Keywords

5.1 Introduction

Approximately 40% of CD patients with ileal disease will develop clinically apparent strictures throughout their lifetime [1]. The frequency of fibrostenosing complications has still remained significant despite immunosuppressive therapy in CD patients in the form of steroids or immunomodulators [2, 3]. Since a myriad of genetic and epigenetic variables are thought to contribute to fibrostenosing disease, including those that affect cytokine biology, the investigation of specific therapeutics targeting those pathways has become prevalent. The potential adverse effects of inhibiting pathways involved in tissue repair and mucosal healing, as well as the relatively slow evolution of fibrosis in CD has made precise targeting of fibrosis difficult. Despite these potential deterrents, cytokine-targeted therapy has become the pillar of treatment for many inflammatory conditions and is being evaluated for fibrotic disorders. The question of whether anti-cytokine therapy will prove useful for intestinal fibrosis still remains, however. This chapter will review current cytokines involved in fibrosis and their potential targeting for treatment.

5.2 “Inflammatory” Cytokines

Targeting inflammation has been the mainstay of therapy in IBD. As such, anti-inflammatory cytokine therapeutics have provided significant advances in treating IBD patients. In many cases, inflammatory disease is associated with significant fibrotic change, as increased inflammation perpetuates the cascade of mucosal repair. Thus, since fibrogenesis may be a consequence of increasing inflammation, the hope of treating resulting fibrosis by preventing and suppressing inflammatory insults has emerged.

5.2.1 TNFα

TNFα is a multifunctional cytokine, often considered proinflammatory (but with important immunomodulatory properties, as well). A variety of cell types can secrete TNFα, including activated macrophages, B cells, T cells, keratinocytes, and fibroblasts. Depending upon the conditions, TNFα can trigger either pro-inflammatory or anti-inflammatory pathways by engaging one or both of two distinct transmembrane receptors: TNF-Receptor 1, and TNF-Receptor 2. In addition to its pro-inflammatory effects, TNFα may potentiate fibrosis via induction of tissue inhibitor of metalloproteinase-1 (TIMP-1) and reduce MMP-2 activity and collagen degradation [4]. Treatments targeting TNFα are some of the most widely used anti-cytokine therapies for inflammatory disorders, but mixed evidence has surfaced for using these agents in pro-fibrotic diseases. In some animal models of liver and renal fibrosis, TNF blockade reduced organ inflammation and fibrogenesis [5, 6], but a recent clinical study investigating adalimumab for fibrotic kidney disease (FSGS) failed to meet its primary outcome [7]. An open-label pilot study in 16 systemic sclerosis patients demonstrated improvement in skin scores with reduction in collagen secretion noted from cultured lesional fibroblasts (Table 5.1) [8,9,10].

Table 5.1 Cytokine and drug targets in fibrosis
Full size table

In contrast, there is evidence suggesting that TNFα is an antifibrogenic cytokine and its blockade might therefore promote fibrosis. In some studies, TNFα can exhibit antifibrotic properties by reducing the expression of collagen and connective tissue growth factor in dermal fibroblasts [11], and via suppression of TGFβ signaling through NFΚB induction of Smad 7 in other cell types [12]. The differing results may separate at the level of the individual TNF receptors on specific cell-types. Diminished TNFR1 signaling accelerates wound-healing, increases collagen deposition, and angiogenesis at wound sites in TNFR1-deficient mice [13]; whereas TNFR2-deficient intestinal myofibroblasts demonstrate reduced cell proliferation and decreased collagen synthesis [4].

Pertaining to intestinal fibrosis per se, the evidence for utilizing TNF antagonists as anti-fibrotic agents has remained questionable. Initial studies of TNF blockade reported concerns due to obstructive complications in some patients that accompanied mucosal healing. In vitro studies with myofibroblasts from CD patients treated with infliximab, however, showed that TNF blockade decreased collagen production [14]. Later multivariable analyses from the observational TREAT registry and the ACCENT I multicenter trial determined that disease duration, severity, location, and new corticosteroid use are factors associated with stricture formation, rather than TNF-antagonist use [15]. Some efficacy has now been seen in a few patients with inflammatory or mixed stenoses [16, 17], as well as small case series reporting intralesional injection of infliximab [18]. Cohort studies suggest that these agents may reduce the need for surgery, as rates of surgery ranged between 27 and 61% within the first 5 years after diagnosis before the use of TNF antagonists, and between 25 and 33% after the introduction of these agents [19, 20]. Indeed, anti-TNF agents are recommended to reduce the risk of post-operative recurrence after surgery. Discerning between unique antifibrotic effects in these cases and modification of the fibrotic program due to reduction in inflammation may be difficult.

