For most gastroenterologists, TNF-α is the most recognized cytokine due to the increasing use of the monoclonal anti-TNF-α antibodies, infliximab, adalimumab, certolizumab, and golimumab, for the treatment of CD and UC. TNF-α is secreted by macrophages, monocytes, neutrophils, T cells, and NK cells following their stimulation by bacterial lipopolysaccharides. CD4⁺ T-lymphocytes secrete TNF-α, while CD8⁺ T cells do not. The synthesis of TNF-α is induced by many different stimuli including interferons, IL-2, and GM-CSF. The production of TNF-α is inhibited by IL-6 and TGF-β.

TNF-α is an agonist of the p38 and c-Jun N-terminal kinase (NK) cascades, two important signaling pathways of the MAP kinase family involved in the generation of the inflammatory responses [5]. It is a potent pro-inflammatory cytokine that exerts its stimulatory effect on cells that produce IFN-γ. Indeed, TNF-α in synergy with factors from non-lymphocyte lamina propria mononuclear cells can act with prostaglandin E2 to stimulate IL-12-mediated T-cell production of IFN-γ. In resting macrophages, TNF-α induces the synthesis of IL-1 and prostaglandin E2_, which can act in concert to potentiate the inflammatory cascade. TNF-α can also enhance the proliferation of T cells induced by various stimuli in the absence of IL-2; in fact some subpopulations of T cells only respond to IL-2 in the presence of TNF-α. Beyond its effect on the immune response, TNF-α activates osteoclasts and thus induces bone resorption, and this effect may be associated with decreased bone mineral density in patients with CD.

Although TNF-α is required for normal host immune responses, the overexpression can have severe pathologic consequences as exemplified by mice in which the overexpression of TNF by a transgene is associated with a severe colitis [6].

In animal models, TNF-α knockout mice do not develop significant colitis [7], and as proof of principle that TNF-α is important for the pathogenesis of IBD, TNF-α-neutralizing antibodies have been shown to be effective in ameliorating intestinal inflammation. Associated human studies have reported elevated levels of TNF-α in serum, stool, and mucosal tissue [8, 9] correlating with clinical and laboratory indices of intestinal activity. Furthermore, dramatic effects have been noted in clinical studies in patients with Crohn disease treated with infliximab [10, 11]. These effects have been observed in both disease amelioration and induction of clinical remission. Important for the understanding of some of the critical side effects of infliximab, TNF-α mediates a part of the cell-mediated immunity against obligate and facultative bacteria and parasites by stimulating phagocytosis and the synthesis of superoxide dismutase in macrophages. It confers protection against Listeria monocytogenes infections and tuberculosis. Anti-TNF-α antibodies have been shown to weaken the ability of mice to cope with these infections. Infection with these organisms is a possible risk of using anti-TNF-α monoclonal antibody therapy in the treatment of IBD and a reason that patients are screened for tuberculosis prior to initiation of therapy with infliximab.

IFN-γ is produced mainly by CD4⁺, CD8⁺ T-lymphocytes, and natural killer cells activated by antigens and mitogens. IFN-γ synergizes with TNF-α and in inhibiting the proliferation of various cell types; however, the main biological activity of IFN-γ appears to be immunomodulatory in contrast to the other interferons (IFN-α or IFN-β), which are mainly antiviral. IL-2 and IFN-γ appear to be intricately interwoven in their functions. In T-helper cells, IL-2 induces the synthesis of IFN-γ and other cytokines. IFN-γ acts synergistically with IL-1 and IL-2 and appears to be required for the expression of IL-2 receptors (CD25) on the cell surface of T-lymphocytes. Blocking of the IL-2 receptor by specific antibodies inhibits the synthesis of IFN-γ. IFN-γ is a modulator of T-cell growth and functional differentiation, a growth-promoting factor for T-lymphocytes, and it potentiates the response of these cells to growth factors. Most importantly, IFN-γ can increase the expression of MHC class molecules allowing greater antigenic recognition. Furthermore, it can increase permeability at epithelial tight junction barriers, thereby allowing further antigenic exposure [12]. Finally, in concert with TNF-α, IFN-γ can cause direct tissue destruction as it increases local inflammation [12, 13].

