# Asthma - Immunologic Mechanisms and Phenotyping

## Overview

Asthma is a chronic inflammatory airway disease characterized by the triad of variable airflow obstruction, bronchial hyperresponsiveness, and progressive airway remodeling. It affects approximately 8 percent of US adults and 7 percent of children, with roughly 300 million individuals affected globally. Modern understanding recognizes asthma as a heterogeneous disease comprising multiple phenotypes and endotypes rather than a single entity, and this conceptual shift has transformed therapeutic approaches. Despite declining mortality with improved therapies, approximately 3,500 asthma-related deaths occur annually in the United States, with persistent disparities across racial, ethnic, and socioeconomic groups.

## Immunopathology of Asthma

### Type 2 (T2) Inflammation

Type 2 inflammation is the dominant immune pathway in approximately 50 to 70 percent of asthma patients and has been the most extensively characterized. The inflammatory cascade is initiated at the epithelial surface, where airborne allergens, pollutants, and viruses damage the airway epithelium and trigger release of epithelial-derived alarmins. Thymic stromal lymphopoietin (TSLP) activates dendritic cells toward a Th2-priming phenotype. Interleukin-33, released from necrotic or damaged epithelial cells, is a potent activator of group 2 innate lymphoid cells (ILC2s). Interleukin-25 similarly activates ILC2s and Th2 memory cells, amplifying the type 2 cascade.

Th2 cells and ILC2s produce the canonical type 2 cytokines that drive the core pathologic features of asthma. Interleukin-4 promotes IgE class switching in B cells, drives Th2 differentiation, and induces mucus metaplasia. Interleukin-5 is the principal cytokine governing eosinophil maturation in the bone marrow, activation, tissue recruitment, and prolonged survival. Interleukin-13 drives mucus hypersecretion through upregulation of the MUC5AC mucin gene, induces goblet cell metaplasia, promotes airway hyperresponsiveness, and contributes to subepithelial fibrosis. Importantly, IL-4 and IL-13 share the IL-4 receptor alpha chain (IL-4Ralpha) as a signaling component, which serves as the therapeutic target of dupilumab.

Eosinophilic inflammation is a hallmark of T2-high asthma. IL-5-driven eosinophil production in the bone marrow and subsequent tissue recruitment results in eosinophil infiltration of the airway wall, where eosinophil granule proteins, including major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil-derived neurotoxin (EDN), and eosinophil peroxidase (EPX), directly damage the airway epithelium. Eosinophils are also a significant source of cysteinyl leukotrienes, which amplify bronchoconstriction. IgE-mediated mast cell activation in the airway submucosa further contributes to the inflammatory milieu. In severe asthma, a unique pathologic finding is the infiltration of mast cells directly into airway smooth muscle bundles, creating a microenvironment of persistent smooth muscle activation.

### Non-Type 2 Inflammation

Approximately 30 to 50 percent of asthma patients exhibit non-type 2 inflammatory patterns, which remain less well characterized and present greater therapeutic challenges. Neutrophilic asthma is driven by IL-17A and IL-17F produced by Th17 cells, which recruit neutrophils via the chemokines CXCL1 and CXCL8. This phenotype is associated with severe asthma, obesity, cigarette smoking, and corticosteroid resistance. The steroid insensitivity of neutrophilic asthma is partly explained by the observation that corticosteroids paradoxically prolong neutrophil survival, in contrast to their pro-apoptotic effects on eosinophils.

Paucigranulocytic asthma, in which neither eosinophils nor neutrophils are present in induced sputum, may represent a phenotype in which structural changes and smooth muscle dysfunction predominate over active inflammation. This pattern may reflect end-stage airway remodeling or intermittent inflammation that is quiescent at the time of assessment.

### Airway Remodeling

Chronic airway inflammation in asthma leads to structural changes that are collectively termed airway remodeling and are partially irreversible. The key components include subepithelial fibrosis, characterized by collagen deposition beneath the basement membrane resulting in reticular basement membrane thickening; smooth muscle hypertrophy and hyperplasia, which increase contractility and contribute to fixed airflow obstruction; goblet cell metaplasia, resulting in mucus overproduction with a shift toward the more viscous MUC5AC mucin at the expense of MUC5B; neovascularization with new vessel formation in the submucosa; and epithelial damage with loss of ciliated cells and increased permeability.

