Residency · Residency · Allergy Immunology

Allergic Bronchopulmonary Aspergillosis

Overview

Allergic bronchopulmonary aspergillosis (ABPA) is a complex hypersensitivity reaction to Aspergillus fumigatus colonizing the airways. It occurs in patients with underlying asthma, affecting an estimated 2 to 32% of those with severe disease, and in patients with cystic fibrosis, where the prevalence ranges from 2 to 15%. A critical conceptual distinction must be maintained: ABPA is not an invasive infection but rather an exaggerated immunologic response to fungal colonization of the airway lumen. The pathogenesis involves a convergence of Type I (IgE-mediated), Type III (immune complex-mediated), and Type IV (cell-mediated) hypersensitivity mechanisms, producing a distinctive clinical syndrome of recurrent pulmonary infiltrates, mucoid impaction, and progressive bronchiectasis. Left untreated, ABPA leads to central bronchiectasis, pulmonary fibrosis, and permanent, irreversible lung damage.

Pathophysiology

Immune Response to Aspergillus

The pathogenesis of ABPA begins with the inhalation of Aspergillus fumigatus spores, which measure 2 to 3 micrometers in diameter and are small enough to reach the distal airways. In patients with asthma or cystic fibrosis, viscid and poorly cleared airway mucus traps these spores, creating an environment conducive to germination. As the spores germinate into hyphae, the growing fungal elements release proteolytic enzymes that directly damage the airway epithelium, disrupting the epithelial barrier and initiating an innate immune response through the release of alarmins including thymic stromal lymphopoietin (TSLP), interleukin-33, and interleukin-25.

These epithelial-derived signals drive a robust type 2 helper T cell (Th2) response that is central to the immunopathology of ABPA. Interleukin-4 and interleukin-13 promote B cell class switching to IgE, resulting in the production of both total IgE and Aspergillus-specific IgE, as well as IgG antibodies directed against Aspergillus antigens. Interleukin-5 drives eosinophilic inflammation, producing both peripheral blood and tissue eosinophilia. Interleukin-9 promotes mast cell expansion within the airways. The simultaneous activation of complement through Type III immune complex deposition in the bronchial walls, combined with a Type IV granulomatous response involving T cells and macrophages, amplifies tissue inflammation and damage.

The cumulative effect of these overlapping hypersensitivity mechanisms is the formation of eosinophil-rich "allergic mucin" containing fungal hyphae, which produces characteristic mucus plugging of the airways. Over time, this chronic inflammatory process results in central bronchiectasis -- dilation of predominantly proximal airways with sparing of the distal bronchi -- a pattern that is pathognomonic for ABPA and distinguishes it from other causes of bronchiectasis.

Risk Factors

Several predisposing factors increase the likelihood of developing ABPA. Underlying asthma, particularly moderate-to-severe persistent disease, and cystic fibrosis represent the primary predisposing conditions. Genetic susceptibility is conferred by specific HLA types, with HLA-DR2 and HLA-DR5 associated with increased risk, as well as polymorphisms in Toll-like receptors including TLR9 T-1237C and TLR4 variants. Heterozygosity for CFTR mutations may increase ABPA susceptibility even in patients without cystic fibrosis. Corticosteroid use, while essential for asthma management, may paradoxically predispose to fungal colonization of the airways.

Clinical Features

Presentation

The clinical presentation of ABPA typically involves poorly controlled asthma with recurrent exacerbations despite what appears to be adequate therapy. Patients experience a productive cough with characteristically brownish mucus plugs containing fungal hyphae and eosinophils. The expectoration of mucoid impactions, when present, is pathognomonic. Systemic symptoms may include low-grade fever, malaise, and weight loss, particularly during flares. Respiratory symptoms encompass wheezing and dyspnea that may be difficult to distinguish from the underlying asthma, and hemoptysis occurs occasionally.