5.2.2 Th1 Cytokines

Despites its proinflammatory potential, IFNγ may also have anti-fibrotic effects. IFNγ can inhibit fibroblast proliferation and migration [21]. Treatment with IFNγ reduces collagen deposition associated with chronic granuloma formation in schistosomiasis-induced fibrosis [22]. Similar results were obtained in models of pulmonary and kidney fibrosis [23, 24]. IFNγ may exert some if its anti-fibrotic activity by suppressing profibrotic cytokines such as TGFβ through the action of Y box-binding protein YB-1 (YB-1). An orally administered agent that promotes nuclear translocation of YB-1 resulted in the improvement of murine liver fibrosis and TNBS-induced murine chronic colitis [25,26,27]. These outcomes were not replicated in human studies, however. A randomized trial of subcutaneously injected recombinant IFNγ did not demonstrate improvement in survival of patients with idiopathic pulmonary fibrosis (Table 5.1) [28].

5.2.3 IL-1 Cytokines

There are 11 members of the IL-1 family of ligands; IL-1α, IL-1β, IL-1 receptor antagonist (IL-1Ra), IL-18, and recently, IL-33, have been studied in vitro, in animal models of disease, and in humans. In humans, IL-1β blockade has been utilized clinically. IL-1β is a cytokine with major roles in inflammation and innate immune response. Activated monocytes, macrophages and dendritic cells produce IL-1β , which can then induce the production of additional pro-inflamamtory cytokines such as TNF-α and IL-6, or chemokines, as well as proteases associated with proliferation of resident fibroblasts [29]. Cell assembly of the NLRP3 inflammasome containing caspase 1 is required to cleave pro-IL-1 into active IL-1 [30]. Studies using KO mice for several components of inflammasome pathway including NLRP3, showed a reduction of IL-1β and consequent reduction of experimental pulmonary fibrosis induced by bleomycin [31]. In an alveolar basal epithelial cell line, IL-1β stimulates transcription of TGF-β [32]. Notably, collagen deposition is reduced in interleukin-1 (IL-1) receptor deficient mice [29]. IL-1β and inflammasome pathway have been reported to play an important role in chronic liver inflammation leading to fibrosis and cirrhosis [33]. In rats, IL-1Ra administration attenuated dimethylnitrosamin-induced liver cirrhosis [34]. In contrast to these data, however, in the gut, IL-1β has been shown to downregulate collagen production [35]. Moreover, it has been shown that corticosteroids repress the IL-1β-induced secretion of collagenase in human intestinal cells [36]. Thus, with regards to fibrosis, IL-1β may have tissue-specific, differing effects regarding fibrosis. Canakinumab, a human anti-IL-1β monoclonal antibody that neutralizes IL-1β signaling, has been developed for suppression of inflammation in patients with disorders of autoimmune origin. In 2009, the drug was approved by the FDA for the treatment of familial cold auto-inflammatory syndrome and Muckle-wells syndrome, which are inflammatory diseases associated with elevated IL-1β levels. It is currently undergoing clinical trials for a variety of inflammatory disorders, but not for yet fibrotic diseases [37].

IL-33 is a member of the IL-1 family, which behaves as both an extracellular cytokine and nuclear transcription factor [38], signaling through a unique receptor: suppression of tumorigenicity 2 (ST2) [39]. IL-33 was initially considered a potent activator of type 2 immune responses integral to adaptive immunity. However, IL-33 is now known play a role in both innate and adaptive immunity. IL-33 is released by epithelial and endothelial cells in response to cell injury and necrosis, thereby acting as an ‘alarmin’ to initiate the innate immune response. Recent studies have demonstrated nuclear IL-33 is important in synovial fibroblasts, skin keratinocytes, and bone-marrow-derived mast cells [39]. A recent study demonstrated that intestinal IL-33 expression is localized to the pericryptal fibroblasts during homeostasis and is increased during infection [40]. ST2 is expressed by various immune cells, most notably T cells, including Th1 cells, Th2 cells, group 2 innate lymphoid cells (ILC2s), regulatory T (Treg) cells, and CD8+ T cells.

