Celastrol increases osteosarcoma cell lysis by γδ T cells through up-regulation of death receptors

gamma delta T cells has been shown to exhibit profound antitumor effects in a broad range of tumor entities, including OS. However, resistance to gamma delta T cells is a serious problem in the management of OS. This study investigates the impact of celastrol on the expression of death receptors 4/5 (DR4/5) on OS cell lines (HOS, U2OS) and cancer cell lysis by gamma delta T cells. The results showed that celastrol increased transcription of DR4/5 in HOS and U2OS, leading to increased cell surface, and total DR4/5 protein expression. Celastrol sensitizes OS cell lines or autologous OS cells to healthy donorsderived or OS patient-derived gamma delta T cell cytotoxicity in vitro. The induction of DR4/5 molecules increased lysis of HOS and U2OS by gamma delta T cells which was abolished by addition of a blocking TRAIL antibody. Importantly, the cytotoxic activity of gamma delta T cells was unaltered by small-dose celastrol. Taken together, our data show that celastrol up-regulated DR4/5 on OS cells to be responsible for intercellular TRAIL/APO-2L crosslink that confers increased cancer cell lysis by gamma delta T cells. These results suggest the clinical evaluation of celastrol in OS, especially in combination with immunotherapy approaches employing adoptive gamma delta T cell transfer.