Low Responsiveness of Basophils via FcεRI Reflects Disease Activity in Chronic Spontaneous Urticaria

BTK signaling-a crucial link in the pathophysiology of chronic spontaneous urticaria

Chronic spontaneous urticaria (CSU) is an inflammatory skin disorder that manifests with itchy wheals, angioedema, or both for more than 6 weeks. Mast cells and basophils are the key pathogenic drivers of CSU; their activation results in histamine and cytokine release with subsequent dermal inflammation. Two overlapping mechanisms of mast cell and basophil activation have been proposed in CSU: type I autoimmunity, also called autoallergy, which is mediated via IgE against various autoallergens, and type IIb autoimmunity, which is mediated predominantly via IgG directed against the IgE receptor FcεRI or FcεRI-bound IgE. Both mechanisms involve cross-linking of FcεRI and activation of downstream signaling pathways, and they may co-occur in the same patient. In addition, B-cell receptor signaling has been postulated to play a key role in CSU by generating autoreactive B cells and autoantibody production. A cornerstone of FcεRI and B-cell receptor signaling is Bruton tyrosine kinase (BTK), making BTK inhibition a clear therapeutic target in CSU. The potential application of early-generation BTK inhibitors, including ibrutinib, in allergic and autoimmune diseases is limited owing to their unfavorable benefit-risk profile. However, novel BTK inhibitors with improved selectivity and safety profiles have been developed and are under clinical investigation in autoimmune diseases, including CSU. In phase 2 trials, the BTK inhibitors remibrutinib and fenebrutinib have demonstrated rapid and sustained improvements in CSU disease activity. With phase 3 studies of remibrutinib ongoing, it is hoped that BTK inhibitors will present an effective, well-tolerated option for patients with antihistamine-refractory CSU, a phenotype that presents a considerable clinical challenge.

Basophils from allergy to cancer

Human basophils, first identified over 140 years ago, account for just 0.5-1% of circulating leukocytes. While this scarcity long hampered basophil studies, innovations during the past 30 years, beginning with their isolation and more recently in the development of mouse models, have markedly advanced our understanding of these cells. Although dissimilarities between human and mouse basophils persist, the overall findings highlight the growing importance of these cells in health and disease. Indeed, studies continue to support basophils as key participants in IgE-mediated reactions, where they infiltrate inflammatory lesions, release pro-inflammatory mediators (histamine, leukotriene C₄: LTC₄) and regulatory cytokines (IL-4, IL-13) central to the pathogenesis of allergic diseases. Studies now report basophils infiltrating various human cancers where they play diverse roles, either promoting or hampering tumorigenesis. Likewise, this activity bears remarkable similarity to the mounting evidence that basophils facilitate wound healing. In fact, both activities appear linked to the capacity of basophils to secrete IL-4/IL-13, with these cytokines polarizing macrophages toward the M2 phenotype. Basophils also secrete several angiogenic factors (vascular endothelial growth factor: VEGF-A, amphiregulin) consistent with these activities. In this review, we feature these newfound properties with the goal of unraveling the increasing importance of basophils in these diverse pathobiological processes.

IgE receptor responsiveness of basophils in chronic inducible urticaria

Introduction

Chronic inducible urticaria (CIndU) is a subgroup of chronic urticaria induced by a specific stimulus. We evaluated basophil characteristics in patients with CIndU and compared with those in patients with chronic spontaneous urticaria (CSU) and healthy controls (HCs).

Methods

Blood was collected from patients, and a basophil activation test (BAT) was performed. Basophil responsiveness and surface marker expression in patients with CIndU were compared with those in patients with CSU and HCs. For some patients with CIndU, blood was collected before and after wheals were induced. In these cases, we compared the responsiveness of basophils before and after the appearance of wheals.

Result

HCs (n=23) and patients with CIndU (n=24) or CSU (n=38) were enrolled in the study. The degree of basophil activation at steady state in patients with CIndU was higher than in HCs. Basophil responsiveness via high-affinity IgE receptor (FcϵRI) stimulation with anti-IgE or anti-FcϵRI antibody in patients with CIndU was equivalent to that in HCs, and higher than that in patients with CSU. No abnormalities in IgE and FcϵRI expressions on the surface of basophils in patients with CIndU were observed. When we induced wheals in some patients with CIndU and performed a BAT before and after the appearance of wheals, no significant changes were found.

