Depletion of Jak2V617F MPN Stem Cells by IFNα in a Murine Model of Polycythemia Vera
Authors: Mullally Ann , Bruedigam Claudia , Heckl Dirk , Poveromo Luke , Heidel Florian H. , Hill Geoff , Ebert Benjamin L. , Lane Steven W. 
Published: October 2019

Abstract
Abstract 806
Interferon alpha (IFNα) is an effective treatment for patients with myeloproliferative neoplasms (MPN). In addition to inducing hematological responses in most MPN patients, IFNα reduces the JAK2V617F allelic burden and can render the JAK2V617F mutant clone undetectable in some patients. The mechanism underlying these responses is incompletely understood and whether the molecular responses that are seen occur due to the effects of IFNα on JAK2V617F mutant stem cells is debated.
Using a murine model of Jak2V617F MPN, we investigated the effects of IFNα on Jak2V617F MPN stem cells in vivo. Chimeric transplant recipients were generated with purified stem cell enriched populations (lin−Kit+Sca1+) and these were treated for 4 weeks with either IFNα or vehicle control.
IFNα treatment caused a reduction in extramedullary hematopoiesis (spleen weight, vehicle 262mg vs IFNα 192mg, p<0.01), hematocrit (vehicle 76.0% vs IFNα 65.5%, p<0.05) and white blood cell count (vehicle 13.9×109/L vs IFNα 7.5×109/L, p<0.01) in this disease model. IFNα treatment caused a reduction in early (CD71+Ter119+) erythroid progenitors that had accumulated in the spleen of Jak2V617F mice. IFNα treatment caused selective depletion of Jak2V617F MPN hematopoietic stem cells (HSC, lin−kit+Sca1+CD150+CD48−) over time and this was associated with reduced Jak2V617F chimerism in the long-term HSC compartment (Jak2V617F chimerism Vehicle 41.4% vs. IFNα 23.9%, p<0.05). IFNα treatment impaired the transmission of Jak2V617F-MPN and reduced Jak2V617F chimerism in transplanted recipient mice, demonstrating functional depletion of disease-specific stem cells.
Mechanistically, IFNα treatment preferentially induced cell-cycle activation of Jak2V617F mutant long-term HSCs. Gene expression profiling revealed relative enrichment of cell cycle genes and depletion of quiescence related genes in IFNα treated Jak2V617F HSC compared to IFNα treated WT HSC. IFNα treatment promoted a predetermined terminal erythroid-lineage differentiation program within myeloid progenitor cells. The effects on Jak2V617F long-term HSC were absent in Jak2V617F+/−IFNAR1−/− (lacking the type 1 interferon receptor) chimeric mice demonstrating that the effects of IFNα treatment were cell autonomous and specific for type 1 interferon signalling.
These findings provide insights into the differential effects of IFNα on Jak2V617F mutant and normal hematopoiesis and suggest that IFNα achieves molecular remissions in MPN patients through its effects on MPN stem cells. Furthermore, these results support combinatorial therapeutic approaches in MPN, by concurrently depleting dormant JAK2V617F MPN-propagating stem cells with IFNα and targeting the proliferating downstream progeny with JAK2-inhibitors or cytotoxic chemotherapy.

Disclosures:
Heidel: Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees. Ebert:Celgene: Consultancy; Genoptix: Consultancy.