The Role of Eosinophilic Inflammation in Inflammatory Bowel Diseases: Conductor or “First” Violin?

Abstract

Eosinophils, one of the subgroups of leukocytes, are present in the gastrointestinal tract, with the exception of the esophagus (their presence in quantities of 15 or more is considered eosinophilic esophagitis). Much of the research on eosinophils has focused on their responses against helminths and type II immune system disorders. However, information on the role of eosinophils in the development and maintenance of inflammatory processes, as well as in the formation and progression of fibrotic changes in patients with inflammatory bowel diseases is limited. With increasing interest in innate immunity and the fact that eosinophil granules contain certain inflammatory mediators, eosinophils are becoming one of the current objects of study in inflammatory bowel diseases. In this paper, the authors presented already known data on the functions of eosinophils in inflammatory bowel diseases and some other chronic inflammatory conditions, and also presented the results of their own research on the role and influence of eosinophils on the course of inflammatory bowel diseases.

1. Introduction

Crohn’s disease (CD) and ulcerative colitis (UC) belong to inflammatory bowel diseases (IBD), are idiopathic heterogeneous diseases, characterized by a relapsing and continuous course [1]. These pathologies occur in genetically predisposed patients as a result of the formation of an inadequate immune response to the intestinal microbiota of the “host” [1]. To date, there is some knowledge about some genetic risk factors, for example, polymorphisms in nucleotide-binding oligomerization domain-2 (NOD-2), the exact pathogenesis unfortunately still remains unclear [1, 2].

In response to the loss of the integrity of the epithelial cover, an excessive immune reaction occurs, which, along with the formation of an inflammatory process, leads to tissue damage [3, 4]. As a result of stable relapse and continuous progression, a similar inflammatory “storm” in inflammatory bowel disease creates the conditions for excessive deposition of extracellular matrix (ECM), leading to intestinal fibrosis (CD is a clear example of this condition). The outcome of fibrosis is the formation of strictures, often leading to intestinal obstruction, one of the most common indications for urgent surgical intervention in patients with CD [4].

According to available literature data, intestinal fibrosis manifests itself only in areas characterized by high activity of the inflammatory process, which once again confirms that an important condition for the development of fibrosis is the presence of inflammation [5, 6, 7]. Due to the above, for many years research has focused not on the resulting fibrosis, but on the inflammatory process leading to fibrosis. The characteristics of the main immune components and mediators involved in intestinal fibrogenesis are not fully understood.

Although in recent years the emphasis in the pathogenesis of Crohn’s disease and ulcerative colitis has been based more on changes in innate immunity, the main basic research on the pathogenesis of inflammatory bowel diseases has focused on excessive adaptive immune responses [8]. In this aspect, the search for a potential role and “niche” for eosinophils in inflammation and fibrosis has again become relevant [9, 10].

Eosinophils have been identified as important cells contributing to immune cell infiltration in IBD, such as eosinophilic infiltration in the lamina propria in interpretations of the Geboes histological score for UC [11]. It is known that basal plasmacytosis associated with eosinophilia is considered an early distinctive histological sign of the diagnosis of IBD and also closely correlates with histological verification [12]. One of the predictors of lack of response to therapy in IBD is the presence of eosinophilic infiltration in the lamina propria in biopsies from patients with UC [13]. The abundance of mediators found in eosinophilic granules, which play a role in inflammation and/or fibrosis, makes these cells appealing for addressing fibrostenosis in IBD. Thus, they are significant in the quest for new treatment “targets” [8]. Eosinophilic granulocytes have been suggested to be associated with increasing levels of inflammation and the development of fibrosis, but the causative role or mechanism of this process is still unclear [11].

2. Eosinophils and the gastrointestinal tract

2.1 General characteristics

Eosinophils, belonging to the leukocyte family, are a specific type of white blood cell primarily situated in the lamina propria of the gastrointestinal tract [11]. They function as immune cells residing throughout the gastrointestinal tract, except for the esophagus [14].

In response to IL-3, IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF), FOG-1 transcription factor levels decrease, while the expression of GATA-1, ID2, and XBP1 transcription factors increases. This cascade facilitates the transformation of pluripotent hematopoietic stem cells in the bone marrow into mature eosinophils [11, 15]. Once stimulated by IL-5, eosinophils are released into the bloodstream. From there, they can migrate into the gastrointestinal tract by binding to specific chemoattractant molecules like ligand 11CC (CCL11, eotaxin-1), CCL24 (eotaxin-2), CCL26 (eotaxin-3), CCL5 (RANTES), CCL7 (MCP-3), and CCL13 (MCP-4), through their CC chemokine receptors (CCR) such as CCR1, CCR3, and CCR4 [16]. Activation of these receptors prompts both the recruitment and activation of eosinophils, leading to the production of various cytokines (IL-4, IL-5, IL-13, interferon-gamma (IFN-γ), etc.) and chemokines (CCL3, CCL5, CCL11, etc.) [11]. When stimulated by cytokines, in particular IL-4, IL-5 and IL-13, eosinophils undergo activation [17]. Eosinophil activation leads to their degranulation, releasing harmful substances like oxygen radicals, eosinophil cationic protein (ECP), and transforming growth factor β (TGF-β) [10, 18]. This process is pivotal in both starting and prolonging inflammation, often in collaboration with other inflammatory cells. Particularly, Th2 lymphocytes, identified by their expression of the membrane receptor CCR3, play a crucial role, as they associate with eosinophils in clusters during inflammation [11]. Eosinophils were thought to act solely as effector cells of the Th2 immune response, but recently it was discovered that eosinophils have their own functions but are closely related to Th2 lymphocytes [11]. In this scenario, Th2 lymphocytes secrete IL-4, IL-5, IL-13, and eotaxins, facilitating the activation and recruitment of eosinophils [19]. Conversely, eosinophils, by releasing IL-4 and IL-5, prompt the maturation of naive Th0 lymphocytes into Th2 cells, and they also activate pre-existing Th2 lymphocytes [19].

2.2 Chemotaxis of eosinophils

In IBD’s active inflammation, eosinophils migrate to the gastrointestinal tract and this migration is driven by chemoattractant molecules binding to receptors on the surface of eosinophils. Chemoattractant molecules not only play a key role in eosinophil recruitment, they also partially serve as eosinophil activators [10]. There are several known pathways for eosinophil chemotaxis.