Conclusion

Peripheral blood basophils in CIndU were slightly activated at steady state, but no abnormalities in basophil responsiveness. In future, a higher number of cases should be enrolled to confirm the role of basophils and refine therapeutic targets for CIndU.

Basophils as a potential therapeutic target in cancer

Basophils, which are considered as redundant relatives of mast cells and the rarest granulocytes in peripheral circulation, have been neglected by researchers in the past decades. Previous studies have revealed their vital roles in allergic diseases and parasitic infections. Intriguingly, recent studies even reported that basophils might be associated with cancer development, as activated basophils synthesize and release a variety of cytokines and chemokines in response to cancers. However, it is still subject to debate whether basophils function as tumor-protecting or tumor-promoting components; the answer may depend on the tumor biology and the microenvironment. Herein, we reviewed the role of basophils in cancers, and highlighted some potential and promising therapeutic strategies.

Activated steady status and distinctive FcεRI-mediated responsiveness in basophils of atopic dermatitis

BACKGROUND

Although basophils are considered to play an important role for maintenance of type 2 inflammation in atopic dermatitis (AD), studies on basophils in AD patients are limited. Some studies have reported the activation status, including CD203c and CD63, of peripheral blood basophils in AD patients.

METHODS

We examined the features of circulating basophils in AD patients, assessed cell surface marker expressions and total serum IgE, and compared basophil responsiveness to stimulation between AD patients and healthy controls (HCs). In addition, the correlations among AD severity, laboratory factors, and features of basophils were examined. Blood samples from 38 AD patients and 21 HCs were analyzed. Basophil response markers CD203c and CD63, and expression of surface-bound IgE and FcεRI on basophils were measured. CD203c and CD63 expressions induced by stimulation with anti-IgE and anti-FcεRI antibodies were measured. Clinical/laboratory factors including total serum IgE were examined for correlations with these basophil parameters.

RESULTS

Baseline CD203c and CD63 expression on basophils were significantly higher in AD patients compared with HCs. The CD203c/CD63 response ratio to anti-FcεRI stimulation was higher than that to anti-IgE stimulation in AD patients, but not HCs. FcεRI expression on basophils was higher in AD patients than in HCs, although surface-bound IgE on basophils was equivalent. Total serum IgE had negative correlations with surface-bound IgE and CD63 responsiveness to anti-IgE stimulation.

CONCLUSIONS

Basophils were spontaneously activated under steady-state conditions in AD patients and responsiveness to anti-IgE stimulation was lower than in HCs. Despite high serum IgE and high basophil FcεRI expression, surface-bound IgE on basophils remained relatively low. Basophils might be suppressed or exhausted regarding FcεRI signaling via IgE in severe AD.

Basophils and their effector molecules in allergic disorders

Basophils are the rarest granulocytes which represent <1% of peripheral blood leukocytes. Basophils bear several phenotypic similarities to tissue-resident mast cells and therefore had been erroneously considered as blood-circulating mast cells. However, recent researches have revealed that basophils play nonredundant roles in allergic inflammation, protective immunity against parasitic infections and regulation of innate and acquired immunity. Basophils are recruited to inflamed tissues and activated in an IgE-dependent or IgE-independent manner to release a variety of effector molecules. Such molecules, including IL-4, act on various types of cells and play versatile roles, including the induction and termination of allergic inflammation and the regulation of immune responses. Recent development of novel therapeutic agents has enabled us to gain further insights into basophil biology in human disorders. In this review, we highlight the recent advances in the field of basophil biology with a particular focus on the role of basophils in allergic inflammation. Further studies on basophils and their effector molecules will help us identify novel therapeutic targets for treating allergic disorders.

Improved FcεRI-Mediated CD203c Basophil Responsiveness Reflects Rapid Responses to Omalizumab in Chronic Spontaneous Urticaria

BACKGROUND

Omalizumab is effective in patients with chronic spontaneous urticaria (CSU) although its mechanism of action is poorly understood. Several studies reported that decreased high-affinity IgE receptor (FcεRI)-mediated histamine release and/or responsiveness was characteristic of basophils in patients with CSU. However, few studies have focused on the relationship between changes in basophil responsiveness via FcεRI after omalizumab treatment and the therapeutic effect in patients with CSU.

