Chronic Spontaneous Urticaria: New Understanding and Breakthrough Treatments

What Are the New Insights into Chronic Spontaneous Urticaria?

Chronic spontaneous urticaria (CSU) continues to challenge clinicians due to its complex pathogenesis and variable treatment response. Recent advances in understanding the molecular and cellular mechanisms underlying this condition have led to the development of promising new therapies targeting specific pathways involved in mast cell activation and inflammatory responses. This comprehensive review from Miot and colleagues synthesizes decades of research on CSU, examining its genetic basis, immunological mechanisms, and the expanding therapeutic landscape, providing valuable insights for clinical researchers and healthcare professionals managing this challenging condition.

CSU affects approximately 0.5-1.5% of the global population, with significant regional variations. Latin America shows the highest prevalence at 1.5%, followed by Asia (1.4%), Europe (0.5%), and North America (0.1%). The condition disproportionately affects women, with studies from Brazil indicating that 80-86% of adult CSU patients are female. The natural history of CSU reveals that approximately 6% of all urticaria cases progress to chronic forms, with 50% of CSU patients experiencing remission after 11 months. However, the disease can persist for five years or longer in severe cases, significantly impairing quality of life and imposing substantial healthcare costs. Cumulative remission rates range from 9-38% in the first year, 29-71% in the fifth year, and 52-93% by the twentieth year of disease. Factors associated with persistent disease include hypothyroidism and higher UAS7 severity scores, highlighting the multifactorial nature of CSU progression and resolution.

The genetic underpinnings of CSU provide critical insights into disease susceptibility and potential therapeutic targets. Various HLA associations have been identified, including HLA-Bw35 in Japanese populations and HLA-DRB1*04 (HLA-DR4) in Caucasian patients with histamine-releasing activity in their serum. Genome-wide association studies have implicated several genes in CSU pathogenesis, including TDGF1, HLA-G, PTPN22, LILRA3, and IGHG1/IGHG2, which collectively explain approximately 8% of CSU heritability. Epigenetic factors also play a role, with Chinese individuals with CSU showing hypomethylation in 439 genes, particularly on chromosome 6, suggesting autoimmune mechanisms similar to those in rheumatoid arthritis and lupus. The sphingolipid pathway, especially sphingosine-1-phosphate (S1p), appears involved in CSU pathogenesis through effects on capillary permeability, immune cell recruitment, and Th17 differentiation. These genetic and epigenetic factors interact with exogenous triggers to determine the phenotypic spectrum of CSU, influencing both disease predisposition and association with other autoimmune conditions.

What Drives the Complex Pathogenesis of CSU?

The pathogenesis of CSU centers on mast cell activation and degranulation in the skin and subcutaneous tissues, involving various immune mechanisms including autoimmunity, inflammation, coagulation factors, and activation of mast cell membrane receptors. Two main autoimmune mechanisms have been identified in CSU: type I autoimmunity (autoallergy) mediated by IgE specific to self-antigens, and type IIb autoimmunity mediated by IgG, IgM, and IgA directed against IgE or its high-affinity receptor FcεRI. Type I mechanisms involve IgE antibodies against antigens such as thyroperoxidase (TPO), thyroglobulin, tissue factor, and interleukin-24 (IL-24), with IgE anti-IL-24 present in 70-80% of CSU patients and correlating with disease activity. Type IIb mechanisms involve IgG anti-FcεRI in 20-50% of patients, characterized by high disease activity, concomitant autoimmune diseases, low serum IgE, high anti-TPO antibodies, basopenia, eosinopenia, poor response to antihistamines and omalizumab, and good response to cyclosporine. Many patients show overlapping mechanisms, with studies from Brazil indicating 38% of patients with type I, 51% with both type I and IIb, 9% with type IIb alone, and 2% with neither mechanism. Complement activation through the alternative pathway amplifies mast cell degranulation, particularly in type IIb CSU, with C5a acting on C5aR receptors expressed only on cutaneous mast cells, explaining the skin's role as the target organ in CSU.

