Abstract
The role of interleukin-13 in mediating ulcerative colitis remains under scrutiny. Compelling evidence from both man and mouse suggests that IL-13 not only contributes to the pathology associated with disease but is also involved in mediating the inflammatory response. These studies have led to the approach of targeting IL-13 as a promising treatment strategy in alleviating ulcerative colitis disease. Despite this evidence, recent clinical trial data suggests that specifically blocking the receptor through which IL-13 signals, IL-4 receptor-alpha (IL-4Rα) in ulcerative colitis patients, is insufficient in protecting them from disease outcome. This challenges the importance of IL-13 as a therapeutic target. This review describes the role of IL-13 in ulcerative colitis and current treatment strategies that target IL-13. The potential role of IL-13 signaling independently of IL-4Rα in mediating ulcerative colitis is highlighted as an important consideration when targeting the signaling mechanisms of IL-13 for therapeutic approaches.
Introduction
Since the discovery of IL-13 it has been shown to be a key cytokine in controlling pathogens such as helminthic parasites, but also as a prominent feature in allergic and inflammatory diseases. IL-13 is a 10–14 kDa immune-regulatory cytokine first described as a protein preferentially produced by activated T helper-Type (Th) 2 cells (Brown et al., ; McKenzie et al., ; Minty et al., ; Hershey, ). However, it has since been established that IL-13 is in-fact produced by a wide variety of cell types, including innate immune cells, with diverse biological activities (Wynn, ; Mannon and Reinisch, ). These include basophils, eosinophils, mast cells, natural killer cells, epithelial cells, smooth muscle cells, fibroblasts, and NK T cells. This highlights the role of cells other than T and B cells that produce IL-13; however the role of these cells specifically in the gut is yet to be established. Once produced IL-13 initiates a cascade of immunological process which aid in parasite clearance. Here IL-13 can act directly on macrophages, driving the differentiation toward the M2 phenotype resulting in a Th2 response (Gordon, ). Furthermore, IL-13 drives beneficial responses such as, the IgE isotype switch, eosinophil recruitment, mucus production and muscle contraction. However, these are the very responses that contribute to pathology in an inflammatory response. Our own studies have shown that blocking IgE in an animal model of ulcerative colitis reduces the severity of disease (Hoving et al., ). This was associated with reduced mast cell activation as described below. The pleiotropic nature of IL-13 quickly became evident by the myriad of diseases in which it plays either a beneficial or detrimental role. IL-13 is a prominent mediator of allergic lung disease, including pulmonary inflammation, asthma and anaphylaxis (Gour and Wills-Karp, ). More recently, IL-13 has been linked to enhancing brain function in mice by increasing cognitive memory (Brombacher et al., ). Here, IL-13 was able to stimulate primary astrocytes to produce brain-derived neurotrophic factor, known to enhance cognitive function. Furthermore, a new SNP in the regulatory region of il13 (rs1881457C) has been associated with an increased risk of Coronary Artery Disease in a Chinese Han co-hort (Zha et al., ). Here the functional mechanisms remain unknown.
IL-13 and its role in colitis
IL-13 is an interesting cytokine in the role it plays in mediating Th2 inflammatory diseases. Initially IL-13 was a target for host directed therapy for asthma, dermatitis and other allergic diseases. However, IL-13 has also been linked to mediating the host inflammatory cascade responsible for the pathogenesis of ulcerative colitis. Combining evidence from mice and man, the mechanism of IL-13 mediated colitis is illustrated in Figure 1. Essentially, a defect in defect in antigen recognition triggers an inappropriate and exaggerated immune response. This is further aggravated by the disruption of epithelial tight junctions, increasing permeability of the intestinal epithelium and resulting in an increase in the uptake of luminal antigens. In a mouse model of ulcerative colitis using the hapten, oxazolone to induce a transient disease phenotype, blocking IL-13 (Heller et al., ) or using IL-13 gene-deficient mice (Weigmann et al., ) has been shown to ameliorate or prevent disease induction. While IL-13 production by NK T cells has been shown to play a major role in mediating disease, our own studies have implicated additional components of the immune response that contribute to the onset and maintenance of disease. These include IL-4Rα-responsive CD4+ T cells and IgE production by B cells which contribute to oxazolone-induced pathology in mice. Depleting IgE was linked to a reduction in the number of activated mast cells and reduced pathology (Hoving et al., ).
