Interleukin 13

Functions

IL-13 has effects on immune cells that are similar to those of the closely related cytokine IL-4. However, IL-13 is suspected to be a more central mediator of the physiologic changes induced by allergic inflammation in many tissues.

Although IL-13 is associated primarily with the induction of airway disease, it also has anti-inflammatory properties. IL-13 induces a class of protein-degrading enzymes, known as matrix metalloproteinases (MMPs), in the airways. These enzymes are required to induce egression of effete parenchymal inflammatory cells into the airway lumen where they are then cleared. Among other factors, IL-13 induces these MMPs as part of a mechanism that protects against excessive allergic inflammation that predisposes to asphyxiation.

IL-13 is known to induce changes in hematopoietic cells, but these effects are probably less important than that of IL-4. Furthermore, IL-13 can induce immunoglobulin E (IgE) secretion from activated human B cells. Interestingly, deletion of IL-13 from mice does not markedly affect either Th2 cell development or antigen-specific IgE responses induced by potent allergens. In comparison, deletion of IL-4 abrogates these responses. Thus, rather than a lymphoid cytokine, IL-13 acts more prominently as a molecular bridge linking allergic inflammatory cells to the non-immune cells in contact with them, thereby altering physiological function.

IL-13 induces its effects through a multi-subunit receptor that includes the alpha chain of the IL-4 receptor (IL-4Rα) and at least one of two known IL-13-specific binding chains. Most of the biological effects of IL-13, like those of IL-4, are linked to a single transcription factor, signal transducer and activator of transcription 6 (STAT6). This can be resulted from an allergic reaction brought about when facing an Ala gene.

Clinical significance

IL-13 specifically induces physiological changes in parasitized organs that are required to expel the offending organisms or their products. For example, expulsion from the gut of a variety of mouse helminths requires IL-13 secreted by Th2 cells. IL-13 induces several changes in the gut that create an environment hostile to the parasite, including enhanced contractions and glycoprotein hyper-secretion from gut epithelial cells, that ultimately lead to detachment of the organism from the gut wall and their removal.

The eggs of the parasite Schistosoma mansoni may lodge in a variety of organs including the gut wall, liver, lung and even central nervous system, inducing the formation of granulomas under the control of IL-13. Here, however, the eventual result is organ damage and often profound or even fatal disease, not resolution of the infection. An emerging concept is that IL-13 may antagonize Th1 responses that are required to resolve intracellular infections. In this immune dysregulated context, marked by the recruitment of aberrantly large numbers of Th2 cells, IL-13 inhibits the ability of host immune cells to destroy intracellular pathogens.

IL-13 induces many features of allergic lung disease, including airway hyperresponsiveness, goblet cell metaplasia and mucus hypersecretion, which all contribute to airway obstruction. IL-4 contributes to these physiologic changes, but is less important than IL-13. IL-13 also induces secretion of chemokines that are required for recruitment of allergic effector cells to the lung. Studies of STAT6 transgenic mice suggest the interesting possibility that IL-13 signaling occurring only through the airway epithelium is required for most of these effects. While no studies have yet directly implicated IL-13 in the control of human diseases, many polymorphisms in the IL-13 gene have been shown to confer an enhanced risk of atopic respiratory diseases such as asthma.