3. The role of eosinophils in inflammation

3.1 Eosinophil activation

When stimulated, activated eosinophils undergo degranulation and release granular contents into the environment. Several such mechanisms of eosinophil activation are known, for example, tissue damage, bacterial and viral infections, binding of cytokines (IL-4, IL-5, IL-13, IL-33, etc.) and some chemokines (eotaxin, RANTES, etc.) [20, 21, 22]. This eosinophilic activation is characterized by increased expression of the surface markers CD44, CD11c, CD11b and CD18. As eosinophil activation is already known to be highly dependent on the cytokine milieu, these markers may help shed light on the incompletely understood role of eosinophils in intestinal inflammation [11, 23]. A graphical description of the eosinophil activation pattern is presented in Figure 1.

Figure 1.

Eosinophil activation pattern (adapted from Jacobs et al. [11]). Upon contact with certain cytokines (IL-4, IL-5, IL-13, IL-33, etc.), chemokines (eotaxin-1,2 and − 3, RANTES, etc.), as well as tissue damage, bacterial and viral infections, eosinophils are activated. This activation is characterized by increased surface expression of CD18, CD44, CD11b, and CD11c (moderate to high expression). CD25 and CD69 are absent on inactive eosinophils but present on active eosinophils (low and high expression, respectively). On the other hand, CD162 and CD31 are highly expressed on inactive eosinophils but only moderately on active eosinophils, and CD62L is moderately expressed on inactive eosinophils but becomes lowly expressed upon eosinophil activation [11].

3.2 IL-4

IL-4, a cytokine, is mainly synthesized by basophils, T lymphocytes, mast cells, type 2 innate lymphoid cells (ILC2), eosinophils, and neutrophils [24]. Its crucial function lies in promoting the differentiation of Th0 cells into Th2 cells. As a result, Th2 cells produce IL-4, setting up a “vicious circle” that enhances the differentiation of Th2 lymphocytes [25]. This pro-inflammatory cytokine is known to be a stimulator of eosinophilic transmigration across the endothelium and Th2 lymphocyte differentiation, which also leads to the release of cytokines [11]. By enhancing the expression of eotaxin, IL-4 also promotes the accumulation of eosinophils and their chemotaxis [11]. IL-4 plays a critical role in asthma and allergic inflammation. Its involvement in the pathogenesis of IBD has been extensively studied, where it contributes significantly to inflammation and immune response activation. Increased IL-4 expression has been noted in patients with UC [26]. Research on IL-4 deficiency has demonstrated its ability to prevent colitis development in IL-10 knockout mice, which typically develop colitis spontaneously [11]. Additionally, evidence from models like dextran sodium sulfate (DSS)-induced colitis and the T-cell transfer model suggests that IL-4 may also participate in the development of colitis [27, 28]. A dual IL-4/IL-13 antagonist evaluated in a mouse model of oxazolic colitis was shown to reduce overall disease activity [29].

3.3 IL-5

IL-5 is a chemotactic agent that promotes the differentiation of eosinophils in the bone marrow and may differ in its ability to activate them. During degranulation, eosinophils produce and secrete IL-5, thereby promoting their own differentiation and activation [11]. IL-5 is synthesized in greater quantities by Th2 lymphocytes and ILC2s, and in smaller quantities by NKT cells, mast cells and eosinophils [30]. ILC2 promote eosinophil activation by synthesizing IL-4 and IL-5 (interacts with eotaxins), as well as IL-13 [31, 32]. Specific inhibition of IL-5 attenuates the type 2 immune response and the clinical severity of the disease in patients with eosinophilic asthma, indicating an important role for IL-5 in eosinophil-associated pathologies [33, 34].

3.4 IL-13

The cytokine IL-13 is produced by Th2 lymphocytes, CD4 and NKT cells, basophils, mast cells and eosinophils. Previously, as a mediator of allergic inflammation, it was associated with airway hyperresponsiveness and the development of fibrosis [11]. Due to the common receptor formed by IL-4Rα and IL-13Rα1, IL-13 and IL-4 share certain functional similarities. Activation of this shared receptor initiates STAT6 signaling and supports type 2 immunity. Furthermore, IL-13 can bind to IL-13Rα2, acting as a decoy receptor and thus blocking IL-13 signaling [35].

This cytokine also binds IL-13Rα2 400-fold higher than IL-4Rα/IL-13Rα1, thereby inhibiting STAT6 signaling and attenuating the subsequent type 2 immune response [36]. IL-13Rα2 knockout mice are protected from induction of colitis in a DSS-induced colitis model [36]. However, previous data revealed that IL-13Rα2 knockout mice were not protected from the development of colitis, but recovered rather faster and had faster colonic mucosal repair [35]. Evidence suggests elevated IL13Rα2 mRNA expression levels in mucosal biopsies from IBD patients during active disease, which has been proposed as a biomarker of α-TNF insensitivity [37].

While the exact function of IL-13 in IBD development is not fully understood, there is a possibility that it could be a promising target for IBD therapy [38].

3.5 IL-33

IL-33 is a pro-inflammatory cytokine produced by various intestinal cells, including ILC2, Th2 lymphocytes, and epithelial cells. It interacts with the suppression of tumorigenicity 2 (ST2) receptor present on the eosinophil membrane, thereby activating the ST2/IL-33 signaling pathway [39]. IL-33 is released upon epithelial damage and can independently influence eosinophils, ILC2, and Th2 cells. ILC2 and Th2 cells can enhance the proliferation of eosinophils by producing IL-5 and IL-13, as well as synthesizing IL-4 [40]. Some studies have suggested a potential role for IL-33 in the development of colitis: in UC patients, it was demonstrated that activated eosinophils together with increased IL-33 mRNA expression levels in the colon were correlated with increased eotaxin-1 expression [41]. In both intestinal biopsies from IBD patients and the colons of SAMP/YitFc mice, which develop colitis spontaneously, ST2/IL-33 signaling initiates activation and eosinophilic infiltration. This aligns with a Th2-mediated immune response, facilitating the release of IL-4, IL-5, and IL-13 [42, 43]. Blocking ST2 in these SAMP/YitFc mice reduced Th2 cytokine production and reduced eosinophil recruitment to the ileum [43].

3.6 Eosinophil degranulation

When eosinophils are activated and their subsequent degranulation, various substances can be released into the environment, often characterized by toxicity. According to specific literature, the release of eosinophil-specific proteins such as eosinophil cationic protein (ECP), eosinophil peroxidase (EPO), eosinophil-derived neurotoxin (EDN), and eosinophil major basic protein (MBP) contributes to tissue damage due to their cytotoxic effects. This cytotoxicity results in the destruction of the epithelial layer. According to other data, the TGF-β1 protein released from eosinophil granules also stimulates inflammatory activity and the formation of fibrosis [11].

