Hypereosinophilia in Summary

Abstract

Eosinophils are white blood cells. They are found in various cellular arrays. Eosinophils play a role in the fight against many parasitic infections. Eosinophilic asthma, nasal polyps, eosinophilic gastrointestinal disorders, polyangiitis, and eosinophilic granulomatosis are diseases referring hypereosinophilic syndrome. Eosinophil granules participate in tissue healing, damage, repair and restructuring processes thanks to proteins and chemical mediators. Interleukin (IL)-5, IL-4, and IL-13′ play a role in the proliferation, maturation, activation, and recruitment of eosinophils. Eosinophils have receptors for various cytokines, chemokines, and adhesion molecules that allow them to participate in inflammatory activities. In response to stimuli, eosinophils may release a range of granule proteins, including major basic proteins (MBPs) 1–2, eosinophil cationic protein (ECP), eosinophil peroxidase (EPX), eosinophil-derived neurotoxin (EDN), cytokines, and cytosolic Charcot-Leyden crystal protein/ galectin-10 (CLC/Gal-10). Eosinophils participate in a variety of biological processes and contribute to both normal and pathological processes. Improvements can be made in understanding the pathophysiological mechanisms of these diseases. It has led to the development of new therapeutics for eosinophilic inflammatory diseases.

1. Introduction

Eosinophils are cells, often living in tissues, whose function is not fully understood. Eosinophils in peripheral blood or tissues can be increased in a wide sequence in severity from mild to life-threatening, and as a result of various mechanisms. When activated, eosinophils can secrete mediators and substances that can damage tissues and contribute to disease pathology (Figure 1).

• Eosinopenia, a decrease in eosinophil blood count

• Eosinophilia, an increase (>500 cells per microliter) in eosinophil blood count. Eosinophils generally constitute less than 7% of circulating leukocytes [1]. A significant increase in the number of non-blood tissue eosinophils in histopathological examination is diagnostic of tissue eosinophilia [2]. Various causes are known; the most common is some type of allergic reaction or parasitic infection. Diagnosis of eosinophilia is made through complete blood count (CBC), but diagnostic procedures for the underlying cause vary depending on the suspected conditions. If the complete blood count shows significant eosinophilia, an absolute eosinophil count is usually not necessary [3]. The location of the causal factor can be used to classify eosinophilia into two general types: extrinsic, in which the factor is located outside the eosinophil cell lineage, and intrinsic eosinophilia, which indicates etiologies within the eosinophil cell lineage [2]. Specific treatments are determined by the causative condition, but in idiopathic eosinophilia, the disease can be controlled with corticosteroids.

• Hypereosinophilia is characterized by an extreme increase (>1500 cells per microliter) in eosinophil blood count. Hypereosinophilic syndrome (HES) due to overproduction of eosinophils is a disturbance characterized by an increased number of eosinophils in the blood and tissues. Sometimes, eosinophilic infiltration and released mediators may cause damage to end organs. The term HES was coined in 1968 and was introduced by Hardy and Anderson [1, 2, 3, 4]. Diagnostic criteria for idiopathic HES were developed by Chusid in 1975. These criteria are as follows:

  1. Eosinophilia in peripheral blood is below (>1500/μL) lasting more than a month

  2. There is no other cause of eosinophilia

  3. End-organ damage or dysfunction

Figure 1.

Functions of eosinophils.

Clonal hypereosinophilia, also called primary hypereosinophilia or clonal eosinophilia, is a group of hematological disorders characterized by the development and growth of a premalignant or malignant population of eosinophils, a type of white blood cell that invades the bone marrow. This population consists of a group of genetically identical eosinophils derived from an eosinophil clone, that is, a sufficiently mutated progenitor cell [1]. The eosinophil clone carries a mutation in any of several genes that encode proteins that regulate cell growth. Mutations cause these proteins to be constantly active, thus stimulating growth in an uncontrolled and continuous manner. The gradually increasing population of eosinophils, which initially forms in the bone marrow, spreads into the blood and can enter various tissues and organs and damage them [1]. Clinically, clonal eosinophilia resembles various types of chronic or acute leukemia, lymphoma, or myeloproliferative hematological malignancy. However, most clonal hypereosinophilias are distinguished from these other hematological malignancies by the genetic mutations underlying their development and, more importantly, by their sensitivity to specific treatment regimens. That is, many types of these disorders are highly sensitive to relatively nontoxic drugs [1, 2].