IFN-γ secretion is one of the few cytokines that correlates with severity of disease in patients with CD. As a known pro-inflammatory cytokine, it would appear to be an obvious choice to target for treatment of IBD. IFN-γ has been targeted in CD using fontolizumab, a humanized monoclonal antibody against IFN-γ [14, 15]. In studies using these antibodies, positive results were found in patients with moderate-to-severe CD when compared to placebo. Although the studies did not reach statistical significance, the results did indicate a trend toward effect. This suggests a potential benefit of blocking IFN-γ in patients with CD.

This cytokine consists of IL-1α and IL-1β subunits, which both are produced by monocytes, macrophages, and endothelial cells. In addition to these pro-inflammatory cytokines, there is an IL-1 receptor antagonist (IL-1RA) produced by intestinal epithelial cells, which is capable of inhibiting the pro-inflammatory actions of IL-1 by binding the IL-1 receptor and competitively blocking the interaction with IL-1. IL-1RA is considered to be one intestinal mechanism for downregulation of the immune response and has been shown to be elevated in the serum of patients with CD. Stimulation of IL-1RA secretion is activated by IL-1, forming a negative feedback loop.

IL-1α and IL-1β are essentially biologically equivalent pleiotropic factors that act locally and systemically. IL-1 has a multitude of effector functions, some of which are mediated indirectly by the induction of the synthesis of other mediators including ACTH, PGE₂, IL-6, and IL-8 (a chemotactic cytokine in the chemokine family). The main biological activity of IL-1 is the stimulation of T-helper cells, which are induced to secrete IL-2 and to express IL-2 receptors. IL-1 can also act on B cells, promoting their proliferation and the synthesis of immunoglobulins. IL-1 stimulates the proliferation and activation of other immune cells such as NK cells, fibroblasts, and thymocytes. The IL-1-mediated proliferative effects can be inhibited by the suppressive cytokine, TGF-β.

The synthesis of IL-1 can be induced by other cytokines including TNF-α, IFN-α, and β or γ and also by bacterial endotoxins and viruses. Furthermore, IL-1 activity is not limited to stimulation of T cells, but it also promotes the adhesion of neutrophils, monocytes, T cells, and B cells by enhancing the expression of adhesion molecules such as ICAM-1 (intercellular adhesion molecule) and ELAM (endothelial leukocyte adhesion molecule). All of which can contribute to the pathogenesis of CD. IL-1 is also a strong chemoattractant for leukocytes, as demonstrated by the local accumulation of neutrophils at the site of injection of tissue with IL-1. Beyond activation by other cytokines, IL-1β is secreted in response to select microbial components via the NLR inflammasomes, specifically P. mirabilis [16]. This and other supporting data demonstrate a strong link between the microbiome and modulation of the immune response in the gut.

Finally, in combination with TNF-α, IL-1 appears to be involved in the generation of lytic bone lesions. IL-1 activates osteoclasts, thereby suppressing the formation of new bone, suggesting another etiology for decreased bone density in CD. Low concentrations of IL-1, however, can promote new bone growth.

IL-1 was one of the first cytokines targeted for therapy in animal colitis models. In these studies, administration of IL-1RA led to amelioration of colitis, in a rabbit model. Thus, it was also one of the first demonstrations that blockade of a single cytokine could be effective in therapy of colitis [17]. In patients with IBD, increased serum levels of IL-1 are seldom detected. However, in intestinal lesions in patients with both CD and UC, IL-1 levels are elevated [18]. IL-1RA is a possible intestinal mechanism for downregulation of the immune response and is elevated in the serum of patients with CD. IL-1RA determines the biological effects of IL-1, as increased concentrations of this mediator will decrease IL-1 activity. In the inflammatory lesions of IBD patients, levels of this mediator are increased, although not as much as IL-1, leading to a disproportionate increase in IL-1 activity [19] overcoming competitive inhibition.

IL-2 is a major T-cell growth factor, secreted by activated T cells, and acts via the high-affinity IL-2 receptor (CD25) on T cells. This binding to CD25 promotes cell proliferation. Under physiological conditions, IL-2 is produced mainly by CD4+ T-lymphocytes following cell activation. Resting cells do not produce IL-2. In T-helper cells, IL-2 induces the synthesis of IFN-γ and other cytokines. IFN-γ acts synergistically with IL-1 and IL-2 and appears to be required for the expression of IL-2 receptors on the cell surface of T-lymphocytes. Blocking of the IL-2 receptor by specific antibodies also inhibits the synthesis of IFN-γ. IFN-γ in return is a modulator of T-cell growth and functional differentiation. It is a growth-promoting factor for T-lymphocytes and potentiates the response of these cells to growth factors.

