Combination of Interferon α, Delivered By Engineered Mesenchymal Stromal Cells, and Cytarabine Limits the Development of Acute Myeloid Leukemia, Potentially Targeting Leukemic Stem Cells

Background: While most acute myeloid leukemia (AML) patients achieve remission, approximately 50% will relapse which is generally attributed to the persistence of leukemic stem cells (LSCs). Indeed, those cells, regarded as the origin of leukemia, are chemoresistant. The Side Population (SP) cells of AML are known to be enriched in LSCs and to be more resistant to chemotherapy compared to non-SP AML cells. So, they are good candidate to target in the establishment of new protocol of treatment. Interestingly, it has been demonstrated that bone marrow mesenchymal stem/stromal cells (MSC) nesting promotes the proportion of AML SP cells (Malfuson et al., 2013).

Aims: In this proof-of-concept study, we developed a two-step treatment based on the use of MSCs to activate the SP AML cells and of cytarabine, commonly used to treat AML, to eliminate them. However, as MSCs are suspected to stimulate leukemic growth and to protect against chemotherapies, MSCs have been engineered to deliver locally IFNα directly to an AML chloroma to enhance the anti-proliferative effect of cytarabine and to overcome the potential MSC pro-tumoral effect.

Methods: Human MSCs were isolated and transduced with a lentiviral vector expressing human IFNα (IFNα MSCs) or a control vector (Ctrl MSCs). Flow cytometric analysis and ELISA confirmed the secretion of bio-active IFNα by transduced MSCs. The anti-AML activity of these MSCs was assessed by co-culture with the KG1a AML cell line. The in vivo validation was performed by subcutaneous injection of KG1a cells (chloroma model) with or without Ctrl MSCs or IFNα MSCs in NSG mice and monitoring of tumor volume.

Results: MSC presence doubled the SP phenotype among AML cells in the co-culture system as expected. The IFNa MSCs significantly decreased the leukemic population by 30% by inducing more early and late apoptosis. The in vivo experiment confirmed this anti-leukemic activity with a lower tumor growth characterized by a tumor volume decreased by 40% compared to no MSC and Ctrl MSC conditions after 40 days post-injection. The effect of cytarabine on the IFNα MSC action was explored by the addition of the drug to the co-culture system or in the chloroma model. Thus, we showed that cytarabine treatment reduced by 7-fold leukemic SP cell proportion in regular culture of AML cells and by 26-fold in the IFNα MSC co-cultures demonstrating that MSC induced SP cells are more sensitive to the drug than basal SP cells. Combination of cytarabine with IFNa secreted by the modified MSCs enhances the anti-leukemic activity of those MSCs in vitro with clear additive effects on AML cell number reduction and early and late apoptosis induction, and with a synergistic effect on cell cycle arrest. The beneficial effect of using IFNa MSCs in addition to cytarabine was demonstrated in vivo with a 40% tumor volume reduction after 40 days post-injection and a longer survival compared to cytarabine or IFNa MSCs alone.

Summary/conclusions: This study showed that IFNα MSCs in combination with cytarabine present a strong anti-leukemic effect, decreasing the leukemic burden and potentially leukemic stem cell pool. This new two-step therapeutic protocol represents a promising alternative for AML treatment.