Targeted Anti-IL-5 Therapies and Future Therapeutics for Hypereosinophilic Syndrome and Rare Eosinophilic Conditions

Differentiation and regulation of CD4+ T cell subsets in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease, and its hallmark pathological features are the loss of dopaminergic (DA) neurons in the midbrain substantia nigra pars compacta (SNpc) and the accumulation of alpha-synuclein (α-syn). It has been shown that the integrity of the blood-brain barrier (BBB) is damaged in PD patients, and a large number of infiltrating T cells and inflammatory cytokines have been detected in the cerebrospinal fluid (CSF) and brain parenchyma of PD patients and PD animal models, including significant change in the number and proportion of different CD4+ T cell subsets. This suggests that the neuroinflammatory response caused by CD4+ T cells is an important risk factor for the development of PD. Here, we systematically review the differentiation of CD4+ T cell subsets, and focus on describing the functions and mechanisms of different CD4+ T cell subsets and their secreted cytokines in PD. We also summarize the current immunotherapy targeting CD4+ T cells with a view to providing assistance in the diagnosis and treatment of PD.

Post-marketing safety of anti-IL-5 monoclonal antibodies (mAbs): an analysis of the FDA Adverse Event Reporting System (FAERS)

BACKGROUND

Anti-IL-5 monoclonal antibodies (mAbs) targeting IL-5 or IL-5 R α (including mepolizumab, benralizumab, and reslizumab) are widely used for inflammatory diseases such as asthma, eosinophilia, and polyangiitis. However, real-world data regarding its safety in a large sample population are incomplete. So, we evaluated the safety of anti-IL-5 mAbs by pharmacovigilance analyzes based on related adverse events (AEs) from the FDA Adverse Event Reporting System (FAERS).

METHODS

In disproportionality analysis, four algorithms were employed to detect the signals of anti-IL-5 mAbs from the FAERS between 2016 and 2022. In addition, we also used MYSQL 8.0, Navicat Premium 15, and Microsoft EXCEL 2019 to analyze the signals of anti-IL-5 mAbs systematically.

RESULTS

There are 9,476,351 reports collected from the FAERS database, of which 22,174 reports listed anti-IL-5 mAbs as the 'primary suspected (PS)' drug. A total of 59 (20 new signals, mepolizumab) and 62 (19 new signals, benralizumab) significant disproportionality preferred terms (PTs) conforming to the four algorithms were retained synchronously. Finally, we detected that the anti-IL-5 mAbs-induced AEs occurred in 31 organ systems (mepolizumab) and 30 organ systems (benralizumab). For mepolizumab and reslizumab, unexpected and new significant PTs of AEs were found, such as asthmatic crisis, chronic obstructive pulmonary disease (COPD), pneumonia, COVID-19, pneumothorax, adrenal insufficiency and so on. Notably, the risk signal of asthmatic crisis for mepolizumab was stronger than benralizumab (ROR 108.04 [95%CI, 96.09-121.47] vs 26.83 [95%CI, 18.91-38.06]). Comparing with mepolizumab and benralizumab, we found the proportion of serious adverse events in mepolizumab was both greater than benralizumab in each age group (≤20, 20-65, and ≥ 65). The median onset time of mepolizumab was 280 days (interquartile range [IQR] 1-367 days).

CONCLUSION

Analysis of FAERS data identified anti-IL-5 mAbs-associated AEs, and our findings supported continuous clinical monitoring, pharmacovigilance, and further studies of anti-IL-5 mAbs. In addition, clinicians may be more aware of the limitations of use in package inserts of anti-IL-5 mAbs: Not for relief of acute bronchospasm or status asthmaticus. Because of some limitations in the FAERS such as self-reports from patients and other confounding factors, the safety of anti-IL-5 mAbs needed more studies in different dimensions, especially the risk of cancer.

