Role of ex vivo Expanded Mesenchymal Stromal Cells in Determining Hematopoietic Stem Cell Transplantation Outcome

Abstract

Overall, the human organism requires the production of ∼1 trillion new blood cells per day. Such goal is achieved via hematopoiesis occurring within the bone marrow (BM) under the tight regulation of hematopoietic stem and progenitor cell (HSPC) homeostasis made by the BM microenvironment. The BM niche is defined by the close interactions of HSPCs and non-hematopoietic cells of different origin, which control the maintenance of HSPCs and orchestrate hematopoiesis in response to the body's requirements. The activity of the BM niche is regulated by specific signaling pathways in physiological conditions and in case of stress, including the one induced by the HSPC transplantation (HSCT) procedures. HSCT is the curative option for several hematological and non-hematological diseases, despite being associated with early and late complications, mainly due to a low level of HSPC engraftment, impaired hematopoietic recovery, immune-mediated graft rejection, and graft-versus-host disease (GvHD) in case of allogenic transplant. Mesenchymal stromal cells (MSCs) are key elements of the BM niche, regulating HSPC homeostasis by direct contact and secreting several paracrine factors. In this review, we will explore the several mechanisms through which MSCs impact on the supportive activity of the BM niche and regulate HSPC homeostasis. We will further discuss how the growing understanding of such mechanisms have impacted, under a clinical point of view, on the transplantation field. In more recent years, these results have instructed the design of clinical trials to ameliorate the outcome of HSCT, especially in the allogenic setting, and when low doses of HSPCs were available for transplantation.

Keywords: bone marrow niche; hematopoietic (stem) cell transplantation; hematopoietic stem and progenitor cell; immunoregulation; mesenchymal stromal cells.

FIGURE 1

Schematic representation describing the clinical use of MSCs in the context of hematopoietic stem cell transplantation (HSCT). (A) MSCs has been successfully employed to reduce the risk and to treat graft-versus-host disease (GvHD) in transplanted pediatric and adult patients. Thanks to their ability to sense inflammatory stimuli, MSCs are capable to modulate T-cell proliferation and activation through the release of specific immunomodulatory cytokines (IL10, TGFb, and PGE2). (B) MSCs have been used as a feeder to expand and maintain UCB-CD34⁺ HSCs before transplantation due to their ability to secrete HSC supportive factors. (C) The co-infusion of MSCs has been demonstrated to promote HSC engraftment and accelerate hematological reconstitution in transplanted patients. Despite only a small percentage of infused MSCs reaches the BM niche, the production and release of supportive factors by co-infused MSCs ameliorates the outcome of HSCT. (D) The ability of MSC to repair and differentiate in bone cells makes them an attractive cell candidate to restore a proper BM niche, with MSCs and MSC-derived osteoblasts capable of supporting HSCs. (BM, bone marrow; UCB, umbilical cord blood). For each clinical application, works describing clinical trials for the use of MSCs are reported.

FIGURE 2

Schematic representation of the hematopoietic supportive activities of human mesenchymal stromal cells (MSCs). MSCs support hematopoietic stem and progenitor cell (HSPC) homeostasis by cell-contact, through the interaction of specific ligands expressed on MSCs surface with HSPC receptors (N-cadherin and Notch). Adherent junction, such as connexin43, also play a role in the control of HSPC metabolism to protect cells from excessive activation. However, MSCs exert their hematopoietic function mainly through the secretion of supportive factors and the release of extracellular vesicles and exosomes. HSC models are licensed by https://creativecommons.org/licenses/by/3.0/legalcode.

FIGURE 3

In vitro strategies to fully exploit the hematopoietic supportive capacity of ex vivo expanded MSCs. Schematic representation of culture strategies aimed at improving the hematopoietic supportive capacity of MSCs, which is impaired upon ex vivo expansion. (A) Several strategies (mesensphere, low oxygen, and low glucose cultures) have been developed to avoid the activation of a senescence program altering the secretory phenotype of ex vivo expanded MSCs. (B) MSC priming with inflammatory cytokines has been demonstrated to increase the release of anti-inflammatory cytokines by MSCs. Similarly, exposure to culture stress increases the survival rate of MSCs when transplanted in vitro. The immunomodulatory activity of MSCs has been exploited for the treatment of GVHD by inducing cell apoptosis in ex vivo expanded cells. (C) Ex vivo reprogramming by the overexpression of specific transcription factors has been studied as a strategy to render ex vivo expanded MSCs similar to primary cells, with a potentiated hematopoietic supportive ability.