Bronchoalveolar lavage fluid in immune checkpoint inhibitor-related pneumonitis: from pathophysiological window to a precision diagnostic tool.

Immune checkpoint inhibitor related pneumonitis is a serious adverse reaction with diverse clinical and radiologic patterns, which make both diagnosis and treatment challenging. Therefore, understanding its underlying biology is crucial for improving clinical management. Bronchoalveolar lavage fluid provides a minimally invasive way to explore the lung immune environment, and it supports cytologic, molecular, and multi-omics analyses. In particular, BALF lymphocytosis serves as a key diagnostic sign. Furthermore, single-cell sequencing has revealed that abnormal T-cell activation and myeloid cell reprogramming play central roles in the development of CIP. These findings have, in turn, led to the identification of potential biomarkers such as CCL18, IL-6, and IP-10 for early detection and disease monitoring. However, the absence of standardized sampling and interpretation methods still limits the reproducibility of results and the broader application of BALF analysis in clinical practice. In the future, integrating BALF-derived data into artificial intelligence frameworks may enhance diagnostic precision and guide personalized therapy. Overall, BALF represents a valuable platform for refining CIP classification, clarifying disease mechanisms, and supporting the development of targeted treatments.