An update on mechanisms of pruritus and their potential treatment in primary cutaneous T-cell lymphoma

Primary cutaneous T-cell lymphomas (CTCL), which include mycosis fungoides (MF) and Sézary syndrome (SS), are a group of lymphoproliferative disorders characterized by clonal accumulation of neoplastic T-lymphocytes in the skin. Severe pruritus, one of the most common and distressing symptoms in primary CTCL, can significantly impair emotional well-being, physical functioning, and interpersonal relationships, thus greatly reducing quality of life. Unfortunately, effectively managing pruritus remains challenging in CTCL patients as the underlying mechanisms are, as of yet, not fully understood. Previous studies investigating the mechanisms of itch in CTCL have identified several mediators and their corresponding antagonists used for treatment. However, a comprehensive overview of the mediators and receptors contributing to pruritus in primary CTCL is lacking in the current literature. Here, we summarize and review the mediators and receptors that may contribute to pruritus in primary CTCL to explore the mechanisms of CTCL pruritus and identify effective therapeutic targets using the PubMed and Web of Science databases. Studies were included if they described itch mediators and receptors in MF and SS. Overall, the available data suggest that proteases (mainly tryptase), and neuropeptides (particularly Substance P) may be of greatest interest. At the receptor level, cytokine receptors, MRGPRs, and TRP channels are most likely important. Future drug development efforts should concentrate on targeting these mediators and receptors for the treatment of CTCL pruritus.

A web server for predicting and scanning of IL-5 inducing peptides using alignment-free and alignment-based method

Interleukin-5 (IL-5) can act as an enticing therapeutic target due to its pivotal role in several eosinophil-mediated diseases. The aim of this study is to develop a model for predicting IL-5 inducing antigenic regions in a protein with high precision. All models in this study have been trained, tested and validated on experimentally validated 1907 IL-5 inducing and 7759 non-IL-5 inducing peptides obtained from IEDB. Our primary analysis indicates that IL-5 inducing peptides are dominated by certain residues like Ile, Asn, and Tyr. It was also observed that binders of a wide range of HLA alleles can induce IL-5. Initially, alignment-based methods have been developed using similarity and motif search. These alignment-based methods provide high precision but poor coverage. In order to overcome this limitation, we explore alignment-free methods which are mainly machine learning-based models. Firstly, models have been developed using binary profiles and eXtreme Gradient Boosting-based model achieved a maximum AUC of 0.59. Secondly, composition-based models have been developed and our dipeptide-based random forest model achieved a maximum AUC of 0.74. Thirdly, random forest model developed using selected 250 dipeptides and achieved AUC 0.75 and MCC 0.29 on validation dataset; best among alignment-free models. In order to improve the performance, we developed an ensemble or hybrid method that combined alignment-based and alignment-free methods. Our hybrid method achieved AUC 0.94 with MCC 0.60 on a validation/independent dataset. The best hybrid model developed in this study has been incorporated into the user-friendly web server and a standalone package named 'IL5pred' (https://webs.iiitd.edu.in/raghava/il5pred/).

Hypereosinophilic Syndrome Following the BNT162b2 (BioNTech/Pfizer) Vaccine Successfully Treated with Mepolizumab: A Case Report and Review of the Literature

Although an increasing number of real-life data confirm large-scale clinical trial findings on the efficacy and safety of SARS-CoV-2 vaccines, rare but severe adverse reactions have begun to emerge. Here, we report a full-blown hypereosinophilic syndrome (HES) following a BNT162b2 (BioNTech/Pfizer) vaccine. A 48-year-old man developed, 5 days after the first shot of the SARS-CoV-2 vaccine, erythematous and painful nodular lesions in the lower and upper limbs accompanied by widespread itching, acrocyanosis with gangrenous lesions at the tips of the first and fourth fingers of the right hand, as well as paresthesia in the right hand and foot. Investigations revealed isolated eosinophilia, occlusion of the right ulnar artery, and electromyography alteration compatible with multifocal sensory neuropathy, as well as minimal accentuation of the interstitial texture with some ground glass appearance. Despite treatment with prednisone in combination with warfarin, he developed thrombosis of the left ulnar artery. Therefore, therapy with an IL-5 inhibitor and acetylsalicylic was successfully added. Given the time interval between the onset of clinical manifestations and the vaccine shot, we believe that the mRNA vaccine triggered the eosinophilic response. This case evidences a possible link between HES and the SARS-CoV-2 vaccination. Mepolizumab, an IL-5 inhibitor, might be considered in steroid refractory cases.

