Abstract 2263: Microenvironmental rewiring in giant cell tumor of bone: Stromal lineage hierarchy, immune diversion, and novel osteoclastogenic and angiogenic circuits.

Abstract Giant cell tumor of bone (GCTB) is a locally aggressive osteolytic neoplasm characterized by neoplastic stromal cells and abundant osteoclast-like giant cells, yet the microenvironmental architecture and regulatory networks that sustain tumor progression remain incompletely understood. Here, we performed integrated single-cell RNA sequencing and spatial transcriptomics on six treatment-naïve primary GCTB specimens to generate a high-resolution atlas of stromal, immune, myeloid, osteoclastic and vascular compartments. We identify a structured mesenchymal hierarchy linking pericytes, osteoblast precursors and tumor stromal cells through a continuous transcriptional trajectory, providing evidence that the neoplastic stromal population arises from, or co-evolves with, osteogenic and perivascular lineages. Tumor cells display marked inter- and intra-patient heterogeneity, characterized by discrete EMT-, coagulation-, myogenesis- and androgen-responsive subclusters, indicating diverse functional specializations within the stromal tumor compartment.Despite substantial cytotoxic T-cell infiltration across all patients, ligand-receptor inference revealed that effector immune signals—including CCL and PARs pathways—are preferentially directed toward osteoclast-like giant cells rather than tumor cells, suggesting an “immune diversion” or “immune sink” mechanism that may blunt anti-tumor immunity despite preserved T-cell functional states. Beyond confirming the canonical tumor-derived RANKL engagement of giant cells, we uncover two additional regulatory circuits: (i) a tumor-specific CHAD-ITGA2 signaling axis, with ITGA2 expression uniquely restricted to giant cells, implicating a selective osteoclastogenic mechanism with therapeutic potential; and (ii) a multisource CSF1/IL34-CSF1R network arising from tumor cells, osteoblast precursors and pericytes, cooperatively shaping myeloid recruitment and giant-cell formation, further supporting a shared stromal origin among these lineages.Finally, we identify a tumor-exclusive APELA-APLNR (APJ) angiogenic axis linking ligand production strictly in tumor cells to receptor expression exclusively in endothelial cells. Spatial transcriptomics confirmed that APELA is enriched within RANKL-high tumor niches, revealing a spatially confined angiogenic microdomain functionally distinct from the broadly distributed VEGF pathway.Together, this atlas defines the cellular logic and intercellular circuitry underlying GCTB progression and identifies multiple stromal, immune, osteoclastogenic and vascular vulnerabilities—including the CHAD-ITGA2 and APELA-APLNR axes—that may guide therapeutic strategies beyond RANKL inhibition. Citation Format: Chao Zhang, Jilong Yang, Xiangchun Li, Kexin Chen, Hongru Shen, Zhichao Liao. Microenvironmental rewiring in giant cell tumor of bone: Stromal lineage hierarchy, immune diversion, and novel osteoclastogenic and angiogenic circuits abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2263.