4. What is the key role of these proteins in the formation of inflammation in the intestines?

4.1 Transforming growth factor β1 (TGF-β1)

TGF-β1 is a cytokine produced by epithelial and immune cells, as well as fibroblasts, and is the most common form in the gastrointestinal tract (compared to TGF-β2 and TGF-β3) [44]. Current literature contains rather contradictory results regarding the role of TGF-β1 in acute intestinal inflammation [45].

4.2 Eosinophil cationic protein (ECP)

Degranulation of eosinophils causes the release of eosinophil-specific ECP, also known as ribonuclease-3, weighing between 18 to 22 kDa. ECP has the ability to harm cell membranes by creating pores in transmembrane channels, facilitating the entry of toxic molecules into the cell. Eosinophils store significant amounts of ECP, which is expelled upon degranulation, eliminating the need for new synthesis during this process [11]. Patients with active IBD had increased serum EPC levels compared with healthy controls or patients with IBD in remission [46]. One study found that an increase in fecal EPC was the same in both CD and UC. However, compared with fecal calprotectin, the diagnostic accuracy was lower in differentiating between active and inactive IBD. However, it was high levels of fecal EPC that correlated with the need for changes in treatment strategy, indicating that this marker can be used as a diagnostic tool to monitor remission in patients with IBD [47]. Moreover, increased deposition of ECP and MBP in the small intestine has been demonstrated in patients with eosinophilic gastroenteritis and correlated with disease severity [11]. Exactly, the evidence suggests a correlation between elevated ECP levels and relapse in inflammatory bowel disease, but further research is needed to determine whether ECP directly contributes to inflammation or if its elevation is merely a consequence of the inflammatory process.

4.3 Eosinophil peroxidase (EPO)

Absolutely, toxic cationic eosinophil peroxidase (EPO) indeed generates hypohalous, bromous, and hypochlorous acids by utilizing hydrogen peroxide, halide ions, and bromide. This process results in the formation of these acids, which have the potential to induce cell damage [48]. Biopsies from the colon mucosa of patients with CD and perfusion fluids from the colon of patients with UC show increased levels of EPO during relapse [49]. EPO causes structural damage by oxidizing nitrates and thus producing toxic reactive oxygen species. These reactive oxygen species have previously been associated with renal inflammation and fibrosis [11].

4.4 Eosinophil-derived neurotoxin (EDN)

Contrary to its name, EDN is not neurotoxic to the human body (neuropathological reactions were detected in a mouse model) [48]. Amcoff K. and his co-authors reported increased levels of EDN protein in the feces of patients with UC not only at the time of relapse, but also 3 months before relapse. Indeed, fecal eosinophil-derived neurotoxin (EDN) has been proposed as a biomarker or predictor of relapse [50]. This prognostic role of EDN in eosinophil-mediated intestinal inflammation has been particularly suggested in pediatric patients [51]. Thus, EDN holds promise as a diagnostic tool or biomarker for gastrointestinal inflammation. However, the question of whether this protein directly contributes to inflammation or fibrosis is still a matter of debate.

4.5 Eosinophil major basic protein (MBP)

Exactly, MBP, also known as proteoglycan 2 (PRG2), is encoded by the PRG2 gene and manifests in two forms: MBP1, which is present in eosinophils, basophils, and mast cells, and MBP2, exclusive to eosinophils [52]. Due to its cationic nature, MBP can disrupt the permeability and function of cell membranes. It is believed that MBP, because of its toxicity, directly increases the permeability of the epithelial layer [52].

Thus (summarized in Table 1), eosinophils are involved in the inflammatory process in patients with inflammatory bowel disease [10, 11]. Studies demonstrate an increase in the number of activated eosinophils in both active and inactive UC compared to healthy individuals [18]. Since the presence of activated eosinophils was more pronounced in remission of UC compared to relapse of UC, it is assumed that eosinophils are also involved in tissue repair and remodeling mechanisms [18]. Moreover, tissue samples from patients with inflammatory bowel disease showed an increase in the number of degranulated eosinophils and the level of eosinophil granules. Peripheral eosinophilia is associated with worse clinical outcomes and more severe UC [53, 54]. Indeed, it has been demonstrated in vivo that IL-4 production by eosinophils contributes to the development of colitis in both chemically induced dextran sodium sulfate (DSS) and T cell transfer models [55]. Although some similar studies have suggested a role for eosinophils in inflammation, conclusive evidence is lacking.

	Pre-clinical evidence	Clinical evidence
IL-4	IL-4 blocking in IL-10 deficient mice: protected from colitis development No IL-4Rα: no disease development	UC patients: ↑ IL-4 expression levels in inflamed mucosa; CD patients: ↓ IL-4 levels in intestinal tissue due to lower numbers of IL-4 producing cells in mucosal biopsies
IL-13	IL-4/IL-13 dual antagonist in oxazolone colitis model; Reduced overall disease activity IL-4/IL-13 blocking trough a shared receptor Reduced overall disease severity {IL-13Rα2 KO model -IL-13Rα2 antibody mediated depletion DSS model: mice protected from colitis introduction IL-13Rα2 KO model: not protected from colitis development but recovered faster	CD and UC patients: ↑ IL-13Rα2 in mucosal biopsies; Potential biomarker for anti-TNF non-responsiveness; Clinical trial with Tralokinumab and Anrukinzumab: no therapeutic effects
IL-5		{Mepolizumab & Reslizumab Benralizumab; Attenuates type 2 response + used and shown effective in eosnophl eosinophilic asthma patients; UC patients’ rectal perfusion fluids: ↑ IL-5 levels
IL-33	SAMP/YitFc colitis model and antibody mediated ST2 blocking: ↓ Th2 cytokine production and ↓ eosinophil recruitment into the ileum C57BL/6 ST2 KO mice and ST2 antibody mediated depletion in C57BL/6 mice alleviated disease symptoms	UC patients: ↑ colonic IL-33 mRNA levels and activated eosinophils; BD patients’ intestinal biopsies: ST2/IL-33 signaling Eosinophil infiltration which coincided with Th2 mediated immune response IL-4, IL-5 and IL-13 release
TGF-β1	TGF- β1 deficient mice: spontaneously develop colitis	Active inflammation in IBD patients: ↑ ↓}TGF – β1 protein levels
EDN		UC patients: ↑ f(EDN) protein levels during and 3 months prior to relapse: possible prognostic role; Suggested as a prognostic marker in pediatric patients
ECP		Active CD and UC: ↑ serum ECP compared to HC; Eosinophil gastroenteritis: ECP and MBP deposition in small bowel
MBP	MBP KO mice: no colitis development upon oxazolone exposure; In vitro co-culture of eosinophils and epithelial cells decreased functioning of the epithelial barrier; attributed to MBP	MBP directly increases epithelial layer permeability via its toxicity

Table 1.

