Chemokine-Driven Intercellular Crosstalk in the Osteosarcoma Microenvironment After Neoadjuvant Chemotherapy: A Single-Cell RNA Sequencing Study

Background/Objectives: Osteosarcoma (OS) is an aggressive bone malignancy with a complex tumor microenvironment (TME) that influences therapeutic outcomes and resistance. How neoadjuvant chemotherapy (NACT) reshapes the OS TME at single-cell resolution remains largely undefined. This study aimed to characterize cellular heterogeneity in the OS TME after NACT and identify chemokine-mediated intercellular crosstalk driving chemoresistance. Methods: Single-cell RNA sequencing was performed on surgical specimens from 12 OS patients (6 treatment-naive and 6 post-NACT). After quality control, 77,616 cells (36,214 from naive patients, 41,402 from post-NACT samples) were analyzed through the Seurat pipeline. Unsupervised clustering, differential expression analysis, and cell–cell communication network construction were performed, and candidate signaling axes were validated using transwell assays and Western blotting. Results: Cells were classified into 10 major cell types. Osteoblasts, identified as malignant cells, were partitioned into 11 subpopulations with marked transcriptional heterogeneity and differential PI3K/Akt pathway activity. Post-NACT, stromal and vascular components underwent molecular and functional remodeling, shaping an immune-activated microenvironment. Mononuclear phagocytes resolved into three discrete clusters—monocytes, macrophages, and dendritic cells—with differentiation gradients. Endothelial cells maintained robust CXCL2 expression throughout the therapeutic course. Functional validation via transwell assays and Western blotting confirmed that endothelial-derived CXCL2 promoted macrophage chemotaxis via CXCR2, with corresponding CXCR2 upregulation in macrophages. Conclusions: Collectively, these findings suggest the complex cellular and transcriptional heterogeneity of the OS microenvironment and its chemokine-driven molecular remodeling after NACT, indicating that TME dynamics may be a determinant of therapeutic response and chemoresistance.