Therapeutically Enhancing Stem Cell Engraftment

Hematopoietic stem cell transplantation is often the only curative treatment modality for a number of hematologic diseases. There are more than 50,000 bone marrow transplants performed each year worldwide. Despite this fact, there are virtually no therapies in clinic today to support or enhance the engraftment of transplanted stem and progenitor cells. In 2015, several high-profile studies were published, which have begun adding to the toolkit of potential therapies to enhance stem cell engraftment.

One therapeutic option that is currently being explored is treating the stem cell graft ex vivo with 16,16-dimethyl prostaglandin E2 (dmPGE2)¹  — a long-acting analogue of the naturally occurring prostaglandin, which is derived from arachidonic acid via the cyclooxygenase enzymes, COX1 and COX2 (Figure). Two articles in 2015 have now expanded the realm of manipulating arachidonic acid-derived eicosanoids to enhance stem cell engraftment. Work published from the laboratory of Dr. Leonard Zon utilized a novel competitive transplantation system in transparent zebrafish, allowing for direct visualization of stem cell engraftment.²  They screened 480 chemicals with known biologic activity for the ability to alter hematopoietic stem cells, and several hits targeted eicosanoids, including 11,12 epoxyeicosatrienoic acid (11,12-EET) and 14,15-EET, which are derived from arachidonic acid via cytochrome P450 epoxygenases (Figure). Mouse bone marrow was treated ex vivo with 11,12-EET for four hours and then washed away (similar to the dmPGE2 protocol), prior to transplantation into lethally irradiated recipients. This short pulse with 11,12-EET resulted in enhanced homing and in increases in both short and long-term hematopoietic reconstitution. These studies highlight the broad role of eicosanoids in regulating stem and progenitor function, and they suggest yet another potential therapeutic strategy utilizing short, ex vivo pulses with small molecules to enhance engraftment. In the future it may be possible to treat a hematopoietic graft with a cocktail of various factors targeting multiple pathways leading to super enhancements.

While the 11,12-EET strategy followed the aforementioned concept of an ex vivo pulse, a team of collaborators at Case Western and UT Southwestern utilized an approach of altering the environment within the recipient of a transplant.³  Many therapeutics that we have today, most notably nonsteroidal anti-inflammatory drugs (NSAIDs), focus on inhibiting COX enzyme activity to decrease PGE₂ levels. However, as PGE₂ enhances stem cell function, the authors evaluated methods to increase the natural levels of PGE₂. They accomplished this enhancement with the small molecule SW033291, which inhibits 15-hydroxyprostaglandin dehydrogenase (15-PGDH), a key enzymatic step in the breakdown of PGE₂ (Figure). When mice were treated with SW033291 prior to transplantation, there were enhancements to the microenvironment, including elevated stromal derived factor-1 (SDF-1), a chemoattractant and supportive molecule for stem cells, and the growth factor c-kit ligand (stem cell factor [SCF]). Additionally, there was improvement in stem and progenitor cell homing to the bone marrow niche, accelerated recovery of neutrophils and platelets, and enhanced survival in a limited cell transplantation assay. These studies outline a potential therapeutic strategy to augment engraftment by altering the recipient environment those cells are entering, rather than altering the stem cell graft itself.

This past year has not only brought exciting new studies showing that stem cell engraftment can be enhanced by ex vivo manipulation of the hematopoietic graft, or by in vivo enhancement of the recipient environment, but also by simply harvesting the graft with enhancing protocols. Earlier this year in The Hematologist,⁴  we highlighted work from the Broxmeyer lab that demonstrated exposure to ambient air could lead to a phenomenon they termed “extraphysiologic oxygen shock/stress” or EPHOSS, which led to reductions in stem cell function.⁵  When bone marrow was harvested in the continuous presence of Cyclosporin A, there was a reversal of the deleterious effects of ambient air exposure, with significant increases in stem cell number and competitive transplantation. These results also extended to human cord blood samples harvested in Cyclosporin A. Given the recurring interest in stem cell expansion and ex vivo manipulation (i.e. gene therapy), the discovery of EPHOSS effects, and more importantly, strategies to mitigate the negative effects on stem cells, represented important contributions to therapeutic enhancement of stem cell engraftment in 2015.