(P 74) TERTIARY LYMPHOID STRUCTURES PREDICT PROGNOSIS AND IMMUNE LANDSCAPE IN OSTEOSARCOMA: A MACHINE LEARNING-BASED TRANSCRIPTOMIC STUDY

Objective: Osteosarcoma (OS) is a highly aggressive bone malignancy with poor prognosis in advanced cases. Tertiary lymphoid structures (TLS), ectopic lymphoid aggregates that orchestrate anti-tumor immunity, have shown prognostic and therapeutic relevance in various solid tumors. However, their presence and function in OS remain unexplored.

Methods: We analyzed transcriptomic data from 286 primary OS samples across public databases and Peking University People's Hospital cohort to identify TLS-related gene expression patterns. A TLS prognostic index (TLSPI) was developed through the integration of 99 machine learning model combinations. TLS presence was validated by immunohistochemistry and multiplex immunofluorescence. Functional enrichment, immune landscape profiling, immunotherapy response prediction, and drug sensitivity analysis were performed using multi-omics bioinformatics tools. The immunoregulatory role of CXCL13 in TLS formation was further investigated through single-cell analysis and in vitro assays.

Results: The spatial structure of TLS was detected in OS specimens. The TLSPI signature (CXCL13, IL2RA, STAT5A) demonstrated superior prognostic performance compared to existing OS models. Low TLSPI was associated with immune-active tumor microenvironment (TME), enriched cytotoxic lymphocyte infiltration, higher expression of immune checkpoint genes, and enhanced predicted responsiveness to immune checkpoint blockade. Mechanistically, CXCL13 promoted B-cell migration, activation, and survival, suggesting its critical role in TLS formation within the OS microenvironment.

Conclusion: Our study provides the first evidence of TLS in OS and introduces a robust TLS-based prognostic model with strong clinical and immunological relevance. TLSPI holds promise for patient risk stratification, immunotherapy response prediction, and personalized treatment guidance. Targeting TLS-associated pathways, particularly CXCL13-mediated B-cell recruitment, may offer novel therapeutic avenues in OS.