Involvement of naïve T cells in the pathogenesis of osimertinib-induced pneumonitis.

Osimertinib, a third-generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor, effectively treats EGFR-mutated non-small cell lung cancer. Drug-induced interstitial lung disease, a potentially fatal complication of osimertinib, involves an increase in lymphocytes in the bronchoalveolar lavage fluid (BALF). As the precise role of these lymphocytes is unknown, we investigated the pathogenesis of osimertinib-induced pneumonitis using BALF obtained from patients, and an osimertinib-induced pneumonitis mouse model. Mass cytometry revealed the presence of CCR7+ CD45RA+ naïve T cells in the BALF of patients with osimertinib-induced pneumonitis. Body weight measurements, BALF analysis, histopathological evaluation and RNA sequencing of mouse lung tissue were performed to investigate immune cell involvement and the mechanisms underlying osimertinib-induced pneumonitis. In the mouse model, administration of osimertinib after naphthalene-induced damage to the bronchiolar epithelium exacerbated lung inflammation and resulted in significant weight loss. Immunofluorescence staining revealed the infiltration of CCR7+ cells into the lungs of mice treated with naphthalene and osimertinib. Bulk RNA sequencing identified an upregulation of chemokine-related biological processes, with increased expression of C-C motif chemokine ligand 21 (Ccl21) and C-C motif chemokine ligand 8 (Ccl8) in the lung tissue. Additionally, immunofluorescence staining confirmed elevated expression of Ccl21 and Ccl8 in the distal bronchiolar epithelium. This study provides insights into the mechanisms underlying osimertinib-induced pneumonitis.