Novel Evidence That the Ectonucleotidases CD39 and CD73, Which Are Expressed on Hematopoietic Stem/Progenitor Cells (HSPCs), Regulate Mobilization and Homing - Studies in CD39-/- and CD73-/- Mice and with Small-Molecule CD39 and CD73 Inhibitors

Background. Adenosine triphosphate (ATP) is an important nucleotide involved in intracellular energy transfer, but it is also released from activated cells into the extracellular space as a crucial component of the purinergic signaling network. Purinergic signaling is an ancient form of extracellular signaling that is mediated by extracellular nucleotides (EXNs), including most importantly the purine ATP and its nucleoside metabolite, adenosine. Purinergic receptors for EXNs are expressed on the surface of all cells in the body; are represented by several families of P1, P2X, and P2Y receptors; and are among the most abundant receptors in living organisms. ATP, the most important EXN, is secreted from cells through pannexin and connexin channels and is processed in the extracellular space to ADP, AMP and finally adenosine by the cell-surface-expressed ectonucleotidases CD39 and CD73. We recently reported that the ATP-adenosine axis regulates the mobilization of hematopoietic stem progenitor cells (HSPCs) (Leukemia 2018, in press,doi: 10.1038/s41375-018-0122-0). Hypothesis. We hypothesized that the ectonucleotidases CD39 and CD73 are important modulators of the ATP-adenosine axis and are new and underappreciated modulators, not only of mobilization but also of bone marrow (BM) homing of HSPCs. Materials and Methods. To shed more light on the role of purinergic signaling in the trafficking of HSPCs, we i) phenotyped murine and human HSPCs for expression of CD39 and CD73 ectonucleotidases by FACS, ii) performed mobilization and homing studies in CD39^-/- and CD73^-/- mice, and iii) blocked CD39 and CD73 activity by employing the small-molecule inhibitors ARL 67156 and AMP-CP, respectively. CD39- and CD73-deficient mice were mobilized with G-CSF or AMD3100, and wild type (WT) animals were exposed to CD39 and CD73 inhibitors in addition to mobilizing agents. Following mobilization, we measured i) the total number of white blood cells (WBCs) and ii) the number of clonogenic colony-forming unit granulocyte/macrophage (CFU-GM) progenitors and Sca-1⁺c-kit⁺lineage^- (SKL) cells circulating in PB. To address the involvement of CD39 and CD73 expressed on the surface of HSPCs or in the hematopoietic microenvironment, we created irradiation chimeras that were subsequently mobilized with G-CSF or AMD3100. Bone marrow (BM) transplantation studies were performed by transplanting WT mice with WT BM cells or BM cells from ectonucleotidase-deficient mice. Homing efficiency was evaluated by determining i) the number of fluorochrome-labeled cells in BM 24 hours after transplantation ii) the number of CFU-S and CFU-GM progenitors that had engrafted in BM on day 12, and iii) the recovery of peripheral blood counts. Results. We found that CD39 and CD73 are expressed by murine and human HSPCs and process ATP to adenosine, which as we observed, is a novel and potent inhibitor of HSPC trafficking. In support of this finding, CD73^-/- and CD39^-/- mice, with reduced extracellular adenosine levels, mobilize HSPCs much better than their normal control littermates. A similar effect was observed in WT mice exposed to CD39 and CD73 small-molecule inhibitors during mobilization. Studies in irradiation chimeras revealed that this effect depended on expression of CD39 and CD73 on the surface of HSPCs and not in the BM microenvironment. Moreover, adenosine also inhibited the homing of HSPCs to BM, as cells from CD39^-/- or CD73^-/- mice engrafted better than WT cells. Furthermore, homing was also inhibited in the presence of exogenously added adenosine. That ectonucleotidase-deficient mice engrafted BM cells from WT littermates as efficiently as did WT recipients indicates the involvement of HSPC-expressed CD39 and CD73 in regulating the homing process. Conclusions. We demonstrate for the first time that HSPC-expressed CD39 and CD39 ectonucleotidases modulate the trafficking of HSPCs. By processing ATP to adenosine, these enzymes negatively affect both the mobilization and the homing processes. Based on this finding, future strategies employing small-molecule inhibitors of both ectonucleotidases could find practical application in improving mobilization in poor mobilizers and accelerating the engraftment of HSPCs after transplantation.