The role of eosinophil-activating mediators and compounds from eosinophil-specific granules in intestinal inflammation [11].

5. The role of eosinophils in fibrosis

Since eosinophil infiltration has already been identified in other diseases with characteristic fibrosis (endomyocardial fibrosis, idiopathic retroperitoneal fibrosis, pulmonary fibrosis, etc.), therapeutic effects on eosinophils may be quite useful in a number of other fibrotic manifestations [11]. Absolutely, the involvement of eosinophils in the development of intestinal fibrosis in IBD is still not fully understood. Therefore, gaining a precise understanding of the mechanism or function of eosinophils in these fibrotic changes requires further comprehensive investigation. We provide a description of the participation of the previously mentioned eosinophil activators and eosinophil granule proteins in the process of fibrosis.

5.1 IL-4

IL-4, known for its ability to induce TGF-β1, stimulates fibroblast expression and the release of inflammatory cytokines, thus contributing to inflammation. Elevated IL-4 expression is correlated with pulmonary fibrosis. While IL-4 has been implicated in idiopathic pulmonary fibrosis, liver fibrosis, and cardiac fibrosis [11], its involvement in the development of intestinal fibrosis remains inadequately understood.

5.2 IL-5

This cytokine has also been studied in various chronic fibrotic diseases, for example, in liver fibrosis, which is the most relevant today [11]. Using IL-5 knockout C57BL/6 mice, Reiman et al. pointed out a significant reduction in the development of liver fibrosis, determined using pathomorphology, and came to the conclusion that IL-5 is an important factor in this process [11]. According to the authors, IL-5 stimulates the response of Th2 lymphocytes, indirectly increasing the level of IL-13, which is a key mediator in the development of fibrosis. IL-5 exhibits both direct and indirect effects on eosinophil-associated liver fibrosis [11]. The significance of such a Th2 response has been demonstrated in experimental models of pulmonary, renal, and intestinal fibrosis [56, 57]. Anti-IL-5 therapy has been demonstrated to reduce intestinal eosinophils and suppress the development of radiation-induced intestinal fibrosis (RIF) in mice, highlighting the importance of eosinophils and IL-5 in RIF development [58]. However, research on the involvement of IL-5 in the development of intestinal fibrosis remains limited, emphasizing the necessity for further studies.

5.3 IL-13

IL-13 is also involved in the formation of fibrosis (fibrosis of the lungs, kidneys, liver and intestines), and has been identified as a possible inducer of airway remodeling in patients with bronchial asthma [59]. IL-13, together with IL-4, is responsible for eosinophil activation and can activate and proliferate fibroblasts [60]. This cytokine promotes pulmonary fibrosis, and an increase in IL-4 and IL-13 receptors on lung fibroblasts was observed in patients with idiopathic pulmonary fibrosis [60]. IL-13 is also involved in intestinal fibrosis. Fibrosis in mice chronically treated with TNBS appears to be mediated by IL-13 through the production of TGF-β1, and blocking IL-13 results in the prevention of intestinal fibrosis [61, 62]. Increased levels of IL-4Rα, IL-13Rα1 and IL-13Rα2 were found in ileal strictures from patients with CD, indicating the possible involvement of IL-13 in this process [26]. Despite its involvement in wound healing, tissue remodeling, and fibrosis, the precise contribution of IL-13 to the development of strictures in patients with Crohn’s disease (CD) is not fully understood [36]. Even if anti-IL13 therapy was not able to suppress inflammation in patients with UC, its effect on inflammation and especially fibrosis in CD has not been studied [26].

5.4 IL-33

Activation of eosinophils by IL-33 and subsequent co-culture with intestinal fibroblasts resulted in increased levels of certain proteins and molecules associated with inflammation and fibrosis. This includes IL-13Rα2, the cytokines TNF-α, IL-1β and IL-6, and the chemokines CCL24 and CCL26. The release of these molecules, especially CCL24 and CCL26, may enhance eosinophil influx. Fibroblasts isolated from this culture were cultured with IL-13 and began to produce fibronectin, collagen 1α2 and periostin, which are characteristic signs of fibrosis. Thus, eosinophils may play a dual role in inflammation and fibrosis. There is also an increase in IL-33 levels in ileal samples from children with Crohn’s disease compared to healthy children [63].

5.5 TGF-β

Indeed, TGF-β, recognized as a profibrotic cytokine, is involved in fibrosis in multiple organs [1, 11, 64]. It has an impact on various airway structural cells, including fibroblasts, smooth muscle cells, and epithelial cells, and is associated with fibrotic processes such as airway remodeling observed in individuals with bronchial asthma [59]. TGF-β prompts fibroblasts to transform into myofibroblasts, consequently contributing to the progression of fibrosis [59]. In vitro cultivation of mucosal fibroblasts obtained from patients with UC during relapse revealed increased synthesis of TGF-β1 and TGF-β3, while mucosal fibroblasts obtained from patients with CD during relapse showed increased production of TGF-β1 and decreased TGF-β3 production. Elevated levels of TGF-β1 are similarly observed in mucosal biopsies from patients with CD [11, 64]. Given that TGF-β is produced by epithelial cells, fibroblasts and immune cells, the role of eosinophils in this condition is not specifically known [44].

5.6 Eosinophilic cationic protein

As previously discussed, this protein has recently been proposed as a potential mediator of tissue remodeling in patients with allergic asthma and eosinophilic esophagitis [65]. In the lungs, tissue remodeling involves the release of collagen and proteoglycans from interstitial fibroblasts. Eosinophils, where ECP plays a significant role, contribute to this process by producing and releasing TGF-β [65]. ECP also triggers collagen gel contraction and intracellular proteoglycan accumulation, potentially affecting fibroblast activation indirectly [11, 65]. However, conclusive evidence is still lacking, and further extensive research is necessary. Additionally, the precise role of ECP in the development of intestinal fibrosis has not yet been fully elucidated.

5.7 EPO

Absolutely, the association of these factors with fibroblast activation suggests a potential involvement of EPO in fibrosis development. However, definitive evidence to confirm this hypothesis is still lacking [11].