IL-2 is a growth factor for all subpopulations of T-lymphocytes including importantly suppressive T regulatory (Treg) cells. It is an antigen-unspecific proliferation factor for T cells that induces cell cycle progression in resting cells and thus allows clonal expansion of activated T-lymphocytes.

In patients with CD, it has been demonstrated in many studies that IL-2 secretion from lamina propria cells is decreased as compared to normal patient samples. Daclizumab, a humanized monoclonal antibody to CD25, produced in an effort to block the binding of IL-2 to the IL-2R, was tested in patients with UC and initially appeared promising in a small open-label study [20], but upon testing in a placebo-controlled study, the therapy did not show efficacy [21]. This effect could be related to the fact that IL-2R (CD25) is also present on T regulatory cells. The inhibition of binding of IL-2 to its receptor present on Treg cells thereby inhibits the proliferation of these cells, which are important in downregulation of the immune response. This highlights a common problem in the targeting of the cytokine pathway for treatment of inflammatory diseases, in that cytokines frequently have multiple effects and can function in both a pro-inflammatory as well as an anti-inflammatory capacity.

IL-6 is a pleiotropic cytokine considered to be a major player in inflammation, regulation of T-cell responses, and apoptosis. Many different cell types produce IL-6. The main sources in vivo are stimulated monocytes, fibroblasts, endothelial cells, macrophages, T cells, and B-lymphocytes. IL-6 is a B-cell differentiation factor in vivo and in vitro and an activation factor for T cells. In the presence of IL-2, IL-6 induces the differentiation of mature and immature T cells into cytotoxic T cells. IL-6 also induces the proliferation of thymocytes and likely plays a role in the development of thymic T cells. Most significantly IL-6 and TGF-β together can induce the development of the inflammatory T-helper-17 (Th17) cell lineage. Finally, in opposition, if IL-6 is present, there is decreased propensity to development of Foxp3-positive Treg cells.

Interestingly, IL-6 levels are increased in the serum of patients with active CD and UC compared to normal controls. A study looking at a known functional polymorphism of the IL-6 gene and the site of disease in CD patients did not demonstrate an association of IL-6 functional polymorphisms with CD or protection from CD. It did demonstrate that patients with the high-producer genotype were more likely to have ileocolonic disease, while those with the low-producer genotype had primarily colonic-type disease, whereas those with intermediate-producer genotype were more likely to have isolated ileal disease. These studies indicated an association of IL-6 production and site of disease [22]. The activity of IL-6 has made it an obvious target for clinical trials not only due to its pro-inflammatory effects but also due to its involvement in T-cell apoptosis [23]. A pilot study was performed [24] to investigate safety and efficacy of a humanized anti-IL-6R monoclonal antibody in patients with CD. This target appeared to be promising in these studies with 80% of the patients treated for 12 weeks demonstrating clinical improvement as compared to 31% treated with placebo.

IL-12 is a heterodimeric molecule composed of IL-12 p40 and IL-12 p35 subunits. IL-12 is secreted by antigen-presenting cells such as monocytes, macrophages, and dendritic cells and to a lesser extent by NK cells. The most powerful inducers of IL-12 are bacteria, bacterial products, and parasites.

IL-12 is a pro-inflammatory cytokine that is important in the differentiation of naïve T cells into IFN-γ-producing pathogenic CD4⁺ Th1 cells [13, 25]. In peripheral lymphocytes of the Th1 T-helper cell type, IL-12 induces the synthesis of IFN-γ, IL-2, and TNF-α. TNF-α also appears to be involved in mediating the effects of IL-12 on natural killer cells since an antibody directed against TNF-α inhibits the effects of IL-12. IL-12 and TNF-α are costimulators for IFN-γ production with IL-12 maximizing the IFN-γ response; the production of IL-12, TNF-α, and IFN-γ is inhibited by IL-10. In Th2 T-helper cells, IL-12 reduces the synthesis of IL-4, IL-5, and IL-10.

