PS1448 LIN‐CD133+CD34+CD41+ HSPC REPRESENT A MEGAKARYOCYTE‐PRIMED NEOPLASTIC FRACTION IN MPN PATIENTS WITH MYELOFIBROSIS
Authors: V. Koehl, O. Bhatt, S. Zeschke, N. Kröger, I. Triviai
Journal: HemaSphere (2019)

Background: Primary myelofibrosis (PMF) is a clonal stem cell disorder characterized by aberrant differentiation of all myeloid lineages. Atypical megakaryopoiesis is considered a dominant factor in myelofibrosis (MF) development, however the driver mechanisms of MF progression in Myeloproliferative neoplasms (MPN) remain poorly understood. Recent studies reported the expansion of CD41+ myeloid progenitors in another MPN, exhibiting commitment to megakaryocytic differentiation (Miyawaki, Iwasaki et al. 2017). Our previous work indicates that xenotransplantation of Lin‐CD133+CD34+ HSPC from MF patients recapitulates aberrant human megakaryopoiesis and MF progression in vivo (Triviai, Stubig et al. 2015). Aims: This prompted us to investigate whether a subfraction of the neoplastic Lin‐CD133+CD34+ HSPC in MF comprises a megakaryocyte‐primed population. We hypothesize that Lin‐CD133+CD34+CD41+ cells may be sustaining aberrant megakaryopoiesis and play an important role in MPN‐related fibrogenesis. Methods: We investigated the expression of CD41+ in the HSPC fractions of MF patient peripheral blood and normal bone marrow (BM). Moreover, we assessed the megakaryocytic potential of different CD41+ HSPC fractions from MF patients in comparison to normal BM in suspension cultures and colony assays. Peripheral blood samples from MF patients (post‐PV, post‐ET, PMF) and healthy BM samples were used to characterize various CD41 expressing HSPC populations by FACS. To determine their clonogenic potential, Lin‐CD133+CD34+CD41+ and CD41‐ populations were seeded in semi‐solid colony assays, suspension cultures with added thrombopoietin (TPO), as well as in long‐term colony‐initiating cell assays. Results: Lin‐CD133+CD34+CD41+ cells could be detected in peripheral blood of all three MF patient groups, as well as in healthy donor BM. No significant expansion of total CD41+ HSPC cells could be detected in patient blood samples compared to healthy BM. Our analysis indicates that the Lin‐CD41+ fraction is dominated by CD41+ CMP/MEP progenitors in normal BM. In contrast to BM controls, these early progenitors are under represented in circulating Lin‐CD41+ cells of MF patients and the expansion of Lin‐CD34 mid CD41+ population is observed. Further phenotypic analysis of the Lin‐CD41+ fraction circulating in MF patients indicated the constant presence of a Lin‐CD133+CD34+CD38‐CD90+CD41+ HSC population ranging within 3–35% of the Lin‐CD133+CD34+CD41+ fraction, suggesting the presence of early HSC CD41+ within the neoplastic population in patients. Colony forming assays of patient Lin‐CD133+CD34+CD41+ cells suggest lower overall clonogenic potential compared to CD41‐ cells of the same population. Prior stimulation with TPO did not increase the CFU‐Mk potential in CD41+ or CD41‐ cells, but did result in a megakaryocyte phenotype for both fractions. LTC‐IC assays revealed a restricted proliferation and colony formation after six weeks of co‐cultures of CD41 expressing neoplastic HSPC compared to the CD41‐ fraction. Summary/Conclusion: Our preliminary results suggest the presence of CD41+ cells within the Lin‐CD133+CD34+ HSPC population found in the peripheral blood of MPN patients with MF. This fraction exhibits limited clonogenic potential and predominantly differentiates towards the MEP lineage under TPO stimulation. Additionally, CD41+ HSC within the neoplastic fraction appear exhausted in vitro and correlate with the expansion of early megakaryocyte progenitors in patient blood. Our work sets the basis to further investigate the neoplastic HSC source of aberrant megakaryopoiesis in MPN and its role in MF progression.