OBJECTIVE

To assess basophil responsiveness via FcεRI stimulation, as well as FcεRI expression and IgE binding on blood basophils from patients with CSU before and after omalizumab treatment and its possible association with the clinical response.

METHODS

We analyzed 34 patients with CSU treated with omalizumab who were categorized as fast responders (FRs) (n = 20) and non or slow responders (N/SRs) (n = 14). CD203c expression induced by FcεRI stimulation, and IgE and FcεRI expressions on blood basophils from patients with CSU before and after omalizumab treatment were analyzed. Basophil responsiveness via FcεRI stimulation was observed in vitro using basophils pretreated with omalizumab.

RESULTS

FRs had increased CD203c responsiveness after treatment with omalizumab compared with N/SRs. This improvement of basophil responsiveness via FcεRI stimulation in FRs was not observed in peripheral blood basophils preincubated with omalizumab in vitro, suggesting that omalizumab does not directly affect circulating pre-existing abnormal basophils.

CONCLUSION

Increased basophil responsiveness via FcεRI after omalizumab treatment is associated with the therapeutic effect and mechanism of action of omalizumab.

Relationship between changes in the 7-day urticaria activity score after treatment with omalizumab and the responsiveness of basophils to FcεRI stimulation in patients with chronic spontaneous urticaria

Background

About one-half of all patients with chronic spontaneous urticaria have low or less reactivity of the basophils to FcεRI stimulation. However, the differences in the clinical characteristics between patients who show normal and attenuated basophil reactivities to FcεRI stimulation are still unclear. Furthermore, it also remains unknown as to what factors induce the poor reactivity of basophils to FcεRI stimulation.

Objective

The aim of the study is to investigate the differences in the clinical characteristics between patients who show normal and attenuated basophil reactivities to FcεRI stimulation.

Methods

We compared the clinical characteristics, including the autologous serum skin test-positive rates, serum concentrations of anti-IgE and anti-FcεRIα autoantibodies, and the FcεRI-crosslinking ability of these autoantibodies between patients with a negative basophil activation test (BAT) (≤10% CD203c^high basophils, n = 9) and positive BAT (>10% CD203c^high basophils, n = 13). We also monitored the changes in the 7-day urticaria activity scores after treatment with omalizumab, as compared to the score at the baseline, between the BAT-positive and BAT-negative patients.

Results

The BAT-negative patients showed a significantly higher urticaria control test score than the BAT-positive patients (p = 0.01). There were no significant differences in the autologous serum skin test-positive rates, concentrations of anti-IgE and anti-FcεRIα autoantibodies, and the FcεRI-crosslinking ability of these autoantibodies between the 2 groups. After treatment with omalizumab for 35 days, the score decreased to under 15 (corresponding to controlled or mild chronic spontaneous urticaria) in all of the BAT-negative patients, whereas in 6 out of the 13 BAT-positive patients, the scores remained over 16 (corresponding to moderate or severe chronic spontaneous urticaria).

Conclusions

The weak reactivity of basophils to FcεRI stimulation may not be due to the desensitization of basophils by anti-IgE or anti-FcεRIα autoantibodies. The time to response to omalizumab might differ between BAT-negative and BAT-positive patients with chronic spontaneous urticaria.

The Journal of Allergy and Clinical Immunology: In Practice 2019 Highlights

This article provides highlights of the clinically impactful original studies and reviews published in The Journal of Allergy and Clinical Immunology: In Practice in 2019 on the subjects of anaphylaxis, asthma, dermatitis, drug allergy, food allergy, immunodeficiency, immunotherapy, rhinitis/sinusitis, and urticaria/angioedema/mast cell disorders. Within each topic, practical aspects of diagnosis and management are emphasized. Treatments discussed include lifestyle modifications, allergen avoidance therapy, positive and negative effects of pharmacologic therapy, and various forms of immunologic and desensitization management. We designed this review to help readers consolidate and use this extensive and practical knowledge for the benefit of their patients.