5.8 What factors can influence eosinophil function?

Exactly, neutrophil extracellular traps (NETs), which are composed of a complex network of extracellular fibers primarily made up of neutrophil DNA, are indeed associated with both inflammatory and fibrotic diseases. Similar to what has been proposed for neutrophils, they have been found to be involved in tissue damage in the airways of asthmatic patients, which is of interest in the context of the development of fibrosis [66]. Some studies have found a relationship between intestinal eosinophils and the microbiome. Indeed, previous findings have shown significantly higher eosinophil counts in germ-free mice compared to pathogen-free mice, suggesting that the microbiome suppresses eosinophil proliferation [67]. Furthermore, when germ-free mice were exposed to a complex microbiome, a significant decrease in eosinophil counts was observed [67]. There is evidence that the high presence of eosinophils resulting from helminth infections can lead to tissue fibrosis [11]. The microbiome clearly has a direct influence on the number and function of eosinophils. Because eosinophils are present in the gastrointestinal tract under normally homeostatic conditions, it is believed that they play a positive role in maintaining tissue homeostasis. This occurs by maintaining plasma B cells that produce IgA, thereby regulating the composition of the intestinal microbiota and promoting the development of Peyer’s patches. Eosinophils enhance the secretion of intestinal mucus and thus help maintain the integrity of the epithelial barrier. Eosinophils release the IL-1 receptor antagonist IL-1Rα, which in turn inhibits IL-1β production, resulting in reduced Th17 differentiation. Given that Th17 cells are key producers of the profibrotic cytokine IL-17A, eosinophils may exert antifibrotic activity [68].

6. Eosinophilic inflammation through the eyes of a gastroenterologist, endoscopist and pathologist

One of the most “difficult” moments in the diagnosis of inflammatory bowel diseases is the correct interpretation of the endoscopic and pathomorphological picture and the corresponding activity of the mucosal lesion.

And first of all, this affects the issues of eosinophilic lesions of the mucous membranes of the small and large intestine. The most commonly differentiated pathology is eosinophilic colitis.

But before touching on the topic of eosinophilic colitis, it is necessary to clarify some aspects of primary and secondary eosinophilic lesions of the gastrointestinal tract (Table 2).

Primary eosinophilic gastrointestinal disorders	Secondary gastrointestinal eosinophilic disorders
Eosinophilic esophagitis	Gastroesophageal reflux disease
Primary eosinophilic gastroenteritis	Infections and parasitic infestations: protozoal, helminthic, fungal and other pathogens
Primary eosinophilic colitis	Drugs (e.g., naproxen, clozapine, rifampicin, enalapril, carbamazepine, gold drugs, interferons, tacrolimus)
Food allergies: IgE-mediated non-IgE-mediated	Systemic and autoimmune diseases: systemic connective tissue diseases (for example, systemic lupus erythematosus, scleroderma, Wegener’s granulomatosis, rheumatoid arthritis, periarteritis nodosa, eosinophilic fasciitis) vasculitis Churg-Strauss syndrome Tolosa-Hunt syndrome transplant rejection reaction Helicobacter gastritis; Celiac disease; Inflammatory bowel diseases (ulcerative colitis, Crohn’s disease); Malignant neoplasms; Iatrogenic pathology (for example, induced by the use of drugs, the use of medical equipment and instruments)
Eosinophilic (allergic, food protein-induced) colitis of early childhood

Table 2.

Classification of primary and secondary eosinophilic gastrointestinal disorders.

Eosinophilic colitis is an extremely rare inflammatory disease of the large intestine. The pathology is characterized by peripheral hypereosinophilia and swelling of the mucous membranes of the gastrointestinal tract. Typically, this condition is associated with individual allergic reactions and/or autoimmune diseases.

6.1 Symptoms of eosinophilic colitis

The causes of eosinophilic colitis are not fully known. Chronic eosinophilic colitis is extremely rare. It is believed to occur due to an allergic reaction to dairy products, eggs and soy. It is believed that the nature of the disease lies in the individual reaction to food allergies and other autoimmune diseases (bronchial asthma, eczema), therefore, when making a diagnosis, it is first of all important to exclude: infectious pathogens of gastrointestinal inflammation, the presence of parasites.

Pathology can affect one or more parts of the intestine. Depending on the depth of penetration of leukocytes into the intestinal walls, the following symptoms of eosinophilic colitis occur:

• when eosinophils affect only the outer surface of the mucosa, problems with malabsorption begin (diarrhea, steatorrhea, deficiency of vitamins (primarily fat-soluble), and a significant decrease in body weight). Along with the above symptoms, associated problems may appear: iron deficiency anemia, hypoalbuminemia, mineral deficiency;

• if the lesion reaches the submucosa and muscular lining of the intestine, the problems worsen. Complete or partial intestinal obstruction is possible, which is accompanied by bloating, abdominal pain, nausea and vomiting;

• if eosinophils completely saturate the intestinal lining, then ascites occurs, which is accompanied by severe swelling, abdominal pain, flatulence, nausea and other gastrointestinal problems.

6.2 Diagnosis of eosinophilic colitis

The broad clinical picture and rarity of the disease oblige doctors to collect many tests to make a diagnosis. In addition to general data on blood, feces and urine, it is necessary to conduct a morphological examination of the walls of the rectum. This requires a colonoscopy with biopsies taken.

In some cases, additional examinations are possible (irrigoscopy, radiography, CT, MRI, etc.).

The endoscopic picture can often be normal or have nonspecific signs of chronic inflammation. Mucosal erythema, fragility, erosion or ulceration, mucous “inclusions” and/or whitish spots may be detected.

Essential for diagnosis is a biopsy, which reveals eosinophilic infiltration of the colon mucosa. Given the nonspecific symptoms of abdominal pain, constipation, diarrhea, and rectal bleeding, the absence of distinctive clinical findings, and the relapsing course, the diagnosis of eosinophilic colitis should be substantiated by examining a colon biopsy [69, 70, 71, 72, 73]. Examination of a biopsy specimen usually demonstrates layers of eosinophilic infiltration in the lamina propria of the colon mucosa, less often spreading to the submucosal and muscular layers. Other histological findings that can be observed in the colon are eosinophilic microabscesses, eosinophilic cryptitis and intraepithelial eosinophils located predominantly in the superficial layers [70]. Multiple biopsies are necessary because not only is the eosinophilic infiltration unevenly distributed in eosinophilic colitis, but the normal eosinophil count usually has a wide range in different segments of the colon, showing a “proximal-distal distribution” of 35 eosinophils in the cecum, decreasing to 8–10 eosinophils in the rectum (high magnification microscopy field ×200) [74]. In addition to counting total eosinophil density, degranulation is assessed as an indicator of eosinophil activation. Degranulation can be observed in routinely stained media and assessed semiquantitatively, but it is unknown whether biopsy trauma may otherwise provoke degranulation of inactive eosinophils. In principle, the process of eosinophil degranulation could initiate cellular damage through mechanisms involving lysosomal, oxidative, and cytotoxic pathways. This damage, if sustained over time, might contribute to the development of localized fibrosis [70]. However, it’s worth noting that such fibrotic changes have not been documented in the gastrointestinal tract.