Beyond autoantibodies, inflammatory processes contribute significantly to CSU pathogenesis. Patients with active CSU show increased mast cell numbers in the skin compared to those in remission or healthy controls. Histopathologically, more than 90% of CSU patients exhibit a non-necrotizing perivascular infiltrate with increased mononuclear cells and a ninefold increase in mast cells showing signs of degranulation. The infiltrate typically consists of 47% T lymphocytes and 22% monocytes, resembling late-phase allergic reactions. Various cytokines, cell adhesion molecules, chemokines, and enzymes are elevated in the blood of CSU patients, including TGF-β1, IL-2, IL-4, IL-5, IL-6, IL-10, IL-17, IL-31, INF-γ, TNF-α, sICAM-1, and sVCAM-1. Alarmins such as IL-33 and TSLP are also elevated and correlate with disease severity and quality of life impairment. The skin of CSU patients shows increased CD4+/CCR5+ lymphocytes, monocytes, neutrophils, eosinophils, basophils, and M2 macrophages, with cytokine patterns indicating both type 1 and type 2 inflammatory responses. Activation of autoreactive CD4+ T cells targeting FcεRI precedes autoantibody development, with increased Th17 cells and IL-17A expression in both lesional and non-lesional skin of severe CSU patients.

The coagulation cascade plays a significant role in CSU pathogenesis, with elevated plasma levels of Prothrombin Fragment 1+2, fibrin degradation products, and D-dimers correlating with disease activity and severity. Tissue factor expressed by eosinophils and dermal endothelial cells activates the extrinsic coagulation pathway, generating thrombin and fibrin. This process activates protease-activated receptors (PARs) on various cell types, including mast cells, promoting inflammatory mediator release, vascular hyperpermeability, and cell recruitment. Activated coagulation factors also trigger the alternative complement pathway, generating anaphylatoxins C3a and C5a, which amplify mast cell and basophil stimulation. Importantly, this coagulation activation occurs locally with active fibrinolysis and poses no risk of thromboembolic events. The intestinal microbiota further influences CSU pathogenesis through dysbiosis, with reduced beneficial bacteria from Firmicutes and Bacteriodetes strains and increased pro-inflammatory members like Enterobacteriaceae. Lipopolysaccharides (LPS) from dysbiotic gut bacteria can reach the bloodstream and lower mast cell activation thresholds, while short-chain fatty acids (SCFAs) like butyrate and propionate can stabilize mast cells and promote intestinal barrier integrity, preventing LPS translocation.

Pruritus, the cardinal symptom of CSU, involves both histaminergic and non-histaminergic pathways. While histamine released by mast cells and basophils binds to H1 and H4 receptors on sensory nerve endings, causing the perception of itch, chronic disease activates additional mechanisms. The Th2 immune pathway, with increased IL-4 and IL-5, activates mast cells and reduces their threshold for degranulation. IL-31 and IL-33, crucial in hive formation and pruritus induction, bind to receptors in non-histaminergic sensory pathways. Proteases like tryptase act on pruritogenic receptors (PAR1 and PAR2) in cutaneous sensory fibers, while serotonin and cytokines (IL-4, IL-13, TSLP) released by mast cells contribute to itch sensation. Circulating basophils synthesize IL-31 in response to IgE-dependent stimuli, creating an autocrine feedback loop releasing IL-4 and IL-13. This explains why second-generation H1 antihistamines, even at quadruple doses, fail to control pruritus in many patients, as the non-histaminergic pathway plays a significant role.

Can Clinical Markers and Comorbidities Guide CSU Diagnosis?

