Cellular immunotherapies have made striking advances in cancer treatment. Frustratingly, however, tumor cells shape their environment, including blood and immune cells, which limits clinical efficacy. How can this be counteracted? An attractive way would be to tackle this at the source of all blood and immune cells: the blood stem cells in the bone marrow. Our research therefore focuses on the impact of tumor cells on blood stem cells and their interaction with a specific environmental cell type in the bone marrow: the mesenchymal stromal cell (MSC). In the laboratory we investigate bone marrow from patients with neuroblastoma, a childhood cancer that arises from precursor cells of the nervous system and can rapidly spread to bone marrow. Moreover, we mimic the bone marrow environment, including blood flow, in a small chip. In this so-called bone marrow-on-a-chip we inject tumor organoids, which are grown from tumor material. Through a better understanding of interactions of tumor cells with bone marrow cells, we expect to ultimately find new targets to restore blood stem cells and limit immunosuppression, for more effective cellular immunotherapy.