Interleukin-27 as a Novel Therapy for Inflammatory Bowel Disease: A Critical Review of the Literature.

In contrast to the evidence presented above, a collection of studies have reported deleterious effects of IL-27 in intestinal inflammation based on inhibiting IL-27 receptor signaling. 53 – 56 Genetic deletion of IL-27Rα on either transferred T cells or in recipient mice effectively prevented the intestinal inflammation typical of the T cell transfer model of enterocolitis. 53 , 54 Increased numbers of transferred T cells lacking IL-27Rα became Foxp3 + , suggesting that IL-27 may negatively regulate the development of Foxp3 + regulatory T cells. 53 The second study also noted that inhibition of IL-27Rα in recipient mice prevented Th17 cell development by decreasing production of IL-1β and IL-6 by antigen presenting cells. 54 Similarly, IL-27Rα knockout mice reportedly develop less severe DSS-induced colitis than wild type mice, characterized by reduced expression of IL-6, TNF-α, and IFN-γ in intestinal lamina propria mononuclear cells. 55 IL-10 deficient mice, which spontaneously develop intestinal inflammation, also reportedly have both delayed pathology and a survival advantage when IL-27Rα is concurrently genetically deleted. 56 Interestingly, a number of these findings attributed to IL-27Rα deletion, including reduced inflammation in both the T cell transfer and DSS models of colitis and reductions in IL-1β, IL-6, and TNF- α, have also been reported as beneficial effects of IL-27 treatment, leaving more questions than answers for the interpretation of these aspects of IL-27 function. A summary of the reported roles for IL-27 in the models of murine colitis detailed above is presented in Table 1 .

While a few studies suggest that inhibition of IL-27 receptor signaling can ameliorate intestinal inflammation, most of the literature supports IL-27 as a suppressor of intestinal inflammation caused by a variety of insults ( Figure 1 ). It is important to note that all of the studies favoring a proinflammatory role for IL-27 in the intestine examined specific consequences of genetic deletion of the IL-27 receptor. In contrast, to our knowledge no report to date has demonstrated deleterious effects of exogenous IL-27 treatment on intestinal inflammation. Additionally, based on the complex biology of IL-27 discussed above, it is not hard to imagine how complete inhibition of IL-27 signaling could eliminate subtle checks and balances inherent to IL-27 function in wild type animals, as IL-27 signaling elicits distinct responses based on cell type and timing. For example, genetic deletion of IL-27Rα predicted that IL-27 would be protective in influenza infection. However, while the administration of IL-27 late in the course of influenza infection was beneficial, IL-27 therapy early in the infection resulted in more severe disease, 32 an outcome impossible to predict by complete genetic inhibition of IL-27 signaling. Based on the ability of IL-27 to inhibit both Th2 and Th17 cell differentiation and promote Tr1 cell development, 17 – 19 , 23 – 27 it is also reasonable to speculate that genetic ablation of IL-27 signaling could alter the differentiation and subsequent phenotype of multiple cell types, thereby creating an inflammatory microenvironment that is not representative of that of wild type animals. Additionally, the timing of IL-27 signaling and the evaluation of its result are critical. In human mast cells, IL-27 rapidly elicits transcription of inflammatory cytokine genes, but elicits a slow response in human monocytes. 12 As a result, conclusions taken at a single or limited time points may not fully illustrate the results of IL-27 therapy or elimination and may contribute to discrepancies between studies. Finally, different concentrations of IL-27 have been shown to elicit distinct patterns of cytokine expression. 25 Different methods of IL-27 administration may produce different concentrations within the intestine, and may be an explanation for differing results between some studies. Furthermore, the different patterns of cytokine expression induced by different concentrations of IL-27 suggest that genetic deletion of IL-27 signaling in an entire animal or subsets of its cells may create a misleading “all or nothing” microenvironment unrepresentative of physiologic reality. When interpreting literature describing the biology of IL-27, it is critical to acknowledge that each subunit of the heterodimeric IL-27 cytokine has unique biologic activities of its own. 57 , 58 As a result, gene and/or protein expression of either of these subunits, even if coexpressed, is not necessarily indicative of the presence of the complete IL-27 dimer, and may in fact represent production of these subunits independently or some combination of complete IL-27 and free subunits. Interestingly, the existence of a soluble form of IL-27Rα has been described that is able to antagonize IL-27 and is thought to originate by cleavage of the membrane-bound receptor. 