EGPA Phenotyping: Not Only ANCA, but Also Eosinophils

Background: Eosinophilic Granulomatosis with Polyangiitis (EGPA) is a small-vessel necrotizing vasculitis. The anti-neutrophil cytoplasmic antibodies’ (ANCA) role in defining clinical EGPA phenotypes is well established. Although the role of eosinophils in disease pathogenesis has been clearly demonstrated, the value of blood eosinophil count (BEC) as a biomarker of disease phenotypes is currently uncertain. Methods: We retrospectively analyzed EGPA patients referred to our Immunology Clinic. Demographic, laboratory and clinical features were retrieved from clinical records, and a Logistic Regression was fitted to evaluate the predictive power of all baseline clinical and laboratory features to define EGPA phenotypes. Results: 168 patients were recruited. BEC ≤ 1500 cells/mL was predictive of a clinical involvement characterized by asthma, chronic rhinosinusitis with nasal polyps (CRSwNP) and lung opacities (OR 0.18, 95% CI 0.07–0.43; respiratory-limited phenotype); BEC > 3500/mL was predictive of extrapulmonary organ involvement (OR 3.5, 95% CI 1.7–7.1; systemic phenotype). BEC was also predictive of peripheral nervous system (PNS) involvement, with a positive trend with increasing BEC (<1500/mL: OR 0.17, 95%CI, 0.06–0.47; >3500/mL: OR 2.8, 95% CI, 1.5–5.28). ANCA positivity was also predictive of extrapulmonary involvement (OR 4.7, 95% CI 1.9–11.99). Conclusions: according to BEC and irrespective of the ANCA status, two EGPA phenotypes could be identified, named systemic and respiratory-limited phenotypes, with different organ involvement and possibly different prognoses.

Novel targeted inhibition of the IL-5 axis for drug reaction with eosinophilia and systemic symptoms syndrome

Background

The drug reaction with eosinophilia and systemic symptoms (DRESS) syndrome represents a severe hypersensitivity reaction. Up-to-date treatment is based on withdrawal of medication, supportive care, and immunosuppression using high-dose corticosteroid (CS) therapy. However, evidence-based data are lacking regarding second-line therapy for steroid-resistant or steroid-dependent patients.

Objectives

We hypothesize that the interleukin (IL)-5 axis plays a critical role in the pathophysiology of DRESS; hence, inhibition of this signaling pathway could offer a potential therapy for steroid-dependent and/or steroid-resistant cases, and it may offer an alternative to CS therapy in certain patients more prone to CS toxicity.

Methods

Herein, we collected worldwide data on DRESS cases treated with biological agents targeting the IL-5 axis. We reviewed all cases indexed in PubMed up to October 2022 and performed a total analysis including our center experience with two additional novel cases.

Results

A review of the literature yielded 14 patients with DRESS who were treated with biological agents targeting the IL-5 axis as well as our two new cases. Reported patients are characterized by a female-to-male ratio of 1:1 and a mean age of 51.8 (17-87) years. The DRESS-inducing drugs, as expected from the prospective RegiSCAR study, were mostly antibiotics (7/16), as follows: vancomycin, trimethoprim-sulfamethoxazole, ciprofloxacin, piperacillin-tazobactam, and cefepime. DRESS patients were treated with anti-IL-5 agents (mepolizumab and reslizumab) or anti-IL-5 receptor (IL-5R) biologics (benralizumab). All patients have clinically improved under anti-IL-5/IL-5R biologics. Multiple doses of mepolizumab were needed to achieve clinical resolution, whereas a single dose of benralizumab was often sufficient. Relapse was noted in one patient receiving benralizumab treatment. One patient receiving benralizumab had a fatal outcome, although mortality was probably related to massive bleeding and cardiac arrest due to coronavirus disease 2019 (COVID-19) infection.

Conclusion

Current treatment guidelines for DRESS are based on case reports and expert opinion. Understanding the central role of eosinophils in DRESS pathogenicity emphasizes the need for future implementation of IL-5 axis blockade as steroid-sparing agents, potential therapy to steroid-resistant cases, and perhaps an alternative to CS treatment in certain DRESS patients more prone to CS toxicity.

Wells Syndrome Presenting as Atypical Periorbital Cellulitis

A 62-year-old man presented with diffuse, painless, left-sided preseptal edema, erythema, and woody induration extending to the left temple. The induration generated an orbital compartment syndrome with markedly elevated intraocular pressure necessitating lateral canthotomy and cantholysis. Although atypical for an infectious etiology, empiric broad-spectrum intravenous antibiotics were initiated with no improvement. A tissue biopsy demonstrated extensive perivascular and interstitial eosinophils with focal flame figures, and the patient was diagnosed with a severe hypersensitivity reaction or eosinophilic cellulitis (Wells syndrome). The disease process remitted rapidly upon initiation of oral prednisone. Wells syndrome is a rare inflammatory eosinophilic dermatosis, most often presenting in the limbs and trunk, with few reports of facial and periorbital involvement. This case highlights the importance of considering Wells syndrome in the differential diagnosis of atypical periorbital cellulitis that is nonresponsive to antibiotics and reviews the clinicopathologic nature of this disease.