ABPA Staging (Patterson-Greenberger, Modified)

Staging (Patterson-Greenberger, modified)

The Patterson-Greenberger staging system, though not strictly sequential, provides a useful framework for conceptualizing the clinical course and guiding treatment decisions. Stage I (Acute) represents the initial presentation, characterized by elevated total IgE, pulmonary infiltrates on imaging, and peripheral eosinophilia. Stage II (Remission) is marked by clinical and radiographic improvement, with no new infiltrates for six months or more, though total IgE typically remains above normal. Stage III (Exacerbation) denotes a recurrent acute episode, recognized by a doubling of total IgE from the established baseline and the return of pulmonary infiltrates. Stage IV (Corticosteroid-Dependent) describes patients who require continuous systemic corticosteroids to maintain disease control. Stage V (Fibrotic) represents the end stage of the disease, with irreversible pulmonary fibrosis, fixed airflow obstruction, and honeycombing changes on imaging. It is important to recognize that progression through these stages is not inevitable or linear; patients may present at any stage and may fluctuate between stages over time.

<image>A clinical staging diagram for ABPA showing the five Patterson-Greenberger stages. Arranged as a horizontal progression with arrows between stages (noting that progression is not always linear). Stage I (Acute): chest CT showing fleeting pulmonary infiltrates, lab values showing markedly elevated total IgE (>1000 IU/mL), eosinophilia, positive Aspergillus IgE/precipitins. Stage II (Remission): clear chest imaging, declining IgE (still above normal), clinically stable. Stage III (Exacerbation): return of infiltrates, doubling of IgE from baseline, worsening symptoms. Stage IV (Steroid-dependent): patient requiring daily prednisone, partially controlled imaging. Stage V (Fibrotic): CT showing central bronchiectasis, upper lobe fibrosis, honeycomb changes, fixed airflow obstruction on spirometry. Below: treatment recommendations at each stage (oral corticosteroids for I and III, monitoring for II, low-dose steroids + antifungal for IV, supportive for V).</image>

Diagnosis

ISHAM Diagnostic Criteria (2013)

The International Society for Human and Animal Mycology (ISHAM) criteria provide the most widely accepted diagnostic framework for ABPA. A predisposing condition -- either asthma or cystic fibrosis -- must be present. Two obligatory criteria must both be met: the total serum IgE must exceed 1,000 IU/mL (or 500 IU/mL in cystic fibrosis patients), and Aspergillus-specific IgE must be elevated, or the skin prick test to A. fumigatus must be positive. In addition, at least two of three supportive criteria are required: elevated Aspergillus-specific IgG (precipitins), characteristic radiographic findings (fleeting infiltrates, mucoid impaction, central bronchiectasis, or high-attenuation mucus), and blood eosinophilia exceeding 500 cells per microliter (which may be suppressed by concurrent corticosteroid therapy).

Key Laboratory Findings

Total serum IgE is the single most important laboratory parameter for both diagnosis and monitoring of ABPA. Levels are typically above 1,000 IU/mL at diagnosis and can reach extraordinary values of 10,000 to 50,000 IU/mL. Serial total IgE measurements are essential for monitoring treatment response and detecting exacerbations: a decline of 35 to 50% from baseline indicates a therapeutic response, while a doubling of total IgE from the nadir value suggests an exacerbation warranting re-evaluation and treatment intensification. Aspergillus-specific IgE is usually markedly elevated, generally above 0.35 kUA/L. Aspergillus-specific IgG (precipitins) is positive in approximately 70 to 90% of ABPA patients. Peripheral blood eosinophilia exceeding 500 cells per microliter supports the diagnosis, though it may be masked by concurrent corticosteroid use. Sputum cultures for Aspergillus are positive in approximately 40 to 60% of cases, which supports but is not required for the diagnosis.

