Enhanced Human Hematopoietic Stem Cell Self-Renewal Enabled by Controlling Feedback Signaling From Lineage Committed Cells
Authors: Csaszar Elizabeth , Kirouac Daniel , Yu Mei , Ito Caryn , Zandstra Peter W. 
Published: October 2019

Abstract
Abstract 1274
Clinical outcomes of hematopoietic stem cell (HSC) transplantation are correlated with infused progenitor cell dose. Limited cell numbers in a typical umbilical cord blood (UCB) unit restricts the therapeutic potential of UCB and motivates ex vivo expansion of these cells. Strategies to grow HSCs have relied on the supplement of molecules acting directly on the stem cell population; however, in all cases, sustained HSC growth is limited by the concurrent growth of more mature cells and their endogenously produced inhibitory signaling factors. Despite increasing evidence for the important role of intercellular (between cell) communication networks, the identity and impact of non-stem cell autonomous feedback signaling remains poorly understood. Simultaneous kinetic tracking of more than 30 secreted factors produced during UCB culture, including TGF-b1, MIP-1b, and MCP-1, in combination with computational simulations of cell population dynamics, enabled us to develop a global control strategy predicted to reduce inhibitory paracrine signaling and, consequently, increase HSC self-renewal. By maintaining endogenously produced ligands at specified levels using a tuneable fed-batch (automated media dilution) strategy, we achieved significant improvements in expansions of total cell numbers (∼180-fold), CD34+ cells (∼80-fold), and NOD/SCID/IL-2Rgc-null (NSG) repopulating cells (∼11-fold, detected at limiting dilution). The fed-batch strategy has been integrated into an automated bioreactor, allowing for the generation of a clinically-relevant cell product after 12 days of culture, with minimal user manipulation. As this strategy targets the HSC environment and not the stem cells directly, it has the ability to act in combination with other expansion strategies to produce synergistic results. Unexpectedly, supplementation of the soluble protein, TAT-HOXB4, to the system, yielded the expected boost in progenitor expansion only in “sub-optimal” control conditions but not in the fed-batch system. Hypothesizing that the efficacy of HOXB4 may be dependent on the skewing of supportive vs. non-supportive cell populations, and the consequent impact of paracrine ligand production, we performed kinetic tracking of 20 hematopoietic cell types during several supportive (fed-batch, HOXB4 supplemented, Notch ligand Delta1 supplemented) vs. non-supportive (control) cultures. Meta analysis of these data revealed a non-autonomous link between HOXB4, increased megakaryocyte production, and stem cell proliferation, as well as between Notch delta-1 ligand, decreased myeloid cell production, and a decrease in the growth inhibition of stem cells. These predictions have been experimentally validated using co-cultures of sorted purified HSCs and CD41+ megakaryocykes and CD14+ monocytes. Our results identify complex connections between mature cell lineages and stem cell fate decisions and we expect to report a direct link between cell-cell interactions emerging from culture manipulations and the resulting impact on HSC self-renewal. Collectively, these studies support a dominant role for non-stem cell autonomous feedback signaling in the regulation of HSC self-renewal. Overcoming cell non-autonomous inhibition of HSC self-renewal has allowed for novel strategies to enhance HSC numbers ex vivo, thereby facilitating the production of clinically relevant quantities of stem and progenitor cells and enabling more effective strategies to treat hematologic disease.

Disclosures:
No relevant conflicts of interest to declare.