This cytokine is considered a driving force behind chronic intestinal inflammation. Evidence for this comes forth from murine models of colitis by demonstrating that disease development could be inhibited by treatment with anti-IL-12 p40 monoclonal antibodies [25]. In human studies, this master T-cell-differentiating cytokine has been shown to be produced in large amounts in the intestines of patients with CD [26]. In addition, this cytokine has been targeted in human CD using various anti-IL-12 p40 monoclonal antibodies and found to be effective in phase 2 and phase 3 multicenter trials [27, 28]. In the latter, significant clinical response and remission could be achieved in patients with moderate-to-severe active Crohn disease. Furthermore, the phase 3 UNITI trial also included a cohort of patients which failed TNF-α mAb, with significant response and remission rates demonstrated in this patient population. The long-lasting clinical effect observed may be due in part to the induction of apoptosis of the inflammatory effector cells. These studies suggest that in addition to IL-2, IL-12 is a necessary growth and survival factor for T cells [29]. It also brings forth the point that the mechanism of action of the various anti-biologic therapies lies not only in their capability to neutralize their respective cytokines but due to their ability to induce cell death of the inciting inflammatory effector cells. Interestingly, the p40 subunit is also found to be a portion of another significant pro-inflammatory master cytokine, IL-23. The positive effects observed of the anti-IL-12 p40 antibody may indeed be due to both the effect on IL-12 and IL-23 [26]. Further studies in models of colitis indicate that IL-23 is important in the inflammatory response in IBD in that it plays a significant role in the maintenance of Th-17 effective inflammatory cells [30].

The discovery of the Th17 cell lineage revolutionized our understanding of IBD pathogenesis. The Th17 type secretes IL-17 and IL-22. IL-17 has been associated with multiple immune regulatory functions. Most notably, IL-17 is involved in inducing and mediating pro-inflammatory responses. IL-17 induces the production of many other cytokines, such as IL-6, G-CSF, GM-CSF, IL-1β, TGF-β, and TNF-α; chemokines including IL-8, GRO-α, and MCP-1; and prostaglandins (e.g., PGE₂) from many cell types (fibroblasts, endothelial cells, epithelial cells, and macrophages). IL-17 expression is stimulated and/or maintained by IL-23 expression. These IL-17-expressing cells appear to be derived by a subset of CD4+ T Cells called T-helper-17 (Th17) cells, which are distinct from Th1 and Th2 cell lineage and need to be derived in the presence of IL-23; in addition IL-17 may be derived to a lesser degree from monocytes and neutrophils [31]. Increased expression of IL-17 has been reported in the intestinal mucosa of IBD patients [32]. Some reports suggest that IL-17 alone is capable of inducing autoimmune tissue reactivity, whereas other groups suggest that IL-17 and IFN-γ synergize to stimulate this autoimmune reactivity [33, 34]. In these studies it was indicated that T cells and monocytes in the intestinal mucosa produce IL-17. IL-17 binds to the IL-17 receptor on endothelial cells and epithelial cells to promote secretion of pro-inflammatory substances that recruit inflammatory cells to the site [35]. In studies where the gene for either IL-17A or IL-17F was deleted, mice continued to develop severe colitis, but when RORγτ (the transcription factor important for expression of all IL-17) genes were deleted, minimal inflammation occurred in colitis models which suggests that the different forms of IL-17 are redundant, but IL-17 together are important for the development of colitis. Unfortunately, as noted with other cytokines, it appears that IL-17 is not just simply an inflammatory cytokine. Recent murine studies in both chemically induced colitis and adoptive transfer colitis indicate that IL-17 plays a complex role in the inflammatory response. These studies showed that transfer of IL-17-deficient T cells into an immunodeficient mouse led to more rapid onset of colitis than transfer of cells from WT mice. One explanation of this could be that Th1 cells bear IL-17 receptors, and signaling through these receptors inhibits Th1 differentiation by suppressing the transcription factor Tbet. Thus, IL-17 may have pro- and anti-inflammatory properties.