Imaging

Chest radiography may reveal fleeting or migratory pulmonary infiltrates, which are a hallmark of ABPA, as well as finger-in-glove opacities representing mucoid impaction, with a predilection for the upper lobes. High-resolution computed tomography (HRCT) is more sensitive and can identify several key findings. Central bronchiectasis, in which proximal airways are dilated while distal airways remain normal in caliber, is present in 50 to 100% of ABPA patients. High-attenuation mucus (HAM), defined as mucus plugs with density exceeding 70 Hounsfield units on CT, is highly specific for ABPA. This hyperdense mucus contains calcium, manganese, and iron complexes, and its presence predicts a higher risk of recurrence and poorer lung function outcomes. Mucoid impaction produces characteristic "finger-in-glove" or "toothpaste" shadow appearances. Additional findings may include tree-in-bud opacities, consolidation, ground-glass opacities, and upper lobe fibrosis in advanced disease.

Differential Diagnosis

The differential diagnosis of ABPA includes severe asthma without ABPA, Aspergillus-sensitized asthma (in which Aspergillus-specific IgE is positive but total IgE remains below 1,000 IU/mL and bronchiectasis is absent), invasive pulmonary aspergillosis (occurring in immunocompromised patients with actual tissue invasion by the fungus), chronic pulmonary aspergillosis (characterized by cavitary disease and aspergilloma formation), eosinophilic granulomatosis with polyangiitis (EGPA, a systemic vasculitis with eosinophilia), and mucoid impaction from other causes.

Treatment

Oral Corticosteroids

Oral corticosteroids remain the mainstay of therapy for acute ABPA and exacerbations. The standard regimen begins with prednisolone at 0.5 mg/kg/day for two weeks, followed by the same dose on alternate days for six to eight weeks, with a gradual taper over three to five months. The goals of corticosteroid therapy are to suppress the inflammatory response, clear pulmonary infiltrates, and reduce total IgE levels. Monitoring of total IgE every six to eight weeks is essential during and after treatment, with a target decline of at least 35 to 50% from baseline. Long-term oral corticosteroid therapy may be necessary in Stage IV disease, though the cumulative side effects of chronic steroid use underscore the importance of steroid-sparing strategies.

Antifungal Therapy

Antifungal therapy serves an adjunctive role in ABPA management by reducing the fungal burden in the airways, thereby decreasing the antigenic stimulus driving the inflammatory response and facilitating corticosteroid tapering. Itraconazole, administered at 200 mg twice daily for 16 weeks in either pulsed or continuous regimens, is the primary antifungal agent used. Therapeutic drug monitoring is important, with a target trough concentration of 1 to 2 micrograms per milliliter at steady state, which is typically achieved at two weeks. Itraconazole is a potent CYP3A4 inhibitor, and clinically significant drug interactions must be considered, particularly with inhaled corticosteroids such as budesonide and fluticasone, which may accumulate to cause iatrogenic Cushing syndrome. Hepatotoxicity is a concern, necessitating monthly liver function test monitoring. Voriconazole is an alternative with better bioavailability, though the risk of phototoxicity limits its use in long-term therapy. Posaconazole may be employed in refractory cases. It is essential to understand that antifungals are adjunctive to corticosteroids, not a replacement for them.

Biologic Therapy

The use of biologic agents in ABPA represents an evolving area of practice. Omalizumab, the anti-IgE monoclonal antibody, has been used in ABPA patients with coexisting severe allergic asthma, and multiple case series and small trials have demonstrated reduced exacerbation rates and steroid-sparing effects. Dosing presents a challenge because total IgE levels in ABPA often far exceed the standard omalizumab dosing table range; some clinicians base dosing on pre-treatment IgE values below 1,000 IU/mL or use empiric high-dose protocols. Anti-IL-5 agents, including mepolizumab and benralizumab, have shown promise in case reports and small series for refractory ABPA, reducing eosinophilic inflammation and offering steroid-sparing potential. Dupilumab, which blocks IL-4 receptor alpha and thus inhibits both IL-4 and IL-13 signaling, has demonstrated efficacy in case reports of refractory ABPA, reducing IgE levels, eosinophil counts, and exacerbation frequency. No biologic agent has received FDA approval specifically for ABPA, and their use remains off-label in this context.

