Osteosarcoma is a malignant bone tumor commonly found in adolescents aged 10-25. The underlying mechanism of osteosarcoma progression is not completely understood, but it is associated with genetic mutations, abnormal growth factors, and pro-survival signaling pathways activation (Misaghi et al., 2018). Treatment options for osteosarcoma include surgery, chemotherapy, and radiation therapy. While surgical resection combined with chemotherapy or radiation therapy can yield desirable outcomes for the early-stage osteosarcoma, the efficacy of chemotherapy and radiation therapy decreases as the condition worsens (Smrke et al., 2021). Therefore, exploring the pathogenesis of osteosarcoma and proposing novel, effective treatments is vital for treating osteosarcoma in a clinical setting.

MPs are small cell membrane-bound particles commonly found in living organisms with a diameter typically ranging between 200-1000 nanometers (Burger et al., 2013). MPs are released by various types of cells and play diverse roles in both physiological and pathological states (Herring et al., 2013). Recent research has shown that MPs play a vital role in the occurrence and development of tumors by participating in the information transmission of tumor cells (Rak, 2010). MPs carry a negative charge, which allows them to counteract cations and specific proteases in the surrounding environment. These MPs contain effective components that include DNA, RNA, lipids, proteins, and other biomolecules, enabling rapid transmission to nearby tissues and cells through vesicles in the body, thereby affecting the activity of tumor cells (Amin et al., 2008). While MPs have various biological functions, including transferring biological information, promoting tumor invasiveness and metastasis, regulating the tumor microenvironment, and inhibiting immune responses, their specific role in tumor progression remains unclear. In-depth research on MPs is critical to clarify the tumor's mechanistic occurrence, improve treatment strategies, and develop new anti-cancer treatments.

Macrophages, an essential component of the immune system, play a vital role in tumor treatment (Ruffell, Coussens 2015). They can inhibit tumor growth by clearing cancer cells, and simultaneously activate tumor cells by secreting growth factors to promote their growth and metastasis (Nardin, Abastado 2008). Macrophages exhibit significant heterogeneity and can be categorized into two subtypes - M1 and M2. M1 macrophages primarily possess inflammatory and immune response-related functions, enabling them to eliminate bacteria, viruses. In contrast, M2 macrophages primarily perform anti-inflammatory and regenerative functions, promoting wound healing. However, in tumors, macrophage subtypes undergo transformation with a reduction in M1 macrophages, and an increase in M2 macrophages, which help to attract and activate macrophages to transform into the M2 subtype and participate in tumor growth and metastasis. This results in the promotion of tumor angiogenesis as M2 macrophages secrete vascular endothelial growth factors, providing essential nutrients and oxygen required for tumor growth and development (Cendrowicz et al., 2021). Recent studies indicate that tumor cells secrete MPs to regulate macrophage polarization (Ma et al., 2016). However, the precise biological mechanisms underlying the interactions between tumor-MPs-macrophages remain unclear.

Our study found that osteosarcoma cells could secrete MPs to stimulate tumor metastasis and chemoresistance in an immune dependent manner. Mechanistically, T-MPs promoted the M2-like phenotype transition of macrophages through TBK1/STAT6 signaling pathway. Meanwhile, those M2-like macrophages promoted osteosarcoma cell migration and chemoresistance through CCL18/STAT3 signaling pathway. Our study explored the role of T-MPs/macrophages axis in promoting osteosarcoma progression, and demonstrated that inhibition of STAT3 signaling could suppress the chemoresistance induced by T-MPs/macrophages axis, which descried novel insight for clinical osteosarcoma therapy.