In most cases, only evaluation of a mucosal biopsy is available to diagnose eosinophilic colitis. However, eosinophilic infiltration in this case is also possible in the deeper layers of the colon wall. Thus, in three cases of pseudo-obstruction of the colon in eosinophilic colitis, eosinophilic ganglionitis was detected in the Meissner plexus [75].

Currently, the diagnosis of eosinophilic colitis remains quite difficult due to the nonspecificity of clinical manifestations and the lack of generally accepted criteria for distinguishing the eosinophilic density of the colon mucosa in the upper normal range from a diagnostically significant pathological increase in the number of eosinophils.

If we proceed from the ECCO recommendations of August 2023 (Definitions of Histological Abnormalities in Inflammatory Bowel Disease: an ECCO Position Paper) [76], then there is some data on pathological assessment associated with eosinophils (Table 3).

Type of abnormality	Example
Unequivocally abnormal feature	Crypt abscess; Granuloma
Features whose occurrence in normal mucosa is a subject of controversy	Lamina propria neutrophils; Eosinophil cryptitis
Increase in a feature that is usually absent or sparse	Crypt branching
Increase or reduction in a feature that is normally present	Plasma cells in the basal mucosa [increase]; Lymphoid aggregates [increase]; Epithelial mucin [reduction]
The severity or magnitude of an abnormality	Mild, moderate, or severe increase in eosinophils / neutrophils / plasma cells
Presence of a feature that is normal at one site but would be abnormal at another site	Paneth cells in right colon [normal]; Minor crypt distortion in caecum and rectum [normal]

Table 3.

Broad categories of histological abnormality [76].

In this document in position 3.4. “Eosinophils” published guidelines for the assessment of eosinophils in biopsy material in inflammatory bowel diseases.

ECCO Position 4.1. [70]

Eosinophils do not define chronic inflammation or acute inflammation reliably in inflammatory bowel disease.

Agreement: 93%.

ECCO Position 4.2. [70]

There is no widely accepted definition of a significant increase in colorectal mucosal eosinophils in inflammatory bowel disease.

Agreement: 93%.

ECCO Position 4.3. [70]

Eosinophilic cryptitis is defined as the presence of at least one eosinophil in the crypt epithelium.

Agreement: 93%.

ECCO Position 4.4. [70]

An eosinophilic crypt abscess is defined as eosinophils in a crypt lumen without the presence of luminal neutrophils.

Agreement: 100%.

There is a scarcity of data regarding the typical count of eosinophils in the intestinal mucosa, the precise definition of an eosinophil crypt abscess, [77, 78] and the criteria for categorizing an increase in intestinal mucosal eosinophils as mild, moderate, or severe (Figures 2 and 3). Additionally, the significance of focal eosinophil cryptitis in the absence of other alterations remains uncertain. The consensus among experts is that an eosinophilic crypt abscess involves the presence of at least two eosinophils. If neutrophils are also present, the lesion is termed a crypt abscess (i.e., neutrophil crypt abscess; Figure 4).

Figure 2.

Basal plasmacytosis, comprising an increase in plasma cell numbers at the mucosa base [‘crypts with their feet in pools of plasma cells’] and elsewhere [e.g., within large ellipse; small ellipse surrounds three plasma cells] (adapted Feakins et al. [76]). Other inflammatory cells [e.g., eosinophils] are also apparent. The base of a crypt [arrow] shows cryptitis [i.e., at least one neutrophil in the crypt epithelium] [79].

Figure 3.

Eosinophils accompany basal plasma cells in this biopsy [circles identify four eosinophils] (adapted Feakins et al. [76]). A crypt shows eosinophil cryptitis [arrow; ie, at least one eosinophil in the crypt epithelium without accompanying neutrophils]. The maximum number of lamina propria eosinophils and of foci of eosinophilic cryptitis in normal mucosa is uncertain.

Figure 4.

This crypt abscess [arrow] consists mainly of eosinophils but also includes neutrophils (adapted Feakins et al. [76]). Therefore, it is a crypt abscess rather than an eosinophil crypt abscess.

To treat eosinophilic colitis and alleviate the patient’s condition, different methods are used: the use of medications (glucocorticosteroids, antihistamines, mesalazine, leukotriene inhibitors, azathioprine and biological treatments); a diet excluding allergens is prescribed.

The effectiveness of treatment lies in reducing the density of eosinophilic infiltration, that is, the area of damage to the tissues of the gastrointestinal tract by leukocytes. Therefore, repeated intestinal biopsies are necessary during treatment.

7. The influence of elevation of the level of eosinophilic cationic protein on the course of inflammatory bowel diseases (results of our own research)

We conducted a small study related to the effect of increased eosinophil cationic protein in patients with inflammatory bowel diseases on disease activity.

Study objective: To investigate the impact of elevated levels of eosinophilic cationic protein on the progression of inflammatory bowel diseases.

Materials and methods: The study was conducted between January 2016 and April 2022, involving a cohort of 400 patients diagnosed with inflammatory bowel disease (Crohn’s disease (CD): 238 patients, Ulcerative Colitis (UC): 150 patients, Microscopic Colitis (MC): 12 patients). Each participant underwent endoscopic examination with biopsy sampling for histopathological analysis, along with a series of laboratory tests including complete blood count, CRP, homocysteine, vitamin D levels, ANCA markers, α-TNF and IL-1β, IL-2, IL-4, IL-6, IL-8, IL-10, IL-18 levels, total protein, albumin, iron, ferritin, eosinophil cationic protein (ECP), IgE levels, urinalysis with urine albumin, and measurement of fecal calprotectin and lactoferrin levels.

Results: Among the 250 patients (62.5%) exhibiting inadequate response to standard therapy, eosinophilic cationic protein levels were assessed in the blood. Among them, 69 cases (27.6%) tested positive. Notably, 190 patients (76%) demonstrated positive endoscopic findings. Furthermore, 95 patients (38%) showed positive histopathological findings, characterized by the presence of more than 15–20 eosinophils per field of view in the biopsy samples. Additionally, 14 patients (5.6% / 20.2%) were diagnosed with concurrent allergic diseases. The level of eosinophilic cationic protein in the blood ranged from 29 to 228 ng/ml (N < 24 ng/ml). In the group of patients with elevated levels of eosinophilic cationic protein, the number of patients with Crohn’s disease increased (41 patients), including patients with phenotype B2 (24 patients), and patients with phenotype B3 (17). Basic patient data and changes in follow-up are presented in the Figures 5–7.

Figure 5.

Distributions of patients. (a) General distribution of patients. (b) Distribution of patients by nosology and phenotypes.

Figure 6.

Distribution of patients after 6-24 months.

Figure 7.

Distribution by EPC level.

