Eosinophils in the pathogenesis of pancreatic disorders

We reported the role of eosinophils during CP pathogeny-associated fibrotic remodeling using an experimental model [ 19 ]. We found pancreatic IL-18 was induced during CP [ 19 ], which was in accordance with previous clinical observations [ 21 – 23 ]. Eosinophilic gastroenteritis (EG) is also linked to EP, and we showed that IL-18 overexpression via recombinant IL-18 or transgene insertion promotes EG disorders [ 24 ] and that IL-18 plays an important role in generating and transforming naïve eosinophils to pathogenic eosinophils [ 19 , 25 ]. Eosinophil infiltration and accumulation has been reported during different stages of pancreatitis onset (AP), progression (RAP, CP), and pancreatic malignancy. However, the molecular mechanisms involved in eosinophil accumulation during pancreatitis pathogenesis remain elusive. Eotaxins serve as a chemoattractants for eosinophils [ 116 ], and activation of eotaxin-3 via involvement of the STAT6 signaling pathway has been reported in human pancreatic myofibroblasts. Furthermore, in a study pancreatic myofibroblasts are reported as the cellular source of eotaxin-3 and its induction is correlated with Th2 cytokines (IL-4 and IL-13) [ 117 ]. We have recently reported eosinophil accumulation and degranulation in the pancreas along with induced mRNA and protein expression levels of proinflammatory cytokines IL-5, IL-18, and chemokines eotaxin-1 and eotaxin-2 in the cerulein-induced experimental mouse model of CP. We also showed that the eosinophil-deficient GATA1 and IL-5-deficient mice were protected from the induction of CP [ 19 ]. Furthermore, we also showed induced accumulation and degranulation of eosinophils and mast cells presence in the malignant pancreatitis [ 15 ]. Several reports also indicate that nerves expressing VIP are induced in the pancreas [ 118 – 120 ], and we recently showed that VIP has chemoattraction activity for eosinophils [ 105 ]. Therefore, the induced expression of VIP and eosinophils in pancreatitis indicates that VIP may be a critical molecule that promotes eosinophilic pancreatitis. Notably, even though eosinophils are detected in pancreatic cancer, including pancreatic ductal carcinoma [ 7 , 121 – 124 ]; but its role in cancer or pancreatic cancer is yet not established. Therefore, it is critical to detect and establish eosinophil’s role in pancreatitis patients based on their eosinophil count, immunoglobulin E levels, and eosinophilic granules in blood and biopsies.

The diagnosis of EP is very important but challenging [ 108 ] because of the similar clinical symptoms, tests, and radiological examinations between EP and pancreatic neoplasia [ 6 , 109 ]. An EP diagnosis should exclude parasitic infections and eosinophilia during pathological samples evaluation [ 110 , 111 ]. Eosinophil-associated pancreatic disorders mainly include several cases of acute pancreatitis, recurrent acute pancreatitis, chronic pancreatitis [ 19 , 89 ], pancreatic cancer [ 15 ], and autoimmune pancreatitis [ 92 ]. Even in pancreatic cancer a marked eosinophilia is observed [ 15 , 109 ]; therefore, EP is sometimes misdiagnosed as pancreatic cancer [ 6 , 69 , 112 , 113 ]. Several lines of evidence indicate that EP is mostly diagnosed for suspected pancreatic tumors that mimic a pancreatic neoplasm [ 65 , 66 , 113 ]. Routine endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) for resectable pancreatic lesions is controversial [ 110 ], but recently, a 37-year-old man was diagnosed with a rare nononcological pancreatic mass representing eosinophilic pancreatitis [ 114 ]. In 2019, Pinte et al [ 115 ] analyzed 36 publications in a systemic review consisting of case reports and case series. They found that out of 45 patients, 20 subjects with eosinophilic gastroenteritis developed pancreatitis, 20 had eosinophilic pancreatitis, and 5 had hypereosinophilic syndromes involving the pancreas. None of these three groups showed significant differences in clinical, laboratory, or imaging features, despite the multiple theories that explain the association of pancreatic and gastrointestinal eosinophilic infiltration. Overall, the diagnosis of EP is very important, and it may be assumed that pancreatic tissue biopsy procedures (i.e., ERCP, endoscopy) with fine needle aspiration may not provide sufficient tissue for pathological examination, leading to missed diagnosis of EP.