A critical recognition is that airway remodeling begins early in the disease process, with structural changes detectable in preschool-age wheezers before a formal diagnosis of asthma is established. While traditionally considered irreversible, emerging evidence suggests that biologic therapies, including mepolizumab and dupilumab, can reduce airway wall thickening as measured by computed tomography, offering hope that targeted intervention may partially reverse established remodeling.

<image>A detailed cross-sectional comparison of a normal bronchiole versus an asthmatic bronchiole. Left panel (normal): thin epithelium with ciliated cells and scattered goblet cells, thin basement membrane, relaxed smooth muscle layer, minimal submucosal glands, patent lumen. Right panel (asthmatic): thickened epithelium with goblet cell metaplasia and mucus plugging, thickened reticular basement membrane (subepithelial fibrosis), hypertrophied and hyperplastic smooth muscle, submucosal edema with eosinophilic and T cell infiltrate, mast cells within smooth muscle bundles, increased submucosal vascularity, narrowed lumen with mucus. Inset boxes showing magnified views of: (1) eosinophil releasing MBP granules onto epithelium, (2) Th2 cell and ILC2 releasing IL-4/IL-5/IL-13, (3) mast cell in smooth muscle releasing histamine and PGD2. Measurements indicated: normal BM ~5 micrometers vs asthmatic BM 10-25 micrometers.</image>

## Asthma Phenotyping and Endotyping

### Clinical Phenotypes

Several clinically recognizable asthma phenotypes have been defined through cluster analyses and clinical observation. Early-onset allergic asthma is the classic childhood-onset phenotype, characterized by atopy, positive family history, good corticosteroid responsiveness, and a course that may be outgrown or persist into adulthood. Late-onset eosinophilic asthma presents in adulthood, is often non-atopic, frequently associated with chronic rhinosinusitis with nasal polyps (CRSwNP) and aspirin-exacerbated respiratory disease (AERD), is less IgE-driven, and demonstrates high blood and tissue eosinophil counts. Obesity-related asthma, predominantly seen in females with a body mass index exceeding 30, tends to be non-eosinophilic, poorly responsive to corticosteroids, and driven by both mechanical effects of obesity and inflammatory mediators from adipose tissue. Exercise-induced bronchoconstriction (EIB) may present as an isolated entity or occur within any asthma phenotype.

Aspirin-exacerbated respiratory disease, also known as Samter's triad, is a distinctive phenotype defined by the triad of asthma, chronic rhinosinusitis with nasal polyposis, and sensitivity to cyclooxygenase-1-inhibiting nonsteroidal anti-inflammatory drugs. The underlying pathophysiology involves dysregulated arachidonic acid metabolism with overproduction of cysteinyl leukotrienes and underproduction of the bronchoprotective prostaglandin E2. Elevated urinary leukotriene E4 is a biomarker, and the tissue pathology is characterized by mast cell and eosinophil predominance. Disease-modifying treatment includes aspirin desensitization, achieving maintenance doses of 325 to 650 mg twice daily, in addition to standard asthma therapy.

### Biomarkers for T2 Endotyping

| Biomarker | T2-High Threshold | What It Reflects | Limitations |
|-----------|-------------------|-----------------|-------------|
| Blood eosinophils | >=150 cells/mcL (biologic eligibility); >=300 (strong T2 signal) | IL-5-driven eosinophil production | Not specific to airways; elevated in parasites, EGPA |
| FeNO | >=25 ppb (clinical threshold); >=50 ppb (high specificity) | IL-13-driven iNOS expression in epithelium | Suppressed by ICS and smoking; elevated by atopy without asthma |
| Total IgE | Variable (guides omalizumab dosing) | Allergic sensitization | Poor correlation with disease severity |
| Sputum eosinophils | >=3% | Direct airway eosinophilic inflammation | Gold standard but limited to specialized centers |
| Serum periostin | Elevated | IL-13 activity on epithelial cells | Research use primarily; limited clinical availability |

Several biomarkers are available to identify the T2 endotype in clinical practice. Blood eosinophil counts serve as the most accessible marker, with a threshold of 150 cells/mcL representing the lower cutoff for some biologic eligibility criteria, 300 cells/mcL indicating a strong T2 signal that predicts robust biologic response, and counts exceeding 500 cells/mcL raising concern for eosinophilic granulomatosis with polyangiitis (EGPA) if accompanying clinical features are present.

