Modulation of Purinergic Signaling Prior to Transplantation Both in Hematopoietic Stem Progenitor Cells (HSPCs) or Recipient Bone Marrow (BM) Microenvironment - As Novel Strategies to Accelerate and Facilitate HSPCs Homing and Engraftment
Authors: Adamiak Mateusz , Thapa Arjun , Bujko Kamila , Brzeźniakiewicz-Janus Katarzyna , Ratajczak Janina , Kucia Magda , Ratajczak Mariusz Z 
Published: November 2021

Abstract
Background. The success rate of hematopoietic stem cell transplantation strongly depends on the number of transplanted hematopoietic stem/progenitor cells (HSPCs) and their speed of engraftment after infusion to the myeloablated transplant recipient. Therefore, clinical outcomes will benefit from accelerating the speed of homing and engraftment rate of transplanted HSPCs. This is important when the number of available HSPCs is low, as seen after poor harvest from BM, poor mobilization efficiency of the donor, and a low number of HSPCs present in the available umbilical cord blood (UCB) unit for an adult recipient. Our recent research demonstrated that purinergic signaling involving extracellular adenosine triphosphate (eATP) and its extracellular metabolite adenosine (eAdo) play a significant opposite role in homing/engraftment of HSPCs - reviewed in Curr Opin Hematol 2021, 28:251-261. To explain this eATP released from the cells of conditioned for transplantation by myeloablation recipient's BM facilitates homing of HSPCs, and subsequently becomes metabolized by cell surface ectonucleotidases CD39 and CD73 to eAdo, that inhibits this process. Therefore, eATP and eAdo upregulated in PB and BM modulate homing/engraftment in i) infused to the recipient donor-derived HSPCs and ii) in recipient BM microenvironment - in an opposite way. We also reported that the beneficial effect of eATP on homing/engraftment of HSPCs depends on the promotion of membrane lipid raft formation on the surface of HSPCs that incorporate homing receptors for their optimal interaction with BM released homing chemoattractants. This process is promoted by eATP activated Nlrp3 inflammasome. On the other hand, Nlrp3 inflammasome and membrane lipid raft formation are inhibited by eAdo in heme oxygenase-1 (HO-1)-dependent manner (Leukemia 2020; 34:1512-1523). Hypothesis. We hypothesized that proper modulation of eATP - eAdo signaling both at the level of transplanted HSPCs and recipient BM microenvironment will speed up the seeding efficiency of transplanted cells to BM niches. Material and Methods. We exposed HSPCs before transplantation ex vivo to i) exogenous eATP or ii) small molecular CD39 and CD73 inhibitors. We also inhibited CD39 and CD73 in transplant recipients BM at the time of myeloablative conditioning. In addition, we also activated ex vivo Nlrp3 inflammasome in HSPCs to be transplanted by specific activator nigericin. In control experiments, eATP stimulated Nlrp3 inflammasome activity was inhibited by the HO-1 activator that is CoPP. Homing of HSPCs was evaluated by measuring a number of donor-derived fluorochrome-labeled cells and clonogenic progenitors in BM of myeloablated hosts at 24 hours after transplantation. Early engraftment was assessed by counting the number of CFU-S and clonogeneic progenitors 12 days after transplantation and by evaluating kinetics of recovery of PB hematopoietic cell counts. Finally, while activation of Nlrp3 inflammasome was assessed by immunofluorescence assay, membrane lipid raft formation was evaluated by confocal microscopy. Results. We noticed that homing and engraftment of HSPCs was significantly accelerated after i) short exposure before transplantation to eATP, ii) inhibition of eAdo formation by CD39, and CD73 inhibitors, and iii) activation of Nlrp3 inflammasome by nigericin. Similarly, inhibition of eAdo formation in recipient BM microenvironment of transplanted mice by CD39 and CD73 inhibitors also improved homing and engraftment efficiency. This correlated with activation in the eATP-dependent manner of Nlrp3 inflammasome in HSPCs followed by membrane lipid raft formation. In the BM microenvironment, upregulation of eATP and inhibition of eAdo also enhanced expression of homing chemoattractants. Conclusions. Since all purinergic signaling modifiers employed in our studies are non-toxic against HSPCs, our data obtained in the animal model indicates that modulation of purinergic signaling before transplantation in HSPCs as well as in BM of the myeloablated recipient would significantly accelerate hematopoietic recovery after hematopoietic transplantation.

Disclosures
No relevant conflicts of interest to declare.