59 Dietrich et al. reported that varying levels of soluble IL-27Rα were produced by human CD4 + and CD8 + T cells, B cells, monocyte-derived dendritic cells, and monocytes. 59 While only speculation, it is interesting to question whether genetic deletion of IL-27Rα in only specific cells, such as the transferred T cells or recipient cells in the studies detailed above, could also eliminate or reduce this negative regulator of IL-27, resulting in dysregulated IL-27 signaling in remaining cells with functional IL-27Rα. The discovery of this endogenous, soluble antagonist for IL-27 is intriguing, and further research into its function and regulation could impact concepts of IL-27 biology and potential therapeutic manipulation. IL-23 promotes the development of Th17 cells, 60 , 61 and a genome wide association study demonstrated a significant association between variants in the IL-23R gene and both Crohn’s disease and ulcerative colitis. 62 However, while multiple studies have associated Th17 cells with the pathogenesis of IBD, the literature remains conflicted over whether their signature cytokines, IL-17A and IL-17F, drive intestinal inflammation or are protective (or both). 63 , 64 In contrast to the efficacy of anti-tumor necrosis factor-α antibody treatment in a subset of IBD patients, 3 antibody neutralization of IL-17A was surprisingly inefficacious, and in some cases, deleterious in patients with moderate to severe Crohn’s disease. 65 Similarly, a clinical trial investigating antibody blockade of IL-17 receptor A in moderate to severe Crohn’s disease was terminated due to a lack of efficacy and worsening of disease in some patients. 66 However, two other therapies known to inhibit Th17 responses have shown promise as treatments for Crohn’s disease and ulcerative colitis. 67 , 68 Vidofludimus, an inhibitor of the enzyme dihydroorotate dehydrogenase known to also inhibit the production of both IL-17A and IL-17F, induced steroid free remission in at least 50% of both Crohn’s disease and ulcerative colitis patients in a clinical trial. Furthermore, of the remaining patients in the trial, partial remission was achieved in an additional 28.6% and 41.7% of patients with Crohn’s disease or ulcerative colitis, respectively. 67 Two trials have investigated the use of tofacitinib in IBD, demonstrating that it may be an effective therapy for ulcerative colitis, but not Crohn’s disease. 68 , 69 Tofacitinib inhibits Janus kinases 1, 2 and 3, and in doing so also blocks the differentiation of Th17 cells. Patients with moderately to severely active ulcerative colitis showed dose dependent clinical responses and clinical remission with tofacitinib therapy. Among patients receiving the highest dose, 78% showed a clinical response and 41% achieved clinical remission after 8 weeks of treatment. 68 A shorter 4 week study found no significant reductions in Crohn’s disease activity index in patients with moderate to severe Crohn’s disease given tofacitinib; 69 however, whether this lack of efficacy is due to differences between the two diseases or the shorter time of treatment (or both) is yet to be determined. Additionally, patients receiving placebo treatment in this study had a higher rate of response and remission than anticipated, leaving the authors to question whether this could have contributed to the nonsignificant difference in treatment results. 69 Interestingly, a recent study reported that antibody neutralization of both IL-17A and IL-17F, but neither cytokine alone, reduced colon histopathology scores in the murine T cell transfer model of enterocolitis. 70 This could explain why vidofludimus and tofacitinib were more successful in clinical trials for IBD than antibody blockade of IL-17A alone, but offers little insight as to why anti-IL-17 receptor A treatment, which should inhibit signaling of both IL-17A and IL-17F, 71 was inefficacious. This is particularly relevant to the discussion herein, as IL-27 is able to block the expression of both IL-17A and IL-17F through inhibition of the transcription factor RORγt. 