Biomarkers in CSU can be divided into clinical and serological markers related to disease activity/severity and response to treatment. Clinical markers of increased disease activity include older age at onset, female sex, presence of angioedema, poor antihistamine response, positive autologous serum test, hypersensitivity to NSAIDs, and long-lasting disease. Serological markers include low eosinophil and basophil counts, low vitamin D levels, elevated CD203c expression on basophils, and increased levels of eotaxin, VEGF, MMP-9, IL-6, IL-18, IL-33, C-reactive protein, D-dimers, antithyroid antibodies, and mean platelet volume. Regarding therapeutic response, poor antihistamine response is associated with elevated UAS7, concomitant inducible urticaria, positive ASST/BAT, elevated CRP/ESR, and elevated D-dimers. Omalizumab response is poorer in patients with autoimmune markers (positive ASST, BAT, ANA) and better in those with high FcεRI expression on basophils and elevated total IgE. Cyclosporine shows good response in patients with autoimmune markers and low total IgE but poor response with elevated D-dimers.

CSU has a significant negative impact on quality of life, with approximately 40% of patients having a DLQI > 10, indicating severe or very severe impairment. The disease imposes considerable healthcare costs ($907-$2,084 annually per patient) and is associated with sleep disturbances, reduced productivity, and psychiatric comorbidities including anxiety, depression, and stress. Stress can exacerbate CSU through both rapid responses mediated by the Sympathetic-Adrenomedullary (SAM) system and slower responses via the Hypothalamic-Pituitary-Adrenal (HPA) axis. Chronic stress increases Th2 cell responses, reduces NK cell activity, and can lead to hypocortisolism and sustained sympathetic nervous system hyperactivity. Psychological stress triggers the release of substance P and CGRP from nerve endings in the skin, activating MRGPRX2 and Neurokinin-1 Receptors on mast cells, creating bidirectional communication between mast cells and sensory neurons. The increased expression of Corticotropin-releasing Hormone Receptor 1 in CSU lesions suggests a close connection between the skin and the HPA axis.

CSU must be differentiated from induced forms of chronic urticaria (pressure, symptomatic dermographism, cholinergic, solar, aquagenic, vibratory, cold) and from urticarial vasculitis (UV), which is characterized by hives lasting more than 24 hours, post-inflammatory purpura/hyperpigmentation, vesicles/blisters, residual desquamation, and systemic symptoms like arthralgia and fever. Skin biopsy is indicated when urticaria or angioedema are not characteristic or when warning signs are present: absence of pruritus, presence of pain or burning, systemic symptoms, violaceous halo, purpura or residual hyperpigmentation, laboratory abnormalities (elevated ESR/CRP, monoclonal gammopathy, elevated ferritin, complement consumption), or lack of response to appropriate treatment. Diascopy and dermoscopy can help visualize purpura in urticarial lesions, with linear vessels more common in CSU (86%) and red-purplish dots or globules (90%) more evident in early UV. Other differential diagnoses include drug reactions, atypical bullous pemphigoid, urticarial dermatitis, neutrophilic urticarial dermatosis, autoinflammatory diseases, Schnitzler syndrome, and Adult-Onset Still's Disease.

Are Current and Emerging Therapies Revolutionizing CSU Treatment?

The therapeutic landscape for CSU has evolved significantly with improved understanding of disease mechanisms. Current international guidelines recommend a stepped approach starting with second-generation H1 antihistamines, followed by omalizumab and cyclosporine. However, 25-50% of patients fail to achieve complete control even with quadruple antihistamine doses, particularly those with severe disease, concomitant inducible urticaria, and elevated inflammatory markers. Second-generation H1 antihistamines act as inverse agonists at histamine receptors, inhibiting vasodilation, reducing capillary permeability, and suppressing inflammatory cell recruitment through effects on NF-κB and calcium channels. Omalizumab, a humanized anti-IgE monoclonal antibody, sequesters IgE and reduces FcεRI expression on mast cells and basophils, showing efficacy in 72% of patients in real-world studies. Cyclosporine inhibits T-cell activation and mast cell mediator release, with response rates of 54% at 4 weeks, 66% at 8 weeks, and 73% at 12 weeks using doses of 2-5 mg/kg/day, though with dose-dependent adverse effects including hypertension and elevated creatinine.