Elevated expression of both IL-33 and ST2 has been reported in inflamed mucosa from IBD patients. Intestinal epithelial cells (IEC) and sub-epithelial myofibroblasts (SEMFs) have been identified as the principal source of IL-33 in UC, along with smooth muscle cells, endothelial cells and adipocytes [41, 42]. Studies in colitis mouse models have suggested a mixed role for IL-33/ST2 in disease, with IL-33 administration attenuating chronic colitis, but neutralization of ST2 resulting in amelioration of disease [43, 44]. Interestingly, although fibrosis is usually associated with CD, it has been reported that IL-33 is expressed in activated SEMFs situated below ulcerative lesions predominantly in UC, as opposed to in CD [42, 45]. Recently, however, IL-33 has been associated with pediatric fibrostenosing CD patients [46].

As in experimental colitis, there have been mixed data regarding the effects of IL-33/ST2 on various fibrotic disaeases. Inhibition of IL-33 in mice suppressed bone marrow-derived fibroblast accumulation and myofibroblast formation in the kidneys after ischemia-reperfusion stress injury, which was associated with less expression of extracellular matrix proteins [47]. Increased hepatic IL-33 expression was noted in the murine bile-duct ligation (BDL) model of fibrosis and in surgical samples obtained from patients with liver fibrosis. Liver injury, inflammatory cell infiltration and fibrosis were reduced in the absence of ST2, and the activation of hepatic stellate cells (HSCs) was decreased in ST2-deficient mice. Interestingly, however, while administration of recombinant IL-33 significantly increased hepatic inflammation in sham-operated mice, it did not enhance BDL-induced hepatic fibrosis [48]. Similarly, endogenous IL-33 had no effect on the progression of fibrosis during experimental steatohepatitis [49]. Thus, mixed data and partially disparate roles for ST2 and IL-33 with regards to liver fibrosis have been demonstrated recently. Further studies are warranted to evaluate the impact of IL-33/ST2 on intestinal fibrosis.

5.2.4 Th2 Cytokines

Th2 cytokines, IL-4 and IL-13, promote fibroblast activation, proliferation, and collagen synthesis [50, 51]. IL-4 is increased in the bronchoalviolar lavage of patients with idiopathic pulmonary fibrosis [52]. IL-4 also increases the expression of collagen in cultured hepatic fibroblasts [53]. IL-13, which shares overlapping functions with IL-4 due to a common receptor subunit (IL-4-Receptor alpha), is involved in many Th2-mediated diseases and has a role in fibrosis as well. IL-13 signals through a complex receptor system comprised of IL-4Ralpha and two IL-13 binding proteins, IL-13Rα1 and IL-13Rα2. Many cell types express IL-13 receptors, including human hematopoietic cells, endothelial cells, fibroblasts, multiple epithelial cell types, and smooth muscle cells [54]. Intestinal samples from fibrotic CD patients expressed increased IL-13 mRNA. Fibroblasts from these samples expressed elevated levels of IL13Rα1 and subsequently down-regulated MMP in response to IL-13 [55]. Interestingly, in another study, elevated IL-13 production was not detected in UC or strictured CD [56]. These associations led to experimental IL-13 pathway targeting. In vivo inhibition of IL-13Rα2 decreased collagen deposition in bleomycin-induced lung fibrosis and reduced production of TGFβ1 in oxazolone-induced colitis [57]. In TNBS-induced colitis, similar inhibition of IL-13 signaling by targeting the IL-13Rα2 with small interfering RNA, reduces fibrosis and expression of TGFβ [58]. In another animal study, IL-13 blockade reduced experimental hepatic fibrosis [59]. With the experimental benefits of IL-13 antagonism, clinical trials with anti-IL-13 antibodies lebrikizumab and tralokinumab have been launched for pulmonary fibrosis (NCT01872689, NCT01629667). The study with lebrikizumab is ongoing, but the trial with tralokinumab was terminated early due to lack of efficacy. Clinical studies targeting IL-13 or IL-13 receptor may be anticipated for fibrosis in CD.