As a result of these roles, the IL-17 family has been linked to many immune/autoimmune-related diseases including rheumatoid arthritis, asthma, and lupus. IL-17 expression is increased in patients with a variety of allergic and autoimmune diseases, such as RA, MS, inflammatory bowel disease (IBD), and asthma, suggesting the contribution of IL-17 to the induction and/or development of such diseases. It must be stated that IL-17 may not appear to be the main cytokine important for inflammation in IBD in that studies evaluating the effect of anti-IL-17A antibody secukinumab for the treatment of Crohn disease have been disappointing and do not appear to have a therapeutic effect. In a realm that is of interest in the development and progression of IBD, IL-17 has been identified as a key mediator of fibrosis in multiple organs including the intestine. As fibrosis is an important issue in IBD, this makes understanding of IL-17 even more critical. Recently, Biancheri et al. demonstrated that IL-17 is upregulated in strictured tissue and that myofibroblasts express receptors for IL-17A [36]. It remains the current hypothesis that while IL-17 plays a role in inflammation in Crohn disease, the role is complex, and it appears that Th1 cytokines such as IFN-γ may play a greater role.

IL-23 and IL-17 changed our view of the cytokines important in the development of IBD. Multiple murine colitis studies demonstrated that development of colitis appeared to be more dependent on IL-23 than on IL-12. IL-23 is a pro-inflammatory cytokine secreted by activated dendritic cells and macrophages that shares structural homology with IL-12; specifically it is composed of the p40 subunit and a unique p19 chain. Initial studies indicating an ameliorating effect of an anti-p40 antibody in murine models of inflammation were felt to be due to its effect on IL-12. However, this effect was reevaluated, and studies suggest that this ameliorating effect may be due to a decrease in IL-23 mediating effect. In these studies, mice deficient in the p19 subunit of IL-23 displayed attenuated inflammation in colitis models, whereas mice deficient in the p35 chain of IL-12 (therefore deficient in IL-12 but not IL-23) had no effect on colitis. These studies together suggest that the initial effects observed with anti-p40 in a variety of animal models may have been due to a decrease in IL-23. IL-23 promotes and stabilizes a novel subset of CD4⁺ T cells (Th17 cells) that is characterized by the production of IL-17, IL-6, and TNF-α and has been associated with autoimmune tissue inflammation [33]. Without IL-23, it has been noted that Th17 cells produce the anti-inflammatory cytokine IL-10. The exact mechanism by which IL-23 promotes the Th17 response has not been defined, but it appears TGF-β and IL-6 are important for the commitment into a Th17 cell, and IL-23 is important for the proliferation of this cell type [37, 38]. Furthermore, recent studies may indicate a separate role for IL-23 in the occurrence of IL-17-expressing cells [39], whereby IL-23 may have a direct effect on regulatory T-cell development. Thus, in these animal studies, mice that lack IL-23 fail to develop colitis; however, this may not be secondary to the inability to produce IL-17 but rather because of the development of a dominant regulatory T-cell response. Moreover, Sunjino et al. demonstrated a dominant role for T regulatory cells in the suppression of colitis by blocking differentiation of Th17 into alternative Th1 type cells, therefore establishing a significant role for this suppressive pathway [40].

IL-23 effect is not limited to Th17 cells but appears to have an effect of the innate immune system inducing monocytes and macrophages to produce pro-inflammatory cytokines such as IL-1, IL-6, and TNF-α as well. In murine colitis studies where either IL-23 or the IL-23 receptor was deleted, it was shown that IL-23 plays a major role in the development of colitis. These studies also have shown an increase in the number of anti-inflammatory Treg cells suggesting that IL-23 may play a role in suppressing this cell type.

In addition, in a genome-wide association study in adults [41] as well as in a pediatric population [42], the IL-23 receptor (IL-23R) gene on chromosome 1p31 has been shown to have a highly significant association with CD; specifically, an uncommon coding variant of the IL-23R gene was shown to confer protection. These data indicate that the IL-23 pathway may have a causal link to CD.

This cytokine initially identified as interferon-γ-inducing factor (IGIF) is similar to the IL-1 family in structure, processing, receptor, and pro-inflammatory properties. It is produced by intestinal epithelial cells and induces other pro-inflammatory cytokines and Th1 polarization. IL-12 and IL-18 have a synergistic relationship. Their production by activated macrophages appears to drive the development of Th1 CD4+ T-cell predominance in the intestinal mucosa. Recombinant IL-18 alone is able to induce a proliferative response in vitro in freshly isolated mucosal lymphocytes from patients with CD. The synergistic effect is likely due to the upregulation of the IL-18 receptor by IL-12.

