Abstract Denosumab, a RANKL inhibitor, is primarily used to prevent osteoclastogenesis for the treatment of conditions like osteoporosis, bone metastasis, and giant cell tumor of bone (GCTB). RANKL is also key in immunity, activating NFκB and its target genes, including the osteopontin-coding gene SPP1, which is linked with CXCL9:SPP1 macrophage polarization and prognosis. Retrospective studies of bone metastasis have demonstrated improved clinical outcomes when immune checkpoint inhibitors (ICIs) are combined with denosumab. We undertook a study to explore denosumab's direct role in anti-tumor immunity. Single-cell RNA sequencing was performed on nine primary human GCTB samples, including three untreated and six treated solely with denosumab, to exclude confounding treatment factors linked with bone metastasis samples. Furthermore, we utilized immunohistochemistry staining from paired pre- and post-denosumab treated samples from a cohort of nine GCTB patients and conducted pan-cancer analysis on primary tumors, spanning 31 distinct types of cancers, sourced from the Genomic Data Commons Data Portal. Our single-cell analysis of GCTB resulted in a comprehensive cell atlas of this rare disease, revealing denosumab's role in inhibiting SPP1 expression and augmenting cytotoxic T cell infiltration. This was confirmed in nine pairs of pre- and post-denosumab treated GCTB samples using immunohistochemistry staining. The pan-cancer analysis indicated a negative correlation between SPP1 expression and CD8A levels across most tumor types, with the CD8A/SPP1 ratio correlating with clinical outcomes in 14 cancer types. Our research provides robust clinical evidence that denosumab improves anti-tumor immunity by inhibiting SPP1 expression, presenting a significant step towards patient selection for combined ICI and denosumab therapy. Citation Format: Zezhuo Su, Maximus Chun Fai Yeung, Jason Pui Yin CHEUNG, Kelvin Sin Chi Cheung. Denosumab Enhances Cytotoxic T Cell Infiltration by supressing SPP1 Expression in Giant Cell Tumour of Bone [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor Immunology and Immunotherapy; 2024 Oct 18-21; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2024;12(10 Suppl):Abstract nr B005.