Extracellular vesicles in osteosarcoma: bridging resistance, immunity, and clinical translation.

Osteosarcoma (OS), the most common primary malignant bone tumor in children, adolescents, and young adults, has experienced only limited improvement in 5-year survival over the past four decades, with survival rates remaining approximately 60-70% for localized disease and below 30% for metastatic or refractory cases. Chemoresistance and an immunosuppressive tumor microenvironment (TME) constitute mutually reinforcing barriers to durable disease control, while extracellular vesicles (EVs) have emerged as key mediators linking tumor-intrinsic resistance, microenvironmental remodeling, and metastasis. Under the MISEV2023 framework, OS-derived EV studies face unique methodological challenges, including mineralized matrix interference, non-vesicular particle contamination, platelet-derived EV interference, and variability in isolation workflows. Nevertheless, standardized and OS-adapted EV workflows may improve the reliability of EVs as candidate liquid biopsy biomarkers for chemotherapy-response assessment, prognostic stratification, immune-status evaluation, and minimal residual disease (MRD)-oriented surveillance. Mechanistically, EVs can propagate chemoresistance by transferring ATP-binding cassette (ABC) transporters and regulatory non-coding RNAs (ncRNAs), reprogram metabolism and cell fate through processes such as ferroptosis resistance, lipid synthesis, autophagy-induced dormancy, and stemness acquisition, remodel bone marrow and pulmonary pre-metastatic niches, and contribute to immunosuppression through macrophage polarization, vascular abnormalization, and restricted T-cell infiltration. Emerging EV-based therapeutic strategies include engineered EVs or exosome mimetics for drug and nucleic acid delivery, inhibition or clearance of tumor-derived EVs, and EV-based vaccine concepts combined with immunotherapy. However, most therapeutic evidence remains preclinical or hypothesis-generating, and translation is constrained by stochastic cargo loading, EV heterogeneity, scalable clinical-grade production, age-specific considerations, and limited clinical validation. This review integrates current evidence on EV-mediated chemoresistance, immune remodeling, and clinical translation in OS, aiming to provide a critical framework for future EV-based biomarker development and therapeutic exploration.