18 This potential therapeutic effect has been demonstrated experimentally in murine enterocolitis, in which decreased expression of RORγt, IL-17A, and IL-17F was reported in the colons of mice following mucosal administration of IL-27. 44 IL-27 also inhibits the development of Th17 cells by inducing expression of programmed death ligand 1 on naïve T cells, which when cultured with naïve CD4 + T cells, prevents their differentiation into Th17 cells. 72 However, it is unclear what effect, if any, IL-27 may have on the secretion of IL-17 by immune cells other than T cells in the inflamed intestine, such as neutrophils and mast cells, which have been shown to be important sources of IL-17 in arthritis and psoriasis. 73 , 74 In contrast to antibody neutralization of a cytokine or its receptor, it is interesting to question whether the administration of another cytokine, potentially more subject to endogenous regulation, could create a more physiologically relevant anti-inflammatory microenvironment that might be able to maintain balance in the pro- and anti-inflammatory effects of the immune mediators it regulates. For example, the elimination of cytokine signaling by antibody neutralization or genetic deletion could block both its beneficial and harmful effects, while modulating a cytokine with one of its physiologic regulators could potentially preserve its beneficial functions. Although grouped together as IBD, Crohn’s disease and ulcerative colitis are distinct conditions that vary both clinically and immunopathologically. 75 The lesions of Crohn’s disease may be located anywhere throughout the gastrointestinal tract and are characterized by transmural infiltrates of macrophages and lymphocytes that in many patients multifocally organize to form granulomas. In contrast, ulcerative colitis is limited to the colon and features histopathologic changes of the mucosa only, including infiltrates of granulocytes and lymphocytes. Mucosal ulceration may be present in both conditions. Based on characterizations of cytokine expression and signaling, Crohn’s disease is considered to be driven by Th1 responses, while ulcerative colitis is a Th2-mediated disease. 75 Despite differences in their immunopathology, there are abundant mechanisms by which IL-27 administration could be an effective therapy for both Crohn’s disease and ulcerative colitis. Perhaps the most obvious mechanism by which IL-27 could reduce inflammation in IBD is through its stimulation of the immunoregulatory cytokine IL-10, which can be induced by IL-27 signaling in CD8 + and regulatory, Th1, Th2, and Th17 cells. 6 , 19 , 21 – 27 IL-10 boasts an incredible number of anti-inflammatory functions, including blocking Th1 and Th2 responses; inhibition of inflammatory cytokine and chemokine production by monocytes and neutrophils; limiting the recruitment of dendritic cells, T cells, neutrophils, and monocytes; inducing anergy in activated T cells; stimulating the production of interleukin-1 receptor antagonist and soluble tumor necrosis factor receptor; and reducing monocyte activation of T cells. 76 , 77 Complementing these functions of IL-10, IL-27 itself also limits tissue infiltration by neutrophils and monocytes, suppresses chemokine production, and inhibits the generation of reactive oxygen intermediates by both macrophages/monocytes and granulocytes. 32 , 33 As demonstrated in a mouse model of experimental autoimmune encephalomyelitis, IL-27’s ability to induce CD39 on dendritic cells could contribute to the suppression of pathogenic T cell responses in IBD. 30 As ulcerative colitis has been shown to be driven by Th2 immunopathology, IL-27’s capacity to block Th2 cell differentiation and cytokine expression makes it well suited to treat this condition. 17 , 75 Additionally, while the treatment of Th1-driven Crohn’s disease with a cytokine known to promote Th1 responses sounds counterintuitive, 6 , 75 IL-27 is also critical for the development of T-bet + CXCR3 + regulatory T cells in the gut-associated lymphoid tissue that are specialized for regulating Th1 cells. 16 IL-10 induced by IL-27 treatment could further contribute to the regulation of exuberant Th1 immunity in Crohn’s disease. 6 , 19 , 21 – 27 How IL-27’s ability to curb Th17 responses may influence its efficacy as a treatment for IBD is uncertain. However, in contrast to the failed trials investigating direct targeting of IL-17 signaling alone as a treatment for Crohn’s disease, inhibition of Th17 responses is only one of many anti-inflammatory functions IL-27 exerts on adaptive and innate immunity. In this way IL-27 treatment would be more analogous to vidofludimus and tofacitinib therapy, which were effective in treating ulcerative colitis and/or Crohn’s disease, 67 , 68 and, like IL-27, exert diverse anti-inflammatory functions in addition to the blockade of IL-17A and IL-17F. Both subunits of the IL-27 receptor, IL-27Rα and gp130, are expressed at low levels in normal intestinal epithelial cells, but are upregulated in inflammation in both epithelial cells and infiltrating leukocytes. 