The Metastatic Bone Marrow Niche in Neuroblastoma: Altered Phenotype and Function of Mesenchymal Stromal Cells

SIMPLE SUMMARY: Patients with metastatic neuroblastoma are at risk for relapse after conventional therapy. It is believed that residual neuroblastoma cells in the bone marrow (BM) are responsible for this process after becoming resistant to chemotherapy. Studies in various tumor types suggest that cells in the BM environment (specifically the so-called mesenchymal stromal cells (MSCs)) can promote survival and chemoresistance of tumor cells. We characterized the MSC-compartment within the BM of patients with neuroblastoma, with and without metastases. Our data indicate that the quantity and functionality of the MSC population is altered in the presence of tumor cells. We furthermore identify a specific MSC-subpopulation which is exclusively detected in the presence of tumor cells. These data contribute to the understanding of the interactions between neuroblastoma cells and the environment, which is essential in order to target the tumor cells in the BM more effectively, thereby improving therapy success and preventing relapse. ABSTRACT: Background: The bone marrow (BM) is the main site of metastases and relapse in patients with neuroblastoma (NB). BM-residing mesenchymal stromal cells (MSCs) were shown to promote tumor cell survival and chemoresistance. Here we characterize the MSC compartment of the metastatic NB BM niche. Methods: Fresh BM of 62 NB patients (all stages), and control fetal and adult BM were studied by flow cytometry using well-established MSC-markers (CD34−, CD45−, CD90+, CD105+), and CD146 and CD271 subtype-markers. FACS-sorted BM MSCs and tumor cells were validated by qPCR. Moreover, isolated MSCs were tested for multilineage differentiation and Colony-forming-unit-fibroblasts (CFU-Fs) capacity. Results: Metastatic BM contains a higher number of MSCs (p < 0.05) with increased differentiation capacity towards the osteoblast lineage. Diagnostic BM contains a MSC-subtype (CD146+CD271−), only detected in BM of patients with metastatic-NB, determined by flow cytometry. FACS-sorting clearly discriminated MSC(-subtypes) and NB fractions, validated by mRNA and DNA qPCR. Overall, the CD146+CD271− subtype decreased during therapy and was detected again in the majority of patients at relapse. Conclusions: We demonstrate that the neuroblastoma BM-MSC compartment is different in quantity and functionality and contains a metastatic-niche-specific MSC-subtype. Ultimately, the MSCs contribution to tumor progression could provide targets with potential for eradicating resistant metastatic disease.