Fractional exhaled nitric oxide (FeNO) reflects IL-13-driven upregulation of inducible nitric oxide synthase (iNOS) in the airway epithelium. A level of 20 ppb or higher is suggestive of T2 inflammation in steroid-naive patients, 25 ppb is the commonly employed clinical threshold, and levels exceeding 50 ppb provide a strong T2 signal with high specificity for eosinophilic airway inflammation. FeNO is elevated by allergic inflammation and eosinophilic bronchitis but suppressed by inhaled corticosteroids and cigarette smoking, which must be considered in interpretation.

Total and allergen-specific IgE levels serve as markers of allergic sensitization and guide omalizumab dosing. Serum periostin, a marker of IL-13 activity, is used primarily in research settings and is less clinically available. Sputum eosinophil counts of 3 percent or higher remain the gold standard for identifying eosinophilic airway inflammation but are limited to research and specialized centers.

### Treatable Traits Approach

Contemporary asthma management is increasingly moving beyond phenotypic classification toward identification and targeting of individual treatable traits. Pulmonary traits include eosinophilic inflammation, fixed airflow limitation, bronchial hyperresponsiveness, and mucus impaction. Extrapulmonary traits encompass gastroesophageal reflux disease, obesity, vocal cord dysfunction, obstructive sleep apnea, anxiety and depression, and chronic rhinosinusitis. Behavioral traits include medication adherence, inhaler technique, active smoking, and ongoing allergen exposure. This multidimensional approach ensures that all modifiable contributors to disease burden are addressed systematically.

## Diagnosis and Assessment

### Spirometry

Spirometry is the cornerstone of asthma diagnosis, with an FEV1/FVC ratio below 0.70 (or below the lower limit of normal) supporting the presence of airflow obstruction. Bronchodilator reversibility, traditionally defined as an improvement of 12 percent and 200 mL in FEV1 following short-acting beta-agonist administration, was updated by the American Thoracic Society in 2022 to remove the requirement for meeting both percentage and absolute change criteria; either criterion is now considered sufficient. Peak expiratory flow variability exceeding 10 percent diurnal variation provides additional supportive evidence for the diagnosis of asthma.

### Bronchial Provocation Testing

Bronchial provocation testing is indicated when asthma is suspected but spirometry is normal. The methacholine challenge test measures the provocative concentration of methacholine causing a 20 percent fall in FEV1 (PC20). A PC20 below 4 mg/mL indicates moderate to severe bronchial hyperresponsiveness with high sensitivity for asthma. A PC20 between 4 and 16 mg/mL represents borderline or mild hyperresponsiveness. A PC20 exceeding 16 mg/mL constitutes a negative test and effectively rules out current asthma with a negative predictive value exceeding 95 percent. Patients must withhold short-acting beta-agonists for 8 hours and long-acting beta-agonists for 24 to 48 hours before testing, while inhaled corticosteroids do not require discontinuation.

Eucapnic voluntary hyperventilation (EVH) is the most specific test for diagnosing exercise-induced bronchoconstriction. Mannitol challenge testing provides an alternative osmolar provocation, with a fall in FEV1 exceeding 15 percent considered a positive result.

### Severity and Control Assessment

Asthma severity is classified retrospectively based on the treatment step required to achieve disease control. The Global Initiative for Asthma (GINA) control assessment evaluates symptom frequency, nighttime awakening, reliever medication use, and activity limitation. Well-controlled asthma is defined as daytime symptoms occurring no more than twice per week, no nighttime symptoms, no activity limitation, and reliever use no more than twice per week. Validated patient-reported instruments include the Asthma Control Test (ACT), with a score of 19 or below indicating suboptimal control, and the Asthma Control Questionnaire (ACQ).