Eosinophils typically measure from 10–16 μm in diameter and possess bilobed nuclei with approximately a 30% ratio of nucleus to cytoplasm. The most prominent feature of eosinophils is their stain with acidophilic dye, first observed by Paul Ehrlich in 1879 who described eosinophils for the first time and noted their higher numbers in patients with asthma and helminth infections [ 1 , 26 ]. Eosinophils have a beneficial role in host defense against nematodes and other parasitic infections and have four different types of eosinophil granules: major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil peroxidase (EPO), and eosinophil-derived neurotoxin (EDN) [ 27 , 28 ]. Once activated, eosinophils synthesize and release their granules, which individually mediate potential regulatory effects on various cell types [ 29 ]. The structures of normal and deregulated eosinophils are shown in Figure 1 . Eosinophils reside mainly in the GI tract but are also found in mammary glands, uterus, thymus, bone marrow and adipose tissues in a healthy state [ 30 , 31 ] and represent ≅1–4% of leukocytes in circulating blood [ 32 ]. Human eosinophils can produce, store, and secrete over 30 cytokines; therefore, eosinophils can be distinguished from other immune cells by their capacity for rapid cytokine secretion [ 33 , 34 ]. Eosinophil-secreted cytokines, chemokines, and growth factors have a profound effect on the progression of immune and inflammatory responses and play roles in tissue regeneration, wound healing, and host defense. Human eosinophils constitutively express immunoregulatory cytokines, such as interleukin (IL)-4, IL-13, IL-6, IL-10, IL-12, interferon (IFN)-γ, and tumor necrosis factor (TNF)-α [ 34 ]. Eosinophils also have several regulatory functions to modulate lymphocyte recruitment and homeostasis [ 35 ]. Eosinophils may behave as antigen-presenting cells (APCs), they can drive Th2 polarization with the help of dendritic cells (DCs) or alone, and they can regulate T cell development in the thymus. Eosinophils express several surface receptors, including IL-5Rα, CC-chemokine receptor 3 (CCR3), Siglec-8 in humans, Siglec-F in mice, and the chemoattractant receptor homologous molecule CRTH2. With the help of these receptors the Eosinophils can survive, proliferate and chemoattract to targeted tissues in disease conditions through their ligands [ 28 ] ( Figure 2 ).

This review mainly provides insights into eosinophils associated with pancreatic disorders, focusing on pancreatitis (acute, recurrent acute, and chronic), diabetes, and pancreatic cancers. The information summarized in this review will be highly useful for future studies focusing on the development of novel treatment options for eosinophils associated with disease. The case reports and experimental models reviewed here indicate the roles of IL-5, IL-18, and VIP in promoting EP; therefore, based on the evidence provided in this review, clinicians should be aware of the role of eosinophils in promoting chronic pancreatitis. Furthermore, the detection of eosinophils in pancreatic tumors indicates that eosinophilic pancreatitis progresses into pancreatic neoplasia. This rationale is based on some recent reports that show (i) the role of eosinophils in the progression of chronic pancreatitis and fibrosis in experimental mouse models and (ii) eosinophil degranulation promotes human pancreatic malignancy. Taken together, evidence indicates that eosinophilic pancreatitis is an independent disease entity that requires further attention from basic scientists and clinicians.

Various reports have indicated that infants can also receive EP from diabetic mothers [ 94 – 96 ], indicating the role of diabetes in the development of EP. EP was reported in premature newborn infants of diabetic mothers [ 95 ]. Eosinophilic pancreatitis and anencephaly were found in the 34-week-old fetus of a mother suffering from type I diabetes (T1D). The histological analysis of the tissue section revealed eosinophilic infiltrate in the fibrous islets of Langerhans that causes hypertrophy and hyperplasia due to the induced maternak IgG [ 96 ]. T1D is an autoimmune disease mainly involving T-cell-mediated destruction of beta cells in the endocrine pancreas. Eosinophils (CD15(dull)/CD14(weak) cells) have been identified from patients with type 1 diabetes mellitus [ 97 ] and express induced levels of myeloid alfa-defensins and myeloperoxidase. The presence of eosinophils in T1D further indicates that eosinophils could be part of an intricate innate immune cellular network involved in the development of diabetes [ 98 ]. Induced subcutaneous eosinophilia in adipose tissue in metabolic syndrome patients with type 2 diabetes mellitus and cardiovascular disease has been reported to play an important role in promoting inflammation-mediated disease pathogenesis [ 99 ]. Blood and renal infiltrated eosinophils are prevalent and associated with the severity of diabetic nephropathy (DN), which is associated with induced blood and renal eosinophilia, and urinary eosinophil cationic protein (ECP) levels are suggested as a biomarker for DN [ 100 ]. A diagrammatic representation of all eosinophilic diseases of the pancreas is shown in Figure 4 .