Cyclosporine A increases the intensity of Toxocara canis infection in swiss mice

Toxocariasis is a zoonotic disease of worldwide distribution. The connection between parasitic diseases and conditions that depress the immune system, such as the use of immunosuppressive drugs, has been studied. The purpose of this study was to evaluate the effect of Cyclosporine A (CsA) on the intensity of infection, humoral response and gene transcription of interleukins IL-4, IL-10 and IL-12 in mice experimentally infected with Toxocara canis. To this end, mice were divided into two groups treated with CsA (G1: 10 mg/Kg and G2: 50 mg/kg), the G3 and G4 group received PBS. After the last administration of the drug or PBS (orally every 48 hours for 15 days), groups G1, G2 and G3 were inoculated with 1200 eggs of T. canis. Was collected blood samples on days zero, 15 and 30 days post-inoculation (PI), for ELISA test and the mice were euthanized 30 days PI. The organs and striated muscle tissue were collected for the recovery of larvae. The splenocytes were analyzed by RT-PCR. The intensity of infection in the mice treated with 50 mg/kg of CsA was 65.5% higher than in the control group (p=0.001). An analysis of the kinetics of anti-Toxocara antibody revealed that the groups treated with CsA showed significantly higher mean levels of antibodies on day 15 PI. The transcription of the three tested interleukins showed no statistical difference between G2 and G3 (control). It was concluded that the immunosuppression triggered by CsA (50 mg/Kg) favored the establishment of a larger number of T. canis larvae without, however, altering immunoglobulin production and IL-4, IL-10 and IL-12 transcription on day 30 PI.

Monoclonal antibodies used for the management of haematological disorders

INTRODUCTION

Monoclonal antibodies Ab (MoAb) are increasingly becoming part of therapeutic armamentarium for haematologists and haemato-oncologists. This review brings together commonly used antibodies in one place for brevity and novel understanding.

AREAS COVERED

Pubmed and Scopus databases were explored focusing on MoAb in clinical haematological practice. Emphasis was given to current review articles. The data base was searched from 1997 till present. 24 different antibodies, most of which are in use were discussed. Antibodies are used for diverse conditions i.e. malignant and benign haematological conditions, treatment at various phases of stem cell transplantation. These antibodies were used both alone or in combination with various chemotherapy, targeted small molecules or as immunoconjugates. Some of the side effect profiles of these antibodies were common and some were unique. Unusual infections or organ dysfunctions were noted. Improved function of antibodies by protein engineering is also advancing rapidly. Dosage, frequency and route of administration depended on the convenience and condition for which the antibody is used.

EXPERT OPINION

: MoAbs are increasingly used in haematology practice either alone or in combination with other types of therapy for improved out come in various haematological conditions.

Modulation of surface CD11c expression tracks plasticity in murine intestinal tissue eosinophils

Intestinal eosinophils are implicated in the inflammatory pathology of eosinophilic gastrointestinal diseases and inflammatory bowel diseases. Eosinophils also contribute to intestinal immunologic and tissue homeostasis and host defense. Recent studies in allergic airway disease suggest functional subphenotypes of eosinophils may underly their pathogenic versus protective roles. However, subphenotypes of intestinal eosinophils have not been defined and are complicated by their constitutive expression of the putative eosinophil inflammatory marker CD11c. Here, we propose a framework for subphenotype characterization of intestinal eosinophils based on relative intensity of surface CD11c expression. Using this flow cytometry framework in parallel with histology and BrdU tracing, we characterize intestinal eosinophil subphenotypes and monitor their plasticity at baseline and within the context of acute allergic and chronic systemic inflammation. Data reveal a conserved continuum of CD11c expression amongst intestinal eosinophils in health and acute disease states that overall tracked with other markers of activation. Oral allergen challenge induced recruitment of eosinophils into small intestinal lamina propria surrounding crypts, followed by in situ induction of CD11c expression in parallel with eosinophil redistribution into intestinal villi. Allergen challenge also elicited eosinophil transepithelial migration and the appearance of CD11c^lo CD11b^hi eosinophils in the intestinal lumen. Chronic inflammation driven by overexpression of TNFα led to a qualitative shift in the relative abundance of CD11c-defined eosinophil subphenotypes favoring CD11c^hi -expressing eosinophils. These findings provide new insights into heterogeneity of intestinal tissue eosinophils and offer a framework for measuring and tracking eosinophil subphenotype versatility in situ in health and disease.

