Punnonen et al. (1993) demonstrated that IL13 induces IgG4 and IgE synthesis by human B cells. The activity of IL13 was shown to be independent of IL4 (147780), but these 2 cytokines may have common signaling pathways. Like IL4, IL13 induced CD23 expression on B cells, enhanced CD72, surface IgM, and class II MHC antigen expression, and induced germline IgE heavy-chain gene transcription in highly purified B cells. Zurawski and de Vries (1994) reviewed the differences and similarities in the actions of IL13 and IL4 and the differences in IL13 in human and mouse cells. Because IL4 and IL13 and their specific signaling pathways are considered attractive targets for the treatment of allergy and asthma, Kelly-Welch et al. (2003) reviewed the signaling connections of these cytokines. IL4 interacts with IL4R (147781) with high affinity, leading to dimerization with either the common gamma chain (IL2RG; 308380), a component of receptors for a number of cytokines, to create a type I receptor, or with IL13RA1 (300119) to form a type II receptor. IL13, on the other hand, binds with high affinity to IL13RA1, which induces heterodimerization with IL4R to form a complex identical to the type II receptor. Alternatively, IL13 may bind with even greater affinity to IL13RA2 (300130), which fails to induce a signal, indicating that it acts as a decoy receptor. The C-terminal tails of the IL4 and IL13 receptor subunits interact with tyrosine kinases of the Janus kinase family (e.g., JAK1; 147795), leading to interaction with STAT6 (601512), which binds to consensus sequences in the promoters of IL4- and IL13-regulated genes. Kelly-Welch et al. (2003) proposed that subtle differences in IL4 and IL13 signaling due to polymorphisms near docking sites in IL4R may have profound implications for allergy and asthma. Zhu et al. (2004) demonstrated that acidic mammalian chitinase (AMCase; 606080) is induced via a T helper-2 (Th2)-specific, IL13-mediated pathway in epithelial cells and macrophages in an aeroallergen asthma model in mice and expressed in exaggerated quantities in human asthma. AMCase neutralization ameliorated Th2 inflammation and airway hyperresponsiveness in mice, in part by inhibiting IL13 pathway activation and chemokine induction. Zhu et al. (2004) concluded that AMCase may be an important mediator of IL13-induced responses in Th2-dominated disorders such as asthma. They found that expression of AMCase was not readily apparent in human lung samples derived from control patients with nonpulmonary disease, but was readily detected in epithelial cells and macrophages in tissue samples taken from patients with asthma. In these studies, the percentage of the epithelium with AMCase mRNA expression and the number of AMCase-positive subepithelial cells in samples from asthmatics were significantly greater than seen in controls. Zhu et al. (2004) also found that AMCase contributes downstream during IL13-induced pathology. Type 2 immunity, which is responsible for protective immune responses to helminth parasites and is the underlying cause of the pathogenesis of allergic asthma, consists of responses dominated by the cardinal type 2 cytokines IL4, IL5 (147850), and IL13. T cells are an important source of these cytokines in adaptive immune responses, but the innate cell sources remained to be comprehensively determined. Using Il13-eGFP reporter mice, Neill et al. (2010) identified and functionally characterized a novel innate type 2 immune effector leukocyte that they called the nuocyte. Nuocytes expand in vivo in response to the type 2-inducing cytokines IL25 (605658) and IL33 (608678), and represent the predominant early source of IL13 during helminth infection with Nippostrongylus brasiliensis. In the combined absence of IL25 and IL33 signaling, nuocytes failed to expand, resulting in a severe defect in worm expulsion that was rescued by the adoptive transfer of in vitro cultured wildtype, but not Il13-deficient, nuocytes. Thus, Neill et al. (2010) concluded that nuocytes represent a critically important innate effector cell in type 2 immunity. Wu et al. (2011) showed that eosinophils are the major IL4-expressing cells in white adipose tissues of mice and, in their absence, alternatively activated macrophages are greatly attenuated. Eosinophils migrate into adipose tissue by an integrin-dependent process and reconstitute alternatively activated macrophages through an IL4- or IL13-dependent process. Mice fed a high-fat diet developed increased body fat, impaired glucose tolerance, and insulin resistance in the absence of eosinophils, and helminth-induced adipose tissue eosinophilia enhanced glucose tolerance. Wu et al. (2011) concluded that eosinophils may play an unexpected role in metabolic homeostasis through maintenance of adipose alternatively activated macrophages. Nussbaum et al. (2013) showed that serum IL5 levels are maintained by long-lived type 2 innate lymphoid (ILC2) cells resident in peripheral tissues. ILC2 cells secrete IL5 constitutively and are induced to coexpress IL13 during type 2 inflammation, resulting in localized eotaxin production and eosinophil accumulation. In the small intestine where eosinophils and eotaxin (see 601156) are constitutive, ILC2 cells coexpress IL5 and IL13; this coexpression is enhanced after caloric intake. The circadian synchronizer vasoactive intestinal peptide (VIP; 192320) also stimulates ILC2 cells through the VPAC2 receptor (VIPR2; 601970) to release IL5, linking eosinophil levels with metabolic cycling. Tissue ILC2 cells regulate basal eosinophilopoiesis and tissue eosinophil accumulation through constitutive and stimulated cytokine expression, and this dissociated regulation can be tuned by nutrient intake and central circadian rhythms.