BDNF is a prognostic biomarker involved in immune infiltration of lung adenocarcinoma and is associated with brain metastasis.

Non-small cell lung cancer (NSCLC) is one of the leading causes of death worldwide. Brain metastases are a common complication of a wide range of human malignancies, particularly lung adenocarcinoma (LUAD). Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, has been linked to several human malignancies and has been shown to promote LUAD tumorigenesis. However, its function in the tumour immune microenvironment (TIME) remains largely unexplored, especially in complex brain tissue environments. In this study, BDNF was found to be particularly increased in patients with advanced tumour stage, lymphatic metastasis, and distant metastasis, indicating a correlation with LUAD progression. We characterized the prognostic value of BDNF and defined BDNF as an unfavourable prognostic indicator through a common driver gene-independent mechanism in LUAD. Furthermore, patients with increased BDNF levels in primary LUAD might have a higher risk of developing brain metastasis (BM), and central nervous system (CNS) metastasis showed an elevated expression of BDNF compared to their matched primary lesions. Additionally, we investigated the interaction between BDNF and infiltrating immune cells in both primary lesions and paired BM using multiplex immunostaining. The results showed that BDNF might drive an immunosuppressive tumour microenvironment (TME) by re-education of tumour-associated macrophages (TAMs) toward a pro-tumorigenic M2 phenotype, particularly in BM. Our findings demonstrate that BDNF serves as an independent potential prognostic marker and correlates with BM in LUAD. As it is closely related to TAM polarization, BDNF may be a promising immune-related biomarker and molecular target in patients with LUAD.