HES is a heterogeneous hematological disorder. In patient groups with different clinical findings, malignancies belonging to the myeloid or lymphoid series may develop. The development of these disorders suggests that there may be different underlying pathologies. HES can affect both myeloid and lymphoid series. Eosinophils belong to the myeloid series. Clonal eosinophilic differentiation may occur from the myeloid early differentiation stage or from the T-cell series that undergo abnormal differentiation in the T-cell stage. Although demonstrating eosinophil clonality is not easy, it has been revealed recently. An interstitial deletion on chromosome 4q12 leads to the fusion of the FIP1L1 and PDGFRα genes (Fip1-like 1-platelet-derived growth factor receptor alpha). Eosinophil growth factor and apoptosis inhibitor cytokines that cause inflammation are GM-CSF (granulocyte-macrophage colony-stimulating factor), IL-3, and IL-5 [3, 4, 5].

Eosinophils are white blood cells (WBCs) of the granulocytic lineage, which also includes neutrophils and basophils. The physiological functions of eosinophils are poorly understood, but they are involved in the host immune response to infection, tissue remodeling, tumor surveillance, and maintenance of other immune cells [1, 2]. Eosinophils develop and differentiate in the bone marrow under the influence of interleukin (IL) 5, IL-3, and GM-CSF. There may be different subsets of eosinophils that play different roles in inflammation and homeostasis [3, 4].

Pulmonary involvement is mainly in the form of chronic dry cough, and infiltration can be seen on chest radiography in <25% of cases. Other target organs where eosinophilic infiltration may be present include the GI tract, liver, spleen, joints, and kidneys. Clinical findings also vary according to involvement. Coagulation system peripheral thromboembolism endovascular damage by eosinophils and peripheral vasculopathy may occur. HES is more common in men than in women (9:1). It is most common between 20 and 50 years of age. Clinical findings vary from patient to patient depending on end-organ involvement. It often has a silent onset, and eosinophilia may be detected incidentally. In some cases, the initial findings may be severe and life-threatening with rapid progression of cardiac and neurologic complications [4, 5, 6, 7]. Involvement of the heart, skin, nervous system, lung, and spleen is present in approximately 45–60% of cases. Liver, eye, and GIS involvement is less (20–30%). Directly and independently acts on beige adipocytes via the release of enkephalin peptides. Eosinophils, directly and indirectly, cause blood vessel relaxation in perivascular adipose tissue through adiponectin and catecholamine release, respectively. The catecholamines signal through β3-adrenergic receptors (β3-AR) on adipocytes to cause vessel relaxation via nitric oxide (NO) and adiponectin [5, 6, 7, 8].

Eosinophilic myocarditis in cardiac involvement is the main cause of mortality and morbidity. Heart damage develops in three stages. The early necrotic stage is rarely symptomatic. This stage is followed by the thrombotic stage with thrombus formation in the damaged endocardium and emboli emanating from it. In the final fibrotic stage, endomyocardial fibrosis and damage to the atrioventricular valves lead to congestive heart failure. Skin manifestations are usually urticaria and angioedema or erythematous, pruritic papules, and nodules. Treatment-resistant mucosal ulcers may also be seen. In neurological complications, the central (encephalopathy) and peripheral nervous system (polyneuropathy) may be affected [6, 8]. The clinical features of different HES subgroups differ. PDGFRα-positive HES occurs almost exclusively in men, often in the heart involvement, and untreated cases have a high mortality rate. In contrast, L-HES does not differ between genders and skin, is prone to soft tissue involvement, and has a rather silent course. Diagnosis in M-HES cases is acute leukemia (eosinophilicor AML), whereas in L-HES cases, there is a risk of developing T-cell lymphoma over a long period [9, 10, 11].