13 Increased expression of IL-27 in inflamed segments of the intestinal mucosa has been demonstrated in both Crohn’s disease and ulcerative colitis. 13 , 78 Patients with active Crohn’s disease have also been shown to have both significantly increased serum IL-27 and soluble IL-27Rα relative to healthy controls; however, despite an overall positive correlation between these two values, the ratio of cytokine to soluble receptor varied widely among patients. 59 Table 2 highlights the evidence for IL-27 as a factor in human IBD. The question remains whether elevations of IL-27 in the inflamed intestinal mucosa and serum of IBD patients are contributing to inflammation in these patients or represent an anti-inflammatory response. However, based on the evidence presented herein, it seems most likely that these elevations in IL-27 represent an (inadequate) anti-inflammatory response in diseased segments of intestine. While IL-27 signaling in the intestine presumably acts on inflammatory cells, inflamed intestinal epithelial cells upregulate the IL-27 receptor and are therefore also capable of responding to IL-27.

Inflammatory bowel disease (IBD) refers to a collection of idiopathic inflammatory disorders of the intestine, the most common of which are Crohn’s disease and ulcerative colitis. IBD is thought to result from an abnormal inflammatory response to commensal organisms and other antigens normally confined to the intestinal lumen due to compromise of the mucosal epithelial barrier and subsequent inappropriate exposure of resident intestinal immune cells to luminal antigens. A single cause for IBD has not been identified. Development of the disease is likely multifactorial, as a variety of etiological factors, including hygiene status, previous gastrointestinal infection, genetics, diet, and various other lifestyle factors, have been implicated in its pathogenesis. 1 Geographically, the highest incidence of IBD has been reported in northern Europe and North America, where an estimated 329 per 100,000 people in the United States suffer from IBD. 1 , 2 Treatment for IBD typically involves some form of immunosuppression, necessitating a balance between achieving remission and managing potential adverse effects. Additionally, likely due to the complex pathophysiology and multifactorial etiology of IBD, rates for remission induction and maintenance with many of the current standard treatments for IBD remain modest and are often variable. 3 , 4 As a result, investigation into novel therapeutic targets is sorely needed. The success of anti-tumor necrosis factor-α treatment in a subset of IBD patients demonstrated that therapy targeting a single cytokine could be efficacious in IBD, and antibodies against a variety of cytokines, including interleukin (IL)-13, IL-18, and IL-21, have entered clinical trials. 3 IL-27 is a relatively recently discovered type I cytokine with established roles in infectious disease, autoimmunity, and cancer in a variety of organs, including the central nervous system, lung, skin, and gastrointestinal tract. 5 , 6 However, it has not been definitively determined whether IL-27 ameliorates or promotes intestinal inflammation, as seemingly contradictory roles for IL-27 have been reported in murine models of IBD. 6 Several excellent reviews have recently discussed the biology of IL-27. 6 – 9 The purpose of this review is to highlight recent literature investigating the role of IL-27 in IBD, and to discuss possible explanations for the conflicting results of these studies. Evidence supporting IL-27 therapy as a potential treatment for IBD will also be discussed.

While complex, the majority of the literature investigating the role of IL-27 in mouse models of IBD complements knowledge gained from human patients and supports an anti-inflammatory role for IL-27 in IBD. However, the studies suggesting a more pro-inflammatory role for IL-27 should not be ignored, but should rather inform future investigations into the complex physiology of this cytokine. While it’s unlikely that a single “silver bullet” treatment for all cases of IBD will ever be found, IL-27 is a promising potential therapy that warrants further investigation.