Efficacy of Endoscopic Posterior Nasal Neurectomy in Treating Eosinophilic Chronic Rhinosinusitis

OBJECTIVES

Recent guidelines have revealed that eosinophilic chronic rhinosinusitis (ECRS) exhibits a strong tendency for recurrence after surgery and impairs quality of life. Neuropeptides play an important neuroimmunological role. The aim of this study was to determine the efficacy of posterior nasal neurectomy (PNN) for the treatment of ECRS by inhibiting type 2 cytokine expression.

METHODS

Forty-six patients were divided into group A and group B according to a random number table. Group A underwent conventional functional endoscopic sinusitis surgery (FESS) combined with PNN, and group B underwent conventional FESS alone. The subjective and objective symptoms included a 10-cm visual analog scale (VAS), 22-item SinoNasal Outcome Test (SNOT-22) score, nasal speculum Lund-Kennedy score, and paranasal sinus computed tomography (CT) Lund-Mackay score at the 1-year postoperative follow-up.

RESULTS

Postoperative VAS (10.33 ± 2.18 vs. 8.38 ± 2.11, p < 0.01) and Lund-Kennedy score (1.95 ± 1.32 vs. 3.14 ± 1.35, p < 0.01) were significantly improved. The rhinorrhea score (1.76 ± 0.83 vs. 2.90 ± 1.14, p < 0.001) in the VAS and the discharge (0.43 ± 0.51, vs. 0.95 ± 0.67, p < 0.01) and edema (0.57 ± 0.60 vs. 0.95 ± 0.59, p < 0.05) scores in the Lund-Kennedy score were observed to have improved significantly in group A compared with those in group B.

CONCLUSIONS

FESS combined with PNN suppresses edema symptoms, which might significantly decrease the surgical recurrence rate of ECRS in the long term.

Development of Rheumatoid Arthritis During Anti-Interleukin-5 Therapy in a Patient with Refractory Chronic Eosinophilic Pneumonia

Purpose

To report the case of a patient with refractory chronic eosinophilic pneumonia who developed rheumatoid arthritis during anti-interleukin (IL)-5 therapy.

Case Report

The case of a 66-year-old male ex-smoker with allergic rhinitis who had dyspnea and chronic cough for 6 months and who was ultimately diagnosed with chronic eosinophilic pneumonia is reported. Long-term corticosteroid therapy was necessary due to recurrence of the chronic eosinophilic pneumonia during tapering of the corticosteroid. As a steroid sparing strategy, mepolizumab was initiated, and the steroid was tapered gradually. When the dose of prednisolone was 2 mg/day, he developed polyarthralgia. Mepolizumab was changed to benralizumab considering the possibility that arthralgia was a side effect of mepolizumab; however, the arthralgia continued and he was ultimately diagnosed with rheumatoid arthritis. Methotrexate was initiated and his arthritis improved. Thereafter, benralizumab was discontinued after 5 injections, and he subsequently required neither systemic corticosteroids nor biologics.

Conclusion

The present case may suggest that suppression of IL-5 induces rheumatoid arthritis in certain patients; however, it is also possible that initial steroid therapy improved subclinical RA and made it remain undiagnosed, and the parallel OCS tapering during IL-5 therapy could have contributed to unveil the underlying RA. Further studies are required to establish guidelines on the optimum use of anti-IL-5 therapy and to understand the interactions between chronic eosinophilic pneumonia, anti-IL-5 therapy, tapering of corticosteroid and development of rheumatoid arthritis.