5.2.5 Th17 Cytokines

IL-17A-F act through the IL-17 receptor and make up the IL-17 family of cytokines. IL-17 is a significant cytokine involved in chemokine production for granulocyte activation and increasing inflammation [60]. IL-17 has demonstrated pro-fibrotic function by enhancing activation pathways in human colonic myofibroblasts [61]. It also sustains fibrotic activity in a number of cells such as stellate cells [62] and lung epithelial cells [63]. Anti-IL-17A monoclonal antibody administered after the onset of myocarditis in mice mitigates cardiac fibrosis and maintains ventricular function [64]. As IL-17A supports the synthesis and secretion of collagen via epithelial-mesenchymal transition in alveolar epithelial cells, its blockade resolves bleomycin-induced acute inflammation, attenuates pulmonary fibrosis, and increases survival [63]. IL-17’s contribution to CD is complicated, however, as both clinical and experimental data suggest divergent inflammatory and regulatory functions. IL-17’s effects on clinical disease activity in animal models of IBD has resulted in contrasting findings depending on the model used [65]. In vitro experiments on human samples showed that IL17-stimulated myofibroblasts from CD strictures generate more collagen and TIMP-1 than controls, and intestinal tissues expressed elevated levels of IL-17A [66]. In a clinical trial of patients with inflammatory CD, blockade of IL-17A by administration of the anti-IL-17A antibody, secukinumab, failed to meet its primary endpoint [67]. A subgroup of patients who demonstrated clinical benefit from anti-IL-17 carried a TNFSF15 (rs4263839) SNP in post hoc analysis, however. The potential functional consequences of this allele include elevated production of TL1A protein. Under TL1A-upregulated conditions in adoptive transfer-induced colitis, IL-17A deficiency ameliorated colonic inflammation via reducing Th1 and Th9 effector responses while enhancing regulatory responses [68]. Thus, there exists a subset of patients (those that overexpress TL1A due to e.g. a TNFSF15 variant) who could potentially benefit from IL-17 blockade. As TL1A overexpression in this subset of patients may promote their fibrotic disease, IL-17 blockade may have a positive impact both on inflammation and fibrosis.

5.2.6 TL1A

TL1A (a protein encoded by TNFSF15) is a member of the TNF superfamily, modulates numerous cellular functions by binding to death domain receptor 3 (DR3, also known as TNFRSF25), which is expressed on a broad array of cells [69,70,71]. TL1A is produced by endothelial cells induced by IL-1β and TNFα, macrophages and dendritic cells in response to Toll-like receptor stimulation, as well as in some lymphoid lineage cells [72,73,74,75].

Developmental, immunoregulatory and pro-inflammatory effects have been described for DR3, which shares homology to TNFR1. Early work on DR3-deficient mice demonstrated that it is required for negative selection in the thymus and in embryonic cells, it can induce FADD- and caspase-8-dependent apoptosis [76, 77]. Conversely, however, DR3 activation of NF-ΚB in human cell lines upregulates c-IAP2, an NF-ΚB-dependent anti-apoptotic protein, which protects against apoptosis [78]. DR3 can also be upregulated on Th17 cells, promote T cell expansion, and cytokine production during immune responses [79,80,81]. The pro-inflammatory effects of TL1A-DR3 likely contribute to this pathway’s effect on fibrosis, but more direct evidence has shown that DR3 is an important receptor for fibroblast development, maturation and function. Owing to the fact that DR3 is expressed on intestinal fibroblasts, DR3-deficient mice display reduced number of colonic fibroblasts, reduced fibroblast activation (as evidenced by decreased expression of alpha smooth muscle actin) and expression of collagen induced by TL1A stimulation [82].

Human IBD studies found that a TNFSF15 haplotype is associated with higher TL1A expression, increased risk of CD, intestinal fibrostenosis, and greater need for surgery [83,84,85]. Consistent with these findings, TL1A overexpression in mice causes spontaneous ileitis with increased collagen deposition [86, 87]. Under induced colitogenic conditions by chronic DSS treatment or adoptive T cell transfer, increased inflammation, fibrosis, and fibrostenotic lesions in the gut are seen [88]. These results support the role of TL1A in induction of intestinal inflammation and suggest its contribution to fibrogenesis in the gut. The potential for TL1A as a therapeutic target in intestinal fibrosis was demonstrated in a study evaluating the effect of anti-TL1A Ab in chronic DSS and adoptive T-cell transfer models of IBD. Treatment with neutralizing TL1A Ab attenuated disease and reversed colonic fibrosis. Additionally, TL1A blockade reduced the number of fibroblasts and myofibroblasts in colonic cell isolates and lowered expression of CTGF, TGFβ1 and IGF-1 [82]. The promising data with TL1A blockade in experimental IBD and increasing evidence as to its relevance in human disease makes TL1A a potential novel target for fibrosis.

5.3 “Regulatory” Cytokines

As mentioned previously, the frequency of fibrostenosing complications has still remained significant despite immunosuppressive therapy in CD patients in the form of steroids or immunomodulators. This may be due to some fibrotic pathways being separate from inflammatory pathways, or alternatively, the ability of some cytokines to promote inflammatory and anti-fibrotic effects simultaneously. Consequently, significant attention has been devoted to targeting those cytokines that might be involved in the “aftermath” of the inflammatory assault, regulating immune function and stimulating tissue repair.