During the dynamic follow-up of patients with elevated eosinophilic cationic protein levels over 6–24 months, several developments were observed. In patients with Crohn’s disease, new instances of fibrosis emerged. Similarly, cases of unresponsiveness to standard therapy and subsequent surgical interventions were noted in patients with ulcerative colitis. Importantly, the levels of eosinophilic cationic protein elevation were higher in the group of patients experiencing complications compared to those without complications. The results of some laboratory parameters, indicators of endoscopic and histological activity of the course of inflammatory bowel diseases, depending on changes in the level of eosinophilic cationic protein are presented in Tables 4 and 5.

Activity indicators	EPC+(N < 24 ng/mL)			EPC-
	Average value	UC n = 35	CD n = 34	Average value	UC n = 35	CD 11 = 34
h/s CRP (N < 1 mg/L)	16.85	15.1	18.6	6.35	6.9	5.8
Plateles (N 150–400,000/mm^3)	461.65	436.1	487.2	335.75	319.9	351.6
Homocystein (N < 12 μmol/L)	17.05	15.9	18.2	10.25	9.6	10.9
Vitamin D (N 30-100 ng/mL)	4.9	4.0	5.8	20.9	23.1	18.7
Calprotectin (N < 50 μg/g)	1791	2354	1228	1269	1563	975
Albumin (in urina) (N < 2 mg/L)	37.25	32.1	42.4	8.3	10.2	6.4
Laktoferrin (N < 7.5 μg/g)	135.5	144	127	57.5	76	39
α-TNF (N 4.6–12.4 pg./ml)	12.5	12.1	12.9	11.25	10.4	12.1
Il-1β (N < 11 pq/ml)	10.45	10.2	10.7	10.4	10.3	10.5
Il-2 (N < 10 pq/ml)	3.2	2.9	3.5	2.85	2.9	2.8
İl-4(N < 4 pq/ml)	3.85	3.9	3.8	2.25	2.2	2.3
İl-6(N < 10 pq/ml)	8.7	8.5	8.9	8.8	8.7	8.9
İl-8(N < 10 pq/ml)	11.1	10.9	11.3	10.35	10.1	10.6
İl-10(N < 20 pq/ml)	21.5	21.2	21.8	18.75	18.4	19.1
İl-18(N < 261 pq/ml)	265.3	264.5	266.1	252.1	258.4	245.8
Endoscopic activity	++*	+++	++++	++	+++	++
Morphological activity	+++**	++++	+++	++	++	++

Table 4.

Changes in activity in some laboratory, endoscopic and pathological parameters.

^*

moderate activity.

^**

high activity.

Activity indicators	ECP+; Average value	ECP−; Average value
Total protein (N 66–87q/l)	62.1	74.3
Albumen (N 35–52 g/L)	29.3	39.7
Iron (N 27–150 μg/dL)	10.8	46.1
Ferritin (N 10–154 ng/mL)	7.5	62.7
IgE (N < 100 IU/mL)	89.6	67.3

Table 5.

Changes in total protein, albumin, iron, ferritin, IgE.

Images from endoscopic examinations (colonoscopy) and pathological examinations of biopsies taken from the mucous membranes of the colon and terminal ileum in patients with elevated blood levels of eosinophil cationic protein are presented in Figures 8 and 9 (the presentedmaterials are archival data of doctors Gulustan H. Babayeva, Gunay V. Asadova, Jamal S. Musayev).

Figure 8.

Patient C.E.N., male, born in 1988. She has been ill since 2018; in 2019, the diagnosis of Crohn's disease (L3, B2, narrowing in the area of the terminal ileum) was verified. (a) Data from endoscopic examination of the colon and terminal ileum. Endoscopic conclusion: Terminal ileitis. Inflammatory bowel disease with ulcerative lesions of the periappendicular and rectosigmoid areas, high endoscopic activity, subcompensated stenosis (Crohn's disease). (b) Data from a pathomorphological study of a biopsy taken from the mucous membrane of the colon. Conclusion of the pathomorphological study: against the background of swelling and ulcerative defect of the colon mucosa, lymphocytes, plasma cells and eosinophilic infiltration were detected in the lamina propria (over 60-65 eosinophils per 1 HPF).

Figure 9.

Patient M.S.A., female, born in 1968. She has been ill since 2017; in 2018, the diagnosis of ulcerative colitis was verified, with total damage to the colon mucosa. (a) Data from endoscopic examination of the colon. Endoscopic conclusion: inflammatory bowel disease, ulcerative colitis, endoscopic remission phase. (b) Data from a pathomorphological study of a biopsy taken from the mucous membrane of the colon. Conclusion of the pathomorphological study: in the biopsy material from the mucous membrane of the colon, the presence of mononuclear cells was revealed against the background of a small number of basal plasma cells; eosinophilic infiltration was detected in the lamina propria (over 35-40 eosinophils 1HPF). The visible picture corresponds to the inactive period of ulcerative colitis (Geboes score 2A.1).

Conclusion: Thus, the increase in the level of ECP in the main group of patients was 27.6%, in this group of patients there were higher indicators of laboratory, endoscopic and morphological activity of IBD; that is, additional determination of eosinophil cationic protein may help identify one of the main reasons for the lack of response in patients with IBD to basic therapy.

8. Is it possible to influence eosinophil activation in IBD?

At diagnosis, patients with IBD are often treated with corticosteroids on a tapering schedule to rapidly improve symptoms (Table 6) [11, 86, 87]. Corticosteroids prevent eosinophil accumulation, reduce eosinophil chemotaxis, and may block other eosinophil factors, including eosinophil adhesion in vitro [88, 89]. Corticosteroids are recognized for their antifibrotic properties, achieved through the reduction of collagen synthesis, albeit at the cost of impeding wound healing [90]. This antifibrotic action has been validated across various conditions including idiopathic pulmonary fibrosis, systemic sclerosis, and retroperitoneal fibrosis [91, 92, 93, 94, 95]. Nonetheless, prolonged corticosteroid use is discouraged due to its systemic side effects [86].

