells were stained in some tissues. These results provide clues to elucidate the known pathological roles of IL-13 in atopic dermatitis and allergic rhinitis, as well as its unknown physiological roles.     

Mechanism of action of interleukin-13 antagonist (IL-13E13K) in cells expressing various types of IL-4R

As interleukin (IL)-13 and IL-4 play a major role in various diseases including asthma, allergy, and malignancies, it is desirable to generate a molecule that blocks the effects of both cytokines. We previously generated a human IL-13 mutant (IL-13E13K), which is a powerful antagonist of IL-13, blocking the biological activities of IL-13. We now show that IL-13E13K also competitively inhibits signaling and biological activities of IL-4 through type II and partially through type III IL-4 receptor (R) system. IL-13E13K completely blocked the IL-4-induced phosphorylation of STAT6 and IL-4-dependent protein synthesis in cells expressing type II and partially type III IL-4R but not type I IL- 4R. Consistent with the inhibition of biological activities, IL-13E13K inhibited IL-4 binding to type II IL-4R-expressing cells but not to type I IL-4R-expressing cells. The inhibition efficiency of IL-4 binding by IL-13E13K was relatively lower compared to wtIL-13 even though IL-13E13K bound to IL-13Ralpha1 positive cells with a similar affinity to wtIL-13. These results indicate that Glu13 in IL-13 associates with IL-4Ralpha, and mutation to lysine decreases its binding ability to IL-4Ralpha chain. IL-13E13K binds to IL- 13Ralpha1, which is shared by both IL-13R and IL-4R systems. Consequently, IL-13E13K inhibits IL-4 binding to these cells and prevents heterodimer formation between IL-13Ralpha1 and IL-4Ralpha chains. This interference by IL-13E13K blocks the biological activities of not only IL-13 but also partially of IL-4. Thus, IL-13E13K may be a useful agent for the treatment of diseases such as asthma, allergic rhinitis, and cancer, which are dependent on signaling through both IL-4 and IL-13 receptors.     

IL-13 signaling through the IL-13alpha2 receptor is involved in induction of TGF-beta1 production and fibrosis

Interleukin (IL)-13 is a major inducer of fibrosis in many chronic infectious and autoimmune diseases. In studies of the mechanisms underlying such induction, we found that IL-13 induces transforming growth factor (TGF)-beta(1) in macrophages through a two-stage process involving, first, the induction of a receptor formerly considered to function only as a decoy receptor, IL-13Ralpha(2). Such induction requires IL-13 (or IL-4) and tumor necrosis factor (TNF)-alpha. Second, it involves IL-13 signaling through IL-13Ralpha(2) to activate an AP-1 variant containing c-jun and Fra-2, which then activates the TGFB1 promoter. In vivo, we found that prevention of IL-13Ralpha(2) expression reduced production of TGF-beta(1) in oxazolone-induced colitis and that prevention of IL-13Ralpha(2) expression, Il13ra2 gene silencing or blockade of IL-13Ralpha(2) signaling led to marked downregulation of TGF-beta(1) production and collagen deposition in bleomycin-induced lung fibrosis. These data suggest that IL-13Ralpha(2) signaling during prolonged inflammation is an important therapeutic target for the prevention of TGF-beta(1)-mediated fibrosis.     

The negative-feedback regulation of the IL-13 signal by the IL-13 receptor alpha2 chain in bronchial epithelial cells

Bronchial asthma is a complex disease characterized by airway inflammation involving Th2 cytokines. Among Th2 cytokines, the significance of IL-13 in the pathogenesis of bronchial asthma has recently emerged. Particularly, the direct action of IL-13 on bronchial epithelial cells (BECs) is critical for generation of airway hyperresponsiveness. IL-13 has two binding units; the IL-13 receptor alpha1 chain transduces the IL-13 signal comprising a heterodimer with the IL-4R alpha chain, whereas the IL-13 receptor alpha2 chain (IL-13Ralpha2) is thought to act as a decoy receptor. However, it remains obscure how expression of these molecules is regulated in each cell. In this article, we analyzed the expression of these components in BECs. Either IL-4 or IL-13 induced intracellular expression of IL-13Ralpha2 in BECs, which was STAT6-dependent and required de novo protein synthesis. IL-13Ralpha2 expressed on the cell surface as a monomer inhibited the STAT6-dependent IL-13 signal. Furthermore, expression of IL-13Ralpha2 was induced in lung tissues of ovalbumin-induced asthma model mice. Taken together, our results suggested the possibility that IL-13Ralpha2 induced by its ligand is transferred to the cell surface by an unknown mechanism, and it down-regulates the IL-13 signal in BECs, which functions as a unique negative-feedback system for the cytokine signal.