Emerging therapies target specific pathways in CSU pathogenesis. Dupilumab, an IL-4Rα inhibitor that blocks IL-4 and IL-13 signaling, showed significant improvements in the LIBERTY-CSU CUPID trials and received approval in Brazil for CSU unresponsive to antihistamines. Anti-IL-5 agents like mepolizumab have shown promise in case reports, while benralizumab's development for CSU was discontinued after disappointing phase 1 results. Off-label use of secukinumab (anti-IL-17A) in treatment-resistant CSU showed 55% improvement at 30 days and 82% at 3 months, while tildrakizumab (anti-IL-23) significantly improved disease activity and control in refractory patients. Tezepelumab, an anti-TSLP monoclonal antibody, demonstrated sustained treatment effects in the INCEPTION study, with continued reductions in IL-5 and IL-13 independent of IgE changes. Bruton's tyrosine kinase (BTK) inhibitors like remibrutinib and fenebrutinib target intracellular signaling in mast cells, basophils, and B cells, with phase 2 trials showing rapid and significant improvements in CSU symptoms. Remibrutinib achieved UAS7=0 in 55.8% of patients by week 52 in extension studies, with a favorable safety profile. Other promising approaches include MRGPRX2 receptor inhibitors (EP262), tyrosine kinase receptor inhibitors (barzolvolimab, briquilimab), and anti-Siglec medications that target inhibitory receptors on mast cells.

The goal of CSU treatment is to achieve complete disease control, reflected by UAS7=0, UCT=16, and DLQI=0, restoring normalcy to patients' lives and potentially interrupting the natural course of the disease. Various factors can trigger CSU exacerbations during treatment, including stress (13.9%), NSAIDs (6.7%), infections (5.5%, especially viral upper respiratory infections including COVID-19), and rarely certain foods. First-generation antihistamines have been banned from CSU treatment due to their low H1 receptor selectivity and significant side effects, including sedation, psychomotor impairment, and potential cardiac toxicity. For patients not responding to measures in the international consensus, medications like colchicine, dapsone, methotrexate, montelukast, hydroxychloroquine, H2-blockers, and doxepin may be used exceptionally, though evidence for their efficacy is limited.

What Direction Will Future CSU Management Take?

The expanding therapeutic arsenal for CSU reflects the complex pathophysiology of this condition and offers hope for patients with refractory disease. As our understanding of CSU endotypes improves, treatment may become increasingly personalized based on biomarker profiles, genetic factors, and specific pathogenic mechanisms. Could the identification of specific endotypes through biomarker profiling become standard practice in CSU management, allowing for more targeted therapeutic approaches? How might the development of oral small molecule inhibitors like BTK inhibitors change the treatment paradigm compared to injectable biologics? These questions highlight the dynamic nature of CSU research and the potential for significant advances in patient care in the coming years.

For clinical researchers and healthcare professionals, this review underscores the importance of considering the multifaceted nature of CSU when developing and implementing treatment strategies. The integration of genetic, immunological, and clinical data promises to enhance our ability to predict disease course, select appropriate therapies, and ultimately improve outcomes for patients with this challenging condition. As new agents targeting specific pathways continue to emerge from clinical trials, the therapeutic landscape for CSU is likely to undergo further transformation, offering additional options for patients who remain symptomatic despite current treatments.

Summary

This comprehensive review examines recent developments in chronic spontaneous urticaria (CSU), covering epidemiology, genetic factors, pathogenesis, and therapeutic advances. The condition affects 0.5-1.5% of the global population, predominantly women, with varying remission rates. The research highlights complex genetic and immunological mechanisms, including autoimmune responses, inflammatory processes, and coagulation cascade involvement. The article details current treatment approaches, from antihistamines to biologics like omalizumab, and explores emerging therapies targeting specific pathways. New treatments include dupilumab, BTK inhibitors, and various monoclonal antibodies, offering hope for improved patient outcomes. The review emphasizes the importance of personalized treatment approaches based on biomarker profiles and specific pathogenic mechanisms.