NK Cells Contribute to Therapeutic Effects of Type I Interferons in Polycythaemia Vera

Myeloproliferative neoplasms (MPN) are clonal hematopoietic stem cell (HSC) disorders resulting in massive overproduction of mature peripheral blood cells, e.g. myeloid cells in chronic myeloid leukemia (CML), erythrocytes in polycythemia vera (PV) or platelets in essential thrombocythemia (ET), eventually occurring with myelofibrosis (MF). The JAK2V617F mutation is highly abundant in PV and has triggered the development of JAK inhibitors relieving PV induced symptoms and splenomegaly. However, neither JAK inhibitors nor phlebotomy or chemotherapy as other treatment options affect the allelic burden of mutated JAK2 as indicator of the clone size of malignant HSCs and thus cannot alter the natural course of the disease. In contrast, interferon alpha (IFNa) is well known to be effective in all MPNs and can even induce long-term remissions but have only been recently approved in the EU for the treatment of PV. Concerning the mode of action, type I interferons (IFNs) can directly inhibit proliferation of the malignant hematopoietic stem cell (HSC) clone. As being an immune stimulatory, type I IFNs drive the activation of immune and stromal cells in general. Thus, it is reasonable to assume that indirect effects of IFNs may also contribute to therapeutic effects. However, the cell specific contribution of type I IFNs on normal and malignant HSCs is unexplored so far. To investigate the immunological mechanisms by which type I IFNs attack malignant HSCs in MPNs, we use a mouse model for PV, in which mice suffer from increased hematocrit and enlarged spleens and in which we can reduce disease symptoms by the injection of IFNa. Performing high-dimensional flow cytometry and single cell sequencing we show here a comprehensive analysis of the contribution of cells and pathways in the therapeutic effect of IFNa in P. vera mice. We identified NK cell frequencies and numbers to be increased in the bone marrow of P. vera mice upon IFNa therapy. In the absence of NK cells after antibody mediated depletion the reduced disease symptoms in IFNa treated P. vera mice were completely reversed, indicating that NK cells play a critical role in the beneficial effects of IFNa treatment of P. vera mice. Thus, NK cells may represent a previously unexplored target in the development of future treatments of MPNs.