Abstract Purpose: Therapy resistance is a major clinical hurdle in bone cancer treatment and seems to be largely driven by poorly understood microenvironmental factors. Recent evidence suggests a critical role for a unique subpopulation of mesenchymal stem cells with inflammatory features (iMSCs), though their origin and function remained unexplored. We demonstrate that cancer-secreted extracellular vesicles (EVs) trigger the development of iMSCs, which hinder therapy response in vivo, and set out to identify strategies to counteract their function. Experimental Design: The role of iMSCs in therapy resistance was evaluated in an orthotopic xenograft mouse model of osteosarcoma. EV-induced alterations of the MSC transcriptome were analyzed and compared with scRNA-seq data of osteosarcoma and multiple myeloma patient biopsies. Functional assays identified EV components driving iMSC development. We assessed the efficacy of clinical drugs in blocking iMSC-induced resistance in vivo. Results: We found that iMSCs are induced by interaction with cancer EVs and completely abrogate the antimetastatic effect of TGFb signaling inhibition. Importantly, EV-induced iMSCs faithfully recapitulate the inflammatory single-cell RNA signature of stromal cells enriched in multiple myeloma and osteosarcoma patient biopsies. Mechanistically, cancer EVs act through two distinct mechanisms. EV-associated TGFb induces IL6 production, while the EV-RNA cargo enhances TLR3-mediated chemokine production. We reveal that simultaneous blockade of downstream EV-activated pathways with ladarixin and tocilizumab disrupts metastasis formation and overcomes iMSC-induced resistance. Conclusions: Our observations establish iMSCs as major contributors to drug resistance, reveal EVs as physiological triggers of iMSC development and highlight a promising combination strategy to improve therapy response in bone cancer patients.

Tumor-secreted extracellular vesicles counteract therapy response by triggering inflammatory mesenchymal stem cell development