(P 13) TRANSCRIPTIONAL DIVERSITY AND NICHE REMODELING AT THE METASTATIC BOTTLENECK IN OSTEOSARCOMA

Objective: Metastasis in osteosarcoma is highly inefficient; although many tumor cells enter circulation, few succeed in colonizing the lung. Emerging evidence points to a subpopulation of “anchor cells” that establish paracrine signaling with the lung microenvironment, producing cytokines such as IL-6 and CXCL8 that recruit proliferative tumor cells. We aimed to characterize the transcriptional landscape of these anchor cells and define how they remodel the metastatic niche.

Methods: We performed single-cell RNA sequencing (scRNA-seq) on OS-17 lung metastases isolated at early and midpoint timepoints to define a transcriptional program of “anchorness.” We then performed scRNA-seq on lung stromal cells to identify the earliest changes in stromal phenotype and composition at the metastatic bottleneck.

Results: Tumor cells present at early metastatic timepoints were transcriptionally diverse and lacked a uniform anchor cell signature previously described in functional studies. Notably, early tumor–lung interactions rapidly induced persistent fibrogenic remodeling of the stroma: fibrogenic pathways were upregulated in macrophages and fibroblasts by day 14. These changes may help create a scar-like matrix that supports tumor cell survival and proliferation despite the metastatic bottleneck.

Conclusion: Heterogeneity in tumor cells is preserved even at early stages of lung colonization, and this heterogeneity contributes to niche remodeling. Our findings highlight the importance of shifting focus from purely tumor-intrinsic phenotypes to understanding how anchor cells condition the lung microenvironment. Identifying key mediators of tumor–matrix crosstalk could inform new therapeutic strategies to limit metastatic progression.