Ex vivo expansion of human hematopoietic stem and progenitor cells

Abstract

Despite progress in our understanding of the growth factors that support the progressive maturation of the various cell lineages of the hematopoietic system, less is known about factors that govern the self-renewal of hematopoietic stem and progenitor cells (HSPCs), and our ability to expand human HSPC numbers ex vivo remains limited. Interest in stem cell expansion has been heightened by the increasing importance of HSCs in the treatment of both malignant and nonmalignant diseases, as well as their use in gene therapy. To date, most attempts to ex vivo expand HSPCs have used hematopoietic growth factors but have not achieved clinically relevant effects. More recent approaches, including our studies in which activation of the Notch signaling pathway has enabled a clinically relevant ex vivo expansion of HSPCs, have led to renewed interest in this arena. Here we briefly review early attempts at ex vivo expansion by cytokine stimulation followed by an examination of our studies investigating the role of Notch signaling in HSPC self-renewal. We will also review other recently developed approaches for ex vivo expansion, primarily focused on the more extensively studied cord bloodderived stem cell. Finally, we discuss some of the challenges still facing this field. (Blood. 2011;117(23):6083-6090) IntroductionThe hierarchical development of the hematopoietic system has become progressively better understood over the past few decades, aided in part by significant advances in identifying and isolating hematopoietic stem cells (HSCs) and their progeny. 1 Although advances have been made in understanding the hematopoietic growth factors that support the progressive maturation of the various cell lineages, less is known about factors that govern the self-renewal of hematopoietic stem cells and multipotent progenitor cells (MPPs) that consist of short-term repopulating stem cells and give rise to the different cell lineages, thereby impacting the ability to expand HSC and MPP (hematopoietic stem and progenitor cell [HSPC]) numbers ex vivo. Initial attempts at ex vivo expansion of HSCs focused on the use of soluble cytokines known to support lineage committed cells with the expectation that some of these factors also supported HSC proliferation. 2 These studies were based on the belief that cell lineage determination was a stochastic process combined with positive and negative cytokinemediated regulatory responses controlling survival and expansion of the stem cell population. 3 More recently, recognition of factors critical for embryologic development as well as discovery of other novel pathways that may influence HSC self-renewal have led to renewed interest in ex vivo expansion, which has been heightened by the increasing importance of HSPCs in the treatment of both malignant and nonmalignant diseases as well as their use in gene therapy.To date, most attempts to expand HSPC ex vivo for enhanced in vivo engraftment in patients have been clinically unsuccessful because of generation of insufficient cell numbers or improper...

Regulation of human hematopoietic stem cell self-renewal by the microenvironment’s control of retinoic acid signaling

“…In contrast, assayable primitive hematopoietic cells, such as cells capable of forming cobblestones after 8 wk of coculture with stromal cells (week 8 cobblestone area forming cells or CAFCW8) (18) and SRCs, are highly enriched in the CD34 + CD38 − cell fraction (19)(20)(21). Upon culture in the presence of growth factors, CD34 + CD38 − cells will proliferate and differentiate with the generation of "de novo" CFU-Cs (21); however, little self-renewal occurs, and HSC activity is rapidly exhausted (22).…”

Evaluating Interaction of Cord Blood Hematopoietic Stem/Progenitor Cells with Functionally Integrated Three-Dimensional Microenvironments

“…Nevertheless, the challenge of expanding long‐term repopulating HSC from a single unit, in such a way that sufficient numbers of HSC will be obtained to engraft an adult, while simultaneously increasing progenitor populations that are able to shorten time to engraftment, has yet to be solved 1, 2.…”

Niche-less maintenance of HSCs by 2i

“…In summary, dual inhibition (2i) of GSK-3 and mTOR allows for the maintenance of human and mouse LT-HSCs in vitro ( Figure 1C), and this may resolve the difficulty in culturing HSCs, which in turn, may improve basic research of HSCs (e.g., gene editing in vitro) and human HSC transplantation outcomes. Furthermore, although the effect of 2i on expansion of HSCs is relatively small, a combination of 2i drugs may increase human clinical trials [1,6,7] that use 2i in vivo for the aim of increasing the number of LT-HSCs, since 2i drugs are known as clinically tolerated medications. Insights gained from the discovery of 2i for HSC maintenance may lead to great benefits for patients with hematologic disorders, hopefully in the near future.…”