Effective Management of Severe Asthma with Biologic Medications in Adult Patients: A Literature Review and International Expert Opinion

Severe asthma often remains uncontrolled despite effective treatments and evidence-based guidelines. A group of global experts in asthma and biologic medications from 9 countries considered the most relevant clinical variables to manage severe asthma in adult patients and guide treatment choice. The resulting recommendations address the investigation of biomarker levels (blood eosinophil count along with fractional concentration of exhaled nitric oxide [FeNO]), clinical features (oral corticosteroid [OCS] dependence, specific comorbid disease entities associated with severe type 2 asthma), and safety considerations. Current evidence suggests that biomarkers, including both blood or sputum eosinophil counts as well as FeNO, add prognostic and predictive value and should be measured in all patients with severe asthma. OCS use is an important factor in biologic selection, especially given the documented ability of some biologics to reduce OCS dependence. Comorbid diseases and relevant safety considerations to each biologic should also be considered. More data are needed to determine whether biomarker profiles identify patients suited to one biologic versus another as limited data support differential predictors of response. Further prospective head-to-head trials and post hoc analyses of clinical trial data are warranted. The authors believe that these recommendations have value as they offer expert opinion to assist health care providers in making difficult decisions regarding the quality of care in severe, type 2 asthma with biologic medications. They remain conditional and are based on limited data owing to a lack of head-to-head comparisons.

Eosinophilic esophagitis, Barrett's esophagus and esophageal neoplasms in the pediatric patient: a narrative review

There are several esophageal disorders that can occur in the pediatric population. Eosinophilic esophagitis (EoE) is an eosinophil predominant inflammatory disease of the esophagus that was first characterized in the early 1900's. EoE is the most common pediatric esophageal inflammatory condition after gastroesophageal reflux disease (GERD). Longstanding GERD is a known risk factor for the development of Barrett's esophagus (BE) in both children and adults. BE is associated with the development of dysplasia and, if left undiagnosed, may progress to the development of esophageal adenocarcinoma (EAC). EAC and esophageal squamous cell carcinoma (ESCC) comprise the majority of childhood esophageal malignant neoplasms. The prevalence of EoE continues to rise within the pediatric population. On the other hand, both BE and esophageal neoplasms remain extremely rare in children. The relationship between a chronic inflammatory condition like EoE to BE and/or esophageal neoplasms remains unclear. The current research of these disease entities is prioritized to further understanding the disease pathogenesis and disease progression, exploring new diagnostic modalities, and developing novel treatments or less invasive therapeutic options. The focus of the following narrative review is to provide a summary of the current clinical practices, future research and their implications on these various esophageal disorders.

Cost-Effectiveness of Biologics for Allergic Diseases

The introduction of specific humanized monoclonal antibodies over the past 20 years has dramatically changed the treatment of allergic diseases. At present, 5 mAbs are licensed for treating moderate to severe allergic and eosinophilic asthma, atopic dermatitis, chronic spontaneous urticaria, chronic sinusitis with nasal polyps, and eosinophilic granulomatosis with polyangiitis. Given the high costs of biologics, understanding their cost-effectiveness is critical. As new biologics are developed and new indications are approved for existing biologics, the use of biologics for allergic diseases will increase. Conducting cost-effectiveness evaluations in parallel to efficacy and effectiveness trials will help patients, providers, payers, and policymakers in decision making.

Driver Mutations in Leukemia Promote Disease Pathogenesis through a Combination of Cell-Autonomous and Niche Modulation

Studies of patients with acute myeloid leukemia (AML) have led to the identification of mutations that affect different cellular pathways. Some of these have been classified as preleukemic, and a stepwise evolution program whereby cells acquire additional mutations has been proposed in the development of AML. How the timing of acquisition of these mutations and their impact on transformation and the bone marrow (BM) microenvironment occurs has only recently begun to be investigated. We show that constitutive and early loss of the epigenetic regulator, TET2, when combined with constitutive activation of FLT3, results in transformation of chronic myelomonocytic leukemia-like or myeloproliferative neoplasm-like phenotype to AML, which is more pronounced in double-mutant mice relative to mice carrying mutations in single genes. Furthermore, we show that in preleukemic and leukemic mice there are alterations in the BM niche and secreted cytokines, which creates a permissive environment for the growth of mutation-bearing cells relative to normal cells.

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Ubiquitous loss of Tet2 followed by expression of Flt3^ITD/ITD results in lethal AML

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Tet2^−/− cells when exposed to leukemic environment manifest MPN-like features

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Hyperproliferation of Flt3^ITD donor cells in preleukemic Tet2^−/− microenvironment