Treatment	Influence on eosinophil presence	Influence on fibrostenosis development
Corticosteroids	Prevent eosinophil accumulation and reduce eosinophil chemotaxis and can block other eosinophil factors.	Demonstrated in idiopathic pulmonary fibrosis, systemic sclerosis and retroperitoneal fibrosis: affects wound healing and reduces collagen synthesis.
Anti-α4β7 integrin (Vedolizumab)	Possible ↓ in influx of eosinophils, but inconclusive evidence [80]; Vedolizumab: no effect on eosinophil circulation Natalizumab: ↑ in circulating eosinophils ↓ eosinophil accumulation at site of inflammation	No effects described in literature.
Anti-TNF (infliximab and adalimumab)	No effect described in literature.	Infliximab: suggested to be effective in the early stages of fibrosis development; ↓ in bFGF and VEGF levels in serum In vitro exposure of myofibroblasts, isolated from CD patients, to infliximab: ↓ collagen production. Adalimumab: CREOLE study; CD patients with small bowel strictures: beneficial effect [81]
Anti-IL-12/IL-23 (Ustekinumab)	No effect described in literature.	No effect described in literature.
JAK inhibitor (Tofacitinib)	Effective in several eosinophil related disorders [82, 83, 84]; ↓ in eosinophil numbers ↓ in disease symptoms BAL (Bronchoalveolar lavage) fluid in mice treated with Tofacitinib [85]: eosinophil presence reduced. ↓ in [TGF-β] ↓ myofibroblasts deposited in pulmonary arteries

Table 6.

Conventional treatment options for IBD patients [11].

In recent decades, the treatment landscape for IBD has undergone substantial changes due to the introduction of various biological therapies and small molecules (see Table 6). However, the specific impacts of these biologics on eosinophil function, recruitment, and degranulation remain largely unclear. Mucosal addressin cell adhesion molecule 1 (MadCAM-1), located on the endothelial cells lining the blood vessels in the intestinal mucosa, interacts with α4β7 integrin receptors present on the surface of eosinophils. This interaction between α4β7 integrin and MadCAM-1 facilitates the migration of eosinophils into the gastrointestinal tract [86, 87].

Thus, it is expected that anti-α4β7 integrin therapy could influence the recruitment of intestinal eosinophils. However, the evidence in the literature is inconclusive: Bochner BS. reported no effect on eosinophils following vedolizumab treatment [80], whereas natalizumab, a humanized antibody targeting α4β1 and α4β7 integrin approved for systemic sclerosis treatment, resulted in increased circulating eosinophils and decreased eosinophil accumulation at the inflammation site [80]. Non-responders to vedolizumab exhibited a higher mean colonic mucosal eosinophil count at baseline. Further research is warranted to ascertain if this elevated baseline eosinophil count can predict vedolizumab treatment failure [96].

Although the effect of infliximab treatment on the presence and activation status of eosinophils has not been described, it is suggested that infliximab is effective in the early stages of fibrosis. Decreases in serum levels of basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) were observed in patients treated with infliximab [11]. These elements are acknowledged for their roles in the development of intestinal fibrosis. Basic fibroblast growth factor (bFGF) fosters the proliferation of fibroblasts, while vascular endothelial growth factor (VEGF) spurs the activation of fibroblasts and the production of extracellular matrix (ECM) components [97]. In the CREOLE investigation, 97 individuals diagnosed with Crohn’s disease (CD) and experiencing small bowel strictures underwent treatment with adalimumab. Findings revealed that around two-thirds (63.9%) of CD patients responded positively to adalimumab, with a sustained response rate of 45.7% at a span of 3.8 years. This implies that anti-tumor necrosis factor (TNF) therapy might offer advantages in managing intestinal strictures [81]. Tofacitinib, the initial JAK inhibitor approved for treating moderate to severe UC, has demonstrated effectiveness in various disorders linked to eosinophils, such as hypereosinophilic syndrome, drug hypersensitivity syndrome, and eosinophilic esophagitis [82, 83, 84]. In an experimental model of pulmonary eosinophilic vasculitis, the administration of tofacitinib to 8-week-old C57BL/6 mice led to decreased levels of eosinophils in the bronchoalveolar lavage fluid. Furthermore, there was a reduction in the concentration of TGF-β in the lavage fluid, along with a decrease in the deposition of myofibroblasts in the pulmonary arteries. This suggests that tofacitinib may not only affect eosinophil infiltration but also function as an antifibrotic agent [85]. However, tofacitinib did not advance beyond phase II trials in patients with luminal CD [98]. In contrast, the JAK-1 inhibitor filgotinib has displayed promising efficacy in CD, including a notable reduction in VEGF [99, 100].

Because the precise role of eosinophils in inflammation and fibrosis remains unclear, therapies specifically targeting eosinophils are not currently part of the treatment regimen for IBD patients. However, studies in murine colitis models have shown that treatments aimed at eosinophils can reduce inflammation and reshape tissue structure [63]. Targeting CCR3 or eotaxin emerges as a promising therapeutic strategy due to their involvement in eosinophil accumulation. In experimental colitis models, blocking eotaxin-1 with an anti-eotaxin-1 monoclonal antibody (mAb) reduced disease severity and showed efficacy in allergic inflammation models [101]. Inhibition of eotaxin has demonstrated significant benefits in DSS colitis, suggesting its potential for IBD treatment development [102].

Benralizumab, a monoclonal antibody that targets IL-5R and reduces eosinophil levels, has shown effectiveness in asthma patients [103]. A similar antibody has been developed and demonstrated significant improvement in radiation-induced intestinal fibrosis in mice, suggesting its potential as a treatment option for certain inflammatory bowel disease patients [58].

Targeting the ST2/IL-33 pathway could offer benefits for managing symptoms in IBD patients. Several antibodies that block IL-33 are currently undergoing investigation for treating asthma and chronic obstructive pulmonary disease. However, caution is advised when targeting ST2, as it activates various cell types, including ILC2s, T lymphocytes, mast cells, basophils, and other immune cells, indirectly affecting other pathways [39].

In conclusion, as highlighted earlier, extensive eosinophil infiltration in the colon’s lamina propria serves as a key predictor of inadequate response to drug therapy among UC patients. This underscores the critical role of monitoring eosinophil levels in individuals with IBD [13].

9. Conclusion

So, what conclusion can be summarized?

Eosinophils and their associated components are significant factors in various inflammatory and fibrotic conditions, including IBD. Despite the elevation of important molecules responsible for eosinophil activation and recruitment in IBD patients, the precise mechanism by which eosinophils exert their effects remains uncertain. The specific impacts of eosinophil-related proteins like ECP, EPO, EDN, and MBP are still unclear. However, the release of TGF-β, which promotes fibrosis, by eosinophils might contribute to intestinal fibrosis in these patients. Present studies mostly provide descriptive findings rather than establishing a definitive cause-and-effect relationship. Therefore, further investigation is crucial to understand the involvement of eosinophil activation and degranulation in inflammation and fibrosis, especially within the intestine. This could lead to the identification of novel therapeutic approaches for managing IBD by targeting inflammation and fibrosis.

And answering the previously asked question: Eosinophilic inflammation in inflammatory bowel diseases – the conductor or the “first” violin, so far the answer is the same: both the conductor and the “first” violin, or, for now, a “one-man show”.