This prospective observational study aims to evaluate whether immune profiles of T-cell subsets in bronchoalveolar lavage fluid can complement tumor PD-L1 expression, assessed by immunohistochemistry in tumor tissue specimens, in predicting clinical outcomes in patients with advanced lung cancer. Although tumor PD-L1 expression measured on tissue biopsies is widely used to guide immunotherapy decisions, its predictive value is limited by spatial heterogeneity and sampling variability. By analyzing activated, exhausted, and regulatory T-cell populations in bronchoalveolar lavage fluid and examining their association with tissue-based tumor PD-L1 expression, this study seeks to determine whether combining local immune biomarkers with PD-L1 expression improves the prediction of treatment response and survival in patients receiving standard-of-care systemic therapy.

Tumor PD-L1 expression assessed by immunohistochemistry on tumor tissue specimens is an established biomarker for selecting immune checkpoint inhibitor therapy in lung cancer; however, its clinical utility is limited by intratumoral heterogeneity, temporal variability, and dependence on the size and location of tissue biopsy samples. As a result, tissue-based PD-L1 expression alone does not fully capture the immune context of the tumor microenvironment or reliably predict treatment response and prognosis in all patients.

Bronchoalveolar lavage fluid provides access to the local pulmonary immune microenvironment and contains immune cells that may reflect tumor-immune interactions more directly than peripheral blood. Prior studies suggest that T-cell populations in bronchoalveolar lavage fluid share phenotypic characteristics with tumor-infiltrating lymphocytes and may serve as surrogate indicators of local antitumor immune activity. However, the relationship between bronchoalveolar lavage T-cell immunophenotypes, tissue-based tumor PD-L1 expression, and clinical outcomes has not been comprehensively evaluated in a prospective setting.

This single-center prospective observational cohort study enrolls adult patients with suspected or confirmed stage IV lung cancer who are undergoing clinically indicated bronchoscopy as part of routine diagnostic or management procedures. Residual bronchoalveolar lavage fluid obtained during standard-of-care bronchoscopy is analyzed using multicolor flow cytometry to quantify T-cell subsets, including activated, exhausted, and regulatory populations defined by surface marker expression. Tumor PD-L1 expression is evaluated by immunohistochemistry performed on diagnostic tumor tissue specimens obtained as part of routine clinical care and analyzed both as a continuous variable and by predefined expression categories.

Participants receive standard systemic anticancer therapy according to current clinical guidelines, including immune checkpoint inhibitor-based regimens when indicated. Clinical outcomes, including objective response rate, progression-free survival, and overall survival, are prospectively collected from electronic medical records. The study examines correlations between bronchoalveolar lavage T-cell immunophenotypes and tissue-based tumor PD-L1 expression, as well as whether stratification based on combined immune profiling provides improved prognostic information compared with tumor PD-L1 expression alone.

By integrating local immune profiling from bronchoalveolar lavage fluid with tumor PD-L1 expression assessed in tissue specimens, this study seeks to identify complementary biomarkers that may enhance risk stratification and refine prognostic assessment in advanced lung cancer, potentially informing future approaches to personalized immunotherapy.

Inclusion Criteria:

• Adults aged 19 years or older
• Patients with suspected or histologically confirmed stage IV lung cancer based on clinical and radiologic findings
• Scheduled to undergo clinically indicated bronchoscopy with bronchoalveolar lavage as part of routine clinical care
• Able and willing to provide written informed consent

Exclusion Criteria:

• Receipt of treatment for acute lower respiratory tract infection (including pneumonia or fungal infection) within 4 weeks prior to bronchoscopy
• History of solid organ transplantation or hematopoietic stem cell transplantation
• Current use of systemic immunosuppressive therapy
• Current or planned use of biologic immunomodulatory agents
• Known autoimmune disease requiring systemic immunosuppressive treatment
• Pregnant women
• Patients with recurrent stage IV lung cancer after prior definitive therapy

The study population comprises adult patients evaluated at SMG-SNU Boramae Medical Center in Seoul, Republic of Korea. Participants are drawn from routine clinical practice and include patients undergoing clinically indicated bronchoscopy as part of the diagnostic evaluation or management of suspected or confirmed stage IV lung cancer. All clinical assessments, treatments, and follow-up are performed according to standard-of-care practice based on current clinical guidelines, and no additional diagnostic or therapeutic procedures are introduced solely for research purposes.

Individual participant data will not be shared publicly. The study includes sensitive clinical, immunologic, and imaging data derived from bronchoalveolar lavage samples and other patient-specific assessments, and no formal data-sharing infrastructure or governance plan has been established at the time of trial registration. However, de-identified individual participant data that underlie the results reported in peer-reviewed publications may be made available after publication upon reasonable request to the corresponding author, subject to appropriate data use agreements, institutional approval, and compliance with applicable ethical and privacy regulations.