Work

56 Myeloid architecture and CD8 + T-cell-to-tumor distance dictate immunotherapy outcomes in osteosarcoma

Year: 2026

Type: conference-abstract

Abstract: Background Metastatic osteosarcoma (OSA) is largely refractory to immune checkpoint blockade due to an immunosuppressive, myeloid-dominated tumor microenvironment (TME). Reprogramming the TME with total tumor (tt) mRNA-lipid particle aggregate (mRNA-LPA) vaccines drives systemic immunity, but adaptive resistance limits monotherapy efficacy. Understanding TME spatial topology, specifically the distribution of CD8 + T cells relative to tumor and suppressive myeloid populations, is critical to deciphering resistance mechanisms and optimizing combinations.Methods A murine OSA lung metastasis model (K7M2, 1.25 × 10 6 cells/mouse i.v.) was treated with anti-PD-1 monotherapy, tt-mRNA-LPA monotherapy, combination therapy, or left untreated. FFPE whole lung sections (4 µm) were stained using the Akoya PhenoCycler panel (i.e., CD3, CD8, FoxP3, F4/80, Arg1, CD206, S100A9, Vimentin). High-dimensional spatial analysis, neighborhood mapping, and distance thresholding were performed via QuPath and Python-based Leiden clustering.Results Spatial profiling of mRNA-LPA monotherapy revealed an immunosuppressive TME dominated by M2-like tumor-associated macrophages (TAMs) and PMN-MDSCs. Tumor nests geographically excluded CD8 + T cells (exclusion z = +51), which were instead sequestered within myeloid niches and co-localized preferentially with M2 TAMs (z = +21). Combining mRNA-LPA with anti-PD-1 partially broke this exclusion. The median CD8-to-tumor distance decreased dramatically from 50 µm to 19 µm, and the proportion of tumor cells directly contacted by CD8+ T cells increased from 14% to 35%. This spatial engagement correlated with a reduced overall tumor fraction (22% to 15%).Conclusions The OSA TME spatial architecture acts as a primary barrier to immunotherapy. While mRNA-LPA monotherapy drives infiltration, T cells remain geographically excluded by suppressive myeloid networks, which were observed across osteosarcoma-bearing lungs from multiple treatment groups, suggesting that their persistence may represent a mechanism of adaptive resistance. Adding PD-1 blockade bridges this spatial gap, enabling CD8 + T cells to physically engage the tumor. The CD8-to-tumor relative distance serves as a crucial predictive spatial biomarker, suggesting that adding myeloid-targeting therapeutics to the mRNA-LPA/anti-PD-1 regimen is necessary to fully overcome TME resistance.

Source: Journal for ImmunoTherapy of Cancer

DOI: 10.1136/jitc-2026-sitc2026.0056