5.3.1 TGFβ

TGFβ is a pleiotropic cytokine inducing proliferation, differentiation, inflammation, immunoregulation, wound healing and fibrosis [89]. TGFβ is perhaps the most widely studied cytokine relevant to fibrosis. Elevated levels of TGFβ and its receptors have been described with numerous fibrotic including heart, lungs, liver, kidney, skin, and intestines. Likewise, genetic over-expression or exogenous administration of TGFβ in animals promotes wide-spread fibrotic disease [90]. TGFβ supports activation and differentiation of fibroblasts and production of collagen and fibronectin, expression of adhesive receptors and contractile elements, and inhibition of matrix metalloproteinases [89, 91, 92]. TGFβ can also induce fibrogenesis via additional mechanisms of fibrosis including epithelial to mesenchymal transition and endothelial to mesenchymal transition [93]. Role of TGFβ and therapeutic targets in fibrosis is further described below and summarized in Table 5.1.

Three main isoforms of TGFβ exist: TGFβ1, TGFβ2, and TGFβ3. These isoforms are secreted as latent precursor molecules containing a latency associated peptide region (LAP), and complexed with latent TGFβ binding proteins (LTBP). The cytokine is active when LTBP is removed extracellularly via proteolytic cleavage by proteases such as plasmin or thrombin; or by interactions of LAP with other proteins such as thrombospondin-1 or integrins [89]. TGFβ signals through two receptors, TGFβR1 and TGFβR2. These receptors form transmembrane serine/threonine kinase, hetero- or homo-dimeric complexes that induce phosphorylation of Smad 2 and Smad 3 proteins. Once phosphorylated, Smad 2 and 3 complex with Smad 4, translocate to the nucleus, and activate transcription. Smad 7 regulates Smad 2/3, by inhibiting binding of Smad 2/3 to the receptor complex. TGFβ can also signal through ERK1/2, c-Jun N terminal kinase, p38 kinases and members of the JAK/STAT family [89, 94].

TGFβ, is also a potent immune modulator central to immune tolerance and development of innate and adaptive immunoregulatory cells. Systemic blockade of TGFβ might therefore upset vital immune homeostasis resulting in troublesome effects. Alternatively, complete antagonism of TGFβ might be ineffective due simultaneous blockade of fibrogenic and regulatory functions. Thus, several direct TGFβ antagonists were found to be ineffective or led to possible drug associated mortality [95, 96]. In an attempt to inhibit TGFβ-driven fibrosis while sparing its immunomodulatory effects, alternative strategies have focused on specific pathways in TGFβ signaling, synthesis, activation, or other downstream mediators. Accordingly, blockade of TGFβR1 signaling by an injectable inhibitor (SD-208) was evaluated in two experimental animal models of intestinal fibrosis: anaerobic bacteria- and trinitrobenzensulphonic acid-induced colitis (TNBS). SD-208 reduced fibroblast activation, phosphorylation of Smad 2 and Smad 3 proteins, and intestinal wall collagen deposition in both models [97]. Similarly, more recent studies on blockade of TGFβR1 with oral inhibitors have demonstrated efficacy in animal models of renal fibrosis, carbon tetrachloride- or bile duct ligation-induced cirrhosis [98, 99], pressure-overload-induced cardiac fibrosis [100], and bleomycin-induced pulmonary fibrosis [101]. These agents are being investigated in oncologic trials, with testing ongoing for fibrotic disorders.

Pirfenidone (5-methyl-1-phenyl-2-[1H]-pyridone) is another orally-administered molecule that has demonstrated anti-fibrotic effects partly by inhibiting synthesis of TGFβ. This agent has been efficacious in patients and experimental models of pulmonary and renal fibrosis [102, 103]. Pirfenidone has been evaluated in randomized, double-blind, placebo-controlled clinical trials where it reduced the rate of decline in lung function as well as improved mortality [104, 105]. Consequently, it has been approved in Europe and by the FDA for treatment of IPF. Pirfenidone, however, has not been consistently efficacious in all trials. No clinical or histologic benefits were observed in myelofibrosis [106], or primary sclerosing cholangitis, while being associated with increased adverse events [107].

