Interpretation of the Role of Eosinophils in Diffuse Interstitial Pneumopathies

From a pathophysiological perspective, the development of interstitial lung disease (ILD) involves progressive inflammation and fibrosis. These processes gradually remodel the structure of the pulmonary parenchyma-comprising alveoli, alveolar ducts, and bronchioles-into fibrotic tissue. The etiology of ILDs is multifactorial: they may be genetic , environmental, or idiopathic.

Fibrosis leads to clinical symptoms such as progressively worsening dyspnea, dry cough, diffuse fine crackles on auscultation, oxygen desaturation, and general deterioration of the patient's condition. Pulmonary function tests (PFTs) typically reveal a restrictive ventilatory defect combined with impaired alveolar diffusion, indicating reduced gas exchange capacity.

Radiological findings may vary and commonly include one or more of the following: ground-glass opacities, reticulations, traction bronchiectasis, honeycombing, and septal thickening. Although high-resolution chest CT is a key tool in ILD diagnosis, histopathological examination of lung biopsies remains the gold standard. The presence of inflammatory and fibrotic tissue infiltration is the hallmark diagnostic criterion. However, this approach requires bronchoscopy and is highly invasive, particularly for frail patients.

Indeed, ILD patients are susceptible to acute exacerbations, which are characterized by sudden and severe clinical deterioration in respiratory function and can be life-threatening. This phenomenon is well-documented in idiopathic pulmonary fibrosis (IPF).

Most diagnoses are established by multidisciplinary discussion (MDD), involving specialists from various fields. While MDD provides a working diagnosis, etiological classification remains suboptimal for many interstitial diseases. For instance, distinguishing between hypersensitivity pneumonitis, IPF, and ILDs associated with rheumatoid arthritis can be challenging-even with advanced imaging techniques.

This has led to more pragmatic classifications such as progressive fibrosing interstitial lung diseases (PF-ILDs), where the focus shifts from precise etiology to the presence of a treatable fibrotic phenotype responsive to antifibrotic therapies.

When an etiological diagnosis is straightforward, treatment is targeted: immunosuppressants for connective tissue disease-related ILDs, or antifibrotic agents such as pirfenidone or nintedanib for IPF or fibrosing forms of other ILDs. Unfortunately, despite these therapies, prognosis remains poor for most ILDs-largely due to the occurrence of exacerbations, which are typically managed with high-dose corticosteroids. Over time, patients may progress to chronic respiratory failure requiring long-term oxygen therapy.

This underscores the need for novel therapeutic strategies to improve both functional status and survival.

Given the fibrotic component of the inflammatory response, one avenue of investigation is to identify the biological drivers of ILD onset from a mechanistic and functional standpoint. In this context, some studies have suggested that inflammation may involve the recruitment of eosinophilic polymorphonuclear cells (E-PMNs). While the role of E-PMNs in ILD pathogenesis has been strongly hypothesized, it has yet to be definitively demonstrated.

E-PMNs are circulating granulocytes derived from the hematopoietic lineage. Produced in the bone marrow, they are recruited into tissues following activation of the type 2 (T2) immune response, orchestrated by Th2 lymphocytes and type 2 innate lymphoid cells (ILC2), primarily via the secretion of IL-5.

This T2 response is triggered by parasitic antigens, allergens, or tissue injury. Although beneficial against parasites, eosinophil recruitment and activation are central to allergic and respiratory pathophysiology and have multiple clinical consequences. E-PMNs release pro-fibrotic mediators such as TGF-β and have been implicated in conditions like chronic eosinophilic pneumonia (Carrington disease) and even endomyocardial fibrosis.

In humans, hypereosinophilia (eosinophilic polymorphonuclear cell ≥ 1500/mm³) is a known cause of pulmonary fibrosis. However, new diagnostic thresholds are being considered, as levels ≥150 cells/mm³ may already indicate pathological eosinophilia.

Our clinical observations have revealed the coexistence of ILD and bronchial disease, as well as characteristic "bronchial-type" exacerbations in ILD patients with unexpected eosinophilia. Investigators believe that both this association and its clinical relevance are currently underestimated in the management of ILD.

There is no established consensus on the management of ILD exacerbations, which often result in short- or medium-term mortality. However, recent studies suggest that high-dose corticosteroid therapy may improve outcomes, supporting our hypothesis of a T2-driven "bronchial-like" inflammatory component in some ILD patients.

Nonetheless, corticosteroids are not always effective and may lead to significant side effects, including heightened infection risk and cardiovascular complications.

In this context, E-PMNs could represent the common pathological link between ILD and bronchial inflammation and may serve as an actionable therapeutic target. This would open the door to existing biotherapies, already available in clinical practice.