Within the bone marrow stroma there exists a subset of nonhematopoietic cells referred to as mesenchymal stem or mesenchymal progenitor cells. These cells can be ex vivo expanded and induced, either in vitro or in vivo, to terminally differentiate into osteoblasts, chondrocytes, adipocytes, tenocytes, myotubes, neural cells, and hematopoietic-supporting stroma. The multipotential of these cells, their easy isolation and culture, as well as their high ex vivo expansive potential make these cells an attractive therapeutic tool. In this work we will review the information dealing with the biology of mesenchymal progenitors as it has been revealed mainly by ex vivo studies performed with bone marrow-derived cells. The discussed topics include, among others, characteristics of mesenchymal progenitors, evidence for the existence of a vast repertoire of uncommitted and committed progenitors both in the bone marrow and in mesenchymal tissues, a diagram for their proliferative hierarchy, and comments on mobilization, microenvironment, and clinical use of mesenchymal progenitors. Despite the enormous data available at molecular and cellular levels, it is evident that a number of fundamental questions still need to be resolved before mesenchymal progenitors can be used for safe and effective clinical applications in the context of both cell and gene therapies.

Mesenchymal stem cells

Recent studies describe beneficial effects of bone marrow-derived mesenchymal stem cell infusion in animal models as well as in patients. However, data on the homing abilities of primary and culture-expanded MSC are lacking. In order to systematically investigate MSC homing we compared the fate of both primary and cultured MSC in a syngeneic mouse model. Twenty-four hours after transplantation of uncultured EGFP-transgenic MSC into sublethally irradiated mice, as many as 55-65% of injected CFU-F were recovered from the BM and 3.5-7% from the spleen. In the subsequent 4 weeks these donor CFU-F expanded 100-fold, which resulted in a normalization of femoral and splenic CFU-F numbers. This highly efficient homing of primary CFU-F contrasted with the defective homing of MSC following culture. Following their infusion immortalized multipotent syngeneic stromal cells were undetectable in BM, spleen, lymph nodes or thymus. Remarkably, following transplantation of primary MSC that had been cultured for only 24 h the seeding fraction in the BM was reduced to 10%, while after transplantation of 48 h cultured primary MSC no CFU-F were detected in the lymphohematopoietic organs. These data suggest that in vitro propagation of BM-derived MSC dramatically decreases their homing to BM and spleen.

Primary murine MSC show highly efficient homing to the bone marrow but lose homing ability following culture.

Hematopoietic stem cell homing and engraftment are crucial to transplantation efficiency, and clinical engraftment is severely compromised when donor-cell numbers are limiting. The peptidase CD26 (DPPIV/dipeptidylpeptidase IV) removes dipeptides from the amino terminus of proteins. We present evidence that endogenous CD26 expression on donor cells negatively regulates homing and engraftment. By inhibition or deletion of CD26, it was possible to increase greatly the efficiency of transplantation. These results suggest that hematopoietic stem cell engraftment is not absolute, as previously suggested, and indicate that improvement of bone marrow transplant efficiency may be possible in the clinic.

https://science.sciencemag.org/content/sci/305/5686/1000.full.pdf

Modulation of Hematopoietic Stem Cell Homing and Engraftment by CD26

Prolonged exposure of human hematopoietic stem cells (HSC) to growth factors for efficient transduction by murine oncoretroviral vectors has major detrimental effects on repopulating activity. In this study, we have used a vesicular stomatitis virus G envelope protein (VSV-G)-pseudotyped human immunodeficiency virus type 1 (HIV-1) lentiviral-based vector system to transduce cord blood (CB) CD34+ cells over a limited time period (≤24 hours). Under these conditions, significant gene marking was observed in engrafted human lymphoid, myeloid, and progenitor cells in all transplanted Severe Combined Immunodeficient (SCID) mice. To enhance the level of gene expression in hematopoietic cells, we also generated a series of lentiviral vectors incorporating the spleen focus forming virus (SFFV) long terminal repeat (LTR) sequences, and the Woodchuck hepatitis virus posttranscriptional regulatory element (WPRE). By including the central polypurine tract (cPPT) sequence of HIV-1 we were then able to achieve high leve...

High-level transduction and gene expression in hematopoietic repopulating cells using a human imunodeficiency virus type 1-based lentiviral vector containing an internal spleen focus forming virus promoter

Isolation of a highly quiescent subpopulation of primitive leukemic cells in chronic myeloid leukemia.

Cell Cycle-Related Changes in Repopulating Capacity of Human Mobilized Peripheral Blood CD34+ Cells in Non-Obese Diabetic/Severe Combined Immune-Deficient Mice

J. C. M. Van der loo

Papers

Cell Cycle-Related Changes in Repopulating Capacity of Human Mobilized Peripheral Blood CD34+ Cells in Non-Obese Diabetic/Severe Combined Immune-Deficient Mice