Downregulation of TGFβ without known adverse immunological effects has been demonstrated by two classes of medications currently in widespread use in primary care: HMG-CoA reductase inhibitors (statins) and antagonists of Renin-Angiotensin system (RAS). Statins may reduce fibrosis, in part, through decreasing expression of TGFβ. Simvastatin reduces TGFβ1 expression in human fibroblasts by inhibition of Smad 3 phosphorylation [108]. In TNBS-induced colitis, it had anti-fibrotic effects by decreasing the level of connective tissue growth factor (CTGF) and inducing apoptosis in fibroblasts [109]. As the primary mediator of the RAS, Angiotensin may contribute to fibrogenesis via induction of TGFβ expression and promotion of collagen production [110]. With regards to intestinal fibrosis, early studies have reported that Angiotensin is increased in the mucosa of CD patients [111]. In TNBS-induced colitis, administration of the ACE inhibitor, captopril, or the angiotensin receptor blocker, losartan, reduced colonic inflammation and fibrosis via reduction in TGFβ [112, 113]. Given the safety and ubiquity of statins and RAS antagonists, future investigations will be feasible and determine if they are capable of favorably impacting fibrogenesis.

An important regulatory step in TGFβ signaling, which might be targeted therapeutically, is the activation of TGFβ from its latent precursor state. AlphaV (αV)-type integrins can bind LAP and activate TGFβ. Integrin-blocking therapeutics such as vedolizumab, have proven effective with regards to inflammation in IBD. These agents may reduce fibrosis via their effects on TGFβ activation. For example, αVβ6 integrin is upregulated in various fibrotic diseases and its blockade has been effective in models of pulmonary fibrosis and liver fibrosis [114]. Similarly, αVβ3 integrin contributes to excess smooth muscle cell proliferation and hyperplasia in intestinal strictures of CD [115]. Cilengitide, an αVβ3 inhibitor, reduces the development of fibrosis in chronic TNBS-induced colitis [116]. Future studies will determine if integrin inhibitors will be effective at treating fibrosis in IBD.

Targeting specific mediators in the TGFβ signaling cascade represents another possibility. This option may provide more specificity by focusing on individual mediators of TGFβ signaling, rather than TGFβ itself. Two such potential strategies are Smad 3 antagonism and Smad 7 agonism. Increased Smad 3 and decreased Smad 7 expression have been observed in intestinal strictures in CD [117]. Furthermore, in multiple animal models, loss of Smad 3 or increase in Smad 7 confers resistance to fibrosis in several organs [118,119,120]. There has been focus on inhibition of Smad 7 in IBD via antisense oligonucleotides (and subsequent increase in Smad 3 transduction with potential TGFβ-mediated shift towards immune-regulation). This strategy may be problematic with regards to fibrogenesis, however. An ideal solution might be to clearly identify those patients that would be more prone to develop fibrotic/stricturing disease vs predominantly inflammatory pathology through functional, genetic and epigenetic studies.

5.3.2 IL-10

As a product of regulatory T cells, IL-10 has an established role with regards to immune regulation [121]. In contrast to TGFβ, however, IL-10 has been shown to inhibit fibrosis. Mice treated with IL-10 develop less liver and lung fibrosis when administered carbon tetrachloride or bleomycin [122, 123]. Similarly, IL-10 deficiency aggravates kidney inflammation and fibrosis in the unilateral ureteral obstruction mouse model [124]. With regards to human IBD, however, although polymorphisms in the IL-10 locus have been associated with IBD [125], treatment of CD patients with recombinant IL-10 has not been significantly effective (Table 5.1) [126].

5.4 Concluding Remarks

Cytokine targeting has proven to be effective in treating inflammation in IBD. Cytokine blockade for intestinal fibrosis has been challenging, however, given the multiple diverging and converging pathways of many cytokines. The genetic heterogeneity present across patient populations may also cause diverse pathogenesis of disease; broad cytokine targeting may then result in contrasting rates of response. Indeed, this has been observed with anti-TNF agents in terms of inflammation, and may be one source of failure of some clinical trials with newer anti-cytokine agents. A potential approach to overcome this difficulty may involve careful selection of patients based on genetic or biochemical characteristics. Additionally, there are promising targets being explored for other fibrotic conditions that may be of benefit in CD and warrant investigation. Given the variables that contribute to fibrostenosis in CD, targeting of multiple culprits in the fibrotic process, in addition to the cytokines themselves, may be an option. Future investigations into novel fibrogenic pathways may lead to more selective therapeutic targets, as well as the identification of specific patient groups that could best benefit from precision treatment.