Osteosarcoma (OS) is a highly malignant mesenchymal tumor composed of cells that produce bone-like and immature bone tissue. The peak incidence occurs in the second decade of life after birth. Although relatively rare, the social impact of this tumor is particularly relevant. Unlike other cancers, the molecular genetics of OS progression and the role of its unique tumor microenvironment (TME)—the bone—remain largely unclear.
While the TME has been extensively studied in other solid tumors to describe its role in tumor progression and, in part, in distant metastasis, research into tumor-stroma interactions in OS has been somewhat neglected for decades. This is partly due to the rarity of OS tumors, their unique location within the skeleton, and the lack of genetically engineered mouse models that allow the study of the involvement of innate and acquired immune cells.
However, recent technological advancements in transcriptomics, proteomics, and bioinformatics have provided a new wave of knowledge about the OS TME.
Recently, researchers from the IRCCS National Cancer Institute Foundation published a study titled “Rewiring innate and adaptive immunity with TLR9 agonists to treat osteosarcoma” in the journal J Exp Clin Cancer Res. The study data suggest that TLR9 agonists act as in-situ anti-tumor vaccines, activating innate immune responses sufficient to inhibit local tumor growth while inducing systemic adaptive immunity by selectively amplifying CD8 T-cell clones, which are essential for exerting non-local effects.
Osteosarcoma (OS) is the most common primary bone tumor in children and adolescents. Standard treatment involves surgery and various chemotherapy regimens, achieving event-free survival rates of 60%–70% for localized disease at diagnosis. However, the prognosis is discouraging for metastatic disease. Harnessing the immune system’s activation in the context of this challenging mesenchymal tumor is a novel therapeutic endeavor.
In an immunocompetent OS mouse model with two contralateral lesions, we tested the efficacy of TLR9 agonists on treated and untreated contralateral lesions to assess non-abscopal effects. Changes in the tumor immune microenvironment were detected using multi-parameter flow cytometry. In experiments with immunocompromised mice, the role of adaptive T cells in the effects of TLR9 agonists was studied, while T-cell receptor sequencing was used to assess the expansion of specific T-cell clones.
TLR9 agonists strongly inhibited the growth of both locally treated and untreated contralateral tumors. Multi-parameter flow cytometry showed significant changes in the immune landscape of the OS immune microenvironment after TLR9 engagement, including a reduction in M2-like macrophages and increased infiltration of dendritic cells and activated CD8 T cells in both tumor lesions.
Notably, CD8 T cells were essential for inducing an abscopal effect but not strictly required to prevent tumor growth after treatment. T-cell receptor (TCR) sequencing of tumor-infiltrating CD8 T cells revealed the selective expansion of specific TCR clones in treated tumors, with notable selective expression in untreated contralateral lesions, providing the first evidence of a reconfigured structure of tumor-related T-cell clones.
In summary, this study demonstrates the anti-tumor activity of TLR9 in OS, a high-grade, invasive tumor for which there are no targeted therapies or effective immunotherapies available. The evidence provided by this research suggests that reprogramming the tumor immune microenvironment may be a valuable approach to improving the effectiveness of standard or novel treatment approaches for poorly infiltrated cold tumor types, such as OS.