In mice, eosinophils mainly develop from granulocyte/monocyte progenitors (GMP) cells in the bone marrow from intermediate eosinophil lineage-committed progenitors (EoPs) [ 36 ]. However, human eosinophil lineage-committed progenitors are derived from common myeloid progenitors (CMPs) or their upstream multipotent progenitors [ 37 ]. Expression of the IL-5 receptor on EoPs is a result of the commitment of GMPs to the eosinophil lineage, which means that IL-5 supports eosinophil development from EoPs rather than instructing GMPs to commit to the eosinophil lineage. Eosinophil differentiation in the bone marrow is promoted by the protein tyrosine phosphatase SHP2 [ 38 ]. Eosinophil lineage fate is regulated by the interplay of several key transcription factors, including CCAAT/enhancer-binding protein (a C/EBP family member), GATA-1 (a zinc finger family member), PU.1 (an Ets family member), and friend of GATA (FOG) [ 39 , 40 ], as shown in Figure 3 . Recently, it has been shown that the protein tyrosine phosphatase SHP2 promotes eosinophil differentiation [ 38 ]. They acquire IL-5Rα on their surface at a very early stage during eosinophilopoiesis and differentiate under the strong influence of IL-5 [ 40 ]. We earlier reported two distinct subsets of CD101 + [ 41 ] and CD274 + [ 42 ] eosinophils that exhibit differences in their function. Earlier C101 + eosinophils are reported as IL-5-dependent inflammatory eosinophils in the lungs of house dust mite (HDM) exposed mice, which is functionally different from Il-5-independentt residential eosinophil. However, we found that the blood of IL-5 transgenic mice do not express CD101, which contradict the finding of earlier published report that CD101 is IL-5 dependent eosinophils [ 41 ]. Our experimentation indicated that the eosinophils isolated from the spleen of IL-5 transgenic mice, when exposed to IL-18 only then they express CD101 as well CD274 [ 42 ]. Similarly, we also found that normal human blood also express CD101 and in disease state another subset of eosinophils is detected that express that increases with the severity of allergic diseases [ 42 ]. Interestingly, the nasal or lung lavage of asthmatic individuals and biopsies of EoE patients express both CD101 + CD274 + eosinophils [ 42 ]. Of note, both CD101 and CD274 expressing eosinophils ae also detected in lungs of asthmatic mice [ 42 ]. Apart from the role of IL-5 in eosinophil generation, maturation and differentiation, IL-18 has recently been reported in the transformation and maturation of naïve eosinophils to CD274-expressing pathogenic eosinophils in allergic diseases [ 9 , 25 ], suggesting a synergistic role of IL-5 and IL-18, as shown in Figure 3 .

A clinical report showed eosinophils in 68 years old female pancreatic adenocarcinoma patients with no allergic or autoimmune disease [ 101 ]. The computed tomography of the abdomen indicated mass in the neck of pancreas and fine needle aspiration biopsy both confirms the adenocarcinoma in patient. The patient was treated with FOLFIRINOX which led to resolution of the eosinophilia. Our research findings demonstrate that eosinophil and mast cell accumulation and degranulation are critical in promoting pancreatitis pathogenesis, which may lead to the development of pancreatic fibrosis and malignancy [ 15 ]. The role of eosinophils in CP-mediated pancreatic malignancy is not yet clearly understood or explored. In a recent report, inflammatory cells, including mast cells and eosinophils, were implicated in the progression of pancreatic remodeling in cerulein-induced chronic inflammation and the progression of pancreatic cancer in Akt1Myr/KRasG12D mice [ 102 ]. The results indicated that pancreas-localized IL-5 induces eosinophils that stimulate IL-5Rα-expressing ductal tumor cells and increase pancreatic tumor cell motility. In response to prolonged eosinophilic inflammation, acinar cells begin to undergo acinar-to-ductal metaplasia and convert these cells into pancreatic intraepithelial neoplasia (PanIN) ( Figure 5 .) [ 102 ]. Furthermore, a case report of a 47-year-old patient showed granular eosinophilic cytoplasm with sporadic pancreatic vasoactive intestinal peptide-producing tumor (VIPoma). VIPoma is a rare disease that is reported in 1 in 10 million people per year [ 103 ]. High vasoactive intestinal peptide (VIP) expression is reported in the pancreas [ 104 ] and is implicated in the chemoattraction of eosinophils in allergic diseases such as asthma and eosinophilic esophagitis [ 105 , 106 ]. In another clinical report, eosinophilic pancreatitis-mediated malignancy was shown to be associated with chronic myocarditis that mimics acute coronary syndrome [ 107 ].