Figure 1
Increasing evidence demonstrates that IL-13 is responsible for initiating the detrimental inflammatory cascade in colitis. While orchestrating an inflammatory response by immune cells IL-13 can also act directly on epithelial cells. In ulcerative colitis, IL-13 has been described as a key effector cytokine acting on epithelial cell function and initiating apoptosis (Heller et al.,
Additional studies have expanded on the current understanding of the role IL-13 plays in colitis and describe additional mechanisms associated with IL-13 during colitis. For example, in the oxazolone colitis mouse model, blocking IL-25 derived from intestinal epithelial cells improved the clinical aspects of disease (Camelo et al.,
IL-13 signaling mechanisms
Both IL-4 and IL-13 cytokines use the IL-4Rα chain as a component of their receptors (Figure 2). This was shown in mice treated with anti-IL-4Rα antibodies or IL-4 antagonists (Aversa et al.,
Figure 2

IL-13 signaling mechanisms and associated host directed targets in the treatment of ulcerative colitis. IL-13 signals through the type II (IL-4Rα and IL-13Rα1) receptor complex and activates the JAK1/STAT6 pathway. In addition, IL-13 has also been shown to signal through IL-13Rα2, activating AP-1 to induce the secretion of TGF-β. This pathway is, in part dependent on the production of TNFα. Various drug targets have been directed at IL-13 including the IL-4Rα signaling pathway to block the immune response that mediate Th2-driven inflammatory diseases such as allergy and colitis. Drug treatments that were beneficial or improved disease outcome are depicted in green and treatments that were unsuccessful or exacerbated disease outcome are depicted in red (Rutgeerts et al.,
IL-4 and IL-13 not only share common subunits, they also share common signaling pathways (Figure 2). The components of both receptor complexes are associated with Janus kinases (JAK). JAK1 has been proposed to associate with the IL-4Rα chain, γc with JAK3 and IL-13Rα1 with JAK2 (Nelms et al.,
Host directed treatment strategies against IL-13
Considering the compelling link between IL-13 and ulcerative colitis, various clinical trials have been implemented to target IL-13 as a treatment strategy. A previous review by Mannon and Reinisch elegantly summarized drug targets for IL-13 in the treatment of colitis, however at the time most of these trials were ongoing and the outcomes unknown (Mannon and Reinisch,
Another recent candidate in ulcerative colitis treatment is tralokinumab, a human IL-13-neutralizing IgG4 monoclonal antibody. This monoclonal antibody has a very high affinity for IL-13 and is at stage III clinical trials for both allergic asthma and atopic dermatitis. A recent study characterizing the structure of tralokinumab Fab in complex with IL-13 demonstrates the inhibition of binding to both IL-13Rα1 and IL-13Rα2 (Popovic et al.,
Many treatment approaches against ulcerative colitis target the IL-4Rα signaling pathway. This is based on the fact that both IL-4 and IL-13 signal through the IL-4Rα. However, in light of recent studies describing the potential role of IL-13Rα2 in IL-13 signaling, this treatment approach may not be appropriate in ulcerative colitis (Fichtner-Feigl et al.,
Lastly, anti-TNFα has proven to be effective in patients that do not respond to convention treatment strategies. In fact this therapeutic approach was the first approved for inflammatory bowel disease treatment more than 50 years ago. As TNFα production is traditionally associated with a Th1 response, it is therefore not a likely candidate for ulcerative colitis treatment. None-the-less, monoclonal antibodies against TNFα such as infliximab and adalimumab have shown promising outcomes (Rutgeerts et al.,
Conclusion
The potential benefits of ameliorating IL-13 production in ulcerative colitis remains an interesting approach in treating disease. Furthermore, these treatment strategies could potentially be extended beyond the pathology of colitis. Targeting IL-13 in allergic diseases and dermatitis has already proven to be promising. If the optimal treatment strategy and correct targets can be identified for ulcerative colitis, this could even translate to preventative treatment of colitis-associated colorectal cancer. The link between ulcerative colitis and colorectal cancer is widely accepted, however the extent of the risk is difficult to determine as reports describe widely varying rates (Eaden et al.,
Statements
Author contributions
The author confirms being the sole contributor of this work and has approved it for publication.
Funding
JH was supported by the National Research Foundation of South Africa, Carnegie Corporation and CIDRI-Wellcome Trust [084323].
Conflict of interest
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
ulcerative colitis, interleukin-13, IL-4 receptor-alpha, drug targets, Inflammatory Bowel Disease, T-helper type 2 immune response
Citation
Hoving JC (2018) Targeting IL-13 as a Host-Directed Therapy Against Ulcerative Colitis. Front. Cell. Infect. Microbiol. 8:395. doi: 10.3389/fcimb.2018.00395
Received
07 June 2018
Accepted
22 October 2018
Published
06 November 2018
Volume
8 - 2018
Edited by
Abhay Satoskar, The Ohio State University, United States
Reviewed by
Muhammad Ammar Zafar, Wake Forest School of Medicine, United States; Tonyia Eaves-Pyles, The University of Texas Medical Branch at Galveston, United States
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Copyright
© 2018 Hoving.
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*Correspondence: J. Claire Hoving jennifer.hoving@uct.ac.za
This article was submitted to Clinical Microbiology, a section of the journal Frontiers in Cellular and Infection Microbiology
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