T cell exhaustion drives osteosarcoma pathogenesis

Osteosarcoma (OS) is a rare cancer with a bimodal age distribution that peaks in children and young adults. It has been shown that the expression of programmed cell death protein 1 (PD-1) and programmed death ligand-1 (PD-L1) on tumor-infiltrating immune cells negatively correlates with prognosis of OS patients. However, a comprehensive assessment of the tumor-infiltrating immune cells in OS and their function has not been performed.

CD8⁺ T cells were isolated from biopsy tissue samples collected from OS patients and control subjects. Mass cytometry, Treg suppression assay, mixed lymphocyte reaction assay, and effector T cell functional assay were performed to analyze the function of tumor-infiltrating T cells. A xenograft metastasis model was established in BALB/c nude mice.

Macrophages and CD3⁺ T cells comprised most of the tumor-infiltrating immune cells in OS, with a disproportionately higher number of helper CD4⁺ T cells than effector CD8⁺ T cells. Whereas the tumor-infiltrating regulatory T cells were functionally intact, the CD8⁺ T cells showed increased expression of the immune checkpoint receptor (ICR) PD-1 and T cell immunoglobulin and mucin-domain containing 3 (TIM3) and were functionally inactive. TIM3 blockade using a monoclonal antibody restored the T cell alloreactive function of the CD8⁺ T cells ex vivo. TIM3 blockade in a xenograft model of OS impaired tumor growth in vivo. TIM3 blockade decreased the number of tumor-infiltrating CD4⁺ T cells while increasing the numbers and functional activation of tumor-infiltrating CD8⁺ T cells in vivo.

These results highlight that TIM3 blockade might be a viable therapeutic option and should be tested in additional preclinical models.