<image>A comprehensive asthma phenotyping and biomarker diagram. Central hub labeled "Asthma" with radiating spokes to different phenotypes arranged in a semicircle: (1) Early-onset allergic (childhood onset, atopic, elevated IgE), (2) Late-onset eosinophilic (adult onset, CRSwNP, high eosinophils), (3) AERD (triad shown: asthma + nasal polyps + NSAID sensitivity, with elevated urinary LTE4), (4) Obesity-related (BMI>30, non-eosinophilic, female predominance), (5) Neutrophilic (smoking, severe, steroid-resistant). Below each phenotype, key biomarker profiles are shown in a table format: blood eosinophils, FeNO, IgE, sputum cell type, steroid responsiveness. A color gradient from red (T2-high) to blue (T2-low) spans across the phenotypes. Available biologic therapies are mapped to the T2-high phenotypes with connecting lines.</image>

## Stepwise Management (GINA 2023+)

### Track 1 (Preferred - ICS-Formoterol Maintenance and Reliever)

The preferred management track employs ICS-formoterol as both maintenance and reliever therapy. At Steps 1 and 2, low-dose ICS-formoterol (budesonide-formoterol) is used on an as-needed basis only. This approach, validated by the SYGMA 1 and 2 trials and the Novel START study, significantly reduces exacerbation rates compared with as-needed SABA alone by providing anti-inflammatory therapy with every reliever dose. At Step 3, low-dose ICS-formoterol is used as regular maintenance therapy with additional doses as needed for relief. Step 4 escalates to medium-dose ICS-formoterol maintenance with as-needed dosing. At Step 5, add-on therapies include a long-acting muscarinic antagonist (LAMA), high-dose ICS-formoterol, or referral for phenotyping and consideration of biologic therapy.

### Track 2 (Alternative)

The alternative track uses ICS in combination with SABA at Steps 1 and 2, ensuring that every reliever dose is accompanied by anti-inflammatory therapy even if delivered through separate inhalers. Step 3 introduces low-dose ICS-LABA with as-needed SABA. Step 4 uses medium- to high-dose ICS-LABA with as-needed SABA. Step 5 adds LAMA and initiates referral for phenotyping and biologic consideration.

### Key Changes from Earlier Guidelines

A paradigm-shifting change in current GINA guidelines is the recommendation that SABA-only treatment is no longer appropriate at any step, even in mild or intermittent asthma. The rationale is that anti-inflammatory reliever therapy reduces exacerbation risk compared with SABA alone, including in patients with mild disease who were traditionally managed with rescue SABA only. Additionally, leukotriene receptor antagonists such as montelukast have been demoted in the treatment algorithm in light of the FDA neuropsychiatric adverse event warning.

## Key Clinical Pearls
- Asthma is heterogeneous: T2-high (~50-70%) and T2-low (~30-50%) endotypes require different management strategies
- Blood eosinophils >=300 cells/mcL and FeNO >=25 ppb are practical bedside biomarkers identifying T2-high inflammation amenable to biologics
- GINA no longer recommends SABA-only treatment at any step; anti-inflammatory reliever therapy (ICS-formoterol PRN) is now standard of care
- FeNO reflects IL-13-driven iNOS expression; it is suppressed by ICS and smoking (false low) and elevated in atopy even without asthma (false high)
- Methacholine challenge has high negative predictive value: PC20 >16 mg/mL effectively rules out current asthma
- AERD is characterized by overproduction of cysteinyl leukotrienes and underproduction of PGE2; aspirin desensitization is disease-modifying
- Airway remodeling begins early in disease; early ICS use may attenuate but not fully prevent structural changes

## References
1. Global Initiative for Asthma (GINA). Global Strategy for Asthma Management and Prevention. Updated 2023. www.ginasthma.org.
2. Woodruff PG, et al. T-helper type 2-driven inflammation defines major subphenotypes of asthma. *Am J Respir Crit Care Med*. 2009;180(5):388-395.
3. O'Byrne PM, et al. Inhaled combined budesonide-formoterol as needed in mild asthma (SYGMA 1). *N Engl J Med*. 2018;378(20):1865-1876.
4. Fahy JV. Type 2 inflammation in asthma - present in most, absent in many. *Nat Rev Immunol*. 2015;15(1):57-65.
5. Laidlaw TM, Boyce JA. Aspirin-exacerbated respiratory disease - new prime suspects. *N Engl J Med*. 2016;374(5):484-488.