In chronic rhinosinusitis with nasal polyps (CRSwNP), which inflammatory pathway is predominantly activated?

Explanation

## Immunopathology of Chronic Rhinosinusitis with Nasal Polyps ### Th2-Mediated Inflammation in CRSwNP **Key Point:** Chronic rhinosinusitis with nasal polyps (CRSwNP) is characterized by a Th2-dominant inflammatory response, with elevated IL-5 and IL-13 driving eosinophilic inflammation and mucus hypersecretion. ### Inflammatory Phenotypes in Chronic Rhinosinusitis | Type | Inflammatory Profile | Key Cytokines | Clinical Features | | --- | --- | --- | --- | | **CRSwNP** | Th2-dominant, eosinophilic | IL-4, IL-5, IL-13 | Nasal polyps, eosinophilia, mucus hypersecretion | | **CRSsNP** (without polyps) | Mixed Th1/Th2 or Th17 | IFN-γ, IL-17 | No polyps; often bacterial biofilm | | **Aspergillus-related** | Th2-dominant | IL-5, eotaxin | Fungal colonization, eosinophilia | ### Pathogenic Mechanisms in CRSwNP 1. **Th2 Cytokine Axis:** - IL-5 → eosinophil recruitment and survival - IL-13 → mucus hypersecretion, epithelial remodeling - IL-4 → IgE production (allergic component) 2. **Eosinophil-Driven Tissue Damage:** - Release of major basic protein (MBP) and eosinophil peroxidase (EPX) - Epithelial barrier dysfunction - Polyp formation via chronic inflammation and edema 3. **Impaired Epithelial Barrier:** - Tight junction protein loss (claudins, occludin) - Increased permeability → antigen uptake → Th2 skewing ### High-Yield Distinctions **High-Yield:** CRSwNP = Th2 + eosinophils + polyps; CRSsNP (without polyps) = mixed Th1/Th17 + bacteria + biofilm. This distinction is critical for understanding pathogenesis and guides emerging biologics (anti-IL-5, anti-IL-4R). **Clinical Pearl:** Patients with CRSwNP often have elevated serum IgE and eosinophilia. Many respond to topical corticosteroids (which suppress Th2) and emerging biologic agents targeting IL-5 (mepolizumab) or IL-4 receptor (dupilumab). **Mnemonic:** **"POLYP = Th2 + Eos"** — Polyps are driven by Th2 inflammation and eosinophilic infiltration. ### Why Not Th1 or Th17? - **Th1 (IFN-γ):** Associated with CRSsNP (without polyps) and bacterial biofilm-dominated disease; not the primary driver in polyp formation. - **Th17 (IL-17):** Emerging evidence in some CRSsNP phenotypes; not the dominant pathway in CRSwNP. - **Treg:** Impaired Treg function may contribute to loss of tolerance, but Treg activation is not the predominant pathway. [cite:Harrison 21e Ch 146]