Intestinal mucosa from patients with CD have been evaluated and found to have increased expression of IL-18 [43], and this was also noted in experimental murine colitis [44]. Tissues from CD patients have been shown in vitro to decrease suppressive cytokine IL-10 expression after treatment with IL-18 indicating one possible effector mechanism. IL-12 and IL-18 together appear to synergize to drive the lamina propria lymphocytes into a Th1-type response. IL-12 appears to induce increased IL-18 expression, thus the synergistic effect [45, 46]. Using models of colitis, multiple laboratories have tried to block IL-18, and the results indicate that IL-18 may have a role in the initiation of intestinal inflammation, while others have shown that IL-18 acts to reduce inflammation.

An additional source for IL-18 production is the inflammasome pathway. The role, however, of the inflammasome to induce secretion of cytokines such as IL-1β and IL-18 is complex. While IL-1β appears to function as a pro-inflammatory cytokine in murine models of colitis [47–50], the function of IL-18 remains a duality. Thus, whereas studies have demonstrated that IL-18 is necessary for the induction of DSS colitis [47, 51, 52], further studies have shown that a deficiency in IL-18 secretion affords mice more susceptibility rather than more resistance to DSS colitis [44, 53, 54]. This correlated to studies which show that a deficiency in NLRP3 inflammasome pathway leads to increased susceptibility to DSS colitis, which appears to be secondary to decreased IL-18 expression [44, 53]. Thus, although IL-18 may have pro-inflammatory properties, it also has an important role in epithelial cell restitution and repair after injury [54, 55].

In a separate but similar role IL-6, a cytokine that can also affect epithelial cells acts as a tumor promoter by affecting the carcinogenicity of these intestinal epithelial cells [56]. IL-18 can have effects on these cell types since IL-18^-/- and IL-18R1^-/- mice display increased susceptibility to DSS colitis-associated cancer [54]. This effect of IL-18 may be through the cytokine IL-22 and its IL-binding protein (22 bp), the latter a decoy protein that neutralizes IL-22. The interplay between these various cytokine pathways is shown by the fact that IL-22 and IL-22 bp can regulate epithelial cell growth/repair and control tumorigenesis, while these aforementioned factors can be regulated by IL-18 and the NLRP3 or NLRP6 inflammasomes [55].

IL-13 can have a dual functional role in that it can down-modulate macrophage activity, reducing the production of pro-inflammatory cytokines (IL-1, IL-6, IL-8, IL-10, IL-12) and chemokines (MIP-1, MCP) in response to IFN-γ or bacterial lipopolysaccharides. IL-13 can also enhance the production of the IL-1 receptor antagonist and decrease the production of nitric oxide by activated macrophages, leading to a decrease in parasiticidal activity. Yet, it appears that IL-13 is important in the development of Th2-type colitis such as the murine model of colitis oxazolone and its human component, UC [57]. In these studies, it was found that IL-13 produced by natural killer (NK) T cells, when neutralized, led to decreased inflammation in the oxazolone model of colitis. Furthermore and most importantly, in human studies, these IL-13-secreting NK T cells exhibited an increased cytolytic function against epithelial cell lines. Moreover, IL-13 itself has been shown to be directly toxic to epithelial cells as well as to cause increased permeability barrier functional defects [58]. Thus, in the oxazolone model of colitis and its human counterpart ulcerative colitis, it is believed that IL-13-secreting NK T cells play a role in the etiology of this disease entity. This is in contrast to the Th1/Th17 disease process discussed in the pathogenesis of CD. Although IL-13 can function as a pro-inflammatory molecule in UC, it may also play a role in innate tumor surveillance pathways. In studies by Schiechl et al., tumor formation was accompanied by the coappearance of F4/80+CD11bhigh Gr1low macrophages, cells that undergo differentiation and activation by IL-13 and subsequently produce a source of tumor-promoting factor such as IL-6 after such activation [59]. In a similar vein, F4/80+CD11bhigh Gr1intermediate macrophages after activation through IL-13 produced increased amounts of TGF-beta, a cytokine that inhibits tumor immunosurveillance.