In the past several decades, the accumulation of eosinophils during different stages of pancreatitis (AP, RAP and CP) has been noted and reported as eosinophilic pancreatitis (EP). In 1955, circulatory eosinophils were found in chronic relapsing pancreatitis patients [ 18 ], and several cases of EP are currently known [ 63 – 67 ]. The influx of eosinophils in the pancreas is also reported in other pancreatic diseases such as lymphoplasmacytic sclerosing pancreatitis, pancreatic allograft rejection, pancreatic pseudocyst, inflammatory myofibroblastic tumor, and histiocytosis X [ 7 ]. A patient with alcohol-related pancreatitis also showed induced pancreatic eosinophilia while suffering from a pancreatic pseudocyst, and the patient was atopic with a high level of IgE [ 68 ]. Abraham et al. [ 7 ] noted and explained two types of histological patterns in EP patients: One pattern features diffuse periductal, acinar, and septal eosinophilic infiltrates with eosinophilic phlebitis and arteritis; the other is characterized by localized intense eosinophilic infiltrates associated with pseudocyst formation. In another case, a patient was diagnosed with pleural effusion-associated eosinophilia in the pancreas following the execution of pancreatic resection [ 69 ]. In addition to humans, EP is also reported in parasitic infections in dogs [ 6 , 70 ], nematode infection in horses [ 71 ], and scorpion envenomation in rodents [ 72 ]. Several cases of eosinophil infiltration have been reported during AP, RAP, CP, pancreatic neoplasia, and diabetes, which we briefly discuss below.

Eosinophils, a subset of immune cells were first identified by Paul Ehrlich in 1879 [ 1 ], they originate from multipotent hematopoietic stem cells (HSCs) in the bone marrow and can be distinguished phenotypically by their bilobed nuclei and cytoplasmic granules [ 2 ]. Eosinophils are involved in diverse immune responses associated with innate and adaptive immunity [ 3 ] and play an important role in the mucosal immune responses of the gastrointestinal (GI) tract. Under normal conditions, eosinophils reside in the the mucosa of the intestine that maintain innate immunity and serve as effector and immunoregulatory functions [ 4 ]. Eosinophil-associated allergic disorders are characterized by induced levels of tissue eosinophila that promotes the dysfunction of several organs like blood, lung, skin, and gastrointestinal tract. These disorders include hypereosinophilic syndrome (HES), asthma, endocrine disorders, eosinophilic dermatitis, eosinophilic esophagitis (EoE), and numerous eosinophilic gastrointestinal disorders (celiac disease, inflammatory bowel disease and allergic colitis) and eosinophilic pancreatitis [ 5 – 10 ]. Repeated bouts of acute pancreatitis (AP), an inflammatory disease of the pancreas which can leads to recurrent AP (RAP) and chronic pancreatitis (CP)[ 11 ]. AP is associated with significant morbidity and cost and is one of the leading causes of hospital admission in the US [ 12 ]. AP is characterized by the release of pro- and anti-inflammatory signals from injured acinar cells and an influx of leukocytes (monocytes, macrophages, and lymphocytes) [ 13 – 17 ]. Several case reports show the accumulation of eosinophils in pancreatic inflammation, known as eosinophilic pancreatitis (EP) [ 6 , 18 ]. This infiltration of eosinophils in the pancreas may not be noticed initially, as most pancreatic biopsies are performed using endoscopic retrograde cholangiopancreatography and endoscopic ultrasound with the purpose of detecting only pancreatic malignancy. These commonly used procedures may not provide sufficient tissue for pathological examination; therefore, EP often goes undetected. Notably, pancreas is devoid of eosinophils in healthy state and the mechanism of eosinophils infiltration in the pancreas is yet not understood. However, it remains unclear how eosinophils are recruited to the pancreas under inflammatory conditions; thus, more in-depth investigations are needed to help develop novel targeted therapies for EP patients. Access to pancreatic tissue is not possible in AP or RAP patients and is very limited for CP patients, so we rely on experimental models to study the detailed pathogenesis of EP. Interleukin (IL)-5 is a well-known growth and differentiation factor for the development, differentiation, and maturation of eosinophils [ 19 ]; however, we did not observe induced IL-5 in an experimental model of CP, a recent report observed induced IL-3, IL-4 and IL-5 levels by performing highly sensitive Luminex analysis in an experimental model of chronic pancreatitis [ 20 ]. These findings are not surprising, as endogenous IL-5 is always present, even in healthy states. We observed that pancreatic IL-18 was induced during CP [ 19 ] which was in accordance with previous clinical observations [ 21 – 23 ]. Eosinophilic gastroenteritis (EG) is also linked to EP, and we showed that IL-18 overexpression via recombinant IL-18 or transgene insertion promotes EG disorders [ 24 ] and that IL-18 plays an important role in generating and transforming naïve eosinophils to pathogenic eosinophils [ 19 , 25 ]. Additionally, we also observed that accumulated macrophages are the source of NLRP3 regulated induced IL-18 in pancreatitis and the eosinophilic inflammation via intracellular upregulation of NF-κB promotes a series of activation pathway that enhance pancreatic inflammation leading to acinar cell death, acinar to ductal cells metaplasia and pancreatic edema.