<image>A diagnostic and treatment algorithm for ABPA. Starting point: "Poorly controlled asthma with suspicion for ABPA (recurrent infiltrates, mucus plugging, elevated eosinophils)." Step 1: Measure total IgE and Aspergillus-specific IgE/skin test. If total IgE >1000 AND Aspergillus IgE positive: assess supportive criteria (precipitins, eosinophilia, radiology). If ISHAM criteria met: Diagnosis of ABPA. Branch by stage: Acute/Exacerbation: oral prednisolone 0.5mg/kg + itraconazole 200mg BID. Monitor: IgE every 6-8 weeks, imaging at 6-8 weeks. If responds (IgE drops >35%, infiltrates clear): taper steroids over 3-5 months. If refractory/steroid-dependent: add/switch biologic (omalizumab for allergic phenotype, anti-IL-5 for eosinophilic). Side panel: imaging examples of central bronchiectasis and high-attenuation mucus. Red flag box: if IgE doubles from nadir = suspect exacerbation, re-image and increase treatment.</image>

ABPA in Cystic Fibrosis

ABPA occurs in 2 to 15% of cystic fibrosis patients, and its diagnosis is particularly challenging because many of the hallmark features of ABPA -- including mucus plugging, bronchiectasis, and elevated IgE -- may also be present as intrinsic features of cystic fibrosis itself. To account for this overlap, modified diagnostic criteria with a lower total IgE threshold of 500 IU/mL are applied in the CF population. The Cystic Fibrosis Foundation has published consensus guidelines for ABPA screening in CF patients. Treatment follows the same general principles as in non-CF ABPA, with corticosteroids and itraconazole forming the therapeutic backbone. Omalizumab has been used in CF-ABPA based on case series data.

Key Clinical Pearls

• ABPA should be suspected in any asthma patient with recurrent pulmonary infiltrates, mucoid impaction, or central bronchiectasis; check total IgE and Aspergillus-specific IgE
• Total IgE >1000 IU/mL is a key diagnostic threshold; serial IgE monitoring guides treatment (target >=35% decline, watch for doubling as marker of exacerbation)
• High-attenuation mucus (HAM) on CT is highly specific for ABPA and predicts more severe disease with higher relapse rates
• Central bronchiectasis with sparing of distal airways is characteristic of ABPA and distinguishes it from other causes of bronchiectasis
• Itraconazole is adjunctive to corticosteroids, not a replacement; monitor drug levels and hepatic function
• Omalizumab is increasingly used for ABPA comorbid with allergic asthma; dosing is challenging when total IgE exceeds standard dosing tables
• ABPA is an immunologic disease, not an infection; invasive aspergillosis is a separate entity occurring in immunocompromised patients

References

1. Agarwal R, et al. Allergic bronchopulmonary aspergillosis: review of literature and proposal of new diagnostic and classification criteria. Clin Exp Allergy. 2013;43(8):850-873.
2. Patterson R, Greenberger PA, Radin RC, Roberts M. Allergic bronchopulmonary aspergillosis: staging as an aid to management. Ann Intern Med. 1982;96(3):286-291.
3. Agarwal R, et al. A randomised trial of itraconazole vs prednisolone in acute-stage allergic bronchopulmonary aspergillosis complicating asthma. Chest. 2018;153(3):656-664.
4. Moss RB. Treatment options in severe fungal asthma and allergic bronchopulmonary aspergillosis. Eur Respir J. 2014;43(5):1487-1500.
5. Voskamp AL, et al. Clinical efficacy and immunologic effects of omalizumab in allergic bronchopulmonary aspergillosis. J Allergy Clin Immunol Pract. 2015;3(2):192-199.

Read this lecture as Markdown