2. Diagnosis and laboratory features

Eosinophil count in peripheral blood is >1500/μL. They are usually mature in appearance and rarely can be seen in young cells. White blood cell count >90,000/μL is an indicator of poor prognosis. Anemia may be observed in 50% of cases. Thrombocytosis or thrombocytopenia may be seen. There may be a significant increase in eosinophils and eosinophil precursors in the bone marrow. Chromosome examination is normal in the majority of cases [9, 10, 11, 12].

The diagnosis of HES is based on the exclusion of causes of reactive eosinophilia, according to WHO criteria, as reactive causes, infections, hypersensitivity and allergic diseases, immune system disorders, metabolic abnormalities, connective tissue, heart, respiratory system, skin, and GIS diseases. After excluding the causes of eosinophilia, then, at least twice, eosinophil count >1500/μLsymptomatic cases, even in the absence of symptoms should be examined for possible HES. ECG, echocardiography, and respiratory tests should be performed as baseline tests to evaluate end-organ involvement. Chest radiography, and clinical indication, if present, a tissue biopsy should be performed [9, 10, 11, 12, 13].

In the third step, the potential downstream of the HES bone groups for the determination of bone cytogenetic examination of marrow, reticulin fibrosis, and special staining for mast cells, lymphadenopathy, and computed tomography of the lung, abdomen, and pelvis should be performed to look for splenomegaly. To these, whether HES is of myeloid or lymphoid origin to establish that it is not. Clinical based on the findings; without cardiac involvement, prominent skin manifestations, high serum IgE levels, or L-HES in patients with hypergammaglobulinemia should be considered [1, 8, 9, 10, 11, 12]. Splenomegaly, cardiac involvement, elevated vitamin B12 levels, anemia or thrombocytopenia, and precursor myeloid cells in peripheral blood M-HES should be considered. However, these features are not specific to HES variants, and further testing is needed. This FIP1LI-PDGFRα fusion by FISH or PCR, lymphocyte phenotyping, and T-cell receptor gene reconstitution needs to be demonstrated. Demonstration of T-cell clonal dysfunction for abnormal lymphocyte immunophenotyping, detection of T-cell subsets (CD3- CD4+ and CD3 + CD4- CD8-), or investigation of TCR gene reconstitution by PCR is sufficient. However, in some patients, the etiology is undetectable. This is still a major factor in the pathogenesis of HES [12, 13, 14].

Some tests may also help to differentiate between M-HES and L-HES. These include the detection of eosinophilopoietic cytokines (IL-3 and GM-CSF) of the T-cell subset and Th2 (IL-4, IL-5, IL-13) cytokines, and serum tryptase, and measurement of TARC levels is a conventional cytogenetic examination. However, in general, IL-5 levels have no value in classification. High serum tryptase levels or atypical mast cells in bone marrow biopsy cases should be examined for the 816 V ckit mutation for systemic mastocytosis. Conditions that can cause eosinophilia in peripheral blood and tissue may mimic HES. Known etiology eosinophilic diseases (such as helminthic diseases) and those of unknown etiology (Wells syndrome or eosinophilic cellulitis) should be differentiated from HES. One eosinophilic syndrome limited to the organ (eosinophilic pneumonia or eosinophilic gastroenteritis) usually does not show the multiorgan involvement characteristic of HES. Chronic eosinophilic leukemia (CEL), acute eosinophilic leukemia, Churg-Strauss syndrome and systemic mastocytosis with episodic angioedema can be difficult to distinguish from HES [12, 13, 14, 15].

Since excessive eosinophil activity can cause end organ damage, the aim of treatment is to reduce eosinophilia. Regardless of the cause and the number of eosinophils (especially in symptomatic patients), the secondary aim is to reduce the damage or the correction. The onset of end-organ involvement is silent and does not always correlate with the degree of eosinophilia. Therefore, monitoring eosinophil count is not sufficient for successful treatment evaluation. Both treatment and follow-up of symptom-free cases, organs, and functions likely to be affected should be monitored. Echocardiography at six-month intervals, pulmonary function tests, and additional tests should be performed. In addition, the underlying disease should be assessed, allowing different treatment approaches to be applied if the disease changes its form [13, 14, 15, 16].

