Resistance to neoplastic transformation of ex-vivo expanded human mesenchymal stromal cells after exposure to supramaximal physical and chemical stress

The risk of malignant transformation of ex-vivo expanded human mesenchymal stromal cells (huMSCs) has been debated in the last years; however, the biosafety of these cells after exposure to supramaximal physical and chemical stress has never been systematically investigated. We established an experimental in vitro model to induce supramaximal physical (ionizing radiation, IR) and chemical (starvation) stress on ex-vivo expanded bone marrow (BM)-derived huMSCs and investigated their propensity to undergo malignant transformation. To this aim, we examined MSC morphology, proliferative capacity, immune-phenotype, differentiation potential, immunomodulatory properties and genetic profile before and after stressor exposure. Furthermore, we investigated the cellular mechanisms underlying MSC response to stress. MSCs were isolated from 20 healthy BM donors and expanded in culture medium supplemented with 5% platelet lysate (PL) up to passage 2 (P2). At this stage, MSCs were exposed first to escalating doses of IR (30, 100, 200 Gy) and then to starvation culture conditions (1% PL). With escalating doses of radiation, MSCs lost their typical spindle-shaped morphology, their growth rate markedly decreased and eventually stopped (at P4-P6) by reaching early senescence. Irradiated and starved MSCs maintained their typical immune-phenotype, ability to differentiate into adipocytes/osteoblasts and to inhibit mitogen-induced T-cell proliferation. The study of the genetic profile of irradiated/ starved MSCs did not show any alteration. While the induction of supramaximal stress triggered production of ROS and activation of DNA damage response pathway via multiple mechanisms, our data indicate that irradiated/starved MSCs, although presenting altered morphology/growth rate, do not display increased propensity for malignant transformation.