Finally, clinical trials aimed at the IL-13 pathway have been performed. Although these trials did not meet their primary endpoints, they did reveal interesting findings concerning the IL-13 signaling pathway. In an initial trial, anrukinzumab, an agent that binds to the IL-4/IL-13Rα1 complex and blocks signaling of IL-13 via the IL-13Rα1 pathway, was utilized [60]. As noted by the authors, these complexes consist of study drug and IL-13, which may be subsequently cleared through another IL-13 receptor pathway, IL-13Rα2. More recent findings demonstrate that the latter IL-13 receptor pathway, IL-13Rα2, and not the IL-13Rα1 pathway appears to be involved in the activation and secretion of IL-13 in ulcerative colitis [61]. Thus, the decreased efficacy of this trap molecule antibody directed against the IL-13Rα1 pathway may be expected based upon the former findings. The dose-response curves demonstrate some efficacy at low doses but not that at higher levels. This might be explained by clearance of IL-13 initially but subsequently binding and activation of the aforementioned IL-13Rα2 pathway leading to decreased responses at higher doses. Finally, another monoclonal antibody, tralokinumab, directed at IL-13 itself had a significant remission rate as compared to placebo but did not achieve significance for response rate [62]. These results may demonstrate that a subgroup of patients may achieve a remission response; however, additional screening markers are necessary to evaluate these responder patients.

IL-33 is part of the IL-1 family and is expressed in various non-hematopoietic cells as well as in inflammatory cells (e.g., macrophages and dendritic cells) [63]. Similar to other IL-1 family members such as IL-1 and IL-18, IL-33 was originally thought to be synthesized as a 30-kDa precursor molecule then subsequently cleaved by caspase-1 upon inflammasome activation to its mature/bioactive 18-kDa form [64]. However, more recent studies have suggested that the full-length 30-kDa IL-33 (f-IL-33) is the bioactive form with decreased active forms (20–22 kDa) resulting from caspase cleavage [65, 66]. In addition, further reports indicate that the bioactive form may not depend upon any caspase cleavage [67]. Thus, IL-33 bioactive form can be regulated by cleavage through proteases, in particular neutrophil serine proteases cathepsin G or elastase c, both released from neutrophils. Therefore, the inflammatory milieu may play a role in the generation of highly active mature forms of IL-33.

The IL-1 receptor-related protein, ST2, is the IL-33 receptor and exists in two different splice variants. ST2L is a transmembrane receptor that confers IL-33’s biological effects, and sST2 is a soluble molecule that serves as a decoy receptor [64]. Signaling through ST2 receptor can drive cytokine production in a host of cell populations, which include type 2 innate lymphoid cells (ILCs) (natural helper cells, nuocytes), T-helper lymphocytes, mast cells, basophils, eosinophils, and natural killer (NK) and invariant natural killer T (iNK T) cells [68, 69]. Thus, the IL-33/ST2 axis appears to play an important role in several chronic inflammatory disorders through the induction of Th2 and/or Th17 cytokines responses such as IL-5, IL-13, and IL-17 [63, 64, 70, 71].

Increased IL-33 production has been noted in murine models of colitis (i.e., oxazolone colitis, SAMP1-yit) as well as in ulcerative colitis [72, 73]. Further studies of active UC patients reveal IL-33 production was localized to intestinal epithelial cells (IEC) and cells in colonic inflammatory infiltrates [70–73]. This increase appears to be regulated in part by TNF-α as the latter can upregulate both IL-33 and sST2 and treatment of patients with anti-TNF-α monoclonal antibody decreases circulating levels of these molecules [70].

IL-9 is another Th2-related cytokine that appears to be involved in IBD pathogenesis. Production of IL-9 is induced in naïve T cells by TGF-β and IL-4 in concert with additional cytokines (i.e., IL-1β and IL-25). This cytokine was initially identified as a Th2-type cytokine by its ability to induce Th2 inflammation in disease states such as parasitic infection, allergy, or autoimmune states [74–76]. Recent studies have elucidated the role of IL-9 in IBD, which demonstrated increased levels of this cytokine in UC (and to a lesser extent in CD) [77]. Studies of the murine colitis model, oxazolone colitis, revealed that mice lacking IL-9 develop no or reduced disease. However, mice deficient in IL-9 also manifest amelioration of several Th1/Th17 murine colitis models, including cell transfer colitis; thus, IL-9 contributes to inflammation in a variety of Th1/Th2/Th17 intestinal inflammatory conditions [78]. The mechanism by which IL-9 may have broad effects on intestinal inflammation is the ability to alter epithelial barrier function via effects on tight junction proteins.