Asymptomatic cases with eosinophilia are benign and do not require treatment. In contrast, M-HES, especially in the presence of FIP1L1/PDGFRα mutation, has a highly aggressive course and is fatal without treatment. Therefore, the treatment decision should be based on the patient’s clinical and laboratory findings and the results of mutation analysis. For suppression of eosinophilia in FIP1L1/PDGFRa-negative diseases, prednisone (60 mg/day or 1 mg/kg/day) is initially replaced for 1–5 days. This information may be useful if the patient develops rapidly progressive organ involvement requiring treatment. In addition, glucocorticoid response is usually indicative of a good prognosis. Glucocorticoids are the first-line treatment in symptomatic patients or those with end-organ damage. Prednisone 1 mg/kg/day (60 mg/day) may be used in initial treatment. In the majority of cases, it leads to a rapid improvement in eosinophilia and organ infiltration, which leads to the arrest of the inflammatory event and prevents further organ damage. If eosinophilia is suppressed, the dose is slowly reduced and treatment is continued with the lowest dose every other day that controls eosinophilia [14, 15, 16, 17].

Hydroxyurea and other chemotherapeutic drugs: hydroxyurea is the drug of choice in steroid-resistant cases, especially in cases of M-HES or CEL. It effectively reduces eosinophil count, improves organ infiltration and hepatosplenomegaly, and delays end-organ damage. An initial dose of 1–2 g is sufficient. The maintenance dose is adjusted according to myelosuppression. Chlorambucil, vincristine, cyclophosphamide, etoposide, 2-chlorodeoxyadenosine, and cytosine arabinoside have also been used in hydroxyurea-resistant, myeloproliferative, and end-organ damage cases. The aim of treatment is the control of organ damage rather than the elimination of eosinophilia. Cyclosporine A: L-HES is characterized by the proliferation of immunophenotypically abnormal T cells. The release of growth factor by these cells, which stimulates eosinophil-formation, is thought to be responsible for eosinophilia. Some of these cases may respond to T-cell suppressive drugs such as cyclosporin or cladribine. Interferon-alpha: efficacy in some case groups has been reported. It can be used as an initial dose of 1–5 million units/m2/day or 5 days a week. The dose can be adjusted according to patient tolerance. The mechanism of action may be based on suppression of eosinophil differentiation and proliferation. However, in two patients with clonal T cells, IFNa has been shown in vitro to inhibit spontaneous apoptosis of clonal Th2 cells [15, 16, 17, 18].

Therefore, its use alone in L-HES should be avoided, as it could theoretically lead to the growth of an abnormal clonal community. In addition, in M-HES, imatinib is less toxic than IFNa. A trial of imatinib should be performed before using IFNa. Except for these two patient groups, IFNa is the drug of choice in corticosteroid-refractory patients. Tyrosine kinase inhibitors: the success of the tyrosine kinase inhibitor imatinib in HES led to a search for its target in HES. As a result, the FIP1L1/PDGFRα fusion on chromosome 4 and its product tyrosine kinase were identified. The activity of this mutation can be suppressed with imatinib [16, 17, 18].

All HES and CEL patients with FIP1L1/PDGFRA mutations respond to imatinib regardless of disease stage. Clinical, hematologic, and molecular remission is achieved in the majority of patients. Clinical symptoms improve rapidly and eosinophil counts normalize within 1–2 weeks. No improvement is expected in end-organ damage, but it may halt progression. Significant improvement in bone marrow fibrosis is promising. In imatinib treatment, there is a risk of left ventricular dysfunction and cardiogenic shock in patients with cardiac involvement. This has been reported to be reversible with systemic glucocorticoids, intensive support, and discontinuation of imatinib. In patients who are found to have cardiac disease by echocardiography and evaluation of serum troponin levels, it is appropriate to use systemic glucocorticoids (1–2 mg/kg/day) for 1–2 weeks when imatinib is started [15, 16, 17]. Some patients without FIP1L1/PDGFRα mutation also responded to imatinib. This suggests that there may be undetected mutations or other targets of imatinib in patients with response. In some patients with no response, the mutation may affect a different PDGFRα fusion partner. As there is currently no test for different mutations, imatinib therapy should also be evaluated in patients with HES with evidence of myeloproliferative disease. Imatinib can also be tried in cases where standard therapy has failed. The appropriate dose and duration of imatinib use have not been established. In patients with FIP1L1/PDGFRα mutation, although it can control clinical symptoms and eosinophilia at a dose as low as 100 mg, it is often higher in patients without mutation. In patients with FIP1L1/PDGFRα mutation, a dose as low as 100 mg can control clinical symptoms and eosinophilia, but higher doses are often required in patients without the mutation. In addition, at low doses, molecular signs of the mutation may persist. According to information from CML, clinical relapse is more frequent if the disease persists molecularly. Therefore, starting treatment at a high dose (400 mg/day) may be a wiser approach [16, 17, 18, 19, 20].