In several allergic diseases, the human body produces a large number of eosinophils in response to some inflammatory cytokines, such as IL-5, IL-13, and IL-18, that can cause chronic tissue inflammation that reduces organs’ functional capabilities [ 25 , 52 – 54 ]. Eosinophilic disorders are diagnosed and named according to the location where the levels of eosinophils are elevated. Eosinophil accumulation in the lungs is called asthma, a complex and diverse disease characterized by several processes at the cellular, molecular and genetic levels. Mouse models have been used to explore in-depth mechanisms operational in the development of eosinophil-mediated asthma [ 55 , 56 ]. Mouse models of asthma are valuable tools to study the physiological, intact respiratory, and immune systems of the disease [ 57 , 58 ] and provide target molecules for the development of new therapeutic interventions. Similarly, EoE is a disease observed globally over the past two decades in both developed and underdeveloped countries. EoE pathogenies that develop via allergen-induced inflammatory cells, mainly eosinophils, play a crucial role in disease pathogenesis [ 59 ]. Nevertheless, the etiology of EoE is not well understood, because eosinophils have very different functions: In the healthy state, they are involved in parasite excretion, whereas in the disease state, they induce inflammatory responses. EoE differs from gastroesophageal reflux disease (GERD) based on eosinophil accumulation in mucosa, intraepithelial eosinophil levels, and acid suppression response [ 5 , 9 , 28 , 60 ]. In the healthy state, in both humans and mice, eosinophils exist in all segments of the gastrointestinal tract except the esophagus from the prenatal to the adult stage [ 61 ]. Eosinophil accumulation has been observed in several gastrointestinal diseases, collectively known as eosinophilic gastrointestinal disorder (EGID). EGID include celiac disease (CE), eosinophilic colitis (EC), eosinophilic gastroenteritis (EG), and gastroesophageal reflux disease (GERD). EGID is a complex digestive disorder in which eosinophil numbers increase during inflammatory conditions, drug reactions, and malignancy in the GI tract. EGID is caused by genetic factors, poor food sanitization, and often by atopic disease conditions [ 61 ]. Several experimental mouse models have been developed to understand the pathogenesis of EGID. These mouse models mimic human EGID disease characteristics such as induction of allergen-specific IgE, cytokines (IL-4 and IL-5), and intestinal eosinophilia. A case report diagnosed eosinophilic infiltration of the pancreas with no hypereosinophilia or high levels of immunoglobulin E, but the patient was concomitantly diagnosed with ulcerative colitis, indicating that eosinophilic pancreatitis may also be associated with that of gastrointestinal disorders [ 62 ]. However, the eosinophils associated pancreatitis is not yet well established or well recognized and is considered a rare disease. Therefore, the current review is focused mainly on eosinophilic pancreatitis.