ヒト骨髄から単離することMesangiogenic前駆細胞（MPCは）

In the last decades, MSCs have been extensively researched and pre-clinically evaluated for possible application in the treatment of various bone/articular, immunological, neurological, cardiovascular, gastrointestinal and hematological disorders^14,15. The easy and inexpensive isolation of multipotent MSCs, from many different tissues, together with their lack of significant immunogenicity¹⁶, contribute to make these cells one of the most interesting cell population to be applied in cell based therapies. Nonetheless, the very low frequency in the tissue of origin represents a great limitation to the MSCs application in clinics, forcing the expansion of these cells, in vitro, before the infusion or transplantation.

Expanded MSC cultures have revealed high grades of heterogeneity and variability^17-19 making it difficult to reach a consensus about MSC production and characterization protocols. Moreover, recent investigations suggested the presence of multiple in vivo MSC ancestors in a wide range of tissues, which contribute to culture heterogeneity^10,20. In fact, it has been proposed that particular culture conditions possibly select or simply promote specific sub-populations of MSCs progenitors present, in various percentages, in “crude” and unprocessed samples like bone marrow (hBM-MNCs) or adipose tissues (stromal vascular fraction)². Thus, the variability in MSC-initiating cell populations together with the great number of different enrichment/isolation and culture protocols applied, represent a great obstacle to the definition of feasible MSC-based therapies.

A crucial factor affecting heterogeneity of MSC cultures is serum supplementation²¹. In our hands replacement of FBS with PhABS in primary cultures from hBM-MNCs, combined with high density seeding on hydrophobic plastics, led to the isolation of a novel highly adherent cell population with distinct biological features named MPCs^5,6. We observed that the addition of small percentages of PhABS to FBS primary cultures also allowed MPC isolation, suggesting the presence of MPC inducing agents in the human serum⁶. At the moment, the MPC isolation/characterization protocol is a unique method available to obtain almost pure MPCs. The protocol has been carefully adjusted and it is highly reproducible for quality screening of MPC preparations before further applications.

MPCs could be used as a source for MSC production, thus limiting the variability introduced by use of unfractionated starting material. The precise definition of the multiple steps characterizing MPC mesengenic differentiation reported⁹ would allow synchronized mesenchymal cell expansion. Nonetheless, this latest condition could be realized exclusively applying highly purified MPC population, as a consequence the characterization of the cell products obtained by the protocol described here, results of crucial importance. This isolating method has been reported allowing MPC recovery with purity generally around 95%. However, donor/patient variability together with the variability related to the different batches of human pooled serum applied, could lead to a significant percentage of MSC-like cells co-isolated together with MPCs, under selective conditions.

It is not clear if these “contaminating” MSC-like cells could arise from the other different in vivo progenitors described in bone marrow²² or from uncontrolled and spontaneous MPC differentiation. In any case, a consistent percentage of MSC-like cells in the MPC products nullify the possibility to applying these cells as homogeneous starting material for the MSC expansion. Thus, here it has been suggested a simple and inexpensive method, based on the MPC resistance to trypsin digestion, increasing the purity of the MPC products. Similar or even better results in purifying MPC cultures could be achieved by fluorescent or magnetic cell sorting performing CD73 and/or CD90 depletion, but significantly prolonging the process time and increasing the costs.

Moreover, MPCs showed expression of pluripotency-associated markers and Nestin, all rapidly lost during mesengenic differentiation⁷. Sprouting assay revealed MPC ability to invade murine ECM protein gel. Taken together these results indicate that MPCs have to be considered a more immature progenitor, retaining angiogenic potential. Nonetheless, the initial enthusiasm about mesodermal differentiation potential of MPCs is actually waning. In fact, after more than 7 years of studies on MPCs, mesengenic and angiogenic potential have been extensively described^5-9, but differentiation toward any other cells of mesodermal origin is still lacking. Thus, here we propose a new, and more rigorous, definition of these cells as “Mesangiogenic Progenitor Cells”, maintaining the acronym MPCs.

We also believe that most controversies about MSC angiogenic potential could be related to the heterogeneous composition of expanded cultures consisting of sub-populations of MPCs and MSCs in variable percentages²³.

Finally, MPCs could also play a crucial role for the implementation of CBMPs applicable for tissue reconstruction, as these cells could also support the neo-vascularization. In fact, future studies on regeneration should take in consideration that the newly formed tissue growth should be supported by concomitant neo-angiogenesis. The co-existence of mesengenic and angiogenic potential in MPCs could significantly improve the regeneration potential of new therapeutic approaches that involve these interesting cells.