In addition, it can be started at 100 mg/day and monitored by fusion gene PCR. If there is no complete response within four weeks, the dose may be increased. In cases where a complete response is achieved but the disease is still present by PCR after 6–12 months, the dose may be increased according to tolerance. Some studies have shown that imatinib is not effective in eliminating early progenitor cells in CML. If this information is adapted to HES, there will be a need for lifelong imatinib use. Serum tryptase level is an indicator of the presence of the FIP1L1/PDGFRα mutation. It can be useful when genetic testing is not possible. Although primary resistance has not been described to date, there have been a few cases of acquired resistance to imatinib based on a single base change (T6741). Clinical trials are ongoing with some new drugs for the treatment of acquired imatinib resistance. Stem cell transplantation is another treatment modality that can be used in imatinib-resistant cases [18, 19, 20].

Anti-IL-5(anti-interleukin-5): anti-IL-5 antibodies (Mepolizumab, SCH55700) have been used in small patient series, and positive results have been reported. No side effects have been reported, and studies with larger patient groups are ongoing. Anti-CD52: eosinophils carry CD52 on their surface. Although response was obtained in treatment-resistant and FIP1L1/PDGFRα-negative cases with alemtuzumab, the disease recurred in some cases following drug discontinuation. Stem cell transplantation: stem cell transplantation, including a reduced regimen, has been used in the treatment of HES in a limited number of cases. It may be useful in treatment-resistant, especially imatinib-resistant cases [18, 19, 20, 21]. Findings that may indicate a good prognosis include the following:

• Prolonged eosinopenia on corticosteroid administration

• Elevated serum IgE levels

• Absence of signs of myeloproliferative disease [20, 21, 22, 23, 24]

3. Conclusion

Knowledge about the function of eosinophils in health and disease is increasing. Besides their original definition as cytotoxic effector cells, eosinophils are now well-known to be involved in host protection. Eosinophils, immune regulation, priority due to their role in homeostasis and the pathogenesis of allergic diseases may be therapeutic targets. In nonallergic inflammatory diseases, eosinophils represent a breakthrough and are a therapeutic target. Accordingly, eosinophil number and levels are used as biomarkers of many eosinophil-related mediators. Additionally, the prevalence of eosinophils in the tumor microenvironment is associated with outcomes of different types of cancer, and so, promising research continues. Glucocorticoids and monoclonal antibodies targeting therapy mediators associated with eosinophilic inflammation such as IL-4, IL-13, and IL-5 may be a therapeutic target for treatments aimed at reducing eosinophil counts and relieving pain. Emerging evidence suggests that eosinophils have high plasticity potential. Development of new diagnostic and treatment strategies for patients with eosinophilia remains an important requirement.

Abbreviations

Anti-IL-5	anti-IL-5 antibodies mepolizumab, SCH55700
β3-AR	β3-adrenergic receptors
CLC/Gal-10	cytokines, cytosolic Charcot-Leyden crystal protein/ galectin-10
ECP	eosinophil cationic protein
EPX	eosinophil peroxidase
FIP1L1-PDGFRA	Fip1-like 1-platelet-derived growth factor receptor alpha
GM-CSF	granulocyte-macrophage colony-stimulating factor
HES	hypereosinophilic syndrome
IFN	interferon
(MBPs)1-2	major basic proteins
NO	nitric oxide